Improved stimuli-response and mechanical properties of nanostructured poly(N-isopropylacrylamide-co-dimethylsiloxane) hydrogels generated through photopolymerization in lyotropic liquid crystal templates

Abstract

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogels are widely studied stimuli-responsive systems due to their significant volume changes at biologically relevant temperatures and a potential wide range of applications including drug delivery, cell cultures, chemical sensors, and desalination. The successful performance of PNIPAM gels often requires a rapid response rate with a significant degree of deswelling when heated above the lower critical solution temperature. However, it is often difficult to design PNIPAM hydrogels with appropriate mechanical strength for the gels to remain functional in a working environment. Herein, lyotropic liquid crystals (LLCs) are utilized to generate a hexagonal nanostructure in PNIPAM hydrogels in order to improve material properties and transport characteristics. Cross-linked methacrylated poly(dimethylsiloxane) (PDMS) was incorporated into PNIPAM gels at varying concentrations through photopolymerization in the hexagonal LLC phase in order to modulate mechanical properties. The hexagonal LLC nanostructure dramatically increases the hydrogel deswelling rate compared to traditional isotropic PNIPAM–PDMS hydrogels of the same chemical composition. Additionally, the ordered LLC structure allows for considerable incorporation of PDMS into the hydrogel without significantly decreasing the water content of the gels. Interestingly, the hexagonal nanostructured hydrogels exhibit similar compressive moduli compared to isotropic hydrogel controls despite having considerably higher water content. These results may be utilized to generate stimuli-sensitive hydrogels with an appropriate rate and degree of deswelling while maintaining necessary mechanical integrity of the gel for use in numerous biomedical and industrial applications.