We have previously presented “nastic” actuators based on electroosmotic (EO) pumping of fluid in microchannels using high electric fields for potential application in soft robotics. In this work we address two challenges facing this technology: applying EO to meso-scale devices and the stability of the pumping fluid. The hydraulic pressure achieved by EO increases with as 1/d2, where d is the depth of the microchannel, but the flow rate (which determines the stroke and the speed) is proportional to nd, where n is the number of channels. Therefore to get high force and high stroke the device requires a large number of narrow channels, which is not readily achievable using standard microfabrication techniques. Furthermore, for soft robotics the structure must be soft. In this work we present a method of fabricating a three-dimensional porous elastomer to serve as the array of channels based on a sacrificial sugar scaffold. We demonstrate the concept by fabricating small pumps. The flexible devices were made from polydimethylsiloxane (PDMS) and comprise the 3D porous elastomer flanked on either side by reservoirs containing electrodes. The second issue addressed here involves the pumping fluid. Typically, water is used for EO, but water undergoes electrolysis even at low voltages. Since EO takes place at kV, these systems must be open to release the gases. We have recently reported that propylene carbonate (PC) is pumped at a comparable rate as water and is also stable for over 30 min at 8 kV. Here we show that PC is, however, degraded by moisture, so future EO systems must prevent water from reaching the PC.
Neural probe insertion methods have a direct impact on the longevity of the device in the brain. Initial tissue and
vascular damage caused by the probe entering the brain triggers a chronic tissue response that is known to attenuate
neural recordings and ultimately encapsulate the probes. Smaller devices have been found to evoke reduced
inflammatory response. One way to record from undamaged neural networks may be to position the electrode sites away
from the probe. To investigate this approach, we are developing probes with controllably movable electrode projections,
which would move outside of the zone that is damaged by the insertion of the larger probe. The objective of this study
was to test the capability of conjugated polymer bilayer actuators to actuate neural electrode projections from a probe
shank into a transparent brain phantom.
Parylene neural probe devices, having five electrode projections with actuating segments and with varying widths (50 -
250 μm) and lengths (200 - 1000 μm) were fabricated. The electroactive polymer polypyrrole (PPy) was used to bend
or flatten the projections. The devices were inserted into the brain phantom using an electronic microdrive while
simultaneously activating the actuators. Deflections were quantified based on video images.
The electrode projections were successfully controlled to either remain flat or to actuate out-of-plane and into the brain
phantom during insertion. The projection width had a significant effect on their ability to deflect within the phantom,
with thinner probes deflecting but not the wider ones. Thus, small integrated conjugated polymer actuators may enable
multiple neuro-experiments and applications not possible before.
Most implantable chronic neural probes have fixed electrode sites on the shank of the probe. Neural probe shapes and
insertion methods have been shown to have considerable effects on the resulting chronic reactive tissue response that
encapsulates probes. We are developing probes with controllable articulated electrode projections, which are expected
to provoke less reactive tissue response due to the projections being minimally sized, as well as to permit a degree of
independence from the probe shank allowing the recording sites to "float" within the brain. The objective of this study
was to predict and analyze the force-generating capability of conducting polymer bilayer actuators under physiological
settings.
Custom parylene beams 21 μm thick, 1 cm long, and of varying widths (200 - 1000 μm) were coated with Cr/Au.
Electroplated weights were fabricated at the ends of the beams to apply known forces. Polypyrrole was
potentiostatically polymerized to varying thicknesses onto the Au at 0.5 V in a solution of 0.1 M pyrrole and 0.1 M
dodecylbenzenesulfonate (DBS). Using cyclic voltammetry, the bilayer beams were cycled in artificial cerebrospinal
fluid (aCSF) at 37 °C, as well as in aqueous NaDBS as a control. Digital images and video were analyzed to quantify the
deflections. The images and the cyclic voltammograms showed that divalent cations in the aCSF interfered with
polymer reduction.
By integrating polypyrrole-based conducting polymer actuators, we present a type novel neural probe. We demonstrate
that actuating PPy(DBS) under physiological settings is possible, and that the technique of microfabricating weights onto
the actuators is a useful tool for studying actuation forces.
Dielectric elastomer actuators (DEAs) consist of an elastomer sandwiched between two electrodes, and they undergo a
large in-plane expansion upon the application of an electric field. They therefore require compliant electrodes that can
stretch tens of percent. The most commonly used electrode material is carbon grease, which smears easily and is
difficult to pattern. This paper outlines the fabrication and performance of a novel polydimethylsiloxane (PDMS)
composite having a 15 wt% loading of exfoliated graphite (EG). This new material has a Young's modulus of only 0.9
MPa and a conductivity of 0.15 S/cm. Unlike other composite electrode materials, the Young's modulus of the
PDMS/EG increases only slightly, by a factor of two, upon addition of the EG. Furthermore, the PDMS/EG composite
is patternable and will not rub off. DEAs were fabricated with 20:1 PDMS as the elastomer using this new electrode
material. The actuation strains were equal to those of 10:1 PDMS DEAs with carbon grease electrodes under the same
electric field. Elastomer/EG composites may also find applications in areas such as flexible electronics, robotics, strain
gauges, and sensors.
Dielectric elastomer actuators (DEAs) have been demonstrated for meso- and macro-scale applications, but only a few
devices have been shown at the micro-scale, the most common of which have been diaphragms that bulge out of the
plane of the wafer. Microscale DEAs would be of value in a wide range of small devices, including micro-robots, micropumps,
and micro-optical systems. An additional advantage of miniaturizing is a reduction in the required driving
voltage from kilovolts to tens of volts because the layers are thinner. However, fabrication of micro-scale DEAs remains
challenging, due in part to the fact that the vast majority of macro-scale materials and/or fabrication methods cannot be
adapted to the micro-scale. On the micro-scale, the elastomers must be deposited as thin films, they must be patternable,
and they must be compatible with the other materials used during fabrication, such as sacrificial layers. The realization
of compliant electrodes must also be handled in a new way. To fully realize the potential of micro-DEAs, it would also
be desirable to develop fabrication procedures for integrating the micro-scale DEAS with complementary metal-oxidesemiconductor
(CMOS) driver circuits and other micro-electro-mechanical systems (MEMS). This article addresses the
progress that has been made thus far in making microfabricated DEAs, as well as the challenges and the key areas in
which additional research needs to be pursued.
A new type of polymeric actuator has been developed based on a micro-scale hydraulic mechanism, in which electroosmotic
flow (EOF) is used to pump a fluid from one place to another in the device. This "nastic" actuator is in principle capable of producing both large displacements and high forces at reasonable speeds. Prototypes were fabricated from polydimethylsiloxane (PDMS) by micro-molding a fluid supply chamber, an expansion chamber, and
connecting channels, and then topping this layer with a thin PDMS membrane. Upon applying a voltage across the two chambers, fluid flowed into the expansion reservoir, deflecting the membrane upward by hundreds of μm within a few seconds. The performance of these prototypes have been characterized in terms of deflection under load at various applied voltages, deflection vs. time upon input of a step potential, and repeatability. The performance of the actuator
has been modeled, and the experimental and theoretical results are in reasonable agreement. The modeling work predicts that as the channel size is scaled down, the actuation stress will increase substantially, up to GPa for nanochannels, rivaling piezoelectrics and shape memory alloys but with much higher strain. Future applications of these actuators may include valves, shape-changing materials, and soft robotics.
Cell-based sensors are being developed to harness the specificity and sensitivity of biological systems for sensing
applications, from odor detection to pathogen classification. These integrated systems consist of CMOS chips
containing sensors and circuitry onto which microstructures have been fabricated to transport, contain, and nurture the
cells. The structures for confining the cells are micro-vials that can be opened and closed using polypyrrole bilayer
actuators. The system integration issues and advances involved in the fabrication and operation of the actuators are
described.
A compliant electrode material is presented that was inspired by the electroding process used to manufacture ionic polymer-metal composites (IPMCs). However, instead of an ion-exchange membrane, a UV-curable acrylated urethane elastomer is employed. The electrode material consists of the UV-curable elastomer (Loctite 3108) loaded with tetraammineplatinum(II) chloride salt particles through physical mixing and homogenization. The composite material is made conductive by immersion in a reducing agent, sodium borohydride, which reduces the salt to platinum metal on the surface of the elastomer film. Because the noble metal is mixed into the elastomer precursor as a salt, the amount of UV light absorbed by the precursor is not significantly reduced, and the composite loses little photopatternability. As a result meso-scale electrodes of varying geometries can be formed by exposing the precursor/salt mixture through a mask. The materials are mechanically and electrically characterized. The percolation threshold of the composite is estimated to be 9 vol. % platinum salt, above which the compliant electrode material exhibits a maximum conductivity of 1 S/cm. The composite maintains its electrical conductivity under axial tensile strains of up to 40%.
Bilayer microactuators of gold and polypyrrole doped with dodecylbenzene sulfonate, PPy(DBS), are characterized with respect to their response times and the influence of operation temperature. These parameters are needed for biomedical applications such as microvalves. To fully open and close the valves, the bilayer hinges must be able to rotate within a few seconds at body temperature. Bilayers were subjected to potential steps to switch the PPy between the oxidized and reduced states. Actuation was viewed through an optical microscope and recorded by a digital camera. The velocity profiles during reduction and oxidation follow the same trends. Two different phases of actuation can be identified. In the first phase there is rapid movement, and in the second phase the velocities slowly decrease until the position reaches steady-state. In order to investigate the effects of elevated temperature on the actuators, the operation temperature was varied stepwise from 25 °C to 55 °C. The curvature increased irreversibly by up to 45% at elevated temperatures, and the output force dropped.
Previously, we presented a model for ion transport in conjugated polymers during electrochemical reduction. In this paper, we will present a more advanced model that includes hole transport, which was neglected in the first-cut model. This addition takes into account the interactions between holes and cations during transport. The result is that the front between oxidized and reduced material now propagates with constant velocity, instead of slowing down over time. Also, an electrolyte layer has been added to the model, and as a result the ion concentration behind the phase front is more accurately predicted.
The ions present in the electrolyte in which a conjugated polymer actuator is cycled are known to affect performance. Understanding how force, response time, and strain are affected by ion size and other ion characteristics is critical to applications, but is not yet well understood. In this paper, we present the effect of alkali cation size on transport velocity and volume change in polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), which is a cation- transporting material. Volume change measured by mechanical profilometry is greatest for Li+ and decreases in order of atomic mass: Li+ > Na+ > K+ > Rb+ > Cs+. Ion transport, measured by phase front propagation experiments, is also fastest for Li+, contradicting the expectation that larger species would move more slowly.
It is important to increase the switching speed of conjugated polymers between oxidized and reduced states for a wide range of devices, including capacitors, electrochromic displays, and actuators. In this paper, we compare the in-plane and the out-of- plane ion transport speed during electrochemical reduction of a conjugated polymer, polypyrrole doped with dodecylbenzenesulfonate. Results show that the in-plane ion transport is approximately 50 times faster than out-of-plane transport. The anisotropy is likely induced by the dodecylbenzenesulfonate, which has been shown previously to form layers parallel to the surface. An engineering method is presented to enhance the in-plane ion transport by etching pores into the polymer.
Polypyrrole/gold bilayer microactuators are being developed in our laboratory for biomedical applications such as microvalves. To fully open and close the valves, the bilayer hinges must be able to rotate between 0° and 180° within a few seconds against external forces. The layer thicknesses and hinge lengths must therefore be properly designed for the application. However, existing models fail to predict the correct behavior of microfabricated PPy/Au bilayer microactuators. Therefore, additional experimental data are needed to correctly forecast their performance. Bilayer actuators were fabricated with ranges of PPy thicknesses and hinge lengths. Bending angles were recorded through a stereomicroscope in the fully oxidized and reduced states. Isometric forces exerted by the hinges were measured with a force transducer, the output of which was read by a potentiostat and correlated with the applied potentials.
The transport of charged species, including both polarons/bipolarons and charge-compensating ions, occurs when conjugated polymers switch between oxidized and reduced states. However, physics-based models of the charge transport processes have not yet been developed. Previously, we presented an electrochromic device that made the path for ion transport much longer than that for electrons, ensuring that ion transport was the rate-limiting step so that the constitutive equation for ion transport could be formulated. Ion concentration profiles and velocities could be tracked by color changes. In this paper, we present the correlation between ion transport and volume change, measured in this device using a mechanical profilometer to scan height profiles during electrochemical reduction. In addition, the effects of electrolyte concentration, electrolyte temperature, film thickness, and ion barrier stiffness on ion transport velocities are explored.
Improving the lifetime of conjugated polymer-based devices that undergo repeated cyclic electrical stimulation, such as
actuators, is important for commercialization. In general, conjugated polymers are contacted by metal electrodes; strain
from volume change can cause the polymers to delaminate, which slowly deteriorates performance or results in sudden
device failure. In this paper, we used polypyrrole on gold to investigate methods for improving adhesion. Gold electrode surfaces were roughened through electroplating, and the adhesion of polypyrrole deposited on these surfaces was tested upon extended electrochemical cycling. Delamination was quantified using a tape test and followed versus cycle number until the polypyrrole was removed or was no longer electroactive. Untreated control surfaces were also monitored. The most effective method for improving adhesion was a 1 μm thick layer of electroplated Au, which
extending the lifetime of the interface beyond the 50,000 cycle lifetime of PPy in aqueous solutions.
We present the use of electroactive polymer actuators as components of a biolab-on-a-chip, which has potential applications in cell-based sensing. This technology takes full advantage of the properties of polypyrrole actuators as well as the wide range of CMOS sensors that can be created. System integration becomes an important issue when developing real applications of EAP technologies. The requirements of the application and the constraints imposed by the various components must be considered in the context of the whole system, along with any opportunities that present themselves. In this paper, we discuss some of these challenges, including actuator design, the use of complementary actuation techniques, miniaturization, and packaging.
Electron transport and ion transport are two critical processes taking place during electrochemical oxidation/reduction of conjugated polymers. Because they accompany and depend on each other, research on the individual processes is difficult. We present a device that allows us to measure ion transport directly and independently from electron transport in conjugated polymers. The device geometry makes the ion path much longer than the electron path, ensuring that ion transport is the rate-limiting step. Ion transport is also visualized directly through the color change of the film (electrochromism) as the electrochemical reaction proceeds, allowing one to precisely and quantitatively track the ion velocity. During reduction at sufficiently negative potentials, a phase front between the oxidized and reduced states was observed to travel into the film, the speed of which was proportional to the applied voltage, demonstrating that migration (rather than diffusion) is the key driving force. At less negative reducing potentials, the film gradually and more uniformly changed color, indicating that diffusion plays a large role. A simple first-cut model with drift and diffusion terms is presented. The simulated ion concentration profile matched the experimentally measured intensity profile strikingly well.
By utilizing strain gage technology it is possible to directly and continuously measure the electrochemically induced strain response of EAP actuators. Strain sensitive actuators were constructed by directly vapor depositing gold (EvAu) on polyimide strain gages which are capable of measuring strain with an accuracy of +/- 1(mu) (epsilon) . Strain sensitive actuators were used to evaluate the strain response of polypyrrole (PPy), poly(3,4-ethylenedioxypyrrole) (PEDOP) and poly(3,6-bis(2-(3,4-ethylenedioxy)thienyl)-N-carbazole) (PBEDOT-Cz). PPy was shown to produce significantly higher strain when compared to PEDOP and PBEDOT-Cz. The resulting overall strain for the materials was: 236, 33, and 35 (mu) (epsilon) respectively. From the initial investigation, adhesion of the EAP to the EvAu layer was identified as a major factor in the resulting lifetime and strain response of these actuators. Therefore an electrochemically deposited Au layer (EcAu) was deposited on top of the EvAu layer to improve the adhesion of the EAP to the working electrode. By changing the surface roughness from requals3.43 (EvAu) to requals8.26 and 18.00 (EcAu) the normalized strain response after 2000 cycles increases from 45% to 60% and 68% respectively. Also by changing the surface roughness from 5 to 23, the resulting strain response increases from ~100 (mu) (epsilon) to 600-800 (mu) (epsilon) for Ppy.
In this work, we investigated the electrochemical actuation of gilded polyaniline bilayers in acidic aqueous electrolytes. Gilding was found to be a useful method to ensure a uniform potential distribution across polyaniline films so that well-defined electrochemistry and electrochemical actuation could be obtained. Electrochemical actuation of gilded polyaniline bilayers was studied by means of bending and linear actuation. Actuation could be obtained by a number of electrical stimulation modes including cyclic voltammetry (CV), square wave potential (SWP) and square wave current (SWC). Within the potential range of -0.2 ~ 0.6 V (vs Ag/AgCl), the polyaniline films expanded upon oxidation and contracted upon reduction, which corresponds to the first redox process of polyaniline between the leucomeraldine and emaraldine oxidation states. Actuation obtained in this potential range is related to the insertion/deinsertion of the electrolyte anion upon oxidation/reduction of polyaniline. It was found that, due to the thin thickness of the gold layer, not only fast bending actuation but also linear actuation could be achieved for the resulting gilded polyaniline bilayers. Extending the applied potential to more positive potentials, polyaniline degradation and oxidation of gold layer were observed.
The volume change that the conducting polymer polypyrrole (PPy) undergoes upon electrochemical oxidation and reduction can be used to make microactuators. We have made microactuators based on a PPy/Au bilayer. These actuators have been combined with other micromachined structures to make biomedical microdevices. Using an area of bilayers one can potentially arrest (nerve) fibers. They can also be used to close a micrometer sized cavity with a lid. In addition, we demonstrate a microrobotic arm that may be developed for the manipulation of small particles.
We present the results of our structural measurements on smectic liquid crystal films deposited on photolithographed gratings on both glass and silicon substrates. These gratings have periods in the range of 664 nm - 200 micrometers , and the depths of up to 2 micrometers . We found that both silicon and glass gratings are able to align the liquid crystal in the period range 664 nm - 24 micrometers , as determined using both x-ray diffraction and optical microscopy. This result sets upper and lower boundaries for grating preparation. The results of our measurements were fit to a multilayer orientational model in order to determine the thickness of the observed aligned layer. We discuss the possible impact of our results on large area display packaging and preparation.
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