Synthesis of NVCL-NIPAM hydrogels using PEGMA as a chemical crosslinker for controlled swelling behaviours in potential shapeshifting applications

Abstract

Stimuli-responsive hydrogels have recently gained interest within shapeshifting applications due to their capabilities to expand in water and their altering swelling properties when triggered by stimuli, such as pH and heat. While conventional hydrogels lose their mechanical strength during swelling, most shapeshifting applications require materials to have mechanical strength within a satisfactory range to perform specified tasks. Thus, stronger hydrogels are needed for shapeshifting applications. Poly (N-isopropylacrylamide) (PNIPAm) and poly (N-vinyl caprolactam) (PNVCL) are the most popular thermosensitive hydrogels studied. Their close-to-physiological lower critical solution temperature (LCST) makes them superior candidates in biomedicine. In this study, copolymers made of NVCL and NIPAm and chemically crosslinked using poly (ethylene glycol) dimethacrylate (PEGDMA) were fabricated. Successful polymerisation was proven via Fourier transform infrared spectroscopy (FTIR). The effects of incorporating comonomer and crosslinker on the LCST were found minimal using cloud-point measurements, ultraviolet (UV) spectroscopy, and differential scanning calorimetry (DSC). Formulations that completed three cycles of thermo-reversing pulsatile swelling are demonstrated. Lastly, rheological analysis validated the mechanical strength of PNVCL, which was improved due to the incorporation of NIPAm and PEGDMA. This study showcases potential smart thermosensitive NVCL-based copolymers that can be applied in the biomedical shapeshifting area. Keywords: hydrogels; soft materials; smart materials; biomaterials; hydrophilic; temperature-responsive; lower critical solution temperature; photopolymerisation; chemically crosslinking; copolymers