A team of electrical, computer, and mechanical engineering students at Rice University, calling itself Carpal Diem, has developed a testing suite to validate how well 3D-printed prosthetic hands transfer force from the wearer, usually a child born without a fully formed hand, to the prosthetic intended to help pick up and manipulate small objects.

Fig. 1 – Rice University engineering student Rachel Sterling monitors as a teammate lines up an object to test the force applied by a 3D-printed hand. (Credit: Jeff Fitlow, Rice University)

“Makers” of these 3D-printed hands freely trade designs online and print hands for children who need them. But, the Rice students say that the new designs are not always as efficient as they could be.

“Children born without full hands are forced to adapt to the world and figure out how to go about their daily routines,” said student Amber Wang. “If a prosthetic hand is not absolutely perfect in its function, the child will probably discard it and return to his or her own adaptive ways.”

Carpal Diem’s suite consists of a motorized wrist-and-palm assembly that can move up to 60 degrees in either direction, a set of objects (a cylinder, a sphere and a rectangular prism) with embedded force sensors and a control program with a graphic user interface. An operator uses the program to bend the wrist and close the printed hand’s fingers and thumb around an object. Sensors in the object send feedback on force strength and distribution to the computer. The Rice team developed their design as their senior project. (See Figure 1)

Not Another Hand, a Better Tester

At first, the team thought they would simply design a better hand. But, their research led them to realize that there are many, many designs out there and available. What was really needed was a design for a force-testing device.

“If a kid has to put in five pounds of force to only get one pound of grip, that’s a lot of lost efficiency because of how these hands are designed,” said team member and mechanical engineering major Rachel Sterling. “Until we reach a force efficiency of 100 percent, the hands aren’t going to be useful.”

Computer engineering major Nirali Desai, explained: “The industry standards for testing these kinds of devices are not very well established.” “We had to get very creative about how we were going to test the accuracy and precision of our device.”

The team said that they are designing the tester so that someone working with e-NABLE (the global network of volunteers who design and print these prosthetics) can have one in the lab, print three different prototypes, and test them in rapid succession,” Dimoff said.

They hope to put the first prototype of the testing device and a detailed protocol for its use into the hands of their mentor, Dr. Gloria Gogola, a pediatric hand surgeon at Shriners Hospital for Children-Houston, who has worked with many Rice engineering teams in recent years, by the end of the school year. Eventually, they plan to have specs for people who want to make these devices themselves.

For more information, visit http://news.rice.edu .