Additively manufactured biodegradable porous iron.

Additively manufactured (AM) topologically ordered porous metallic biomaterials with the proper biodegradation profile offer a unique combination of properties ideal for bone regeneration. These include a fully interconnected porous structure, bone-mimicking mechanical properties, and the possibility of fully regenerating bony defects. Most of such biomaterials are, however, based on magnesium and, thus, degrade too fast. Here, we present the first report on topologically ordered porous iron made by Direct Metal Printing (DMP). The topological design was based on a repetitive diamond unit cell. We conducted a comprehensive study on the in vitro biodegradation behavior (up to 28 days), electrochemical performance, time-dependent mechanical properties, and biocompatibility of the scaffolds. The mechanical properties of AM porous iron (E = 1600-1800 MPa) were still within the range of the values reported for trabecular bone after 28 days of biodegradation. Electrochemical tests showed up to ≈12 times higher rates of biodegradation for AM porous iron as compared to that of cold-rolled (CR) iron, while only 3.1% of weight loss was measured after 4 weeks of immersion tests. The biodegradation mechanisms were found to be topology-dependent and different between the periphery and central parts of the scaffolds. While direct contact between MG-63 cells and scaffolds revealed substantial and almost instant cytotoxicity in static cell culture, as compared to Ti-6Al-4V, the cytocompatibility according to ISO 10993 was reasonable in in vitro assays for up to 72 h. This study shows how DMP could be used to increase the surface area and decrease the grain sizes of topologically ordered porous metallic biomaterials made from metals that are usually considered to degrade too slowly (e.g., iron), opening up many new opportunities for the development of biodegradable metallic biomaterials. STATEMENT OF SIGNIFICANCE: Biodegradation in general and proper biodegradation profile in particular are perhaps the most important requirements that additively manufactured (AM) topologically ordered porous metallic biomaterials should offer in order to become the ideal biomaterial for bone regeneration. Currently, most biodegradable metallic biomaterials are based on magnesium, which degrade fast with gas generation. Here, we present the first report on topologically ordered porous iron made by Direct Metal Printing (DMP). We also conducted a comprehensive study on the biodegradation behavior, electrochemical performance, biocompatibility, and the time evolution of the mechanical properties of the implants. We show that these implants possess bone-mimicking mechanical properties, accelerated degradation rate, and reasonable cytocompatibility, opening up many new opportunities for the development of iron-based biodegradable materials.

Additively manufactured biodegradable porous magnesium.

An ideal bone substituting material should be bone-mimicking in terms of mechanical properties, present a precisely controlled and fully interconnected porous structure, and degrade in the human body to allow for full regeneration of large bony defects. However, simultaneously satisfying all these three requirements has so far been highly challenging. Here we present topologically ordered porous magnesium (WE43) scaffolds based on the diamond unit cell that were fabricated by selective laser melting (SLM) and satisfy all the requirements. We studied the in vitro biodegradation behavior (up to 4 weeks), mechanical properties and biocompatibility of the developed scaffolds. The mechanical properties of the AM porous WE43 (E = 700-800 MPa) scaffolds were found to fall into the range of the values reported for trabecular bone even after 4 weeks of biodegradation. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), electrochemical tests and µCT revealed a unique biodegradation mechanism that started with uniform corrosion, followed by localized corrosion, particularly in the center of the scaffolds. Biocompatibility tests performed up to 72 h showed level 0 cytotoxicity (according to ISO 10993-5 and -12), except for one time point (i.e., 24 h). Intimate contact between cells (MG-63) and the scaffolds was also observed in SEM images. The study shows for the first time that AM of porous Mg may provide distinct possibilities to adjust biodegradation profile through topological design and open up unprecedented opportunities to develop multifunctional bone substituting materials that mimic bone properties and enable full regeneration of critical-size load-bearing bony defects. STATEMENT OF SIGNIFICANCE: The ideal biomaterials for bone tissue regeneration should be bone-mimicking in terms of mechanical properties, present a fully interconnected porous structure, and exhibit a specific biodegradation behavior to enable full regeneration of bony defects. Recent advances in additive manufacturing have resulted in biomaterials that satisfy the first two requirements but simultaneously satisfying the third requirement has proven challenging so far. Here we present additively manufactured porous magnesium structures that have the potential to satisfy all above-mentioned requirements. Even after 4 weeks of biodegradation, the mechanical properties of the porous structures were found to be within those reported for native bone. Moreover, our comprehensive electrochemical, mechanical, topological, and biological study revealed a unique biodegradation behavior and the limited cytotoxicity of the developed biomaterials.