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Bioderived cellulose nanocrystals (CNCs) are used to create light, flexible, biocompatible, and biodegradable electronic devices. Herein, surface modification of cellulose nanocrystals was employed to fabricate cationic and anionic CNCs. Subsequently, we demonstrated rectification behavior from a fixed junction between two agarose hydrogels doped with cationic and anionic cellulose nanocrystals. The current rectification ratio reaches 70 reproducibly, which is significantly higher than that for analogous diodes generated with microfibrillated cellulose (∼15) and the first polyelectrolyte gel diode (∼40). The current-voltage characteristics of the CNC-hydrogel diode are influenced by concentration, gel thickness, scanning frequency, and applied voltage. The high surface area of CNC resulted in high charge density after surface modification, which in turn resulted in good rectification behavior from only small amounts of dopant material.

Electronically conducting polymer (ECP) actuators and sensors are ionic EAPs, whose working principle relies on the motion of ions, usually contained in an electrolyte, toward or from the electroactive polymer. We demonstrate here that linear deformation and sensing in open-air are accessible for ionic EAPs if classical trilayer devices are made electromechanically asymmetric by tuning the properties of PEDOT:PSS electrodes according to an electromechanical model. We also present the first results on “dry” ionic conducting membranes based on polymeric ionic liquids. This “dry” electrolyte allows developing ionic EAPs without any liquid component and still presenting large and fast bending deformation

We address a challenge in the engineering of proteins to redirect electron transfer pathways, using the bacterial photosynthetic reaction centre (RC) pigment–protein complex. Direct electron transfer is shown to occur from the QA quinone of the Rhodobacter sphaeroides RC containing a truncated H protein and bound on the quinone side to a gold electrode. In previous reports of binding to the quinone side of the RC, electron transfer has relied on the use of a soluble mediator between the RC and an electrode, in part because the probability of QB quinone reduction is much greater than that of direct electron transfer through the large cytoplasmic domain of the H subunit, presenting a ~ 25 A barrier. A series of C-terminal truncations of the H subunit were created to expose the quinone region of the RC L and M proteins, and all truncated RC H mutants assembled in vivo. The 45M mutant was designed to contain only the N-terminal 45 amino acid residues of the H subunit including the membrane-spanning α-helix; the mutant RC was stable when purified using the detergent N-dodecyl-β-d-maltoside, contained a near-native ratio of bacteriochlorophylls to bacteriopheophytins, and showed a charge-separated state of $${{\text{P}}^{\text{+}}}{{\text{Q}}_{\text{A}}^-}$$ . The 45M-M229 mutant RC had a Cys residue introduced in the vicinity of the QA quinone on the newly exposed protein surface for electrode attachment, decreasing the distance between the quinone and electrode to ~ 12 A. Steady-state photocurrents of up to around 200 nA/cm2 were generated in the presence of 20 mM hydroquinone as the electron donor to the RC. This novel configuration yielded photocurrents orders of magnitude greater than previous reports of electron transfer from the quinone region of RCs bound in this orientation to an electrode.

Electronic Conducting Polymers (ECP) have been widely studied in a tri-layer configuration as soft, bending actuators. These electroactive materials have also been reported to behave as mechanical strain sensors able to convert mechanical stimulation into electrical signals. This sensing behavior is attributed to the so-called piezoionic effect and is observed and reported in most ionic electroactive polymers (EAPs). However, ambiguities remain on the origin of this effect, being attributed either to stress gradient induced ion motion or to Donnan potentials arising at the ECP/electrolyte interface. In this work, the sensor mechanism of trilayer ECP actuators is studied and discussed as a function of different physical and chemical parameters thanks to the versatile synthesis of conducting interpenetrating polymer networks. Results demonstrate that the main mechanism relies on stress gradient, as in other ionic EAPs, instead of Donnan potential. Moreover, a deep investigation of the electrolyte nature and its concentration is performed. Mobile ions deduced from actuation experiments are correlated with the sign of voltage output during sensing experiments. An interesting inversion point is demonstrated at 2.5 M using 1-ethyl-3-methylimidazolium bis-(trifluoromethyl-sulfonyl)-imide electrolyte in propylene carbonate concentration where simultaneous charge compensation (no sensing) and volume compensation (no actuation) occur for mobile cations and anions, while electrochemical behaviour remains unchanged.

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.

Metals and glass are excellent for containing electrolytes and liquids in general, but their rigid mechanics limits their application for mechanically active ionic actuators or flexible/ stretchable electrochemical devices such as batteries and supercapacitors. In this study, we evaluate the performance of spray-coated poly (styrene-block-isobutylene-block-styrene) (SIBS) as a stretchable encapsulant, which suggests that it offers a better combination of compliance and impermeability than any other barrier. We examined the drying time of 360-&micro, m thick encapsulated tri-layer conducting polymer (CP) actuators, comprised of poly(3,4-Ethylenedioxythiophene) (PEDOT) as the CP electrode and an interpenetrated polymer network of polyethylene oxide (PEO) and nitrile butadiene rubber (NBR) as the separator layer, which operates with a 1 M solution of Lithium bis(trifluoromethanesulfonyl)imide (Li+TFSI&minus, ) in propylene carbonate (PC). A 100-&micro, m thick SIBS encapsulation layer is anticipated to help these devices to retain 80% of stored PC for more than 1000 times longer compared to when there is no encapsulation (from less than 0.5 days to over 1.5 years). This low permeability combined with the low Young&rsquo, s modulus of the SIBS film, its biocompatibility, biostability, and FDA approval, as well as ease of fabrication, make this thermoplastic elastomer a promising candidate as an encapsulant for flexible ionic devices such as flexible batteries and supercapacitors, ionic-electrode capacitive sensors, and ionically electroactive actuators. This paves the way for using these devices in implantable and in vivo applications.

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.

A novel pressure sensor is proposed exhibiting generative properties from displacement-induced ionic charge separation in gel electrolytes. A mechano-ionic or ‘piezo-ionic’ effect, in analogy to the well-known piezoelectric effect, is hypothesized to originate from a difference in mobilities between cationic and anionic species causing a localized ionic charge gradient upon application of pressure. This gradient can be detected as a voltage or current by using copper electrodes placed at the sides or at regular intervals along a surface of the gel. The voltage generated may be a result of the local concentration gradient induced by the deformation of the gel or perhaps is the result of some ions moving faster through the porous gel than others. In this work, ionic polymer gels based on Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) co-polymer were synthesized in situ to incorporate an organic electrolyte consisting of bis(trifluoromethane)sulfonimide lithium salt in propylene carbonate. With two electrodes placed under the gel, the samples were subjected to a sinusoidal mechanical force while open circuit voltage was measured to determine the relationship between electrical signal and mechanical input. The voltages generated are 10’s of mV in magnitude at 1 kPa. Results suggest a maximum sensitivity of 25 μV/Pa at 10 mHz, comparable to the voltages expected in piezoelectric polymers such as PVDF (44 μV/Pa for similar dimensions). The non-aqueous, solid-state ionic gels presented in this work provide improved stability and is less prone to evaporation than its aqueous, hydrogel based counterpart. The new mechanism of sensing provides an alternative to the more rigid and less stretchable piezoelectric sensors.

Several studies have been reported on the development of controllable catheters in the biomedical field. Electronic conductive polymers (ECP) actuators appeared to be among the most suitable systems thank to their biocompatibility, low operating potential (± 2V) with a reasonable deformation (2%)[1–3]. Electroactive catheters, especially in neurosurgery, should have two levels of properties: strong deformations tip in order to reach, for example the aneurysms and sweep the total volume of the pouch, and sufficient rigid middle part for getting forward in the tortuous vessels network. We designed an electroactive catheter, constituted of two parts with different deformation ability and modulus. The high deformations tip can be obtained with a weak modulus actuator. On the other hand, the second part needs to possess high modulus where small deformations are sufficient. In this work, interpenetrating polymer networks (IPN) will be used as the structural material of the catheter. The IPN architecture allows the synthesis of actuators containing the ions necessary for the redox process and thus avoiding any interference of the position control due to the exchange with the ions from the physiological medium. In addition, the fact that the catheter can be synthesized in a customized way allows modulating its mechanical properties. By introducing a rigid polystyrene network into a specific part of the actuator, it is possible to locally increase the rigidity of the device while keeping reasonable deformation. First, we will describe the synthesis and the characterization of a beam shape actuators with different local stiffnesses. Then, the first steps for the elaboration of tubular actuator will be presented.

Electrochemical actuators operating in open-air are trilayer electrochemical devices based on an ionically conducting membrane sandwiched between two electrodes of electronic conducting polymers (ECP). Tuning functional properties of the actuator, i.e. the output force, is demonstrated via the modification of the ECP’s surrounding macromolecular architecture. Theoretical models have suggested that the output force of trilayer actuators is related to the Young’s modulus of the electrodes. As a consequence, we designed Interpenetrating Polymer Network (IPN) membranes combining three different polymer networks with a co-continuous morphology to act as a host matrix for ECP electrodes. Each of these polymer networks is chosen for a specific role: (i) poly(ethylene oxide) network providing ionic transport medium within the ECP electrodes, (ii) Nitrile Butadiene Rubber providing endurance, flexibility and robustness to the final device and (iii) polystyrene to increase the Young’s modulus of the ECP electrodes. The synthesized IPNs have been thoroughly characterized. The performances of resulting actuators have been assessed. The output force is almost doubled due to the stiffening effect of the polystyrene phase, as predicted by the beam theory.

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.

Ionic resistance is often a rate limiting factor in electrochemical systems such as batteries, supercapacitors, and ionic polymer actuators. Such cells typically contain laminated and interpenetrated structures consisting of one or two electronically conductive layers and an ionically conductive separator layer. Here we analyze the role of electrodes in determining ionic resistance, and study the effects of interpenetrating ionic and electronically conducting phases. We also show that conducting polymer electrodes that are polymerized on separator layers, as have been applied to create actuators, supercapacitors and electrochromic device, can be widely and unevenly distributed through the device. Swept sine and DC measurements are used with a four-point diffusion cell probe to characterize the phase dependent ionic impedances in 1. polypyrrole (PPy) coated polyvinylidene fluoride (PVDF) and 2. poly(ethylene dioxythiophene) (PEDOT) interpenetrated poly(ethylene oxide) (PEO) − nitrile butadiene rubber (NBR) co-polymer with various PEDOT content levels. A finite Warburg-based model is introduced to explain the frequency dependence, enabling a time constant for ion transport within membrane to be estimated. It is found that the separator layers in the composite membranes are shorted at higher frequencies (10 ∼ 100 Hz). This effect is likely due to interpenetration of the electronic phases into the bulk of the separator layers, providing a means of reducing internal resistance and increase power at short times. Finally, a non-uniform impedance distribution model is introduced to predict the effective composite ionic conductivity in terms of the ionic conductivities of each phase, and their non-uniform volume fractions. Taken together, the approaches presented provide a means of probing the influence of ionic conductivities of various phases on the rate of charging in electrochemical devices.

The rate of charging of supercapacitor and battery electrodes is often limited by the transport of ions through the active electrode material. One way to accelerate this transport is to increase the porosity of the electrode, at the cost of electrode capacity. This tradeoff is studied through the fabrication and characterization of porous carbon nanofibre/polypyrrole films used as storage electrodes. Electrospun poly (acrylonitrile-co-acrylamide) fibres were carbonized and then electrochemically coated with variable amounts of polypyrrole. The resulting hybrid materials were characterized using both conventional methods such as cyclic voltammetry, electron microscopy and conductivity, as well as ionic conductivity and pulsed-field-gradient nuclear magnetic resonance which directly probe ion transport. It is found that with modest porosity, these materials retain much of the capacitance (∼50%) of bulk polypyrrole with dramatically increased (∼300 times) charge and discharge rates, suggesting the potential of the approach for increasing the useful frequency range of polypyrrole-based supercapacitors, as well as other storage materials whose porosity can be varied. Simple transport models based on series or parallel arrangements of electrolyte and active electrode are developed to explain the results and identify the factors limiting charge/discharge rates.

Electrodes that selectively exchange charge with only certain redox couples in a mixture could improve the performance of photogalvanic and bio-photovoltaic cells described in the literature. One avenue to achieve selectivity is through the use of semiconducting rather than metallic electrodes to exploit the presence of the bandgap to control reaction rates. In this paper, Fluorine doped Tin oxide (F:SnO2), Copper(II) oxide (CuO) and Nickel oxide (NiO) electrodes are investigated as a means to achieving selective redox reactions. The reactions of methyl viologen, ferricyanide/ferrocyanide and ferric/ferrous couples on the three semiconducting electrodes were studied using cyclic voltammetry and sampled current voltammetry. The rate of electron transfer between the electrodes and the redox couples depended on the difference between the semiconductor majority carrier band edge and the standard redox potential of the redox couple. Most noticeably, the rate constant of methyl viologen on F:SnO2 was two orders of magnitude higher than that for the ferric/ferrous ion. Similar results were obtained with the NiO electrode while electrochemical instability hampered the tests of the CuO electrode.

Polypyrrole and other conducting polymers are of interest in actuators, sensors, energy storage devices and organic electronics. The patterning of these polymers can be challenging, particularly polypyrrole due to its insolubility. This paper reports the first micropatterning of polypyrrole using high-frequency pulses of extremely miniaturized electrical discharge. Microstructures with surface roughness of 70 nm are produced in the polymer with sub-micron depth control. Patterning of polypyrrole film deposited on a commercial medical catheter is demonstrated toward enabling smart catheters that use the patterned film as integrated actuators. This novel micropatterning capability opens up new possibilities for polypyrrole and likely other polymers, promoting micro-device applications for them.

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.

Ionically electroactive devices with no encapsulation dry out in air if a solvent-based electrolyte is used, and exchange ions in wet environments, both of which cause the performance of the device to vary over time. In this paper, we investigate the behavior of bare poly(3, 4-ethylenedioxythiophene) trilayer actuators both in intermittent use and continuous cycling in open air and in water, in order to understand how their response changes with time and solvent loss. Not surprisingly, the devices slow as solvent evaporates, but, unexpectedly, the active displacement increases until a large fraction of the solvent is gone. The electrolyte used in these 360 μm thick devices is a 1 M solution of Bis(trifluoromethane)sulfonimide lithium salt (Li+TFSI−) in propylene carbonate (PC). The trilayers lose all their solvent within 8 d, with ~40% loss within a day, when stored in an environment with controlled temperature and relative humidity of (23 ± 2)°C and (50 ± 3)%, respectively. The devices' speeds slow as the PC evaporates from the device (staying within 10% of their initial value after losing ~20% of its PC content). Intermittent testing shows displacement of the device actually increases until only ~14% of the PC content remains which would take almost 4 d if the device is stored in the controlled conditions mentioned above. This is largely due to the reduction of thickness in the trilayers, which then leads to higher curvature. Cycling in open air or in water leads to immediate displacement decrease: dropping 60% over one hour cycling in air and over 12 min cycling in water-due to the reduction of charge transfer rate. Overall, for applications where speed is not critical and operation is only needed for a matter of hours or days, encapsulation may not necessary. We expect that encapsulation will be beneficial to maintain the intermittent operation of the device and to maintain speed for longer periods of use (beyond 4 d and 12 h, respectively in the case studied). Encapsulation should also allow a stable displacement amplitude over time.

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.

Electrochemically driven actuation of polypyrrole in aqueous sodium hexafluorophosphate (NaPF 6 ) solution has been shown to produce repeated large strains (>6%) at low voltages and with high conductivity, making it one of the most promising electroactive conducting polymers. Little is known about the voltage dependent stiffness of this version of the polymer. This information is important in determining the strain as a function of load. In this paper the complex Young's modulus (storage and loss components) of a hexafluorophosphate-doped polypyrrole film in aqueous NaPF 6 electrolyte at different oxidation states, under various loads and as a function of the frequency of the applied load, is investigated. Uniformity of doping was ensured by allowing enough time to reach steady state charge levels, and the creep during measurements was minimized by using preconditioning cycles. The results of this study show that storage modulus decreases (from 1 GPa to 0.80 GPa) as the polypyrrole oxidation potential increases (from −0.4 V to +0.4 V versus Ag/AgCl reference electrode). The loss modulus, on the other hand, increases from 55 MPa to 80 MPa. An increasing trend in the Young's modulus is also observed with the applied load. The storage modulus increases from 0.65 GPa to 1 GPa by increasing the applied load from 0.2 MPa to 2.5 MPa. The modulus is found to increase with time through the experiment, which may be due to stretch alignment of the polymer. It is also observed that complex Young's modulus increases in proportion to the logarithm of frequency.

A metallic conductor layer on an elastomeric substrate has a limited stretchability. Application of a conductive rubber layer underneath the metal can enhance the flexibility and preserve the electrical connection even at high strains. Silicone and polydimethyl siloxane (PDMS) have been tested as elastic substrates. A repeatable resistance-strain behavior up to 100% strain is achieved for a multilayer conductor with PDMS substrate. It is found that this type of substrate has a large influence on the size of cracks on the metal layer.

A time-dependent electrochemical and mechanical model of the bending of a conducting polymer actuator-driven structure is presented, and the predicted response is compared with experimental results. The model uses time constants obtained from a transmission line model to describe the electronic and ionic charge transport into the polymer actuator. It then relates the charge transport to the mechanical deflection of the actuator structure. The model is used to predict the time-dependent bending of an active catheter, which is ultimately intended for use in intravascular imaging. A commercial catheter is coated with polypyrrole and laser micromachined into electrodes, which are electrochemically activated, leading to bending of the catheter. The time-dependent bending is compared with the dynamic beam bending model, with measured physical properties including elastic moduli, strain to charge ratio, electronic conductivity, ionic conductivity and capacitance used in the model to successfully describe the dynamics. The results of this comparison are used to determine the primary factors limiting catheter bending speed, and to suggest an optimal polypyrrole geometry and electrochemical parameters in order to achieve faster response. Copyright © 2010 Society of Chemical Industry

Minimally invasive surgical tools using catheter based technology plays an important role in biomedical diagnostics and treatments. Much research has focused on producing an actively controllable tip catheter to improve accuracy and efficiency over traditional passive catheters when navigating inside patients. In this work, we describe the design, fabrication and characterization of a novel laser-micromachined polypyrrole (PPy) based active catheter. Two-dimensional controlled bending motion of a four-electrode catheter is demonstrated. Combining such catheter with optical coherence tomography, which can provide subsurface visualization of biological tissue, imaging capability using the active catheter tip is also demonstrated.

The performance of conducting polymers as actuators is determined by a complicated series of molecular processes that occur as a result of oxidation and reduction of the polymer. In particular, the amount of actuation strain generated for a given voltage stimulus is determined by the number and type of ions (and solvent) that enter and leave the polymer and changes in the mechanical properties (particularly elastic modulus) of the polymer. In this paper, we present the effects of cyclic voltammetry on the shear modulus and volume of polypyrrole in two different electrolytes: propylene carbonate and an ionic liquid. An electrochemical quartz crystal microbalance has been used to study simultaneous volume and modulus changes occurring during redox cycling of polypyrrole. The results demonstrate that the modulus generally increases due to oxidation of the polymer, although initial oxidation from the fully reduced state first produces a decrease in modulus followed by a larger increase. The modulus shift and volume changes were smaller in the ionic liquid electrolyte, probably because of the absence of solvent. Comparison of the results obtained in the two electrolytes suggest that interchain interactions dominate in the determination of modulus, so that modulus is higher in the oxidised state even when the polymer is swollen with counterions and solvent.

The phenomenon of voltage generated from a soft sensor using polypyrrole in response to mechanical deformation is described and investigated. The sensor consists of two polypyrrole layers in contact with an electrolyte and operates in bending mode in air. The magnitude and sign of the induced voltage was found to depend on the type of dopant counter-ions and the nature of the surrounding electrolyte. The mechanical sensor response is shown to be a “reverse actuation”, generating millivolt signals for millimeter sized deflections or ∼ 1000 C m–3 charge for 1 % strain in the polypyrrole layer. A model based on ‘Deformation Induced Ion Flux' has been proposed whereby the strain induced volume change in the polymer produces a shift in the Donnan equilibrium between mobile dopant ions inside the polymer and in the external electrolyte. A simple thermodynamic model provides reasonable estimates of the size of the voltage and charge produced.

A variety of NMR techniques are employed to examine the abundance, dynamics, and translational diffusion of PF 6 − ions in doped polypyrrole films in various oxidation states. NMR is employed as a non-invasive alternative to elemental analysis for quantifying ion content, which is found to decrease linearly with decreasing electrochemical potential, directly confirming the ion intercalation mechanism of polymer actuation, and demonstrating the capacitive nature of the polypyrrole film/electrolyte system. With known ion content, sample mass, and deposition current, the relative amounts of pyrrole, ions, and solvent in the films can be determined. A T 1 relaxation study along with 1D nuclear Overhauser effect (NOE) difference experiments reveal that the rotational correlation time and solvent accessibility of PF 6 − ions in the oxidized films are similar to those in the solvent, indicating the ions experience a solvated environment, and do not sit at stable sites in the polymer matrix. A drastic decrease of the NOE enhancement, along with changes in relaxation behavior and electrical conductivity in reduced films implies that polypyrrole undergoes a significant structural change when reduced. This change leads to a much less solvated ion environment, and may be responsible for the expansion of films sometimes observed upon reduction. Translational motion of the PF 6 − ions in the oxidized films is probed via diffusion measurements made using pulsed-field gradient NMR. The diffusion coefficients found ( ∼ 5 × 1 0 − 9 cm2/s) are an order of magnitude smaller than those typically extracted from impedance spectroscopy measurements; this is explained in terms of field driven ion migration in the impedance spectroscopy case.

Conducting polymer actuators are of interest due to their low voltage operation, and their relatively high strains and forces. However, information is incomplete regarding the appropriate operating loads, the extent of creep and cycle life. We report cycle life and creep response in polypyrrole actuators operated in propylene carbonate. Polypyrrole films are found to extend passively by 2% after 100 min at 20 MPa, including about 1% elastic elongation. Results of creep tests at stresses of up to 60 MPa are presented, showing a non-linear and history dependent response at very high loads. The magnitude of the creep suggests that in situations where position control is desired under varying loads and at times of longer than tens of minutes that the polymer is best operated at loads of

The electronic, structural, and chemical properties of polypyrrole doped with the hexafluorophosphate ion are investigated as a function of oxidation state. These properties are found to be highly correlated; specifically, they all experience a rapid transition at approximately −0.1 V versus SCE. The electronic conductivity of the film changes by two orders of magnitude through the transition potential, in the well-known doping-induced metal–insulator transition. Also at −0.1 V versus SCE, X-ray diffraction and macroscopic actuation measurements reveal a structural change in the polymer. Results suggest a loss of pi-stacking in the polymer crystals, and a reordering of the dopant ions in the matrix. The dopants appear to exist in an isotropic liquid-like state in the highly oxidized film, changing to a channel structure at the transition point. This change in structure is also consistent with the transition observed in the electrochemical impedance spectrum. The equilibrium charge on the polymer with oxidation state is consistent with a constant DC capacitance, however the AC impedance spectrum shows increased constant phase element behavior in the reduced state. This could be due to a greater range of diffusion constants in the reduced state, which we associate with the heterogeneous ion configuration.

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.

The increasing understanding of the advantages offered by fish and insect-like locomotion is creating a demand for muscle-like materials capable of mimicking nature's mechanisms. Actuator materials that employ voltage, field, light, or temperature driven dimensional changes to produce forces and displacements are suggesting new approaches to propulsion and maneuverability. Fundamental properties of these new materials are presented, and examples of potential undersea applications are examined in order to assist those involved in device design and in actuator research to evaluate the current status and the developing potential of these artificial muscle technologies. Technologies described are based on newly explored materials developed over the past decade, and also on older materials whose properties are not widely known. The materials are dielectric elastomers, ferroelectric polymers, liquid crystal elastomers, thermal and ferroelectric shape memory alloys, ionic polymer/metal composites, conducting polymers, and carbon nanotubes. Relative merits and challenges associated with the artificial muscle technologies are elucidated in two case studies. A summary table provides a quick guide to all technologies that are discussed.

Conducting polymer-based actuators are capable of producing at least 10 times more force for a given cross-sectional area active . stress than skeletal muscle, and potentially 1000 times more, with strains typically between 1% and 10%. Low operating voltages make them particularly attractive for use in micro-electromechanical systems, in place of electrostatic and piezoelectric actuators. A drawback of conducting polymer actuators is their relatively slow speed, and hence low power-to-mass ratio. In this paper, shaped voltage pulses are applied to generate strain rates of up to 3% s y1 , with peak power to mass ratios of 39 W kg y1 of polymer, nearly matching mammalian skeletal muscle. Results are obtained from polypyrrole linear and bilayer actuators and employ both liquid and gel electrolytes. q 2000 Elsevier Science S.A. All rights reserved.

Conducting polymer-based actuators undergo volumetric changes as they are oxidized or reduced, from which mechanical work can be obtained. Polypyrrole [Q. Pei, O. Inganas, Advanced Materials 4 (1992) 277; Q. Pei, O. Inganas, Journal of Physical Chemistry 96 (1992) 10508; E. Smela, O. Inganas, Q. Pei, I. Lundstrom, Advanced Materials 5 (1993) 630; E. Smela, O. Inganas, I. Lundstrom, Science 268 (1995) 1735; J.D. Madden, S.R. Lafontaine, I.W. Hunter, Proceedings – Micro Machine and Human Science 95, Nagoya, Japan, October 1995] and polyaniline-based actuators have attracted recent interest because of the high forces per cross-sectional area (stress) and relatively large strains generated at low activation voltages; however, with the notable exception of some bilayers, the operation of these actuators has largely been constrained to bulk liquid environments. Operation out of solution is clearly desirable for many potential applications. Linear actuators that contract in length like muscle fibers fully exploit the high forces produced by conducting polymers. Results presented here demonstrate the operation in air of polypyrrole linear actuators. These actuators are capable of generating stresses exceeding those of mammalian skeletal muscle.

Polypyrrole-based actuators are of interest due to their biocompatibility, low operation voltage and relatively high strain and force. Modeling and simulation are very important to predict the behaviour of each actuator. To develop an accurate model, we need to know the electro-chemo-mechanical specifications of the Polypyrrole. In this paper, the non-linear time-variant model of Polypyrrole film is derived and proposed using a combination of an RC transmission line model and a state space representation. The model incorporates the potential dependent ionic conductivity. A function of ionic conductivity of Polypyrrole vs. local charge is proposed and implemented in the non-linear model. Matching of the measured and simulated electrical response suggests that ionic conductivity of Polypyrrole decreases significantly at negative potential vs. silver/silver chloride and leads to reduced current in the cyclic voltammetry (CV) tests. The next stage is to relate the distributed charging of the polymer to actuation via the strain to charge ratio. Further work is also needed to identify ionic and electronic conductivities as well as capacitance as a function of oxidation state so that a fully predictive model can be created.

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

The effect of cycling on charge-storage, actuation and sensing behavior of a polypyrrole is studied, having its application for an electroactive catheter in mind. It is shown that the electrochemical capacitance of a polypyrrole film decreases by about 15 % over the course of 100 cycles, while the per cycle rate of this decrease drops by 75 % between the first and the last ten cycles, implying that a steady-state value may exist. The decrease in capacitance is shown to have a significant effect on actuation strain. In order to achieve a more constant capacitance and more robust actuation performance, it is proposed to pre-cycle the potential of the film to exhaust the effect of processes that contribute to the decrease in capacitance and allow it to reach a more constant value. The ability of a polypyrrole film to generate currents corresponding to applied external load during actuation is verified and the cycle life time of such a sensor is studied. It is shown that after an initial decrease, the sensor current reaches a steady-state value as well, and maintains that value at least over 5600 cycles. Copyright 2009 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.

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.

An active catheter intended for controllable intravascular maneuvers is presented and initial experimental results are shown. A commercial catheter is coated with polypyrrole and laser micromachined into electrodes, which are electrochemically activated, leading to bending of the catheter. The catheter's electro-chemo-mechanical properties are theoretically modeled to design the first prototype device, and used to predict an optimal polypyrrole thickness for the desired degree of bending within approximately 30 seconds. We compared the experimental result of catheter bending to the theoretical model with estimated electrochemical strain, showing reasonable agreement. Finally, we used the model to design an encapsulated catheter with polypyrrole actuation for improved intravascular compatibility and performance.

Actuation of polypyrrole in aqueous sodium hexafluorophosphate solution has been shown to produce relatively large strains. However little has been published on appropriate potential range of actuation in this electrolyte. This information is clearly crucial for applications. Our particular interest is in disposable applications where a relatively small number of cycles are needed, and maximum strain is desired. The electrochemical degradation as a function of voltage is investigated by cycling the film between fixed voltages and measuring the charge transfer. The experiment was done on a glassy carbon substrate in order to reduce effects of change in resistance with oxidation state, preventing actuation. The dependence of charging on voltage and the rate of reduction in the extent of charging are measured. The voltage range for effective operation of the device was found to be -0.4 V to 0.8 V versus a Ag/AgCl reference electrode in order to achieve stable performance over at least 30 minutes. The mechanisms of degradation at potentials beyond 0.8 V appear to be the substitution of hydroxyl ions in the polymer backbone, as suggested in reports on degradation of polypyrrole in other electrolytes. An observed reduction in charge transfer rate at potentials lower than -0.4 V is consistent with a reduction in ionic conductivity at highly reduced states, as has also been suggested in the literature. Copyright 2007 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.

A novel anti-fouling mechanism based on the combined effects of electric field and shear stress is reported. A lead zirconate titanate (PZT) composite is used to generate an electric field and an acoustic streaming shear stress that increase nanomolecule desorption. In vitro characterization showed that (1) 58 ± 5.5% and 39 ± 5.2% of adsorbed bovine serum albumin (BSA) proteins can be effectively removed from fired silver and titanium coated PZT plate, respectively; and (2) 43 ± 9.7% of the anti-mouse immunoglobulin G (IgG) can be effectively removed from a fired silver coated PZT plate. Theoretical calculations on protein-surface interactions (van der Waals (VDW), electrostatic, and hydrophobic) and shear stress describe the mechanism for protein desorption from model surfaces. We have shown that the applied electric potential is the major contributor in reducing the adhesive force between protein and surface, and the desorbed protein is taken away by acoustic streaming shear stress. We strongly believe that the present method offers the possibility of minimizing nanomolecule adsorption without further surface treatment.

Device designers are continually confronted with the challenge of selecting the best actuator for a task and developers of new actuators are seeking applications for which their technologies are suitable. A web-based interface is presented that enables designers to input basic needs (force, displacement, frequency, cycle life, dimensions, voltage and power available) and retrieve an initial evaluation of the suitability of the various actuator technologies in the database. The prototype contains data for a number of emerging technologies including conducting polymers, dielectric elastomers, ferroelectric polymers, thermal and magnetic shape memory alloys, carbon nanotube actuators, liquid crystal elastomers, ionic polymer metal composites and mammalian skeletal muscle. The system is very early in the development process, and it is hoped that feedback from the EAP community will help guide the growth of and establish the need for this tool.

Polypyrrole actuators offer attractive possibilities due to their large electrochemical stress (>5MPa), moderate to large strain (>2 %) and low voltage operation (6(propylene carbonate) underwent a 4.8% irrecoverable contraction during the first cycle and lost half it’s actuation each cycle. Polypyrrole in NaPF6(aq) showed a 2.1% initial expansion in length on the first cycle followed by a 2.1% contraction on the next cycle, while active strain amplitude dropped from 7.7% to 5.9% to 4.9%. Polypyrrole in NaCl(aq) has a net contraction of 6% over two cycles with no significant change in it’s original 3.5% actuation amplitude. This suggests that although films in NaPF6(aq) have the best initial strain, films in NaCl(aq) maintain the most consistent strain amplitude in response to temperature on the timescales observed. Strain to charge ratio was found to decrease slightly, but the majority of the loss in actuation for films is correlated with a reduction in charge transfer each cycle. This reduction may result from a reduction in the active volume of film.

Polypyrrole exhibits actuation under electrochemical doping and undoping of the polymer matrix. Active strain and x-ray diffraction measurements are performed in order to correlate the microscopic and macroscopic structural changes occurring during actuation. Under galvanostatic reduction, the film shows first a linear contraction with oxidation state due to expulsion of small anions. At 50% of the as-grown doping level, the film reverses direction and begins to expand, possibly due to incorporation of the larger cations. X-ray diffraction, however, does not show a linear contraction or expansion in the crystalline portions of the polymer. The polymer crystallites go through a sudden contraction in one dimension of approximately 14%, and otherwise remain constant. The contraction in the polypyrrole crystals occurs at the 50% doping level, and may be correlated with the reversal of macroscopic actuation.

A typical limitation of polypyrrole based conducting polymer actuators is the low achievable active linear strains (2 % recoverable at 10 MPa, 7 % max) that these active materials exhibit when activated in a common propylene carbonate / tetraethylammonium hexafluorophosphate electrolyte. Mammalian skeletal muscle, on the other hand, exhibits large recoverable linear strains on the order of 20%. Such large linear strains are desirable for applications in life-like robotics, artificial prostheses or medical devices. We report herein the measurement of recoverable linear strains in excess of 14 % at 2.5 MPa (20 % max) for polypyrrole activated in the 1-butyl-3-methyl imidazolium tetrafluoroborate liquid salt electrolyte. This advancement in conducting polymer actuator technology will impact many engineering fields, where a lightweight, large displacement actuator is needed. Benefits and trade offs of utilizing ionic liquid electrolytes for higher performance polypyrrole actuation are discussed. Copyright 2004 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.

A new class of molecular actuators where bulk actuation mechanisms such as ion intercalation are enhanced by controllable single molecule conformational rearrangements offers great promise to exhibit large active strains at moderate stresses. Initial activation of poly(quarterthiophene) based molecular muscles, for example, show active strains in the order of 20%. Molecular rearrangements in these conjugated polymers are believed to be driven by the formation of pi-dimers (e.g. the tendency of pi orbitals to align due to Pauli’s exclusion principle) upon oxidation of the material creating thermodynamically stable molecular aggregates. Such thiophene based polymers, however, suffer from being brittle and difficult to handle. Polymer composites of the active polymer with a sulfated polymeric anion were therefore created and studied to increase the mechanical robustness of the films. This additional polyelectrolyte is a Sulfated Poly-Beta-Hydroxy Ether (S-PHE) designed to form a supporting elastic matrix for the new contractile compounds. Co-deposition of the polyanion with the conducting polymer material provides an elastic mechanical support to the relatively stiff conjugated polymer molecules, thus reducing film brittleness. The active properties of such poly(quarterthiophene)/S-PHE polymer actuator composites based on intrinsic molecular contractile units are presented and discussed.

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.

Freestanding films of poly(3,4-ethylenedioxythiopene), PEDOT, were synthesized electrochemically from a solution containing EDOT monomer, tetrabutylammonium hexafluorophosphate, and water in propylene carbonate. The films were tested mechanically under constant stresses ranging from 0.6 to 2.1 MPa and subjected to various electrochemical waveforms while immersed in a bath containing propylene carbonate and an electrolyte. The characterization resulted in observations of ultimate linear strains of 2%, strain rates of 0.003 Hz, and strain to charge densities of 4 x 10 -10 m 3 /C, comparable to the conventional conducting polymer polypyrrole. In addition to the quantitative analysis, evidence of both anionic and cationic intercalation into the polymer is presented with a discussion of prospective mechanisms and consequences. Keywords: poly(3,4-ethylenedioxythiophene), PEDOT, PEDT, actuator, artificial muscle 1. INTRODUCTION Conventional electroactive polymer actuators exploit volume changes driven by the flux of ions in and out of the bulk material. These ions are forced either chemically or electrochemically and can be used to perform work against an applied load. To date numerous polymers have shown such capability, including polyacetylene, polypyrrole and polyaniline

Electrochemical deposition has traditionally been employed to grow metal coatings (plating) and to fill molds (electroforming). However, by localizing electric field or inducing local convection on a conducting substrate it is possible to write patterns without employing masks or molds. Here we present techniques capable of depositing high aspect ratio, truly 3D structures such as columns and helical springs. Conducting polymers such as polyaniline, polyacetylene, and polypyrrole are routinely employed to fabricate batteries, capacitors, colour displays, and transistors. Here we demonstrate the electrodeposition of polypyrrole films to form bilayer actuators.

Traditional conducting polymer actuators such as polypyrrole offer tremendous active stress at low actuation voltages but with moderate strain, strain-rate and efficiency. We report the synthesis of novel thiophene based conducting polymer molecular actuators, exhibiting electrically triggered molecular conformational transitions. In this new class of materials, actuation is the result of conformational rearrangement of the polymer backbone at the molecular level and is not simply due to ion intercalation in the bulk polymer chain upon electrochemical activation. Molecular actuation mechanisms results from (pi) $min(pi) stacking of thiophene oligomers upon oxidation, producing a reversible molecular displacement which is expected to lead to surprising material properties such as electrically controllable porosity and large strains. The hypothesis of active molecular conformational changes is supported by in situ electrochemical data. Single molecule techniques are considered for molecular actuator characterization. Mechanical properties of these new materials are currently being assessed.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Conducting polymer actuators generate forces that exceed those of mammalian skeletal muscle by up to two orders of magnitude for a given cross-sectional area, require only a few volts to operate, and are low in cost. However application of conducting polymer actuators is hampered by the lack of a full description of the relationship between load, displacement, voltage and current. In an effort to provide such a model, system identification techniques are employed. Stress-strain tests are performed at constant applied potential to determine polypyrrole stiffness. The admittance transfer function of polypyrrole and the associated electrolyte is measured over the potential range in which polypyrrole is highly conductive. The admittance is well described by treating the polymer as a volumetric capacitance of 8*107 F*m3 whose charging rate is limited by the electrolyte resistance and by diffusion within polypyrrole. The relationship between strain and charge is investigated, showing that strain is directly proportional to charge via the strain to charge density ratio, (alpha) = 1*10+-10 m3*C-1, at loads of up to 4 MPa. Beyond 4 MPa the strain to charge ratio is time dependent. The admittance models, stress/strain relation and strain to charge relationship are combined to form a full description of polypyrrole electromechanical response. This description predicts that large increases in strain rate and power are obtained through miniaturization, yielding bandwidths in excess of 10 kHz. The model also enables motor designers to optimize polypyrrole actuator geometries for their applications.

Photosynthetic reaction centres (RCs) convert light into separated charges with nearly perfect quantum efficiency, and have been used to generate photocurrent. Previous work has shown that electron tunnelling rates between redox centres in proteins depend exponentially on the tunnelling distance. In this work the RC from Rhodobacter sphaeroides was genetically modified with the aim of achieving the shortest tunnelling distances yet demonstrated between the RC's electron-accepting P site and underlying graphite and gold electrodes, and between the electron donor Q site and graphite electrodes. Opposite charges are carried to counter electrodes using mobile mediators, as in dye-sensitised solar cells. Native RCs are bound to graphite surfaces through N-(1-pyrene)iodoacetamide. Although the linker's length is only 4 A, the electron transfer pathway between the Q electron donor site on the RC and the electrode surface is still too large for current to be significant. A mutant version with the electron acceptor P side close to the graphite surface produced currents of 15 nA cm−2 upon illumination. Direct binding of RCs to a gold surface is shown, resulting in currents of 5 nA cm−2. In both cases the current was unaffected by mediator concentration but increased with illumination, suggesting that direct electron transfer was achieved. The engineering of an RC to achieve direct electron transfer will help with long term efforts to demonstrate RC-based photovoltaic devices.