Fitting an Exoskeleton

Exoskeletons derive their name from the exoskeletons we find in nature.  Those seen on insects fit precisely along the body, enhancing the strength of the insect while allowing for motion.  The walking stick shown here gains strength from this supportive structure, but the joints are intricately developed to work with the muscles and tendons to allow for a very large range of motion at the joint.

photos: ©Matthew Strausser IG: @strausserphotography

A rehab exoskeleton must likewise become an extension of the patient, working with both their own muscles and the actuation of the device.  Therefore, the joint alignment and sizing is critical to comfort and functionality.  But our challenge of time remains and we must consider the tradeoffs between accurate sizing and speed.  If we adjust every aspect of the device perfectly, the device will fit snugly, but each patient will either need custom pieces or set up times will become prohibitive.  If we do not have sufficient resolution, however, we risk pressure sores, abrasion, discomfort, or muscle strain.  We as engineers are thus challenged to develop adjustments that are fast, accurate, and secure and use compliance where rigid attachment may not be necessary.

EksoNR utilizes adjustments at the upper and lower leg and hip width as well as velcro soft goods to achieve a safe and comfortable fit while being relatively quick to set up. 

Next, the patient must get into the device.  For some devices, the patient can put on pieces while sitting or may need to put an AFO or similar brace into their shoe.  In others, they transfer into the device, and still for others a bodyweight support system may be used to help get that patient in. Locking in the adjustments may require specialized tools, and we must consider their maintenance as well.  It is advantageous to have adjustments that an untrained user, such as a therapy aid, can make as this decreases the time out of the therapists’ session.  We also consider what adjustments may need to be made during the session and how to accomplish those with minimal disturbance.  We also have to provide the therapist a way to determine if the adjustments are correct whether through visual clues, messages from the device itself, or tests that can be run to check.

Currently, most devices use traditional mechanisms for locking and positioning, such as nuts and bolts, friction clamps or pins.  Improved materials and manufacturing processes will provide us new opportunities for improving and streamlining adjustments. For example, printing carbon fiber or even metal components allows us to gain strength while using intricate patterns for tool-less locking.  There are also soft sensors and soft actuators in development that will improve fit and self-adjust support and bracing.  The challenge here becomes balancing the complexity and cost with the benefits gained in terms of therapy outcomes.  As these technologies advance, the balance may come to tip in the favor of additional sensing and robotic controlled adjustment, but I will leave that for a future design project.

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