Evaluating Material Options for Orthopedics

Orthopedic device manufacturing continues to rebound from the COVID-19 pandemic. Elective surgeries are still on the upswing and orthopedic technology companies are also investing heavily in R&D. Although new implants, devices, materials, and instruments are in high demand, supply chain delays continue to slow down production and extend lead times. This, however, is starting to stabilize. “Most of the supply chain issues are behind us, although lead times on average have increased by eight to 12 weeks,” said Omar Hameed, marketing director of Magnum Medical, a Chandler, Ariz.-based provider of medical, surgical, and diagnostic products for the medical device industry.

Resin suppliers, compounders, and processors are still struggling to catch up with manufacturing demand. Vexing labor shortages are also contributing to longer lead times. “Lead times continue to extend and currently, many specialty materials, including medical grade stainless and cobalt alloys, are now well over one year,” said Stephen Smith, director of marketing for Banner Industries, a Chicago, Ill.-based provider of centerless grinding, turning, straightening, and associated value-added services to the medical device industry.

In addition, he noted the Russian invasion of Ukraine and subsequent decision by many governments and companies to not purchase materials from Russia have stressed the supply of titanium from U.S. domestic and European sources.

Determined to shield their customers from global supply chain disruptions, CaP Biomaterials, an East Troy, Wis.-based contract manufacturer of calcium phosphates for medical devices, decided to manufacture its own high-purity raw materials from base chemicals.

“Although this took considerable time and effort, we managed to keep our customers’ products going out on time,” said Onno Visser, managing director for CaP Biomaterials. “This solution also deepened our understanding of certain aspects of our own manufacturing process, which we have been able to apply to gain even more control and efficiency over how we manufacture materials.”

Latest Material Trends

The major trend in medical-grade materials is the continuing development of additive manufacturing (AM) and powder metallurgy. This is especially true for titanium and titanium alloys, where powder manufacturers are working to produce more generic grades that can be used for a greater variety of powder processes. Demand for bioabsorbables and bioresorbables, ketone-based materials such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), and thermoplastic polyurethane (TPU) continues to grow, driven by a growing variety of implantable applications “and the ability to 3D-print customized implantable applications,” said Larry Johnson, president of Foster Distribution for Foster Corporation, a Putnam, Conn.-based custom compounder of medical thermoplastic compounds.

Biomaterials, once considered too sensitive for AM methods, can now be 3D-printed, opening up a wide range of new potential applications. For example, 3D-printing equipment maker Boston Micro Fabrication (BMF) and 4D Biomaterials (AM printing materials) have collaborated to develop a 3D-printing process for micro-scale geometries that uses specialized bioresorbable materials.¹ The technology combines BMF’s projection micro-stereolithography (PµSL) process with 4D Biomaterials’ polycarbonate-urethane-based resin inks. This new AM capability could enable a variety of new designs, including rigid orthopedic devices and micro-scale soft-tissue structures. The system can also be tuned to create a range of highly specific mechanical properties.

The material science behind bone void fillers (injectable and moldable) for bone growth and remodeling continues to advance. “Key features such as surface area, extremely high purity, specific surface topologies, inclusions of silica, zinc, magnesium, and other elements have kept us developing new processes and tighter process controls,” said Visser. “We have had an increase in demand for powders in the <10 micron range, with tight particle size distribution requirements. In response, we have invested in specific equipment to meet these customer needs.”

The increased popularity of robotic-assisted surgeries, new designs for minimally invasive products, lightweight handheld technologies, and disposable instrumentation are prominent trends in orthopedics. “Material characteristics are important for these specialized devices, such as strong wear characteristics and being lightweight,” said Jeff Ribley, district sales manager for the Midwest U.S. for Omniseal Solutions, a Saint-Gobain business that provides advanced materials and solutions for the medical device and life sciences industries. “Materials that offer these advantages include composite materials with high mechanical strength, custom polyimides that are self-lubricating and high strength, and polytetrafluoroethylene [PTFE] for low friction with outstanding chemical and wear resistance.”

Metal parts can also be replaced with lighter plastic parts made from specifically engineered materials that meet or exceed the same material properties that metal parts have. Plastic components that replace their metal counterparts can be custom-fit and designed for enhanced fluid, pressure, friction, and wear control. Examples include surgical- and dental-powered hand pieces for rotary or reciprocating motion, as well as in-vitro diagnostics for wear and friction control.

What OEMs Want

Ketone-based materials, bioabsorbable materials, and TPUs currently dominate the orthopedic device market, simply because they have long been available for implantables and have proven track records of success. “The ortho market is, however, interested in other materials as they become available for implantable applications,” said Johnson. “As a result, material companies are getting more comfortable with these applications and are starting to market materials for use in implantables.”

Selecting the right implantable material depends on the application and the environment that the product will be in when inside the body. For applications that require very high strength, a ketone-type product or TPU is likely the best choice. Materials with highly specific, customized properties can also be engineered by materials manufacturers to meet proprietary applications. “TPUs are a good example,” said Johnson. “A few companies work directly with customers to make proprietary TPU materials that only a particular customer can use for its application. Foster works with customers to functionalize materials for specific purposes as well. These technologies can be developed in the synthesis stage or after synthesis in the compounding stage.”

There is comfort (and lower cost) in using highly proven materials, as well as faster time to market. To keep things simple, OEMs look for off-the-shelf biocompatible medical and implant polymers for added strength, wear, and performance. These are often standard, validated materials with long histories of safe use, which is especially important when considering the recently implemented European Union Medical Device Regulations (EU-MDR), which have compelled OEMs to request additional information on material certifications for their products. “One specific issue from a raw material standpoint is the need to report the cobalt content for stainless steel grades, as any components containing over 0.1% cobalt content by weight must have this specifically stated as part of the labeling for the finished device,” said Smith.

Medical device manufacturers (MDMs) are increasingly interested in self-lubricious materials such as silicone formulations or ultra-high molecular weight polyethylene (UHMPE)—a biocompatible and self-lubricating polyolefin. Using self-lubricating materials can streamline production and eliminate secondary processing steps. A lubricious material has a low coefficient of friction, which helps improve flow rates in fluid systems, resistance to abrasion, and ease of pushability for inserting rods or shafts, providing an “overall slickness of the material’s exterior for applications where this type of surface smoothness is particularly crucial,” said Ribley. “Utilizing lubricious materials also allows companies to drastically reduce the amount of lubricants or oils that they would otherwise use.”

Because of increased demand for material certifications and for processing of materials in controlled environments, CaP Biomaterials recently completed an 11,000-square-foot expansion that allows CaP to perform post-sinter processing in a cleanroom environment for highly demanding applications. “These upgrades in our manufacturing capabilities, as well as the expansion of our leadership team, allow us to work even more effectively with our customer’s R&D teams during all stages of their development process,” said Visser.

Manufacturing Considerations

New manufacturing methods, such as high-precision computer numerical control (CNC) machining, 3D printing, and micro-injection molding allow for rapid prototyping, faster design iterations, and manufacturability of advanced designs—ultimately simplifying overall device architecture. Omniseal Solutions provides engineering resources for testing and finite element analysis (FEA) to create materials that withstand unique or challenging environments. “Many of our customers require 1 million-plus life cycles and our materials and products must be able to perform in these harsh environments and still reduce equipment maintenance,” said Ribley. “We create and test material solutions until we find the right combination that meets or exceeds a customer’s performance requirements.”

Many OEMs present ideas to their contract manufacturers (CMs) for products and processes that they developed at lab scale, with non-medical grade raw materials. They are hopeful that their material partners can develop a process that delivers materials as close to the “original” as possible, but also scalable to commercial quantities and with the proper quality controls to support their product approval process.

“This is often challenging, since these processes do not scale up quite as easily as they were hoping for, which is why CaP does all R&D work on full-scale manufacturing equipment,” said Visser. “Our in-house testing capabilities and broad range of production processes and equipment enable us to come up with a suitable process fairly quickly. Time and budget are definitely of the essence, as there is typically a strict deadline on getting product to market.”

Omniseal Solutions has developed over 500 unique material formulations, many of them co-developed or developed for specific life science applications. Some of the environment or application requirements include type of energy (e.g., electric, pneumatic, ultrasound, laser), temperature or pressure, cycle or lifetime requirements, sterilization process, and biocompatibility. “For example,” said Ribley, “high-speed surgical handheld devices need to be easy to handle and lightweight—our engineering team has developed custom materials and designs for these applications, such as the self-lubricating materials that are in our Meldin bearings.”

Miniaturization is a key trend that increasingly impacts material selection for orthopedic device designs, which typically require specific formulations. A good starting approach is selecting plastic raw materials and powder particle sizes that are appropriate for the wall thicknesses that need to be molded/extruded, which can be very thin for micro-molded parts. “These micro-molded parts can be difficult to fill while still maintaining the strength needed for the application,” said Johnson. “Some of the particle sizes used as fillers in micro-molded parts need to be small enough so that the particles do not create a rough surface—for example, nanoparticles in at least one direction.”

Cap Biomaterials has seen more requests for very small particles with tight particle size definitions for inclusion in polymer composites or putty products. Companies are looking to find the optimum material for peak biological and mechanical performance “and are open to assessing the different properties of materials that are, for example, manufactured with different sintering temperatures or various calcium/phosphate ratios—each of which can have significant effects on the handling, mechanical, and biological properties of the final device,” said Visser.

Miniaturization and increased tool functionality are also in high demand for robotics and motorized, minimally invasive surgical equipment to improve maneuverability, access, and precision during surgeries. These requirements often call for “unique materials properties, such as the low friction and low wear of advanced polymer compounds and composites, combined with very high precision manufacturing that allows higher design flexibility and integration, such as for miniature motor components,” said Ribley. “And while materials are key for the clinical application, from the manufacturing point of view, it is also important that the material can be processed/manufactured accurately for miniaturization components.”

Material Magic

Material challenges can generally be solved through creative processing techniques, as well as material engineering input—for example, using appropriate fillers that adequately functionalize the material and processing techniques that preserve raw material properties and allow for proper mixing of the constituents—this is especially true for miniaturized devices.

As a custom compounder of medical thermoplastic compounds, Foster Corporation recently collaborated with a large medical OEM to develop a minimally invasive medical device, utilizing a complex set of ingredients (multi-functional reinforcements, additives, and pigments) in the form of a compounded pellet. In addition to the complexity of the injection-molded compound, “we were able to work with the OEM and the processor to supply a small minimum order quantity [MOQ] of development samples [25 pounds up to production quantities of 20,000 pounds] in support of the program,” said Johnson. “Foster also provided product and process validation work, material regulatory support, compliance support, unique certificate of analysis [COA] testing, and sourcing of the raw material.”

Evonik has succeeded in formulating PEEK to have osteoconductive properties by incorporating an additive that makes bone cells adhere to the implant. “The functional specialty additive is biphasic calcium phosphate,” said Bing Carbone, president of Modern Plastics, a Shelton, Conn.-based provider of high-performance, medical-grade polymer stock shapes for the medical device industry. “Calcium phosphates are a natural component of bone. If osteoblasts find body-like substances at the implant, they can dock there more easily. This positively influences osteointegration, the interfacial fusion between bone and implant, and patient outcomes.” Evonik has also developed VESTAKEEP PEEK, a filament for the 3D-printing of custom implants. VESTAKEEP is more rigid and durable than steel and weighs less, making it ideal for replacing metal parts and part consolidation.

CaP Biomaterials’ patented composite of hydroxyapatite and calcium carbonate is going through final testing, leading up to the expected FDA 510(k) clearance by one of CaP’s customers. CaP has been granted patents for this material in both the U.S. and Europe. “Testing this material as a stand-alone bone void filler in highly challenging pre-clinical trial models has shown that this composite material outperforms other bone void fillers when it comes to the speed and quality of bone growth,” said Visser. “Our patent portfolio also includes novel calcium phosphate materials coated with bioglass, which aligns with our business model of exploring applications with customers that are looking to develop high-performance materials. They can leverage such materials to offer excellent performance and allows them to clearly differentiate their products from standard calcium phosphate products.”

Most surgical handpieces require materials that protect the internal electronics during operation. Smaller seals and bearings for minimally invasive devices require smaller components that can be maintained at higher speeds (10,000 to 15,000 rpms for orthopedic drills, for example). Some applications require longer running times than others; therefore, each surgical handpiece is unique and requires a custom design and materials. “Omniseal uses two different seal materials for unique friction and pressure requirements on the rotary shaft,” said Ribley. “The unique properties of the two different materials provides ideal wear and friction control. These combination materials also complement each other, with one having stronger friction and the other stronger wear characteristics. We can also provide miniature vanes for pneumatic motors with self-lubricious and lightweight materials also providing noise reduction during usage.”

DSM Biomedical has developed polycarbonate polyurethane-urea (PCPU)-based Biomerix, a non-degradable medical-grade thermoset polyurethane that has passed ISO-10993 requirements for biocompatibility and is biostable, biodurable, reticulated, elastomeric, and proven to support tissue in-growth in a variety of vascular and soft tissue applications. For rotator cuff repairs, Biomerix is an ideal material for the reinforcement of scaffolds that help distribute loads over the fibrous tendons. Biomerix provides a unique three-dimensional, open-cell microporous structure with an interconnecting network of pores ranging in sizes from 200 to 500 µm. Biostable scaffolds with highly accessible void content (>90%) allows cellular infiltration and bio-integration with the surrounding tissue, improving strength and stability. Biomerix can also be preloaded with collagen or active pharmaceutical ingredients such as antimicrobials or inductive signaling molecules, and also be customized to the geometry, mechanical needs, and properties for the application.

Moving Forward

Material science will continue to play a critical role in the development of new orthopedic devices that are smaller and more complex than ever before. Using the most advanced manufacturing processes, such as AM and rapid prototyping, these devices will be made with the highest precision for complex applications such as robotic-assisted surgeries and minimally invasive procedures. As AM evolves, so will its materials, resulting in one-of-a-kind devices that expand the range of orthopedic applications. For example, new generations of surgical robots will require advanced self-lubricious and stronger-wear materials.

Despite the impressive rate of innovation and new product releases in the orthopedic market, increasingly tough regulatory requirements in the U.S. and across the globe could slow down the development of new orthopedic materials. Even with these concerns, materials innovation continues. For example, using a novel polymerization process, scientists at the Massachusetts Institute of Technology (MIT) have developed a new polymer that is twice as strong as stainless steel but as light as plastic.² It can also be easily manufactured in large quantities. The new MIT material is a two-dimensional polymer that self-assembles into sheets, unlike all other polymers, which form one-dimensional, spaghetti-like chains.

“This new material will enable us to do new things,” said research leader Michael Strano, professor of chemical engineering at MIT. “It has very unusual properties and we’re very excited about that.”

Johnson thinks that perhaps the biggest breakthrough with materials is that more material suppliers are becoming interested in developing new solutions for the orthopedic device market and thereby will expand materials science for these unique and challenging ortho applications.

“More material availability and deeper material science means a wider range of design possibilities and more innovative material solutions, which will make medical devices more functional and easier to use,” said Johnson. 

References

1. bit.ly/odt221191
2. bit.ly/odt221192

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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.