Metallic Make Up for Orthopedic Materials

Rebounding at a steady pace from the COVID-19 pandemic, orthopedic device companies are busy, investing in new product development and doing their best to manage their supply chains. Even though elective surgeries are on the rise and demand for implants, materials, and components is high, supply chain delays for some products can be a year or longer, which makes forecasting extremely frustrating. This, of course, slows down production and makes it harder for resin suppliers, compounders, and processors to catch up with manufacturing demand; this is especially true for specialty materials, including medical grade stainless and cobalt alloys.

For example, some mill lead times for medical grade stainless steels and cobalt alloys extend as far out as a year, “with current mill lead times out until mid-2024 at the earliest,” said Stephen Smith, director of marketing for Banner Industries, a Carol Stream, Ill.-based value-added processor and distributor of medical grade bar, coil, plate, and sheet for the orthopedic, dental, spine, and trauma markets.

Mill lead times for titanium grades have jumped from about 20 weeks to 50-56 weeks over the same period due to the Russian invasion of Ukraine and sanctions against Russia, causing termination of contracts for titanium with Russian producers, which account for about 35% of global titanium consumption. The resurgence of air travel is causing the need for new aircraft, which further depletes available titanium resources. “Both Boeing and Airbus have expanded production targets for 2023 and 2024, which creates a greater demand for titanium and corresponding increased mill lead times,” Smith added.

Another trend in the orthopedic market is the increased use of biomaterials for customized, patient-specific implants—not only by MDMs, but also hospitals and ambulatory surgery centers at point of care. For metals, researchers are looking at ways to improve osseointegration with bone by enhancing surface treatments and coatings. Although additive manufacturing (AM) and ceramic materials for medical implants continue to advance, key challenges such as weight, cost, and microporous structure remain. For synthetic polymers, MDMs are using “minerals like hydroxyapatite to improve the bone-like interface,” said Robert Joyce, president and founder of FibreTuff, a Toledo, Ohio-based provider of biomedical-grade materials and parts for 3D printing, molding, and extrusion of orthopedic products, including anatomical bone models, surgical guides, and temporary implants. “However, processing challenges with the 3D-printed customized synthetic polymer implants with minerals include loss in implant mechanical properties—for example, tensile and flexural strength, with the minerals being resorbed by the body after two to three years.”

Bioresorbable implant materials are relatively new biomaterials in the orthopedic industry, but are gaining plenty of interest (and R&D). For example, in a recent study, high-density MgB2 and a composite material consisting of polylactic acid (PLA) matrix and MgB2 powder were tested in vivo as candidates for biodegradable materials for orthopedic implants. Both materials proved to be biocompatible with bone and adjacent soft tissue 90 days after implantation, with no cellular dysplastic changes. The authors concluded these magnesium materials “show excellent biocompatibility, optimal biodegradation, and good tissue regeneration.”¹

Although supply chains are settling down, there are still plenty of delays that worry material providers and their customers. To prepare for future shortages, companies are adding to their raw materials inventories whenever they can. “We have definitely increased our safety stock, not just for raw materials and spare parts, but also for intermediate and final products,” said Onno Visser, managing director for CaP Biomaterials, an East Troy, Wis.-based manufacturer of high-quality, custom-fabricated calcium phosphate products for biomedical applications. “Our customers have reacted mostly positively to risk-sharing proposals that support these stock adjustments.”

Latest Material Trends

Key performance factors such as improved biocompatibility, corrosion resistance, and high fatigue strength are in demand by MDMs for manufacturing devices that improve patient outcomes. “Trends such as patient-specific solutions, minimally invasive surgeries, and robotic-assisted surgeries build on these critical material requirements,” said Gaurav Lalwani, strategic business and applications development lead (medical) for Carpenter Technology, a Philadelphia, Pa.-based melter and manufacturer of bar, wire, strip, plate, and powder in iron, nickel, cobalt, and titanium alloys for numerous ultra-critical components.

There is also enhanced focus on nickel allergies and the health risks of cobalt in materials. MDMs are looking to their suppliers for guidance and compliance concerning restrictions on chemistry brought about by the new EU MDR requirements, “including reporting and controlling the residual levels of cobalt in stainless steels,” said Smith.

Several areas in orthopedics and cardiology present significant opportunities for new material development or enhancement of existing materials, such as spine and robotics. “In spine, there is growing adoption of additively manufactured titanium components using Ti64ELI 3D printing,” said Lalwani. “Within robotics, OEMs are looking for tighter tolerances and stronger materials to develop instrumentation tools to support minimally invasive surgeries (MIS) procedures.”

Extremities is another hot market.

“We are seeing double-digit growth for extremities,” said Smith. “Even large joints are expected to have mid-single-digit growth in 2023, almost double the average of the previous decade.”

Calcium phosphate is a favorite go-to compound for implant developers, especially as they deepen their understanding of how these materials can be tuned to create customized handling and performance characteristics. For example, calcium phosphates are often used as filler materials in polymer composites to create very specific mechanical and chemical properties for joint implants. MDMs seek out knowledgeable material specialists with the capabilities to optimize the materials they want to use in next-generation compounds. “Details really matter for applications like this,” said Visser. “Our ability to fine-tune such parameters as surface area, particle size, and chemical composition allows MDMs to test a range of composites to find the ideal formulation for their application—for example, adding precise amounts of elements such as strontium or zinc.”

Developing materials to meet evolving design criteria and component functionality has always been a critical part of the medical device industry. There is a constant need for high-strength and tough materials to support MIS by reducing their implant or instrument footprints and improving wear resistance. There is also a growing need for materials that are compatible with next-generation methods of production, such as “lights-out” manufacturing, six-axis CNC (computer numerical control) machining, Swiss-style machining, and hybrid equipment, which can combine AM with CNC machining.

“Properties that are most attractive for implants include a Young’s modulus similar to bone, biocompatibility, hydrophilicity, minimal or no degradation, bioactivity, and good cell regeneration,” said Joyce. “Most OEMs seek out these properties to create superior implant-to-bone interfaces that will accelerate healing time and reduce infection for customized implants.”

MDMs do investigate material innovations with the intention of either re-engineering and/or developing proprietary materials and alloys with their material partners. Increasingly, MDMs work directly with material compounders to formulate unique or proprietary materials for their applications. Sometimes, this may only require slight modifications of patented formulas to create the specific properties required for an MDM’s device.

Most OEMs have their own internal specifications for medical-grade materials. “Many of these are identical to the ASTM specifications, but there are often slight modifications, which require that order requirements are very carefully analyzed to ensure compliance with the OEM specifications,” added Smith.

Carpenter Technology works directly with OEMs to help solve their material challenges with R&D resources that align with the customer’s design team. Carpenter’s R&D facility provides small melt capability (about 300 pounds), mini-mill production, and a full testing lab to help develop both the material and the validation data to support adoption. “This greatly reduces the barriers to evaluating material innovation and builds confidence in potential performance characteristics,” said Lalwani.

Additive Manufacturing

The major trend in medical-grade materials is the continuing development of 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. “Some material producers are taking a creative approach for making second-generation biomaterials with synthetic polymers,” said Joyce. “One specific focus includes giving PEEK hydrophilicity qualities to improve the implant-to-bone interface.”

Materials for orthopedic implants have steadily advanced, utilizing creative surface designs, textures, and even minerals for improving osseointegration. “For example,” said Joyce, “hydrophobic synthetic implants made with PEEK have incorporated bone minerals like hydroxyapatite and calcium phosphate in the compounded formulations. These minerals provide an improved bone interface and promote bioactive performance.”

Typically, for alloys such as nitinol, chemistry changes during the additive manufacturing process. These changes can be due to vaporized elements such as nickel or oxygen pickup from the ambient atmosphere, depending on the printing environment. “These changes can mean big differences in how a material and its final component performs,” said Lalwani. “There are challenges with cracking, build failures, and undesirable mechanical properties that can result. In addition to the chemical variation, anisotropic grain size and mechanical properties can also vary depending on the printing parameters. Lastly, some alloys with reduced weldability have additional hurdles to overcome in additive processes and require alternatives or unique processing strategies, such as additive powders with customized chemistry.”

Fiber alignment and the behavior of fillers can also be a challenge. For example, very small fillers can lose their effectiveness in additive manufacturing. Medical engineers are intent on designing smaller pores in orthopedic implants, which more effectively emulate cancellous bone. This also builds more mass into the part, increasing strength and improving performance.

Overall, the adoption of AM processes across a variety of medical applications will only continue to expand. The ability to produce custom-to-patient components via additive manufacturing is the key to improving patient recovery and overall outcome/quality of life. In addition, AM combined with machining (subtractive) provides unique options for the producer. “As AM evolves, we will see more dual machines that incorporate AM build-up and machining practices, driving tolerances, speeds, and surface finishes in both series and parallel operations,” said Lalwani. “This, combined with unique AM processes, enables the creation of functionally graded implants with carefully tuned material properties for each application.

“Also,” noted Smith, “many AM parts in the medical market require proprietary analyses to meet the OEM requirements or those of the 3D printing equipment. This limits availability and increases costs. However, as AM becomes more entrenched in medical devices, generic specifications are more likely to become prevalent, thereby reducing costs and increasing the cost-effectiveness for AM.”

New Materials and Processes

Carpenter Technology is currently developing a minimum residual stress 17-4PH bar stock for some unique medical machining applications to ensure consistency from bar to bar. This will ensure minimal deflection during the machining operation and open the door to advanced, hands-off manufacturing of complex and thin-wall geometries. “Furthermore, some secondary operations may be eliminated due to first-time-through acceptability,” said Lalwani. “This would be a significant improvement for several medical component machining practices, especially after Carpenter continues this development project with plans to leverage to other alloy systems.”

Additionally, with the increased focus on low-cobalt material options and interest in low-nickel alloy options for allergies, Carpenter has developed BioDur 108. This high-strength implantable stainless steel has seen increased demand in recent years, with designers looking to leverage the material’s unique capabilities to next-generation applications.

FibreTuff has created a synthetic polymer composite without hydroxyapatite that is biocompatible, nonresorbable, and bioactive. This advanced composite is easy to 3D-print and has a novel feature that allows 3D-printed FibreTuff parts to be visible in radiographic images to track healing process. “The filament setup and operation is not processing-intensive and easy to manage at 260°C,” said Joyce. “Parts can be fabricated quickly and can be cut, sawed, and drilled without melting.”

Many alloys have been developed at Carpenter to support the needs of the medical community. For cobalt chrome molybdenum (see ASTM F1537), Carpenter Technology has developed seven variants that support improved machinability, forgeability, and diffusion bonding characteristics of surface coatings. These advancements have positively affected the large joint and spinal segments. Other materials in the Carpenter portfolio include high-strength PH stainless materials for critical instrumentation applications and a novel high-strength Ti-6/4 powder metal named Grade 23+ that allows OEM design engineers to achieve 15% to 20% increased strength with a lower starting oxygen content for AM-made devices.

FibreTuff is piloting new biomaterials to be 3D-printed with FibreTuff technology. The company is also focusing on developing a lower-cost printing platform and improving functionality. “We will continue to work toward commercial viability to produce a permanent implant with improved bone interface, evidence-based healing, and disruptive behavior to lower patient risk with better outcomes,” said Joyce.

R&D teams continue to investigate the possible applications of nanomaterials and nanotechnology in building medical devices—for example, increasing part strength or enhancing coating performance. In many cases, nanotechnology requires special processing techniques, couplings, or compression. Although many developers and their MDM partners are interested in the properties of nano-sized materials, most companies opt for larger particle sizes to reduce risk, as their behaviors and safety profiles are better understood. For example, it can be challenging to process nano-sized calcium phosphate particles because they tend to agglomerate (providing these materials in a solvent could prevent this issue).

The primary “nanoscale” concern is how these materials will interact with the body and adhere to the foreign material. “Nanostructured surfaces promote increased host-implant integration, improve cell adhesion, and have been proven to positively impact the surgical outcomes,” said Lalwani. “Within metals, nanostructured surfaces can either be accomplished via coating applications [thermal spray] or via additive manufacturing.”

Moving Forward

Innovative new biomaterials continue to be formulated or discovered. Researchers at Tohoku University have discovered a super-elastic metal alloy that could be a perfect material for joint implants.² Titanium, perhaps the most popular metal for hip or knee implants, is stiffer than bone, which can lead to bone loss; flexible materials in implants, however, lead to faster wear over time. This new material—Co-Cr-Al-Si—bends like bone, exhibits superior wear resistance, and has a strain recovery rate twice that of commercially available titanium alloys—making it an extremely attractive material for joint replacements.

A new polymer engineered by scientists at the Massachusetts Institute of Technology is twice as strong as stainless steel but as light as plastic. Manufactured using a novel polymerization process, the new material is a two-dimensional polymer that self-assembles into sheets, unlike other polymers, which form one-dimensional, spaghetti-like chains.³ The new material’s elastic modulus—a measure of how much force it takes to deform a material—is four to six times greater than that of bulletproof glass, with a yield strength twice that of steel, but at only one-sixth the density.

Tantalum has intrigued material scientists for decades because of its inert bioactivity and ability to enhance macrophage responses. Tantalum-based products that are already in use in the healthcare industry include staples, stents, and coatings. Recent studies show the living cell density of human osteoblasts is up to six times higher on tantalum as compared to titanium.⁴ AM techniques will soon be able to produce fully dense open-celled structures for load-bearing implants with high normalized fatigue strength and good ductility.

Inspired by the unique structures of adhesives created by mussels and flatworms, engineers at McGill University have created a medical adhesive that outperforms standard glues and other sealants.⁵ Its unique internal structure allows it to easily adhere to the wound area (even if the surface is slick with mucus or blood) without the need for applied pressure. The adhesive immediately stimulates coagulation and can be removed easily and painlessly. The macroporous and dehydrated nature of the adhesive is what creates the strong bond at the wound interface, accelerating the uptake of fluid. “This work opens new avenues for the development of bioadhesives and hemostatic sealants,” said lead researcher Jianyu Li, a professor of mechanical engineering at McGill University.

AI is quickly replacing the expensive trial-and-error approach to developing novel biomaterials. Guided by parameters entered into the computer system, AI can access databases to rapidly test thousands of combinations of materials to generate new formulations that are unique, effective, and sustainable. For example, using AI, DAN*NA—a Barcelona-based green bio-engineering company—has created a bio-PLA [polylactic acid] that is more flexible than similar materials on the market, while still retaining its transparency.⁶ Other AI-generated bio-based materials will be used for tissue regeneration and bioprinting.

Finding the right material does not always require AI to test thousands of formulations, or the use of the newest technologies that challenge the limits of material science.

Sometimes just a tweak or two will do it.

“Generally, it is our experience that when we are asked to push the boundaries of what is possible, answers lie in assessing each individual manufacturing step,” said Visser; “from the synthesis of raw materials [when there are no raw materials available on the market to meet our requirements] to using several types of micronizing such as ball milling, ultra-centrifuge milling, and planetary milling. Going back to the basics is quite often the best way to make progress.”

By optimizing CaP Biomaterials’ processes, the details of known materials and processes goals that seem impossible then become achievable, Visser noted.

“For example, calcium phosphates span a wide range of materials,” he said. “Being able to precisely engineer their surface area, chemical profile, sintering temperature, and more still allows devices to be developed that handle better, and are as biologically responsive as the customer expects.” 

A One-on-One on Material Shortages and Potential Options Based in Rockaway, N.J., Titanium Industries is a global provider of specialty metals for the medical device industry and other high-tech markets. With over 26 years of experience in the high-performance metals industry, Titanium’s vice president of sales and marketing Greg Himstead is a top expert in this field. Orthopedic Design and Technology (ODT) recently sat down with Greg to discuss the material trends that are impacting the orthopedic market. ODT: What is the current state of material availability for orthopedic device manufacturers? Are there still pandemic-related supply chain issues for materials? Himstead: Yes, there is still a scarcity of specialty metals used in orthopedic implants and instrumentation applications. The supply of titanium 64 ELI has been significantly impacted due to Russian titanium no longer being supported. As a result, many OEMs have switched to non-Russian sources for titanium sponge input feedstock, as well as non-Russian ingot melt capacity and downstream forging of bars, plates, sheet, etc. Many other specialty medical grades—including cobalt-chrome-moly, Custom 455, and Custom 465—also have long lead times of approximately 70 weeks. Traditional instrumentation stainless steels are also experiencing extended lead times, which can be attributed to labor shortages and the loss of generational know-how due to increasing early retirements. Robust and resilient supply chain solutions are needed to offset frequent mill production remakes and other push-outs. All alloy families have experienced significant increases in base prices over the last two years, with higher base prices remaining stable while demand continues to outpace supply. ODT: Will these frustrating lead-time delays slow down production this year? Himstead: On the demand side, it is the perfect storm with key specialty metals market segments all in growth mode, including medical implant, airframe, jet engine, defense, oil and gas, electrification of mobility, and space flight all forecast to grow this year between 6% and 15%. In this high raw material demand cycle, the best supply solutions involve 1) aggregating demand across the sub-tier supply network, 2) aggregating demand across the global OEM production site network, 3) locating raw material strategically in region, 4) agreeing upon safety stock quantities, 5) emphasizing forecasting accuracy and looking out over longer time frames, 6) creating flexible price adjustment models, and 7) establishing long-term commitments with the mill supply base that account for the current long lead times. ODT: What are OEMs asking for the most when it comes to materials? Himstead: OEMs are currently encountering a mounting challenge when it comes to managing their production schedules and having enough raw materials. In order to tackle this issue, resilient and risk-mitigating supply chain solutions are being explored. These solutions may include safety stocks, backup plans, and redundancy measures. Although creating fixed, firm price multi-year contracts has proven to be difficult, pricing adjustment systems do offer a viable solution. These systems usually involve a fixed base price on an annual basis, tied to a volume commitment, and a monthly or variable raw material surcharge. ODT: What are some of the advances in materials and material processing that are making a difference in orthopedic device manufacturing? Himstead: The field of materials science is constantly evolving, and recent developments have been particularly promising. With the advent of 3D printing for materials, machine learning for materials discovery, and nanotechnology, the discipline is becoming more efficient and effective than ever before. These cutting-edge technologies are also opening up new doors in engineering material properties, further expanding possibilities in the field. Additive manufacturing is rapidly expanding in almost all market segments. Although producers of powder or wire input stock preserve their proprietary nature, other types of consumable raw material products are no longer limited to prototype and development quantities. Instead, they are now being produced at serial production rates. Through our own unique specifications and best value production routing, we are now providing machined build plates (also called base plates) to the companies that are expanding additive 3D printing production. ODT: OEMs are always looking for ways to save time and accelerate time to market. What is your best advice for making this happen? Himstead: OEMs are increasingly looking for supply chain innovation and resiliency, and are finding that a total true cost of acquisition and risk mitigation approach is best. In this approach, response time, reliability, geographic proximity, release quantities tailored to optimize production, just-in-time delivery, plus up-to-date usage data versus forecast and alloy family health reports about the global specialty metals producers are all included in a multi-year long-term agreement.

References

1. bit.ly/odt230541
2. bit.ly/odt230542
3. bit.ly/odt221192
4. bit.ly/odt230544
5. bit.ly/odt230545
6. bit.ly/odt230546

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Mark Crawford is a full-time freelance business and marketing/communications writer based in Corrales, N.M. His clients range from startups to global manufacturing leaders. He has written for MPO and ODT magazines for more than 15 years and is the author of five books.