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JIF=4.806; International audience; Low cost soft, flexible and stretchable sensors are being explored that can be applied to virtually any surface, and promise to enable tactile sensing on robots, prosthetics, skin and stretchable displays. Previous work has shown that capacitive sensors that are both stretchable and transparent can be created by using ionically conductive hydrogel electrodes. In this study, the electrodes are interpenetrating polymer networks swollen with ionic liquid, enabling high transparency and stretchability, without evaporation. The interpenetrating network is synthesized from poly(ethylene oxide) and nitrile butadiene rubber. Electrodes are patterned using femtosecond laser machining, creating stretchable electrodes. These were encapsulated in a polydimethylsiloxane substrate, producing a 95% transparent sensor (450 nm). A 4x4 tactile array shows the ability to sense proximity and multi-touch, as well as be robust to variations in environmental temperature (from 4 °C to 72 °C). Temperature change has a large effect on the resistance of the electrodes - an effect that could be used to measure device temperature. In addition, the sensor is able to detect proximity and touch while on skin, when covered by a layer of fabric or during stretch.

An integrated solar energy conversion and storage system is presented using a dye sensitized electrode in a redox battery structure. A stable discharge voltage is shown with high areal energy storage capacity of 180 µW h cm−2 by choosing iodide/polysulfide as the pair of active materials matched with permeable porous electrodes. The solar rechargeable battery system offers a higher round-trip efficiency and potential cost savings on fabrication compared to individual devices.

At the mention of nylon, you may think of shirts, stockings, fishing line, and sutures. A recent discovery, first reported in February 2014 in Science, is that the strong, tough, and inexpensive threads that make up fabrics and filaments can actively contract and expand?with much greater force than muscle [1]. This newly discovered musclelike action creates opportunities in medical devices?among other areas?that are just beginning to be explored.

Diverse means have been deployed for harvesting electrical energy from mechanical actuation produced by low-grade waste heat, but cycle rate, energy-per-cycle, device size and weight, or cost have limited applications. We report the electromagnetic harvesting of thermal energy as electrical energy using thermally powered torsional and tensile artificial muscles made from inexpensive polymer fibers used for fishing line and sewing thread. We show that a coiled 27 μm-diameter nylon muscle fiber can be driven by 16.7 °C air temperature fluctuations to spin a magnetic rotor to a peak torsional rotation speed of 70 000 rpm for over 300 000 heating–cooling cycles without performance degradation. By employing resonant fluctuations in air temperature of 19.6 °C, an average output electrical power of 124 W per kg of muscle was realized. Using tensile actuation of polyethylene-based coiled muscles and alternating flows of hot and cold water, up to 1.4 J of electrical energy was produced per cycle. The corresponding per cycle electric energy and peak power output, per muscle weight, were 77 J kg−1 and 28 W kg−1, respectively.

International audience; Conducting polymer actuators have long been of interest as an alternative to piezoelectric and electrostatic actuators due to their large strains and low operating voltages. Recently, poly (3,4-ethylenedioxythiophene) (PEDOT) - based ionic actuators have been shown to overcome many of the initial obstacles to widespread application in micro-fabricated devices by demonstrating stable operation in air and at high frequencies, along with microfabrication compatible processing using a layer by layer method that does not require any handling. However, there is still a need for characterization, prediction, and control of the actuator behavior. This paper describes the fabrication and characterization of thin trilayers composed of a 7 mu m thick solid polymer electrolyte (SPE) sandwiched between two 2.1 mu m thick PEDOT-containing layers. Beam properties including capacitance, elastic moduli of the layers, and the extent of charge driven strain, are applied to predict curvature, frequency response and force generation. The actuator is represented by an electrical circuit, a mechanical system described via dynamic beam theory, and a strain-to-charge ratio for the electro-mechanical coupling matrix, which together predict the actuator curvature and the resonant response. The success of this physical model promises to enable design and control of micro-fabricated devices.

There is increasing interest in creating bendable and stretchable electronic interfaces that can be worn or applied to virtually any surface. The electroactive polymer community is well placed to add value by incorporating sensors and actuators. Recent work has demonstrated transparent dielectric elastomer actuation as well as pressure, stretch or touch sensing. Here we present two alternative forms of sensing. The first uses ionically conductive and stretchable gels as electrodes in capacitive sensors that detect finger proximity. In this case the finger acts as a third electrode, reducing capacitance between the two gel electrodes as it approaches, which can be detected even during bending and stretching. Very light finger touch is readily detected even during deformation of the substrate. Lateral resolution is achieved by creating a sensor array. In the second approach, electrodes placed beneath a salt containing gel are able to detect ion currents generated by the deformation of the gel. In this approach, applied pressure results in ion currents that create a potential difference around the point of contact, leading to a voltage and current in the electrodes without any need for input electrical energy. The mechanism may be related to effects seen in ionomeric polymer metal composites (IPMCs), but with the response in plane rather than through the thickness of the film. Ultimately, these ionically conductive materials that can also be transparent and actuate, have the potential to be used in wearable devices.

A stretchable, transparent touch pad and proximity sensor made using silicone and gel operates while being bent and stretched., The development of bendable, stretchable, and transparent touch sensors is an emerging technological goal in a variety of fields, including electronic skin, wearables, and flexible handheld devices. Although transparent tactile sensors based on metal mesh, carbon nanotubes, and silver nanowires demonstrate operation in bent configurations, we present a technology that extends the operation modes to the sensing of finger proximity including light touch during active bending and even stretching. This is accomplished using stretchable and ionically conductive hydrogel electrodes, which project electric field above the sensor to couple with and sense a finger. The polyacrylamide electrodes are embedded in silicone. These two widely available, low-cost, transparent materials are combined in a three-step manufacturing technique that is amenable to large-area fabrication. The approach is demonstrated using a proof-of-concept 4 × 4 cross-grid sensor array with a 5-mm pitch. The approach of a finger hovering a few centimeters above the array is readily detectable. Light touch produces a localized decrease in capacitance of 15%. The movement of a finger can be followed across the array, and the location of multiple fingers can be detected. Touch is detectable during bending and stretch, an important feature of any wearable device. The capacitive sensor design can be made more or less sensitive to bending by shifting it relative to the neutral axis. Ultimately, the approach is adaptable to the detection of proximity, touch, pressure, and even the conformation of the sensor surface.

This project outlines the design of an innovative wearable wrist orthosis which potentially can be used for rehabilitation therapy and for assisting with daily tasks. The orthosis uses recently discovered nylon artificial muscles as a means to actuate the device. Constructed from silver coated nylon sewing thread, these newly developed actuators have been shown to exhibit power to weight ratios that exceed that of human skeletal muscle. In this paper, we develop an electromechanical model for the force generated by the actuators and propose a simple closed loop controller to control the actuators. This system is then implemented in a wrist exoskeleton prototype to demonstrate the potential use of nylon actuators in biomedical applications.

Ultrafast laser micromachining was optimized for microstructuring polypyrrole as a facile new approach towards tailoring electrochemical and mechanical responses desirable for microactuator, sensors, neural probing, and nerve conduit applications. Laser perforation of high-density and high aspect ratio through-holes generated greater than 5-fold increase in surface area. The flexible machining technique offers micron-size resolution and fast prototyping capability for optimizing properties and opening new directions for polypyrrole-based devices.

An analytical model is presented to describe the electrochemical impedance of conducting polymer based devices. The analytical expression of the impedance is obtained from a two dimensional finite transmission line equivalent circuit. The model relates impedance to cell geometry, electrolyte conductivity, polymer ionic and electronic conductivities and capacitance. These parameters were measured for a hexafluorophosphate (PF6-) doped polypyrrole material (the conducting polymer used in this study) and entered to the model to predict its impedance as a function of frequency. The model is unique in representing the two dimensional charging of the polymer, namely ionic mass transport through the thickness of the polymer structure and electronic resistance along its length. Close agreement is observed between impedance spectroscopy results and model prections of the charging of a polypyrrole film electrically connected at one end. The provides a means of modeling the electrochemical charging of conducting polymers and electrochemical double layer capacitor electrodes having significant ionic and electronic conductivities.

In the existing theory for organic field-effect transistors (OFETs) the effect of molecular order on the subthreshold characteristics is ignored. This effect is studied analytically through the potential profile and subthreshold swing at a weak accumulation mode. The developed theory predicts a wide accumulation region and a low subthreshold swing for a poorly ordered semiconductor. Simulation of transistors with various distributions of density of states is performed to mimic OFETs made of regioregular-Poly 3 hexylthiophene (rr-P3HT). The simulation results support the analytical equation for the potential profile. Furthermore, the effect of molecular order on the subthreshold swing matches with the theory. However, the analytical value for the subthreshold swing is greatly underestimated mainly due to an oversimplified model used for mobility.

The performance of bio-photovoltaic devices with a monolayer of the immobilized photosynthetic reaction center (RC) is generally low because of weak light absorption and poor charge transfer between the RC and the electrode. In this paper, a new bio-photovoltaic device is described in which the RC is dissolved in the electrolyte of an electrochemical cell. The charges generated by the illuminated RC are transferred to electrodes via mediators. The difference between the reaction rates of two types of mediator at the electrode surfaces determines the direction of the photocurrent in the device. Experimental results show that the magnitude of the photocurrent is proportional to the incident light intensity, and the current increases nonlinearly with an increase in the RC concentration in the electrolyte. With further optimization this approach should lead to devices with improved light absorption.

Squeezing a water droplet between two electrodes can generate a potential difference by converting some of the mechanical energy in vibrations into electrical energy. By utilizing the high capacitance inherent to electric double layers, and the surface charging at a polymer/water interface, we demonstrate a sensor that generates up to 892 mV peak-to-peak between 1 and 100 Hz, in response to a 250 μm deformation. This frequency response is described and explained using a linearized model in which the interfacial charge acts as the priming voltage, removing the need for external charging normally required in capacitive generators. The model suggests how to design the cell for maximum power output and provides an intuitive understanding of the high pass nature of the sensor. It successfully predicts the point of maximum power transfer.

Polypyrrole actuators offer high stresses, moderate strains, low voltage operation and biocompatibility. These materials also act as force and displacement sensors. In this paper the properties of polypyrrole sensors are investigated, enabling the first quantitative demonstration of the link between actuation and sensing, and suggesting a new mechanism of electromechanical coupling that can be exploited in materials that are both ionically and electronically conducting. Polypyrrole actuates when electrochemically charged, with the strain being proportional to the charge density transferred. It has also been shown that applying force to polypyrrole generates a proportional change in voltage. It is shown herein that the same constant of proportionality describes the response in both cases. Based on this finding a semi-empirical model of the electromechanical coupling is proposed. The model suggests that the insertion of ions into the polymer requires mechanical work to elastically deform the matrix. The external application of stress alters the internal stress on ions, helping insert or remove charge, and change voltage. The frequency dependence of the relationship between stress and voltage is also investigated. The sensor response is relatively stable at frequencies between 0.1 Hz and 100 Hz. The results and models presented enable the prediction of polypyrrole sensor and actuator responses.

In spite of a high quantum efficiency in the bacterial photosynthetic reaction center (RC) the overall efficiency in a RC-based photovoltaic device is very poor partly because of an inefficient collection of charges by electrodes. To explain charge transport between the RC and an electrode a diffusion model is proposed. The numerical solution of the diffusion process describes the measured photocurrent well. An approximation of the initial condition is also made to obtain analytical expressions for the photocurrent. The model suggests that the slow transient response of the photocurrent is due to the diffusion in a biological photovoltaic device.

Organic semiconductor to metal Schottky contacts have been widely used in electronic devices and to investigate the properties of organic semiconductors. In designing and characterizing these devices the full depletion approximation is used. The analytical and numerical simulations presented in this paper suggest that this approximation is not generally valid. Simulations of a Schottky contact between regioregular-poly 3 hexylthiophene (rr-P3HT) and aluminum show that this approximation becomes worse as molecular order decreases, with the potential profile increasingly deviating from the expected quadratic function of position. Also the depletion width decreasing well below that predicted using the approximation. In this work the slope of the band tail is used as a measure of disorder.

The use of printing methods to deposit organic semiconductors promises to enable low-cost electronics. However, printing processes deposit thick and amorphous semiconductor layers that result in poorly performing organic field-effect transistors (OFETs) that generally are not appropriate for incorporation into commercially viable circuits. Another undesirable property of OFETs is their high operating voltage (~40 V). Organic metal-semiconductor FETs (OMESFETs) are proposed as alternatives to OFETs for use with printing methods. OMESFETs operate at low voltages (~5 V) and are expected to show better on/off current ratios than OFETs in a thick-film semiconductor. Simulations of OFETs and OMESFETs are performed assuming regioregular poly (3-hexylthiophene) (rr-P3HT) as the amorphous semiconductor layer with localized states close to the band edge. The results of the simulations show a current ratio of 104 in the OMESFET and of 700 in the OFET for a 400-nm-thick semiconductor layer. Because the OMESFET operates in the depletion mode, versus the accumulation mode in the OFET, the calculated mobility in the OMESFET is two orders of magnitude smaller than that in the OFET. Simulations suggest that the OMESFET design offers performance advantages over printable OFETs, where low-voltage operation is demanded.

In fabricating organic field-effect transistors (OFET) the deposition of a very thin and electrically continuous semiconductor layer using a low-cost process such as a printing method is a challenge. A simple model is proposed which relates performance to thickness, and shows that the thick layers typical of low-cost methods lead to poor device properties. The analytical model of thickness dependence is shown to match OFET simulation results for a range of thickness. These results indicate a change in the threshold voltage and drops in the output impedance and the current ratio with an increase in the semiconductor thickness. As a solution a dual gate structure is suggested for organic transistors, in which the secondary gate controls the effective thickness of the organic layer through a Schottky contact with the semiconductor. Simulation results for a 200 nm thick dual gate OFET show a performance much better than is observed in a near optimal 20 nm thick OFET, by achievement of a current ratio of 10 6 , versus 2500 in the OFET.

Bolted-down robots labor in our factories, performing the same task over and over again. Where are the robots that run and jump? Equaling human performance is very difficult for many reasons, including the basic challenge of demonstrating motors and transmissions that efficiently match the power per unit mass of muscle. In order to exceed animal agility, new actuators are needed. Materials that change dimension in response to applied voltage, so-called artificial muscle technologies, outperform muscle in most respects and so provide a promising means of improving robots. In the longer term, robots powered by atomically perfect fibers will outrun us all.

A polyurethane hydrogel based touch sensor with high transparency and conformability is demonstrated. Polyurethane hydrogels swollen with various electrolytes were compressed at a pressure of 30 kPa, simulating a fingertap on a conventional touch screen device. Unlike ionic polymer metal composite and conducting polymer trilayer sensors, where electrodes render the sensors opaque and relatively rigid, the electrodes used in this work are metal wires or strips, separated from each other by regions of transparent film, enabling transparency and compliance. The voltages and currents observed when the perturbation is above one electrode are on the order of 10-2 V and 10-7 A, relative to a second electrode that is approximately 1 cm away. The sign of voltage and current signals detected from perturbations made between electrodes is determined by relative proximity to each electrode, and the magnitude appears to decrease with increasing distance from the electrodes. These observations suggest that it may be possible to discriminate the location of touch based on signals transmitted to the edges of an ionically conductive film. A model to describe the inhomogeneous ionic distribution and predict the resultant voltage and current is presented to qualitatively explain the sensing, based on the Donnan potential.

Conducting polymers demonstrate effective capacitances of more than 100 Farads per gram, 5 orders of magnitude higher than traditional capacitors. However polymer discharge times tend to be on the order of seconds, as opposed to the milli or microseconds of conventional capacitors, so that the overall power density is still at least an order of magnitude lower. The polypyrrole devices are essentially electrolytic capacitors in which charging times appear to be limited by rates of ionic mass transport and RC charging times. Electrochemical impedance measurements suggest diffusion time constants of 33 ms in 158 nm thick polypyrrole films with volumetric capacitances of 107 F/m3. The impedance of a highly porous polypyrrole/carbon composite was measured to investigate the achievement of similarly fast response times in much thicker materials. It is shown that increasing the polypyrrole content of the film increases capacitance up to 60 F/g, but also increases the charging time constant. Analysis of the rate limiting factors suggests a method of optimizing capacitor geometry in order to maximize rates, including the prediction of device geometries that will lead to power delivery matching those of traditional capacitors.

A decade of research into electroactive polymer actuators is leading to the exploration of applications. These technologies are not ready to compete with the internal combustion engine and electric motors in high power propulsion systems but are suitable for intermittent or aperiodic applications with moderate cycle life requirements, providing an alternative to solenoids and direct drive electric motors. Polypyrrole, an emerging actuator material, is applied to drive hydrodynamic control surfaces and in particular to change the camber of a foil. The foil is intended for use in the propeller blade of an autonomous underwater vehicle. A scaled prototype is constructed which employs polypyrrole actuators imbedded within the blade itself to vary camber. The kinematics required to generate camber change are demonstrated, with >30/spl deg/ deflections of the trailing edge being observed from both bending bilayer and linear actuator designs. Forces developed in still conditions are five times lower than the 3.5 N estimated to be required to implement variable camber. The observed 70 kJ/m/sup 3/ polypyrrole work density however is more than sufficient to produce the desired actuation from within the limited blade volume, enabling an application that is not feasible using direct drive electric motors. A key challenge with the polypyrrole actuators is to increase force without sacrificing speed of actuation.

Impedance spectroscopy, charge/discharge curves and pulsed current measurements are used to identify cell response as a function of voltage and frequency, and in turn to create a non-linear time domain circuit model of a cell. The cell studied incorporates a lithium nickel manganese cobalt electrode. Linear finite transmission line models are fitted to the frequency response to identify model parameters, including equivalent series resistance, charge transfer resistance, double layer capacitance and mass transfer resistance. The frequency response is obtained at a number of states of charge so that the dependence of these properties on state of charge can be estimated. Constant current charging is used to estimate effective capacitance as a function of state of charge. A non-linear capacitor in series with an equivalent circuit resistance effectively predicts cell time response to pulsed current inputs, except where gradients in local state of charge exist within electrodes.

Photogalvanic solar cells, the original dye based solar cell, have yet to fulfill their promise as a low fabrication cost, scalable energy conversion system. The efficient performance of photogalvanic cells relies on high dye solubility and selective electrodes with fast electron transfer kinetics. A new configuration is proposed for the photogalvanic cells that removes these impractical requirements. Instead of illuminating the device through the electrode, as is the conventional approach, a new vertical configuration is employed with light coming between the two electrodes. This way, the light absorption and hence electron generation is spread through the depth of the device. As a result, unreasonably fast electrode kinetics are no longer required. The depth can be adjusted according to the concentration of the dyes, and thus deeper cells enable low solubility dyes to be employed. The proposed configuration is mathematically modeled and the advantages over the conventional cell are shown. A numerical model is built for more detailed analysis that gives practical guidelines for working towards device parameters with high power conversion efficiency. The analysis suggests that upon the realization of highly selective electrodes and an improved dye/mediator couple, an efficiency of 13% should be achievable from the new configuration., 23 pages, 5 Figures, 4 Tables

A microfabrication technology capable of electrodepositing truly three-dimensional metal structures is introduced. Micrometer-scale nickel structures including a multicoiled helical spring have been fabricated. Electrodeposition is localized by placing a sharp-tipped electrode in a plating solution, near a substrate, and applying a voltage. Structures are built by moving the electrode appropriately with respect to the substrate. Vertical deposition rates of 6 /spl mu/m/s are observed, two orders of magnitude greater than those of conventional electrodeposition. The theory of mass transport to a region of localized field is discussed, and a model of deposition profile is presented. The process can potentially produce submicrometer feature sizes using a range of materials including pure metals, alloys, and polymers.

Finding low cost methods of storing renewable power is a key obstacle facing wider use of solar and wind technologies. Creation of combined solar cells and batteries or supercapacitors has recently received attention, but thus far energy densities have been low. In this work the highest volumetric and areal energy density for an integrated solar energy harvesting and storage device are demonstrated in a scalable solar rechargeable redox flow battery. The integrated solar power rechargeable battery shows higher energy storage yields compared to having solar cells coupled to separate batteries due to fewer energy conversion steps. The device promises to enable cost and space efficiencies and reduce the complexity of solar energy harvesting systems. The solar-battery is designed and demonstrated based on two known technologies: the redox flow battery; and the dye-sensitized solar cell. The redox flow design enables independent scaling of power and energy rating of the system and thus makes the device potentially applicable for large scale energy storage purposes, unlike non-flow energy storage solutions [1]. Being an electrochemical cell and having structures similar to batteries with photoconversion efficiencies more than 13%, dye sensitized solar cells are good candidates to be integrated in a solar battery. A porous dye-sensitized TiO2 electrode is inserted into one half cell of a redox flow battery structure, resulting in a three electrode device as is depicted in Figure 1. The cell is charged by connecting the sensitized electrode to the battery's charge extraction electrode in the other half cell, causing the oxidation of ions on the sensitized electrode and reduction in the second half cell. A charge balancing ion neutralizes the charge across the two half cells by transporting through the membrane. The stored energy can later be extracted by connecting a load between the two non-sensitized electrodes. Iodide/triiodide and tetrasulfide/disulfide are chosen as the two charge storage redox couples in the battery. During discharge, disulfide is oxidized to tetrasulfide in one half cell and triiodide is reduced to iodide in the illuminated half cell. The selected redox couples have standard reduction potentials close to the common N719 dye HOMO level and the TiO2conduction band respectively and therefore can efficiently transfer electrons with the electrodes. They are highly soluble in the electrolytes, with this high concentration enabling high energy density. Both redox species are negatively charged which enables the utilization of a cationic exchange membrane and minimal self discharge. Nickel foam electrodes are treated with polysulfide to form a highly catalytic high surface area electrode for polysulfide. Photoconversion and photocharge-discharge characteristics of the solar battery show an areal energy density of 180 µWh/cm2, a volumetric energy density of 1.42 Wh/L, a charge capacity of 3.2 Ah/L, a round trip energy storage efficiency of 78% and almost perfect Coulombic efficiency. These values represent a 100 times increase in charge capacity and a 30 times improvement in areal energy density compared to the previous work on solar redox batteries [2], and unlike previous work shows nearly constant voltage during discharge, as expected for a battery. In a broader comparison to all integrated solar energy harvesting and storage devices, this work shows the highest areal energy density and charge capacity. Given an expected improvement in solar power conversion efficiency from the current 2 %, this approach has the prospect of offering a low cost method of both capturing and storing solar energy. References: [1] T. N. Murakami et al. Chem. Commun., 2005, 3346-3348 [2] N. F. Yan et al. J. Electrochem. Soc., 2014, A736–A741 Figure 1

Reaction centers (RCs) from natural photosynthetic cells are photoactive proteins, which generate electron-hole pairs in presence of light. In a new approach presented in this work, a solution of suspended RCs with mediators has been applied as the electrolyte to build electrochemical based photovoltaic (PV) devices. In this approach, the mediators transfer charges from the RCs to the electrodes (indirect charge transfer). Various metallic and wide bandgap semiconducting materials, including Carbon, Au, Indium Tin Oxide (ITO), SnO2, WO3, have been tested as the electrodes. Among all WO3, which is a semiconductor, have shown the largest photocurrent density with an amount of ∼5.1 μA/cm2. The results show that the material of the electrode can affect the rates of the reactions in the cell. Choosing an appropriate material for the electrode, the charge transfer from the mediators to the electrode would be rectified to achieve a large photocurrent.

The high quantum efficiency (~100%) in the bacterial photosynthetic reaction center (RC) has inspired research on the application of RCs to build protein based solar cells. Conventionally, applying RCs as the photosensitive layer on the surface of a carbon electrode has shown poor photocurrents in the cells. The low photocurrent is partly due to the weak absorption of light in the monolayer of RCs. Also, an Atomic Force Microscopy image of the electrode shows lots of defects on the immobilized RCs at the electrode surface. In this work, we have built a bio-photoelectrochemical cell in which the RCs are floating in the electrolyte instead of being attached to the surface of an electrode. Despite the simple structure of the cell, the photocurrent is significantly higher in the new cell compared to when RCs are attached to an electrode. The amplitude of current reached to ~40 nA for free floating RCs, about five times larger than that in the cell with attached RCs. The aging effect was studied in both cells in a course of a week. The lifetime of attached RCs on electrode surface was slightly better than solubilized RCs in the electrolyte. Also, it is found that the mechanism which governs the charge transfer from RCs to the electrodes is the same in both bio-photoelectrochemical cells.

In this paper conducting polymer based active catheters are presented. Design considerations along with the promise and challenges associated with conducting polymer driven devices are discussed. A conducting polymer driven intravascular catheter is described briefly and its design challenges such as structural rigidity and angle of bending are studied. Then a detailed description of a polypyrrole based active catheter that is ultimately intended for in-vivo imaging applications will be presented. The active catheter contains an optical fibre and is designed to scan the fibre in two dimensions at a speed of 30 Hz to provide real time imaging. The preliminary design was realized by fabricating polypyrrole actuators on a commercially available catheter and patterning the polymer using laser machining technique. The initial device was tested at lower speeds and an image was taken using optical coherence tomography (OCT). The primary challenge to achieving an effective polypyrrole driven catheter for real time imaging is to demonstrate high speed actuation with reasonable liftetime. According to our model, electrochemical characteristics of the conducting polymer such as electronic conductivity, ionic conductivity and electrochemical strain need to be improved to achieve the desired catheter scanning speed.

Imitating nature's mechanisms offers enormous potential for the improvement of our lives and the tools we use. This field of the study and imitation of, and inspiration from, nature's methods, designs and processes is known as biomimetics. Artificial muscles, i.e. electroactive polymers (EAPs), are one of the emerging technologies enabling biomimetics. Polymers that can be stimulated to change shape or size have been known for many years. The activation mechanisms of such polymers include electrical, chemical, pneumatic, optical and magnetic. Electrical excitation is one of the most attractive stimulators able to produce elastic deformation in polymers. The convenience and practicality of electrical stimulation and the continual improvement in capabilities make EAP materials some of the most attractive among activatable polymers (Bar-Cohen Y (ed) 2004 Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges 2nd edn, vol PM136 (Bellingham, WA: SPIE Press) pp 1–765). As polymers, EAP materials offer many appealing characteristics that include low weight, fracture tolerance and pliability. Furthermore, they can be configured into almost any conceivable shape and their properties can be tailored to suit a broad range of requirements. These capabilities and the significant change of shape or size under electrical stimulation while being able to endure many cycles of actuation are inspiring many potential possibilities for EAP materials among engineers and scientists in many different disciplines. Practitioners in biomimetics are particularly excited about these materials since they can be used to mimic the movements of animals and insects. Potentially, mechanisms actuated by EAPs will enable engineers to create devices previously imaginable only in science fiction. For many years EAP materials received relatively little attention due to their poor actuation capability and the small number of available materials. In the last fifteen years, a series of new materials have emerged that exhibit large displacement in response to electrical stimulation. This capability is making them highly attractive as actuators for their operational similarity to biological muscles, particularly their resilience, quiet operation, damage tolerance and ability to induce large actuation strains (stretching, contracting or bending). The application of these materials as actuators involves multi-disciplines including materials, electromechanics, chemistry, computers and electronics. Even though the force of actuation of existing EAP materials and their robustness requires further improvement, there has already been a series of reported successes in the development of EAP-actuated mechanisms. Using EAP to replace existing actuators may be a difficult challenge and therefore it is highly desirable to identify a niche application where EAP materials would not need to compete with existing technologies. EAP materials can be divided into two major groups based on their activation mechanism: ionic or electronic. Electronic EAPs, such as electrostrictive, electrostatic, piezoelectric and ferroelectric, are driven by Coulomb forces. These types of EAP material can be made to hold the induced displacement while activated under a DC voltage, allowing them to be considered for robotic applications. These materials have high mechanical energy density and they can be operated in air with no major constraints. However, electronic EAPs require high activation fields (>10 V/μm) that are close to the breakdown level. In contrast to electronic EAPs, ionic EAPs are materials that involve the transport of ions and they consist of two electrodes and an electrolyte. The activation of ionic EAPs can be achieved by voltages as low as 1–2 volts. Examples of ionic EAPs include gels, polymer–metal composites, conducting polymers and carbon nanotubes. Their disadvantages are a need to maintain wetness and their low electromechanical coupling. Turning EAP materials into actuators-of-choice requires a well established infrastructure. This involves improving the understanding of the basic principles that drive the various EAP materials. It is also necessary to develop a comprehensive material science, as well as effective electro-mechanics analytical tools and material processing techniques. Efforts are underway to study the parameters that control EAP electro-activation force and deformation and many successes have been reported. The processes of synthesizing, fabricating, electroding, shaping and handling are being refined to maximize the actuation capability and robustness of EAP materials. Methods of reliably characterizing the response of these materials are being developed and efforts are being made to establish a database with documented material properties in order to support design engineers who are considering the use of these materials. Grand challenge for the development of EAP-actuated robotics. The technology of artificial muscles is still in its emerging stages but the increased resources, growing number of investigators conducting research related to EAP, and improved collaboration among developers, users and sponsors are leading to rapid advances in this field. In 1999, in an effort to promote worldwide development towards the realization of the potential of EAP materials, Yoseph Bar-Cohen posed an arm-wrestling challenge (http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-armwrestling.htm). A graphic rendering of this challenge is illustrated in the above figure. In posing this challenge, he is seeking to see an EAP-activated robotic arm win against a human in a wrestling match in order to provide a gauge of the level of advances in the development of these materials. Success in wrestling against humans will enable capabilities that are currently considered impossible. It would allow applying EAP materials to improve many aspects of our life where some of the possibilities include effective implants and prosthetics, active clothing and realistic biologically inspired robots, as well as fabricating products with unmatched capabilities and dexterity. The first arm-wrestling match against a human (a 17 year-old female high school student) was held on 7 March 2005 as part of the EAP-in-Action session of SPIE's EAPAD conference. Three robotic arms participated in the contest and the girl won against all these arms. Subsequent contests are now focusing on measuring the performance of the robotic arms compared to the student performance that was recorded in the 2006 contest. In a future conference, once advances in developing such arms reach a sufficiently high level, a professional wrestler will be invited for the next human/machine wrestling match. This issue of the journal is dedicated to publishing recent research advances in the field of EAPs and is the first such dedicated issue ever to be published. The included papers cover the whole spectrum of elements considered critical to the development of the EAP technology infrastructure. The issue ends with a paper from the research group at EMPA describing their work on one of the first three arms that participated in the first historical arm-wrestling match. In the coming year the editors are hoping to see a significant growth in the amount of research and related publications addressing the many challenges that this field still poses.

We present our experimental investigation into 'mechanochemoelectrica' behavior of tri-layer polypyrrole (PPy) -type conducting polymer sensors. One end of the polymer strip is clamped and the other-free end-is excited through a mechanical lever system, which provides sinusoidal displacement inputs. The voltage generated and current passing between the two outer PPy layers as a result of the displacement input is measured to model the output/input behaviour of the sensors through their experimental current/displacement and voltage/displacement frequency responses. We specifically targeted the low frequency behaviour of the sensor as it is a relatively slow system. Experimental transfer function models are generated for three sensors with the dimensions of (7.5 mm times 1 mm times 0.17 mm), (10 mm times 1 mm times 0.17 mm), and (12.5 mm times 1 mm times 0.17 mm). These models are for use in understanding the dynamic behaviour and sensing ability of the polymers as mechanical sensors. The effect of the active sensor length on the voltage and current outputs are investigated that the shorter is the sensor length, the higher are the voltage output and the current passed. Also, their current and voltage responses under an impulse stimulus (i.e. displacement) are experimentally measured to show their dynamic sensing response.

The combination of polymer semiconducting 'inks' and relatively high resolution printing processes promises to make integrated printed circuits commercially viable. These circuits can be flexible and capital costs of production are very low. One of the key challenges to implementation of this technology is that most transistors made using polymer semiconductors have high operating voltages, which is not ideal for low power, low cost, battery compatible technology. We report on an organic metal semiconductor field effect transistor which operates at 3.5 V. In the initial implementation of this transistor poly(3-hexylthiophene) is the semiconductor, with an aluminium layer used as the gate electrode. The on/off current ratio of this device is only 24.6, but simulations suggest that an improvement of up to three orders of magnitude is possible. The fabrication process requires one less step than other organic transistors. It also uses the gate to encapsulate the transistor, limiting exposure oxygen and moisture, which are known to degrade their performance.

The DC characteristics of a Schottky contact between regioregular poly (3-hexylthiophene) and aluminum are studied in forward and reverse bias regimes. Current-voltage curves of the junction in reverse bias show a resistive path in parallel with the expected Schottky contact. This is the sign of a nonuniform junction between the metal and semiconductor that exhibits ohmic behavior in some regions. Reduction of this parallel resistance and degradation of the Schottky junction are observed over a period of two weeks. Accumulation of undesired ions in the polymer or diffusion of aluminum atoms into the semiconductor are two possible mechanisms which may explain the time dependent behavior of these Schottky junctions.

Conducting polymer actuators are employed to create actively shaped hydrodynamic foils. The active foils are designed to allow control over camber, much like the ailerons of an airplane wing. Control of camber promises to enable variable thrust in propellers and screws, increased maneuverability, and improved stealth. The design and fabrication of the active foils are presented, the forces are measured and operation is demonstrated both in still air and water. The foils have a "wing" span of 240 mm, and an average chord length (width) of 70 mm. The trailing 30 mm of the foil is composed of a thin polypyrrole actuator that curls chordwise to achieve variable camber. The actuator consists of two 30 μm thick sheets of hexafluorophosphate doped polypyrrole separated from each other by a gel electrolyte. A polymer layer encapsulates the entire structure. Potentials are applied between the polymer layers to induce reversible bending by approximately 35 degrees, and generating forces of 0.15 N. These forces and displacements are expected to enable operation in water at flow rates of > 1 m/s and ~ 30 m/s in air.

In robots, the power source is usually distant from the actual moving parts, which can be inefficient. One solution is to provide power locally, as in real muscles, by incorporating fuel cell technology directly into artificial muscles.

Conducting polymer devices have been demonstrated to generate a current or voltage in response to a displacement or force, which facilitates their use as mechanical sensors. Trilayer bending sensors are laminated conducting polymer devices that can provide self-contained operation in air, while maintaining many of the desirable conducting polymer properties including low weight and biocompatibility. This paper extends our previous characterization work by identifying the frequency response of the sensor output as the device geometry is varied and at frequencies up to 300 Hz. Current output is found to increase with the volume of conducting polymer across the sensor spectrum, while the usable sensor bandwidth is bounded by low-pass behaviour and a peak at the low frequency end of the spectrum and by a mechanical resonance at high frequencies. On the basis of these findings, suggestions are presented which can be used to optimize the current output and bandwidth of trilayer bender sensors.

A large low frequency inductance is found in a Schottky diode composed of regioregular poly(3-hexylthiophene) and aluminum. This apparent inductance is evident in response to both swept frequency sinusoidal, ramp and step voltage inputs above a threshold voltage. The constant slope of the current in response to a voltage step suggests an incredibly large inductance (a few hundred megahenry) in a device that is only 2000μm3 in size. A number of potential mechanisms including chemical reactions, barrier modulation, and memory effects are evaluated in order to find a suitable explanation for the inductive behavior. Similarity in the dc characteristics of the organic Schottky diode and organic bistable devices that are being applied as memory suggests that the current leads the voltage due to increments in tunneling current that occur as charges are gradually stored in localized states.