Uv and spontaneously cured polyethylene glycol-based hydrogels for soft and hard tissue scaffolds

Abstract: UV-curing is one of the most commonly used methods for producing hydrogels for soft and hard tissue scaffolds. Spontaneous curing is an alternative method which possesses some advantages in comparison to the conventional UV-curing methods; for example, in situ crosslinking and excluding initiators. The main objective of this study was to investigate promising materials for producing UV and spontaneously cured hydrogels, and subsequently to perform a comparison between the produced hydrogels with regard to their different mechanical and physical properties.Seventeen different hydrogels including five UV-cured and twelve spontaneously cured hydrogels were produced by applying thiol-ene chemistry and by varying precursor materials. Hydrogel systems including di- and tetra- functional PEGs of different lengths (2 kDa and 6 kDa) and two different thiol-crosslinkers (ETTMP 1300 Da and DTT) were subsequently characterized and evaluated. The evaluation tests applied in this study were Raman spectroscopy, weight and volumetric swelling test, leaching test, tensile test, and rheology test. Between all the systems, tetra-acrylated PEG (6 kDa) BisMPA was found to be the most promising system. The pH level of the applied solvent (PBS) for spontaneously cured hydrogels was varied from the physiologically relevant level of 7.4 to 7.0 and 7.8 in order to investigate the dependency of physical and mechanical properties of the hydrogels to this parameter.Spontaneous curing of tetra-acrylated PEG (6 kDa) BisMPA with ETTMP 1300 Da as the thiol-crosslinker, was accomplished within 3½ min in PBS with a pH level of 7.4; and it came out to be the fastest spontaneously cured system between all the tested hydrogels. Increasing the PBS pH level resulted in a faster curing process (accomplished in 1½ min). Spontaneously cured hydrogels generally showed decreased mechanical properties, but improved swelling behavior compared to UV-cured hydrogels. Nevertheless, the discussed system still possessed 50% of the elastic modulus in the tensile test in comparison to the UV-cured state; and showed the highest elastic modulus in comparison to other spontaneously cured systems. The storage modulus of the mentioned hydrogel in the spontaneously cured state was very close to the same parameter in the UV-cured hydrogel based on the same precursors. It also possessed the highest storage modulus between all the spontaneously cured hydrogels. Although the obtained swelling properties of this system were not the highest between all the tested hydrogels, these parameters were still in an acceptable range as for a hydrogel proposed for tissue scaffold application (swelling ratio: 9.72, water content: 89.71%, volumetric swelling ratio: 9.05). Furthermore, the system had the lowest weight loss ratio between all the acrylate-based hydrogels (including both UV and spontaneously cured systems), which along with the Raman spectroscopy results shows the high crosslinking efficiency of the system.