Conductive Carbon Fiber-Hydrogel-Composites for Functional Tissue Engineering

Materials for tissue engineering rely on the mimicry of extracellular matrix environments, easily achievable biocompatibility and favorable mechanical properties. Hydrogels are a type of polymer that incorporate these properties and are widely used as tissue scaffolds [1]. In general, hydrogels are a non-conductive material and therefore tailoring their electrical properties to enhance conductivity opens the possibility of extending their applications to include sensing and actuation.<br/>One technique to tailor and modify the conductivity and mechanical properties of hydrogels is to embed carbon fibers in the polymer matrix. To date, carbon fiber-hydrogel-composites (CHC) with random fiber distribution within the hydrogel have been used as flexible and biocompatible sensors [2]. In addition, composites of epoxy-based polymers and unidirectional fiber bundles have been investigated for their properties under high strain and have shown piezoresistive behavior [3,4].<br/>Our work targets the application of unidirectional carbon fibers as a novel approach for tailoring the hydrogel's electrical conductivity. The aim of this research is to study the capabilities of CHCs for directed electric signal transmission along the fibers which could be a first step towards applications as bioelectrical elements, e.g. in nerve grafts. In this regard, two forms of current flow inside the CHC need to be considered: electron- and ion-based for the carbon fibers and hydrogel matrix, respectively. The initial investigation focused on studying the interplay between these two types of conductivity and how they influence the general electric signal transmission capabilities of CHCs.<br/>Therefore, cylindrical polyacrylamide hydrogels containing a bundle of polyacrylonitrile carbon fibers (~1000 filaments) in the middle were fabricated by molding and UV photopolymerization, with the fiber ends protruding from the hydrogel for electrical connection. Two different types of samples were defined: Sample group (SG) 1 features a continuous fiber bundle throughout the hydrogel and SG2 a cut fiber bundle inside the matrix with a gap of a few millimeters. This enables the investigation of the transition between electron- and ion-based current flows. Furthermore, samples from both groups were stored in different concentrations of phosphate buffered saline (PBS) solution to study the influence of the hydrogel’s ion concentration on signal transmission capabilities. Prior to the electrical measurements, the samples were immersed in PBS solutions ranging from 1x to 5x in increments of 1x for at least 24 hours. Please note that the CHC samples were always stored in PBS solution between experiments but the measurements itself were conducted in air.<br/>Predefined signals with a frequency of 1 kHz were applied at one end of the CHC sample and the voltage at the other end captured with a digital oscilloscope. The received signal was evaluated for change in amplitude and phase with respect to the input.<br/>SG1 showed no relation between ion concentration and amplitude or phase but good overall conductivity (~100 S/mm). SG2 samples showed a variation in signal phase shift as a function of increasing ion concentration.<br/>These experimental results were accompanied by the electrical properties of the CHCs being modeled in a circuit model in order to gain insight into the causes of the different transmission behavior. This is a very first step towards characterizing and ultimately utilizing the properties of CHCs for active signal transmission. Further work will focus on different fiber bundle distributions and the influence of mechanical stress on the electrical properties.<br/><br/>[1] S. Naahidi et al., Biotechnol Adv 35:530, 2017; doi:10.1016/j.biotechadv.2017.05.006<br/>[2] Y. Huang et al., ACS Appl Polym Mater 5:5707, 2023; doi:10.1021/acsapm.3c00983<br/>[3] D. D. L. Chung, Carbon 50:3342, 2012; doi:10.1016/j.carbon.2012.01.031<br/>[4] D. Wentzel et al., Int J Eng Sci 130:129, 2018; doi:10.1016/j.ijengsci.2018.05.012