Mechanically tunable conductive interpenetrating network hydrogels that mimic the elastic moduli of biological tissue

Abstract

Conductive and stretchable materials that match the elastic moduli of biological tissue (0.5-500 kPa) are desired for enhanced interfacial and mechanical stability. Compared with inorganic and dry polymeric conductors, hydrogels made with conducting polymers are promising soft electrode materials due to their high water content. Nevertheless, most conducting polymer-based hydrogels sacrifice electronic performance to obtain useful mechanical properties. Here we report a method that overcomes this limitation using two interpenetrating hydrogel networks, one of which is formed by the gelation of the conducting polymer PEDOT:PSS. Due to the connectivity of the PEDOT:PSS network, conductivities up to 23 S m^-1 are achieved, a record for stretchable PEDOT:PSS-based hydrogels. Meanwhile, the low concentration of PEDOT:PSS enables orthogonal control over the composite mechanical properties using a secondary polymer network. We demonstrate tunability of the elastic modulus over three biologically relevant orders of magnitude without compromising stretchability ( > 100%) or conductivity ( > 10 S m^-1).

Fig. 1

Fabrication and structure of C-IPN hydrogels. a Process for fabricating C-IPN hydrogels. First, PEDOT:PSS hydrogels are formed from aqueous solutions of PEDOT:PSS by using ionic liquid to screen the electrostatic repulsions between PEDOT:PSS microgels, enabling them to aggregate into a macroscopically connected network. Second, the PEDOT:PSS hydrogel is infiltrated with acrylic acid, bisacrylamide, and an azo-initiator. Finally, the polyacrylic acid network is formed by polymerizing the monomers in water at 70 °C. b Various large shapes made by casting the PEDOT:PSS/acrylic acid mixture into different silicone soap molds. Scale bars are 1 cm. c, d C-IPN can be micropatterned into pyramidal structures with features as small as 10 μm by casting into silicon molds. Scale bar is 200 μm c and 100 μm d. e Cross-sectional SEM image of freeze-dried C-IPN showing that the final gel is homogeneous and porous. Scale bar is 100 μm. f FTIR spectra of PEDOT:PSS, PAAc, and C-IPN, showing a clear presence of PAAc within the C-IPN composite

Fig. 2

Gelation of PEDOT:PSS. a, b PEDOT:PSS forms a gel a after mixing it with ionic liquid. The gel strength, given by storage modulus (G’), increases with increasing ionic strength b. c Mixing PEDOT:PSS with either ionic liquid or CuCl₂ will cause the solution to gel, although the gelation occurs much more rapidly with CuCl₂. With ionic liquid, the PEDOT:PSS mixture only starts to become more solid-like (G’ > G”) after 13 min. By contrast, the mixture with CuCl₂ gels so quickly that G’ already exceeds G” by the start of the rheology measurement

Fig. 3

Mechanical and strain-dependent properties of C-IPN hydrogels. a Tensile elongation curves of different C-IPN formulations, showing that all formulations can be stretched to over 100% despite large differences in the elastic moduli, which is given by the initial slope of the stress/strain curve. b Picture of a C-IPN 4 gel being stretched to 250%. c Cyclic stress/strain tensile data for C-IPN 2 exhibiting minimal hysteresis, reflecting its excellent elastic properties. d Change in resistance, expressed as a ratio between resistance (R) and initial resistance (R_o), across a C-IPN 2 gel as it is cycled reversibly between 0 and 100% strain for 10 cycles. Despite the large changes in tensile strain, the resistance stays fairly constant near its initial value. e Due to the largely strain independent conductivity of the gels, it is able to keep an LED lit even after being stretched to 50% strain

Fig. 4

Characterization of dual ionic and electronic conductivity. a Equivalent circuit model representing the bulk C-IPN hydrogel. R_e represents electronic resistance, R_i represents ionic resistance, CPE_dl represents the double-layer capacitive phase element (CPE), whereas CPE_g represents the geometric CPE. CPE elements are used to account for inhomogeneous or imperfect capacitance, and are represented by the parameters Q and α, where Q is a pseudocapacitance value and α represents its deviation from ideal capacitive behavior. The true capacitance (C) can be calculated from these parameters by the relationship C = Q ω_max^α-1, where ω_max represents the frequency at which the imaginary component reaches a maximum. R_c represents the total ohmic resistance of the cell assembly. b Nyquist plot obtained from performing electrochemical impedance spectroscopy (EIS) through a bulk C-IPN 2 gel, overlaid with the plot predicted from the equivalent circuit model. Impedance was measured between 500 mHz and 7 MHz, with higher real components of the impedance obtained at lower frequencies. c When a constant DC current of 5 mA is applied through the C-IPN 2 gel, the voltage across the gel plateaus to a value of 0.0247 V. This value can be used to calculate an electronic resistance that is comparable to the value of R_e extracted from the model. d Overlay of Nyquist plots obtained for three C-IPN formulations, where C-IPN 1 is the stiffest and densest, and C-IPN 3 is the softest and least dense. Impedance was measured between 500 mHz and 7 MHz, with higher real components of the impedance obtained at lower frequencies. e Values for all relevant parameters extracted for the three C-IPN formulations by fitting their EIS data with the equivalent circuit model. As the gel stiffness and density increase, the relative ionic resistance within the gel increases as well