Shoulder sensor patch could improve injury recovery

Shoulder sensor patch could improve injury recovery
The motion capture cameras and kirigami sensor pick up first author Erin Evke’s arm movements, enabling the team to match the reading from the sensor to the position of her shoulder. (Levi Hutmacher, University of Michigan Engineering)

A wearable sensor patch that conforms to shoulder contours could help patients recover from injuries, according to an article on the University of Michigan website.

Inspired by a University of Michigan professor’s recovery after a cycling crash, the patch uses electronic sensors to understand the functional range of motion as opposed to today’s static measurements. Influenced by kirigami, the Japanese art of creating 3D structures from cut paper, the sensor can conform to the curves of a joint, yet can be manufactured flat.

“The shoulder in particular moves in a very complex way. It’s one of the most well-articulated joints in the body,” said Max Shtein, a professor of materials science and engineering at the University of Michigan, in a statement. Shtein broke his collarbone after crashing his bike and collaborated with his students on the work.

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During recovery, Shtein was appalled to find that his physical therapist assessed the range of motion in his shoulder with a basic protractor, a process that is prone to errors and cannot be repeated at home.  .

While Shtein realized that a wearable sensor could have recorded the full scope of his motion and tracked his exercises, the current technology of building electronics onto curved structures would be costly. The sensor would have to be manufactured flat but still be able to follow the contour of a patient’s shoulder.

He and Erin Evke, a doctoral student in materials science and engineering, came up with a solution inspired by kirigami. She laser cut a thin sheet of plastic into a labyrinth of concentric ovals. The shape pulls apart like a Slinky, and the cuts open into a lacework over the shoulder.

“If you take a sheet of paper and try to wrap it around a sphere, it’s impossible to do so without folding or wrinkling the paper. This would significantly stress the sensors before your measurement even began,” Evke said. “Our cut pattern avoids this problem.”

Because these structures transform from a flat sheet to a wrinkle-free, 3D shape, they can be manufactured with existing technologies. Shtein estimates that individual sensors could cost less than $10 if they are mass produced.

As a proof-of-concept, the team equipped the kirigami patch with two strain gauges—one along the corner of the shoulder to capture arm movement, and the other at the back of the shoulder to measure cross-body movements.

Shtein’s team collaborated with researchers working under Deanna Gates, an associate professor of movement science at the University of Michigan, to cross-reference the data from the kirigami sensor with arm motions captured by a camera motion tracking system. The system uses reflective markers to track the angular positions of the arm and reconstructs them in a computer simulation.

The researchers envision this sensor being given to physical therapy patients, enabling them to log exercises and see progress through a smartphone app. This could help keep patients stick to an exercise program and also provide more detailed information to therapists about each patient’s progress.

The study is published in the journal Advanced Materials Technologies.

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