Keeping It Covered

Orthopedic implants and medical devices continue to evolve at a rapid pace. They are smaller yet more complex, with higher functionality and shorter life cycles. And speed to market is a top priority for keeping those life cycles as long as possible. The ultimate trick is vastly improving performance and longevity without having to redesign the product to the point that it creates a slowdown of evaluation at the U.S. Food and Drug Administration. Also, with the trend toward outcome-based reimbursement, healthcare providers are instructing OEMs that they want devices and solutions that provide the best possible clinical outcomes—not just for the welfare of the patient, but to keep healthcare costs down.

Coatings can be the magic bullet for all of the above because they greatly can improve performance often without having to go through a complete product redesign. The right coating can provide engineers with more choices for material selection—hundreds of popular coatings and finishes provide desirable characteristics such as improved fatigue strength, heat resistance, conductivity, insulation, water repulsion and osseointegration (bone in-growth). Selecting the right coating can make a component or device stronger, safer, longer-lasting and more cost effective—and more effective than its competitors.

“Design engineers in the medical device industry are becoming more aware of—and familiar with—the innovative possibilities of using high-performance coatings to enhance or alter the surface properties of a product,” said Don Garcia, director of research and development for Boyd Coatings Research Company, a Hudson, Mass.-based provider of high-performance coatings for the medical device industry. “As a result, engineers are designing more instruments and devices that utilize the individual properties of various coatings in technically creative ways.”

These include using coatings as electrical insulators and/or conductors of current.

OEMs and material providers also are developing coatings with the baby boomer in mind. Baby boomers represent the largest retiring generation in the United States to date. They are more affluent, more active and retiring at an earlier age than any previous generation. They also have the highest expectations for the healthcare industry—they want quick, easy medical solutions (especially joint replacements) that will keep them healthy and active as they age.

Bionate II PCU is a medical-grade polymer often used in long-term implants—pictured above in the form of pellets, film, disk and tubing. Photo courtesy of DSM Biomedical.

“The world has an aging, yet active, population,” stated William Fuller, director of business development for DSM Biomedical, the Berkeley, Calif.-based provider of coatings, drug delivery platforms and biomedical materials, and a division of the Netherlands-headquartered Royal DSM. “In orthopedic applications we are seeing a strong clinical demand for faster healing, return to an active lifestyle, reduced infection risk, less follow-up care requirements and reduced potential for revisions. In this regard, coatings are one of many tools aimed at improving the clinical outcome.”

To meet these demands, devices and implants are becoming smaller and more complex. This drives new expectations for the functionality of coatings, as well as the expertise of the coater to apply coatings to smaller, more complex geometries. This often requires coaters to be innovative in their coatings design and application methodologies, sometimes developing their own proprietary technologies.

For example, coating small inside diameters (IDs)—especially those coupled with physical complexities such as bends, flanges, corners or other obstacles—greatly increases the level of difficulty in coating applications. Boyd Coatings Research Company has developed a process that evenly coats an ID as small as 0.007 inches—even if the lumen is long or has physical complexities. “Our borescopes and other specialized inspection devices enable quality assurance technicians to examine small inside diameters to depths of 39 inches,” said Garcia. “Precision go/no-go wire and pin gauges are used to assure customers’ tight circularity and thickness tolerances are achieved.”

Anodic Coating Technologies
Medical customers want the best of both worlds: improved performance that differentiates their products from the competition, but without significant changes in materials and methods that would require extensive revalidation work or higher production costs. This quandary often can be solved by applying anodic coatings to the part or product.

“Anodic coatings are regularly used for instrument handles, cases, containers, and other medical products,” said Jack Tetrault, president of Sanford Process Corporation, a Woonsocket, R.I.-based provider of aluminum anodic coating technologies. “They provide excellent abrasion resistance, are highly flexible in terms of coloring and visual properties and have excellent thermal conductivity to allow for quick drying. They are also well-accepted throughout the industry.”

Anodization is an electrolytic passivation technique that thickens the oxide layer on the exterior of a metal part or product. The process alters the crystal structure of the metal, resulting in a harder, smoother surface with improved wear resistance, corrosion resistance and better adhesion properties for secondary operations such as priming and painting. Anodization is commonly used on aluminum and aluminum alloys, titanium, zinc, tantalum, niobium and other metals.

Chromic acid anodizing (also known as Type I) is the earliest form of the technique. It increasingly is being replaced by Type II anodization, which uses sulfuric acid to create a harder surface with better physical characteristics.

“For example, our Type-2 titanium anodizing process increases lubricity, anti-galling and increased fatigue strength,” said Dean Zentz, vice president of operations for Warsaw, Ind.-based Danco, which provides anodized products for the medical device and aerospace industries. “This coating is becoming increasingly popular within the medical device industry, especially for the finishing of orthopedic implants. Since the coating is biocompatible as well as nontoxic, the process greatly improves implant performance.”

Titanium anodizing also is used to create different colors for cosmetic and/or identification purposes for various components. In the operating room, for example, staff might prefer magenta, blue, gold, green and bronze colors that can help visibly differentiate various lengths of screws that possess the same diameter.

“Titanium is one member of a family of metals, including niobium and tantalum, that color-anodizes because it is reactive,” said Zentz. “It reacts when excited by heat or electricity in an electrolyte by creating a thin oxide layer at the surface. The layer appears in color due to an interference phenomenon. This very thin, transparent coating that derives its ‘color’ when white light reflects off the base metallic surface, only to be interfered with within the coating. Some frequencies of light waves escape and recombine with surface light to be either reinforced or cancelled out—producing the color we see. The metal itself does not change color. Because of the absence of dyes and pigments, anodized titanium implants are both hypoallergenic and biocompatible.”

There also is a movement toward replacing traditional inks and dyes used as marker materials on instruments/devices. Not only is there the risk of contamination from tiny pieces of dyed material that might flake off into the body cavity during surgery, “but most inks and dyes currently used on medical devices cannot withstand multiple sterilizations and autoclaving the way that high-performance fluoropolymers can,” said Garcia. “Instead we can use colored coatings as marker materials to replace the traditional inks and dyes.”

A big problem for conventional anodic coatings is that more aggressive cleaning and sterilization techniques can corrode the coating; this especially is true for medical instruments and ancillary equipment made from aluminum. High pH detergents and autoclave, and various sterilization methods can strip and discolor anodic coatings, delaminate epoxy printing and cause other in-field failures.

To tackle this challenge, Sanford Process Corporation developed a process that changes the rate of solubility of the inorganic anodic coating by changing the material from an amorphous to a partially crystalline phase without altering the basic coating chemistry, creating a much greater resistance against high- and low-pH cleaners.

“An added bonus is that the coating protects anodic colors from fading and eliminates dye migration and pore contamination, resulting in exceptional embedded printing,” said Tetrault.

This technology, called micro-crystalline anodizing, is offered on the market as Micralox. Depending on the pH of the detergent, Micralox can make products last up to ten times longer than traditional aluminum hard coatings. Because clients also want products that can perform well in medical settings around the world, Tetrault indicated that anodic coatings must be developed that can meet all regional and global requirements.

“For instance, parts of Europe and certain other parts of the world are known for the use of high-pH cleaners (> 10),” said Tetrault. “This needs to be taken into account when determining coating selection. Another example is Sterrad sterilization, which is increasingly used for tools that cannot be wet-cleaned and sterilized due to electronics. Sterrad works through oxidation that can lead to loss of color.

Micralox coatings have much higher resistance to color fading.”

Bio-Derived Coatings
There is an increasing demand in the orthopedic market for bio-derived coatings that promote faster healing after implantation. These include coatings that incorporate antimicrobial or pharmaceutical agents, biologics that are conducive to tissue healing and osseointegration and resorbable materials.
“Many early medical coatings were developed to improve the biocompatibility of biomaterials used in implantable devices,” said Fuller. “These early coatings were developed out of necessity due to certain materials used in implantable devices that did not possess an ideal biointerface with the body. For example, in some cardiovascular devices the materials used caused platelet adhesion on the surface and prevented the device from functioning properly. Therefore device manufacturers began using coatings that contained heparin (or other anticoagulants) to prevent this adhesion.”

DSM has focused on developing materials that provide an optimal biointerface for the intended application, thereby reducing the need to have coatings that simply improve the biocompatibility of the material. DSM’s SME technology, for example, can alter the surface characteristics of the polyurethane materials used in articulating joint implants, cardiovascular electrostimulation devices and total disc replacement devices. Other medical coatings result in hydrophilic and non-biofouling characteristics.

“Hydrophilic coatings can help improve device performance over a wide range of applications and procedures by creating a lubricious, slippery surface, making device or instrument insertion more comfortable for patients,” said Fuller. “Non-biofouling coatings dramatically reduce unwanted protein absorption and cellular adhesion on material surfaces, which helps extend the life and increase the accuracy of biosensors and diagnostic cartridges, among other applications.”

Coatings for faster healing can go beyond implants and devices to high-strength sutures for soft tissue repair. One approach now being tested is the use of collagen as a dip coating on high-strength sutures.

“Since collagen is the main component of connective tissue, applying a collagen coating to sutures used for soft tissue repair has the potential to improve healing,” said Fuller. “Whether this benefit is real or not remains to be clinically proven, but we are seeing interest in the market for this approach.”

Cutting-Edge Research
Research into advanced biomaterials for coatings continues at a breakneck pace. A flurry of recent announcements on new coatings includes Branchburg, N.J.-based Hydromer Inc.’s Base F200 polymer, a cardiac stent coating that reduces cell mitosis and platelet adhesion. It is also available in variations that include non-leachable heparin.

Anodic coatings are used regularly for instrument handles, cases, containers and other medical products. Photo courtesy of Sanford Process Corporation.

The Massachusetts Institute of Technology (MIT) in Cambridge, Mass. is developing a nanoscale (100 nanometers to 1 micron) biofilm that promotes bone growth in and around an orthopedic implant, creating a strong bond between the implant and patient’s own bone. The film consists of layers of materials that promote rapid bone growth, including hydroxyapatite, a calcium-phosphate compound that resembles natural bone. This material attracts stem cells from the bone marrow and provides the framework for the growth of new bone. The MIT researchers control the thickness of the layer and the amount of growth factor released by using a manufacturing process called additive or layer-by-layer assembly, which deposits the desired components in precise layers and concentrations using a computer-assisted drafting (CAD) file. This is an important advance because it is the first manufacturing process that reliably delivers the exact amount of growth factor to the coating (too much growth factor can lead to unwanted side effects).

Researchers at Pacific Northwest National Laboratory (PNNL) in Richland, Wash., have developed the first water-based process that deposits calcium-phosphate, thin-film coatings containing controlled-release bioactive therapeutics, such as antimicrobial agents, on orthopedic implant surfaces. This new thin-film technology is expected to play a major role in dramatically reducing post-surgical infections in implant recipients and wounded military personnel. The simultaneous, sustained delivery of different antimicrobial agents from the coated implant is expected to achieve optimal infection prophylaxis and improved post-operative therapeutic outcomes.

Several companies are developing advanced coatings with specific applications for spinal implants. Spire Medical in Bedford, Mass., is working with the National Institutes of Health’s National Institute of Arthritis and Musculoskeletal and Skin Diseases on a new type of nanotechnology coating for wear-resistant orthopedic devices. The approach is to engineer super-lattice coatings that are comprised of precisely arranged, alternating nano-structures composed of dissimilar materials. Test results indicate the combined materials exhibit hardness and wear resistance that are far superior to the constituent materials. If commercialized, these coatings would be effective in extending the longevity of weight-bearing surfaces in articulating systems, such as spinal implants.

In another development, Carlsbad, Calif.-based Spinal Elements has launched a plasma-sprayed porous titanium coating called Ti-Bond that can be deposited on the company’s Lucent spine interbody implants made with polyetheretherketone to enhance bone healing during fusion. The coating consists of unconnected titanium pores that are biomechanically attached to the superior and inferior surfaces of the implants using a plasma vacuum spray process.

Center Valley, Pa.-based Aesculap Implant Systems has developed an innovative “alternative surface” for its knee revision systems. Called Enduro AS, this PVD (physical vapor deposition) coating of titanium or niobium nitride consists of seven layers, improving the implant’s resistance to wear and reducing the risk of metal ions shedding into the bloodstream. According to in-vitro tests conducted by the company, levels of allergenic ions such as cobalt, chrome, molybdenum and nickel are “below every biologic reaction limit.”

Diamond on Diamond
University of Alabama-Birmingham (UAB) researchers are developing a novel, nanostructured diamond coating for hip and knee implants. The goal is to improve the wear resistance of implants made from metal alloys and improve their overall longevity, reducing the need for replacing compromised or failed implants. Initial testing of this new class of functionalized, nanostructured coating for titanium and cobalt chrome alloys has shown promising results.

Long-term, successful functioning of implants is the ultimate goal of joint-replacement designers. A serious concern, however, is the release of metal ions into the bloodstream over time, and the deterioration of the implant and surrounding tissue resulting from wear debris. UAB researchers hope that, by covering metal-alloy implants with nanostructured diamond coatings, they can reduce wear on the implants and minimize the release of metal debris into the bloodstream and surrounding tissue, thereby reducing inflammation, pain and bone loss.

Even though diamond coatings greatly reduce the release of metal particles, nano-diamond particles themselves will inevitably break away from the implant as well due to the grinding action. The good news is that results show diamond coatings release smaller particles, in smaller numbers, compared to metal-on-metal implants. Laboratory research also indicates that nano-diamond particles do not cause inflammation in human tissue or elicit an immune system response, suggesting they are not toxic at these small sizes and concentrations.

UAB also is testing multilayered nanostructured diamond coatings for a temporomandibular joint (TMJ) implant that will undergo extensive testing in a simulator to 1.125 million cycles (approximately equivalent to 10 years clinical use). The TMJ device will result in less long-term wear at articulating surfaces due to high hardness and very low surface roughness. “We use nanotechnology approaches in these projects to improve the joint implants for dental and orthopedic (hip and knee) applications,” reported the university. “In-vitro response by mesenchymal stem cells and implantation of nano-diamond-coated titanium evidenced the biocompatibility and biointegration characteristics of diamond surfaces. Macrophage cell responses to nano-diamond wear debris exhibited cell viability with very minimal release of pro-inflammatory mediators.”

Meeting Higher Standards
Advancements in coatings materials have also required enhancements or modifications by coating providers to comply with expanding environmental concerns. Initiatives such as the Restriction of Hazardous Substances, Registration, Evaluation, Authorization and Restriction of Chemical Substances (REACH) and others have made it necessary for companies to update or advance some methodologies they use to manufacture and apply coatings materials.

“These modifications affect the entire process and, not surprisingly, changes to the process can sometimes have unforeseen consequences,” said Garcia. “As this directly affects product performance and efficacy, we perform all the necessary due diligence to ensure there are no negative impacts. We are replacing chemistries that are on a list of non-compliant materials by reformulating materials with either different chemicals or different processes to eliminate those chemicals.”

Boyd Coatings Research Company also has developed an early compliance strategy regarding the Environmental Protection Agency’s initiative to phase out perfluorooctanoic acid (PFOA), which is used as a processing aid in the manufacture of polytetrafluoroethylene (PTFE) resin dispersions. The company is working with its vendors to develop alternative technologies to make a new generation of products and processes that eliminate PFOA and its environmental impact, without sacrificing product performance. While the official goal for PFOA phase-out is 2015, Boyd Coatings has set its own phase-out date of spring 2013 to give its medical customers time to conduct their own due diligence regarding the company’s new PFOA-free products and processes. To avoid any negative impact of the phase-out to its customers (especially the risk of lack of supply), Boyd constantly monitors and coordinates affected products for material availability of existing PTFE and new PFOA-free PTFE.

“More than ever, design engineers are reaching a profound understanding of the value of high-performance coatings in the overall instrument/device,” said Garcia. “Instead of viewing the coatings as an additional cost, they are seeing it as a value-added proposition that, in the long run, reduces the cost of the device while improving performance. In outsourcing that competency, they receive specialized engineering, documentation and staffing specializing in operations and processes important to medical device OEMs.”

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Mark Crawford is a full-time freelance business and marketing/communications writer based in Madison, Wis. He can be reached at [email protected].