Stimulatory effect of cobalt ions incorporated into calcium phosphate coatings on neovascularization in an in vivo intramuscular model in goats

Rapid vascularization of bone graft substitutes upon implantation is one of the most important challenges to overcome in order to achieve successful regeneration of large, critical-size bone defects. One strategy for stimulating vascularization during the regeneration process is to create a hypoxic microenvironment by either directly lowering the local oxygen tension, or by applying hypoxia-mimicking factors. Cells compensate for the hypoxic condition by releasing angiogenic factors leading to new blood vessel formation. In the present study, we explored the potential of cobalt ions (Co2+), known chemical mimickers of hypoxia, to stimulate vascularization within a bone graft substitute in vivo. To this end, Co2+ ions were incorporated into calcium phosphate (CaPs) coatings deposited on poly(lactic acid) (PLA) particles with their effect on the formation of new blood vessels studied upon intramuscular implantation in goats. PLA particles and CaP-coated particles without Co2+ ions served as controls. Pathological scoring of the inflammatory response following a 12-week implantation period showed no significant differences between the four types of materials. Based on histological and immunohistochemical analyses, both blood vessel area and number of blood vessels in CaP-coated PLA particles containing Co2+. were higher than in the uncoated PLA particles and CaP-coated PLA particles without Co2+. Analysis of blood vessel size distribution indicated abundant formation of small blood vessels in all the samples, while large blood vessels were predominantly found in PLA particles coated with CaP containing Co2+ ions. The results of this study support the use of CaPs containing Co2+ ions to enhance vascularization in vivo. Statement of Significance In this work, we have investigated the potential of cobalt ions, incorporated into thin calcium phosphate (CaP) coatings that were deposited on particles of poly(lactic acid) (PLA), to induce neovascularization in vivo. Qualitative and quantitative histological and immunohistochemical analyses showed that both the number of blood vessels and the total blood vessel area were higher in CaP-coated PLA particles containing cobalt ions as compared to the uncoated PLA particles and CaP-coated PLA particles without the metallic additive. Furthermore, a wider distribution of blood vessel sizes, varying from very small to large vessels was specifically observed in samples containing cobalt ions. This in vivo study will significantly contribute to the existing knowledge on the use of bioinorganics, which are simple and inexpensive inorganic factors that can be used to control relevant biological process during tissue regeneration, such as vascularization. As such, we are convinced that this manuscript will be of interest to the readers of Acta Biomaterialia.