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Cut sections of aluminum-steel cast foam and 3.7- and 1.4-mm steel-steel powder metallurgy foam are examples of composite metal foams that could potentially be used in orthopedic implant applications.
Scientists at North Carolina State University (NC State; Raleigh; www.ncsu.edu) are developing a novel metal foam that they hope could serve as a replacement material for damaged bone in future orthopedic and dental implant applications. Because the material's modulus of elasticity is similar to that of bone, Professor Afsaneh Rabiei and former NC State PhD student Lakshmi Vendra believe that it can prevent bone rejection, which often occurs with more-rigid implant materials such as titanium.
Lighter than solid metals, composite metal foam (CMF) can be fabricated from a variety of different alloys, says Rabiei, an associate professor of mechanical and aerospace engineering and an associate member of the biomedical engineering faculty at NC State. CMF is made using prefabricated hollow spheres formed in a metallic matrix, both of which are made from the same material or two different materials. It is manufactured by casting molten metal around the hollow spheres or by mixing the spheres with metal powder and baking them in a furnace. The bone implant itself can be manufactured by casting or hot-pressing the CMF in a mold or by machining it to the desired shape.
"At first, we made steel-steel and aluminum-steel versions," Rabiei notes, "but now we are able to make our composite metal foams out of titanium, cobalt-chromium, and other metals or their combinations."
The composite foam is about 65% lighter than the bulk metal from which it is made, Rabiei remarks. "This means that it is about 65% porous. We can change the porosity percentage by controlling the diameter and wall thickness of the spheres. That way, we can match it with patients' bone porosity, considering their age or the condition of their bones."
Because it equalizes the load-bearing ability of the natural bone and the implant, the material is suitable for bone-replacement applications. "When an implant is placed in the bone, the two need to handle the load together," Rabiei explains. "If the bone's modulus of elasticity is lower than that of the implant, the implant will take over the bone's load-bearing function, causing the surrounding bone to die." This phenomenon, known as stress shielding, loosens the implant, resulting in eventual failure and the need for revision surgery. While bone's modulus of elasticity--the measurement of a material's ability to deform under pressure and then return to its original shape when the pressure is removed--lies between 10 and 30 GPa, titanium's modulus is approximately 100 GPa. In contrast, CMF has a modulus that is consistent with bone. In addition, the porous, lightweight material exhibits high-energy absorption capability, and its rough surface fosters bone in-growth.
A major goal of medical research is the development of implants with osseointegrative properties. The NC State scientists' metal foam material fulfills that function by allowing the bone to grow into the implant's porosities, enabling it to become anchored inside the bone. "Even if you use it together with a bone cement, the cement can form a nice interlock with the porosities of the foam," Rabiei says. "That secures the implant in the bone."
In addition to its potential benefits as a bone-replacement material, the CMF could be used in any application requiring a light, strong material, including medical devices for use inside or outside of the body.