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

Mass transfer in rapidly photopoIymerized hydrogel networks of poly(ethylen e glycol) (PEG) was investigated to characterize these materials for potent ial biosensor applications. The rapid polymerization from a concentrated po lymer precursor solution results in a tightly cross-linked hydrogel network that potentially contains microgels, all conditions that can hinder analyt e mass transfer. We examined the mass transfer characteristics of microsphe res fabricated from diacrylated PEG (MW 575), dimethacrylated PEG (MW 1000) , or tetra-acrylated PEG (MW 18,500) mixed with trimethylolpropane triacryl ate, a triacrylated cross-linking agent, whose concentration ranged up to 2 0% (v/v). Swelling behavior was dynamically characterized starting from a d ehydrated state using a CCD-camera integrated with an inverted microscope. Hydrogel swelling was extremely rapid with gel front diffusivities on the o rder of 10(-6) cm(2)/s. Estimated hydrogel mesh sizes ranged from 8.6 to 13 .7 (A) over dot for spheres fabricated using PEG with molecular weights bet ween 575 and 1000, to 103 (A) over dot for spheres fabricated using PEG wit h a molecular weight of 18,500. Dynamic uptake of tetramethylrhodamine was followed using a fluorescence microscope to estimate small analyte diffusiv ities into the hydrogel networks. Tetramethylrhodamine diffusivities were o n the order of 10(-7)10(-9) cm(2)/s. Experimental diffusivities were used t o simulate mass transfer into the gel and thus the potential response time of biosensors based on these systems. (C) 2001 EIsevier Science Ltd. All ri ghts reserved.