The influence of diffusion time on the properties of sequential interpenetrating PEG hydrogels

Journal article, 2012

Four sets comprising a total of 16 sequential interpenetrating network (SeqIPN) hydrogels were efficiently fabricated via UV initiated thiol-ene coupling chemistry and from 2 kDa or 8 kDa primary poly(ethylene glycol) (PEG) networks (S2 and S8). Each primary system delivered four different SeqIPNs constructed after 2, 4, 20, and 44 h diffusion of secondary network PEG precursors, 2 kDa and 8 kDa. This allowed the assessment of both mechanical and swelling properties for a wide range of novel hydrogels ranging from loosely crosslinked SeqIPN 8-8 to densely crosslinked SeqIPN 2-2 systems. All gel fractions of secondary networks were above 83% and 44 h of diffusion was found sufficient to fully saturate the primary networks. Disperse red functionalized PEGs (2 kDa and 8 kDa) were further used as probes to investigate the diffusion mechanisms. The impact of diffusion time on loosely crosslinked S8 network with a swelling degree of 970% and tensile modulus of 175 kPa displayed a significant change in the final properties. For instance, a 2 h diffusion of 2 kDa PEG precursors generated a SeqIPN 8-2:2 comprising a secondary network solid content of 34% with a water swelling degree 580% and a tensile modulus of 365 kPa. On saturation, that is, 44 h of diffusion, SeqIPN 2-8:44 exhibited 64% of secondary network solid content, a swelling capacity of 380% and over fourfold of tensile modulus (758 kPa) when compared with the primary network S8. SeqIPN hydrogel with the highest tensile modulus and lowest degree of water swelling was obtained after 44 h diffusion of 2 kDa PEG precursors within the densely crosslinked S2 primary network. In this case, SeqIPN 2-2:44 noted a water swelling capability of 280% and a tensile modulus over 1 MPa. The latter was twofold when compared with S2 with a tensile modulus of 555 kPa. Consequently, the diffusion time of secondary network is a promising parameter to control and that enables the fabrication of PEG hydrogels with a wider window of mechanical and swelling properties.