The proliferation of wearable robotics and exoskeletons is earnestly underway around the world. In virtually any area of one’s personal travels, encountering someone wearing a device meant to support musculoskeletal movement is no longer the thing of science fiction. Whether wearing an exoskeleton on the job, designed to reduce the risk of soft tissue damage over time, or a prosthetic device meant to restore motion in those who have experienced a loss of mobility, these wearable innovations are here to stay.
There are a burgeoning number of companies developing these high-tech marvels and they are paving the way to improving and, in many cases, even returning quality of life to audiences previously without solutions.
One such company that Carl Stahl Sava Industries partners with is Abilitech™ Medical , makers of the Abilitech Assist — a groundbreaking, upper-limb robotic orthotic. This pioneering, wearable device assists in supporting people with upper-limb limitations, including those who have suffered spinal injuries, muscular dystrophy or multiple sclerosis that have left their day-to-day mobilities restricted. While the Abilitech Assist depends upon a host of intricate mechanical components, critical to the device’s functionality are a series of stainless steel, mechanical cables running through it. As the Assist is intended to compensate for reduced typical motor function, without the cables powering this movement, such motion could not be achieved.
Engineering the cables comprising the Assist required special sensitivity to extreme flexibility, while under significant load conditions and over one million cycles.
To achieve the tremendous flexibility the device demanded, Sava engineers suggested using 7x49, nylon- jacketed, stainless steel cable construction. The 7x49 construction is among the most flexible stranding designs available, allowing for extremely tight bending radii, which is required to contribute to both the desired motion and overall cycle count. Nylon-jacketed cables also made sense for Abilitech Assist because first off, the cable is lubricated beneath the jacket, which has shown to extend the lifetime of a cable by a factor of 2x to 3x.
Sava says that the nylon coating lessens the fatigue and bending stresses on the cable, while preventing metal-to-metal contact between cables and pulleys. To realize the ideal lifespan of Abilitech Assist, all these parameters needed to be carefully balanced with consideration for the loads the device would shoulder. 304 stainless steel was selected as the cable material strictly because stainless steel is known in medical devices markets for transferring product loads reliably and effectively. Stainless steel mechanical cable is also known for its imperviousness to most corrosive conditions. Being uncertain of the environments in which Abilitech Assist may be deployed, Sava says that considerations for corrosion were made to account for a diverse body of users and lifestyles.
Also integral to the choice of mechanical cables used in the manufacture of Abilitech Assist was ensuring proper function over time. Sava, therefore, says it built and tested each mechanical cable assembly via proof loading process to guarantee the strength of the swaged fittings and remove constructional stretch. Leaving constructional stress unaccounted for, the cables inside the Abilitech Assist may stretch too far and degrade the device’s ability to perform correctly. Understretched cables will elongate during usage, making it best to prestretch the cables to mitigate the risk of poor performance in the field.
The urgency with which wearable prosthetics and exoskeletons have been made available to the occupational audiences they are designed to support made Sava’s involvement in this project particularly satisfying. “It’s always our pleasure to participate in creating devices that help the world move,” says Greg Soja, Sava’s senior vice president of engineering.