Bioprinting of cell-laden hydrogels onto titanium alloy surfaces to produce a bioactive interface

Chemical etching of Ti-6Al-4V biomaterials fabricated by selective laser melting enhances mesenchymal stromal cell mineralization

Porous titanium scaffolds fabricated by powder bed fusion additive manufacturing techniques have been widely adopted for orthopedic and bone tissue engineering applications. Despite the many advantages of this approach, topological defects inherited from the fabrication process are well understood to negatively affect mechanical properties and pose a high risk if dislodged after implantation. Consequently, there is a need for further post-process surface cleaning. Traditional techniques such as grinding or polishing are not suited to lattice structures, due to lack of a line of sight to internal features. Chemical etching is a promising alternative; however, it remains unclear if changes to surface properties associated with such protocols will influence how cells respond to the material surface. In this study, we explored the response of bone marrow derived mesenchymal stem/stromal cells (MSCs) to Ti-6Al-4V whose surface was exposed to different durations of chemical etching. Cell morphology was influenced by local topological features inherited from the SLM fabrication process. On the as-built surface, topological nonhomogeneities such as partially adhered powder drove a stretched anisotropic cellular morphology, with large areas of the cell suspended across the nonhomogeneous powder interface. As the etching process was continued, surface defects were gradually removed, and cell morphology appeared more isotropic and was suggestive of MSC differentiation along an osteoblastic-lineage. This was accompanied by more extensive mineralization, indicative of progression along an osteogenic pathway. These findings point to the benefit of post-process chemical etching of additively manufactured Ti-6Al-4V biomaterials targeting orthopedic applications.

Cell seeding via bioprinted hydrogels supports cell migration into porous apatite-wollastonite bioceramic scaffolds for bone tissue engineering

Cell seeding via cell-laden hydrogels offers a rapid way of depositing cells onto a substrate or scaffold. When appropriately formulated, hydrogels provide a dense network of fibres for cellular encapsulation and attachment, creating a protective environment that prevents cells to be washed away by media. However, when incorporating hydrogels into a cell seeding strategy the cellular capacity for migration from a hydrogel network and subsequent biofunctionality must be assessed. Here, we compare cell seeding via a bioprinted hydrogel with conventional manual cell seeding in media. To this end, we use a binder jet 3D printed bioceramic scaffold as a model system for bone tissue engineering and the reactive jet impingement (ReJI) bioprinting system to deliver high cell density cell-laden hydrogels onto the surface of the scaffolds. The bioceramic scaffolds were produced in apatite-wollastonite (AW) glass-ceramic, with a total porosity of ~50 %, with pore size predominantly around 50-200 μm. Bone marrow-derived mesenchymal stromal cells were seeded onto the porous AW substrate both in media and via ReJI bioprinting. Cell seeding in media confirmed the osteoinductive nature and the ability of the scaffold to support cell migration within the porous structure. Cell seeding via ReJI bioprinting demonstrated that the cell-laden hydrogel penetrated the porous AW structure upon hydrogel deposition. Furthermore, cells would then migrate out from the hydrogel network and interact with the bioceramic substrate. Overall, levels of cell migration and mineralisation were significant and comparable for both seeding approaches. However, cell seeding via bioprinted hydrogels may serve as an effective strategy for in situ cell seeding into implants, which is desired in clinical tissue engineering procedures, avoiding the time taken for cell attachment from media, and the requirement to maintain a specific orientation until attachment has occurred.

Platelet-derived extracellular vesicles formulated with hyaluronic acid gels for application at the bone-implant interface: An animal study

Background/Objective

Platelet derived extracellular vesicles (pEV) are promising therapeutical tools for bone healing applications. In fact, several in vitro studies have already demonstrated the efficacy of Extracellular Vesicles (EV) in promoting bone regeneration and repair in various orthopedic models. Therefore, to evaluate the translational potential in this field, an in vivo study was performed.

Methods

Here, we used hyaluronic acid (HA) gels formulated with pEVs, as a way to directly apply pEVs and retain them at the bone defect. In this study, pEVs were isolated from Platelet Lysate (PL) through size exclusion chromatography and used to formulate 2% HA gels. Then, the gels were locally applied on the tibia cortical bone defect of New Zeland White rabbits before the surgical implantation of coin-shaped titanium implants. After eight weeks, the bone healing process was analyzed through biomechanical, micro-CT, histological and biochemical analysis.

Results

Although no biomechanical differences were observed between pEV formulated gels and non-formulated gels, biochemical markers of the wound fluid at the interface presented a decrease in Lactate dehydrogenase (LDH) activity and alkaline phosphatase (ALP) activity for pEV HA treated implants. Moreover, histological analyses showed that none of the treatments induced an irritative effect and, a decrease in the fibrotic response surrounding the implant for pEV HA treated implants was described.

Conclusion

In conclusion, pEVs improve titanium implants biocompatibility at the bone-implant interface, decreasing the necrotic effects of the surgery and diminishing the fibrotic layer associated to the implant encapsulation that can lead to implant failure.