The medical device industry has long sought surface modification solutions that will improve cell growth and proliferation on titanium, polyetheretherketone (PEEK), and polymer artificial implants. In some cases, plasma sprays are applied. However, within the industry there is concern that small particles of the coating could be released into the surrounding tissue. Radiofrequency (RF) plasma is another alternative, but the process generates quite a bit of heat — a distinct disadvantage when processing polymer implants.
Fortunately, there is a new plasma etching process called electron enhanced material processing (EEMP) that provides a unique low-temperature approach that has been shown to improve performance of metal and alloys, but also osseointegration of polymer implants. In EEMP, precisely controlled waves of electrons — not ions — are accelerated to the surface of the material at specific voltages designed to create chemical reactions that release the surface atomic bonds, allowing the material at the surface of the sample to be gently lifted away.
The EEMP process can be used on surgical-grade stainless steel, titanium, titanium alloys, and cobalt-chromium surfaces. With a slight modification, that process can be adjusted to work on a polymer surface. This level of integration is possible because a proprietary process delivers “tuned” electron energies to the bonds that are in the surface to be modified.
Today, the materials used to create artificial implants include surgical-grade stainless steel, titanium, titanium alloys, and cobalt-chromium. Often these materials are utilized to meet specific strength requirements because implants are subjected to high, variable loads based on the body position and movement. These metals have also been demonstrated to be highly biocompatible and corrosion resistant.
There are drawbacks to metals and alloys, however, including the potential to interfere with diagnostic imaging, including MRI and CT scans. The other concern is stress shielding, the reduction in bone density as a result of removal of normal stress from the bone by an implant. This occurs, in part, due to the modulus of elasticity of metals is higher than bone, which can affect load distribution and lead to bone resorption.
That has stimulated research into alternative solutions, namely resorbable and non-resorbable polymer implants that have a similar modulus of elasticity to bone. The most used polymer in orthopedics is ultra-high molecular-weight polyethylene (UHMWP) or high-density polyethylene (HDP). Among the growing options is the organic polymer thermoplastic PEEK, which is already used to create cages in the $1 billion spinal fusion market. But there are downsides to this approach as well. Polymers are typically not as effective in supporting osseointegration, which is the structural and functional connection between bone and the surface of the implant.
To resolve these issues, one option is the application of plasma spray coatings that modify the surface by depositing materials such as titanium on a polymer or hydroxyapatite on titanium. Hydroxyapatite is a calcium phosphate plasma sprayed onto the surface of titanium or some other kind of metal. Titanium plasma spray, on the other hand, is applied to a titanium or a polymer surface to roughen it. Although plasma sprays are used, there is some concern that small particles debris particles could cause an undesirable tissue response with eventual longer-term aseptic loosening of the implant.
The other traditional alternative is RF plasma treatment, which modifies the surface of the implant. Plasma is a state of matter, like a solid, liquid, or gas. When enough energy is added to a gas it becomes ionized into a plasma state. The collective properties of these active ingredients can be controlled to clean, activate, chemically graft, and deposit a wide range of chemistries.
RF plasma essentially rearranges and creates new bonds on the outer surface. For all intents and purposes, it is the same underlying material that is being implanted, it is just the top layer that has been modified. RF plasma is created by applying an RF signal (typically 13.56 MHz) that causes the atoms or molecules of the gases introduced into the chamber to increase in temperature until they ionize into a plasma. A separately controlled RF signal under the item pulls the positive ions down to bombard the surface of the material.
The process generates quite a bit of heat, however. By nature, biocompatible polymers cannot take much heat without altering the underlying structure. In some cases, the flow and melting temperatures of polymers is not very high.
In EEMP, precisely controlled waves of electrons — not ions — are accelerated to the surface. Because electrons have little mass, there is no impact damage to the surface and only nominal heat is generated as a result of the chemical reaction, thus the sample remains at room temperature.
Biocompatible polymers can be processed with EEMP while maintaining a very low temperature profile. The bonds of the polymer chains are excited on the surface and either etched or modified the surface. This is in contrast to the destruction of bonds and surfaces, which is what normally happens when putting temperature sensitive polymers in regular RF plasma etchers.
Unlike RF plasma, which generates a specific result no matter what type of material being processed, EEMP is extremely flexible and adaptable to a variety of applications and materials. The variables that can be manipulated and tuned to achieve specific unique results include the gases utilized in the chamber, the electron energy in the discharge (based on the material to be etched) and the temperature.
It is possible to “tune” the process to attain that ideal surface envisioned — based on the desired physical and/or chemical properties required. A certain level of roughness or a certain level of surface smoothness may be desired — or a surface that is hydrophobic or hydrophilic. An early experiment of the process even produced an extremely rough surface, prompting a biologist who examined it to suggest that it very closely resembled the surface of real human bone.
Among the benefits of EEMP is the ability to achieve atomically smooth surfaces due to the nature of the process, which removes atoms layer-by-layer, beginning with any existing peaks to within one lattice constant of atomic smoothness, which is less than 0.25 nm.
In this way, the roughness can be modulated to achieve the desired outcome, whether that is subatomic smoothness up to hundreds and micron scale roughness. This is important, because there are advantages of rough and smooth surfaces, depending on the application. For example, a polymer surface that will be implanted in an intra-articulating joint would requirement smoothness to minimize joint wear and friction. When trying to stimulate osseointegration, rough surfaces are shown to produce a better outcome.
The EEMP process also ensures that the polymer’s surface is devoid of any type of organic material that could lead to toxicity issues related to the implants.
There are other potentially interesting applications for the medical industry related to using EEMP for surface modification, including supporting bacterial adhesion and biofilm formation that are still being explored. Although the technology is in the early stages of proof-of-concept, there is significant value in the physical chemical surface property changes achieved to date.
This article was written by Michael Barden, Head of Research & Development and Lead Scientist for PVA TePla America, Corona, CA. He designs and develops custom surface coatings for the medical device and electronics industry, specializing in nanoscale plasma enhanced chemical vapor deposition (PECVD), silane chemistries, and self-assembled monolayers (SAMs). For more information, visit here .