Stimulus-Responsive Transport Properties of Nanocolloidal Hydrogels

Applications of polymer hydrogels in separation technologies, environmental remediation, and drug delivery require control of hydrogel transport properties that are largely governed by the pore dimensions. Stimulus-responsive change in pore size offers the capability to change gel's transport properties "on demand". Here, we report a nanocolloidal hydrogel that exhibits temperature-controlled increase in pore size and, as a result, enhanced transport of encapsulated species from the gel. The hydrogel was formed by the covalent cross-linking of aldehyde-modified cellulose nanocrystals and chitosan carrying end-grafted poly(N-isopropylacrylamide) (pNIPAm) molecules. Owing to the temperature-mediated coil-to-globule transition of pNIPAm grafts, they acted as a temperature-responsive "gate" in the hydrogel. At elevated temperature, the size of the pores showed up to a 4-fold increase, with no significant changes in volume, in contrast with conventional pNIPAm-derived gels exhibiting a reduction in both pore size and volume in similar conditions. Temperature-mediated transport properties of the gel were explored by studying diffusion of nanoparticles with different dimensions from the gel, leading to the established correlation between the kinetics of diffusion-governed nanoparticle release and the ratio nanoparticle dimensions-to-pore size. The proposed approach to stimulus-responsive control of hydrogel transport properties has many applications, including their use in nanomedicine and tissue engineering.