Biopolymer Hydrogels: Modulating Microstructure through External and Internal Stimuli

Polysaccharide-based hydrogels are being sought in a range of application because of the wide array of properties that they offer. Of particular interest to us is the ability to tune their microstructure and rheology using external and internal stimuli. We examine three different polysaccharides, guar gum, aliginate and nanocellulose, to illustrate this notion. Guar gum with its mannose backbone and galactose side unit offers a unique opportunity to tailor its characteristics using back-bone of side-chain cleaving enzymes. While backbone hydrolysis leads to a loss in the entanglement, and ensuing network structure, we find that cleaving side-chains using enzymes lead to ‘hyper-entanglement’ between guar polymer chains. This results in creation of gels, the structure of which is dependent on the extent of enzyme action and time. Nanocellulose, on the other hand, intrinsically forms a hydrogel in an aqueous media. However, exposure to different shear histories can lead to hierarchical breakdown of the microstructure, and reformation, leading to gels with different internal structure and rheology. We explore such restructuring the gel microstructure and its relation to modulus and yield stress Finally, alginate hydrogels respond well to UV light. Using calcium carbonate nanoparticles, and UV induced photo acid generators, physical crosslinks can be created to form alginate hydrogels with a variety of elastic modulus. The sol-gel transition is modulated by various factors, including UV dosage and polymer concentration. In some cases, gelation occurs in the ‘dark’ phase, when the UV light is turned off, leading us to propose a two step-mechanism, involving calcium ion formation and its diffusion.