In Vitro Assessment of a Phosphoserine-Modified Calcium Phosphate Bioadhesive for Use in Bone Repair and Regeneration

Bone fractures represent a significant global healthcare burden, particularly in osteoporotic patients. While the use of metal implants for bone fixation is a current gold standard, their widespread use has several limitations. To address these issues, a phosphoserine and calcium phosphate-based adhesive (‘OsStic®’) has been developed. This adhesive is designed to bind fragmented bone together following fracture, facilitating rapid healing and repair. Phosphoserine addition to the adhesive significantly improved the handling and mechanical properties, producing an adhesive that meets clinical requirements. The focus of this research was to further investigate the OsStic® adhesive and carry out an in vitro evaluation of the material. OsStic® samples were fabricated according to a previously optimised composition. HydroSet™ was used as a proprietary comparator, in addition to phosphoserine-free alpha tricalcium phosphate (αTCP) cement. The biocompatibility and osteogenic potential of the OsStic® adhesive was successfully demonstrated. Cytotoxicity testing showed no material-related cytotoxicity (Fig. 1a). Cells readily adhered and became confluent, confirmed through SEM (Fig. 1b) and LIVE/DEAD™ staining (Fig.1c-d). Sustained cell proliferation was confirmed over 4 weeks (Fig. 1e), with superior proliferation kinetics on the OsStic® adhesive observed. Cell-cell interactions were observed using actin staining (Fig.1f). Alizarin red staining confirmed significant biomineralization in all groups, with greater levels on the OsStic® adhesive (Fig. 1g). Alkaline phosphatase (ALP) activity was confirmed via an enzymatic activity assay and corroborated by positive ALP staining (Fig. 1h). The expression and secretion of osteogenic differentiation-related factors (Fig. 1i-k), including osteopontin, osteocalcin, RUNX2 and BMP-2, were observed across the three materials groups highlighting OsStic's potential to actively support bone regeneration and healing processes, in addition to its benefits in stabilising bone fractures. Overall, these results further demonstrate the significant potential of OsStic® as a next-generation bone adhesive, not only in terms of its physical properties but also its excellent biocompatibility compared to proprietary technologies used in orthopaedics. For any figures or tables, please contact the authors directly.