Cleavable Silyl EtherMonomers with Elevated Thermomechanical Properties for Bone Regeneration

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Over the last years, stereolithography has developedto be one of the most promising fabrication techniques in tissue engineering.Posing the possibility of fabricating patient-specific, porous implants,it became especially attractive for scaffold fabrication for the treatmentof critical sized bone defects. State-of-the-art photopolymer systemsmostly consist of potentially cytotoxic compounds, such as (meth)­acrylates,that furthermore show insufficient degradation and lead to acidicdegradation products that could induce adverse tissue reactions. Herein,we introduced trifunctional monomers comprising cleavable silyl ethergroups for thiol–ene photopolymerization to enlarge the materialplatform for printed bone grafts. Polymer networks comprising a highnumber of silyl ether moieties typically tend to be mechanically weakand exhibit low Tg values, especiallywhen combined with thioether bonds, which are a direct result of polymerizationvia thiol–ene click reaction. To push thermomechanical propertiesto a level where they are sufficient for bone grafting (Tg > 37 °C), we introduced rigid bridged alicyclicstructures in the form of norbornane-derived motifs into the silylether monomers, resulting in a norbornene-containing double bond monomerand a norbornane-derived thiol monomer. Together with noncleavablecomonomers, we were able to demonstrate a substantial increase in Tg up to 62 °C, which is well above thevalues reported until now for similar thiol–ene networks. Furthermore,in this study, we demonstrated high photoreactivity for some of themonomers and also successfully performed proof-of-concept printingusing a DLP setup. Besides excellent thermomechanical behavior, themechanical strength of the silyl ether-based polymer network was shownto be outstanding. Cleavability of the silyl ethers was displayedwith a quasi-linear degradation rate of 6.5% per month with moderateswelling. Additionally, the degradation product of the silyl ether-basednetwork was isolated and shown to exhibit no relevant cytotoxicityto mouse fibroblast cells.