Impedance characterization of biocompatible hydrogel suitable for biomimetic lipid membrane applications

Hydrogels, biocompatible and hydrophilic polymeric networks, have been widely applied in, e.g., pharmaceutical and biomedical research. Their physico-chemical properties can be fine-tuned by changing the fraction and molecular structure of cross-linkers. Hydrogel layers with varying thickness have also been used to support biomimetic lipid bilayers on microfabricated electrodes for studies using electrochemical impedance spectroscopy (EIS). To provide deeper understanding of the impedimetric behavior of thick hydrogels that are covalently tethered on microfabricated electrodes and the influence of cross-linking, we present here a thorough EIS characterization of poly(2-hydroxyethyl methacrylate) hydrogels cross-linked with poly(ethylene glycol)-dimethacrylate in ratios 1:100, 1:200, and 1:400. We propose an equivalent circuit model comprising an open-boundary finite-length Warburg element and constant phase element in series to describe the mass transfer differences between the bulk hydrogel and covalently tethered domain at the electrode-hydrogel interface. The results indicated that an increased fraction of the hydrophilic high-molecular weight cross-linker significantly decreased the charge transfer resistance for hexacyanoferrate(III/II), which could be attributed to increased permeability and decreased electrode passivation due to the lower degree of tethering on the acrylate modified electrodes. Cryo-SEM visualization of the structural differences caused by cross-linking showed good agreement with the EIS results, whereas the degree of hydration of the hydrogels did not show any statistically significant differences.