Full-field hygroscopic characterization of tough 3D-printed supramolecular hydrogels

Chain-extended ureido-pyrimidinone poly(ethylene glycol) (CE-UPy-PEG) is a supramolecular hydrogel with excellent mechanical properties and shape memory capabilities, making it highly suitable for 3D printing of complex biomimetic structures to mimic biomaterials. However, its transient hygroexpansion response under environmental change, specifically relative humidity (RH), which is strongly affected by the supramolecular sub-structure, is poorly understood. Therefore, a high-precision full-field fiber-swelling methodology is applied to 3D-printed CE-UPy-PEG fibers, enabling investigation of the influence of PEG chain length (1.5, 3, and 10 kg/mol studied here) and RH rate from wet to dry on the longitudinal and transverse surface strain evolution during multiple RH cycles. The PEG length directly influences the fibers' hygroscopic properties, because only CE-UPy-PEG_3k and CE-UPy-PEG_10k exhibit a phase transformation from semicrystalline to amorphous at higher RH levels, which is fully described by a phenomenological phase transformation model. Furthermore, all fibers display cyclic repeatability (shape memory), increased swelling for longer PEG chains and lower RH rate, and disappearance of sub-millimeter-sized tube-like voids after wetting.