Surgical grade stainless steel, titanium, titanium alloys, and cobalt-chromium allow medical devices to meet specific strength requirements, as implants are subjected to high, variable loads based on body position and movement. These metals are also highly biocompatible and corrosion resistant.
However, metals and alloys can interfere with diagnostic imaging such as MRI and CT scans, and stress shielding – reduction in bone density due to an implant removing normal stress from the bone – can occur, in part, because the modulus of elasticity of metals is higher than that of bone. This can affect load distribution and lead to bone resorption.
Such problems have stimulated research into alternatives such as resorbable and non-resorbable polymer implants with a similar modulus of elasticity to bone, such as ultra-high molecular-weight polyethylene (UHMWP) and high-density polyethylene (HDP). Organic polymer thermoplastic polyetheretherketone (PEEK) 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 – the structural and functional connection between the bone and the implant’s surface – says Keyvan Behnam, an implantable biomaterial researcher at Zimmer Biomet, a medical device company specializing in orthopedic implants.
“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,” Behnam says.
Fortunately, a new surface modification technique that uses electron-driven plasma treatment may improve osseointegration performance in metals, alloys, and polymer implants.
Behnam says he began examining solutions to improve cell growth and proliferation relative to the base material by modifying the surface of titanium, PEEK, and polymers.
One technique, plasma spray coating, deposits materials such as titanium to a titanium or polymer surface to roughen it. Hydroxyapatite, a calcium phosphate, can be plasma sprayed onto the surface of titanium or other metals.
Already used in medical manufacturing, some developers worried that small coating particles could be released into the surrounding tissue due to abrasion or other means, which could cause an undesirable tissue response with eventual longer-term aseptic loosening of the implant.
Another alternative is radiofrequency (RF) plasma treatment. Plasma is a state of matter, like a solid, liquid, or gas. When enough energy is added to a gas, it’s 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,” Behnam explains. “It’s the same underlying material that is being implanted, it’s just the top layer that has been modified.”
RF plasma is created by applying an RF signal (typically 13.56MHz) that causes atoms or molecules of gases introduced into a 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.
However, the process generates heat, a distinct disadvantage when processing polymer implants – biocompatible polymers cannot take much heat without altering the underlying structure. In some cases, the flow and melting temperatures of polymers are not very high.
Electron enhanced material processing (EEMP), a new plasma etching process developed by VelvETch in partnership with PVA TePla America, provides a low-temperature approach for etching sensitive polymers or metals.
Precisely controlled waves of electrons – not ions – are accelerated to the material’s surface at specific voltages, creating chemical reactions that release the surface atomic bonds and allow surface material to be lifted away.
Because electrons have little mass, there’s no impact damage to the surface. Additionally, the chemical reaction generates nominal heat, keeping the sample at room temperature.
“We can process biocompatible polymers with EEMP while maintaining a very low temperature profile. We excite the bonds of polymer chains on the surface and either etch or modify the surface. This contrasts with the destruction of bonds and surfaces, which is what normally happens when you put temperature sensitive polymers in regular [RF plasma] etchers,” says Samir Anz of VelvETch.
Unlike RF plasma, which generates a specific result regardless of the material being processed, EEMP is flexible and adaptable to a variety of applications and materials. The gases in the chamber, the electron energy in the discharge (based on the material to be etched), and the temperature can be manipulated to achieve unique results.
“You can really tune what you are doing to attain that ideal surface you envision – based on the desired physical and/or chemical properties required,” Anz says. “You may want a certain level of roughness or smoothness. You may want a surface that is hydrophobic or hydrophilic. The EEMP process essentially has the capacity to deliver on what you require.”
According to Anz, an early experiment of the process produced an extremely rough surface, prompting the suggestion that it closely resembled the surface of human bone.
“You could use an EEMP process on stainless steel or titanium surfaces. With a slight modification, that process can be adjusted to work on a polymer surface. This level of integration is possible because our proprietary process delivers tuned electron energies to the bonds that are in the surface to be modified,” Anz says.
EEMP also enables creation of atomically smooth surfaces because the process removes atoms layer-by-layer, beginning with any existing peaks to within one lattice constant of atomic smoothness – less than 0.25nm.
There are advantages of smooth and rough surfaces, depending on the application. For example, a polymer implant in an intra-articulating joint requires a smooth surface to minimize joint wear and friction. However, when trying to stimulate osseointegration, rough surfaces produce a better outcome.
The EEMP process also ensures the polymer’s surface is devoid of organic material that could lead to implant toxicity issues. Today, plasma chambers are available for EEMP along with contract processing services through a partnership with PVA TePla.
According to Behnam, there are other potentially interesting applications for EEMP, including supporting bacterial adhesion and biofilm formation, that are still being explored.
“We are in the 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,” Behnam says. “Our initial studies have shown that we can improve the way cells interact with surfaces that have been modified in this way and that the modified material is chemically indistinguishable from the base material.”