3-D printing prostheses

The 3-D hand prosthetic that Erin Mullin designed and printed with the help of open-source files

By Keely Savoie

While prosthetic hands can enable people who are missing digits to regain functionality,  the cost of fully integrated, high-tech hands can be completely out of reach for many who may want them.

Now, with the advent of 3-D printing, prosthetics can be easily replicable and ubiquitously available. Advancing that goal was precisely the aim of Erin Mullin ’17 when she began working on developing a customizable prosthetic hand after downloading a template using open-source files.

With a 3-D printer, a few fiddly bits (screws, guitar strings, bungee cords), countless trips to the hardware store, and lots of ingenuity, Mullin printed and constructed a prosthetic hand that could be made nearly anywhere — and for less than $50. These hands have the potential to radically change the lives of their recipients, who through genetics or happenstance, are missing the digits that so many of us take for granted.

Though she arrived at Mount Holyoke intending to participate in the dual-degree engineering program, Mullin decided to pursue a math major with her advisor, Jessica Sidman, Professor of Mathematics on the John Stewart Kennedy Foundation.

It wasn’t until Mullin spent time between her sophomore and junior year on Easter Island for a summer internship that her interest in engineering was renewed and solidified. Mullin, with a team of Mount Holyoke students, Associate Professor of Mathematics Dylan Shepardson, and engineers Marilla Lamb and Matthew Petney, spent the summer devising renewable energy solutions for the island. Using only landfill salvage, and working with the Easter Island wind blowing against them, the Mount Holyoke students assembled a bike-powered generator powerful enough to blend a smoothie — a sweet reward for their efforts.

“I knew then that I wanted to get back to engineering,” said Mullin.

A hop, skip, and 5,300 miles from Rapa Nui (the native name for Easter Island) Mullin pursued her renewed passion for engineering on the Mount Holyoke College campus. She came upon the prosthetic hand project at the suggestion of Sidman, who thought it would inspire — and challenge — Mullin.

“Using open-source files is a double-edged sword,” said Mullin. “It presupposes that you have the same software and hardware as the developer, and we had neither, so it was more of a guideline and I had to figure out how to actualize it.”

The blue-green plastic prosthetic device looks simple enough: a wrist plate that extends over the back of the hand, secured by a velcro band around the palm, four grasping fingers and a thumb that curl and uncurl. The movements are controlled by flexing the wrist, which tightens or contracts artificial tendons, which are fishing wires and bungee cords threaded through the digits and fed to a raised bar on the wrist brace. The strings can be adjusted via zip-ties to tighten or loosen the grip, another innovation of Mullin’s. She sought to  to decrease the cost while adding flexibility to the original design, which called for tightening screws akin to guitar-tuning screws.

The devil was in the details. The process took months of trial and error. Software issues, printing errors, hardware problems, cracked plastic, holes too small, hardware too big, prosthetic fingers too long to carry to load or to fold properly — Mullin recalled the multiple challenges. But finally she completed construction of a prosthetic functional grasping hand.

“This hand allows people the ability to grasp, and carry objects. It would be great if it enabled more complex movements, but gesture-control prosthetics range from $6,000 to $30,000,” said Mullin. “Because these hands can be made for about $50 they have the potential to give back functionality to many people who otherwise wouldn’t have the option.”

Mullin has saved the file with all of her hard-learned modifications and shared it with the Mount Holyoke community. She has also trained three other students, Nikkole Spencer ’19, Carla Gonzalez-Vazquez ’19, and Janae Davis ’19, who are now working on a different version of the prosthetic hand. Together with Sidman, they are hoping to partner with E-NABLE, an organization that works to connect those in need of prosthetic hands to those with the means to produce them.

“The next step is for the students to send in the hand that they’ve been working on,” Mullin said. “If Mount Holyoke establishes a chapter (of E-NABLE), we can begin to distribute hands because we have a lot of resources and can help people around the world.”

After graduating, Mullin left campus at the end of June to begin a Ph.D. program in engineering at the University of Arkansas.

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