Bioartificial polymeric material: Poly(ethylene glycol) crosslinked with albumin. II: Mechanical and thermal properties

A family of hydrogels based on the crosslinking of bovine serum albumin (BSA) and activated poly(ethylene glycol) (PEG), were characterized. The swelling process was found to be dependent on the molecular weight of the PEG used and on the OH/NH2 molar ratio of the reagents. These hydrogels performed a volume expansion from 266 to 554% when PEG with molecular weights from 3,350 to 20,000 were used. The volume expansion was greater when the OH/NH2 molar ratio increased. Some anisotropy occurred during swelling since thinner devices swelled more than thicker ones, due to the difference of constraints in the hydrogels structure. Mechanical studies showed that these hydrogels were highly deformable and presented good elastic behavior since they broke only after 80% deformation. At this point, they needed a compression force ranging from 165 to 456 g/cm(2) to break. The differentiation of two kinds of water in these highly swollen hydrogels was performed using DSC studies at low temperature. Free water (i.e., bulk water) was found to be the predominant form which filled the network, and bound water (i.e., water engaged in interactions with PEG) tended to form a trihydrate complex (three molecules of water per ethylene oxide repeated unit). Three forms of water with different evaporating rates were detected by DSC studies at high temperature. This other form of water may be important for maintaining the hydrogel's form.