New photoactivators for multiphoton excited three-dimensional submicron cross-linking of proteins: bovine serum albumin and type 1 collagen.

We report the synthesis and optical characterization of two new photoactivators and demonstrate their use for multiphoton excited three-dimensional free-form fabrication with proteins. These reagents were developed with the goal of cross-linking Type 1 collagen. This cross-linking process produces structures on the micron and submicron size scales. A rose bengal diisopropyl amine derivative combines the classic photoactivator and co-initiator system into one molecule, reducing the reaction kinetics and increasing cross-linking efficiency. This derivative was successful at producing stable structures from collagen, whereas rose bengal alone was not effective. A benzophenone dimer connected by a flexible diamine tether was also synthesized. This activator has two photochemically reactive groups and is highly efficient in cross-linking bovine serum albumin and Type 1 collagen to form stable, robust structures. This approach is more flexible in terms of cross-linking a variety of proteins than by traditional benzophenone photochemistry. The photophysical properties vary greatly from that of benzophenone, with the appearance of a new, lower energy absorption band (lambda max approximately 370 nm in water) and broad, visible emission band (approximately 500 nm maximum). This absorption band is highly solvatochromic, suggesting it arises, at least in part, from a charge transfer interaction. Collagens are typically difficult to cross-link photochemically, and the results here suggest that these two new activators will be suitable for cross-linking other forms of collagen and additional proteins for biomedical applications such as the de novo assembly of biomimetic tissue scaffolds.