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The partitioning and diffusion behavior of theophylline, vitamin B12, and cytochrome c in physically crosslinked networks of a genetically engineered silk–elastinlike protein-based (SELP) copolymer with an amino acid sequence of [(GVGVP)4GKGVP(GVGVP)3(GAGAGS)4]12 was investigated. The effect of gelation kinetics on the equilibrium swelling ratio and normalized dimensions of loaded SELP hydrogel disks before and after release studies was also examined. Size dependent release behavior was quantified by diffusion studies with equilibrium loaded SELP hydrogels. Direct loading diffusion studies confirmed that hydrogels produced by direct incorporation of cytochrome c with the aqueous SELP solution did not significantly influence the release behavior compared to equilibrium loaded hydrogels. An overall increase in the equilibrium swelling ratio after the release studies was observed. Analysis of the hydrogel disk dimensions after the release studies revealed no expansion of the disk dimensions. The apparent increase in the equilibrium swelling ratio was most likely due to a decrease in the hydrogel crosslinking density following the removal of the polymer soluble fraction over the course of the release study. [Copyright &y& Elsevier]