Orthopedic and spinal device OEMs are teaming up with suppliers that are developing new and novel technologies for advanced materials, such as PEEK-Optima polymer by Invibio (invitio.com), West Conshohocken, PA. The PEEK-Optima polymer is a high-performance, biocompatible polymer with distinct advantages and benefits over other implant materials such as bone, metals, and other polymers.

With the device industry looking to use PEEK in a myriad of applications, there has been a search to complement the polymer with surface-enhancing technology that would solve one of the material’s limitations. PEEK, though noted for its bio-inert properties, has little direct bone attachment. In other words, bone treats the polymer as inert but does not see it as something that it wants to attach to mechanically or chemically. The search for a process that could enhance the uses of this material has led many OEMs to work with a variety of surface-modification technologies. The basic concept is simple– apply a bioactive coating to a bioinert material in order to enhance bone up-growth or osseointegration. This application fits perfectly into many of the uses of PEEK when applied to fusion-type devices.

The only hurdle in the technology is the always rigorous regulatory issues in getting new and novel ideas through the FDA approval process. Many companies have already decided to bring this technology to the European market where the introduction to new technologies is more prevalent. Depending on the device system used, the FDA may allow for the 510(k) process for this technology. If not, the device companies will be faced with the expensive premarket approval (PMA) process.

An interesting example of this type of development is the Epoch hip stem, developed by Zimmer, Inc, Warsaw, IN. Zimmer introduced a three-part composite consisting of a Co-Cr-Mo alloy core that is essentially overlaid with PEEK through an injection-molding process. The outer layer is Zimmer’s porous titanium fiber mesh. The result has been an implant that mimics the stiffness of bone and in theory eliminates stress shielding.

A more recent example of a breakthrough surface-modification technology is by Surface Dynamics. This method applies plasma-sprayed titanium and or hydroxyapatite (HA) coating to PEEK-Optima. The hydroxyapatite process has been used for many years in dental, orthopedic, and spine application with metal implants. HA has been used on PEEK with success in spinal-fusion applications, showing that the coating maintains the proper properties when applied to a polymeric material such as PEEK.

Applying thermally sprayed materials to PEEK is challenging because the physical and thermal properties of polymer material versus metallic or ceramic materials can be significant. To meet the challenges associated with these characteristics, the titanium coating is applied using a proprietary vacuum process that essentially provides low-temperature stability during processing. Therefore, no chemical or mechanical deterioration occurs during the plasma-spray process. One of the main difficulties in applying a titanium coating to PEEK devices involves the difficulty in using a material that readily oxidizes at elevated temperatures. Therefore, the titanium-coating process is accomplished in a vacuum chamber that has been specifically built for processing implants.

Even though the process is considered high temperature, the PEEK does not degrade during processing. The glass transition temperature of PEEK is reported to be 143°C, well below the processing temperature of the substrate environment. The chemical structure of PEEK confers outstanding chemical and radiation resistance along with resistance to structural degradation at high temperatures.

The plasma-spray process depends initially on thermal (melting point) and secondarily on kinetic energy when being applied to a substrate. The material is essentially melted in the plasma while being accelerated by the high-velocity plasma-gas stream. The molten of semimolten droplets attach to the roughened substrate forming splats of particles one after another. The plasma-spray process utilizes inert gases (argon-nitrogen) that flow over a cylindrical copper anode and a tungsten cathode while a direct-current (DC) arc is maintained, which produces the gas plasma where the core temperature can be as high as 30,000°C. Powder metal or ceramics are injected into the plasma stream either internally or externally where they will be melted and accelerated at the substrate. The entire process is carefully automated and controlled so as not to apply or transfer excessive energy, specifically heat, to the PEEK substrate.

Mechanical properties of the commercially pure titanium (CP-Ti) coating exceed the FDA “Guidance Document for Testing Orthopedic Implants With Modified Metallic Surfaces Apposing Bone or Bone Cement”. Tensile and shear testing were all done per the ASTM F1147 and F1044, with resulting averages of 50.8 MPa and 42.6 MPa respectively. The thickness is controlled by precise programming of a six-axis robot along with mass flow control of the titanium powder feed rate into the plasma gun. The thickness range of the coating was 200-500 microns with a roughness measurement of Ra 32.7µm. The roughness measurement is obtained through image-analysis software that obtains its measurement through a mounted cross-section of the coating on a PEEK substrate. The coating topographically is very rough due to the size of the melted particles, which is the intent for most press-fit applications. A complete FDA Master File describing the process along with all the required physical, chemical, and mechanical properties is on file with the FDA.

With the Surface Dynamics coating process, PEEK properties are protected and complemented by the titanium coating, adding to the performance of the device. This will allow design engineers to seek new applications for a material that is changing how polymers are incorporated into medical devices as market demands run parallel with the patients’ needs: more anatomically and biologically relevant designs made possible by advanced biomaterials and processing technologies.