A dissolvable metal bone implant that gets stronger under pressure

For millions suffering severe fractures, today's reality looks far less elegant. Surgeons rely on permanent titanium or stainless steel plates that remain trapped in the body long after the bone mends. These rigid fixtures can weaken neighboring tissue by shielding it from natural bodyweight, trigger chronic aches, and often require invasive removal operations.

Biodegradable zinc seemed like the ideal candidate to fix this because it dissolves naturally in bodily fluids. Yet metallurgy has long been stuck in a frustrating bind: zinc’s atomic architecture makes it naturally brittle. If we process it to be tough enough to absorb sudden jolts, it becomes too soft to carry bodyweight; if we make it stiff and strong, it snaps like chalk under tension.

A research team has improved this balance by using laser powder bed fusion to create alternating coarse- and fine-grained regions in a zinc–copper alloy. Published in the International Journal of Extreme Manufacturing, the study links this layered structure to improved strength and ductility

Zinc has a hexagonal close-packed crystal structure that can make deformation difficult to coordinate between grains. In the printed alloy, finer grains provide stronger barriers to dislocation motion, while coarser grains deform more readily. Bringing these regions together helps the material accommodate strain while resisting further deformation.

To solve this, the engineers turned to laser powder bed fusion, sweeping an infrared beam across fine metallic dust to melt it layer by layer. The researchers switched between two combinations of laser power and scanning speed after every ten deposited layers, alternating the energy density to control grain growth. Higher energy input favoured grain growth, whereas lower energy input promoted faster cooling and finer grains. The resulting fine- and coarse-grained regions were about 100 and 200 µm wide, respectively, with average grain sizes of 3.7 and 9.2 µm.

This alternating pattern acts like miniature corrugated cardboard hidden inside a solid metal block. When walking or jumping pulls on the implant, the softer, coarse-grained bands stretch first to cushion the blow. As these pliable layers flex, microscopic lattice defects called dislocations slip forward and pile up against the rigid, fine-grained walls. This atomic traffic jam creates an internal back-pressure that spreads peak stress evenly throughout the implant, stopping local tears before they can turn into cracks.

The resulting composite withstood 245.6 MPa of pulling force while stretching 12.7% before failure, a rare combination of strength and ductility for printed zinc. Its compressive resistance jumped past 416 MPa, more than double that of raw zinc and comfortably exceeding the forces generated by a human stride.

The layered architecture also solves a biological timing puzzle. Because grains of different sizes corrode at slightly different rates, microscopic electrical currents form across the layers, allowing the alloy to dissolve smoothly at 0.47 millimeters per year. This pace precisely matches the months-long window needed for a skeleton to rebuild itself. Along the way, the implant safely releases zinc and copper ions that killed 91.2% of Staphylococcus aureus bacteria and encouraged fresh bone to weave through test scaffolds in rabbits without damaging healthy cells.

For the medical device industry, the process is entirely digital, eliminating the need for expensive post-casting presses or high-heat ovens. Before these dissolving supports arrive in orthopedic clinics, the team plans to run long-term durability tests under millions of repetitive impacts to ensure the layered boundaries hold firm inside diverse bone structures.

International Journal of Extreme Manufacturing (IJEM, IF: 25.1) is devoted to publishing articles of the highest quality and significance to pushing the limits of scales, precision, performance and environments in manufacturing.

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