One way in which orthopedic medical device manufacturers can improve functionality and performance in their devices is through enhanced surface modifications and coatings, which can result in better performance, fewer problems and improved longevity. 

For example, design engineers (especially in the spinal market) are making the surfaces of their implants more bone-like to enhance osseointegration (which is the direct structural and functional connection between living bone and the surface of a load-bearing artificial implant). There also is strong interest in modifying the surfaces of challenging materials such as ceramics and advanced polymers to improve osseointegration—one example would be applying titanium coatings to implants made from polyether ether ketone (PEEK). Antibacterial coatings improve functionality, longevity and safety. Also, because a small percentage of implant recipients are allergic to metal, a number of major implant manufacturers are placing non-allergic coatings on their metal parts.

“OEMs are seeking greater wear resistance and ease of processing,” said Chander Chawla, director of biomedical polyurethanes for DSM, a Berkeley, Calif.-based medical device materials developer and process manufacturer, which is a division of Koninklijke DSM N.V., a Dutch multinational life-sciences and materials-sciences company. “We have seen increased requests for reduced friction coefficients for applications in articulating joints, which require special surface modifications.”

Another recent trend has been the desire by orthopedic device firms and end-users to extend the lifetime performance of reusable medical equipment and instruments by improving their surfaces to better withstand cleaning and sterilization.

“Conventional anodic coatings suffer from corrosion, smutting, fading and other related deficiencies brought on by cleaning and sterilization processes,” said Tim Cabot, president of DCHN LLC, a Woonsocket, R.I.-based provider of metal finishing services to the medical device industry. “This is especially true when high-pH cleaning detergents, autoclave sterilization and Sterrad sterilization are used. In addition, these same cleaning and sterilization cycles can degrade marking and printing on instruments, devices and equipment.”

New Materials, New Possibilities
A big challenge regarding advanced coatings is developing a material that is effective and also meets or exceeds regulatory requirements for biocompatibility and other international chemical compliance directives, such as Restriction of Hazardous Substances (RoHS) and Registration, Evaluation, Authorization and Restriction of Chemicals in Europe. Although the research investment for developing and thoroughly testing new coatings is significant, it is necessary for improving functionality and safety, and minimizing failures—especially loosening, wear debris and poor integration with the bone.

New coating materials can solve many of these performance challenges. For example, DSM’s Bionate II thermoplastic polycarbonate-urethane (PCU) is the next generation of the Bionate polyurethane family, a medical grade polymer that has been used successfully for long-term implants for many years. Bionate II PCU provides 10 percent more strength, improved oxidative stability and enhanced processability. This advanced polymer uses built-in surface modification technologies that can produce a wide range of potential surface characteristics, depending on the application. These technologies are covalently bound (hence non-leaching) to the polymer during synthesis and provide more consistent surface properties compared to surface-modifying additives. Because of its exceptional load-bearing capabilities and biostability, Bionate II PCU is used extensively in orthopedic applications, especially in spinal motion preservation devices.

“Bionate PCU can also serve as a biomaterial for the 3-D printing of customized implants,” added Chawla.

Hydroxylapatite (HA) is a common coating material that promotes osseointegration and has a proven track record. Typically used on metal components and implants, HA now is gaining an interest among designers and engineers as a coating on polymers like PEEK. Promimic A.B., a biomaterial company based in Gothenburg, Sweden, has developed a coating called HA Nano Surface that can be applied to various types of substrates, including metals, ceramics and polymers, improving osseointegration by creating a surface that resembles natural human bone.

“Because the coating is only 20 nanometers in thickness, it also has the advantage of not introducing a load-bearing layer between the core implant and the new bone,” said Ulf Brogren, CEO of Promimic. “Nano-thin coatings do not affect insertion friction. Therefore, it is possible to coat screws without damaging the coating when the implant is inserted.”

Boyd Coatings Research Company, a Hudson, Mass.-based firm that designs and applies high-performance coatings, recently developed a new process that enables pure forms of polytetrafluoroethylene (PTFE) and other high temperature-cured fluoropolymer coatings to be applied to nitinol medical devices, without degrading the performance characteristics of the nitinol material.

“Typically, pure formulations of high-temperature-curing fluoropolymers require temperatures of 700 degrees Fahrenheit to cure,” said Don Garcia, director of research and development for Boyd Coatings. “This is a problem for nitinol products because they cannot tolerate such high temperatures without suffering adverse reactions.”

Boyd’s new process enables the application of fluoropolymer coatings to nitinol in their pure form, eliminating the need to use hybrid forms of PTFE with resin binders to stay below the nitinol transition temperature. Being able to use the pure form of these fluoropolymers on nitinol is a major development for users of nitinol devices, because now they can take advantage of all the special properties of pure PTFE, including its low friction and non-sticking properties.

“We are constantly designing and developing new coatings systems and processes using combinations of conventional fluoropolymers,” said Garcia. “Since there are rarely any new fluoropolymers that come into the market, the challenge lies in how to use them in combination with one another, and the processes by which you apply them, to meet the specific requirements.”

Preparing the Surface
Naturally, new coatings get a lot of attention—but achieving a surface finish that meets or exceeds industry standards is equally important. This especially is true for “friction” parts such as artificial joints, where the smoothness of the implants is as important as their dimensional integrity. In general, the smoother the friction area of the part, the longer it lasts. With knee femurs, for example, the current industry standard regarding final surface finish is Ra (surface roughness average) = 4 micro inches or slightly lower. However, Rösler Oberflaechentechnik GmbH, a Germany-based company that specializes in surface finishing for the medical industry, has developed a process that produces a superior surface finish as low as Ra = 1.5-0.8 micro inches.

The process—called drag finishing—can process multiple parts in a single batch without the parts ever touching each other. The drag finishing system is equipped with a rotary carousel (also called a spinner) that contains multiple rotary work stations/spindles. The implants that are mounted to the workstations with special fixtures are immersed into a work bowl filled with grinding or polishing media and “dragged” through the media in a circular manner. The rotary movement of the carousel and spindles is induced by a powerful drive system consisting of two independent drive motors for the individual adjustment of the carousel and spindle speeds.

“This flexible drive system allows a wide variety of different processing alternatives,” said Eugen Holzknecht, recently retired general manager for Rösler Metal Finishing LLC in Battle Creek, Mich. “The grinding and polishing media used for the drag finishing process, and the required compounds, are developed by Rösler and approved and certified by numerous implant manufacturers. The result is an excellent surface finish with an absolutely even metal removal on the complex part surface, without negatively impacting their dimensional integrity.”

The surface-smoothing stage requires processing times between 90 and 150 minutes and generates an isotropic surface with surface readings of as low as Ra = 3-2 micro inches. This process also offers the additional benefit of slightly dimpling the part surface, making it more wear resistant.

Longer-Lasting Reusables
Healthcare systems and orthopedic surgeons use plenty of instruments, devices and other equipment and want them to last as long as possible. DCHN, in partnership with Sanford Process Corporation (SPC), a Woonsocket, R.I.-based developer of aluminum anodizing technologies based on proprietary rectification technology and process development, has developed surface-finishing technologies that extend the life spans of reusable medical devices and equipment. For example, medical equipment made from aluminum commonly uses conventional Type II and Type III anodizing. Although these methods provide a good initial finish to the metal, the coatings are not designed for medical applications and the finish degrades over time. To mitigate this issue, DCHN and SPC have developed a higher-performance coating, based on the same anodic coating chemistry.

“We create a micro-crystalline anodic coating where the amorphous oxide is partially converted into more stable crystalline structures,” said Cabot. “This results in more stable coatings that can better withstand cleaning and sterilization.” This type of anodic coating is a hard coat and is commercially known as Micralox. It has 10 times the chemical and corrosion resistance of conventional hard coat, yet still maintains the same physical properties. It is also fully RoHS- and cytotoxic-compliant.

“Micralox hard coat is especially beneficial for applications where high chemical resistance is required, such as applications that require regular cleaning with detergents,” added Cabot. “It can withstand hundreds of Steris, Sterrad and autoclave cycles and also maintain its integrity at pH 13 for two hours or pH 0.9 for 48 hours.”

Another area of interest for medical device manufacturers is polyvinylidene fluoride (PVDF). This plastic has excellent chemical resistance, high purity and abrasion resistance and is available in U.S. Food and Drug Administration (FDA)-compliant grades. Due to its durability, inertness and resistance to solvents, acids and heat, PVDF is a popular choice for marking reusable medical devices and instruments, which must withstand the rigors of daily handling and sterilization. Additional desirable characteristics of PVDF coatings include a lower cure temperature (500 degrees Fahrenheit) and good dielectric strength (1,500 V/mil—a unit of dielectric strength: volts per mil [a mil is 1/1000 inch]). It also is available in different colors.

“We can coat the entire device with PVDF—or just a selected surface of the device—regardless of complexity of geometry,” said Garcia. “Of particular interest here is our ability to pad print or screen CE marks, UL marks, logos, etc., onto the coated surface using that same PVDF material. By doing so, we can ensure that the device has the performance characteristics of the PVDF coating on every designated surface. Identification markings made with PVDF materials will be legible and as durable as the base performance coating itself. The biocompatibility and other desirable PVDF characteristics will be in every aspect the same throughout the device coating.”

Keeping Them Clean
To get maximum return on their investment, end-users want equipment that lasts a long time, which means instruments and medical equipment must be able to survive repeated cleaning procedures and sterilization cycles without damage. Cleaning and sterilization can be done for products before they are packaged, after they are opened and before they are used, or after use.

Vacuum Processing Systems LLC, an East Greenwich, R.I.-based manufacturer of aqueous and solvent equipment for cleaning, pacifying and surface treatment for medical devices, has developed a patented vacuum cycling nucleation (VCN) process that rapidly can turn over bulk fluid chemistries within porous or high-aspect-ratio parts. VCN typically is used to clean products during manufacturing and prior to sterilization, or for medical-device reprocessing.

“We replace the chemistry often and quickly at the boundary layer between the surface being cleaned and the cleaning liquid,” said Donald Gray, president of Vacuum Processing Systems. “For example, in passivation the acid concentration tends to drop at the boundary layer in a part that is being soaked or even agitated. Our VCN process brings fresh acid, or whatever chemistry is being used, to the surface by disturbing the boundary layer.”

The process works effectively for cleaning, rinsing, passivating, sterilizing and coating hard-to-reach interior surfaces in complex parts. Both internal and external surfaces now can be treated in a fraction of the time using VCN, which forces convection into tight areas, where previously diffusion was the main mechanism for delivering the cleaning solutions. Multiple surface treatments (cleaning, passivation, coating, etc.) can be performed in a single chamber, since VCN rinsing and vacuum drying efficiently recover fluids from the part and processing chamber. VCN also is effective for extracting monomers or fillers from 3-D-printed parts, according to the company. Properly prepped and treated internal surfaces lead to better ligament attachment and lower infection rates, officials noted.

“Customers come to us because they need a level of performance that is not being offered by their present techniques,” said Gray. “With passivation porous stainless steel filters, for example, the typical soak in 50 nitric acid does not reach the internal areas. The VCN system can treat products such as small precision bearings, small lumen needles, long catheter tubing, small blind holes with polishing compound in them, 0.006-inch internal diameter complex micro valves, reprocessed medical devices, laparoscopy equipment, bits, blades and burrs.”

OEMs also are interested in adding ultrasonic equipment to their current process in an effort to reduce part failures and increase throughput.

“Whether it is for the cleaning stages prior to anodize, or in the acid phase in a passivation line, the systems we are building are more advanced than ever,” said Douglas Cole, medical vertical sales manager for Blackstone-Ney Ultrasonics, a Jamestown, N.Y.-based provider of precision cleaning and surface finishing equipment to the medical industry.

These advances largely are being driven by FDA regulations as well as end-user requirements. In the past, a fairly basic tank with ultrasonics and heat is all that was required for cleaning. Now, the FDA is seeking more documentation. To help, Blackstone-Ney Ultrasonics equipment can track data from temperature to ultrasonic energy and pH/resistivity.

The question I am asked the most is, ‘How do I validate that my ultrasonics are working?’” said Cole. “Although a truly reliable tool doesn’t exist, there are many things customers can do that will satisfy the FDA. I always recommend looking at multiple data points, rather than a single number from a single device. Combining results from different tests will give you the best picture of what is occurring in your process tank.”

Cole also points out that a common problem he sees is end-users who mismatch frequency to the level of cleanliness they are trying to achieve.

“A good rule of thumb regarding ultrasonics is that if you can’t clean it in five minutes with your current process, it is not likely to be the best process for you,” he said. “Also consider what size micron particle you’re trying to remove and remember that higher frequencies tend to do a better job at removing smaller particulates.”

Sometimes it is a combination of processes that does the trick. For example, ultrasonics work well on exposed surfaces but not internal cavities. VCN is not as effective on exposed flat surfaces. When these situations arise, Vacuum Processing Systems combines ultrasonics with its VCN system to clean complex devices, in much less time than standard techniques take.

“For example, we have a project where a one-minute VCN replaces a 10-minute process that does not work very well,” said Gray. “Within the first few minutes you will know if the VCN process will work or not for your project.”

Moving Forward
There are plenty of surface modification and coating developments on the horizon that excite OEMs. For example, techniques are being refined that provide the ability to coat complex geometries selectively, including using different coatings on different parts of the same instrument. Advanced polymers continue to be engineered with enhanced physical properties, including improved strength and heat and chemical resistance, which can reduce the need for surface treatments and other secondary operations. 3-D printing equipment also is advancing at a rapid rate.

“Three-dimensional printing holds great promise for making better implants,” said Chawla. “Regulatory institutions such as the CE (Conformité Européenne—mandatory conformity marketing for products sold in Europe) and FDA are already creating guidelines for quality management of 3-D printing.” 

In general, polishing coated implants is a time-consuming process. It especially is important that the surface finishing process does not grind through the coating layer and expose the underlying metal. However, as more medical device manufacturers use new materials that have greater wear resistance, the polishing process will take longer and be more expensive. To offset such cost increases, Rösler is exploring the use of customized robotic loading and unloading systems to improve the efficiency of its drag finishing machines. The company also is further developing its grinding and polishing technologies, including ever finer and harder abrasive particles.

“At the moment, we are developing media containing extremely fine abrasives with a hardness of near 10 on the Mohs scale (of mineral hardness)—in other words, very close to the hardness of diamonds,” said Holzknecht. “These new media materials should yield Ra surface readings of well below one micro inch on the toughest implant or coating materials.”

As the market continues to shift due to changing demographics and health priorities, the demand for higher-performing implants will increase, which means OEMs and their trusted partners must find cost-effective ways to deliver the implants at a reasonable cost.

“Combining novel implant designs with coatings, and adding the right bone chemistry for better and faster integration, will be critical for meeting these growing challenges,” said Brogren. 

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Mark Crawford is a full-time freelance business and marketing/communications writer based in Madison, Wis. His clients range from startups to global manufacturing leaders. He also writes a variety of feature articles for regional and national publications and is the author of five books. Contact him at mark.crawford@charter.net.