Impact of Polyester Dendrimers as Branched Multifunctional Cross-Linking Additives in Triazine-Trione-Based Composites Developed via High-Energy Visible Light Thiol–ene Chemistry

Abstract

Hydroxyapatite (HA) infused triazine-trione (TATO) composites have emerged as an injectable platform for customizable bone fixators due to their fast and benign curing via high-energy visible light-induced thiol–ene chemistry (HEV-TEC), promising mechanical performance, and preclinical outcomes. These composites can overcome many of the existing limitations accompanying metal implants such as poor patient customizability, soft tissue adhesions, and stress shielding. Taking into account that the promising benchmarked TATO composite (BC) is based on stable sulfur–carbon bonds, we herein investigate the impact of introducing polyester dendritic cross-linkers based on bis-MPA as chemically integrated branched additives that display labile esters in a branched configuration. The inclusion of dendrimers, G1 and G3, in concentrations of 1, 3, and 5 wt % in the composite formulations were found to (i) decrease the processing viscosity of the composite formulations, reaching Newtonic and nonshear thinning behavior at 37 °C and (ii) impact the size distribution of bubble cavities in the composite cross sections. The lowest collected T g for the dendrimer-containing composites was noted to be 73.2 °C, a temperature well above physiological temperature. Additionally, all composites displayed flexural modulus above 6 GPa and flexural strength of ca. 50 MPa under dry conditions. The composites comprising 5 wt % of G1 and G3 dendrimers, with ester bond densities of 0.208 and 0.297 mmol/g, respectively, reached a mass loss up to 0.27% in phosphate buffered saline at 37 °C, which is within the range of established polycaprolactone (PCL). Combined with the nontoxic properties extracted from the cell viability study, polyester dendrimers were determined as promising additives which compatibilized well with the TATO formulation and cross-linked efficiently resulting in strong composites suited for bone fracture fixations.

Enhanced Degradability of Thiol–Ene Composites through the Inclusion of Isosorbide-Based Polycarbonates

“…Screws are used to anchor the patches to the bone fragments. The mechanical properties of these TATO composites can be tuned through the choice of monomers, with flexural modulus values ranging from 52 MPa to 7 GPa. − The stiffest of these composites, based on the TATO alkene and thiol monomers 1,3,5-tiallyl-1,3,5-triazine-2,4,6-trione (TATATO) and 1,3,5-triazine-2,4,6-trione, 1,3,5-tris­(mercaptopropyl) (TMTATO), falls within the flexural modulus range of human trabecular bone (3–10 GPa). , The fluid nature of the composite resins and their use of on-demand curing allows for drop-casting of screws and plates and even 3D printing, making them suitable for potential prosthetic implants ,,, while biomechanical studies on human hand cadavers and animal bones suggest that composites containing TATATO, TMTATO, and HA can maintain alignment of the bone against bending forces experienced during rehabilitation flexion exercises …”

“…Recently, allyl-functionalized polyester dendrimers have been used as potential degradable cross-linking additives in TATO composites. Adding up to 5 wt % of these dendrimers into an existing TATO formulation did not jeopardize the strength or modulus of the material, but, unfortunately, it only had a very minor impact on hydrolytic degradation after a time period of 8 weeks . Easier synthetic alternatives could be used to introduce degradation into composite materials such as linear polymers.…”