Hydrogelators that undergo stimuli-responsive sol-gel transitions have attracted attention as biomaterials because of their potential applications, for example as sophisticated cell culture substrates, drug carriers, microvalves, and microactuators. [1, 2] Among the various stimuli possible, light is unique because it allows both spatial and temporal control of a specific reaction without requiring physical contact. Many photochemical sol-gel transitions have been reported, mostly in polymers but also in low-molecular-weight peptides. [1, 3] The importance of peptide systems is their potential biocompatibility and the opportunity for researchers to molecularly design bioactive functions. [4, 5] The advent of two-dimensional self-assembling systems and patterning technologies has generated some examples of stimulus-driven bioactivity on surfaces. [6] Using a peptide amphiphile (PA) containing the fibronectin epitope Arg-Gly-Asp-Ser (RGDS) for cell adhesion, [7] we demonstrate here light-triggered enhancement of bioactivity in a three-dimensional system.
PAs are known as highly versatile molecules that selfassemble into nanostructures such as spherical micelles, fibers, and helices through hydrogen-bond formation and hydrophobic collapse. [4, 5,[8][9][10] Our laboratory first reported on PAs that form high-aspect-ratio nanofibers and therefore gels at very low concentrations. [4] These specific PAs self-assemble into nanofibers as a result of their b-sheet peptide domains, and their self-assembly can be triggered by charge screening through changes in pH or the addition of salts. Molecular changes in the b-sheet domains can disrupt nanofiber formation. For example, Hartgerink and co-workers reported the formation of spherical micelles and not fibers as a result of the N-methylation on the amide nitrogen closest to the alkyl tail of a PA. [9] Our laboratory also reported recently the Scheme 1. Upon irradiation of 1 the 2-nitrobenzyl group is cleaved to afford 2.