Extrusion-based 3D-printed porous iron as a bone substitute scaffold.
Extrusion-based 3D-printed porous iron as a bone substitute scaffold.

Biodegradable metals such as iron, magnesium, and zinc could make ideal temporary bone substitutes because they degrade gradually as new bone regrows. Researchers from Delft University of Technology have taken a systematic look at porous iron, which is stronger than either magnesium or zinc, to assess its potential as a scaffold for bone repair [Putra et al., Acta Biomaterialia (2020), https://doi.org/10.1016/j.actbio.2020.11.022].

"In comparison with other biodegradable metals or polymers for bone implants, iron has a high mechanical strength, which allows for the design and fabrication of porous structures for the treatment of critical bony defects," says Amir A. Zadpoor, who led the study.

Iron is also used by the body to transport oxygen, accelerates enzyme reactions, plays a role in the immune system, and is essential to bone regeneration. But previous attempts to make bone scaffolds using fabrication methods used to, such as powder bed fusion, had limitations. So Zadpoor and his colleagues developed an alternative additive manufacturing technology using extrusion-based 3D printing.

"We wanted to verify the feasibility of applying extrusion-based 3D printing to fabricate porous iron and explore the potential of resolving the fundamental issue of bulk iron, which has a very low biodegradation rate, while maintaining other important properties such as structural integrity and mechanical properties during the bone healing period," say Zadpoor and coauthors Niko E. Putra and Jie Zhou.

In this approach, particulate iron is mixed with a polymer solution to form an ink, which is deposited layer by layer to build up a three-dimensional structure. The scaffold is heated, initially to drive off the polymer, and then at a higher temperature to fuse the iron particles together into a porous solid. The iron forms a hierarchical structure with macroscale pores and micropores within the supporting struts. When immersed in simulated body fluid, the porous iron has an accelerated biodegradation rate, losing 7% of its mass over 28 days, because of its much larger surface area.

Corrosion occurs throughout the scaffold, even inside the pores, creating a mixture of iron-, oxygen-, and carbon-rich products and trace elements including sodium, calcium, and phosphorus. The scaffold’s mechanical properties, however, remain within the range of porous bone.

"[We have confirmed] that extrusion-based 3D printing can deliver porous iron scaffolds with enhanced biodegradability and bone-mimicking mechanical properties for potential application as bone substitutes," say Zadpoor, Putra, and Zhou. "We are now exploiting the capabilities of this 3D printing technology to achieve other functionalities desired for bone-substitution applications."

Nanobioceramics could be fused with the iron scaffold to promote bone growth, as well as antibacterial agents to prevent infections or drugs to treat bone diseases.