Silicone or Polydimethylsiloxane (PDMS) is an important class of organo-silicon synthetic materials based on molecular chains of alternating silicon and oxygen atoms. This hybrid material has been used in a wide range of biomedical applications due to its unique characteristics, including physiological inertness, low toxicity, its similar elasticity to soft biological tissue, good thermal and oxidative stability, optical transparency, high permeation to oxygen and the ease of fabrication[1],[2]. However, as a result of its extremely low glass transition temperature and weak intermolecular forces, PDMS exhibits very low mechanical properties at room temperature and requires a cross-linking to decrease its compliance. One of the well-known chemistry to cross-link PDMS is the use of hydrosilylation addition, which requires an expensive metal catalyst as well as poisoning by traces of compounds containing sulphur, nitrogen, phosphorous or tin[3]. Therefore, to overcome the current drawbacks of platinium-catalysed curing of silicones, photo-induced thiol-ene chemistry is a particularly good candidate for efficiently linking PDMS chains. The reaction is versatile with various choices of -thiol and -ene functional groups and kinetically fast with a simple requirement under mild condition, i.e. room temperature and little limitations to oxygen or moisture presence[4].
This project explores the use of photo initiated thiol-ene chemistry to cross-link the side-chain thiol-functionalised PDMS with the telechelic vinyl PDMS cross-linkers at ambient temperature and generate substrates with storage modulus varying from Pa to sub MPa. Various factors have been studied, which includes ene:thiol ratios, cross-linker’s chain length and UV radiation intensity, to assess their influence on the network properties and gelation kinetics using Rheology, swelling and tensile mechanical characterisation. It has been shown that the curing reaction is rapid and very efficient with gelation obtained within 1-2 seconds, while mechanical properties are easily altered by the combination of ene:thiol ratios and the length of cross-linker. In addition, toxicity and cell behaviour on substrates with low and high stiffness were also characterised. There is no significant toxicity observed for both types of stiffness and the cells were spreading nicely on those substrates.
Furthermore, by introducing the excess of thiol- moieties on the surface of the cross-linked substrate, we investigate the possibility of attaching different biological molecules (e.g. peptides) via a spacing backbone, i.e. PEG or polyoxazoline. The ability of post surface-modification allows us to use this system for various biomedical applications such as studying cell behaviour, micro-patterning and for microfluidics systems.
References:
[1] Liles, D. T. In The Fascinating World of Silicones, The 39th Waterborne Symposium, 2012.
[2] Mark, J. E.; Allcock, H. R.; West, R., Polysiloxanes and Related Polymers. In Inorganic polymers, Oxford University Press: 2005; pp 154-199.
[3] Van den Berg, O.; Nguyen, L.-T. T.; Teixeira, R. F. A.; Goethals, F.; Özdilek, C.; Berghmans, S.; Du Prez, F. E. Macromolecules 2014, 47, (4), 1292-1300.
[4] Hoyle, C. E.; Lee, T. Y.; Roper, T. Journal of Polymer Science Part A: Polymer Chemistry 2004, 42, (21), 5301-5338.