There has been considerable interest in the application of photochromism to photo-responsive systems which has led to the development of new tailored smart materials for photonics and biomedical fields. Within a polymeric matrix photochromic isomerizations can be stimulated by light to reversibly alter the physical and chemical properties of a material such as LC phase, shape, surface wettability, permeability, solubility, self-assembly, size and fluorescence. The underlying principles behind photoresponsive behavior, subsequent applications and relevant examples are discussed in this review.

Reversible photocycloaddition reactions have previously been employed in chemical cross-linking for the preparation of biomaterial scaffolds. However, the processes require activation by high-energy UV light, rendering them unsuitable for modification in biological environments. Here we demonstrate that the [2 + 2] photocycloaddition of styrylpyrene can be activated by visible light at λ = 400–500 nm, enabling rapid and effective conjugation and cross-linking of poly­(ethylene glycol) (PEG) in water and under mild irradiation conditions (I = 20 mW cm–2). Notably, the reversion of the cycloaddition can be triggered by low-energy UV light at λ = 340 nm, which allows for efficient cleavage of the dimer adduct. Using this wavelength-gated reversible photochemical reaction we are able to prepare PEG hydrogels and modulate their mechanical properties in a bidirectional manner. We also demonstrate healing of the fractured hydrogel by external light triggers. Furthermore, we show that human mesenchymal stem cells can be encapsulated within the gels with high viability post encapsulation. This photochemical approach is therefore anticipated to be highly useful in studies of cell mechanotransduction, with relevance to disease progression and tissue regeneration.