Journal Articles

Journal Papers  |  Conference Proceedings  |  Books and Book Chapters  |  Patents

Due to the journals' ownership of the 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 get an article for free via the link, I will be happy to send it to you upon request:  send me an email 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
2009	DEA	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 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	DEA	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 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 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 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 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	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	cell	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	cell	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	DEA	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	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	DEA	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	CP	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 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	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 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	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	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	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 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	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	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] no online access at this time -- email me for a PDF copy 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	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	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 JSTOR 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	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	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	CP	E. W. H. Jager, E. Smela, and O. Inganäs "Microfabricating conjugated polymer actuators" Science, 290, 1540-1545 (2000) [Abstract] link to PDF on JSTOR 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	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	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	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	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	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	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 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 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 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	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	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 EBSCO 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 EBSCO 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	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] no online version available 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] no online version available 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 EBSCO 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	CP	E. Smela, O. Inganäs, Q. Pei, and I. Lundström "Electrochemical muscles: micromachining fingers and corkscrews" Adv. Mater., 5, 630-632 (1993) [Abstract] no online version available 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	CP	E. Smela, O. Inganäs, and I. Lundström "Conducting polymers as artificial muscles: challenges and possibilities" J. Micromech. Microeng., 3 (4), 203 (1993) [Abstract] no online version available 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] no online version available 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] no online version available 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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