Impact of Elastin-like Protein Temperature Transition on PEG-ELP Hybrid Hydrogel Properties

Biomaterial hydrogels are made by cross-linking either natural materials, which exhibit inherent bioactivity but suffer from batch-to-batch variations, or synthetic materials, which have a well-defined chemical structure but usually require chemical modification to exhibit bioactivity. Recombinant engineered proteins bridge the divide between natural and synthetic materials because proteins incorporate bioactive domains within the biopolymer backbone and have a well-defined amino acid structure and sequence. Recombinant engineered elastin-like proteins (ELPs) are modeled from the native tropoelastin sequence. ELPs are composed of repeating VPGxG penta-peptide sequences, where x is any guest residue except proline. ELPs undergo a lower critical solution temperature (LCST) transition above which they aggregate into a coacervate phase in aqueous solution. Here we show that the LCST transition impacts hydrogel microarchitecture which may serve as a useful design feature in engineering ELP-based hydrogels. We investigate how the ELP LCST transition contributes to the properties of hybrid poly(ethylene glycol) (PEG) and ELP (PEG-ELP) hydrogels. PEG-ELP hydrogels gelled below the LCST have a homogeneous distribution of ELP, while gelling above the LCST results in the formation of spherical ELP-rich regions within the bulk hydrogel. The ELP-rich microarchitecture is maintained when an amine-reactive cross-linker is incorporated during the gelation process. The formation of ELP-rich regions reduces PEG-ELP hydrogel bulk stiffness and increases optical density. Our characterizations of hydrogels created by using the LCST transition provide design criteria for incorporating microscale features. This may be a useful technique in understanding the role of localized bioactivity at the microscale level within hydrogel systems.