Hybrid and Multi-Component Hydrogels

Low-molecular-weight gelators (LMWGs) form a network via non-covalent interactions to immobilise the surrounding bulk solvent and form a gel. Whilst such gels are highly responsive and dynamic, they are often mechanically weak. In order to enhance the mechanical strength of such networks, the LMWG network can be supplemented with a second network formed from stronger polymer gelators (PGs) to yield a multi-component, multi-functional material – a hybrid gel. By using this multi-functionality, hybrid gels were made that could demonstrate the following: a) robustness yet responsiveness, b) spatial control over the formation of one network in the presence of another, and c) temporal control over the formation of one network in the presence of another. For the first aim, a pH-responsive LMWG (1,3:2,4-dibenzylidene-D-sorbitol dicarboxylic acid, DBS-CO2H) was combined with the robust PG agarose. The assembly of DBS-CO2H in the presence and absence of agarose was investigated by NMR and CD spectroscopies, whilst materials properties were examined by rheology. DBS-CO2H was found to retain its pH-responsive character, as was demonstrated by cycling the pH within the gel – whilst the DBS-CO2H network could be switched “on” or “off”, the robust agarose network remained intact. Following this, DBS-CO2H was combined with the photo-inducible PG poly(ethylene glycol) dimethacrylate (PEGDM). Spectroscopic methods and electron microscopy showed that the kinetics and morphology of DBS-CO2H assembly were impacted by the presence of PEGDM. The application of a mask during photoirradiation allowed patterning of the PEGDM network to form a material with two distinct, spatially-resolved regions, defined as a “multidomain gel”, achieving the second aim. The different domains had different properties with regards to the diffusion and release of dyes. DBS-CO2H was then combined with another pH-responsive LMWG (1,3:2,4-dibenzylidene-D-sorbitol-dicarbonyl-glycine, DBS-Gly). The two gelators showed a good degree of kinetic self-sorting, their self-assembly being triggered at different pHs. It was possible to use two proton sources – the slow hydrolysis of glucono-δ-lactone, and the more rapid photoacid generator diphenyliodonium nitrate – to achieve a two-step process of network formation. As the second step was UV-initiated, photopatterned multi-component gels were produced; these materials were both spatially and temporally resolved, achieving the third aim. Finally, the combination of DBS-CO2H, DBS-Gly and PEGDM into a three-gelator, multi-component hybrid hydrogel was investigated.