| Summary: | A new and simple method to develop highly stretchable and resilient hydrogels via ultraviolet light-emitting diode (UV LED) photopolymerization was carried out. Firstly, a single network (SN) polyacrylamide (PAAm) hydrogel was prepared. The SN hydrogel had achieved about > 98 % monomer conversion, 66 to 82 % gel fraction and 10.7 ± 0.1 to 4.1 ± 0.01 swelling degree. Differential scanning calorimetry result showed the existence of bound and free water in PAAm hydrogel which have interrelation with the swelling and tensile properties. Nevertheless, the SN hydrogels demonstrated poor tensile properties (tensile strength: ~ 0.06 MPa, Young's modulus: 0.26 ± 0.02 MPa, elongation at break: ~ 32 ± 3.4 % and toughness: ~ 1104 ± 90.5 J/m2) which severely limit their extensive uses for the advanced functional material. Thus, double network (DN) hydrogels were prepared and characterized by adding chitosan hyaluronic acid (ChiHA) to PAAm SN hydrogels at concentration ranging from 20-50 wt%. The optimized DN hydrogel with 30 wt% of ChiHA composition exhibited higher monomer conversion (up to 99 %), gel fraction (~ 73 to 98 %) and tensile properties (tensile strength: ~ 0.16 MPa, Young's modulus: ~ 0.47 MPa, elongation at break: ~ 49 ± 0.1 % and toughness: ~ 1785 ± 58.4 J/m2). However, the resilience property in DN hydrogels was low as indicated by large hysteresis upon loading-unloading cycle. To overcome this problem, the selected compositions of DN hydrogels with 40 and 50 wt% of overall monomer concentration (OMC) were modified to produce elastomeric hydrogels (EH). Silicone urethane acrylate (SUA), an elastomeric and resilient material, was integrated into DN hydrogels in a "sandwich-like" form via photopolymerization. EH was successfully polymerized by reaching up to 99 % of gel fraction. The peaks from Fourier transform infrared spectra at 1262, 1096, 1023 and 802 cm-1 were attributed to Si-CH3, Si-O-Si, -C-O and Si-C stretching mode for SUA network. EH possessed excellent tensile properties where its tensile strength, Young's modulus, toughness and elongation at break were ~ 2-13 times larger than SN and DN hydrogels. EH also exhibited a remarkable compressive strength (~ 1.5 MPa), exceptional fracture toughness (~ 36851 J/m2) and highly resilient (~ 93 %). These exceptional properties were due to the reversible assembly of the strong and flexible SUA chain, which could be explained by the dissipation of the crack energy along the EH network. The temporarily molecular dissociation in EH network which could be instantly reconstructed during unloading process may also be responsible. These fascinating properties of the novel EH had offered an alternative candidate for biomaterial applications.
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