Bioactive Nanopatterning Technology Could Yield Long-Lasting PEEK Implants

Expectations for next-generation orthopedic implants have changed in recent years as such factors as a massive aging population, an increasing volume of younger recipients, physically active patients, and longer life spans have spurred a significant demand for stronger, more durable devices. With an eye toward the future, researchers at the University of Glasgow (Scotland) have developed a technique for creating bioactive nanopatterns on PEEK surfaces that holds the potential for dramatically improving implant fixation and longevity.

Engineers have historically turned to metals to provide the necessary strength and durability for orthopedic implants such as knees and hips. But the use of metals for hip implants, in particular, has begun to fall out of favor in the wake of several high-profile metal-on-metal implant recalls and the identification of such issues as dangerous wear particle production and stress shielding.  Furthermore, conventional orthopedic metals, as well as polymers and ceramics, are biologically inert--a quality that designers formerly prized but are now seeing as a potential drawback.

"[Current] orthopedic components come into contact with the bone marrow--containing mesenchymal stem and progenitor cells--and bone containing osteoblasts. Inert materials contain little 'information' for the cells to differentiate. This can lead to soft-tissue encapsulation and subsequent micromotion of the implant, ultimately leading to failure," explains Matthew Dalby of the university's Institute of Molecular, Cell, and Systems Biology. "Strategies like hydroxyapatite (HA) incorporation have been employed, but HA is osseoconductive rather than inductive; it promotes migration of osteoblasts but not osteoblast formation from mesenchymal stem cells. Also, roughness has been used, but it is hard to control and describe, and this can lead to reproducibility issues." In order to exchange these pitfalls for progress, engineers need to explore the use of bioactive materials to stimulate bone cell response and, ultimately, improve secondary implant fixation, according to Dalby.

Among the most promising materials for next-generation implants, the researchers state, is PEEK, which has experienced success in the spinal implant market but has been slow to penetrate broader orthopedic applications. "PEEK has mechanical properties similar to those of natural bone and thus will allow the body's natural remodeling to continue undisturbed," notes Nikolaj Gadegaard, senior lecturer in biomedical engineering at the university. "This leads to little or no bone loss around the implant, which is a great benefit for the patient." PEEK offers the added benefits of being easily and cost-effectively processed via injection molding.

The catch, however, is that PEEK is also a biologically inert material. To clear this design hurdle, the researchers have developed a technique for nanopatterning PEEK implant surfaces that renders them bioactive. Serving to encourage mesenchymal cells to form bone, the treated PEEK surface consists of a series of tiny pits that measure 120 nm in diameter and 100 nm deep.

"Previously, two strategies had been used for surface preparation: rigid order--techniques such as electron-beam lithography can give subnanometer accuracy--and total randomness, including sanding, blasting, anodizing, etc. Total order gave very little result in terms of osseoinduction, and randomness is hard to describe and control and, thus, results can vary," Dalby says. "What we did was take the middle route. We put our pits in a square lattice with 300-nm center-center spacing and then added 'controlled disorder'--up 50-nm displacement from the center position in x and y. This adjustment gave us the osseoinductive properties we had been looking for."

Imparting these osseoinductive properties to the PEEK surface paves the way for a range of exceptionally long-lasting finger, wrist, shoulder, spine, knee, and hip implants, according to the researchers. On the road to reaching that milestone, however, they are laboring to translate the Petri-dish-proven nanopatterned implants into working prototypes for preclinical evaluation.

"Besides exploring the discovered biological effects at a fundamental level, we are also exploring what other patterns we can define and use for stem cell control," Gadegaard says. "The advantage of our technology is that any new discovery can be directly fed into the pipeline we have developed for translation."