Effect of Divalent Metal Cations on the Conformation, Elastic Behavior, and Controlled Release of a Photocrosslinked Protein Engineered Hydrogel

We investigate the effect of Zn2+, Cu2+, and Ni2+ coordination on the conformation, mechanical properties, contraction, and small-molecule drug encapsulation and release of a photocrosslinked protein-engineered hydrogel, CEC-D. The treatment of the CEC-D hydrogel with divalent metal (M2+) results in significant conformational changes where a loss in structure is observed with Zn2+, while both Cu2+ and Ni2+ induce a blueshift. The relationship of M2+ to mechanical properties illustrates a trend, while the CEC-D hydrogel in the presence of 2 mM Cu2+ reveals the highest increase in G′ to 14.4 ± 0.7 kPa followed by 9.7 ± 0.9 kPa by addition of 2 mM Zn2+, and a decrease to 1.1 ± 0.2 kPa is demonstrated in the presence of 2 mM Ni2+. A similar observation in M2+ responsiveness emerges where CEC-D hydrogels contract into a condensed state of 2.6-fold for Cu2+, 2.4-fold for Zn2+, and 1.6-fold for Ni2+. Furthermore, CEC-D hydrogels coordinated with M2+ demonstrate control over the encapsulation and release of the small molecule curcumin. The trend of release is opposite of the mechanical and contraction properties with a 70.0 ± 5.3% release with Ni2+, 64.2 ± 1.2% release with Zn2+, and 42.3 ± 11.3 release with Cu2+. Taken together, these results indicate that the CEC-D hydrogel tuned by M2+ is a promising drug delivery platform with tunable physicochemical properties.