Volume Change in Polypyrrole Studied by Atomic Force Microscopy

Subsurface Profiling of Ion Migration and Swelling in Conducting Polymer Actuators with Modulated Electrochemical Atomic Force Microscopy

Understanding the dynamics of ion migration and volume change is crucial to studying the functionality and long-term stability of soft polymeric materials operating at liquid interfaces, but the subsurface characterization of swelling processes in these systems remains elusive. In this work, we address the issue using modulated electrochemical atomic force microscopy as a depth-sensitive technique to study electroswelling effects in the high-performance actuator material polypyrrole doped with dodecylbenzenesulfonate (Ppy:DBS). We perform multidimensional measurements combining local electroswelling and electrochemical impedance spectroscopies on microstructured Ppy:DBS actuators. We interpret charge accumulation in the polymeric matrix with a quantitative model, giving access to both the spatiotemporal dynamics of ion migration and the distribution of electroswelling in the electroactive polymer layer. The findings demonstrate a nonuniform distribution of the effective ionic volume in the Ppy:DBS layer depending on the film morphology and redox state. Our findings indicate that the highly efficient actuation performance of Ppy:DBS is caused by rearrangements of the polymer microstructure induced by charge accumulation in the soft polymeric matrix, increasing the effective ionic volume in the bulk of the electroactive film for up to two times the value measured in free water.

Chlorpromazine-Polypyrrole Drug Delivery System Tailored for Neurological Application

Nowadays, drug delivery systems (DDSs) are gaining more and more attention. Conducting polymers (CPs) are efficiently used for DDS construction as such systems can be used in therapy. In this research, a well-known CP, polypyrrole (PPy), was synthesized in the presence of the polysaccharide heparin (HEP) and chlorpromazine (CPZ) using sodium dodecyl sulfate (SDS) as electrolyte on a steel substrate. The obtained results demonstrate the successful incorporation of CPZ and HEP into the polymer matrix, with the deposited films maintaining stable electrochemical parameters across multiple doping/dedoping cycles. Surface roughness, estimated via AFM analysis, revealed a correlation with layer thickness-decreasing for thinner layers and increasing for thicker ones. Moreover, SEM images revealed a change in the morphology of PPy films when PPy is electropolymerized in the presence of CPZ and HEP, while FTIR confirmed the presence of CPZ and HEP within PPy. Due to its lower molecular mass compared to HEP, CPZ was readily integrated into the thin polymer matrix during deposition, with diffusion being unimpeded, as opposed to films with greater thickness. Finally, the resulting system exhibited the ability to release CPZ, enabling a dosing range of 10 mg to 20 mg per day, effectively covering the therapeutic concentration range.

Mapping the nanoscale elastic property modulations of polypyrrole thin films in liquid electrolyte with EC-AFM

Linking structure to mechanical and elastic properties is a major concern for the development of novel electroactive materials. This work reports on the potential-induced changes in thickness and Young modulus of a substrate supported, perchlorate doped polypyrrole thin film (<100 nm) investigated with electrochemical atomic force microscopy (AFM) under in situ conditions. This was accomplished by nanomechanical mapping of potentiodynamically electropolymerized polypyrrole film in electrolyte solution with AFM during redox cycling. The polypyrrole film thickness and Young modulus follow the electrical potential nearly linearly, increasing due to solvent and ion influx as the film is oxidized, and decreasing during reduction. Our measurements also confirm the presence of a potential-independent, passive swelling which is accompanied by softening of the film, likely caused by osmotic effects. Additionally, the heterogeneous distribution of the Young modulus can be directly traced to the typical nodular surface topography of polypyrrole, with the top of the nodular area possessing lower modulus, thus highlighting the complex relationship between topography and elastic properties.

Modeling oxidised polypyrrole in the condensed phase with a novel force field

A novel model potential is developed for simulating oxidised oligopyrroles in condensed phases. The force field is a coarse grained model that represents the pyrrole monomers as planar rigid bodies with fixed charge and dipole moment and the chlorine dopants as point atomic charges. The analytic function contains 17 adjustable parameters that are initially fitted on a database of small structures calculated within all-electron density functional theory. A subsequent potential function refinement is pursued with a battery of condensed phase isothermal-isobaric Metropolis Monte Carlo in-silico simulations at ambient conditions with the goal of implementing a hybrid parametrization protocol enabling agreement with experimentally known thermodynamic properties of oxidised polypyrrole. The condensed system is composed of oligomers containing 12 monomers with a 1:3 dopant-to-monomer concentration. The final set of force field optimised parameters yields an equilibrium density of the condensed system at ambient conditions in excellent agreement with oxidised polypyrrole samples synthesised in wet-laboratories.

Downplaying the role of water in the rheological changes of conducting polymers by using water-in-salt electrolytes

Volumetric changes associated with solvent/electrolyte exchange in electronic conducting polymers (ECPs) play an important role in the mechanical stability of the polymers, as these changes are a critical factor in ECP-based energy storage devices. Thus, the present work explores the hindering of such volumetric deformations for polypyrrole films doped with dodecylbenzenesulphonate (PPy(DBS)) by employing highly concentrated aqueous electrolytes (or water-in-salt electrolytes, WiSEs), and their effects over the corresponding electrochemical capacitor cell energy retention. Electrochemical quartz crystal microbalance with dissipation monitoring measurements for thin PPy(DBS) films in the WiSEs revealed negligible dissipation changes (ΔDn ≈ 0), in contrast with those in dilute aqueous electrolyte (ΔDn ≠ 0), indicating inexpressive structural deformation of PPy(DBS) in the WiSE. This phenomenon is observed for thick freestanding PPy(DBS) films, which presented a maximum bending angle decay from ∼56° (diluted aqueous electrolyte) to 3.5° when working in the WiSE, thus proving the hindering of film bending. The observed trends are reflected in the PPy(DBS) cell energy retention, where the use of a WiSE decreased cell energy fading by 30% after 600 cycles, in comparison with cells based on diluted electrolytes.

Probing the Electrolyte Transfer in Ultrathin Polypyrrole Films by In Situ X-ray Reflectivity and Electrochemistry

This study reports on the potential-induced charge and mass transfer between an ultrathin polypyrrole (PPy) film and an electrolyte by simultaneous in situ X-ray reflectivity (XRR) and electrochemistry (EC) utilizing their sensitivity to electrons. An about 30 nm thin PPy film was deposited on a silicon single crystal by fast potential cycling, providing a dense film of an extraordinary small surface roughness. XRR was recorded from the PPy film in an aqueous 0.1 M perchloric acid at electric potentials between -0.2 V and +0.5 V vs Ag/AgCl. The PPy film shows typical reversible and linear changes in film thickness and electron density arising from the potential-dependent electrolyte incorporation. By introducing EC-XRR, a comprehensive analysis combining in situ XRR and EC, the net number of electrons passing through the PPy-electrolyte interface was deduced along with the potential-induced thickness variations, indicating a complex exchange mechanism. Evidently, along with the anion transfer, parallel charge compensation by protons and a volume and electron compensating counterflow of solvent molecules take place. Complementary time-dependent EC-XRR scans indicate that these exchange mechanisms are individual in two potential ranges. The low actuation along with a high pseudocapacitance suggest the fast potentiodynamically deposited PPy film as a promising supercapacitor material.

A Biomimetic Approach to Increasing Soft Actuator Performance by Friction Reduction

While increasing power output is the most straight-forward solution for faster and stronger motion in technology, sports, or elsewhere, efficiency is what separates the best from the rest. In nature, where the possibilities of power increase are limited, efficiency of motion is particularly important; the same principle can be applied to the emerging biomimetic and bio-interacting technologies. In this work, by applying hints from nature, we consider possible approaches of increasing the efficiency of motion through liquid medium of bilayer ionic electroactive polymer actuations, focusing on the reduction of friction by means of surface tension and hydrophobicity. Conducting polyethylene terephthalate (PET) bilayers were chosen as the model actuator system. The actuation medium consisted of aqueous solutions containing tetramethylammonium chloride and sodium dodecylbenzenesulfonate in different ratios. The roles of ion concentrations and the surface tension are discussed. Hydrophobicity of the PET support layer was further tuned by adding a spin-coated silicone layer to it. As expected, both approaches increased the displacement-the best results having been obtained by combining both, nearly doubling the bending displacement. The simple approaches for greatly increasing actuation motion efficiency can be used in any actuator system operating in a liquid medium.

Concept of an artificial muscle design on polypyrrole nanofiber scaffolds

Here we present the synthesis and characterization of two new conducting materials having a high electro-chemo-mechanical activity for possible applications as artificial muscles or soft smart actuators in biomimetic structures. Glucose-gelatin nanofiber scaffolds (CFS) were coated with polypyrrole (PPy) first by chemical polymerization followed by electrochemical polymerization doped with dodecylbenzensulfonate (DBS^-) forming CFS-PPy/DBS films, or with trifluoromethanesulfonate (CF₃SO₃^-, TF) giving CFS-PPy/TF films. The composition, electronic and ionic conductivity of the materials were determined using different techniques. The electro-chemo-mechanical characterization of the films was carried out by cyclic voltammetry and square wave potential steps in bis(trifluoromethane)sulfonimide lithium solutions of propylene carbonate (LiTFSI-PC). Linear actuation of the CFS-PPy/DBS material exhibited 20% of strain variation with a stress of 0.14 MPa, rather similar to skeletal muscles. After 1000 cycles, the creeping effect was as low as 0,2% having a good long-term stability showing a strain variation per cycle of -1.8% (after 1000 cycles). Those material properties are excellent for future technological applications as artificial muscles, batteries, smart membranes, and so on.

Fullerene Active Layers for n-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are currently being developed for applications ranging from bioelectronics to neuromorphic computing. We show that fullerene derivatives with glycolated side chains can serve as n-type active layers for OECTs with figures of merit exceeding the best reported conjugated-polymer-based n-type OECTs. By comparing two different fullerene derivatives, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and 2-(2,3,4-tris(methoxtriglycol) phenyl) [60]fulleropyrrolidine (C60-TEG), we find that the hydrophilic glycolated side chains in C60-TEG enable volumetric doping of C60-TEG films. In contrast, the hydrophobic nature of PCBM prevents ions from penetrating into the material. Our results demonstrate that small-molecule semiconductors follow many of the same design principles established for conjugated polymers and can function as high-performing mixed electronic/ionic conductors for efficient, fast OECTs.

Hardware and Software Development for Isotonic Strain and Isometric Stress Measurements of Linear Ionic Actuators

An inseparable part of ionic actuator characterization is a set of adequate measurement devices. Due to significant limitations of available commercial systems, in-house setups are often employed. The main objective of this work was to develop a software solution for running isotonic and isometric experiments on a hardware setup consisting of a potentiostat, a linear displacement actuator, a force sensor, and a voltmeter for measuring the force signal. A set of functions, hardware drivers, and measurement automation algorithms were developed in the National Instruments LabVIEW 2015 system. The result is a software called isotonic (displacement) and isometric (force) electro-chemo-measurement software (IIECMS), that enables the user to control isotonic and isometric experiments over a single compact graphical user interface. The linear ionic actuators chosen as sample systems included different materials with different force and displacement characteristics, namely free-standing polypyrrole films doped with dodecylbenzene sulfonate (PPy/DBS) and multiwall carbon nanotube/carbide-derived carbon (MWCNT-CDC) fibers. The developed software was thoroughly tested with numerous test samples of linear ionic actuators, meaning over 200 h of experimenting time where over 90% of the time the software handled the experiment process autonomously. The uncertainty of isotonic measurements was estimated to be 0.6 µm (0.06%). With the integrated correction algorithms, samples with as low as 0 dB signal-to-noise ratio (SNR) can be adequately described.

Electrochemically Controlled Drug Release from a Conducting Polymer Hydrogel (PDMAAp/PEDOT) for Local Therapy and Bioelectronics

In this study, the release of fluorescein from a photo-crosslinked conducting polymer hydrogel made from a hydrogel precursor poly(dimethylacrylamide-co-4-methacryloyloxy benzophenone (5%)-co-4-styrenesulfonate (2.5%)) (PDMAAp) and the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is investigated. Fluorescein, here used as a model for a drug, is actively released through application of an electrical trigger signal. The detected quantity is more than six times higher in comparison to that released from a conventional PEDOT/polysterene sulfonate (PSS) system. Release profiles, drug dose, and timing can be tailored by the application of different trigger signals and pretreatments. To demonstrate that the novel drug release system can be used for a drug relevant for local delivery to a neural interface, experiments are furthermore performed with the anti-inflammatory drug dexamethasone (Dex). The conducting polymer hydrogel facilitates the active release of Dex, in comparison to the previously used PEDOT/Dex. It is suggested that PEDOT/PDMAAp is an interesting alternative for conducting polymer based drug release systems, with the potential to offer more volume for storage, yet retaining the excellent electrochemical properties known for PEDOT electrodes.

Conjugated Polymer Actuators and Devices: Progress and Opportunities

Conjugated polymers (CPs), as exemplified by polypyrrole, are intrinsically conducting polymers with potential for development as soft actuators or "artificial muscles" for numerous applications. Significant progress has been made in the understanding of these materials and the actuation mechanisms, aided by the development of physical and electrochemical models. Current research is focused on developing applications utilizing the advantages that CP actuators have (e.g., low driving potential and easy to miniaturize) over other actuating materials and on developing ways of overcoming their inherent limitations. CP actuators are available as films, filaments/yarns, and textiles, operating in liquids as well as in air, ready for use by engineers. Here, the milestones made in understanding these unique materials and their development as actuators are highlighted. The primary focus is on the recent progress, developments, applications, and future opportunities for improvement and exploitation of these materials, which possess a wealth of multifunctional properties.

Electrically Switchable Monostable Actuatoric Polymer-Based Nanovalve Arrays with a Long-Term Stability

Here we present a novel electrically switchable nanovalve array based on an intrinsic conductive polymer that has the capabilities to change its volume depending on its redox state. The polymer is created by anodic deposition of a sodium dodecylbenzenesulfonate (DBS)-doped polypyrrole (PPy). Optimization of the DBS-doped PPy layers revealed an actuatoric performance of up to 10% out of plane volume change. More interestingly, the electrochemical characterization revealed an actuatoric monostable polymer that could be used to fabricate nanovalve arrays that have a native opened state when no potential is applied and that can be closed when a reductive potential is applied. As a proof of concept, Atto488-labeled biotin (Biotin-Atto488) was used as a model compound and defined nanovalve arrays with nanopores in the range of 10 nm in diameter (opened state) were fabricated. Afterward, we were able to successfully prove the functionality of our nanovalve array by monitoring the flow-through rates of the Biotin-Atto488. More strikingly, we could demonstrate for the first time the robust and long-term stability of our nanovalve array without any performance loss for at least 72 h and retention capabilities of up to 90%. Furthermore, the demonstrated long-term stability was achieved under biocompatible conditions without the need of toxic dopant supplementation of the flow-through solution. Thus, our novel functional long-term stable nanovalve array offers the capabilities for practical applications.

pH/NIR-Responsive Polypyrrole-Functionalized Fibrous Localized Drug-Delivery Platform for Synergistic Cancer Therapy

Localized drug-delivery systems (LDDSs) are a promising approach for cancer treatment because they decrease systematic toxicity and enhance the therapeutic effect of the drugs via site-specific delivery of active compounds and possible gradual release. However, the development of LDDS with rationally controlled drug release and intelligent functionality holds great challenge. To this end, we have developed a tailorable fibrous site-specific drug-delivery platform functionalized with pH- and near-infrared (NIR)-responsive polypyrrole (PPy), with the aim of cancer treatment via a combination of photothermal ablation and chemotherapy. First, a paclitaxel (PTX)-loaded polycaprolactone (PCL) (PCL-PTX) mat was prepared by electrospinning and subsequently in situ membrane surface-functionalized with different concentrations of PPy. The obtained PPy-functionalized mats exhibited excellent photostability and heating property in response to NIR exposure. PPy-coated mats exhibited enhanced PTX release in a pH 5.5 environment compared to pH 7.4. Release was further accelerated in response to NIR under both conditions; however, superior release was observed at pH 5.5 compared to pH 7.4, indicating a dual stimuli-responsive (pH and NIR) drug-delivery platform. More importantly, the 808 nm NIR irradiation enabled markedly accelerated PTX release from PPy-coated PCL-PTX mats and slowed and sustained release following termination of laser irradiation, confirming representative stepwise drug-release properties. PPy-coated PCL-PTX mats presented significantly enhanced in vitro and in vivo anticancer efficacy under NIR irradiation compared to PPy-coated PCL-PTX mats not exposed to NIR or uncoated mats (PCL-PTX). This study has thus developed a promising fibrous site-specific drug-delivery platform with NIR- and pH-triggering that notably utilizes PPy as a dopant for synergistic photothermal chemotherapy.

Electrochemical polypyrrole formation from pyrrole 'adlayer'

In this research study, we investigated the morphology of polypyrrole nanostructures, which were formed during the electrochemical deposition of conducting polymer. An electrochemical quartz crystal microbalance (EQCM) cell equipped with a flow-through system was employed to exchange solutions of different compositions within the EQCM cell. When bare PBS buffer in the EQCM cell was exchanged with PBS buffer with pyrrole we observed a distinct increase in the resonance frequency Δf. This change in the resonance frequency and electrical capacitance, which was calculated from electrochemical impedance spectroscopy (EIS) data, illustrate that pyrrole on the surface of the gold electrode formed an adsorbed layer (adlayer). The formation of a pyrrole adlayer before the potential pulse that induced polymerization was investigated by QCM-based measurements. The electrochemical polymerization of this adlayer was induced by a single potential pulse and a nanostructured layer, which consisted of adsorbed polypyrrole (Ppy) nanoparticles with a diameter of 50 nm, was formed. QCM and EIS data revealed that by the next cycle of the electrochemical formation of Ppy, which was investigated after flow-through-based exchange of solutions, the initially formed Ppy surface was covered by the adlayer of pyrrole. This adlayer was desorbed when pyrrole was removed from the solution. When electrochemical polymerization was performed using 50 potential pulses, a Ppy layer, which had more complex morphology, was formed on the EQCM crystal. Scanning electron microscopy showed that the conductivity of this layer was unequally distributed. We observed that the polypyrrole layer formed by electrochemical deposition, which was performed using potential pulses, was formed out of aggregated spherical Ppy particles with a diameter of 50 nm.

Polypyrrole-coated electrodes show thickness-dependent stability in different conditions during 42-day follow-up in vitro

Electroconductive polypyrrole/dodecylbenzenesulphonate (PPy/DBS) has been proposed as novel electrode coating for biomedical applications. However, as yet, little is known about its long-term stability in moist conditions. This study compares the stability of PPy/DBS-coated platinum electrodes that are either dry-stored, incubated, or both incubated and electrically stimulated. The electrical and material properties of three different coating thicknesses were monitored for 42 days. Initially, the PPy/DBS-coating decreased the low frequency impedance of the platinum electrodes by 52% to 79%. The dry-stored electrodes remained stable during the follow-up, whereas the properties of all the incubated electrodes were altered in three stages with thickness-dependent duration: stabilization, stable, and degradation. The coated electrodes would be applicable for short-term, low-frequency in vitro measurements of up to 14 days without electrical stimulation, and up to 7 days with stimulation. The coating thickness is bound to other coating properties, and should therefore be selected according to the specific target application. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2202-2213, 2018.

Polypyrrole Films with Micro/Nanosphere Shapes for Electrodes of High-Performance Supercapacitors

We demonstrate a simple and efficient one-step procedure for synthesizing a solid state polypyrrole (PPy) thin film for supercapacitor applications using alternating current impedance spectroscopy. By controlling the frequency and amplitude we were able to create unique PPy nano/microstructures with a particular morphology of the loop. Our PPy micro/nanosphere shows extremely high capacitance of 568 F/g, which is close to the theoretical value of 620 F/g and 20-100% higher than that of other reported PPy electrodes. Most of all, this material presents high capacitance and significantly improved electrochemical stability without pulverization of its structure, demonstrating 77% retention of the capacitance value even after 10 000 charge/discharge cycles. These results are a consequence of the larger surface area and adequate porosity generated due to the balance between the nano/micro PPy loops. This created porous structure also allows the favored penetration of electrolyte and high ion mobility within the polymer and prevents the mechanical failure of the physical structure during volume variation associated with the insertion/deinsertion of ions upon cycling.

Electro-mechano responsive properties of gelatin methacrylate (GelMA) hydrogel on conducting polymer electrodes quantified using atomic force microscopy

Electrical stimulation of hydrogels has been performed to enable micro-actuation or controlled movement of ions and biomolecules such as in drug release applications. Hydrogels are also increasingly used as low modulus, biocompatible coatings on electrode devices and thus are exposed to the effects of electrical stimulation. As such, there is growing interest in the latter, especially on the dynamic and nanoscale physical properties of hydrogels. Here, we report on the electro-mechano properties of photocrosslinkable gelatin methacrylate (GelMA) hydrogel applied as coatings on conducting polymer polypyrrole-dodecylbenze sulfonate (PPy-DBSA) electrodes. In particular, Electrochemical-Atomic Force Microscopy (EC-AFM) was used to quantify the nanoscale actuation and dynamic changes in Young's modulus as the GelMA coating was electrically stimulated via the underlying PPy-DBSA electrode. Pulsed electrical stimulation was shown to induce dynamic expansion and contraction, or nanoscale actuation, of the GelMA hydrogel due to the reversible ingress of electrolyte ions and associated changes in osmotic pressure during oxidation and reduction of the PPy-DBSA film. In addition, dynamic changes in the Young's modulus of up to 50% were observed in the hydrogel and correlated with the actuation process and ion diffusion during oxidation and reduction of the underlying PPy-DBSA film. These dynamic properties were investigated for hydrogels with varying degrees of cross-linking, porosity and modulus, the latter ranging from ≈0.2-1 kPa. The study demonstrates an AFM-based approach to quantify the dynamic physical properties of hydrogels, which are shown to be modulated via electrical stimulation. This can enable a better understanding of the electro-mechano mechanisms that are important for the controlled release of drugs or controlling cell interactions at the hydrogel-cell interface.

In Situ Measurement of Voltage-Induced Stress in Conducting Polymers with Redox-Active Dopants

Minimization of stress-induced mechanical rupture and delamination of conducting polymer (CP) films is desirable to prevent failure of devices based on these materials. Thus, precise in situ measurement of voltage-induced stress within these films should provide insight into the cause of these failure mechanisms. The evolution of stress in films of polypyrrole (pPy), doped with indigo carmine (IC), was measured in different electrochemical environments using the multibeam optical stress sensor (MOSS) technique. The stress in these films gradually increases to a constant value during voltage cycling, revealing an initial break-in period for CP films. The nature of the ions involved in charge compensation of pPy[IC] during voltage cycling was determined from electrochemical quartz crystal microbalance (EQCM) data. The magnitude of the voltage-induced stress within pPy[IC] at neutral pH correlated with the radius of the hydrated mobile ion in the order Li(+) > Na(+) > K(+). At acidic pH, the IC dopant in pPy[IC] undergoes reversible oxidation and reduction within the range of potentials investigated, providing a secondary contribution to the observed voltage-induced stress. We report on the novel stress response of these polymers due to the presence of pH-dependent redox-active dopants and how it can affect material performance.

"Bio-switch Chip" Based on Nanostructured Conducting Polymer and Entrapped Enzyme

We report a switchable biochip strategy where enzymes were entrapped in conducting polymer layers and the enzymatic reaction of the entrapped enzymes was controlled in real-time via electrical stimuli on the polymer layers. This device is named here as a "bio-switch chip" (BSC). We fabricated BSC structures using polypyrrole (Ppy) with entrapped glucose oxidase (GOx) and demonstrated the switching of glucose oxidation reaction in real-time. We found that the introduction of a negative bias voltage on the BSC structure resulted in the enhanced glucose oxidation reaction by more than 20 times than that without a bias voltage. Moreover, because the BSC structures could be fabricated on specific regions, we could control the enzymatic reaction on specific regions. In view of the fact that enzymes enable very useful and versatile biochemical reactions, the ability to control the enzymatic reactions via conventional electrical signals could open up various applications in the area of biochips and other biochemical industries.

Mechanoelectrochemistry of PPy(DBS) from correlated characterization of electrochemical response and extensional strain

This paper investigates nanostructured morphology-dependent charge storage and coupled mechanical strain of polypyrrole membranes doped with dodecylbenzenesulfonate (PPy(DBS)). Nanoscale features introduced in PPy(DBS) using phospholipid vesicles as soft-templates create a uniform and long-range order to the polymer morphology, and lead to higher specific capacitance. It is widely stated that nanostructured architecture offer reduced mechanical loading at higher charge capacities, but metrics and methods to precisely quantify coupled localized strains do not exist. Towards this goal, we demonstrate the use of scanning electrochemical microscope with shear force imaging hardware (SECM-SF) to precisely measure charge storage function and volumetric strain simultaneously, and define two metrics--filling efficiency and chemomechanical coupling coefficient to compare nanostructured morphologies and thicknesses. For thin membranes (smaller charge densities), planar and vesicle-templated membranes have comparable mechanoelectrochemical response. For thick membranes (0.4 to 0.8 C cm(-2)), a 15% increase in charge storage is associated with 50% reduction in extensional strain. These results allow for the formulation of rules to design nanostructured PPy(DBS)-based actuators and energy storage devices.

Bioinspired Fabrication of Free-Standing Conducting Films with Hierarchical Surface Wrinkling Patterns

Mechanical instability has been shown to play an important role in the formation of wrinkle structures in biofilms, which not only can adopt instability modes as templates to regulate their 3D architectures but also can tune internal stresses to achieve stable patterns. Inspired by nature, we report a mechanical-chemical coupling method to fabricate free-standing conducting films with instability-driven hierarchical micro/nanostructured patterns. When polypyrrole (PPy) film is grown on an elastic substrate via chemical oxidation polymerization, differential growth along with in situ self-reinforcing effect induces stable wrinkle patterns with different scales of wavelengths. The self-reinforcing effect modifies the internal stresses, hence PPy films with intact wrinkles can be removed from substrates and further transferred onto target substrates for functional device fabrication. To understand the buckling mechanics, we construct a model which reveals the formation of hierarchical wrinkle patterns.

Dynamic mechanoelectrochemistry of polypyrrole membranes via shear-force tracking

Mechanoelectrochemistry is the study of elastic and plastic deformation of materials during reversible reduction and oxidation processes.

Carbide-derived carbon in polypyrrole changing the elastic modulus with a huge impact on actuation

With lowering of the elastic modulus, the CDC–POM–PPy composite shows a 20× improved strain response over PPy/DBS.

Torsional behaviors of polymer-infiltrated carbon nanotube yarn muscles studied with atomic force microscopy

Torsional behaviors of polymer-infiltrated carbon nanotube (CNT) yarn muscles have been investigated in relation to molecular architecture by using atomic force microscopy (AFM). Two polymers with different stiffnesses, polystyrene (PS) and poly(styrene-b-isoprene-b-styrene) (SIS), were uniformly infiltrated into CNT yarns for electrothermal torsional actuation. The torsional behaviors of hybrid yarn muscles are completely explained by the volume change of each polymer, based on the height and full width at half maximum profiles from the AFM morphological images. The volume expansion of the PS yarn muscle (1.7 nm of vertical change and 22 nm of horizontal change) is much larger than that of the SIS yarn muscle (0.3 nm and 11 nm change in vertical and horizontal directions) at 80 °C, normalized by their values at 25 °C. We demonstrate that their maximum rotations are consequently 29.7 deg mm(-1) for the PS-infiltrated CNT yarn muscle (relatively larger rotation) and 14.4 deg mm(-1) for the SIS-infiltrated CNT yarn muscle (smaller rotation) at 0.75 V m(-1). These hybrid yarn muscles could be applied in resonant controllers or damping magnetoelectric sensors.

Controlling the electro-mechanical performance of polypyrrole through 3- and 3,4-methyl substituted copolymers

A unique study of the impact of crosslinking on the electro-mechanical performance of electropolymerised polypyrrole films using laser-scanning micrometry.

Characterization of conjugated polymer actuation under cerebral physiological conditions

Conjugated polymer actuators have potential use in implantable neural interface devices for modulating the position of electrode sites within brain tissue or guiding insertion of neural probes along curved trajectories. The actuation of polypyrrole (PPy) doped with dodecylbenzenesulfonate (DBS) is characterized to ascertain whether it can be employed in the cerebral environment. Microfabricated bilayer beams are electrochemically cycled at either 22 or 37 °C in aqueous NaDBS or in artificial cerebrospinal fluid (aCSF). Nearly all the ions in aCSF are exchanged into the PPy-the cations Na(+) , K(+) , Mg(2+) , Ca(2+) , as well as the anion PO4 (3-) ; Cl(-) is not present. Nevertheless, deflections in aCSF are comparable to those in NaDBS and they are monotonic with oxidation level: strain increases upon reduction, with no reversal of motion despite the mixture of ionic charges and valences being exchanged. Actuation depends on temperature. Upon warming, the cyclic voltammograms show additional peaks and an increase of 70% in the consumed charge. Bending is, however, much less affected: strain increases somewhat (6%-13%) but remains monotonic, and deflections shift (up to 20%). These results show how the actuation environment must be taken into account, and demonstrate proof of concept for actuated implantable neural interfaces.

Electronic control over detachment of a self-doped water-soluble conjugated polyelectrolyte

Water-soluble conducting polymers are of interest to enable more versatile processing in aqueous media as well as to facilitate interactions with biomolecules. Here, we report a substituted poly(3,4-ethylenedioxythiophene) derivative (PEDOT-S:H) that is fully water-soluble and self-doped. When electrochemically oxidizing a PEDOT-S:H thin film, the film detaches from the underlying electrode. The oxidation of PEDOT-S:H starts with an initial phase of swelling followed by cracking before it finally disrupts into small flakes and detaches from the electrode. We investigated the detachment mechanism and found that parameters such as the size, charge, and concentration of ions in the electrolyte, the temperature, and also the pH influence the characteristics of detachment. When oxidizing PEDOT-S:H, the positively charged polymer backbone is balanced by anions from the electrolyte solution and also by the sulfonate groups on the side chains (more self-doping). From our experiments, we conclude that detachment of the PEDOT-S:H film upon oxidation occurs in part due to swelling caused by an inflow of solvated anions and associated water and in part due to chain rearrangements within the film, caused by more self-doping. We believe that PEDOT-S:H detachment can be of interest in a number of different applications, including addressed and active control of the release of materials such as biomolecules and cell cultures.

Exchanged cations and water during reactions in polypyrrole macroions from artificial muscles

The movement of the bilayer (polypyrrole-dodecylbenzenesulfonate/tape) during artificial muscle bending under flow of current square waves was studied in aqueous solutions of chloride salts. During current flow, polypyrrole redox reactions result in variations in the volumes of the films and macroscopic bending: swelling by reduction with expulsion of cations and shrinking by oxidation with the insertion of cations. The described angles follow a linear function, different in each of the studied salts, of the consumed charge: they are faradaic polymeric muscles. The linearity indicates that cations are the only exchanged ions in the studied potential range. By flow of the same specific charge in every electrolyte, different angles were described by the muscle. The charge and the angle allow the number and volume of both the exchanged cations and the water molecules (related to a reference) between the film to be determined, in addition to the electrolyte per unit of charge during the driving reaction. The attained apparent solvation numbers for the exchanged cations were: 0.8, 0.7, 0.6, 0.5, 0.5, 0.4, 0.25, and 0.0 for Na(+), Mg(2+), La(3+), Li(+), Ca(2+), K(+), Rb(+), and Cs(+), respectively.

Redox switching of heteropolyanions entrapped in polypyrrole films investigated by ac electrogravimetry

In this work, the global kinetics and the exchange of protons and free solvent at the film/electrolyte interface were investigated when a PPy film doped with HPA (heteropolyanions) is polarized. The ionic and electronic transfers for the PPy-HPA system are fast because the determined values of the resistances are relatively low (a few ohms). Depending on the applied potential, the global kinetics is controlled by both the ionic and electronic transfers. The transport effects were neglected here because the prepared films were very thin. Protons play an important role in charge compensation whatever the imposed potential on the PPy-HPA film. This phenomenon results from the acidic-basic properties of the SiMo(12)O(40)(4-) reduced species. A small quantity of water was also shown to be involved in the film reaction process.