# Cleavable Silyl Ether Monomers with Elevated Thermomechanical Properties for Bone Regeneration

## TL;DR

This paper introduces new silyl ether monomers for 3D printing bone grafts that are strong, degrade safely, and avoid harmful byproducts.

## Contribution

The novel monomers combine rigid norbornane structures with cleavable silyl ethers to achieve high thermomechanical properties and safe degradation.

## Key findings

- The new monomers achieved a Tg of 62°C, significantly higher than previous thiol–ene networks.
- The material showed a quasi-linear degradation rate of 6.5% per month with low cytotoxicity.
- Proof-of-concept 3D printing was successfully demonstrated using a DLP setup.

## Abstract

Over the last years, stereolithography has developed
to 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 treatment
of critical sized bone defects. State-of-the-art photopolymer systems
mostly consist of potentially cytotoxic compounds, such as (meth)­acrylates,
that furthermore show insufficient degradation and lead to acidic
degradation products that could induce adverse tissue reactions. Herein,
we introduced trifunctional monomers comprising cleavable silyl ether
groups for thiol–ene photopolymerization to enlarge the material
platform for printed bone grafts. Polymer networks comprising a high
number of silyl ether moieties typically tend to be mechanically weak
and exhibit low T
g values, especially
when combined with thioether bonds, which are a direct result of polymerization
via thiol–ene click reaction. To push thermomechanical properties
to a level where they are sufficient for bone grafting (T
g > 37 °C), we introduced rigid bridged alicyclic
structures in the form of norbornane-derived motifs into the silyl
ether monomers, resulting in a norbornene-containing double bond monomer
and a norbornane-derived thiol monomer. Together with noncleavable
comonomers, we were able to demonstrate a substantial increase in T
g up to 62 °C, which is well above the
values reported until now for similar thiol–ene networks. Furthermore,
in this study, we demonstrated high photoreactivity for some of the
monomers and also successfully performed proof-of-concept printing
using a DLP setup. Besides excellent thermomechanical behavior, the
mechanical strength of the silyl ether-based polymer network was shown
to be outstanding. Cleavability of the silyl ethers was displayed
with a quasi-linear degradation rate of 6.5% per month with moderate
swelling. Additionally, the degradation product of the silyl ether-based
network was isolated and shown to exhibit no relevant cytotoxicity
to mouse fibroblast cells.