To produce conductive, biocompatible, and mechanically robust materials for use in bioelectrical applications, we have developed a new strategy to selectively incorporate poly(pyrrole) (Ppy) into constructs made from silk fibroin. Here, we demonstrate that covalent attachment of negatively charged, hydrophilic sulfonic acid groups to the silk protein can selectively promote pyrrole absorption and polymerization within the modified films to form a conductive, interpenetrating network of Ppy and silk that is incapable of delamination. To further increase the conductivity and long-term stability of the Ppy network, a variety of small molecule sulfonic acid dopants were utilized and the properties of these silk-conducting polymer composites were monitored over time. The composites were evaluated using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), optical microscopy, energy-dispersive X-ray (EDX) spectroscopy, cyclic voltammetry, a 4-point resistivity probe and mechanical testing. In addition, the performance was evaluated following exposure to several biologically relevant enzymes. Using this strategy, we were able to produce mechanically robust polymer electrodes with stable electrochemical performance and sheet resistivities on the order of 1 × 10(2) Ω/sq (conductivity ∼1 S/cm).

3866 www.MaterialsViews.com wileyonlinelibrary.com upon oxidation due to anion insertion and contract upon reduction due to anion expulsion or 2) contract on oxidation due to cation expulsion and expand on reduction due to cation insertion. Actuators based on CPs can be electrically controlled at low operating voltages (typically 1-3 V), continuously switched between expanded/ contracted states, and operate well in liquid electrolytes. Interfacing CPs with biological systems is also possible due to the demonstrated biocompatibility in vitro and in vivo . [ 5,6 ] To date, record-breaking CP actuators have been demonstrated to generate stresses as large as 100 MPa [ 7 ] and strains up to 40%, [ 8 ] although the generation of both high stress and high strain has yet to be achieved. Typical CP actuators can generate smaller, yet still notable, stresses of 1-5 MPa with strains on the order of 2%. [ 9 ] These impressive values have led to commercial interest in the development of several types of biomedical devices utilizing CP actuators. [ 1,9 ] However, current optimized device designs are not ideal for applications requiring implantation in vivo . Major diffi culties encountered when fabricating CP-based actuators arise from the fact that the bulk polymers are brittle and insoluble due to the extended conjugated backbone, which restricts the molding or processing of these materials into 3D structures. Therefore, the majority of CP-based actuators are synthesized via electropolymerization directly onto metal foils, [ 1 ] where the metal is often retained in the fi nal device. While metal incorporation helps minimize the iR drop across CP fi lms, these devices are typically limited to 2D fi lm architectures and have signifi cant problems with delamination. [ 10,11 ] While useful for surgical and external biomedical applications, incorporation of non-degradable or rigid components [ 1,12 ] that are incompatible with soft tissues severely limit the possible applications of CP actuators. In addition, device performance in a biologicallyrelevant environment is still unclear as the majority of studies utilize optimized electrolyte systems that employ toxic salts or organic solvents.To avoid the use of metals or rigid inorganic components in the fi nal device, all-polymeric actuators have been constructed by depositing CPs in situ during chemical polymerization onto several types of synthetic backing materials such as PVDF, [13][14][15] crosslinked PEO-based copolymers, [16][17][18][19][20] and polyurethane. [ 21 ] Single-component, metal-free, biocompatible, electromechanical actuator devices are fabricated using a composite material composed of silk fi broin and poly(pyrrole) (PPy). Chemical modifi cation techniques are developed to produce free-standing fi lms with a bilayer-type structure, with unmodifi ed silk on one side and an interpenetrating network (IPN) of silk and PPy on the other. The IPN formed between the silk and PPy prohibits delamination, resulting in a durable and fully biocompatible device. The electroch...

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.

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