This two-part series examines trends in materials development and fabrication. This first installment focuses on how material strength, stiffness, and other variables affect the structural properties and design of orthotic and prosthetic devices.
By Cary Groner
The design and fabrication of devices such as ankle foot orthoses (AFOs) and prosthetic lower limbs have come a long way since the days of leather, wood, and steel.
Thermoplastics, including polypropylene and polyethylene, have made lighter, more flexible AFOs easy to customize for individual patients, and thermosetting polymers such as carbon fiber have offered progress in terms of strength, weight, minimalist design, energy return, and atrophy prevention (see “Strengthening the case for carbon fiber AFOs,” October 2010, page 23).1 In the prosthetic market, advances in plastics and metal alloys are leading to lighter, stronger artificial limbs, which have been adopted enthusiastically by younger and more athletic patients—including soldiers returning from Iraq and Afghanistan.
Polymer engineers and metallurgists are nevertheless keenly aware of the limitations of existing materials and eager to discover ways to meld their best features or develop new compounds.
“The material composition of an ankle foot orthosis has a tremendous impact, not only on the durability of the device, but on how it functions for the patient,” said Roger Marzano, CPO, CPed, who is in private practice at Yanke Bionics in Akron, OH. “Devices made from copolymer, polypropylene, or carbon fiber, having the same trimlines and height, would all function differently as far as range of motion and energy-reflection characteristics.”
Others in the field echo this view.
“Materials have a direct and critical impact on a device’s performance, but this is often overlooked because clinicians aren’t necessarily trained in metallurgy or materials science,” said Gerald Stark, MSEM, CPO/L, a senior clinical specialist at Ottobock Healthcare in Minneapolis, MN. “Knowledge of materials affects what types of forming techniques are used and can ultimately make a critical difference in component selection, structural strength, and functional optimization.”
Marzano added that other factors affecting material selection include the patient’s size and weight, occupational demands, athletic aspirations, dermatological conditions, and insurance coverage, as well as environmental considerations related to fabrication and disposal of devices.
Plastics, son, plastics!
Polypropylene has been the plastic of choice for many years because it’s easy to shape using heat and vacuum-forming devices, it mills well, it’s lightweight and inexpensive, and it’s rela- tively nontoxic to work with. Polypro- pylene has limitations, though: it isn’t as strong as steel or composites like carbon fiber, it provides little energy return, and it’s subject to fatigue.
“Thermoplastic devices are easier to manufacture and modify than thermoset or pre-preg carbon fiber designs,” said Marzano. (“Pre-preg” is an industry term for pre-impregnated composites such as carbon fiber or fiberglass, in which a central core of woven fibers is held together by resin). “However, thermoforming can negatively affect the performance of a device if it’s molded incorrectly or at too high a temperature,” he said.
Research has quantified some of polypropylene’s weaknesses. For example, a study of deformation characteristics published in the Journal of Prosthetics & Orthotics (JPO) last year found that posterior leaf-spring AFOs made of polypropylene and polyethylene, when plantar flexed from 0° to 10° in a testing machine, lost stiffness and became permanently deformed after about 90,000 cycles, whereas the same properties of carbon-fiber AFOs remained unaffected.2 Even plastic AFOs with mechanical ankle joints fatigue; an unpublished study found that, after 22,800 test flexures, stiffness was reduced by 38%.3 Over-shaving the devices as part of customization may be deleterious to longevity as well, according to a 2005 study.4
An earlier JPO paper found, however, that fatigue effects may not be permanent. (The reason for this discrepancy isn’t clear; it may be that other researchers haven’t given their plastics enough time to recover fully.) In this study, the authors subjected 1/8”-thick pediatric polypropylene AFOs to three 24-hour periods of 187,200 load cycles, then measured their mechanical properties just after each cycle and at six different recovery intervals ranging from 15 minutes to nine hours.5 They found a 30% reduction in stiffness and a 4.4% increase in malleolar diameter after just one 24-hour period; however, following 15 minutes of rest the AFOs began to recover their mechanical integrity, which returned completely within an hour.
Nevertheless, in the clinical environment it’s common to bolster thermoplastics just to be sure.
“Thermoplastic devices may begin to cold flow or fatigue over time, and early models sometimes failed as a result,” Stark said. “Since the early nineties, practitioners have preferred to reinforce thermoplastic or polypropylene structural frames with composites.”
The paper provided a primer of the arcane terminology sometimes used to assess the features of thermoplastics and other materials; for example, creep describes the deformation and failure that occur under constant loads, whereas cold flow refers to elongation and elastic recovery signifies what happens when the loads are removed. When both elastic and viscous behaviors are present, viscoelasticity is the term of choice, and the authors summarized polypropylene’s properties as combining the features of an elastic solid and a fluid. “Under short-duration loads, the plastic AFO behaves elastically,” they wrote. “However, with steady loads it will stretch or elongate, recovering only when the load is removed.”
Cranking up the heat—and turning it down
At the molecular level, polypropylene resembles long semiparallel strings of beads (i.e., polymer chains) held together by two types of bonds, primary and secondary. When it is heated or stressed, the weaker bonds dissociate and the plastic becomes flexible. The material provides visual evidence of this altered state by becoming transparent; then, as it cools, the secondary bonds reform and the material turns opaque again.5
Researchers have noted that polypropylene typically contains both crystalline and amorphous regions, and that the polymer chains are more highly organized in the former.6 They report, moreover, that the microstructure of semicrystalline polymers such as polypropylene is affected by thermal history; heating and cooling times and temperatures have profound and lasting effects on strength and stiffness. The lack of industry consensus on thermoforming procedures is one reason different labs and practitioners sometimes get disparate results. The study’s authors described what they determined to be optimal conditions, including proper forming and set temperatures, as well as slow cooling.
Gerald Stark agreed with the importance of this last point.
“Be gentle, love your plastic, don’t shock it with compressed air!” he said. “You freeze plastic in a weaker state when you cool it immediately, but if you let it cool naturally, it maximizes strength and lessens the chance of failure.”
Even when produced under optimal conditions, however, polypropylene still has limitations. In addition to those already mentioned, it becomes brittle in cold weather.
“In Chicago, in the winter, a lot of practitioners will use a copolymer—a blend of polyethylene and polypropylene—so it doesn’t shatter when the temperature drops,” said Thomas Karolewski, CP, supervisor of the Orthotics and Prosthetics Clinic at Hines VA Hospital in that city.
There are several other strategies for optimizing thermoplastics, according to Karolewski, some of which are simple design modifications.
“You can corrugate the plastic, putting in a little ridge line so it becomes harder to flex,” he said. “You can also put in a small carbon-fiber plate at the ankle, or in other areas, to stiffen it up.”
Although carbon fiber as part of a composite material offers the advantages noted earlier, and most practitioners use it for a variety of applications, it too poses problems. First, as a thermosetting polymer, once it’s cured, it’s cured—it can be sanded and minimally shaped, but it can’t be heated and reformed. Second, the resins and sanding dusts are usually toxic and require cumbersome safeguards. (New ecologically friendlier resins are now available, according to Karolewski, though they are not yet in widespread use.)
Karolewski became concerned about the issue when one of his students at Northwestern University in Chicago conducted a study [unpublished] showing that children born to women in the O&P field had higher rates of birth defects than the national average. Carbon fiber is also hard to dispose of because the resins are relatively durable, and carbon, as a chemical element, doesn’t break down into anything else.
“Carbon fiber is extremely strong, but the resin that binds it is brittle,” said Gerald Stark. “It is dependent on the fiber orientation and the upright design. For example, if you limit ankle motion too much with a carbon-fiber AFO, you move the point of bending somewhere else. You see greater localized shear at the metatarsal toe breaks, then the resin in the AFO starts to break down. Soon you see crazing or cracking of the composite, and the carbon delaminates and breaks apart.”
For such reasons, many in the field are looking for ways to combine the strength and energy storage of carbon fiber with the versatility and workability of thermoplastics such as polypropylene.
“Gary Bedard has developed a new plastic in which the carbon fibers are extruded right inside thermoplastic,” Karolewski said. “He designed it for orthotics, but I’ve been using it to bubble-form prosthetic components. How the carbon fibers lined up linearly after the pull was amazing. It added quite a bit of stiffness, although the manufacturing time was a little longer because the carbon fiber slowed the droop of the plastic.”
Bedard, a certified orthotist who lives and works in San Mateo, CA, as a clinical application liaison for Becker Orthopedic, told LER that carbon-infused polypropylene has been a personal project for many years.
“I wanted something with a higher performance value than we’ve offered with polypropylene, but that didn’t have some of the disadvantages of a thermoset resin or pre-preg,” he said.
“Quite a few orthoses are made with thermoset pre-pregs; the problem is that materials are expensive, processing is difficult, and it’s hard to accommodate a patient’s discomfort because postdelivery modifications are limited,” Bedard continued. “What I’ve done is take three-quarter-inch segments of carbon fiber and embed them into the core of the polypropylene so they’re not exposed to the surface. If you vacuum-form with it, due to a proprietary process the carbon fibers stay embedded and don’t protrude through the surface, so there’s no dermal contact. In cross-section, the material is three-sixteenths of an inch thick, which is the common gauge used to make orthoses. There are two discrete layers in the core, separated by a neutral axis of polypropylene. The material is heated and draped over the plaster models we use for fabrication, then vacuum-formed using the same methods you’d use with homopolymer polypropylene.”
The material, called ProComp, provides an upgrade in strength of roughly the same magnitude as that between polyethylene and polypropylene, Bedard said. Other companies have produced similar polypropylene–carbon-fiber sandwiches.
This is a big leap forward, according to Karolewski.
“We’ve gone from polypropylene, polyethylene, copolymers, and copolyesters to adding bits and pieces of carbon to the construct of thermoplastics, to now extruding the plastic with carbon fiber in it, which I think is amazing,” he said. “That’s the kind of technology that’s going to change the field.”
Because Bedard approaches materials engineering from the perspective of a certified orthotist, he’s more attuned to the needs of the patient than your average polymer wonk, and this perspective influences his approach.
“In orthosis design, stiffness is typically a combination of material, thickness, trim lines, and augmentive reinforcing techniques, such as the insertion of corrugations or ribs,” he explained. “You have to select the best combination of those characteristics to satisfy your performance needs for the patient.”
For example, a patient with significant instability in the foot and ankle complex might need more surface coverage for correct alignment, he said.
“But if someone has primarily a neuromuscular defect rather than a skeletal defect, you can minimize the surface component and increase your structural component through the use of materials like [ProComp],” he said.
Researchers aren’t just devising novel ways to combine old materials, of course; they’re also developing new ones.
“In the past, we used to think in terms of thermosets and thermoplastics, but now there are new, reformulated materials that blur the lines between them,” said Gerald Stark. “One example is LPET [low-temperature polyethylene therephthalate], which technically is a thermoplastic, but has the toughness and pliability of a thermoset and is so inexpensive it’s used in drink bottles. We’re getting better at making pre-preg carbon that’s thermomoldable with LPET, so there are lots of innovations headed our way.”
Another promising compound, polyetheretherketone (PEEK), is so tough it’s used to make springs in the automotive industry. Researchers are also creating hybrids that combine thermosets and thermoplastics, such as styrene and butadiene, in nanostructures that offer benefits of both.7,8
Composites are going through evolutionary leaps as well, Stark noted. For example, piezo crystals (crystals that generate electricity when subjected to stress), can be placed in composites to create different stiffness that changes when electrified.
“There’s a lot of voodoo in composite development,” he acknowledged. “You can’t really simulate it with CAD models; you have to make it and test it.”
Designers and fabricators are combining materials in other ways to make the best use of differing properties, according to Stark.
“An orthosis has three elements: structural, fitting, and dynamic response,” he said. “Recently, at the Orthopadie + Reha-Technik [congress] in Leipzig, Germany, I saw a flexible silicone interface with a rigid pre-preg carbon composite strut. This is how we make prosthetic devices, but in this case they’d used the design in an orthosis; it gave the compliance of silicone with the structural rigidity of carbon. That’s pretty exciting.”
Other new systems combine carbon and LPET, Stark said, which will allow them to be thermomolded. Stark is particularly interested in new fabrication techniques that make better designs feasible.
“Before, we could design things that we couldn’t make, but CAM is finally catching up to CAD thanks to micromilling machines,” he said.
One example is new methodology that makes the casting of titanium parts more accurate and less expensive—an advance already used in the defense, aerospace, medical, and prototyping industries.
“Basically, they shoot two lasers into sintered titanium dust and it welds the metal beads together,” he explained. “It’s an additive process rather than subtractive, and titanium in that ‘sponge form’ costs about three dollars a pound, instead of seventy-five dollars a pound.”
Other approaches to alloying metals are enhancing the standard use of aluminum and titanium in prosthetic parts that require exceptional strength, such as pylons.
“These metals have to be very lightweight and corrosion resistant,” Stark said. “The primary alloy we use in O&P is 2024 aluminum, which is a copper alloy that accepts coloring and gives you a shiny well-finished component. Other alloys, such as 7075 aluminum, a zinc alloy, are also very strong. Titanium is a little heavier than aluminum but gives you the strength of stainless steel—though it’s a bit more brittle and five times more expensive. So, as engineers, we have to balance all these things.”
Stainless steel is still important in some cases, for example, in heavier patients for whom component strength is more important than light weight. Stark explained the differences that specialists have to consider, beginning with the distinction between yield strength and fatigue load.
“Aluminum has a slightly higher yield point—the point at which it plastically deforms,” Stark said. “But you can only load it to about half its yield point repeatedly, say for three million cycles, and then you’ll start to see creep or cold flow. Stainless steel, on the other hand, can be loaded all the way up to its yield point, to infinity—and as engineers, we don’t use that word lightly.”
The Iraq and Afghanistan wars, as well as the burgeoning interest in sports for disabled athletes, have greatly advanced the field, he added.
“We have a lot of young kids—Paralympic athletes and guys at Walter Reed—testing the limits of everything we can make,” he said. “Engineers learn a lot when things break. In the future, we’re going to see very high-strength, very lightweight systems, as we get better at fitting all these things together.”
Cary Groner is a freelance writer in the San Francisco Bay Area.
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