Material by Nature
Demands for smaller, more durable medical devices are promptingengineers and scientists to turn to the natural world for solutions.
The best and shortest road toward knowledge of truth [is] nature.
~ ancient Egyptian proverb
Thomas Adams has dedicated virtually his entire professional life to improving the functioning of various machines—from the simplistic workings of skateboards and racing bikes to the more complicated mechanisms of helicopters, fighter jets and space satellites.
There is still one machine, though, that remains out of reach of Adams’ expertise: the human body.
“The body is a very unique and complex machine. It has movements that you don’t normally see in automobiles or in aircraft,” said Adams, founder and president of Tiodize Co. Inc., a Huntington Beach, Calif., developer and manufacturer of coatings and composites that prevent friction, wear and corrosion. “You cannot design anything like the body. It produces torque, it can twist many different ways and it contains fluids that you never see in nature. The body is an amazing machine. It is so much more complex than people understand.”
This complexity has baffled mankind since the dawn of civilization. Ancient Egyptians, for example, drew parallels with the flow of the Nile River (and its annual flooding, which irrigated their farms) to better understand the human anatomy, assuming the body—like the mighty Nile—had channels that flowed in a similar manner.
These channels, the Egyptians surmised, acted much like the basic sluices that watered their arid fields—they carried blood, air and water to the extremities, helping them to grow and remain healthy. This simple concept formed the basis for a loosely structured but nevertheless primeval wellness plan: Clear channels in the body induced good health, while blocked passages portended disease, much in the same way an obstructed irrigation ditch could precipitate the atrophy of a patch of crops. Blockages in the body could be caused by any number of factors, though the ancient Egyptians liked to attribute rotting food to these occlusions. Such a diagnosis led them to believe that nearly all diseases were caused by the improper digestion of food (hence their preferred remedies of laxatives and vomiting).
The ancient Greeks, meanwhile, tried to unravel the body’s assorted mysteries through the identification of its central organ. Aristotle believed it to be the heart; Herophilus of Chalcedon contended it was the brain (as proof, the anatomist and physician provided the primordial medical world with descriptions of the brain, spinal cord and nerves). Several centuries later, physician/writer Galen of Pergamon zeroed in on the body’s circulatory system, arguing that veins connecting the liver to the heart circulate “vital spirits throughout the body via the arteries.”
As they gained more knowledge of the body and its various functions, these primal practitioners of medicine (and ordinary folks as well) looked to the natural world for both remedies and replacement parts. The ancient Egyptians, for instance, used honey and willow bark to prevent infection in wounds, and ostrich eggs triturated with grease for severe forehead injuries. They also used linen sutures to close large lacerations and wood to replace limbs lost to battle or disease.
“A long time ago, in the pyramids, mummies were found, and on the mummies [archeologists] found a few little stitches, or what we would call sutures,” said Abhay Pandit, a professor of mechanical and biomedical engineering at the National University of Ireland, Galway. “On some of the mummies [archeologists] also found a few wooden feet, or what we now call prosthetic feet. The sutures held things together and the prosthetic feet were actually shaped like a person’s foot. The commonality between the sutures and the wooden feet is that they are materials that were put in the body. In some ways, they [ancient Egyptians] were trying to mimic the geometric structure of the foot. They were attempting biomimetics … and people have been doing that for centuries.”
Indeed, homo sapiens have spent generations dabbling in biomimetics, the science of adapting designs from nature to solve modern medical problems. History is littered with examples of the practice: Mayans fashioned nacre teeth from seashells in roughly 600 A.D., remarkably achieving bone integration; Galen described sutures made of gold wire; surgical pioneer J. Marion Sims commissioned a jeweler make silver wire sutures in 1849; and researchers in the United Kingdom developed a cane for the visually impaired that is modeled after the biological sonar bats use to fly in total darkness.
Biomimetics—derived from the Greek words “bios” (life) and “mimesis” (imitate)—has been used in various industries over the years, from political science and car design to computer science and architecture. Perhaps its most fitting application, though, is healthcare, where scientists and engineers are constantly searching for materials that match the body’s natural composition and will survive its attempts at rejection. As Pandit noted, “Our bodies are quite smart. We cannot underestimate the body’s capacity for what it can reject. It may not reject [a material] right away—it may take 15 years to reject, it may take 25 years to reject, but [the body] will reject. We as scientists and engineers have not done a complete job of making these materials completely inert. That is something we need to work on.”
Composition, Cost, and Other Challenges
Material dormancy is just one of the many hurdles engineers and scientists have encountered in their quest for new medical device substances. Two of the most obvious are properties and performance.
Materials used in medical devices perform differently depending on their chemical structure. Polyethylene, for instance, has become an increasingly popular substance in the medical device sector in recent years due to its pliability and its diversity of applications. The material has been used for decades in joint replacements but recently was refined to serve the needs of minimally invasive surgical applications, particularly cardiovascular implants. A certain grade of polyethylene fibers can be shaped into various textile constructions, including braids and woven tubes, while a polyethylene film can be used as a thin barrier or cover for medical devices. The combination of fiber and film has been used to enhance the design of numerous implantable cardiovascular devices, including stent grafts and covered stents.
A thermoplastic polymer consisting of long chains, ordinary polyethylene is produced by combing the ingredient monomer ethylene, a gaseous organic compound.
High density polyethylene (HDPE), which can be produced by chromium/silica catalysts, metallocene catalysts or Ziegler-Natta catalysts, is stronger and more dense than ordinary polyethylene. This kind of material is used in such products as milk jugs, detergent bottles, margarine tubs, garbage containers and water pipes. About one-third of all toys are made with HDPE.
Ultra-high-molecular-weight polyethylene (UHMWPE) is the strongest member of the polyethylene family, with a molecular weight between 2 and 6 million, according to industry data. The material has extremely long chains which enable it to transfer loads more effectively to the “polymer backbone” and sustain more forceful impacts. Besides its strength, UHMWPE is highly resistant to corrosion and abrasion, is self-lubricating, has an extremely low moisture absorption level and has an equally low coefficient of friction. UHMWPE is used in a diverse range of applications, from bulletproof vests and butchers’ chopping boards to sutures and replacement joints.
“Modern medical materials have become very complex. A lot of the materials being used now for medical products are part of a system,” said James Rancourt, Ph.D., founder and CEO of Polymer Solutions Incorporated, an independent polymer testing laboratory based in Blacksburg, Va. “The material itself is important but the structure of the material is also very important. In the earlier days the structure could be something such as a polymer that was part crystalline with the crystalline regions giving the plastic high strength, low compression, resistance to deformity and making it optically opaque. Now it’s more complicated than that.
“With forms like polyester fabrics used for surgeries, you get into issues of what the fiber is made out of molecularly, whether the filaments that make up the fabric are going to be single fibers and whether the yarns are going to be made up of tiny individual little filaments,” Rancourt continued. “Also, if these yarns are made up of the tiny filaments, what will the filament size be, how many filaments will there be per yarn and what will the weave structure be? All the while the material has to handle the manufacturing, it has to handle the sterilization, the packaging, the transportation, the storage prior to use and be accepted by the medical community that has to implement the product. Every single step is highly engineered. If any links along that chain fails, then the material potentially is not going to do what it was supposed to do.”
And that can spawn other challenges, namely, additional costs and time to market. Manufacturing companies can minimize such challenges (and added expenses) by effectively communicating their product visions with materials suppliers. At NuSil Technology, LLC, a Carpinteria, Calif.-based manufacturer of silicone compounds for various industries, materials experts have drafted a flow chart of questions to ask customers to ensure the product development process goes smoothly. “We put ourselves in their position to try and understand what they need from the silicone,” noted Brian Reilly, NuSil’s product director of healthcare materials.
In some cases, extra testing may be required, further delaying the time to market and adding even more costs to the project’s overall price tag. To avoid these types of potential delays, device manufacturers and materials scientists must possess a clear understanding of the characteristics of the implantable substance and how it potentially could react with the body, experts said. “You can’t just put anything into the body,” Tiodize’s Adams told Medical Product Outsourcing. “You have to know where it’s going and what it’s being used for. The body takes tremendous loads. The PSI (pounds per square inch) load on the body can be extremely high. You should check the PSI load on a material and test the wear to make sure it can handle different loads. That’s why you see so many recalls going on with hip implants. The materials being used arewearing out.”
Wear has become a fairly consistent challenge over the last several decades, particularly with orthopedic implants, as aging baby boomers seek to maintain their active lifestyles and stave off the body’s natural descent into osteoarthritis and bone loss. The search for materials that prolong the lifespan of worn-out or damaged joints has precipitated the development of composites and biomaterials.
Composites are engineered from two or more constituent materials with significantly different physical or chemical properties. In recent years, they have become a competitive alternative to the metals and metallic alloys traditionally used in orthopedic implants, such as stainless steel, cobalt chromium, titanium and tantalum.
Advanced composites, also known as high-performance composites, contain a large percentage (about 60 percent by volume) of highly resistant continuous fibers typically composed of carbon, glass or aramid substances. Materials experts claim both types of composites provide device manufacturers with several advantages over metals, including their light weight and strong resistance; their ability to allow different properties and functionalities to be obtained in different part areas or directions; their translucency; and the ability of parts creators to exercise considerable control over their
Biomaterials—substances that interact with the body’s biological systems—have existed for centuries, though it can be argued that such materials predate history. In late July 1996, the skeletal remains of a prehistoric man was found on a bank of the Columbia River in Kennewick, Wash. Through radiocarbon dating, archeologists determined the remains to be 9,300 years old. CPT scans of the man’s pelvis showed a spear point with serrated edges lodged in the ilium; the point appeared to be crafted from a volcanic, siliceous gray stone that was native to southern Washington State at the time. The spear point—obviously the result of an accident or battle wound—apparently had healed within the bone and did not significantly impede the man’s activity. While it was not intended as an implant, materials scientists claim the embedded spear point illustrates the body’s amazing capacity to tolerate foreign objects and substances.
The body, though, is not always so accommodating. Biocompatibility long has been a challenge for materials scientists and medical device designers, as evidenced by the decades-long search for the right combination of substances to replace aging hip and knee joints. Many of the major orthopedic device firms have been forced to recall their hip implant systems due to failures, high revision surgery rates, or high wear particles. Metals such as stainless steel and cobalt chromium historically have been used to craft artificial hip joints, but issues with wear debris has led researchers to toy with different combinations of materials over the last few decades, including titanium, polyethylene and ceramics.
“There’s a lot of push in the biomaterials community to synthetically prepare materials that mimic biological structures and are comparable to [the body’s] biological structures, have dimensions that approach the dimensions of cells and encourage cells to intrude into a matrix or exclude cells in other biological components,” Rancourt noted. “Rather than have a slab of plastic, for example, these synthetically-prepared materials might have the same chemical composition as a slab of plastic but instead of being a slab it’s a very complex porous structure, but the porosity is not random. The pores, ribs and bridges of the scaffold are tailored now to be certain dimensions. People are being very clever in this area just by what they are coming up with to promote healing or promote bone growth.”
Some of that cleverness is coming from researchers at the Fraunhofer Institute for Manufacturing and Advanced Materials in Dresden, Germany, who have developed a titanium “foam” that may prove to be a better material for use in orthopedic implants. The researchers believe the foam’s porous structure will better grow into the implant, which itself would be lighter because less material is used. Having already experimented on defective vertebral bodies and stressed bones, engineers and researchers are now designing full-fledged foam-based implants.
Drivers of Material Innovation
The quest for substances that will solve the many issues associated with artificial hips and knees is one of the main drivers of innovation in the development of new materials.
“The clinical wear issues of hip and knee implants have driven innovation in ultra-high-molecular-weight polyethylene, ceramic composites, metallic alloys as well as surface treatments/coatingsfor orthopedic applications in the past 20 years or so,” said Hai Trieu, Ph.D., Distinguished Scientist and Technical Fellow at Medtronic Inc.’s Spine, Biologics and Kyphon divisions. “As a result, materials used in existing hip and knee implants are significantly more wear resistant than in the past.”
And there’s a good chance they’ll become even more wear-resistant in the future. C5 Medical Werks has developed a new, higher-strength biocompatible ceramic material for use in femoral head, acetabular (hip) cup and femoral knee devices. Company executives claim the material, called cerasurf, “delivers high strength and high fracture resistance in combination with low wear characteristics.” The material is being used in current designs of femoral head and acetabular liner products, according to a news release from the Grand Junction, Colo.-based firm.
Described as an alumina matrix composite ceramic material, cerasurf has proven its worth during testing. Industry standard femoral head burst tests showed values of greater than two times when compared to U.S. Food and Drug Administration guidance figures.
Orthopedics, however, is not the only innovation driver. Other sources of inspiration for new materials (and combination substances) include the miniaturization of medical devices and the rise of minimally invasive surgery. Both trends have begat the need for materials that are flexible, resistant to corrosion and can be made small enough to fit inside the tiniest of orafices.
“Areas such as neurology and orthopedics are gaining a lot of momentum with Nitinol as a material,” said Dave Niedermaier, vice president of sales for Nitinol Devices & Components Inc., a Fremont, Calif.-based, privately held firm that provides full-scale manufacturing, development and technical services for nitinol medical devices. “ In addition, there are new treatments being offered for some of the more established diseases like repairing heart valves. There are companies that are developing ways to deliver heart valves percutaneously rather than the old method of cracking open the chest. To do that, the devices need to have some of the properties that nitinol provides.”
Devices also should have some of the properties that materials such as silicone provides. An ideal medical material due to its rubber-like flexibility, silicone boasts a laundry list of attributes: heat-resistance, low toxicity, the ability to repel water, oxygen resistance and low chemical reactivity. The latter characteristic, in fact, has made silicone an increasingly popular material with companies that produce pharmaceutical products.
“Pharmaceuticals companies are looking at such low concentration levels of active chemicals that anything extractable from a device or the material used to make components in that device causes a problem. This desire for low extractable level materials is the real driver for material development,” explained Steve Glancy, chief chemist at Vernay Laboratories, a Yellow Springs, Ohio-based manufacturer of elastomeric fluid control products. “Everybody’s looking at high consistency silicone or LSR [liquid silicone rubber] today because they have low extractable levels. It’s one of the few polymerization sequences that run neat, using only siloxane. The advantage of silicone is that it cannot incorporate into protein systems or DNA because it’s so dissimilar to biological chemistry. This lack of reactivity is really driving people to use high consistency silicone and LSR for device components.Medical applications are a huge market for silicone right now. If you look at the material requests for medical industry applications, most of the requests can be fulfilled, in general, by silicone, not just LSR. LSR is the generic acronym people always use when requesting silicone even though the silicone selection is more an issue of which component manufacturing process will be used than the differences in component performance.”
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It has perhaps become mankind’s greatest quest—the search for medical materials that will help him live longer, live better, and hopefully, live happier. Generations of doctors, scientists and inventors have tried numerous substances over the years to replace worn-out, damaged or missing body parts but their attempts have had only limited success. The body, apparently, is a complicated machine, constantly on the lookout for foreign objects and materials that don’t intermingle well with its unique array of fluids, tissue, muscle, tendons and bone.
Advances over the last several decades have helped biomaterials become a staple of 21st century medicine. And while those advancements certainly have improved the quality of medical devices, issues of rejection and biocompatibility remain constant challenges for manufacturing firms and materials experts. As a result, scientists and device engineers increasingly are looking to nature to help heal the body, surmising that substances found in the natural world might receive a warmer welcome from the human anatomy.
One of the most frustrating challenges with natural remedies though, is the amount of time it takes to develop a new material from nature. As Tiodize’s Adams’ noted, “Sometimes things show promise right away and sometimes it takes years before a material shows promise. You’d like to snap your fingers and have a cure-all for everything but it doesn’t happen. It takes a lot of effort, time and testing.”