By combining cells, biological factors, and biomaterials the field of tissue engineering has generated technologies capable of supporting regeneration. However, the regulatory hurdles associated with the use of cell-based therapies often hinder translation. Consequently, to meet the growing demand for regenerative technologies new approaches are needed. Emerging evidence suggests that cell-derived extracellular vesicles (EVs) are critical in cell-cell communication and regulation of bone formation. This talk will explore the role of osteoblast EVs in directing stem-cell differentiation in-vitro. EVs were isolated from cell culture media by ultracentrifugation and profiled for size and composition using a range of techniques. Notably, proteomic analysis revealed the presence of calcium channelling annexins and bridging collagens that may be key to their role in mineralisation. To minimise the concentration of EVs required to induce a pro-osteogenic effect we propose that they may be locally delivered. Opportunities to incorporate these pro-osteogenic EVs into injectable biomaterials will be discussed, in particular the formulation of microcapsules and fluid-gels. In summary, incorporation of EVs in tissue-engineered scaffolds has the potential to deliver all the advantages of a cell-based therapy but without using viable cells. The advantages of this approach may represent a new phase of tissue engineering.

Heterotopic ossification is the formation of lamellar bone in soft tissues and is a common complication of high-energy combat injury. This disabling condition can cause pain, joint ankylosis, and skin ulceration in the residua of amputees. This project is aimed at developing a novel treatment to dissolve hydroxyapatite in heterotopic ossification and prevent the crystallisation of this this mineral at sites of ectopic bone formation. Previously reported results demonstrated that hexametaphosphate could dissolve hydroxyapatite at physiological pH. Further work has been undertaken to investigate the mechanism of this dissolution and establish a means of temporal control of action. In addition, physicochemical analyses of samples of human heterotopic ossification have yielded important insights into the nature of this pathological tissue. Techniques include mapped micro X-ray fluorescence, mapped Raman spectroscopy, scanning electron microscopy, and micro computed tomography. Formulation engineering work has begun in order to develop an appropriate delivery vehicle for this agent. This includes rheological testing and hexametaphosphate elution profiles. Finally, micro CT analysis has shown that hexametaphosphate is able to dissolve human heterotopic ossification tissue. In summary, this work has moved us closer towards our goal of a novel injectable agent for the treatment and prevention of heterotopic ossification.

Background

Bone is a hierarchically structured hard tissue that consists of approximately 70 wt% low-crystallinity hydroxyapatite. Intricate tubular channels, such as Haversian canals, Volkman's canals, and canaliculi are a preserved feature of bone microstructure. These structures provide pathways for vasculature and facilitate cell-to-cell communication processes, together supporting viability of cellular components and aiding in remodeling processes. Unfortunately, many commercial bone augmentation materials consist of highly crystalline phases that are absent of the structuring present within the native tissue they are replacing. This work reports on a the development of a novel bone augmentation material that is able to generate biologically analogous tubular calcium phosphate mineral structures from hydrogel-based spheres that can be packed into defects similar to those encountered in vivo.