Biodegradable metals, also known as bioresorbable or bioabsorbable metals, are compatible with human tissues and degrade to nontoxic by-products. The most promising candidates for use as orthopedic and cardiovascular implants are alloys of magnesium, which biodegrade in 6–15 months, and alloys of iron, which biodegrade in 12–36 months. Both types of alloy degrade by corrosion—the oxidation and dissolution of the metals. See also: Alloy; Corrosion; Iron; Iron alloys; Magnesium; Magnesium alloys

Severe bone fractures require the support of stainless steel or titanium implants, such as plates, rods, and screws, to keep the bones properly aligned during the healing process. Because these metals are not biodegradable, implants may need to be removed after fracture healing in cases of growing children or infection, for example. For orthopedic applications, certain polymeric materials, such as poly(lactic acid) [PLLA], have been found to effectively biodegrade but they lack the load-bearing capacity to stabilize and support most bone fractures. Research for higher-strength polymeric materials is proceeding, however. Biodegradable magnesium alloys have comparable strength-to-weight ratios as steel alloys but currently suffer from too rapid degradation for orthopedic applications. See also: Absorbable orthopedic implants; Bone disorders; Surgery

So far, the most advanced biodegradable-metal application is for magnesium alloy stents. Stents are small, expandable mesh tubes that restore or improve blood flow through narrowed, blocked, or weakened arteries. Biodegradable magnesium alloys contain up to 7 percent of other elements, such as aluminum, calcium, cerium, manganese, silver, strontium, yttrium, zinc, and zirconium. Clinical trials have found the mechanical properties of magnesium stents to be similar to those of stainless-steel stents. Compared to PLLA stents, biodegradable metallic stents are thinner and easier to implant at the correct position in the body. In addition, magnesium stents tend to stay attached to the vessel wall more reliably than PLLA stents after 6 and 12 months. See also: Blood vessels; Heart disorders

A second-generation magnesium stent coated with drugs that help prevent an artery from becoming blocked again (restenosis) is being tested. Still, winning acceptance for magnesium stents may be challenging because stainless steel and PLLA stents are already medically proven products. But in a futuristic application, Dae-Hyeong Kim and coworkers at Seoul National University demonstrated a working model of a magnesium stent that incorporated additional biodegradable components, including blood-flow sensors, flexible electronic memory circuits, ceria nanoparticles embedded in a film of PLLA to reduce inflammation, and gold and silica nanoparticles that release drugs when heated with infrared light-emitting diodes (LEDs) to prevent restenosis. See also: Biosensor; Light-emitting diode; Nanoparticles; Polymers for drug delivery; Printable semiconductors for flexible electronics