Lower Critical Solution Temperature Tuning and Swelling Behaviours of NVCL-Based Hydrogels for Potential 4D Printing Applications

Abstract

The phase transitions of poly (N-vinyl caprolactam) (PNVCL) hydrogels are currently under investigation as possible materials for biomedical applications thanks to their thermosensitive properties. This study aims to use the photopolymerisation process to simulate the 4D printing process. NVCL-based polymers with different thermal properties and swellability were prepared to explore the possibility of synthetic hydrogels being used for 4D printing. In this contribution, the thermal behaviours of novel photopolymerised NVCL-based hydrogels were analysed. The lower critical solution temperature (LCST) of the physically crosslinked gels was detected using differential scanning calorimetry (DSC), ultraviolet (UV) spectroscopy, and cloud point measurement. The chemical structure of the xerogels was characterised by means of Fourier transform infrared spectroscopy (FTIR). Pulsatile swelling studies indicated that the hydrogels had thermo-reversible properties. As a result, the effect of varying the macromolecular monomer concentration was apparent. The phase transition temperature is increased when different concentrations of hydrophilic monomers are incorporated. The transition temperature of the hydrogels may allow for excellent flexibility in tailoring transition for specific applications, while the swelling and deswelling behaviour of the gels is strongly temperature- and monomer feed ratio-dependent.

Strategies for Developing Shape-Shifting Behaviours and Potential Applications of Poly (N-vinyl Caprolactam) Hydrogels

“…As reported in our earlier works [ 18 , 24 ], a hydrogel-based monolayer only displayed volume changes under heating and cooling conversions. In contrast, a simple way of creating monolayer structures that can change their conformations was demonstrated in this contribution.…”

“…The active layer of the bilayer was prepared according to the S5 formulation, with a swelling ratio of approximately 500% at room temperature in equilibrium state. Compared with the bilayers developed in other studies [ 24 ], a 500% swelling ratio is relatively large for achieving shapeshifting behaviour. This study thus proposes and practices two schemes to control the expansion of the active layer.…”

“…Additionally, when the temperature was raised above the LCST, the deformed strips failed to return to a perfectly flat formation. This is because the active layer de-swells at high temperature, but the shrunken sample still retains a 200% expansion ratio (the measurement of expansion ratio is described in reference [ 24 ]), so that only a reduction in the bending angle can be observed in the samples.…”

“…The bilayers were fabricated using the Dr. Gröbel UV chamber system for the detection of self-changing behaviours. The formulation S5, which demonstrated the best performance in a previous investigation [ 24 ], was used to form the active layer. The passive layer was prepared with Elastic 50A V1 resin (Formlabs Company, Boston, MA, USA).…”

“…Therefore, the concept of constructing single/bilayer objects via UV chamber systems was proposed. The works presented in our last papers prepared a series of PNVCL copolymers/terpolymers via photopolymerisation and 3D printing [ 18 , 24 ]. The copolymer containing 0.1 wt% Irgacure 2959, 2 wt% poly (ethylene glycol) dimethacrylate (PEGDMA), 30 wt% N, N-dimethyl acrylamide (DMAAm) and 70 wt% NVCL demonstrated a relative short reaction time and excellent swelling behaviours.…”