Photoswitchable Hydrogels for Biomedical Applications

Multi-responsive synthetic hydrogels are very versatile three-dimensional network capable to respond simultaneously to multiple relevant stimuli i.e., temperature, pH, light, redox activity, enzymes, etc. These polymeric tunable materials have the capability to retain large amounts of water, controlling the biodegradability, biocompatibility and mechanical strength, which makes them excellent candidates to be widely used in biomedical applications such as tissue engineering, biosensing and drug delivery, among others. The most outstanding used thermo-responsive monomer in biomedical field is the N-Isopropylacrylamide (NIPAM) whose reversible volume phase transition is shown by swelling at temperature below the lower critical solution temperature (LCST ≈ 32 C) and shrinking above it. NIPAM can be copolymerized with other monomers, for example methacrylic acid (MAA), a pH-responsive monomer whose carboxylic acid moiety deprotonates at pH higher than 4.8, generating the ionic interaction with water and ensuing swelling of the hydrogel. The stimuli, in this case, are found in our body, in the difference of temperature and pH between diseased and healthy tissue, or between organs. However, external stimuli, i.e., magnetic field, electric field, light and ultrasound have advantageous features. From these stimuli, light can be considered the most promising, being non-invasive, tunable and easy manipulated stimulus to produce a change in the hydrogel properties. Photoswitchable monomers are very versatile light-sensitive molecules that can be photopolymerized in presence of other sensitive monomers to modify their response. Spiropyranes, Diaryethenes and Azobenzenes are some of the traditional Photoswitchable molecules used in biomedical applications. Azobenzenes, and more specifically, Push-Pull azobenzenes, are extensively studied due to the thermal isomerization reducing the light cytotoxicity and the possibility of functionalization with biological molecules such as peptides, to regulate the cell attachment and increase the biocompatibility.<br/>In this work, a Push-Pull Azobenzene monomer (PPm) and azobenzene crosslinker (PPc) are produced by Williamson ether synthesis and posterior diazotization reaction. Subsequently, the carboxylic acid moiety of the monomer is functionalized by Steglich esterification with RGD cyclic peptide (PPmp). Moreover, the monomer (PPm) is functionalized with acrylamide by amide formation in presence of DCC to obtain the azobencene crosslinker (PPc). The triple-responsive hydrogels of N-Isopropylacrylamide (NIPAM), Methacrylic acid (MAA) and PPmp or PPc were synthesized using led light in few minutes or by thermal reaction. The exposition of the PPmp containing hydrogels to low energetic light, produces a change in the conformation of the azobenzene molecule modulating the interaction of the peptide with the mammalian cells. However, the exposition of the PPc to light modifies the conformation producing changes on the scaffold structure possibly reducing the pore size and modifying the mechanical properties along with the cell attachment.