Breakthrough and Challenges for Biodegradable Zn Alloys: Grain Coarsening as a Strategy for Superior Strength and Creep Resistance
Biodegradable zinc (Zn) alloys have emerged as viable candidates for medium- to high-load-bearing implants, outperforming magnesium alloys in strength and ductility while exhibiting in vivo degradation rates that align with vascular (3–6 months) and bone (2–3 months) healing. Clinical translation has advanced to maxillofacial fixation, interference screws, and drug-eluting coronary stents. Alloy design has evolved from Zn-Mg and Zn-Li systems to Zn-Mn alloys that achieve 143% room-temperature elongation via surface-roughness-induced plasticity. Conventional strengthening relies on grain refinement, yet fine-grained Zn alloys suffer reduced creep resistance at body temperature (0.45 homologous temperature). A paradigm shift was introduced by Nie’s group: coarse-grained Zn-0.2Mg (47 μm) exhibits higher yield strength than fine-grained (11 μm) counterparts, defying the Hall-Petch relationship. This anomaly arises from accommodation twinning that maintains intergranular cohesion, shifting deformation from intergranular to intragranular mechanisms. Multi-component alloys (Zn-0.2Mg-0.1Cu-0.2Mn) achieve compressive yield strengths exceeding 400 MPa with grain sizes of 10–50 μm, alongside improved creep resistance and acceptable cytotoxicity relative to Mg-Zn-Ca. A 6-week in vivo study confirmed appropriate degradation. These findings establish grain coarsening as a counterintuitive but effective pathway for designing stronger, more durable biodegradable Zn alloys for load-bearing applications.