Bioabsorbable metals hold a lot of potential as orthopaedic implant materials. Three metal families are currently being investigated: iron (Fe), magnesium (Mg) and zinc (Zn). Currently, however, biodegradation of such implants is poorly predictable. We thus used Direct Metal Printing to additively manufacture porous implants of a standardized bone-mimetic design and evaluated their mechanical properties and degradation behaviour, respectively, under in vivo-like conditions.

Atomized powder was manufactured to porous implants of repetitive diamond unit cells, using a ProX DMP 320 (Layerwise, Belgium) or a custom-modified ReaLizer SLM50 metal printer. Degradation behaviour was characterized under static and dynamic conditions in a custom-built bioreactor system (37ºC, 5% CO₂ and 20% O₂) for up of 28 days. Implants were characterized by micro-CT before and after in vivo-like degradation. Mechanical characterization (according to ISO 13314: 2011) was performed on an Instron machine (10kN load cell) at different immersion times in simulated body fluid (r-SBF). Morphology and composition of degradation products were analysed (SEM, JSM-IT100, JEOL). Topographically identical titanium (Ti-6Al-4V, Ti64) specimen served as reference.

Micro-CT analyses confirmed average strut sizes (420 ± 4 μm), and porosity (64%), to be close to design values. After 28 days of in vivo-like degradation, scaffolds were macroscopically covered by degradation products in an alloy-specific manner. Weight loss after cleaning also varied alloy-specifically, as did the change in pH value of the r-SBF. Corrosion time-dependent changes in Young's moduli from 1200 to 800 MPa for Mg, 1000 to 700 MPa for Zn and 48-8 MPa for iron were statistically significant.

In summary, DMP allows to accurately control interconnectivity and topology of implants from all three families and micro-structured design holds potential to optimize their degradation speed. This first systematic report sheds light into how design influences degradation behaviour under in vivo-like conditions to help developing new standards for future medical device evaluation.