Collagen type I is the main structural protein of the body and it plays vital roles in various functions including wound healing, cell signaling, and force transmission. At appropriate pH, temperature, and ionic conditions, the collagen protein monomers can assemble into fibrils that interweave and trap water into a hydrogel in vitro. This property has been used to make hydrogels for wound healing applications, cell culture research, and drug delivery. Current techniques for manufacturing collagen I hydrogels allow fine control over macrostructural features on the order of 100 µm, but have limited control over microstructural features, such as fibril diameter and mesh size, that influence mechanical properties of these gels. We employed the use of a phenylglyoxylate cyclohexylammonium (PGA-CHA) photobase generator (PBG) that dissolves in acidic collagen solutions and releases a base upon interaction with light, raising the pH and triggering the collagen assembly. After exploring the parameter space, we elucidate the design parameters, including light absorptivity, PBG quantum yield, buffering effects, and heat generation, needed assemble collagen at physiologically relevant pH values (pH 6, 7, 8) using a single formula irradiated with 365 nm light for different durations. Areas of the solution masked from light did not form a gel and remained a solution. Collagen gels assembled using the PBG showed increasing storage modulus, decreasing fibril thickness, and decreasing characteristic mesh size with increasing assembly pH. This behavior aligns well with the Morse model of un-crosslinked semi-flexible polymers and agrees with previous literature on the behavior of collagen hydrogels made with the current state-of-the-art techniques. Human foreskin fibroblasts grown on collagen gels assembled at different pH conditions using the PBG exhibited a variety of behaviors in morphology and migration. These behaviors provided insight into the length scale of mechanical properties that cells probe, as well as ways in which subtle changes in the mechanical properties influence cell speed and persistence. In this way, we introduce a new platform for microstructural control over collagen hydrogels using a PBG, which may be useful in influencing cells through their mechanical environment.
Akash S. Dhawan (Tue,) studied this question.