Mass transfer in rapidly photopolymerized poly(ethylene glycol) hydrogels used for chemical sensing

Mass transfer in rapidly photopolymerized hydrogel networks of poly(ethylene glycol) (PEG) was investigated to characterize these materials for potential biosensor applications. The rapid polymerization from a concentrated polymer precursor solution results in a tightly cross-linked hydrogel network that potentially contains microgels, all conditions that can hinder analyte mass transfer. We examined the mass transfer characteristics of microspheres fabricated from diacrylated PEG (MW 575), dimethacrylated PEG (MW 1000), or tetraacrylated PEG (MW 18,500) mixed with trimethylolpropane triacrylate, a triacrylated cross-linking agent, whose concentration ranged up to 20% (v/v). Swelling behavior was dynamically characterized starting from a dehydrated state using a CCD-camera integrated with an inverted microscope. Hydrogel swelling was extremely rapid with gel front diffusivities on the order of 10−6 cm2/s. Estimated hydrogel mesh sizes ranged from 8.6 to 13.7 Å for spheres fabricated using PEG with molecular weights between 575 and 1000, to 103 Å for spheres fabricated using PEG with a molecular weight of 18,500. Dynamic uptake of tetramethylrhodamine was followed using a fluorescence microscope to estimate small analyte diffusivities into the hydrogel networks. Tetramethylrhodamine diffusivities were on the order of 10−7–10−9 cm2/s. Experimental diffusivities were used to simulate mass transfer into the gel and thus the potential response time of biosensors based on these systems.