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Functionally Graded Implants Fuse Strength with a Bioactive Surface

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A computer-controlled solid freeform fabrication technique enables the development of implants that feature a titanium-rich core with an HA-rich surface.
To many experts, the ideal orthopedic implant would be bioactive and durable while facilitating osseointegration. Yet in the quest to develop such an implant, the most favorable combination of materials and coatings for fabricating such a product seems to be elusive. Professors Leon Shaw and Yong Wang from the chemicals, materials, and biomolecular engineering department at the University of Connecticut (UConn; Storrs, CT; have made progress, however, through their research on functionally graded hip implants that exploit only the most desirable properties of hydroxyapatite (HA) and titanium.

Although prized by the orthopedics industry for its bioactive nature, HA is too brittle for use as the primary hip implant material and can experience significant problems during processing. HA powder is vulnerable to fracture, for instance, and can also transform to amorphous calcium phosphate when exposed to extremely high temperatures during thermal spraying, according to Shaw. In contrast, titanium is valued for its corrosion resistance and mechanical strength—a property that is essential for load-bearing implants. But it is inert. And because the material does not promote bone in-growth or cellular adhesion, an implant with a titanium surface is susceptible to loosening over time and often requires revision procedures.

In order to offset titanium’s bioinert nature while benefitting from its mechanical strength, many engineers have been exploring the efficacy of titanium implants featuring an HA-coated surface. But this configuration still does not address HA’s processing challenges or potential for delamination, according to Shaw. What will, he proposes, is the development of functionally graded implants consisting of a titanium-rich core that transitions into an HA-rich surface. “Our idea is to have a bioactive surface and the center is titanium rich so you can carry loads. Because the surface is bioactive, we actually could induce bone in-growth, so adhesion will be good and healing will be very fast,” Shaw says. The researchers have identified 40% HA and 60% titanium as the target composition on the surface of the implant.

Fabrication of these graded implants is achieved through a computer-controlled solid freeform fabrication technique coined by the researchers as the slurry mixing and dispensing (SMD) process. “This [implant] is made by a layer-by-layer technique,” Shaw explains. “The idea is that you want to change the composition gradually, so at different locations, you have different compositions.”

This method is unique, Shaw states, because conventional layering processes produce a plane geometry that requires machining to shape the part. Ultimately, such actions alter the material composition, he adds. The SMD process, on the other hand, does not require machining. Instead, the layered powder is converted into a solid piece through sintering operations, which eliminate the need for machining and preserve the original compositions. Furthermore, the graded construction does not produce sharp interfaces, thereby eliminating the risk of delamination, according to Shaw.

Aided by a three-year grant from the National Science Foundation, the UConn researchers hope to produce a proof of concept within the next two years and then optimize the implant the following year.
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