Controlling Water States in UV LED Curable Polyacrylamide Hydrogels: A Pathway to Stronger Biomaterials

Abstract

Hydrogels, particularly composed of polyacrylamide (PAAm), are extensively utilized for biomaterials, especially in biomedical diagnostics. In biomaterials, the water state is one of the requirements to be eligible for medical use. In this study, PAAm hydrogels were synthesized using UV LED photopolymerization to investigate the effect of the PAAm hydrogel water state towards its overall monomer concentration. This study establishes the direct transition between free water dominant states in low crosslinked networks to non-freezable bound water in highly crosslinked hydrogels, by varying the overall monomer concentration (OMC). The PAAm hydrogel exhibits high monomer conversion efficiency (98%) and high gel fraction (80%) when increasing the OMC. Additionally, the mechanical testing demonstrated that higher OMC enhanced tensile strength by a factor of 6 and Young’s modulus by a factor of 13, confirming the formation of a denser polymer network. However, the elongation at break decreased from 63 to 11%, indicating reduced flexibility. Meanwhile, the toughness increased up to 1104 J/m² for 40 wt.% of OMC, while 50 wt.% OMC resulted in brittleness due to excessive crosslinking. The swelling behavior is more controlled for higher OMC (40 – 50 wt.%) due to lower water uptake. From the thermal approach via differential scanning calorimetry analysis, the water states of the hydrogel are determined, where the free water was almost eliminated and left with non-freezable bound water for a highly crosslinked and rigid network at 40 – 50 wt.% of OMC. Further increasing the OMC to 50%, however, results in brittleness and reduced toughness. Therefore, higher OMC (40%) exhibits hydrogels with higher mechanical strength and controlled swelling behavior. These findings introduce valuable insights for designing mechanically robust hydrogels by modifying water hydrogel interaction, a potential method for next-generation biomaterials for tissue adhesives, drug delivery, and biomedical diagnostics, with superior mechanical strength, durability and hydration control.