Gyroids are oddly shaped objects: their curved geometrical structures contain no straight lines and never self-intersect, and yet they remain infinitely connected. These gyroid structures can also be found in nature, as photonic nanostructures in the wings of the Callophrys rubi butterfly.
The highly curved surfaces in gyroids also make them excellent candidates for artificial bone implants. The only problem? Getting their pore sizes right remains a challenge—if the pores are too big, bone cells cannot attach to the scaffold; if the pores are too small, the transport of nutrients to the bone tissue is hindered.
To enhance the biocompatibility of 3D-printed bone implants, researchers in Singapore designed a heterogeneous gyroid structure that solved both problems: millimeter-scale pores for nutrient and oxygen exchange, and micrometer-scale pores for cell adhesion and growth.
“Biostructural and mechanical compatibility are the most important factors for the success of an artificial implant in the human body,” said Pan Wang, a Scientist at A*STAR’s Singapore Institute of Manufacturing Technology (SIMTech) and the lead author of the study. “In addition, the implants also need to have similar stiffness and strength to bone to avoid stress-shielding effects, which will lead to the loss of surrounding bone mass.”
While searching for an ideal artificial bone implant, the team reviewed an internally developed database for suitable initial gyroid structures. Using a simulation technique called finite element modeling, they then developed modified gyroid 3D lattices with the desired pore size and mechanical properties using detailed visualizations of stress distribution patterns.
Next, the researchers selected a Ti-6Al-4V alloy for their implant material and successfully fabricated five types of gyroid structures by electron beam melting, a type of metal 3D printing. The lattices had variable cell wall thicknesses and pore sizes and possessed a range of Young’s modulus from 8 to 15 GPa and compressive strength from 150 to 250 MPa. These mechanical properties are well within the range observed with human bone, Wang noted.
Introducing numerous pores within the gyroid structures also distributed stress more evenly, allowing the implants to deform more stably and avoid brittle failure. Furthermore, these pores also enhance the biological function of metallic lattices, Wang added. “After implantation, the metallic ‘skeleton’ populated with cells will grow with the femur and its mechanical properties will gradually change to a bone-like one,” he said.
Wang believes that their research findings will support the development of more biocompatible implants as well as further knowledge into lattice design and 3D printing. “Our target is to develop permanent orthopedic implants that can fully eliminate stress shielding and last beyond a patient’s lifetime,” he said.
The A*STAR-affiliated researchers contributing to this research are from the Singapore Institute of Manufacturing Technology (SIMTech).