Ionizable amino acids in protein-based hydrogels can confer pH-responsive behavior. Because elastin-like polypeptides (ELPs) have an established sequence and can crosslink to form hydrogels, they are an ideal system for creating pH-sensitive materials. This study examines different parameters that might affect pH-sensitive behavior and characterizes the mechanical and physical properties between pH 3 and 11 of three ELP-based crosslinked hydrogels. The first finding is that varying the amount of crosslinker affects the overall stiffness and resilience of the hydrogels but does not strongly affect water content, swelling ratio, or pH sensitivity. Second, the choice of two popular tag sequences, which vary in histidine and aspartic acid content, does not have a strong effect on pH-sensitive properties. Last, selectively blocking lysine and tyrosine residues through acetylation significantly decreases the pH-sensitive zeta potential. Acetylated hydrogels also demonstrate different behavior at low pH values with reduced swelling, reduced water content, and higher stiffness. Overall, this work demonstrates that ELP hydrogels with ionizable groups are promising materials for environmentally-responsive applications such as drug delivery, tissue engineering, and microfluidics.
Keywords: elastomeric materials; environmentally responsive materials; ionizable amino acids; recombinant proteins; smart materials.
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