3D Printed Customizable Bistable Finger Orthotic for Arthritis & Injury Recovery

A PhD student at Carnegie Mellon University was inspired by her friend’s arthritis struggles to come up with a better solution than a stiff finger brace. Her idea was a customizable brace that could switch easily between stiff and flexible, without having to be removed, and she used 3D printing to make it.

If you grew up during the 1980s and 1990s like I did, you might remember slap bracelets, which were in just about every birthday party goody bag I ever got. Also known as snap bracelets, kids thought they were fun and adults thought they were annoying, but did you ever think about how they worked? These bracelets were bistable structures, which means they had two stable equilibrium positions: straight and coiled. A bistable mechanism will only switch between these states once sufficient energy or force is applied—like slapping it on your wrist. That’s also sort of how the 3D printed finger brace works.

During an internship, Yuyu Lin, a PhD student at the university’s School of Computer Science Human-Computer Interaction Institute (HCII), noticed that when her friend used the computer, she had to take off her finger braces. She wore these to help relieve and treat the arthritis in her knuckles, but she wasn’t able to bend her fingers to the keyboard while wearing the braces.

Lin wanted to help her friend, and other people who faced similar challenges. She thought it might be possible to make a finger brace that could go easily from stiff to flexible, which would allow the finger to move without having to take the brace off. So she collaborated with colleagues at the Interactive Structures Lab (ISL) to research the problem and come up with a better solution.

Lin explained, “For this work, we were trying to think from the perspective of the patient, and how to get them to wear this brace and complete their rehabilitation routine more easily.”

Alexandra Ion, an assistant professor in the HCII and director of the Interactive Structures Lab, said, “We wanted to understand how we could help people, and what patients needed right now. We wanted to add our expertise to build this new, unexpected thing.”

Typically, finger orthoses are static, which leaves the finger immobile so it can heal. But doctors often ask that they be removed during rehabilitation exercises, and patients have a difficult time maintaining a good balance between immobility and painful but necessary movement. The CMU researchers came up with a fully customizable, bistable finger brace for injury recovery and arthritis treatment that switches from stiff to flexible with a simple push or flex of the finger.

“Orthoses are essential components of rehabilitation, yet limited in functionality. Static braces immobilize joints, which, especially for hand and finger injuries, interfere with users’ daily activities. Additionally, early mobilization schedules require users to take off and reapply their static orthoses frequently, which is cumbersome,” the team wrote.

“To facilitate both rehabilitation and dexterity, we introduce a novel multifunctional yet unpowered finger orthosis design. Our design supports easy switching between two distinct states: a stiff state for immobilization and a flexible state for mobilization.”

The brace was designed as two rigid 3D printed pieces, connected by an elastic band. When the wearer pushes down on the brace, or bends or curls their finger to a certain point, the band is easily released, thus enabling easy finger movement. When the finger is extended and pushed up, the elastic band snaps back into place and immobilizes the finger again. It’s just like the slap bracelets mentioned before, which are rigid until bent to a certain point, and then they curl around your wrist.

Collaborating with medical professionals, the CMU researchers determined that the 3D printed orthotic device would be most useful on the tendons of the second knuckle on the hand. Called the proximal interphalangeal joint, this area is difficult to treat. That’s because if a patient doesn’t get enough early mobilization of the joint once it’s been hurt, post-injury stiffness often occurs.

This orthotic device can be 3D printed in one piece and doesn’t need any assembly. It’s customized using computational design software, which, as the researchers wrote, “supports tailoring the switching thresholds of the brace based on patients’ individual finger strengths and range of motion.”

To customize the braces, a wearer needs to collect certain metrics, including finger strength (measured with a force gauge) and dimensions, and extension angle, which can be measured using a protractor. The software uses this information to simulate a digital version of the brace, which is used to find how much force (torque) is needed to safety switch the brace from stiff to flexible. Based on the results, a 3D design for the brace is generated, and patients can then customize it before printing.

“Following a preliminary study with 10 healthy people that validates the usability and wearability of the brace, our two-week case study with a patient indicates that our brace supports everyday activities and assists with rehabilitation,” the researchers concluded.

By noticing her friend’s difficulty and using available resources to do something about it, Lin and the rest of the team could ultimately make it easier for patients to follow rehabilitation plans, manage chronic conditions like arthritis, and speed up their recovery.

In the future, Lin wants to continue developing braces and adaptive devices that can be worn comfortably by users with limited mobility.