High Performance Flexible Inner Liner Materials - Without Silicone

The Limitations of Traditional EVA Sheet Materials

By Jeff Wilson

When EVA (ethylene vinyl acetate) sheet materials were introduced to the prosthetic market, prosthetists realized they could use these soft, flexible plastics to create flexible inner liners for rigid definitive sockets.

As with many flexible plastics, EVAs have a very high coefficient of surface friction, which means that other materials do not easily slide against them. This is an issue for amputees because a high coefficient of friction makes it more difficult for them to don and doff their prostheses. Plastic manufacturers decreased the high coefficient of friction by blending silicone into the EVA. However, EVA with silicone has several drawbacks. Some patients may experience skin irritation when using sockets made from EVA with silicone. Sockets made from EVA with silicone may also extrude out of prosthetics over time. Additionally, silicone-filled EVA sheet may be difficult to fabricate since it can be extremely soft when it comes out of the oven, it creates weak seams, and it tends to create sockets with inconsistent wall thickness during blister or bubble forming.

OP-TEK® Flex Comfort

OP-TEK® Flex Comfort is a new low friction EVA socket liner material with outstanding fabrication and performance characteristics. OP-TEK® Flex Comfort has a proprietary non-silicone lubricant that greatly reduces friction, making it easy for patients to don and doff their prostheses. The additive used to decrease friction in OP-TEK® Flex Comfort is hypoallergenic1 and FDA compliant for direct skin contact. Along with decreasing the possibility of skin irritation, OP-TEK® Flex Comfort has the additional benefit of having a softer “feel” compared with silicone-filled EVAs.

One common problem with silicone-filled EVAs is that they are susceptible to a phenomenon referred to as cold-flow or creep. This creates a problem for the amputee because the inner liner can cold-flow out of the top of the socket causing an uncomfortable fit. Silicone-filled EVA liners can also grow out of cut-out windows that are fabricated in rigid sockets to provide additional comfort for the amputee. These problems are greatly reduced with Curbell Plastics’ OP-TEK® Flex Comfort because the material has superior cold flow resistance compared with silicone-filled EVAs.

OP-TEK® Flex Comfort solves many of the fabrication problems associated with EVAs with silicone. It has 20% greater melt strength compared with silicone-filled EVAs, which makes OP-TEK® Flex Comfort easier to handle when it comes out of the oven. High melt strength materials are easy to form into finished sockets using the blister or bubble forming technique where the material is placed in a metal frame, heated to forming temperature, and then stretched over a mold of the amputee’s residual limb. Low melt strength materials such as EVA with silicone tend to stretch too easily when heated and formed. This makes them difficult to thermoform since it is very easy to pull the material over the mold unevenly resulting in liner walls that are not uniform in thickness. The increased melt strength of Curbell Plastics’ OP-TEK® Flex Comfort helps fabricators achieve more consistent forming of the material resulting in more uniform wall thickness throughout the liner.

Drape forming is another technique that is used to thermoform inner liners. In drape forming, the material is heated to forming temperature, draped over a mold of the amputee’s residual limb, and then seamed together to form the liner. Because this technique requires a seam, it is critical that the material used can adhere to itself securely. Unfilled EVA plastics are a good choice for drape forming because they adhere to themselves extremely well when at forming temperature. However, silicone-filled EVAs tend to have much weaker seams compared with unfilled EVA materials. OP-TEK® Flex Comfort’s low friction additive does not negatively impact the seaming characteristics of the material. Seam failures among EVA liners containing silicone are more frequent in colder climates. Amputees living in colder climates can really benefit from the superior seaming characteristics of OP-TEK® Flex Comfort.

An additional benefit of the superior seaming characteristics of OP-TEK® Flex Comfort is that a cost savings can be achieved by using the thinner (and less expensive) sheet materials that are required for the drape forming process. The typical sheet thicknesses used in drape forming are 1/8", 3/16", and 1/4", whereas blister or bubble forming requires sheet that is 3/8", 1/2", or 5/8" thick. Thinner gauges of EVA sheet tend to be less expensive (on a dollars per square foot basis) than thicker gauges since thinner gauge sheets are less labor-intensive to manufacture. EVA sheet greater than 1/4" thick has to undergo the additional manufacturing step of being laminated from thinner sheets. This adds manufacturing cost to the thicker gauges.

OP-TEK® Flex BiLam

Definitive sockets are sometimes manufactured from carbon braided fiber. Carbon braid results in lighter, stronger, and stiffer sockets, but the carbon fiber also creates a cosmetic issue because carbon braided fiber sockets are generally black and EVA liner materials are typically white or translucent. The stark contrast of these two colors draws attention to the trim lines and window cut-outs of carbon braided fiber sockets. To decrease this contrast and provide the ability to hide trim lines and window cut-outs, Curbell Plastics offers a BiLam material.

OP-TEK® Flex BiLam (BiLam) is a laminated sheet with Curbell Plastics’ low friction inner liner material, OP-TEK® Flex Comfort, on one side and our OP-TEK® Flex Black EVA (which does not contain the friction-reducing additive) on the other side. Since both materials are EVAs, the bond strength between the two layers is extremely strong. BiLam will not delaminate during the fabrication process. The benefit of this multilayer structure for fabricators is that during bubble or blister forming they can create a flexible inner liner out of two materials with one “pull”. Note: The sheet is formed with the OP-TEK® Flex Comfort side down on the mold and the OP-TEK® Flex Black side facing up toward the fabricator.

Amputees experience the benefits of the OP-TEK® Flex Comfort layer (low friction, soft feel, and hypoallergenic surface) on the patient side of the inner liner. They also benefit from improved aesthetics when the socket is viewed from the outside since the OP-TEK® Flex Black material on the socket side of the liner tends to hide trim lines and window cut-outs in carbon fiber sockets. Another benefit to the amputee is that the OP-TEK® Flex Black layer does not contain the friction reducing additive so the outer side of the BiLam liner will be somewhat tacky, which helps to hold the carbon fiber socket in place.

In conclusion, Curbell Plastics’ OP-TEK® Flex Comfort and OP-TEK® Flex BiLam are two new liner materials that create sockets that are easier to fabricate, more comfortable to wear, and more pleasing in appearance than sockets made from traditional silicone-filled EVA sheet materials. These new materials also offer greater long-term performance since they have superior resistance to cold flow and they have more secure seams compared with silicone-filled EVAs.

1. For the purpose of this article, the term “hypoallergenic” means that historically this material has caused few allergic reactions among patients. It is impossible to guarantee that a plastic material will never cause an allergic reaction, and clinicians are advised to do their own testing to insure that the product is suitable for their applications.

About the author

Jeff Wilson, Senior Business Development Manager for the Curbell O&P team, is a nationally recognized expert on the use of performance plastics for orthotic and prosthetic applications. He brings more than 25 years of plastic materials experience to customers in the O&P device fabrication industry. Jeff’s extensive knowledge of plastics and the way their physical, mechanical, and thermal properties dictate their behavior in various environments helps O&P fabricators develop better, stronger, and more comfortable devices for their patients. Contact Jeff.

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