Degradable implants, which can be broken down and exerted by the body after bone healing, represent a giant leap forward in bone fracture fixation. Imagine if fracture patients no longer needed to be burdened by plates and screws for the rest of their lives. However, such implants would need to be carefully designed to ensure that their degradation does not compromise their ability to stabilize the bone fracture as it heals. Strong, slowly degrading materials are needed, and their development is one of the key aims of the BoneFix project. Previous fixation patches in BoneFix have been constructed with a composite consisting of allyl and thiol monomers and a high concentration of hydroxyapatite, which can be cured within seconds via thiol-ene coupling (TEC) chemistry. While this composite allows for strong and stiff fixations, it does not show any signs of degradability, either in vitro or in vivo.
In the latest paper from the BoneFix consortium, a team from KTH, led by PhD student Jorge San Jacinto Garcia, describes how including degradable polycarbonates within the bone fixation composite’s formulation increases the composite’s hydrolytic degradation without sacrificing its high flexural modulus or strength. The polycarbonates contain repeating isosorbide units and carbonate linkages, which can undergo hydrolytic cleavage. Jorge explains, “the decoration of these polycarbonates with allyl groups allows us to covalently incorporate them into the composite’s polymeric network via TEC chemistry.” Once embedded within the network, the polycarbonates allow for the slow degradation of the composite, which was noticeable over an 8-week period in static in vitro conditions simulating physiological pH and temperature.
Jorge evaluated the impact that the inclusion of these polycarbonates had on the mechanical properties of the composites during hydrolytic degradation over 8-weeks. He also used the new polycarbonate-infused composites to fixate osteotomies on synthetic bones using the AdhFix technique; an innovative approach that involves inserting screws into bone fragments and then constructing a plate around and over these screws in situ with the composite. The system is then cured with high energy visible light to give a strong, rigid fixation. When tested with four-point bending, the new degradable composites showed similar bending stiffness and strength values to the previously studied non-degradable composite. “The use of these degradable composites for fracture fixation implants could reduce the burden of fracture treatments and the recovery time of patients,” explains Jorge, “as their degradation would remove the need for any removal surgery after bone healing.”
The paper, titled “Enhanced Degradability of Thiol–Ene Composites through the Inclusion of Isosorbide-Based Polycarbonates” was published in ACS Applied Materials & Interfaces, and can be found at https://pubs.acs.org/doi/10.1021/acsami.4c09626. The data from the paper can also be found in the following open access data repository: https://zenodo.org/records/12689371.