The use of biomedical materials has a long and fascinating legacy characterized by creativity, innovation, and positive medical outcomes. Since the dawn of civilization, man has been exploring ways in which natural materials might replace or enhance the natural functions of the human body. Archaeological evidence suggests, for example, that the ancient Egyptians used sea shells to replace missing teeth and linen to close wounds — creating what may have been the first dental implants and sutures.

Other examples of natural materials once used in medical applications include coconut shells used to close holes in skulls, as was the practice in Tahiti, and elephant ivory used to create the first recorded hip implant in 1891 in Germany.

The real revolution in biomedical materials began in the second part of the 20th century, with the introduction of synthetic materials that let medical device makers — particularly those focused on orthopedic implants — break free from many of the limitations and risks associated with relying solely on natural materials.

Today, healthcare providers and device makers are finding solutions with the potential to address key problems associated with one of the greatest challenges facing the world today: our aging population.

Companies focused on biomedical materials R&D — particularly in polymer chemistry and polymer technologies — are taking a leading role in the rapidly growing orthopedic market by accelerating technology that makes new and improved treatments possible. As a case study, consider the current demands in the orthopedic world and some of the ways innovation in polymer and fiber technologies are helping device makers meet these emerging needs.

While people don't want to acknowledge the fact, the human body performs optimally for only approximately 45 years. After this time, it is actually past its physical peak. Because current culture encourages staying physically active throughout our lifetimes, many people have increasingly high expectations for medical treatments and devices that will extend their active lifestyles. Add to that cultural expectation, changing demographics (aging populations) and physiques (most individuals are heavier today than in decades past), and you have a recipe for a dramatic increase in orthopedic problems and demand for more effective orthopedic treatments and technologies.

Durability is key

In the orthopedic world, increasing the durability of materials is key, as the number of “repairs” that a patient can undergo on their shoulder, knee, or hip is not without limits. Implants currently being used have progressed enormously during the years, but still there is a considerable risk that they will need replacing after about 15 years. Needless to say, this gives rise to a continuous need for better, stronger, and even more durable materials.

Note that addressing the growing demand for new and more effective orthopedic treatments cannot be addressed solely by creating more durable materials. Biomaterials R&D must take a broader view, and those companies in the field must aim to develop the broadest product portfolios possible.

Orthopedic implants require a tremendous diversity of materials. Using the analogy of an orthopedic implant and “a bridge” helps highlight how and why so many different material properties are necessary. In many cases, the implant is either repairing or replacing skeletal body parts and thus it is “bridging the gap between tissues.” Considering implants from the perspective of “a bridge” also helps to identify the types of materials needed to build it: a strong rigid construction material for the pillars (in orthopedic devices this is usually titanium or cobalt-chrome steel), a strong cable for the suspension (ultra-strong fibers), a surface-bearing material to reduce friction and wear (in hip and knee replacements ultra high molecular weight polyethylene, or UHMwPE, is the gold standard) and, finally, a material for shock absorption to reduce vibrations (polyurethanes are deemed particularly effective).

The different materials each have their specific properties and peculiarities, which determine their influence on the design of materials.

A closer look at UHMwPE

UHMwPE fibers are one of the most recent advances in biomedical materials, and they are proving effective in moving implants beyond the limitations of more traditional orthopedic fibers and sutures. Conventional fibers such as polyesters, polypropylene, or nylon have a moderate strength and show a fairly large stretch (elongation) before they ultimately break. UHMwPE fibers are the opposite - the strength is much higher. The fibers on a weight basis are more than 10 times as strong as steel, and a braid or suture made from this fiber has the potential to be twice as strong as a comparable polyester product. At the same time, the elongation is hardly noticeable - when ultimately reaching the breaking strength, the elongation is just about 3%. This specific factor supports new approaches. For instance, the fibers are used in modern ligament fixations where the very low stretch results in a very rigid fixation, improving the chance for fast re-adhesion of the torn ligaments back to the bone.

The real breakthrough of hip arthroplasty was sparked in the late 1960s by the British surgeon John Charnley, the first to apply UHMwPE as an acetabular liner in artificial hips. Since then, the material has been the undisputed gold standard, despite the fact that its low wear level can cause complications in some patients. Originally referred to as “bone cement disease,” osteolysis, the softening and degradation of bone after hip arthroplasty, has ultimately been linked to the human bodies' response to small UHMwPE wear particles.

To address this issue, research in the past decade has focused on improving the wear resistance of UHMwPE, by highly crosslinking the material. Why crosslinking? By means of intensive gamma radiation, a dense network is created by forming chemical bonds between individual UHMwPE molecules, basically creating a hip-cup comprising one single giant molecule. The upshot is a very large increase in wear resistance, albeit at the expense of some of the other mechanical properties. This is important for the sound design of an implant: fatigue resistance and impact resistance strongly reduce as a result of crosslinking, especially after the added impact of thermal processing. Thermal processing is done after crosslinking the material to reduce the level of remaining free radicals, which would otherwise reduce the resistance to oxidative degradation of the material.

Other properties are important to the design of orthopedic implants as well. For example, fatigue resistance is an important aspect in knee arthroplasty where tibial posts are subjected to intense recurring forces. Also important is the yield strength. It determines the maximum value of the stress that can be applied before non-elastic and therefore non-reversible changes in the shape of the implant happen. Clearly this is a critical limitation for every designer regardless of whether the design is in medical implants or industrial applications. A higher yield strength basically means more design safety but also more design freedom.

This is why leading biomedical materials organizations are focused on a completely new class of UHMwPE polymers known as easily crosslinkable UH. These polymers need just a small dose of radiation for complete crosslinking. The lower radiation lets them be treated much more gently than common UH materials. The final processing of the material is directed at getting optimal mechanical properties after crosslinking so maximum design freedom is maintained.

Inroads with PEEK and elastomers

Moving to rigid implant materials, a clear objective is to better mimic the properties of natural bone. Rigid materials are used as constructive support in applications such as hip and knee arthroplasty, as well as in devices for spinal stabilization or fixation. Historically, as well as today, metals have been the material of choice. Metals such as titanium offer superb strength and stability, though the tremendous rigidity (read: elastic modulus) of these materials also presents great challenges: Sometimes too much is really too much. A phenomenon called stress shielding is often quoted with respect to titanium implants. Over time, the implants soften the bone to which they are attached. Bone is a living material and its health is determined by the balance of both bone growth as well as bone loss. The growth of bones, however, is influenced by the amount of stress that is placed on it. Like many tissues in nature, bone grows in accordance with its needs. When strong, stiff metal plates take over the function of the bone, it simply loses its strength. The solution is to use materials that have an elastic modulus much closer to that of bone.

In recent years, a lot of progress has been made by the use of PEEK (polyether ether ketone) as a substitute for titanium in certain devices. PEEK is a very stable and strong engineering plastic that can be used perfectly well in a number of implants, bringing the stiffness of these implants a step closer to that of bone. Innovative companies are working on materials that have a modulus that can be tuned even more closely to that of bone, with the obvious idea of removing the stress shielding issues as well as potentially leading to better bone quality.

Finally, the use of more elastomeric materials is gaining ground. Silicone polymers have been used for a very long time, being applied in many cardiovascular applications as well as in the use of finger joints, for example. Engineers believe that elastomeric materials have the potential to mimic the soft musculoskeletal tissues of the human body in the way that rigid materials can mimic bone. As such, companies are applying a range of polycarbonate urethane polymers in ways that better mimic biomechanical properties. In addition to their mechanical properties, polyurethanes can be enhanced with specific surface properties for better biointerface performance. Polyurethanes are being used in many different orthopedic applications including spinal stabilization, total disk replacement, and hip/knee arthroplasty to name a few. In the spine, the material has similar damping characteristics as the real human disks and should therefore be a suitable material to develop replacements for the real tissues.

A good example of such a device is the Freedom Lumbar Disc, which recently received CE Mark approval. Developed by AxioMed Spine Corp., Garfield Heights, OH, axiomed.com, the disc uses a specially formulated copolymer to replicate the natural function of a human disc. The characteristics of the polymer, in combination with the implant design, provide 3D motion that mimics the spine's natural biomechanics.