The Effects of Structural & Materials Design on the Mechanisms of Tissue Integration with the 3D Printed Polyether-Ether-Ketone Cranial Implants in Vivo

3D printed polyether-ether-ketone (PEEK) implant has become a popular clinical alternative to implants made of titanium alloy for cranial bone substitutes due to its bone-match properties, however its biological inert hindered the progress of clinical applications of such material. To enhance the tissue integration capability of PEEK implants and promote their long-term stability, cranial implants of gradient porous structures were designed and manufactured via fused filament fabrication (FFF) 3D printing technology in both PEEK and PEEK/HA composites materials, then functionally evaluation of the implants on the tissue in-growth and the osteointegration mechanisms was conducted via animal tests. The 3D printed PEEK scaffold was found to have 2–5 folds of the compressive strength to those of the natural cranial bone. The in vivo studies verified that the porous PEEK/HA scaffold could effectively induce the bone ingrowth to form a stable biointegration boundary surrounding the host bone tissue after 4 weeks of implantation. Moreover, the PEEK/HA scaffold showed no significant advantages in improving the soft tissue in-growth, only making its distribution more evenly. It is also interestingly to find out that the vertically connective pores throughout the implant did not enhance the tissue binding force even though it did promote the nutrient transportation. In conclusion, the use of PEEK/HA composite material and a well-designed porous structure was proved to be an effective approach to improve the biointegration between the implant and host tissues.