Scanning electrochemical microscopy measurements of photopolymerized poly(ethylene glycol) hydrogels

Scanning electrochemical microscopy has been used to examine the molecular transport properties of photopolymerized poly(ethylene glycol) (PEG) hydrogels having different mesh sizes. Both the molecular weight (508 Da or 3000 Da) of the PEG diacrylate macromer and its weight percent (20 wt%, 40 wt%, or 60 wt%) in solution prior to photopolymerization were varied. Mesh size was estimated from equilibrium swelling measurements and a thermodynamic model. Estimated mesh sizes ranged from ca. 10 إ for 60 wt% PEG 508 gels to ca. 100 إ for 20 wt% PEG 3000 gels. The electrochemically active diffusing species, ferrocenemethanol, was detected via oxidation at a platinum microelectrode. For a given hydrogel, multiple approach curves showed a consistent relationship between current and distance. Electrochemically estimated diffusivities followed the same trend as predictions based on mesh size and ranged from 25% to 80% of the diffusivity in aqueous solution. As a proof of concept, scanning electrochemical microscopy was successfully used to map the topography of hydrogels with complex architecture, which are being designed as cell scaffolds.