More Than Scratching the Surface

The coating or surface modification of an orthopedic device is a precise process that can easily break down if the correct procedures are not followed.

It used to be a painstaking undertaking to ensure that all settings and protocols were correctly being carried out. But in recent years, many coating and surface modification processes have become automated, with some even adding robotics to the mix. These advances ensure that inputs are consistent and the same steps are followed every time. This results in a repeatable, validatable and safe process that ensures all devices receive equal treatment. What follows are examples of different forms of orthopedic device coating and surface modification processes, and the ways automation has impacted them.

Metal Processes

On the surface-finishing side, common processes are passivating, electropolishing, and anodizing. They often involve making devices smoother, shinier, safer, cleaner, and more durable. Other uses for which they are employed include protection against corrosion, wear resistance, color coding, and burr elimination.

The purpose of passivating is to remove free iron and foreign contaminants from the surface of stainless steel parts or devices, said Cheryl Larkin, spokeswoman for Miraclean Ultrasonics, an Ashville, N.Y.-based surface finishing firm. Diluted nitric or citric acid is used, the former being more common. Formulas specified by the A967 standard by ASTM International establish a set time range for the parts to be immersed in the acid. That forms a natural passive film on stainless steel, which enhances its corrosion resistance, she said.

In recent years, more applications have called for citric acid because it is safer, more environmentally friendly, and requires shorter cycle times, Larkin noted.

“Some shops now are expanding their capabilities by setting up passivating lines that have both nitric and citric passivating tanks in the same line, followed by dedicated rinses,” she told Orthopedic Design & Technology. “Other shops are beginning to use citric in some cells and nitric in others, depending on the manufacturing stage, environmental considerations, and specification requirements.”

The passivation process has become more automated to reduce human errors and enhance repeatability and verification, Larkin said.

“The automated passivating line is PLC controlled. Loads are advanced through the process steps by the automated hoist at intervals defined per the specification requirement,” she explained. “Temperature, chemistry concentration, ultrasonic operation, pumps, and other mechanical actions are controlled by the PLC [programmable logic controller] and can be programmed to alarm out and pause or terminate the process if an out-of-specification condition occurs. Optional SPC [statistical process control], bar coding, and data logging are also available to meet verification requirements. In addition to the automatic processing of loads through the automated hoist, a number of additional safeties are typically included in these types of lines such as push-pull exhaust, enclosures, light curtains, and secondary containment to capture any leaks or spills.”

Electropolishing (EP) comes into play when there are irregularities on the surfaces of parts or small amounts of metal must be removed to conform to dimensional tolerances on parts with complex geometries, Larkin said.

Manufacturers who need a high-quality surface finish often go for this option. “Electropolishing is what we do if we want to make a bright part, and to remove burrs or discoloration,” said Bill Loucks, general manager of Electrolyzing Corp. of Ohio, a Cleveland-based firm specializing in surface modification.

“An EP system may be as simple as an appropriately sized single tank with a process timer and a suitably sized rectifier with voltage and current control,” Larkin noted. “In the electropolishing tank, the work piece becomes the anode in a process that combines chemistry (electrolyte), cathode, voltage, and process time to create the desired effect.”

ASTM Standard E 1558-09 provides guidelines for the electropolishing of various metals and includes substrate-specific formulas for electrolytes, cell voltages, and process times, Larkin added. At the end of the process, the part is rinsed to remove the electrolyte and any residuals.

As with passivation, more EP operations are now being automated.

Parts are advanced through the line via a PLC-controlled programmable hoist, which improves operator safety by minimizing potential contact with process chemistries or voltage, Larkin said. Automation, she added, can be programmed to control features such as process times, temperature, agitation, pump operation, and rectifier operation.

Other features include alarming and terminating out-of-specification operations, statistical process control, bar coding, and data logging.

Miraclean offers two types of titanium anodizing for the medical device industry: titanium color anodizing and Type II titanium anodizing. In both cases, a clean part is immersed in an electrolyte and exposed to an electric current at a voltage required to produce the desired effect. After being anodized, the parts are thoroughlyrin-sed and dried. Sometimes passivation follows.

Titanium color anodizing is used primarily for color coding titanium parts for easy product identification. It creates a stable oxide layer on the surface of the titanium through the application of the current, according to Larkin. “Different voltages produce discretely different thicknesses of oxide layers which results in different color effects,” she said.

Type II titanium anodizing produces a gray titanium part with such enhanced properties as anti-galling, lubricity, and improved wear resistance, Larkin noted.

Temperature, voltage, and current are the most important factors in the anodizing process, Larkin said. Automation allows these factors to be better monitored and controlled.

“A typical automated titanium anodizing line will consist of some or all of the following: a pre-clean followed by rinsing, an acid etch to expose the titanium surface followed by rinsing, anodizing followed by rinsing, and drying,” Larkin explained. “The parts are moved through the process steps at the appropriate times by the automated hoist in accordance with the process specification. At each station, the automation also controls other critical process parameters such as temperature, chemistry concentration, ultrasonic operation, agitation and pumps, and voltage and current.” As with EP and passivation, automation of anodizing reduces human error and improves safety.

For parts that are color anodized, a scanner can be integrated into the automated system for color-matching purposes, she added.

Another surface modification process is a chrome-based proprietary one from Electrolyzing Corp. of Ohio: Medcoat 2000. The hard chrome process improves both lubricity and wear characteristics.

“Big OEMs sell implants but loan out or give away instruments. So they want to make the instruments last as long as they can. Treating with Medcoat 2000 can add 2-4 times the life to those products,” Loucks said. “The part will hold its aesthetic looks much longer. It makes them all look uniform and adds a wear-resistant coating.”

Executives at Electrolyzing Corp. have turned to automation to ensure that all parts have the same parameters.

Processes that apply metal coatings to devices such as titanium and HA (hydroxyapatite) also have become automated, said Robert Willson, head of research and development for APS Materials Inc., a Dayton, Ohio-based coatings firm.

“Everything goes through a very automated system to ensure more accuracy and more repeatability,” he told ODT. “We have our own development system for depositing coatings on different implants, geometries, and substrates. Part of our strategy is finding ways to apply coatings to different substrates, whether they are PEEK, cobalt chrome, or whatever. Through our development process, we come up with the characteristics that are required. We determine the customer requirements and especially the specific material requirements. And we can adjust our system to attain them.”

Blasting Processes

Great strides also have been made in automating blasting systems, which is another way to modify implant surfaces.

“The past year has been quite an active year within the orthopedic community,” said John Carson, manager of marketing and business development for Guyson Corp., a Saratoga Springs, N.Y.-based firm specializing in blasting. “A lot of new technology is being adopted. For example, having robots involved directly with surface modification, where they work directly with the blasting apparatus, is kind of novel. What’s really occurred to me in the past year is that the other elements of a blasting system can be configured to support the QC [quality control] systems and the validation requirements of the medical manufacturers.”

Blasting systems now can be programmed with specific parameters that can ensure accurate performance at any given moment. That information can be fed directly into the customer’s QC system. “This is a drastic change from only a few years ago. This gives an advantage of greater control and more efficient, repeatable methods,” Carson said. “It has an impact on precision. It’s part of a drive toward better quality and elimination of non-conforming processes.”

Among other things, advances in automation have enabled all blasting material to be screened before the process begins, he noted. This screening ensures that all particles that are bigger or smaller than the specified size are removed before they are sent for blasting. The screening also better controls and improves the documentation of blast pressure, feed rate, media flow, speed and angle.

Besides improving the process, automated systems also can reduce labor costs. For example, when a machine is low on media, a signal will occur so the issue can be taken care of and the process won’t have to stop.

“We had one customer whose labor costs on an orthopedic component were reduced by a factor of 10,” Carson said. “It took one-tenth of the time to do the same process. And in that case, they weren’t even using robotics.”

Companies that use blasting machines with robotics need only one human operator for every two or three machines because tasks such as loading and unloading can be automated, Carson noted. In addition, automation allows companies to reduce their operating unit to a small footprint, which can help keep things clean and organized, and move the product efficiently between processes.”

To continue to take advantage of these benefits, Guyson executives have signed a development agreement with a major robotics firm, Carson said.

Bioceramic Processes

Several processes can be used to apply bioceramic coatings, according to Gene Elwood of Ionbond’s North American Medical Coatings Competence Center. Ionbond is a surface engineering firm with global headquarters in Olten, Switzerland and North American headquarters in Madison Heights, Mich. The processes that Ionbond employs are:

* Physical Vapor Deposition with ARC Evaporation (PVD-ARC)
* Unbalanced Magnetron Sputtering;
* Chemical Vapor Deposition (CVD); and
* Plasma-Assisted CVD (PACVD). 

PVD-ARC is the most common choice for medical applications, Elwood said, because its properties are well-suited to the materials used in medical manufacturing. It also has a relatively low processing temperature.

“You can use it with all varieties of stainless steels on the instrument side,” he said. “It is at the top of the PVD food chain relative to the hardness and adhesion of coating.”

Conversely, CVD cannot be used with stainless steels because it has a relatively high-temperature deposition process, thereby making it an inferior choice for most medical applications.

Unbalanced Magnetron Sputtering was a process developed to improve certain qualities of traditional sputtering, and those improvements have been beneficial to medical-device coating processes. “It gives a more dense and adhesive coating than traditional sputtering,” Elwood noted.

PACVD is used for applications that demand low temperatures but require the hardness and smoothness of a diamond film. Ionbond uses this process for its ADLC diamond coating.

Coating choices usually are made after an extensive consultation with the device company.

“We first have to understand their application and their performance objectives,” Elwood said. “Then we get together and review the materials they are using and the other processing steps involved. That prompts a decision about the correct coating, and the correct processing technique follows from there. The customer rarely, if ever, comes to us and says ‘use this technology.’ They say ‘do you have this coating? We need you to deposit it.’ Or ‘which coating do you recommend?’ The processing part evolves from there.”

As with metal-based processes, bioceramic coating equipment is fully automated. “It is equipment-intense but not labor-intense. You can have multiple systems managed by one skilled operator,” Elwood said.

Device firms should be certain their coating provider’s equipment is fully process-validated and their coatings are compliant with ISO 10993-1. Ideally, providers also should have an analytical lab where difficulties in processing can be worked out. Ionbond uses its lab to perform root-cause analysis to help customers reduce waste, save costs, and improve turnaround.

Such analyses, if done collaboratively, also can help device manufacturers avoid problems that may occur before the coating stage. “We can do a reverse audit of sorts,” Elwood noted. “We go to the customer, who knows all of their processes. We know what causes us issues, but we don’t know what might be causing issues before the product gets to us. We can be a different set of eyes for them and help them sort out their problems at all stages. For example, they might ask us to jump in and help on a problem with corrosion.”

Polymer Processes

Pictured above is a work cell robotic blast system based on a single spindle rotary blast machine model used by orthopedic implant and instrument manufacturers. Photo courtesy of Guyson Corp.

Polymer coatings are applied as liquids or powders, sometimes electrostatically, said Donald Garcia, director of research and development for Hudson, Mass.-based Boyd Coatings Research. But the exact process used depends on a number of factors.

“The geometry of the part, the type of polymer to be applied, the thickness needed, and the intended purpose of the polymer as it relates to the function of the device are all considered,” he said. “There are volume and cost considerations as well. If the process is done with the maximum transfer efficiency, that can drive costs lower.”

Tooling also must be considered. “Do you make an investment in a tooling system, or will each part be custom coated?” Garcia asked. “That is where volume plays a role, as well as how much the device will cost and whether it is reusable or disposable. That gives you a picture of what can be afforded.”

Automation and robotics are common in polymer coating processes, as they help improve repeatability, and both input and environmental controls. Automated data collection programs can help customers integrate inputs, outputs, and inspections, resulting in better control of processes and inputs, Garcia said.

A New Process

What kind of efficiencies could be attained if coating was not a separate process at all, but an integrated part of implant manufacturing? Medical device manufacturers in the United States will discover the answer to this question later this year or in early 2011, when the electron beam melting (EBM) process isexpected to receive 510(k) clearance from the U.S. Food and Drug Administration.

EBM, offered by Arcam AB of Gothenburg, Sweden, is an additive manufacturing process, said Stefan Thundal, Arcam’s industry marketing manager for orthopedic implants and aerospace segments.

“You start with a metal powder. It is melted in thin layers to build up the part, and the process is managed by a 3-D CAD [computer-aided design] model,” he said. “Our most important business area right now is orthopedic implant manufacturing, particularly for implants with porous structures, what we refer to as Trabecular structures. The system builds the entire implant, both the solid section and the porous ‘coating.’ You design the entire implant including the porous section in a CAD environment, and you build it in a single-step process. You don’t need to apply a coating afterward.”

The technology has its roots in rapid prototyping, though EBM produces real standard implants in medical grade materials, Thundal noted. EBM-manufactured standard im-plants have been on the market in Europe since 2006, but they have not yet hit the U.S. market aside from custom implants. That could change within a year, however.

“To achieve the properties you look for in osseointegration, primary fit, etc., you may optimize your design in terms of pore geometry, pore size, relative density, and roughness,” Thundal said. “It provides great design flexibility. You have an opportunity to design what you really want in terms of porosity. That gives OEMs freedom to really differentiate their products. With traditional methods, you are very limited by the particular process or processes you have access to.”

EBM also increases productivity and lowers costs. “Press fit implants are cheaper to produce with our technology largely due to the fact that there is no secondary process to apply a coating,” Thundal added.

So far, the most popular application for EBM manufacturing has been acetabular hip cups, but hip stems and spinal cages recently have hit the European market.

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Automation of coating and surface modification processes is one of the most significant trends in orthopedic medical device manufacturing. Advances constantly are being made, and entirely new paradigms are being offered, as evidenced by the introduction of EBM.

“Whether parts are outsourced or produced in-house, automating the processes by which they are manufactured, and optionally tracking and storing relevant data regarding the processes, reduces risks of operator error and provides a positive and proactive verification response to the stricter regulatory climate,” Miraclean’s Larkin explained.

“Automation of processes also has the additional benefits of reducing manual labor, freeing operators to perform other functions, and improving operator safety, minimizing or eliminating operator contact with potential chemical or equipment hazards.”