Articles
Whatever Happened to Thermoplastic Elastomer?
Published on March 1, 2011 by Tony Wickman in Tony Wickman, CTPO
Remember TPE (thermoplastic elastomer)? I first heard about it when I read an article in 1990 by Ron Sutton, BSE, CO, in the Journal of Prosthetics & Orthotics (JPO). When used to fabricate an AFO, this cool new material was just rigid enough to allow some control, but flexible enough to allow a more natural gait. I thought that the AFO described was revolutionary, and it was for a while, but then it went away. Why?
For those who don’t know about TPE, it is basically a marriage of an olefin material (like polypropylene) and vulcanized rubber. It is a commonly used material in a lot of industries. The automotive industry uses it for the flexible skirting around bumpers and for suspension bushings, and it is used in the medical industry for things like catheters and other types of tubes. TPE is a great material because it offers the thermoplastic characteristics of polypropylene with rubber’s flexibility. This allows you to fabricate devices that are more bio-synergistic. Devices fabricated from TPE offer a broader window for control but will yield to overwhelming forces, making the device far more comfortable even when applying a substantial corrective force.
The patients we switched from a conventional polypropylene AFO to a TPE AFO instantly noted the increased comfort, and the patients we started out in a TPE AFO rarely complained of discomfort. In situations where our objective was to correct foot drop, the TPE AFO would easily accomplish this without having the normal range of motion (ROM) restrictions in the frontal or the transverse plane. By helping the patient to achieve a more normal ROM, compliance goes up, and stride length increases. Oxygen consumption should decrease as well.
Of course it begs the question, if this stuff is so great how come almost nobody uses it anymore? For starters, there is a bit of a learning curve involved in adopting this material.
The O&P profession is famous for being slow to adopt new technology, and TPE is no exception. Since the material typically heats at a lower temperature than conventional polypropylene, one of the first problems people noted is that, when heated, the material quickly goes from a nice workable consistency to a stretchy, stinky, near-liquid mess. Of course, this only happens if you try to cheat by heating the material in an oven set at a steady 400 degrees. If you turn the oven down to 325 degrees and take the time to let the material heat more slowly, it is very workable and easy to handle.
When properly heated, TPE’s consistency is similar to polyethylene, and once it is formed it is very stable. It doesn’t tend to expand or contract in the presence of a foam liner, and it easily molds around even the deepest undercuts commonly found when a thermo-insertion joint is present. Back in the old days, when most of us were using gas or electric convection ovens, taking the time to drop the oven temperature down was a pain, but now that most of us have gone to infrared ovens, this happens pretty quickly, so there really isn’t a good excuse to cheat.
Another turnoff with TPE is the price. Selling at roughly four times the cost of polypropylene, it can be a hard sell in these troubling economic times. The upside is that the actual material cost of an AFO is usually not the dominant factor in the overall production expense, labor is. Since TPE is easy to form, cut, and grind, I think the labor time to fabricate a TPE AFO is actually a little less. This may not fully compensate for the greater material expense, but once you factor in comfort and ease of fit, I think the cost becomes less of a factor than you might think.
No material is perfect for everything, and TPE is no exception. Because it has a tendency to cold forge, or change with repeated impacts even when there isn’t any heat present, it can have some limitations. Typically, you would not want to use this material for a patient with severe frontal plane disorders such as ankle valgus or ankle varus because TPE is too yielding. But for sagittal plane disorders like foot drop, or transverse plane disorders like mild posterior tibial tendon dysfunction (PTTD), this material is perfect—it has just enough control to fix the problem without all the rigidity of a traditional polypropylene AFO.
TPE has one other trick up its sleeve. Like many acrylic plastics, it is hygroscopic. Basically, it slowly but surely absorbs water from the atmosphere. In its cold state, this isn’t really a problem, but when you put a sheet of old TPE in the oven, the heat can cause the water molecules to expand and form nasty-looking little bubbles in the sheet as it heats up. The bubbles generally appear well before the material is up to a high enough temperature to thermoform, and if you think the bubbles will disappear as it cools, think again. They are there to stay, and it can make for a pretty ugly piece of plastic. If you have old material, you have two choices. You can either throw it away and order new, or you can desiccate it.
The desiccation process is simple, but it takes a little time. We just put the TPE in the oven at about 200 degrees, leave it for a few hours and slowly ramp the temperature to 350 degrees. If you do this slowly enough, the water will evaporate without expanding to the point where it causes visible bubbling. Most suppliers ship TPE with a barrier film nowadays, so this is rarely a problem anymore unless it hangs around for quite a while. In Florida, where the humidity is frequently above 80 percent, it can be a struggle to store TPE for long periods, but with a little patience it is a problem we can work around.
If you have ever worn an orthosis, especially an AFO made from TPE, you will quickly notice the advantages. It’s easy to make shapes that are more aggressive and yet more comfortable than with other materials. Rather than rely on padding to add control pressure, you can actually make the device do all the work. It is thinner this way and fits better into the shoe, and it offers much better toughness than materials that are similarly flexible.
Every material we work with in this industry can be problematic on some level. But each material also presents us with an opportunity. I remember when acrylic resins were introduced—they were expensive, stinky, and a real pain to work with. But because they were inherently more rigid and a good bit stronger than the old polyester resins, we stuck with them, worked out the bugs, and never looked back. TPE demands the same basic approach. If we want to offer our patients the best possible sheet material for their particular circumstance, sometimes we have to work through the bugs.
Also published in the March 2011 edition of the O&P Edge. © 2011 O&P Edge