Host responses to implants revealed by intravital microscopy

Biomaterials, biomedical devices and engineered cell grafts can be implanted to restitute tissue anatomy and function. Such implants can either integrate physiologically, with no or minimal scarring, or induce chronic inflammation and the foreign body response, which leads to graft failure. Intravital microscopy in small animal models can be applied to visualize the structure and integration of implanted natural and synthetic fibres, metals, cells within hydrogel carriers and engineered tissues, as well as the stepwise cellular and molecular tissue response. In this Review, we discuss how intravital microscopy can reveal regenerative and pathological responses to implants, including wound healing and graft integration, which depend on the time-controlled activation of macrophages and early neovascularization, and adverse reactions, such as the foreign body response and fibrosis. The combination of improved materials designs, detection of cell signalling using molecular reporters and targeted intervention will allow the development of strategies to improve graft integration and functionality. The integration and function of implanted biomaterials and devices depend on the responses of the host tissue, such as macrophage activation and neovascularization. This Review discusses how intravital microscopy can be applied to assess the mechanisms underlying regenerative and pathological responses to implants.