A systematic study is carried out to understand how physical and covalent crosslinks affect the mechanical properties of an eight-arm poly(ethyleneglycol)-based hydrogel. Heparin and heparin-binding peptide are used as a physical crosslinker, and an enzymatically cleavable peptide with a cysteine on each end serves as a covalent crosslinker. While physical crosslinks alone do not induce gelation due to the low binding affinity between heparin and heparin-binding peptide, the addition of covalent crosslinks leads to gel formation. Strikingly, the addition of the covalent crosslinks not only leads to gel formation, but also enhances the contribution from the physical crosslinks to the overall shear moduli, which are negligible in the absence of covalent crosslinks. The gels, which contain both covalent and physical crosslinks, are able to reversibly respond to external stimuli such as temperature and oscillatory shear unlike the purely covalent gel in which the moduli remain largely insensitive to such stimuli. Two explanations are provided for this striking phenomenon. First, the addition of covalent crosslinks increased the stress relaxation time of the gel enabling the physical interactions to contribute to the moduli. This is contrasted to the case of physically crosslinked material, which relaxes the stress too quickly, preventing the physical interactions from contributing to the low frequency moduli. Second, it is believed that the physical interactions within the covalent network were further enhanced by "macromolecular confinement", which favors the formation of compact conformational structures in the confined space. Quartz crystal microbalance (QCM) was used to measure the dissociation constant (K(d)) within the hydrogel and to demonstrate that the binding between heparin and heparin-binding peptide is stronger within the gel compared to that within the solution phase. Because extracellular matrix (ECM) contains both covalent and physical interactions between its constituents, and the mechanical properties of the ECM are important factors to control cell functions, the findings of this research may have important implications in various fields of tissue engineering and cell biology.
