Recently we have shown that a composite material of silk and the conducting polymer polypyrrole (PPy) has promising characteristics for use as a bending bilayer actuator. In this study, the reaction conditions were varied for the in situ incorporation of polypyrrole into silk films during pyrrole polymerization. While surface morphology and mechanical properties were minimally affected, polymerization conditions were identified where the resistivity, stability of the films during storage and stability during prolonged electrochemical cycling were dramatically improved. When fabricated into bilayer-type electromechanical actuation devices, stress and strain generation, as well as the stability during repeated actuation, was found to be superior for silk-polypyrrole composite films with improved electrical properties.

Biocompatible materials capable of controlled actuation are in high demand for use in biomedical applications such as dynamic tissue scaffolding, valves, and steerable surgical tools. Conducting polymer actuators are of interest because they operate in aqueous electrolytes at low voltages and can generate stresses similar to natural muscle. Recently, our group has demonstrated a composite material of silk and poly(pyrrole) (PPy) that is mechanically robust, made from biocompatible materials, and bends under an applied voltage when incorporated into a simple bilayer device architecture and actuated using a biologically relevant electrolyte. Here we present trilayer devices composed of two silk-PPy composite layers separated by an insulating silk layer. The trilayer architecture allows one side to expand while the other contracts, resulting in improved performance over bilayer devices. Specifically, this configuration shows a larger angle of deflection per volt applied than the analogous bilayer system, while maintaining a consistent current response throughout cycling. In addition, the overall motion of the trilayer devices is more symmetric than that of the bilayer analogs, allowing for fully reversible operation.

A B S T R A C TBiocompatible materials capable of controlled actuation under biologically relevant conditions are in high demand for use in a number of biomedical applications. Recently, we demonstrated that a composite material composed of silk biopolymer and the conducting polymer polypyrrole can bend under an applied voltage using a simple bilayer device. Here, further characterization of these bilayer actuators using time-of-flight secondary ion mass spectrometry is presented. The roles of different electrolyte components and factors affecting device performance and stability are clarified. Results of this study are discussed in the context of strategies for optimization of device performance.

There is an ongoing effort to improve the energy storage capacity of graphene-based supercapacitors. These supercapacitors store energy in the electric field between the charge carriers in the graphene and the counter ions in a liquid electrolyte. To characterize this double layer, we use the Hall effect to determine the charge carrier density in graphene as a function of voltage. We disentangle the separate roles of double-layer capacitance (the electrostatic contribution) and quantum capacitance and compare the performance of different electrolytes. Our results highlight the advantages of Hall effect measurements for probing the electrostatics of graphene–electrolyte interfaces.

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