Journal Articles

Journal Papers |  Books and Book Chapters  |  Conference Proceedings

Due to journal ownership of copyrights, I have not posted pdf copies of the papers here.  Instead, I have posted links to the journal sites, which should work if your institution has access.  If you cannot obtain an article via the link, I will be happy to send it to you upon request: contact me at smela@umd.edu.

Click on Year to sort by year, and click on Code to sort by the paper topic codes given above.

Year	Code	Citation
2022	EIT	E. Smela "EIT for tactile sensing: considerations regarding the injection-measurement pattern" Eng. Res. Expr., 4(4), 045041, (2022) [Abstract] direct link to PDF on IOP site Bipolar current injection and voltage measurement (I–M) patterns are frequently used in electrical impedance tomography (EIT) for tactile sensing. In this work, the total set of 36 unique combinations for 16-electrode systems is investigated using simulations. Performance is evaluated on a circular sensor as a function of hyperparameter and target position with respect to critical performance measures for tactile sensing; these include not only peak amplitude and resolution, but also susceptibility to noise and, importantly, the uniformity of performance over the sensor area. The determination of which pattern to employ can therefore be based on the needs of the particular application. The relative performance of the I–M patterns is determined at small hyperparameters by electrode placement symmetry, but at large hyperparameters by sensitivity at the center of the sensor. Patterns with high spatial symmetry should be avoided; these include electrode pairs on opposite sides of the sensor. Patterns with electrodes in adjacent positions, which have been the norm for tactile sensing, should also not generally be used. If performance on all metrics across a wide range of hyperparameters is needed, then placing both the injection and measurement electrodes 3 spaces apart (the 3–3 pattern) can be a good strategy for the Gauss-Newton 1-step algorithm with the Laplacian prior. The use of 13 electrodes instead of 16 is also examined. The absence of symmetry provides greater flexibility in the choice of I–M pattern, and the loss in performance may be small; furthermore, the reduction in data collection time may be advantageous. Beyond avoiding the worst I–M patterns, the most important measure to take is decreasing noise on the signal to permit the use of a smaller hyperparameter; this is of greater importance than selecting from among the remaining I–M patterns.
2022	EIT	E. Smela "EIT for tactile sensing: considering artefacts in hyperparameter selection" Eng. Res. Expr., 4(3), 035057, (2022) [Abstract] direct link to PDF on IOP site Electrical impedance tomography (EIT) is employed in tactile sensing to create an image of impedance changes within a continuous sensor using electrodes placed only at the perimeter. Noise destabilizes EIT images, and the onset of instability is associated with the appearance of artefacts, which are spurious image features that are not associated with sensor responses to contacts. Artefacts are detrimental because the essential features of contacts, or targets, must be correctly represented, including how many there are and their approximate shapes and locations, yet their presence has not previously been used as a performance measure. Regularization, the extent of which is determined by the hyperparameter lambda, is used to manage the destabilization, but it results in spatially non-uniform defocusing of image features. We therefore introduce an efficient criterion for evaluating tactile sensor image quality based on the onset of artefacts. Using simulated data and the one-step Gauss-Newton reconstruction algorithm with the Laplace prior, the noise level at which artefacts first appear at a given hyperparameter, or noise threshold Nth(lambda), is found. How the relationship depends on target characteristics and other factors is shown, and Nth can vary by orders of magnitude. The conceptually similar BestRes method and the classical L-curve and generalized cross-validation (GCV) methods for determining an optimal hyperparameter are evaluated using the criterion of artefact-free images. The L-curve generates hyperparameters that are well matched to the onset of artefacts, except at high noise; the other two result in artefacts. For high dynamic range tactile inputs, setting the threshold at a fixed value using a method such as Nth is not advisable, and automatic regularization tailored to the input may be needed using a method such as the L-curve or GCV, provided that the computational overhead is tolerable.
2019	sens-cell	B. P. Senevirathna, S. Lu, M. Dandin, E. Smela, and P. Abshire "Correlation of capacitance and microscopy measurements using image processing for a lab-on-CMOS microsystem" IEEE Trans. Biomed. Circ. Syst., 13(6), 1214-1225, (2019) [Abstract] direct link to PDF on IEEE site We present a capacitance sensor chip developed in a 0.35 µm CMOS process for monitoring biological cell viability and proliferation. The chip measures cell-to-substrate binding through capacitance-to-frequency conversion with a sensitivity of 590 kHz/fF. In vitro experiments with two human ovarian cancer cell lines (CP70, A2780) were performed and showed the ability to track cell viability in real-time over three days. An imaging platform was developed to provide time-lapse images of the sensor surface, which allowed for concurrent visual and capacitance observation of the cells. Results showed the ability to detect single cell binding events and changes in cell morphology. Image processing was performed to estimate cell coverage of sensor electrodes, showing good linear correlation and providing a sensor gain of 1.28 ± 0.29 aF/µm2, which agrees with values reported in literature. The device is designed for unsupervised operation with minimal packaging requirements. Only a microcontroller is required for readout, making it suitable for applications outside the traditional laboratory setting.
2019	sens-cell	B. Senevirathna, S. Lu, M. Dandin, J. Basile, E. Smela, and P. Abshire "High resolution monitoring of chemotherapeutic agent potency in cancer cells using a CMOS capacitance biosensor" Biosens. Bioelectron. 142, 111501, DOI https://doi.org/10.1016/j.bios.2019.111501 (2019) [Abstract] direct link to PDF on Science Direct Monitoring cell viability and proliferation in real-time provides a more comprehensive picture of the changes cells undergo during their lifecycle than can be achieved using traditional end-point assays. Particularly for drug screening applications, high-temporal resolution cell viability data could inform decisions on drug application protocols that might lead to better treatment outcomes. We describe a CMOS biosensor that monitors cell viability through high-resolution capacitance measurements of cell adhesion quality. The system consists of a 3 × 3 mm2 chip with an array of 16 sensors, on-chip digitization, and serial data output that can be interfaced with inexpensive off-the-shelf components. An imaging system was developed to provide ground-truth data of cell coverage concurrently with data recordings. Results showed the sensor's ability to detect single-cell binding events, track cell morphology changes, and monitor cell motility. A chemotherapeutic assay was conducted to examine dose-dependent cytotoxic effects on drug-resistant and drug-sensitive cancer cell lines. Concentrations higher than 5 µM elicited cytotoxic effects on both cell lines, while a dose of 1 µM allowed discrimination of the two cell types. The system demonstrates the use of real-time capacitance measurements as a proof-of-concept tool that has potential to hasten the drug development process. - email me for copy if no access through your institution
2019	sens-piezo	A. Ramadan, A. Roy, and E. Smela "Recognizing hemiparetic ankle deficits using wearable pressure sensors" IEEE J. Trans. Eng. Health Med. 7(1-3), DOI: 10.1109/JTEHM.2019.2942922, (2019) [Abstract] direct link to PDF on IEEE site Objective: To provide proof-of-concept for a novel method to recognize impaired push-off and foot-drop deficits in hemiparetic gait using analog pressure sensors. These data may enhance feedback from a modular ankle exoskeleton (such as Anklebot) for stroke rehabilitation, which now employs on/off foot switches under the foot. Methods: A pressure sensor was positioned on the posterior side of the calcaneus. Experiments were conducted on two healthy subjects with normal walking and with hip circumduction and foot drop, the latter to mimic hemiparetic gait post-stroke. Results: Unlike the foot switches, the pressure sensor yielded data during swing. The initial swing and terminal stance readings followed local foot-shoe dynamics and were thus able to detect foot drop swing deficits while also providing push-off information during stance. Discussion: The analog pressure sensors provided more information than foot switches, even during stance. This system may provide clinicians with a tool to monitor foot drop and push-off.
2018	sens-piezo	Y. Chen, M. Yu, H. A. Bruck, and E. Smela "Compliant multi-layer tactile sensing for enhanced identification of human touch" Smart Mater. Struct. 27 (12), 125009, DOI: 10.1088/1361-665X/aaeae4 (2018) [Abstract] direct link to PDF on IOP site - email me for copy if no access through your institution Tactile sensing is of interest for facilitating interactions between robots and humans. To aid the robot’s interpretation of human contact, the use of a multilayer cutaneous tactile sensing architecture that can provide more information and an expanded force sensing range was explored, revealing differences in the signal generated by a machine versus a human. The multi-layer system consisted of two stretchable sensing skins alternating with two foam layers of different stiffness. When human touch was compared with machine indentation, a large variability in human touch was found. Thus, although the topmost skin, placed over a soft foam, was able to better recognize light contacts, a second underlying skin, placed over a harder foam, was required to gauge stronger human contacts. Out-of-plane touch modalities, such as tapping and punching, could be identified using strip-shaped skins having just two electrodes. To provide distributed sensing, the technique of electrical impedance tomography was employed with the multi-layer architecture using larger-area skins having electrodes around the perimeter. Distributed touch modalities, such as multi-point finger presses and sliding, were distinguished from single-point pressing. The distributed multi-layer sensing system also had the ability to assess higher-force touches.
2018	sens-cell	B. Senevirathna, S. Lu, M. Dandin, J. Basile, E. Smela, and P. Abshire "Real-time measurements of cell proliferation using a lab-on-CMOS capacitance sensor array" IEEE Trans. Biomed. Circ. Syst. 12(3), 510-20, DOI: 10.1109/TBCAS.2018.2821060 (2018) [Abstract] direct link to PDF on IEEE site We describe a capacitance sensor array that has been incorporated into a lab-on-CMOS system for applications in monitoring cell viability. This paper presents analytical models, calibration results, and measured experimental results of the biosensor. The sensor has been characterized and exhibits a sensitivity of 590 kHz/fF. We report results from benchtop tests and in vitro experiments demonstrating on-chip tracking of cell adhesion as well as monitoring of cell viability. Human ovarian cancer cells were cultured on chip, and measured capacitance responses were validated by comparison with images from photomicrographs of the chip surface. Analysis was performed to quantify cell proliferation and adhesion, and responses to live cells were estimated to be 100 aF/cell.
2018	sens-piezo	Y. Chen, M. Yu, H. A. Bruck, and E. Smela "Characterization of a compliant multi-layer system for tactile sensing with enhanced sensitivity and range" Smart Mater. Struct. 27:065005, DOI: 10.1088/1361-665X/aabc29 (2018) [Abstract] direct link to PDF on IOP site - email me for copy if no access through your institution To allow robots to interact with humans via touch, new sensing concepts are needed that can detect a wide range of potential interactions and cover the body of a robot. In this paper, a skin-inspired multi-layer tactile sensing architecture is presented and characterized. The structure consists of stretchable piezoresistive strain-sensing layers over foam layers of different stiffness, allowing for both sufficient sensitivity and pressure range for human contacts. Strip-shaped sensors were used in this architecture to produce a deformation response proportional to pressure. The roles of the foam layers were elucidated by changing their stiffness and thickness, allowing the development of a geometric model to account for indenter interactions with the structure. The advantage of this architecture over other approaches is the ability to easily tune performance by adjusting the stiffness or thickness of the foams to tailor the response for different applications. Since viscoelastic materials were used, the temporal effects were also investigated.
2017	sens-piezo	E. M. Barnett, J. J. Lofton, M. Yu, H. Bruck, and E. Smela "Targeted feature recognition using mechanical spatial filtering with a low-cost compliant strain sensor" Sci. Rep. 7:5118, DOI:10.1038/s41598-017-05341-w (2017) [Abstract] direct link to PDF          direct link to supporting information The field of soft robots would benefit from electrically controlled contractile actuators in the form of fibers that achieve a strain of 20% in less than a second while exerting high force. This work explores possible designs for achieving this goal using self-contained electroosmotic fluid pumping within a tube-shaped structure. The most promising configuration is a combination of a bellows and a McKibben-type muscle, since pumping fluid from the former to the latter results in contraction of both portions. Realizing such a device entails challenges in fabrication and electrokinetic fluid pumping in closed systems. Further studies of electroosmotic flow in salt-free organic solvents are needed.
2017	act-nast	E. Smela "Challenges in realizing a self-contained hydraulically-driven contractile fiber actuator" Faraday Discuss. DOI: 10.1039/c6fd00235h (2017) [Abstract] link to PDF on RSC site          direct link to supporting information A tactile sensing architecture is presented for detection of surface features that have a particular target size, and the concept is demonstrated with a braille pattern. The approach is akin to an inverse of mechanical profilometry. The sensing structure is constructed by suspending a stretchable strain-sensing membrane over a cavity. The structure is moved over the surface, and a signal is generated through mechanical spatial filtering if a feature is small enough to penetrate into the cavity. This simple design is tailorable and can be realized by standard machining or 3D printing. Images of target features can be produced with even a low-cost compliant sensor. In this work a disposable elastomeric piezoresistive strain sensor was used over a cylindrical “finger” part with a groove having a width corresponding to the braille dot size. A model was developed to help understand the working principle and guide finger design, revealing amplification when the cavity matches the feature size. The new sensing concept has the advantages of being easily reconfigured for a variety of sensing problems and retrofitted to a wide range of robotic hands, as well as compatibility with many compliant sensor types.
2016	act-nast	D. Sritharan and E. Smela "Fabrication of a miniature paper-based electroosmotic actuator" Polymers 8(11) 400 (2016) [Abstract] link to PDF on MDPI site          direct link to supporting information A voltage-controlled hydraulic actuator is presented that employs electroosmotic fluid flow (EOF) in paper microchannels within an elastomeric structure. The microfluidic device was fabricated using a new benchtop lamination process. Flexible embedded electrodes were formed from a conductive carbon-silicone composite. The pores in the layer of paper placed between the electrodes served as the microchannels for EOF, and the pumping fluid was propylene carbonate. A sealed fluid-filled chamber was formed by film-casting silicone to lay an actuating membrane over the pumping liquid. Hydraulic force generated by EOF caused the membrane to bulge by hundreds of micrometers within fractions of a second. Potential applications of these actuators include soft robots and biomedical devices.
2016	sens-cell	T. Datta-Chaudhuri, E. Smela, and P. Abshire "System-on-chip considerations for heterogeneous integration of CMOS and fluidic bio-interfaces" IEEE Trans. Biomed. Circ. Syst. 10(6)1129-1142 (2016) [Abstract] link to PDF on IEEE site CMOS chips are increasingly used for direct sensing and interfacing with fluidic and biological systems. However, distinct and vexing technical challenges arise from the disparate requirements of biosensors and integrated circuits. The IC design process must take into account operation at 37 °C, electro-thermal effects and corresponding power limitations, additional floorplanning complexity, signal coupling through fluid, and electrochemical effects. System modeling has to address not only circuit design but also physical structures post-fabricated on the surface of the IC and the behavior of biological components. The next step is packaging, which must enable electrical operation while also allowing fluidic integration for cell culture; for MEMS processing it should ideally provide a flat surface. At the final stage, cell culture on an IC chip requires surface preparation and biocompatible materials. The opacity of the chip presents challenges in optical assessment, while incubator-free cell culture presents its own issues. We recommend a two-step modeling approach to overcome the lack of multi-domain modeling tools: using FEM simulation to determine electro-thermal and electrostatic effects, and using those results to modify simulation parameters for IC design. These simulations can also aid in material selection and in ascertaining layer thicknesses for packaging.
2016	sens-cell	T. Datta-Chaudhuri, R. C. Araneda, P. Abshire, and E. Smela "Olfaction on a chip" Sens. Act. B 235 (1 Nov), 74–78 (2016) [Abstract] link to PDF on Science Direct site          direct link to supporting information On-chip sensing of physical signals is now relatively straightforward, with on-chip tilt sensors and cell phone cameras now ubiquitous, but sensing of odorants has proven to be far more challenging. A general purpose, mobile electronic nose would address pressing needs in security and food safety. We demonstrate here an approach that combines living olfactory sensory neurons (OSNs) with integrated circuitry. The hybrid bio-electronic chip harnesses the as-yet unparalleled capabilities of olfactory sensory neurons – in variety, selectivity, and ability to function in the real world – and takes advantage of modern silicon chip technology for detecting and processing electrical signals. OSNs were cultured directly over an array of electrodes on the surface of a custom integrated circuit so that the signals, a series of “spikes” upon odorant binding, could be locally detected and processed. Consistent and selective responses to specific odorants were recorded from individual neurons.
2016	sens-piezo	Y. Chen, M. Yu, H. A. Bruck, and E. Smela "Stretchable touch-sensing skin over padding for co-robots" Sens. Act. B 25, 055006 (2016) [Abstract] link to PDF on IOP site          direct link to supporting information For robots that work collaboratively with people, often referred to as “co-robots”, it would be beneficial for them to be soft or padded and to have a touch-sensing “skin” to enable tactile environmental awareness. However, a sensing skin over a padding material that undergoes large deformations requires “stretchable” materials, which may possess time-dependent or viscoelastic mechanical responses. In this work the roles that a padding layer plays when placed under a stretchable sensing layer was investigated. A strain-sensing skin was formed by coating a thin film of compliant piezoresistive sensing paint, consisting of exfoliated graphite in latex, onto a rubber membrane, and the response of the skin was characterized. The change in resistance was linear with tensile strain. The role of the padding material was then investigated under indentation by examining three foams and two elastomers. As expected, the padding enhanced energy dissipation as shown by hysteresis in the sensor response, which is linked to its protective function; the hysteresis was comparable for the five padding materials. The padding also provided an unexpected advantage: it magnified the change in resistance compared to that obtained under free displacement in air. While hysteresis in viscoelastic materials can largely be handled with an appropriate model, inconsistency cannot be, and the two elastomers were found to have unacceptably high variability because of micro-cracks and other defects in these materials. On the other hand, foams that had few defects and regular cell sizes gave good consistency across trials and different sensor positions over the padding. Combined with their lighter weight and availability in a wide range of stiffness, we conclude that foams make a better choice for padding of co-robots.
2015	act-nast	D. Sritharan, A. S. Chen, P. Aluthgama, B. Naved, and E. Smela "Bubble-free electrokinetic flow with propylene carbonate" Electrophoresis 36, 2622-2629 (2015) [Abstract] link to PDF on Wiley site          direct link to supporting information For electroosmotic pumping, a large direct-current electric field (10+ V/cm) is applied across a liquid, typically an aqueous electrolyte. At these high voltages, water undergoes electrolysis to form hydrogen and oxygen, generating bubbles that can block the electrodes, cause pressure fluctuations, and lead to pump failure. The requirement to manage these gases constrains system designs. This article presents an alternative polar liquid for directcurrent electrokinetic pumping, propylene carbonate (PC), which remains free of bubbles up to at least 10 kV/cm. This offers the opportunity to create electrokinetic devices in closed configurations, which we demonstrate with a fully sealed microfluidic hydraulic actuator. Furthermore, the electroosmotic velocity of PC is similar to that of water in PDMS microchannels. Thus, water could be substituted by PC in existing electroosmotic pumps.
2015	sens-piezo	E. Sauerbrunn, Y. Chen, J. Didion, M. Yu, E. Smela, and H. A. Bruck "Thermal imaging using polymernanocomposite temperature sensors" Phys. Stat. Sol. 212 (10), 2239–2245 (2015) [Abstract] link to PDF on Wiley site Sensing temperature distributions over an area is of interest for many applications, and it is currently performed using sensors that are attached to the structure. In this work a conductive “smart paint”, made fromlatex and exfoliated graphite, is introduced for temperature sensing. This provides, as an alternative to manual fixation, the ability to integrate sensors during fabrication because the approach is amenable to additive manufacturing technologies such as 3D printing. We demonstrate that calibration of the spray-coatable polymer/carbon composite thin film sensors allows accurate temperature measurement over an area. We further demonstrate continuous, distributed temperature sensing by employing electrical impedance tomography to reconstruct a thermal image from measurements at the perimeter of the sensing region.
2015	other	I. D. Jung, M. C. Lee, H. Lim, E. Smela, and J. S. Ko "Microbumpers maintain superhydrophobicity of nanostructured surfaces upon touch" Appl. Surf. Sci., 349, 705–714 (2015) [Abstract] link to PDF on Elsevier site          direct link to supporting information Because of their fragility, nanostructured surfaces have not been used in applications that requiremechanical contact with the environment. This paper demonstrates the utility of an array of“microbumpers” in the form of pillars rising above a nanostructured surface to provide protection. Super-hydrophobic surfaces with micro-pillar arrays of varying pitch were fabricated and subjected to repeatedvertical touch with a PDMS finger replica under different applied forces, such as would be experiencedby a touch screen display. For sufficiently small pitches, the microbumpers maintained the strong waterrepellency and low droplet adhesion, even after 1000 touch cycles, but if the pitch was too large thenanostructures were damaged and the superhydrophobicity lost. In comparison, surfaces comprisingonly nanostructures lost superhydrophobicity almost immediately. To image surface wetting by the waterdroplets, a droplet freeze-fixing, resin-embedding (FFRE) technique was developed. The approach of dec-orating a surface with microbumpers to provide mechanical protection should be applicable to a widerange of substrates with coatings optimized for various functions.
2015	act	B. Balakrisnan, A. N. Nacev, and E. Smela "Design of bending multi-layer electroactive polymer actuators" Smart Mater. Struct., 24(045032 (2015) [Abstract] link to PDF on IOP site           direct link to supporting information direct link to Matlab program The effects of layer thickness and stiffness on multilayer bending actuator performance were investigated with an analytical mechanical model. Performance was evaluated in terms of curvature, blocked force, and work. Multilayer device designs corresponding to dielectric elastomer actuator, ionic polymer metal composite, and conjugated polymer structures were examined. Normalized plots of the performance metrics as functions of relative layer thickness and stiffness are presented that should allow initial, starting-point estimates for designs for particular applications. The results show that to achieve high curvature, layer thickness and stiffness may need to be set above or below particular bounds, or varied together, depending on the device configuration; often there is a broad plateau of combinations that work equally well. There is a conflict between achieving high bending and high force: the former requires the device to behave as much as possible like a simple bilayer with optimal ratios of thickness and modulus, while the latter requires thicker layers and shows little dependence on their moduli. Finally, to maximize work there are areas in the thickness-modulus plane that should be avoided, these areas varying with the configuration in sometimes surprising ways.
2014	fluid	J. M. Burke, R. E. Zubajlo, E. Smela, and I. M. White "High-throughput particle separation and concentration using spiral inertial filtration" Biomicrofluidics, 8(2), 024105 (2014) [Abstract] link to PDF on AIP site A spiral inertial filtration (SIFT) device that is capable of high-throughput (1 ml/min), high-purity particle separation while concentrating recovered target particles by more than an order of magnitude is reported. This device is able to remove large fractions of sample fluid from a microchannel without disruption of concentrated particle streams by taking advantage of particle focusing in inertial spiral microfluidics, which is achieved by balancing inertial lift forces and Dean drag forces. To enable the calculation of channel geometries in the SIFT microsystem for specific concentration factors, an equivalent circuit model was developed and experimentally validated. Large particle concentration factors were then achieved by maintaining either the average fluid velocity or the Dean number throughout the entire length of the channel during the incremental removal of sample fluid. The SIFT device was able to separate MCF7 cells spiked into whole blood from the non-target white blood cells (WBC) with a recovery of nearly 100% while removing 93% of the sample volume, which resulted in a concentration enhancement of the MCF7 cancer cells by a factor of 14.
2014	PCR	M. Armani, M. Tangrea, B. Yang, A. Rosenberg, K. Ylaya, J. Martinez, J. Rodriguez-Canales, J. Hanson, B. Shapiro, M. R. Emmert-Buck, E. Smela, and S. M. Hewitt "Multiplex quantitative measurement of mRNAs from fixed tissue microarray sections" Diag. Molec. Pathol., Published Ahead-of-Print doi: 10.1097/PDM.0000000000000039, (2014) [Abstract]                             link to AIMM site          direct link to unformatted text and supporting information The development of prognostic and diagnostic biomarkers, such as those from gene expression studies, requires independent validation in clinical specimens. Immunohistochemical analysis on tissue microarrays (TMAs) of formalin-fixed paraffin-embedded tissue is often used to increase the statistical power, and it is used more often than in situ hybridization, which can be technically limiting. Herein, we introduce a method for performing quantitative gene expression analysis across a TMA using an adaptation of 2D-RT-qPCR, a recently developed technology for measuring transcript levels in a histologic section while maintaining 2-dimensional positional information of the tissue sample. As a demonstration of utility, a TMA with tumor and normal human prostate samples was used to validate expression profiles from previous array-based gene discovery studies of prostate cancer. The results show that 2D-RT-qPCR expands the utility of TMAs to include sensitive and accurate gene expression measurements.
2014	act-CP	E. D. Daneshvar and E. Smela "Characterization of conjugated polymer actuation under cerebral physiological conditions" Adv. Healthcare Mater., 3(7), 1026–1035 (2014) [Abstract] link to PDF on Wiley site         direct link to unformatted text & supporting information Conjugated polymer actuators have potential use in implantable neural interface devices for modulating the position of electrode sites within brain tissue or guiding insertion of neural probes along curved trajectories. The actuation of polypyrrole (PPy) doped with dodecylbenzenesulfonate (DBS) was characterized to ascertain whether it could be employed in the cerebral environment. Microfabricated bilayer beams were electrochemically cycled at either 22 or 37 °C in aqueous NaDBS or in artificial cerebrospinal fluid (aCSF). Nearly all the ions in aCSF were exchanged into the PPy - the cations Na+, K+, Mg2+, Ca2+, as well as the anion PO43-; Cl- was not present. Nevertheless, deflections in aCSF were comparable to those in NaDBS and they were monotonic with oxidation level: strain increased upon reduction, with no reversal of motion despite the mixture of ionic charges and valences being exchanged. Actuation depended on temperature. Upon warming, the cyclic voltammograms showed additional peaks and an increase of 70% in the consumed charge. Bending was, however, much less affected: strain increased somewhat (6-13%) but remained monotonic, and deflections shifted (up to 20%). These results show how the actuation environment must be taken into account, and demonstrate proof of concept for actuated implantable neural interfaces.
2014	sens-cell	T. Datta-Chaudhuri, P. Abshire, and E. Smela "Packaging commercial CMOS chips for lab on a chip integration" Lab Chip, 14(10), 1753 - 1766 (2014) [Abstract] link to PDF on RSC site           direct link to supporting information Combining integrated circuitry with microfluidics enables lab-on-a-chip (LOC) devices to perform sensing, freeing them from benchtop equipment. However, this integration is challenging with small chips, as is briefly reviewed with reference to key metrics for package comparison. In this paper we present a simple packaging method for including mm-sized, foundry-fabricated dies containing complementary metal oxide semiconductor (CMOS) circuits within LOCs. The chip is embedded in an epoxy handle wafer to yield a level, large-area surface, allowing subsequent photolithographic post-processing and microfluidic integration. Electrical connection off-chip is provided by thin film metal traces passivated with Parylene-C. The Parylene is patterned to selectively expose the active sensing area of the chip, allowing direct interaction with a fluidic environment. The method accommodates any die size and automatically levels the die and handle wafer surfaces. Functionality was demonstrated by packaging two different types of CMOS sensor ICs, a bioamplifier chip with an array of surface electrodes connected to internal amplifiers for recording extracellular electrical signals and a capacitance sensor chip for monitoring cell adhesion and viability. Cells were cultured on the surface of both types of chips, and data were acquired using a PC. Long term culture (weeks) showed the packaging materials to be biocompatible. Package lifetime was demonstrated by exposure to fluids over a longer duration (months), and the package was robust enough to allow repeated sterilization and re-use. The ease of fabrication and good performance of this packaging method should allow wide adoption, thereby spurring advances in miniaturized sensing systems.
2013	sens-piezo	J. Wissman, A. Perez-Rosado, A. Edgerton, B. M. Levi, Z. N. Karakas, M. Kujawski, A. Philipps, N. Papavizas, D. Fallon, H. A. Bruck, and E. Smela "New compliant strain gauges for self-sensing dynamic deformation of flapping wings on miniature air vehicles" Smart Mater. Struct., 22, 085031 (2013) [Abstract]                             link to PDF on IOP site          direct link to supporting information Over the past several years there has been an increasing interest in the development of miniature air vehicles (MAVs) with flapping wings. To allow these MAVs to adjust to changes in wind direction and to maximize their efficiency, it is desirable to monitor the deformation of the wing during flight. This paper presents a step in this direction, demonstrating the measurement of strain on the surface of the wing using minimally invasive compliant piezoresistive sensors. The strain gauges consisted of latex mixed with electrically conducting exfoliated graphite, and they were applied by spray coating. To calibrate the gauges, both static and dynamic testing up to 10 Hz were performed using cantilever structures. In tension the static sensitivity was a linear 0.4 Ohm/microstrain and the gauge factor was 28; in compression, the gauge factor was -5. Although sensitivities in tension and compression differed by a factor of almost six, this was not reflected in the dynamic data, which followed the strain reversibly with little distortion. There was no attenuation with frequency, indicating a sufficiently small time constant for this application. The gauges were thin, compliant, and light enough to measure, without interference, deformations due to shape changes of the flexible wing associated with generating lift and thrust. During flapping the resistance closely tracked the generated thrust, measured on a test stand, with both signals tracing figure-8 loops as a function of wing position throughout each cycle.
2012	stretch	B. Balakrisnan, A. Nacev, J. M. Burke, A. Dasgupta, and E. Smela "Design of compliant meanders for applications in MEMS, actuators, and flexible electronics" Smart Mater. Struct., 21, 075033 (2012) [Abstract] link to PDF on IOP site           direct link to supporting information Meandering beams, crenellated surfaces, and zigzag-shaped electrodes are employed as compliant elements in microscale applications ranging from springs attached to proof masses in microelectromechanical systems (MEMS) to stretchable electrodes in flexible electronics and dielectric elastomer actuators. An understanding of how the meander shape affects the stiffness of these structures would permit preliminary design without the necessity of fabricating or running simulations on each case. In this paper, we present general guidelines for designing meandering cantilevers, showing how the amplitude, angle, length, and thickness affect both the axial and bending stiffnesses. Simple analytical expressions are derived, and the results are compared with those from numerical simulations and experimental measurements. The more complex case of a stiff thin film overlying a crenellated elastomer is also simulated.
2012	MEMS	M. Dandin, P. Abshire, and E. Smela "Polymer filters for ultraviolet-excited integrated fluorescence sensing" J. Micromech. Microeng., 22, 095018 (2012) [Abstract] link to PDF on IOP site           direct link to supporting information Optical filters for blocking ultraviolet (UV) light were fabricated by doping various polymer hosts with a UV absorbing chromophore. The polymers were polydimethylsiloxane (PDMS), a silicone elastomer frequently used in microfluidics, SU-8, a photopatternable epoxy, and Humiseal 1B66, an acrylic coating used for moisture protection of integrated circuits. The chromophore was 2-(2-hydroxy-5-methylphenyl) benzotriazole (BTA), which has a high extinction coefficient between 300 nm and 400 nm. We demonstrate filters 5 µm thick that exhibit high ultraviolet rejection (nearly -40 dB at 342 nm) yet pass visible light (near 0 dB above 400 nm), making them ideal for ultraviolet-excited fluorescence sensing within microsystems. The absorbance of the BTA depended on the host polymer. These filters are promising for integrated fluorescence spectroscopy in bioanalytical platforms because they can be patterned by dry etching, molding or exposure to ultraviolet light.
2012	fluid	J. M. Burke and E. Smela "A novel surface modification technique for forming porous polymer monoliths in PDMS" Biomicrofluidics, 6 (1), 016506 (2012) [Abstract] link to PDF on AIP site           A new method of surface modification is described for enabling the in situ formation of homogenous porous polymer monoliths (PPMs) within poly(dimethylsiloxane) (PDMS) microfluidic channels that uses 365 nm UV illumination for polymerization. Porous polymer monolith formation in PDMS can be challenging because PDMS readily absorbs the monomers and solvents, changing the final monolith morphology, and because PDMS absorbs oxygen, which inhibits free-radical polymerization. The new approach is based on sequentially absorbing a non-hydrogen-abstracting photoinitiator and the monomers methyl methacrylate and ethylene diacrylate within the walls of the microchannel, and then polymerizing the surface treatment polymer within the PDMS, entangled with it but not covalently bound. Four different monolith compositions were tested, all of which yielded monoliths that were securely anchored and could withstand pressures exceeding the bonding strength of PDMS (40 psi) without dislodging. One was a recipe that was optimized to give a larger average pore size, required for low back pressure. This monolith was used to concentrate and subsequently mechanical lyse B lymphocytes.
2011	PCR	M. Armani, M. A. Tangrea, B. Shapiro, M. R. Emmert-Buck, and E. Smela "Quantifying mRNA levels across tissue sections with 2D-RT-qPCR" Anal. Bioanal. Chem., 400 (10), 3383-3393 (2011) [Abstract] link to PDF on Springer site           direct link to supporting information Measurement of mRNA levels across tissue samples facilitates an understanding of how genes function and what their roles are in disease. Quantifying low-abundance mRNA requires a workflow that preserves spatial information, isolates RNA, and performs RT-qPCR. This is complex because these steps are typically performed in three separate platforms. In the present study, we describe 2D-RT-qPCR, a method that quantifies RNA across tissues sections in a single integrated platform. The method uses the grid format of a multi-well plate to macrodissect tissue sections and preserve the spatial location of the RNA; this also eliminates the need for physical homogenization of the tissue. A new lysis and nucleic acid purification protocol is performed in the same multi-well plate, followed by RT-qPCR. The feasibility 2D-RT-qPCR was demonstrated on a variety of tissue types. Potential applications of the technology as a high-throughput tissue analysis platform are discussed.
2011	act-CP	S. Alben, B. Balakrisnan, and E. Smela "Edge effects determine the direction of bilayer bending" Nano Lett., 11 (6), 2280–2285 (2011) [Abstract] link to PDF on ACS site           direct link to supporting information We elucidate the reason for preferential bending along the long edge in thin rectangular bilayers in which one of the layers is isotropically strained. While this preference has been observed previously, the physical basis for this preference has not been understood. We find that the bending direction is determined by the existence of doubly curved regions at the curled edges, which lower the energy. This energy difference between "spiral" and "cigar" shapes increases with aspect ratio.
2010	stretch	M. Kujawski, J. D. Pearse, and E. Smela "Elastomers filled with exfoliated graphite as compliant electrodes" Carbon, 48 (9), 2409-2417 (2010) [Abstract] link to PDF on ScienceDirect site           direct link to supporting information A compliant electrode material has been realized by blending an insulating polydimethylsiloxane (PDMS) elastomer with a conductive exfoliated graphite filler, which was produced by microwave irradiation. The conductivity and stiffness of the electrodes were determined as a function of filler concentration. These materials exhibited a low percolation threshold: above 3 wt% loading they became conductive, with conductivities reaching as high as 0.4 S/cm. They remained elastomeric upon loading up to 25 wt%, having a Young's modulus of only 1.4 MPa. This modulus (corresponding to a 220% increase compared to the unloaded PDMS) is the lowest reported for loaded elastomers above the percolation threshold. Scanning electron microscopy showed that the composites contained small voids, unlike unloaded PDMS, which might be responsible for the low modulus. The performance of these electrodes is comparable to that of PDMS loaded with carbon nanotubes, but the exfoliated graphite material can be produced at a fraction of the cost.
2009	PCR	M. Armani, J. Rodriguez-Canales, J. Gillespie, M. Tangrea, H. Erickson, M. R. Emmert-Buck, B. Shapiro, and E. Smela "2D-PCR: A method of mapping DNA in tissue sections" Lab Chip, 9(24), 3526 - 3534 (2009) [Abstract] link to PDF on RSC site           direct link to supporting information A novel approach was developed for mapping the location of target DNA sequences in tissue sections. The method combines a high-density, multi-well plate with an innovative single-tube procedure to directly extract, amplify, and detect the DNA in parallel while maintaining the two-dimensional (2D) architecture of the tissue. A 2D map of the gene glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was created from a tissue section and shown to correlate with the spatial area of the sample. It is anticipated that this approach may be easily adapted to assess the status of multiple genes within tissue sections, yielding a molecular map that directly correlates with the histology of the sample. This will provide investigators with a new tool to interrogate the molecular heterogeneity of tissue specimens.
2009	act-nast	M. E. Piyasena, R. Newby, T. J. Miller, B. Shapiro, and E. Smela "Electroosmotically driven microfluidic actuators" Sens. Act. B., 141, 263-269 (2009) [Abstract] link to PDF on ScienceDirect site          direct link to supporting information A prototype electroactive polymer actuator has been developed based on electroosmotic (EO) pumping to create hydraulic pressure. The actuator was fabricated from poly(dimethylsiloxane) (PDMS) with embedded micro-scale channels, reservoirs, and electrodes surmounted by a membrane. An applied voltage caused one reservoir to expand as fluid was pumped into it, and the other reservoir to contract, with the membrane above the expansion reservoir rising by 400 mm within a few seconds. Since the prototype was made from PDMS, which is an elastomer, the device was entirely flexible. The actuator performance was characterized, and it agreed well with predicted values. Furthermore, the calculations indicate that, once optimized, such actuators could have high stress as well as high strain and high speed. By combining unit cells such as these into a material and actuating them individually via independently-controlled flexible electrodes, one could realize smart materials that could change shape. Other future applications may include micro-valves, micro-positioners, soft robots, and active camouflage layers.
2009	MEMS	B. Balakrisnan, S. Patil, and E. Smela "Patterning PDMS using a combination of wet and dry etching" J. Micromech. Microeng., 19, 047002 (2009) [Abstract] link to PDF on IOP site PDMS films 10 microns thick can be patterned within 30 minutes by combining dry etching to achieve substantially vertical sidewalls with wet etching to achieve high etch rates and to protect the underlying substrate from attack. Dry etching alone would have taken 5 hours, and wet etching alone would produce severe undercutting. In addition, using either technique alone produces undesirable surface morphologies. The mask used during etching is critical to a successful patterning outcome. E-beam evaporated Al was found to work well, adhering strongly to oxygen plasma treated PDMS, and holding up well during both dry and wet etching. To prevent wrinkling of the PDMS, a fast deposition rate should be used.
2009	CP	X. Wang and E. Smela "Color and volume change in PPy(DBS)" J. Phys. Chem. C , 113 (1), 359-368 (2009) [Abstract] link to PDF on ACS site        link to supporting information, including videos The optical-mechanical-electrochemical coupling in polypyrrole films doped with dodecylbenzenesulfonate, PPy(DBS), is studied during electrochemical switching. A special experimental geometry is used in which the top surface of the film is covered by a transparent ion barrier that constrains charge-compensating cations to enter and exit the film only at the edges. The cation concentration determines the volume of the polymer, and is directly related to the oxidation level of the polymer, which determines its color. During electrochemical reduction, the edges of the film lighten as the cations arrive, and this light color travels as a front to the center of the film. At the same time, the height of the film increases when and where the color lightens, and the increase in height is directly proportional to the change in color intensity. The height and the intensity attain their maximum values as the front passes and do not increase further thereafter, indicating that the ion concentration in the fully reduced state has a maximum value that is not exceeded. The color and volume changes are associated with only the first pair of peaks in the cyclic voltammogram, which represent only a fraction of the consumed charge in a typical electrochemical scan.
2009	CP	X. Wang and E. Smela "Experimental studies of ion transport in PPy(DBS)" J. Phys. Chem. C , 113 (1), 369–381 (2009) [Abstract] link to PDF on ACS site        link to supporting information, including videos Lateral ion transport in conjugated polymer films is studied using a special experimental geometry in which the top surface of the film is covered by a transparent ion barrier. Because of the barrier, when the oxidation level of the polymer is switched electrochemically, charge-compensating ions can only enter and leave the polymer from the edges. Since conjugated polymers are electrochromic, the color of the film changes during switching, and this can be monitored to provide information on the oxidation level of different parts of the film. Since the oxidation level cannot change until the cations arrive, the color also directly maps the positions of the cations. This geometry was employed to study cation transport in polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS). Upon reduction, the ions travel in a front from the edges to the center of the film. During the first-ever reduction, this cation front stays sharp, but in subsequent reduction scans the front moves 20-30 times faster and broadens as it moves. The higher the applied voltage, the faster the front moves, with a linear dependence. The velocity of the front also strongly depends on the initial oxidation level of the polymer. During oxidation, on the other hand, the entire film gradually darkens, with no front and no dependence of the switching speed on the applied potential.
2009	CP	X. Wang, B. Shapiro, and E. Smela "Development of a model for charge transport in conjugated polymers" J. Phys. Chem. C , 113 (1), 382-401 (2009) [Abstract] link to PDF on ACS site        link to supporting information A finite element model for charge transport in conjugated polymers is developed based on transport equations for ionic and electronic charge coupled with the Poisson equation. The model behavior is fully explored, and its complexity is gradually increased to realize a full model that treats non-Fickian diffusion through non-constant coefficients and that includes ion transport in the electrolyte. The simulation results are compared qualitatively with the experimental results for the ion-barrier-covered PPy(DBS) film of Part 2 during electrochemical reduction, and the model is found to successfully account for the dominant behaviors, including the emergence of a front. One of the key findings of the simulations is that migration must be taken into account to correctly describe ion ingress: none of the various simulations in which ion transport was only by diffusion predicted the experimental results. Another is that the front velocity is proportional to the applied voltage, as found experimentally, and that the cation front can move into the polymer with a velocity v ~ sqrt(t) even when the ions move by migration alone.
2009	CP	B. J. West, T. F. Otero, B. Shapiro, and E. Smela "Chronoamperometric study of conformational relaxation in PPy(DBS)" J. Phys. Chem. B, 113 (5), 1277–1293 (2009) [Abstract] link to PDF on ACS site        link to supporting information In conjugated polymer devices that switch from one oxidation level to another, such as artificial muscles, it is important to understand memory effects that stem from conformational relaxation movements of the polymer chains. Chronoamperometry during electrochemical switching of polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), is used to gain insight into the conformational relaxation processes in cation-transporting materials. During oxidation, the current decays exponentially with a time constant that decreases with the anodic voltage. During reduction, there is again an exponentially decaying current with a time constant that decreases with the cathodic voltage, but superimposed on that is a small current peak that increases in size with the voltage. This peak accounts for a maximum of 20% of the total reduction charge, which is approximately the same amount of charge that is in the most cathodic pair of peaks in the cyclic voltammogram. The position of this peak depends logarithmically on the applied cathodic potential (shifting to shorter times with larger Eca) as well as on the anodic potential that was applied just prior to the reduction step (shifting to longer times with Ean). Furthermore, the shoulder position depends logarithmically on the time that the prior anodic voltage was held (shifting to longer times with twait). These results are consistent with the electrochemically stimulated conformational relaxation (ESCR) model.
2008	act-CP	E. Smela "Conjugated polymer actuators" MRS Bull., 33 (3), 197-204 (2008) [Abstract] only available for a fee from MRS -- email me for a PDF copy Conjugated polymer artificial muscles fill a unique niche in the EAP portfolio. They combine high strength, low voltage, and reasonable speed with versatile fabrication and design. This article reviews the actuation mechanism in these materials and presents some of the designs that have been developed for applications such as Braille displays, catheters, and bioMEMS devices.
2008	act-CP	S. B. Prakash, M. Urdaneta, M. Christophersen, E. Smela, and P. Abshire "In situ electrochemical control of electroactive polymer films on a CMOS chip" Sens. Act. B , 129 (2), 699-704 (2008) [Abstract] link to PDF on ScienceDirect/Elsevier site The paper reports, for the first time, an integrated potentiostat for driving electrochemical actuation reactions on a CMOS chip. The integrated microsystem comprised on-chip working, counter, and pseudo-reference electrodes fabricated on top of the necessary control circuitry. To demonstrate its capability, the potentiostat chip was used to electrodeposit films of polypyrrole/dodecylbenzenesulfonate, PPy(DBS), in situ onto the working electrodes. It was also used to cycle PPy(DBS) films between the fully oxidized and fully reduced states. This was confirmed by video recordings of the distinctive electrochromic changes, as well as by the shapes of the cyclic voltammograms. Both showed good repeatability. The demonstrated approach of integrating driver circuitry along with actuation electrodes on a common CMOS substrate eliminates the need for an external potentiostat instrument, and can be widely employed for the fabrication and control of electrochemical microactuators in integrated lab-on-a-chip systems.
2008	DEP	M. Urdaneta and E. Smela "Parasitic trap cancellation using multiple frequency dielectrophoresis, demonstrated by loading cells into cages" Lab Chip, 8 (4), 550-556 (2008) [Abstract] link to PDF on RSC site This paper presents a method to load cells into microvials using dielectrophoresis. This has not been possible until now because such features produce electric field distortions that interfere with the dielectrophoretic manipulation. Multiple frequency dielectrophoresis was used to actively cancel out the effect of these distortions, allowing many cells to be loaded into a vial in a matter of seconds in fluid flows of up to 300 mm/s. Actively cancelling field distortions enables the presence of features that would otherwise be prohibited near regions of dielectrophoretic manipulation, significantly expanding the environments in which dielectrophoresis can be used.
2008	DEP	M. Urdaneta and E. Smela "The design of dielectrophoretic flow-through sorters using a figure of merit" J. Micromech. Microeng., 18 (1), Article Number: 015001(2008) [Abstract] link to PDF on IOP site Dielectrophoretic (DEP) forces produced by a pair of electrodes are used for such applications as particle sorting in microfluidic systems. In order to allow quantitative comparison of different DEP systems, we introduce a new figure of merit: dimensionless characteristic velocity corresponding to the maximum flow that can be used while still successfully deflecting the desired particles. This avoids the necessity of performing numerical simulations in order to compare designs and allows one to focus on general system design questions. The maximum deflection velocity is evaluated versus a characteristic length for four different sorter configurations, yielding curves that can be used as tools to design DEP sorters. Maximum flow predictions are compared with previously published theoretical and experimental results.
2007	DEP	M. Urdaneta and E. Smela "Multiple frequency dielectrophoresis" Electrophoresis, 28 (18), 3145-3155 (2007) [Abstract] link to PDF on Wiley/InterScience site A novel method of modeling multiple frequency dielectrophoresis (MFDEP) is introduced based on the concept of an effective Clausius-Mossotti factor, CMeff, for a particle that is exposed to electrical fields of different frequencies, coming either from one or multiple pairs of electrodes. This analysis clearly illustrates how adding frequencies adds control parameters, up to two additional parameters per frequency. As a result, MFDEP can be used for a wide variety of applications, including separating particles with very similar Clausius-Mossotti spectra, trapping multiple groups of cells simultaneously, and cancelling unwanted dielectrophoretic traps. Illustrating the modeling approach, we determine the CMeffs for live and dead yeast cells, and then predict their equilibrium distribution on a 3-electrode configuration, with two electrodes at different frequencies and the third electrode at ground. This prediction is validated experimentally, using MFDEP to selectively attract live cells to one location and dead cells to another, trapping both. These results demonstrate that the use of multiple frequencies for the manipulation of particles can enhance the performance of dielectrophoretic devices, not only for sorting, but also for such applications as patterning cells in close proximity for the formation of cell consort
2007	sens	M. Dandin, P. Abshire, and E. Smela "Optical filtering technologies for integrated fluorescence sensors" Lab Chip, 7 (8), 955-977 (2007) [Abstract] link to PDF on RSC site Numerous approaches have been taken to miniaturizing fluorescence sensing, which is a key capability for micro-total-analysis systems. This critical, comprehensive review focuses on the optical hardware required to attenuate excitation light while transmitting fluorescence. It summarizes, evaluates, and compares the various technologies, including filtering approaches such as interference filters and absorption filters and filterless approaches such as multicolor sensors and light-guiding elements. It presents the physical principles behind the different architectures, the state-of-the-art micro-fluorometers and how they were microfabricated, and their performance metrics. Promising technologies that have not yet been integrated are also described. This information will permit the identification of methods that meet particular design requirements, from both performance and integration perspectives, and the recognition of the remaining technological challenges. Finally, a set of performance metrics are proposed for evaluating and reporting spectral discrimination characteristics of integrated devices in order to promote side-by-side comparisons among diverse technologies and, ultimately, to facilitate optimized designs of micro-fluorometers for specific applications
2007	stretch	M. Urdaneta, R. Delille, and E. Smela "Stretchable electrodes with high conductivity and photopatternability" Adv. Mater., 19 (18), 2629-2633 (2007) [Abstract] link to PDF on Wiley/InterScience site This communication presents a method for making compliant electrodes that have a maximum conductivity of ~50 S/cm (corresponding to a sheet resistance of 2 Ohm/square) when unstretched and that do not lose conductivity until elongated by over 100%. The electrodes have a Young's modulus under 10 MPa, and they show no sign of electrical or mechanical fatigue even after thousands of strain cycles. Upon mechanical relaxation after strain cycling, their conductivity and length recover nearly to their initial values, signifying a low electrical and mechanical hysteresis. The electrodes are photopatternable, making them compatible with microfabrication technology. This enables the fabrication of meso- to micro-scale electrodes for stretchable circuits, retinal implants, or high-deformation microelectromechanical systems (MEMS).
2007	act-CP	Y. Liu, Q. Gan, S. Baig, and E. Smela "Improving PPy adhesion by surface roughening" J. Phys. Chem. C, 111 (30), 11329-11338 (2007) [Abstract] link to PDF on ACS site Conjugated polymers have found applications as “artificial muscles” because they undergo significant volume change upon electrochemical cycling. However, this large repeated strain also leads them to delaminate from the underlying electrode. Two methods of improving the adhesion of polypyrrole films to Au electrodes during extended electrochemical cycling were quantitatively characterized. Both involved roughening the Au surface, the first by electroplating and the second by etching. The extent of delamination was quantified using a tape test at regular intervals during switching between the fully oxidized and reduced states in an aqueous electrolyte, and the surfaces were characterized by scanning electron microscopy. Untreated smooth control surfaces were simultaneously monitored the same way. Without surface treatment, 3000 Å thick polypyrrole films doped with dodecylbenzenesulfonate, PPy(DBS), delaminated unpredictably between 1 cycle and 20 000 cycles, with almost half the samples failing before 3000 cycles. Electroplating even a thin layer of 1000 Å of Au in a low-concentration plating bath consistently improved lifetimes to at least 10 000 cycles. Thicker plating produced even better adhesion, and a 1 ím thick layer of electroplated Au virtually eliminated delamination, extending the lifetime beyond 60 000 cycles. However, this Au roughness is thicker than the PPy film itself, and is not useful for microactuator work. Etching also improved adhesion, with the best results obtained for the shortest etch times. With a roughness of only 700 Å, an undercut surface morphology was produced that allowed good mechanical interlocking of the PPy. This process is appropriate for use with thin films; however, it will be sensitive to the grain structure of the original Au film. On surfaces with no delamination, it was possible to measure the oxidative degradation of the PPy(DBS) in the aqueous electrolyte. The electroactivity decreased steadily over time, with 20% loss after 10 000 cycles, 50% at 35 000, and virtually no electroactivity by 60 000 cycles.
2007	stretch	R. Delille, M. Urdaneta, K. Hsieh, and E. Smela "Compliant electrodes based on platinum salt reduction in a urethane matrix" Smart Mater. Struct., 16 (2), S272 -279 (2007) [Abstract] link to PDF on IOP site An elastomeric but electrically conducting material is presented that is fabricated using a method inspired by ionic polymer-metal composites. The Young's modulus is only 10 MPa, and yet the conductivity is nearly 1 S/cm and the material remains electrically conductive under uniaxial strains of 30%. Furthermore, this electrode material is photo-patternable. Fabrication begins with mixing the platinum salt tetraammineplatinum(II) chloride into a UV-curable acrylated urethane elastomer precursor (Loctite 3108). The mixture is crosslinked under UV light in less than a minute and can therefore be patterned if it is exposed through a mask. The salt is then chemically reduced with sodium borohydride, which results in the formation of 100 nm sized platinum nodules on the surface of the film.
2006	act	B. Shapiro and E. Smela "Bending actuators with maximum curvature and force and zero interfacial stress" J. Intell. Mater. Syst. Struct., 18 (2), 181-186 (2006) [Abstract] link to PDF on Sage site Stress at the interface of a bilayer actuator can lead to delamination, and this stress increases with the difference in the Young’s moduli of the two layers. In this study, an actuator that includes a third ‘buffer’ layer is modeled, and it is shown how optimization of the thickness and modulus of this layer can reduce stress at that interface to zero, with no loss of curvature and only a small loss in force. A polypyrrole (PPy)/conducting polymer/gold trilayer is used as a model system, with the aim of reducing stress at the weak polymer/gold interface. For a 450nm thick PPy layer with a Young’s modulus of 0.2 GPa, an intermediate layer of 150nm thickness with a modulus of 5 GPa reduces the stress to zero with no loss of curvature and only a 7% loss in force compared to the bilayer case.
2006	FO	P.-S. Jian and E. Smela "Modeling the performance of a long-period Bragg grating ambient-index sensor" Smart Mater. Struct., 15, 821-828 (2006) [Abstract] link to PDF on IOP site A long-period Bragg grating (LPG) is a resonant structure that couples light between the fundamental core mode and specific cladding modes. The propagation constant of the cladding mode is sensitive to the index of refraction outside the fiber, making LPGs applicable as sensors. The particular cladding mode orders that are coupled depend on the grating period, the cladding radius, and the size of the refractive index modulation. However, design of LPG sensors is largely empirical, making the realization of target characteristics difficult. A mathematical model has been developed to understand how to control the mode coupling, which gives rise to a resonant attenuation peak in the transmission spectrum. We show that the depth of the peak is governed by the length of the grating and by the coupling constants between the core and cladding modes. Good agreement with experimental spectra suggests that the model can be used as a practical tool to design long-period fiber grating sensors. The LPG is most sensitive when the ambient index is close to, but less than, the index of the core. We also propose a Mach–Zehnder interferometry technique to enhance sensitivity with not just a shift in the resonant peak, but also an amplitude modulation.
2006	MEMS	R. Delille, M. Urdaneta, S. Moseley, and E. Smela "Benchtop polymer MEMS" J. Microelectromech. Syst., 15 (5), 1108-1120 (2006) [Abstract] link to PDF on IEEE site          direct link to supporting information Loctite photopatternable adhesives 3108, 3340, and 3525 are introduced for MEMS applications. These materials are patterned within minutes by exposure to UV light followed by rinsing with a solvent; no further processing is required. Because the uncured fluid is relatively insensitive to room light, this can be done on any lab bench without the requirement for a clean room. The materials can be spin-coated to obtain films, or cast between spacers for layers up to 1 cm thick, and the cured polymers range from elastomeric to rigid. These adhesives are of interest for rapid, inexpensive fabrication of relatively low-resolution features (tens to hundreds of m) by photo-curing. They can alternatively be molded, like PDMS, to achieve high resolution, as well as irreversibly bonded after an O2 plasma treatment. In addition, like SU8 they can be used as molds for patterning PDMS. Initial characterization of resolution, swelling, and biocompatibility were performed. One of the polymers, 3340, can be used for packaging bioMEMS- on-CMOS chips, exploiting its biocompatibility and its photo-patternability at thicknesses of 1500 m to cover the bond wires while exposing the chip surface. As further demonstrations of the versatility of these materials, multi-level, interconnected channel structures were fabricated with a gelatin sacrificial layer, and magnetic films were prepared, since the polymers remain patternable even with additives.
2006	act-CP	M. Christophersen, B. Shapiro, and E. Smela "Characterization and modeling of PPy bilayer microactuators. Part 1: Curvature" Sens. Act. B, 115, 596-609 (2006) [Abstract] link to PDF on ScienceDirect/Elsevier site          direct link to supporting information Bilayer actuators of gold and polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), were microfabricated with lengths and thicknesses that varied over two orders of magnitude. The actuators were connected to rigid plates. The curvature of the bilayers was examined as a function of PPy and Au layer thicknesses to enable a determination of the actuation strain and Young’s modulus of the PPy(DBS). Four different Au thicknesses were tested, and maximum curvature was obtained in all cases at a PPy:Au thickness ratio of 5:1. The data could not be fit with a constant value for the actuation strain; in addition, the PPy film bent even in the absence of an Au layer. Therefore, a model was developed that included strain and modulus variations along the direction of film thickness. The experimental data were fit with an exponential variation in actuation strain, starting at 20% at the surface of the PPy and decreasing to 3% at the gold interface in thicker films. The Young’s modulus was found to be constant between 0.08 and 0.20 GPa in the oxidized state. These values differ significantly from thick-film values. With this information, micro-actuators can now be designed to achieve a specified radius of curvature.
2005	act-CP	E. Smela and B. R. Mattes "Polyaniline Actuators, Part 2: PANI(AMPS) in MSA" Synth. Met., 151, 43-48 (2005) [Abstract] link to PDF on Elsevier/Science Direct site Actuation of polyaniline (PANI)/Au bending bilayers and stretched polyaniline fibers doped with 2-acrylamido-2-methyl-propane-1-sulfonic acid was studied in aqueous methanesulfonic acid. Electrochemical activity was retained even upon repeated cycling into the pernigraniline state, unlike in HCl. The polyaniline expanded 1.6% from the leucoemeraldine state to the emeraldine state, similar to the strain in HCl, and then contracted 0.2% upon entering the pernigraniline state. The polyaniline was not mechanically stable in the pernigraniline state, however: the ultimate tensile strength decreased so much that the actuators were rendered ineffective for lifting a load. In addition, in this electrolyte the creep was considerable even under small loads; the Young’s modulus was only 0.4 GPa in the oxidized state, compared to 1.7 GPa in HCl. Top of Page
2005	act-CP	E. Smela, W. Lu, and B. R. Mattes "Polyaniline Actuators, Part 1: PANI(AMPS) in HCl" Synth. Met., 151 (1), 25–42 (2005) [Abstract] link to PDF on Elsevier/Science Direct site Drawn polyaniline films and fibers doped with 2-acrylamido-2-methyl-propane-1-sulfonic acid, PANI(AMPS), were electrochemically cycled in HCl and their material properties and actuation performance comprehensively characterized. The Young's modulus was obtained as a function of applied voltage. Actuator figures of merit were derived from isotonic and isometric measurements, including strain, stress, work, power, creep, and efficiency. The effects of sample length, solution pH, electrochemical driving method, frequency, and load were studied, as well as the response of current to applied load for sensing applications. This work presents a complete picture of a polyaniline actuator for the first time. The behavior of the actuator is discussed in terms of the changes in the oxidation and protonation states of polyaniline.
2005	act-CP	S. Maw, E. Smela, K. Yoshida, and R. B. Stein "Effects of monomer and electrolyte concentrations on actuation of PPy(DBS) bilayers" Synth. Met., 155 (1), 18-26 (2005) [Abstract] link to PDF on ScienceDirect/Elsevier site          direct link to supporting information Bilayers of gold-coated Kapton and polypyrrole doped with dodecylbenzenesulfonate PPy(DBS), were prepared electrochemically, and their bending movement was studied under electrochemical stimulation. The effects on actuation of monomer and electrolyte concentration during deposition, as well as electrolyte concentration during subsequent electrochemical cycling, are presented. Lowering the pyrrole concentration decreased hysteresis, somewhat increased actuator speed, and unexpectedly eliminated the so-called capacitive current above the oxidation peak. Neither pyrrole nor electrolyte concentration in the deposition solution affected the extent of movement, despite the fact that the former strongly impacted the cyclic voltammograms. The cyclic voltammograms were, therefore, not predictive of the extent of bending, and strain could not be significantly improved by altering the concentration of either monomer or salt in the deposition solution. Raising the deposition electrolyte concentration, and in particular raising it with respect to the pyrrole concentration, did increase the movement to charge ratio, making the actuators more efficient.
2005	act-CP	X. Lin, J. Li, E. Smela, and S. Yip "Polaron-induced conformation change in single polypyrrole chain: an intrinsic actuation mechanism" Int. J. Quant. Chem., 102, 980-985 (2005) [Abstract] only available for a fee from Wiley/InterScience -- email me for a PDF copy Ab initio calculations show that a neutral polypyrrole chain in the ground state assumes a helical shape resulting from a novel bending mechanism, while upon oxidation the chain becomes planar within the polarons, an effect due to enhanced inter-ring pi bonding. This polaron-induced conformation change leads to an intrinsic potential for inducing macroscopic strains in the single chain, with implications for further theoretical studies and experiments.
2004	CP	X. Wang, B. Shapiro, and E. Smela "Visualizing ion transport in conjugated polymers" Adv. Mater., 16 (18), 1605-1609 (2004) [Abstract] link to PDF on Wiley/InterScience site We report a new technique to study ion transport in materials that are electrochromic, such as PPy(DBS). An experiment is introduced that makes electron transport fast compared with ion transport and that allows the visualization of the ion locations in the film. A first-cut model is introduced which includes ion drift and diffusion, and which accounts for much of the observed inplane ion transport behavior.
2004	act-CP	W. Lu, E. Smela, P. Adams, G. Zuccarello, and B. R. Mattes "Development of solid-in-hollow electrochemical linear actuators using highly conductive polyaniline" Chem. Mater., 16 (9), 1615-1621 (2004) [Abstract] link to PDF on ACS site In this paper, we report the use of highly electrically conductive (400-1000 S/cm) polyaniline fibers for the development of electrochemical linear actuators. The high conductivity of the fibers ensures well-defined electroactivity and actuation without a metal backing. In propylene carbonate (PC), the electrochemical and actuation behavior of the fibers was influenced by the dopants’ solubility and size: solubility in the electrolyte ensured easy expulsion from the polymer, thus facilitating ion transport and resulting in high electroactivity and strain in the fibers. Actuation of fibers was also affected by electrolyte anions. Small anions led to anion-exchange dominated actuation, and large anions led to cationexchange dominated actuation. Electrochemical linear actuators with a unique solid-in-hollow configuration were developed using a polyaniline solid fiber in a polyaniline hollow fiber with a gel electrolyte. These actuators were simple to construct and allowed two-electrode operation. Isotonic strains of 0.9% and isometric stresses of 0.9 MPa were realized.
2003	act-CP	E. Smela "Conjugated polymer actuators for biomedical applications" Adv. Mater., 15 (6), 481-494 (2003) [Abstract] link to PDF on Wiley/InterScience site Conjugated polymers are electroactive, and they have found applications as artificial muscles. Actuators that have been developed over the last decade have now reached the early stages of commercialization, particularly for use in biomedical devices. This article reviews the motivation for using this class of actuator, the types of devices that have been fabricated, and some of the biomedical applications that are being developed. Recommendations are presented for future work on understanding the actuation mechanisms, on actuator design, and on the measurement of metrics. Top of Page
2003	act-CP	M. Pyo, C. C. Bohn, E. Smela, J. R. Reynolds, and A. B. Brennan "Direct strain measurement of polypyrrole actuators controlled by the polymer/gold interface" Chem. Mater., 15 (4), 916-922 (2003) [Abstract] link to PDF on ACS site Electrochemically induced strain and actuation in a polypyrrole/Au/polyimide laminate was measured using a sensitive strain gage during oxidation and reduction (redox) of PPy(ClO4-). The polyimide formed the top layer of the strain gage, allowing direct measurement of strain during redox switching in various electrolytes and potential windows. Strain increased in the order NaCl < NaNO3 < NaClO4. In the NaClO4 solution for oxidizing potentials between 0.1 and 0.5 V vs Ag/AgCl, strain and charge were linearly proportional to voltage. Lower polypyrrole deposition potentials also increased strain. The surface roughness of the Au was varied by electrochemically depositing Au (which we designate EcAu) on top of thermally evaporated Au (EvAu)/polyimide. Greater EcAu deposition times led to greater Au roughness. Strain increased with polypyrrole thickness and Au roughness, which correlated with better adhesion between the polypyrrole and the Au. Au roughness also improved the cycling stability of the actuators.
2002	CP	W. Lu, A. G. Fadeev, B. H. Qi, E. Smela, B. R. Mattes, J. Ding, G. M. Spinks, J. Mazurkiewicz, D. Z. Zhou, G. G. Wallace, D. R. MacFarlane, S. A. Forsyth, and M. Forsyth "Use of ionic liquids for pi-conjugated polymer electrochemical devices" Science, 297 (5583), 983-987 (2002) [Abstract] link to PDF on Science site     pi-Conjugated polymers that are electrochemically cycled in ionic liquids have enhanced lifetimes without failure (up to 1 million cycles) and fast cycle switching speeds (100 ms). We report results for electrochemical mechanical actuators, electrochromic windows, and numeric displays made from three types of pi-conjugated polymers: polyaniline, polypyrrole, and polythiophene. Experiments were performed under ambient conditions, yet the polymers showed negligible loss in electroactivity. These performance advantages were obtained by using environmentally stable, room- temperature ionic liquids composed of 1-butyl-3-methyl imidazolium cations together with anions such as tetrafluoroborate or hexafluorophosphate.
2001	act-CP	E. Smela and N. Gadegaard "Volume change in polypyrrole studied by atomic force microscopy" J. Phys. Chem. B, 105 (39), 9395-9405 (2001) [Abstract] link to PDF on ACS site The electrochemically induced volume change in conjugated polymers is of interest for actuator applications. Important parameters include the magnitude and speed of the volume change and how volume correlates with potential and current. In this paper, atomic force microscopy was used to examine the expansion and contraction of films of polypyrrole doped with dodecylbenzenesulfonate. The films were photolithographically patterned to allow the scan to start on the substrate, go over the film, and return back to the substrate, giving an absolute height measurement. The method is described in detail. This technique allowed us to map the real-time volume change in-situ in the out-of-plane direction, which revealed that the film thickness increased by over 35% in the reduced state compared to the oxidized state. This compares with an inplane strain of similar to2% found in prior studies using bilayers, showing that the volume change is anisotropic. The thickness change was well-correlated with the cyclic voltammograms, and the polymer responded with a 3%/s expansion for an applied reducing potential step. The height change depended on film thickness, with films near 1.5 mm undergoing the greatest strain. The morphologies of the as-grown, oxidized, and reduced states were also examined, but there were no substantial differences. The unexpectedly large out-of-plane strain was confirmed by in-situ mechanical surface profilometry. These measurements also showed that the hardness of the reduced state was lower than that of the oxidized state.
2001	act-CP	S. Maw, E. Smela, K. Yoshida, P. Sommer-Larsen, and R. B. Stein "The effects of varying deposition current density on bending behaviour in PPy(DBS)-actuated bending beams" Sens. Act. A, 89, 175-184 (2001) [Abstract] link to PDF on Elsevier/ScienceDirect site Polypyrrole(dodecylbenzene sulfonate), a type of conjugated polymer that can change volume under electrochemical stimulation, was electrochemically deposited on metal-coated Kapton strips using current densities that ranged from 0.4 to 40 mA/cm2. The resulting bending beams were subjected to various voltage and current stimulation waveforms, and the beam movements and electrochemical activity were recorded. The extent of beam curling during cyclic voltammetry and potential stepping correlated inversely with deposition current density. However, during current stepping, the curling did not depend on deposition current density. Rather, it depended only on the amount of charge exchanged.
2000	PLED	Y. Kaminorz, E. Smela, T. Johansson, L. Brehmer, M. R. Andersson, and O. Inganäs "Characteristics of polythiophene surface light emitting diodes" Synth. Met., 113 (1-2), 103-114 (2000) [Abstract] link to PDF on Elsevier/ScienceDirect site Surface light emitting diodes (SLEDs), in which previously microfabricated electrodes were coated with a conjugated polymer, were made with greatly different electrode spacings (250 nm and 10 or 20 mm) and with different electrode material combinations. The fabrication process allowed us to compare several electrode materials. The SLED structures also enabled imaging of the light emission zone with fluorescence video microscopy. Conventional sandwich structures were also made for comparison (electrode separation 50 nm). In this study, the emitting layer was poly[3-(2',5'-bis(1'',4",7" trioxaoctyl)phenyl)-2,2'-bithiophene] (EO-PT), a conjugated polymer based on polythiophene with oligo(ethyleneoxide) side chains. The current-voltage (I(V)) and light-voltage (L(V)) characteristics of the SLEDs were largely insensitive to electrode separation except at high voltages, at which the current in the devices with the largest separations was limited. Sandwich structures had the same light output at a given current. Light could be obtained in forward and reverse bias from indium tin oxide (ITO)-aluminum, gold silicide-aluminum, and gold silicide-gold SLEDs, but the turn-on voltages were lowest with the ITO-aluminum devices, and these were also the brightest and most reliable. Adding salt to the EO-PT increased the current and brightness, decreased the turn-on voltages, and made the I(V) characteristics symmetric; thus, a device with an electrode separation of 10 mm had the extraordinarily low turn-on voltage of 6 V. The location of the light emission was at the electron-injecting contact.
2000	act-CP	E. W. H. Jager, E. Smela, and O. Inganäs "Microfabricating conjugated polymer actuators" Science, 290, 1540-1545 (2000) [Abstract] link to PDF on Science site Conjugated polymer actuators can be operated in aqueous media, which makes them attractive for laboratories-on-a-chip and applications under physiological conditions. One of the most stable conjugated polymers under these conditions is polypyrrole, which can be patterned by means of standard photolithography. Polypyrrole- gold bilayer actuators that bend out of the plane of the wafer have been microfabricated in our laboratory. These can be used to move and position other microcomponents. Here we review the current status of these microactuators, outlining the methods used to fabricate them. We describe the devices that have been demonstrated as well as some potential future applications. Top of Page
1999	act-CP	E. Smela, M. Kallenbach, and J. Holdenried "Electrochemically driven polypyrrole bilayers for moving and positioning bulk micromachined silicon plates" J. Microelectromech. Syst., 8 (4), 373-383 (1999) [Abstract] link to PDF on IEEE site The conjugated polymer polypyrrole undergoes a volume change of several percent when its oxidation state is changed electrochemically by the application of voltages between 0 and -1 V (versus Ag/AgCl). This volume change is due to ion movements in or out of the polymer film. Bilayers of polypyrrole and gold undergo a large bending and can deliver high force. They can thus function as hinges to lift rigid components. In this work, we demonstrate that silicon plates produced by reactive ion etching can be lifted by such PPy/Au hinges. The strength and efficiency of these bilayers are also determined.
1999	act-CP	E. Smela and N. Gadegaard "Surprising volume change in PPy(DBS): an atomic force microscopy study" Adv. Mater., 11 (11), 953-957 (1999) [Abstract] link to PDF on Wiley/InterScience site The volume change in polypyrrole (PPy) films doped with the bulky dodecylbenzenesulfonate (DBS) anion during redox reactions was measured directly using AFM. PPy patterned films (1-1.5 mm) were grown on a gold layer (on a Si wafer coated with Cr) with the help of photoresists and in situ electrochem. and AFM measurements were carried out. The potential was cycled between 0 and -1 V; freshly deposited PPy/DBS showed a 60-100% vol. increase during the first reduction. and a 30-40% change between oxidized and reduced states thereafter. The volume change was highly anisotropic.
1999	act-CP	E. Smela "A microfabricated movable electrochromic "pixel" based on polypyrrole" Adv. Mater., 11 (16), 1343-1345 (1999) [Abstract] link to PDF on Wiley/InterScience site Polypyrrole (PPy) was doped with dodecylbenzenesulfonate and incorporated into a micro-mechanical device together with e.g. Al, Au, Si. It is possible to supply power to devices fabricated on Si that are moved out of plane of the wafer using a polymer actuator. Due to changes in dimensions of the doped PPy (caused by electrochem. changes) the micro-mech. device (pixel) was moved. Possible applications are discussed.
1999	act-CP	E. Smela "Microfabrication of PPy microactuators and other conjugated polymer devices" J. Micromech. Microeng., 9, 1-18 (1999) [Abstract] link to PDF on IOP site Conjugated polymers have a no. of interesting properties that can be exploited in microfabricated sensors and actuators. For example, polypyrrole is a conjugated polymer that can change vol. to deliver significant stresses and strains. These materials can be patterned using conventional microfabrication techniques. The procedures for doing this are described in this paper, focusing on the microfabrication of polypyrrole microactuators. In addition, other methods for the deposition and patterning of conjugated polymers are reviewed. A special technique for releasing actuators, differential adhesion, is also detailed.
1999	act-CP	E. W. H. Jager, E. Smela, O. Inganäs, and I. Lundström "Polypyrrole microactuators" Synth. Met., 102 (1-3), 1309-1310 (1999) [Abstract] link to PDF on Elsevier/ScienceDirect site The doping and undoping of conjugated polymers is accompanied by a volume change. This volume change can be used to make micro-actuators. We have built polypyrrole (PPy) gold bilayer structures. The volume change of PPy induces a bending of the bilayer. We have made micro-actuators based on this principle, which have been examined with video-microscopy. Some applications for these micro-actuators are proposed, including opening and closing boxes for cell biology and nanoliter chemistry and micro-robotics.
1999	act-CP	E. W. H. Jager, E. Smela, and O. Inganäs "On-chip microelectrodes for electrochemistry with moveable PPy bilayer actuators as working electrodes" Sens. Act. B, 56, 73-78 (1999) [Abstract] link to PDF on Elsevier/ScienceDirect site We present electrochemical microactuators which have all the electrodes necessary for the actuation -- the working, counter, and reference electrodes -- on-chip. This is a first step towards an all-polymer system, i.e., a system that does not require a liquid electrolyte. The microactuators' performance was as good as when standard, off-chip counter and reference electrodes were used. Specifically, the speed of actuation was the same. In addition, we obtained a good cyclic voltammogram, although the oxidation and reduction peaks were shifted and some noise was present. Apart from application in an all-polymer system, we will also use these microactuators for studies on the effect of mechanical stimulation of living cells.
1998	SAM	E. Smela, G. Zuccarello, H. Kariis, and B. Liedberg "Thiol Modified Pyrrole Monomers: 1. Synthesis, Characterization, and Polymerization of 1-(2-Thioethyl)-Pyrrole and 3-(2-Thioethyl)-Pyrrole" Langmuir, 14 (11), 2970-2975 (1998) [Abstract] link to PDF on ACS site          link to supporting information To try to solve the problem of delamination of polypyrrole from gold during electrochemical cycling, 1-(2-thioethyl)pyrrole and 3-(2-thioethyl)pyrrole were synthesized as adhesion promoters. These compounds were designed to bind to gold through the sulfur atom to form monolayers, and their alkyl chains were made short to permit electron transfer to and from the pyrrole moieties. It was hoped that during electropolymerization of unmodified pyrrole in solution, the pyrrole moieties of the monolayer would be incorporated into the growing film. In this first paper of the series, the synthesis and characterization of the monomers and their electrochemical polymerization to form thin films is presented. Nuclear magnetic resonance and infrared, Raman, UV-vis-NIR, and mass spectroscopy measurements form a basis for comparison with monolayers adsorbed on gold surfaces in the following papers. Top of Page
1998	SAM	E. Smela, H. Kariis, Z. Yang, K. Uvdal, G. Zuccarello, and B. Liedberg "Thiol Modified Pyrrole Monomers: 2. As-Deposited Monolayers of 1-(2-Thioethyl)-Pyrrole and 3-(2-Thioethyl)-Pyrrole" Langmuir, 14 (11), 2976-2983 (1998) [Abstract] link to PDF on ACS site          link to supporting information A combination of analytical techniques was used to examine monolayers of 1-(2-thioethyl)pyrrole (1-TEP) and 3-(2-thioethyl)pyrrole (3-TEP) adsorbed on gold, including infrared, UV-vis-NIR, mass spectroscopy, ellipsometry, wetting, and X-ray photoelectron spectroscopy. Both monolayers were found to undergo adsorption-catalyzed reactions. Monolayers of 1-TEP were disordered even after long deposition times, with an isotropic distribution (orientation) of the tails, from perpendicular to parallel to the substrate, as expected for a short chain thiolate with a relatively bulky tail group. Vapor and solution deposition methods produced the same results. The 1-TEP monolayers were quite sensitive to oxygen, and exposure to air resulted in the rapid formation of carbonyl species. In the bulk, 1-TEP did not react in this manner. Solution-deposited 3-TEP monolayers consisted predominantly of short-chain alkanethiolates and/or disulfides that had lost the pyrrole moiety, together with ca. 30% of intact, bound 3-TEP. The cleavage was not reflected in the ethanolic solutions, nor did the amount of intact 3-TEP on the surface decrease with deposition time, so cleavage probably occurred upon adsorption. The intact 3-TEP molecules assumed a conformation with the pyrrole rings essentially parallel to the substrate. A large carbonyl peak indicated that the 3-TEP oxidized even more quickly than 1-TEP when exposed to air.
1998	SAM	E. Smela, H. Kariis, Z. Yang, M. Mecklenburg, and B. Liedberg "Thiol Modified Pyrrole Monomers: 3. Electrochemistry of 1-(2-Thioethyl)- Pyrrole and 3-(2-Thioethyl)-Pyrrole Monolayers in Propylene Carbonate" Langmuir, 14 (11), 2984-2995 (1998) [Abstract] link to PDF on ACS site          link to supporting information The electrochemical oxidation of 1-(2-thioethyl)pyrrole (1-TEP) and 3-(2-thioethyl)pyrrole (3-TEP) monolayers on gold was studied. Their cyclic voltammograms were different, with, for example, 1-TEP having three oxidation peaks and 3-TEP just one. Moreover, the cyclic voltammetry of bothTEPmonolayers was unusual and did not follow the expected behavior for simple electron transfer from an adsorbed monolayer. The chronoamperometry for 1-TEP was also surprising, showing nucleation-like peaks; this was not seen for 3-TEP. Air exposure left the 1-TEP, but not the 3-TEP, surface electrochemically inert. Spectroscopic data on the oxidation products of surface-bound pyrroles are presented for the first time. IRAS, XPS, andUV-vis-NIR showed that, after electrochemical cycling, all the pyrrole rings were absent. This demonstration that the molecules decompose calls into question the generality of the graftingmechanism of adhesion promotion for this type of compound.
1998	SAM	E. Smela "Thiol Modified Pyrrole Monomers: 4. Electrochemical Deposition of Polypyrrole over 1-(2-Thioethyl)-Pyrrole Monolayers" Langmuir, 14 (11), 2996-3002 (1998) [Abstract] link to PDF on ACS site The electrochemical deposition of polypyrrole (PPy) potentiodynamically and potentiostatically over monolayers of 1-(2-thioethyl)pyrrole (1-TEP) is described. The relationship between monolayer oxidation and PPy growth depended strongly on the electrolyte. In propylene carbonate/LiClO4, the PPy polymerization potential was higher than that for 1-TEP oxidation and the monolayer oxidation peak was identical to that seen in monomer- free electrolyte. In water/LiClO4, the oxidation and polymerization potentials were almost the same, with the result that the onset of polymerization was prevented until after the monolayer oxidn. process had been completed. In water/(sodium dodecylbenzenesulfonate), an anionic surfactant, the polymerization potential was lower than that in water/LiClO4, and polymerization apparently occurred without monolayer oxidation. In all cases, PPy growth was found to be essentially the same on 1-TEP, oxidized 1-TEP, and clean gold as evidenced by cyclic voltammograms, chronoamperograms, and film appearance. These results show that the 1-TEP monolayers probably decomposed before PPy polymerization began or were, in the case of dodecylbenzenesulfonate sodium salt, most likely inaccessible because they were covered by the surfactant. This indicates that 1-TEP cannot be used to covalently bind PPy to gold. These results also demonstrate that the general approach to adhesion promotion through the use of thiol-modified pyrroles has conditions and limits that must be recognized. The electrochem. of PPy grafting to surface-bound pyrrole depends critically on the electrolyte, which is important for application of this technique in real devices. In addition, the oxidation reactions undergone by such a monolayer cannot be assumed to be different in the absence and presence of pyrrole. Finally, the technique will probably not work with surfactant anions, which are of most practical interest. The study was conducted in an attempt to try to solve the problem of delamination of PPy from gold during electrochemical cycling, using (1-TEP) as adhesion promoter, however, the results are not encouraging.
1998	PLED	E. Smela, Y. Kaminorz, O. Inganäs, and L. Brehmer "Planar microfabricated polymer light emitting diodes" Semicond. Sci. Tech., 13, 433 (1998) [Abstract] link to PDF on IOP site Conjugated polymers are organic semiconducting materials that can emit light. These polymers have the advantages of being light, cheap and easy to process, and in addition the band gap can be tailored. We report the microfabrication of surface light-emitting diodes (SLEDs) on silicon substrates in which the electrodes are underneath the organic electroluminescent layer. Patterned electrodes are separated by a 2500 A thick insulating layer of silicon oxide or are interdigitated with a separation of 10 or 20 mm; the luminescent polymer is spin coated or solvent cast on top of the electrodes. This fabrication method is completely compatible with conventional silicon processing because the polymer is deposited last and the light is emitted from the upper surface of the diodes. Despite the large spacing between electrodes, and despite the absence of an evapd. top contact, the voltages required for light emission were not much greater than those used in conventional sandwich-type structures. The polymers used were poly(3-(4-octylphenyl)-2,2'-bithiophene) (PTOPT) and polythiophene with oligo(ethylene oxide) side chains (EO-PT).
1998	act-CP	S. Shimoda and E. Smela "The effect of pH on polymerization and volume change in PPy(DBS)" Electrochim. Acta, 44 (2-3), 219-238 (1998) [Abstract] link to PDF on Elsevier/Pergamon site Polymerization and electrochemically induced volume change in polypyrrole doped with the large aromatic surfactant anion dodecylbenzenesulfonate, PPy(DBS), were studied as a function of pH in unbuffered aqueous solutions prepared by mixing DBS acid and NaOH. Volume change was measured during the first, anomalous reduction and in subsequent cycles using a bending bilayer. The results differed completely from those obtained by others for PPy doped with small, mobile anions. Films could be deposited potentiostatically at all nominal pH values between 1 and 11, but growth was fastest at the lowest pH and uneven above pH 10 due to impaired nucleation. However, during potentiodynamic polymerization, deposition and redox were easier at alkaline pH, having a smaller peak separation and greater charge density. At pH less than 3, there were no reduction or oxidation (redox) peaks and the films became insulating. Contrary to expectations, the pH of the growth solution did not affect the performance of the bilayer actuators, but the pH of the cycling solution was important. Bending was fastest in alkaline solutions, although the degree of bending was the same at low and high pH. Except at very low pH, two pairs of redox peaks could be distinguished in the cyclic voltammograms, with volume change restricted to the lower potential pair associated with Na+ movement. There was no volume change associated with the anomalous current peak seen during the first reduction scan, regardless of pH, but the films contracted during re-oxidation. Thereafter, during reduction the films expanded to their original, as-grown volume.
1998	sens-CP	D. Pede, E. Smela, T. Johansson, M. Johanssen, and O. Inganäs "A general purpose conjugated polymer array for imaging" Adv. Mater., 10 (3), 233 (1998) [Abstract] link to PDF on Wiley/InterScience site A method is demonstrated of building microfabricated arrays of sensors based on conjugated polymers focusing on optical imaging with a photodiode array. The photodiode array is assembled by coating the bridge microstructure matrix with a conjugated polythiophene/C60 blend. An array of pixels formed by perpendicular electrodes coated with a polymer layer is used. The upper and lower layers of electrodes have different functions and are separated by an insulating layer where they cross. The deposition of the active material is achieved by spin-coating or solvent-casting of a conjugated polymer. The results of using the bridge devices as chemical sensors for detecting the distribution of I2 using a polymer layer which is oxidized by I2, and the possibility to derive bridge structures to obtain electroluminescence, are briefly described. Top of Page
1998	SAM	H. Kariis, E. Smela, K. Uvdal, M. Wirde, U. Gelius, and B. Liedberg "Thioethyl-pyrrole monolayers on gold. A spectroscopic study in ultra high vacuum" J. Phys. Chem. B, 102 (34), 6529-6538 (1998) [Abstract] link to PDF on ACS site The adsorption of 3-(2-thioethyl)pyrrole (3-TEP) and 1-(2-thioethyl)pyrrole (1-TEP) on gold has been studied in ultrahigh vacuum with infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption mass spectroscopy. Depositions were made from liquid phase at atmospheric pressure, from gas phase in ultrahigh vacuum: and from gas phase at atmospheric pressure. Both molecules were found to chemisorb to the gold surface regardless of the deposition method. Most of the 3-TEP, however, broke up, losing the pyrrole ring as it adsorbed on the surface. Both monolayers were sensitive to oxygen and oxidized quickly if stored in air. Deposition of 1-TEP from solution or vapor in air gave a higher desorption peak temperature than deposition from gas phase in UHV. The masses of the desorbing fragments that were detected from the heated monolayers were smaller than those seen upon introducing 1-TEP gas into the UHV system, revealing that a partial decomposition of the adsorbed molecules occurred prior to or at desorption. Some samples were electrochemically cycled to positive potentials to investigate the stability of the monolayers. A large fraction of the 1-TEP and 3-TEP stayed on the surface when cycled to 0.8 V, but some oxidation, fragmentation, and desorption were seen at higher potentials.
1998	PLED	Y. Kaminorz, E. Smela, O. Inganäs, and L. Brehmer "Sensitivity of polythiophene planar light emitting diodes to oxygen" Adv. Mater., 10 (10), 765-769 (1998) [Abstract] link to PDF on Wiley/InterScience site Results from microfabricated LEDs with electrodes underneath the electroluminescent layer (surface LEDs), here with a rectangular bottom ITO electrode separated by a 2500A thick insulating layer of SiO2 from a comb-shaped top Al electrode, are reported. The luminescent polymer, poly[3-(2',5'-bis(1'',4'',7''-trioxaoctyl)phenyl)- 2,2'-bithiophene] (EOPT), was solvent cast on top of the electrode. The polymer-on-top configuration allowed the gases to diffuse quickly into the film during device operation and enabled PL measurements on the same device. Electroluminescence (EL) was quenched by oxygen, which also increased the electron current in EOPT. Removal of oxygen caused the EL to recover and the current to fall again, but with different time constants. The quenching was not due to degradation. PL was insensitive to oxygen alone, so excitons were not directly involved in the quenching mechanism.
1995	LC	E. Smela and L. J. Martinez-Miranda "Effect of substrate preparation on smectic liquid crystal alignment: III. The significance of thermal history" J. Appl. Phys., 77 (5), 1930-1933 (1995) [Abstract] link to PDF on AIP site X-ray diffraction measurements were done on octylcyanobiphenyl (8CB) films on a glass grating with a 9 mm period. The films were heated from the smectic to the isotropic phase using three different methods. The cooling rate, which determines the time the mols. are able to respond to the influence of the air and substrate interfaces, plays a critical role in the alignment of the liquid crystal molecules. In all cases, the surface tension was strong enough, and the energy cost of an interface with a second alignment was high enough, to produce a single homeotropic alignment in films as thick as 15 mm.
1995	LC	E. Smela and L. J. Martinez-Miranda "Effect of substrate preparation on smectic liquid crystal alignment: II. Further results and modeling" J. Appl. Phys., 77 (5), 1923-1929 (1995) [Abstract] link to PDF on AIP site X-ray diffraction measurements were performed on smectic A octylcyanobiphenyl liq.-crystal (LC) films deposited on glass and Si gratings with periods between 664 nm and 200 mm. The film's two interfaces, air-LC and LC-grating, imposed boundary conditions that required different alignments for the minimum energy configuration; the competition between these two forces was studied in thick (3+ mm) films. Consistent with earlier work, gratings with periods approximately 10 mm and depths of 1.5-3 mm induced a portion of the film into homogeneous alignment in the direction of the grooves. As expected from theory, longer-period 200 mm gratings with the same depth showed an inconsistent ability to produce an inplane alignment; however, contrary to theory, shallower 664 nm gratings also failed to consistently induce in-plane alignment and did not produce an azimuthal preference. The results from the x-ray scans were fit with a multilayer model to give estimates of the thicknesses, orientations, and mosaic spreads of the different regions in the films. Films having an in-plane component have a 5-10 mm thick homeotropic skin at the surface. On 9 and 15 mm gratings, the in-plane scattering arose from a single homogeneous region in which the molecules had a small tilt (up to 5 degrees) with respect to the plane of the substrate. The director was not exactly parallel to the grooves but was offset by up to 20 degrees, and the mosaic spread in the azimuthal direction was typically 20 degrees. In some films there was an additional region that appeared to be disordered.
1995	act-CP	E. Smela, O. Inganäs, and I. Lundström "Controlled folding of micrometer-size structures" Science, 268 (23 June), 1735-1738 (1995) [Abstract] link to PDF on JSTOR site Several types of microactuators were fabricated (from simple paddles to self-assembling and -diassembling cubes). Conducting bilayers made of a layer of polymer and a layer of Au were used as hinges to connect rigid plates to each other and to a Si substrate. The bending of the hinges was electrically controlled and reversible, allowing precise 3-dimensional positioning of the plates. The structures were released from the substrate with a technique based on differential adhesion. This method (which avoids the use of a sacrificial layer and allows the actuators to pull themselves off the surface) may have general applications in micromachining. Possibilities include the manufacutre of surfaces whose light reflection or chemical properties can be switched.
1993	sens	K. Suga, N. Koshizaki, K. Yasumoto, and E. Smela "Gas-sensing characteristics of zinc oxide-nickel monoxide junction structures with intervening ultrathin silica layer" Sens. Act. B, 14 (1-3), 598-599 (1993) [Abstract] link to PDF on journal website The gas-sensing characteristics of junction structures composed of Al2O3-doped ZnO, NiO, and an intervening ultrathin SiO2 layer at the junction interface were investigated. An ultrathin intervening SiO2 layer 30-60 nm thick at the junction interface gives high NO sensitivity and fast response time. A suitable combination of component and intervening layer thickness will be important to obtain higher NO sensitivity. Top of Page
1993	LC	E. Smela and L. J. Martinez-Miranda "X-ray study of substrate-induced alignment of a smectic A liquid crystal" Liq. Cryst., 14 (6), 1877-1883 (1993) [Abstract] only available for a fee -- email me for a PDF copy The authors have performed a structural study of the liq. crystal (LC) octylcyanobiphenyl (8CB), deposited on gratings and flat surfaces, using high resolution x-ray scattering as a function of film thickness. Glass was used as substrate and treated with either one of the organic surfactants MAP or DMOAP. Competing with the LC-air interface, which is a strong aligner, a grating at the LC-substrate interface produces distortions in the smectic layering with an excess of elastic energy, which favors alignment parallel to the substrate and the grooves. The analysis confirmed the authors' earlier observations that surface tension at the air interface plays a dominant role in the alignment of the LC molecules.
1993	LC	E. Smela and L. J. Martinez-Miranda "Effect of substrate preparation on smectic liquid crystal alignment: a structural study" J. Appl. Phys., 73 (7), 3299-3304 (1993) [Abstract] link to PDF on AIP site Octylcyanobiphenyl, a bilayer smectic A liquid crystal at room temperature, was deposited on gratings, grids, and flat surfaces, and was studied using high-resolution x-ray scattering at shallow angles of incidence. One surface of the film was in contact with air and the other was in contact with the glass or Si substrate, which had been treated with one of two different silane solutions. At the air interface, surface tension forces caused the liquid crystal molecules to align perpendicularly with respect to the plane of the substrate. Competing with the liquid crystal-air interface, which is a strong aligner, a grating at the liquid crystal-substrate interface can produce distortions in the smectic layering, which results in excess elastic energy and favors alignment parallel to the substrate and the grooves. In films less than ~30 mm thick, the homeotropic orientation was maintained throughout the film due to the constraint of perpendicular alignment at the air interface. However, for thicker films on gratings, Bragg scattering from molecules lying parallel to the grooves was observed. The free-air surface was thus the strongest aligning force, followed by the surface topography, while surface anchoring was not found to play a role.
1993	act-CP	E. Smela, O. Inganäs, Q. Pei, and I. Lundström "Electrochemical muscles: micromachining fingers and corkscrews" Adv. Mater., 5, 630-632 (1993) [Abstract] link to PDF on Wiley site Artificial fingers, bilayers of gold and polypyrrole (PPy) anchored at one end, were fabricated on a Si wafer surface using microfabrication techniques and electropolymerization of pyrrole. In an aqueous electrolyte the fingers curled upon reduction of PPy, the gold layer being neutral. The response time was ~5 seconds and they operated for thousands of cycles.
1993	act-CP	E. Smela, O. Inganäs, and I. Lundström "Conducting polymers as artificial muscles: challenges and possibilities" J. Micromech. Microeng., 3 (4), 203 (1993) [Abstract] link to PDF on IOP site Microfabrication techniques allow one to construct metal/polymer bilayers anchored at one end to a substrate and free to move at the other end. Electrically controlled "fingers" were fabricated using the conducting polymer as an artificial muscle: doping and undoping the conjugated polymer causes a change in volume of this layer which results in bending of the bilayer. Grasping hands, fingers with sensing tips, and micropumps can be envisioned. Standing in the way of immediate fabrication are several materials challenges, including layer adhesion, material compatibility, sacrificial layer improvement, and low-temperature processing techniques.
1991	LC	E. Smela and L. J. Martinez-Miranda "X-ray study of liquid crystal alignment: Evidence for an alternate mesophase at the free surface?" Mol. Cryst. Liq. Cryst., 203, 1-8 (1991) [Abstract] only available for a fee -- email me for a PDF copy The alignment of the free surface of a liquid crystal film as a function of depth using grazing incidence x-ray diffraction was studied. Well defined gratings were produced on soda lime glass using conventional photolithograpic treatment with MAP. The substrates induced homogeneous alignment with a preferred orientation along the direction of the grooves in the bulk of an 8CB film in the smectic A phase at room temperature. At the air/liq. crystal interface, these molecules align homeotropically. Using conventional Bragg diffraction, a layer spacing of 31.5 A was observed for the perpendicularly aligned molecules near the surface. However, using a collimated beam in the grazing incidence geometry at a nominal incident angle of 0.0 degrees, a diffraction peak with a large angular spread showed a layer spacing of 40.1 A at the surface. The topmost layers are probably in the antiferroelectric smectic A2 phase, in which the molecules lie head to head with a double layer spacing of approximately twice the molecular length. Because the SmA2 phase is not stable in the bulk, it is either a surface or a grating induced effect. With increasing depth, the bulk bilayer spacing is reestablished.
1988	FO	E. Smela and J. J. Santiago-Aviles "A versatile twisted fiber optic sensor" Sens. Act., 13 (2), 117-129 (1988) [Abstract] link to PDF on ScienceDirect site An optical sensor sensitive to changes in light absorption and other optical interactions, and consisting of two fibers twisted around each other, is described. The sensor can be used as a refractometer, is very sensitive to the presence of water in oil, can be used to determine the amount of a solvent in oil, and might be useful in monitoring degradation of internal combustion engine oil. In addition, by surrounding the fibers with a film of solution, and observing the changes in ouput over time, additional information can be obtained. Used in this manner, the sensor can be used to identify specific solutions. Top of Page

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