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The thiol-ene reaction was first suggested by Posner in 1905,1 but academic interest in this potential polymerization reaction remained relatively small, especially as compared to (meth)acrylate polymerization, until the last two decades. Interest in the thiol-ene reaction mechanism increased as distinct advantages over acrylate polymerization were discovered. Various researchers have shown that, unlike acrylates, thiol-ene reactions have reduced oxygen inhibition,2 significant lower shrinkage,3 and better mechanical properties. Despite all this advantages, thiol-ene systems have serious drawbacks, such as limited shelf-life stability and bad odor of the thiol. This problem (the latter on) has been addressed by a number of high molecular multifunctional low-odor thiols that are now commercially available. The main issue appears to be the stability of the formulations with shelf lives varying enormously from a few seconds to months.
Thiol-Ene polymerization has been studied for a wide range of applications such as coatings, printing inks or 3D printed objects. In the last decades, development of polymerizable and cytocompatible hydrogels with tunable properties has also gained significant interests for soft tissue engineering and regenerative medicine. The use of light as the trigger would provide spatiotemporal control over the crosslinking process of hydrogels while providing more geometrical complexities. For instance, photopatterning of bioactive ligands(e.g., RGD motifs) within hydrogel matrices has enabled user-defined manipulation of cell functions in 3D. Acrylate-based monomers are frequently used, but monomer toxicity is a serious concern due to Michael Addition reaction of amino-groups of proteins with the acrylate group. Therefore we have recently proven that vinylesters and vinylcarbonates4 are a suitable alternative, especially as the thiol-ene polymerization gives rapid curing similar to acrylate-based monomers.5,6
3D-printing7 of such hydrogel structures is possible by two-photon polymerization (TPP), a real 3D additive manufacturing technique. With highly effficient initiators it is possible to get resolution well below the micrometer range.
