As coatings evolve, application technologies must rise to the challenge.
Highly specialized coatings increasingly are being used to enhance the performance and appearance of orthopedic implants and instruments. Surface modification science, advanced application techniques and uniformity of surface treatments are evolving as orthopedic OEMs move toward smaller, more complex products that require rigorous testing and study.
Micro-crystalline coating is suitable for orthopedic surgical trays. Photo courtesy of Sanford Process Corporation.
OEMs are looking for higher-performance coatings for their implants and instruments that will make their devices last longer. For example, as reusable devices become more popular there is greater need for more durable and chemically resistant coatings that can withstand multiple high-pH cleanings, disinfection and sterilization, as well as exposure to corrosive conditions. More coatings and technologies are being developed to combat these harsh environments and extend the life of medical equipment—thereby providing a better value proposition to customers and eliminating certain risks in the field.
“As you can imagine, a surgeon performing a delicate operation cannot afford to have instruments bind, chaff or otherwise function improperly because the surface properties were not well selected or applied,” said Don Garcia, research and development director for Boyd Coatings Research Company, a provider of high-performance coatings based in Hudson, Mass. “There are hundreds of coatings on the market that have potential uses in the medical industry and OEMs expect us to bring that knowledge to the table and be able to recommend a ‘good, better, or best’ range of materials for any application.”
Tom Glass, president of Able Electropolishing Co. Inc. in Chicago, Ill., agreed.
“In the past, 99 percent of the electropolishing we performed was done on stainless steel,” said Glass. “Now we process titanium, nickel-titanium, tantalum and tungsten.
Tantalum is good for high-voltage imaging applications; tungsten works well for filaments. Tolerances as tight as 1/10,000 of an inch are often required. Clients are also bringing us smaller components that are more detailed. We continuously invest in R&D to improve processes for new materials, increase production rates and minimize stock removal variation to provide the best options to our clients.”
OEMs are experts in their domains but increasingly rely on supplier expertise when it comes to product design, material selection and manufacturing processes. In most cases, the OEM has determined what type of surface condition is required, as well as the measurement methods for assessing the quality of the surface. Typically, each coating has a recommended range of roughness that is optimum for bond strength, based on previous evaluation and testing.
“The customer may not necessarily have determined exactly what types of abrasive media and blasting equipment are capable of producing the specified surface roughness, so our laboratory testing services can help demonstrate the measurable results using different media or blast process parameters,” said John C. Carson, business development manager for Guyson Corporation in Saratoga Springs, N.Y., a manufacturer of automated and robotic blasting machines. “In cases where a new coating is under development, and the most desirable surface condition has yet to be determined, preliminary lab work can support a methodical program of evaluation by providing test samples with different roughness for follow-up coating and testing.”
OEMs also want their vendors to be current on regulatory requirements and fully compliant with the expectations of the U.S. Food and Drug Administration (FDA) and other regulatory bodies—especially regarding validation (in fact, OEMs expect vendors to know more regulatory fine print than they do).
“Our orthopedic industry customers are definitely looking for equipment features that simplify and support process monitoring and validation,” Carson noted. “For instance, electronic sensors can be installed to generate data on blast process parameters such as blast pressure or media flow rate; increasingly those data are captured and acquired to verify that the process is carried out in conformance with process specifications.”
Contract manufacturers and equipment vendors who want to add value and longevity to their OEM partnerships must have employees who specialize in compliance with FDA; U.S. Environmental Protection Agency; Registration, Evaluation, Authorization and Restriction of Chemicals (REACH, in Europe); and Restriction of Hazardous Substances (RoHS, also in Europe) standards.
“By dealing with these regulations on a day-to-day basis we are able to stay knowledgeable in the latest requirements and have the critical mass to equip ourselves with the latest technologies for applying coatings, using the latest materials, for the best performance while doing so in an environmentally friendly manner,” said Garcia.
A Plethora of Materials
Fluoropolymer coatings such as polytetrafluoroethylene (PTFE), polyvinyl acetate (PFA), polyvinylidene fluoride (PVDF), fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (ETFE) and ethylene chlorotrifluoroethylene (ECTFE), as well as hybrids incorporating polyphenylene sulfide (PPS) and ceramic, are in high demand.
“PTFE and PVDF are two coatings commonly used for medical applications,” said Garcia. “PTFE has non-stick and low-friction surface properties, while PVDF applications tend to be for instruments needing electrical insulation properties and resistance to autoclaving and chemical sterilization. PVDF is also REACH-compliant and RoHS-compliant, presenting a minimal environmental footprint.”
Porous surfaces based on sprayed titanium or sintered titanium beads can support the firm anchorage of uncemented implants to the surrounding bone. Calcium phosphate (CaP) coatings, applied by plasma spray technique or wet-chemical deposition processes, help foster bone formation, thereby shortening the healing process.
DOT GmbH, a provider of surface treatments for orthopedic implants based in Rostock, Germany, was one of the first companies to manufacture CaP coatings on an industrial scale 15 years ago.
“Such thin coatings allow for the complete coating of complex geometries,” said Axel Baumann, director of innovation management for DOT GmbH. “They dissolve within a few weeks and they
Metals, of course, are still popular, have a long history and generally are well understood. However, as the number of metal-sensitive (especially nickel) patients continues to grow, coating engineers are starting to focus on developing coatings with anti-allergy properties. “Ceramic hard coatings, such as titanium nitride (TiN), titanium niobium nitride (TiNbN) or zirconium nitride (ZrN) can increase the abrasion resistance of implants and prevent the emission of these allergy-inducing metal ions,” added Baumann.
Research at the University of California-San Diego (UCSD) has focused on modifying the surface texture of polyetheretherketone (PEEK) to make it more suitable for orthopedic implants. PEEK is well known for it mechanical toughness, resistance to thermal and chemical degradation and non-toxicity.
Advantages compared with titanium include PEEK’s X-ray translucence and an elastic modulus that is similar to bone. It also reduces stress shielding and bone resorption—common problems for implanted metals with mismatched elasticity properties. PEEK even has been shown to be excellent material for articulation in the joint; however, the main drawback to its use in joint implants is that it does not interface well with bone.
Dr. Sungho Jin, a professor of material science at UCSD, Garrett Cale Smith, Ph.D., and post-doc student Chulmin Choi may have solved this problem by modifying PEEK to carry nanostructured surfaces that promote osseointegration in implant applications.
“We’ve developed a novel surface modification technology that transforms PEEK into a bone-bonding surface,” explained Smith, co-inventor and CEO of Nasseo Inc., a medical device start-up company that is licensing the technology from UCSD to help treat dental and orthopedic implant failures. “The technology utilizes nanostructures with unique 3-D topography to promote stem cell differentiation and to increase the surface area of the implant-to-bone contact. The potential applications of this technology are significant, including spinal cages and next generation hip and knee replacements that leverage the favorable mechanical and imaging properties of PEEK.”
Although it’s been around for more than 20 years and is highly regarded in other industries for its superior chemical, optical, electrical and tribological properties, diamond-like carbon (DLC) films are gaining interest in the orthopedic community as possible coatings for implants. Implant designers are intrigued by its low friction coefficient, high hardness and wear resistance, which could make devices last longer and prevent the sticking of blood cells. Recent studies show that DLC-coated PEEK has a surface hardness and elastic modulus much closer to cortical bone—possibly providing a solution to the degenerative bone disease called osteanabrosis, which may be caused by stress shielding effects.
“Another study shows that phosphorus-doped DLC thin films could even be used to produce patterned neuron networks, suggesting DLC is a candidate coating material for implants in the human nervous system,” wrote Tao Lu in an article titled “Surface Modification of Biomaterials” in the March 2012 online issue of Interface FOCUS. “In addition to good tribological qualities, DLC thin films are well known for their biocompatibility and improved cell adhesion as well as chemical inertness, which enhance their utility as a coating candidate for cardiovascular devices.”
Advanced Technologies Move Ahead
Sometimes the best way to create an advanced coating is by improving the existing process technology. For example, even though the conventional anodizing process has many attributes, it does not sufficiently protect products from the repeated physical stresses of regular cleaning, disinfecting and sterilization—a trend that’s definitely on the rise.
“Many of the workarounds to make anodic coatings perform better in the field, such as designating specific detergents within the cleaning protocols, just don’t work in practice,” indicated Jack Tetrault, president of Sanford Process Corporation (SPC) in Woonsocket, R.I., a provider of aluminum finishing technologies. “These solutions are also often expensive or create a different set of problems, such as delamination and chipping.”
Sanford started looking at why anodic coatings fail in the field. The most common cause in medical applications is that frequently used cleaning detergents strip the anodic coating over time and autoclave sterilization causes colors to fade. The temperature cycles of cleaning and sterilization can also cause silkscreen inks to lose adhesion to the substrate and delaminate.
SPC engineers discovered that, by partially converting the amorphous oxide anodic coating into microcrystalline structures, they could significantly change the rate of solubility of the coating in the presence of strong hydroxides and acids or other strong chemistries found in cleaning detergents—without changing the other coating properties. They then developed the Sanford Process Low-Voltage Hard Anodizing Technology for use in applications that require regular high pH cleaning or exposure to corrosive conditions. These include cases, trays, handles, instrument bodies, fixturing devices and hospital accessories.
Electropolished and color-anodized dental tips. Photo courtesy of Able Electropolishing.
Electropolishing has become a popular metal-finishing process to improve the life of metal parts, especially those that flex or bend. During the manufacturing process microdefects can form on the surface of these components, later becoming initiation sites for crack propagation or corrosion.
“These imperfections on the surface of metal parts can be removed through electropolishing,” said Glass. “The electropolishing process is also well-suited for deburring and can reduce microfinish values by 50 percent with a removal of .0005 inches from each surface.”
Electropolishing also enhances corrosion resistance for a wide variety of stainless steels and metal alloys, increasingly replacing the passivation process. Passivation is a chemical procedure that has been used for many years to restore stainless steel to its original corrosion specifications by removing free iron and other contaminants; however, it does not remove all the free iron that can be imbedded from manufacturing operations. It also won’t remove heat tint or oxide scale on stainless steel.
“Engineers looking to pass stringent salt-spray and humidity tests have turned to electropolishing as a more aggressive replacement for passivation,” Glass said. “Electropolishing dissolves the outer skin of metal, removing deeply imbedded contamination. Heavily contaminated surfaces such as machined parts, welded or brazed assemblies or other components that typically respond poorly to passivation alone are good candidates for electropolishing. Unlike passivation, all stainless alloys including the 400 series and precipitating grades can be processed without distortion, flash attack or hydrogen embrittlement.”
The great majority of surface prep work for coatings is applied by thermal spray or physical vapor deposition (PVD), which produce a very specific surface roughness to promote bonding. Thermal spray (and the related field of plasma spray) represents a rapidly evolving science, especially as it applies to coatings. Thermal spray is fast becoming the preferred method for depositing dissimilar materials, such as hydroxyapatites.
Considerable research is being conducted on thermal-spray and plasma-spray deposition of coatings on implantable devices made from a range of materials. Research topics include the effect of coating thickness on stress distribution in hip implants, bioactivity and electrochemical corrosion.
Researchers at Stony Brook University in New York have injected polymeric media into the openings of thermal-sprayed porous alumina coatings in an effort to improve strength and fracture toughness. Testing has shown that the fracture toughness doubled and strength quadrupled compared to a standard sprayed alumina coating. These results suggest that hybrid, synthetic composites can behave similarly (or even outperform) natural composites.
In another development, a low-pressure plasma-spray technology designed to deposit coatings out of the vapor phase has been developed by engineers in Europe. The plasma-spray-physical vapor deposition process uses a high-energy plasma gun that melts and vaporizes the feedstock material.
The vaporized material is incorporated into a supersonic plasma plume that then deposits an even coating on shadowed areas and surfaces that are not in the line of sight to the coating source. This unique process bridges the gap between the conventional PVD technologies and standard thermal spray processes.
For Guyson Corporation, the most dramatic change in its operations has been increased interest in robotic blasting, which is viewed as the most precise and flexible type of automated blasting.
“With other types of automatic blasting different blast guns or nozzles must be set at different angles and distances,” said Carson. “This provides good access to the entire part. But for a complex part, like a hip stem with multiple surfaces, it can be difficult to deliver a consistent surface roughness to all sections of the part. It is nearly impossible to duplicate this every time, to set up the next part with exactly the same angles and distances, so test parts and adjustments are needed to dial-in the process. Otherwise, no matter how skilled the operator, there will be variation in surface roughness.”
Robotic blasting employs a single blast nozzle and advanced motion control. Therefore the relationship between the target surface and blast gun is constant and repeatable, eliminating approximation and guesswork. All parameters are stored and can be immediately recalled for that piece.
“Robotic blasting creates a higher standard of consistency with no wasted motion,” said Carson. “Setup time is much shorter and the quality of the deposited surface is much higher. The energy savings is also significant—one gun consumes about 25 percent less energy than multiple guns, a savings that is passed onto the customer.”
On the Horizon
A chemical engineering team led by professor Paula Hammond at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., has developed a new coating for implants that appears promising for improving adhesion to bone and preventing premature failure. The coating is a very thin film (100 nanometers to one micron in thickness) that consists of layers of materials that promote rapid bone growth, including hydroxyapatite, a natural component of bone containing calcium and phosphate. Hydroxyapatite attracts stem cells from the bone marrow and provides an interface for the formation of new bone. The other layer releases a growth factor that stimulates the stem cells to transform into bone-producing cells called osteoblasts, which generate new bone around the implant.
Previous efforts to coat orthopedic implants with hydroxyapatite have resulted in unstable films that tend to break away from the implant. The MIT team can control the thickness of the film and the amount of growth factor released by using a method called layer-by-layer assembly, in which the desired components are laid down one layer at a time until the desired thickness and drug composition are achieved.
“This provides a significant advantage because other systems so far have really not been able to control the amount of growth factor that is required,” said graduate student Nisarg Shah, a member of the discovery team. “Many devices must use quantities that may be orders of magnitude more than what is really needed, which can lead to unwanted side effects.”
To Shah’s knowledge, this layer-by-layer (LBL) process has not been used in any commercially available implants or devices. Hammond’s lab also has studied how this coating can be applied to various medical devices, including implants and bandages for wound healing.
“We have proposed this method because of the simplicity with which it can be applied to virtually any surface with a complex geometry, including textures and pores,” said Shah.
Fundamentally, electrostatic interactions between different components drive the self-assembly; this process can be controlled to “tune” the amounts of different components in the coating. This is critical for applications that involve the incorporation of biologics such as growth factors, where too much may cause undesirable side effects and too little may have no substantial effect at all.
“Current methods for delivering these highly potent molecules lack the ability to exert tunable control,” said Shah. “Additionally, current methods of coating with ceramics typically result in films that are too thick and unstable, which can break off in patches and negatively impact the success of the implant.
We have demonstrated it is possible to achieve the desired bone regeneration with these nanoscale films, which are tethered to the implant surface via electrostatic interactions and do not have the same drawbacks.”
The LBL coating process can be a post-sterilization procedure that requires no modifications to the existing manufacturing process. For example, the femoral stem of the hip implant is coated using LBL under sterile conditions in a clean room by dipping or spraying the implant with different solutions containing the osteophilic materials. Once the coating process is completed the implant can be packaged and shipped.
“This technology is highly versatile,” concluded Shah. “It can be applied to any geometry and the result is a uniform coating all around. The coating can be used for joint replacements, as well as fixation plates and screws for setting bone fractures.”
Mark Crawford is a full-time freelance business, marketing and communications writer based in Madison, Wis. His clients range from startups to global manufacturing leaders. He can be reached at firstname.lastname@example.org.