Army Researchers Invent Device To Improve Orthoses

Army scientists have developed rate-activated tethers (RAT) to incorporate into orthotics, braces, and compression garments that provide variable support depending on the wearer’s speed and movement intensity. The tethers stretch and recover quickly during moderate extension rates and resist during extreme extension rates, providing a unique balance of comfort, performance, and support.

Ankle, knee, and back injuries are as common as they are debilitating. A wide range of commercial products exists to reduce the probability or severity of these injuries. Unfortunately, orthotic devices, braces, and compression garments can be highly uncomfortable, inhibit performance or inadvertently increase the likelihood of other injuries. 

The Army’s new orthotic device design includes adaptable coupling elements and rate-activated tethers incorporated into elastic materials, such as spandex, comprising the body of orthotics and braces. The tethers themselves are cable-like devices that are filled with fluid and filaments. The tethers can be made of materials that are both elastic and able to seal fluid inside, such as rubber, silicone, polymer, or latex. The filaments inside the tethers can be made of steel, polymer, glass, or carbon. The filaments can be attached to one or both sides of the tether or be free-floating. At low extension rates, the filaments can easily slide through the fluid or past each other. At higher rates, the fluid transforms to a more rigid material which significantly reduces filament movement.

The resistance force to extension is designed to increase as the extension rate increases, but the ideal rate-dependent response depends on the specific application. The fluid used can be Newtonian or non-Newtonian. Non-Newtonian fluids change viscosity with the strain rate, which enables more customization. A shear thickening fluid (STF) that flows like a liquid at low deformation rates but becomes highly resistant to flow at high deformation rates can cause severe stiffening to lock the device. For other applications, a more gradual resistance may be preferred.

The tethers can be incorporated into various orthotics, braces, and compression garments, such as ankle and knee braces and lower leg compression socks. For example, in an ankle brace, the tethers will support regular daily activities without causing the joint's weakening due to accommodation to the brace over time. However, during fast movements, the device will limit motion to prevent supination of the ankle.

Please contact our knowledgeable staff at Teter Orthotics & Prosthetics for any questions. We’ve been providing expert orthotic and prosthetic services in Michigan since 1955 and have grown to 22 locations, including Traverse City, Alma, Alpena, and Ludington!

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