Today, the materials used to create artificial implants include surgical grade stainless steel, titanium, titanium alloys and cobalt-chromium. Often the materials utilized to meet specific strength requirements because they 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 Polyetheretherketone (commonly known as 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, says Keyvan Behnam of Zimmer Biomet, a medical device company specializing in orthopedic implants from the spine to joints, dental, trauma and even for sports medicine.
“That is the dilemma: if you use a polymeric material you may be able to match the modulus of elasticity of bone better, but the downside is it doesn’t integrate into the host bone in the same way,” says Keyvan Behnam, whose focus is on implantable biomaterial research at Zimmer Biomet.
Fortunately, a new technique in surface modification using a unique form of electron-driven plasma treatment has been shown to improve performance of metal and alloys, but also improve osseointegration of polymer implants.
Approaches to Surface Modification
Behnam says his initial interest in examining potential solutions to the issue was driven by a desire to modify the surface of titanium, PEEK, polymers and other materials to improve cell growth and proliferation relative to the base material.
One available technique is the use of plasma spray coatings, where the surface is modified 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 polymer surface to roughen it.
Although plasma sprays are used within the industry, there is some concern that small particles of the coating could be released into the surrounding tissue, due to abrasion or some other means. 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,” explains Behnam. “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 a radio frequency signal (typically 13.56 Megahertz) 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 radio frequency 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, a distinct disadvantage when processing polymer implants. By nature, bio-compatible polymers cannot take much heat without altering the underlying structure. In some cases, the flow and melting temperatures of polymers is not very high.
Electron Enhanced Material Processing (EEMP), a new plasma etching process developed by VelvETch in partnership with leading plasma equipment manufacturer, PVA TePla America provides a different low temperature approach for etching of sensitive polymers or metals.
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.
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.
“We can process biocompatible polymers with EEMP, while maintaining a very low temperature profile. We just excite the bonds of the polymer chains on the surface and do things to them or modify them with certain chemicals, destroying them, which is what happens when you put temperature sensitive polymers in regular [RF plasma] etchers,” explains Samir Anz of VelvETch.
Unlike RF plasma, which generates a specific result no matter the 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.
“You can really ‘tune’ what you are doing to attain the ideal surface you are trying to create, based on the physical chemical properties,” explains Anz. “You may want a certain level of roughness or certain level of smoothness. You may want a surface that is hydrophobic or hydrophilic. The EEMP process essentially has the capacity to do that.”
According to Anz, an early experiment of the process produced an extremely rough surface, prompting a biologist that examined it to suggest it very closely resembled the surface of real human bone.
“You could use EEMP on a stainless-steel or titanium surface, and the adjust it to work on a polymer surface, because you are tuning it to energy of the bonds that are on those specific surfaces,” adds Anz.
Among the additional 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 nanometers.
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 the polymer’s surface is devoid of any type of organic material that could lead to toxicity issues related to the implants.
Today, plasma chambers are available for EEMP along with contract processing services through a partnership with industry leader PVA TePla, a company that designs and manufactures plasma systems.
According to Zimmer Biomet’s Behnam, there are other potentially interesting applications related to using EEMP for surface modification, including supporting bacterial adhesion and biofilm formation that are still being explored.
“We are in early stages of proof-of-concept, but we see significant value in the physical chemical surface property changes we have been able to achieve so far,” says Behnam.
“Our initial studies have shown that we can improve the way cells interact with the surfaces [of implants] that have been modified in this way and that the modified material is chemically indistinguishable from the base material,” he adds.
