Hydrogels have been used as one of the novel soft materials in many biomedical applications such as drug delivery and tissue engineering for recent decades.

In the main part of this work, bi-functional poly(ethylene glycol) (PEG) precursors with either thiols (PEG-SH) or allyls (PEG-Al) , covering molecular weights from 3 kDa to 8 kDa were synthesized and thoroughly characterized by ¹H NMR, ¹³C NMR, FT-Raman and MALDI-TOF techniques. By combining PEG precursors with complementary trifunctional crosslinkers, a library of well-defined single-network hydrogels was efficiently constructed via the robust UV-initiated thiol-ene coupling (TEC) chemistry. Novel sequential interpenetrating network (seqIPN) hydrogels based on PEG were fabricated by diffusing and afterwards crosslinking secondary-network precursors within dense (2 kDa) to loose (8 kDa) primary networks. The impacts of polymer chain length and diffusion time on the swelling and mechanical properties were assessed for the seqIPN hydrogels. Additionally, disperse red 13 decorated PEG 2 kDa and 8 kDa were synthesized and used as probes to monitor the secondary-network precursor diffusion rate by UV/Vis spectroscopy. 

FT-Raman and leaching tests were conducted to evaluate the efficiency of the TEC reaction for the development of PEG networks and their gel fractions. All gels were fully crosslinked within 5 minutes and with the gel fraction above 84%. The chain length of PEG, location of functional groups of PEGs, solvents, solid content were found to have directly influence on the mechanical and swelling properties of PEG single-network hydrogels. The utilization of the diffusion time dependent seqIPN strategy enabled further freedom to control the swelling and mechanical properties of PEG hydrogels, with the degree of water swelling ranged from 280 – 870% and the tensile modulus ranging from 1135 kPa to 175 kPa.

Furthermore, the seqIPN strategy was utilized for fiber reinforced free radical polymerized hydrogels. N, N-dimethylacrylamide (DMA) with crosslinker poly(ethylene glycol) diacrylate were diffused in bacterial cellulose (BC) aerogel thereafter UV crosslinked to form BC-DMA hydrogels. FT-Raman and leaching tests were conducted to evaluate the efficiency of the free radical polymerization and the BC-DMA gel fractions. After UV cure for 10 minutes, robust DMA networks were formed within BC aerogels with over 94% gel fraction. The high porosity and robust interpenetrating DMA network within BC fibers were further analysed with FE-SEM. Compression tests showed that fiber reinforced DMA hydrogels have higher compression modulus than DMA hydrogels, ranging from 4.4 to 8.3 MPa with water content from 78 to 70%.