Three years of research and development have resulted in a clinical trial where 50 amputees will receive prosthetic devices with a Gripper Thumb Hand for a Terminal Device. 

Here’s the story:

A year ago, the design was released for Beta Testing

In 2014 we updated our elbow-powered RIT Arm designs by adding the Raptor Adaptor to accommodate the e-NABLE Raptor Hand.   This began a  thread of exploratory research, collaborations and iterative design that lead us to the Gripper Thumb Hand, a one moving part “passive prehensor” that eliminates the need for strings, cables, motors or harnesses.

The Raptor Adaptor let the wrist-actuated hand work on the elbow-actuated RIT Arm.

A few weeks later, we modified the Raptor Adaptor for a boy named Wilhe in Mexico who had a conventional split hook prosthesis. 

The Raptor Adaptor easily morphed into a terminal device that could be attached right over the split hook on a traditional prosthesis and it arrived just in time for Wilhe’s Christmas celebration.

On New Year’s weekend, 2015, Jon Schull, Nick Hall and I visited Jeff Erenstone at his Orthotics and Prosthetics shop in Lake Placid, NY.  Jeff showed us how licensed O&P practitioners created prosthetic devices, and we showed him some experimental techniques we were developing.  It became apparent to all of us that custom-made medical prostheses could be comprised of modular components that clinicians could mix and match to  for each individual patient.

Jeff demonstrates his craft.

Since our team in the MAGIC ACT Lab at R.I.T did not include clinicians, Jon and I decided to focus our research on “Terminal Devices” that would be compatible with the sockets and harnesses that were provided by the trained clinicians.

Using a heat gun, Jeff forms a 3D printed cone over a plaster casting of a residual arm.

In the following months, we worked to explore how our 3D printing techniques might be combined  with Jeff’s thermoforming techniques.  In February 2015 we tested the resulting “Monette Socket” with our favorite “test pilot” Lusie.   We used our Ultimaker 2 to print PLA cones to Jeff’s specifications; he  thermoformed them over a plaster cast of Lusie’s residual arm.

Jeff fits the Monette socket on Lusie.

These two experiences led us to an insight.  The traditional split hook is a “voluntary opening device” that maintains a firm grasp on its own, thanks to strong rubber band or spring.   Body-powered split hook devices are incredibly functional.  But for many people, especially children, they have little cosmetic appeal.  We decided to develop a terminal device that looked like a hand, with a spring-driven grasp that opens via an elbow-powered cable, but that was easier said than done.   After swapping the flexor tendons and the extensor elastics in a Raptor Reloaded hand, we discovered that the 10 finger joints of a traditional e-NABLE hand generated too much friction.  

The Normally Closed Raptor Reloaded worked, but it took a lot of force.

We needed  a block and tackle mechanism to provide enough power from elbow movement to open the grip, as demonstrated at a 3D printing show in New York City in April, 2015.

Modular Training Arm for Raj and for Lusie.

In the summer of 2015, we worked with Jeff to develop the “Modular Training Arm” with a forearm that could be printed flat and then thermoformed for a precise fit (ideally by a clinician).  We developed one for Lusie in Rochester and one for a bilateral amputee in Hyderabad, India named Raj.

We tested it with Lusie and Nate Ramsey (an Occupational Therapist with a prosthetic arm).  This led to our next big insight.  Nate showed us one of the devices he used: a voluntary-opening “prehensor” with one moving part.  We realized that we could make a more natural looking device on the same principle.  The concept for the Gripper Thumb was born, and developed rapidly using Tinkercad in a  Modular Training Arm Forum that we started in September.

 The Gripper Thumb Hand 2.0  was printed flat and then thermoformed using a technique we had developed for the gauntlets of wrist-powered hands.  Printing flat then thermoforming results in a plywood-like strong printed structure that is not susceptible to layer adhesion failures that had resulted in easily broken fingers.

The device was printed flat and thermoformed at the knuckles, making the fingers very strong.

In September, we started the  Modular Training Arm Forum.  In this forum, we started developing the Gripper Thumb in a series of experiments using Tinkercad. 

We knew we had something by November, 2015, when Nate and Lusie joined us at the Rochester MakerFaire.  Nate used it for two-handed tasks like zipping his jacket and tying his shoe.   and Lusie used one designed by R.I.T. student Joe Clifford to ride her bicycle.

Joe helps Lusie try a Gripper Thumb on her bicycle.

In December of 2015, a number of e-NABLErs converged at Autodesk University in Las Vegas where several hundred attendees joined us in the fabrication of 50 wrist-powered Raptor Reloaded hands.  Jeff and I discussed next steps for the Modular Training Arm.  He showed me his concept for a “Paper Cup Arm” that would combine a prosthetist-fitted socket with volunteer-made 3D printable modular components.  We called it the Paper Cup Arm because Jeff demonstrated his idea with a prototype that had a paper cup to represent the 3D-printable cover.

Lusie using “Paper Cup Arm.”

A few months later I attended a summit with prosthetists and e-NABLE volunteers at Autodesk’s Pier 9 studios in San Francisco.   One theme that emerged very clearly was the importance of cosmesis (the appearance of the device) in many parts of the world.  In Haiti, for instance, we had learned that an arm and hand that would not draw attention in public was more important to the amputee than a prosthesis that was functionally useful.

So we began working on Gripper Thumb Hands that looked more realistic than our initial flat-printed and thermoformed devices.  In the Cosmetic Terminal Devices forum, we used Peter Peter Binkley ‘s “Circle Hand” design to create a more natural-looking Gripper Thumb.

Peter Binkley’s Circle Hand Design.


Heart Hand Love Hand sculpture could be remixed in Tinkercad.

I also found a sculpture called the Heart Hand Love Hand on Thingiverse and remixed that STL file with Tinkercad to make a design that eventually evolved into the design that we published as the Gripper Thumb Terminal Device in November of 2016. This publication included instructional videos and STL files to use for Beta testing in the community.


When it was published in Thingiverse, the Gripper design had been iterated until it could grasp a range of objects from a 77mm dia. food or beverage container to a 26mm bicycle handlebar.


At the end of 2016, Jon Schull retired from R.I.T.  and shut down the MAGIC ACT Lab there.  We set up the Rochester e-NABLE Lab at Vertus High School where we have continued our work with high school lab technicians and adult interns from around the world.

To see what evolved in the subsequent 18 months, see the blog post:

Adjustowrap Gripper Arm System


This post is a work in progress.

Categories: Minds at Work

Skip Meetze

In the Rochester e-NABLE Lab at Rochester. NY.  Mentor of students in rapid prototyping and iterative design, Collaborator with other e-NABLE volunteer designers around the world, Advancing the technology of assistive devices and the evolution of open-source hardware.

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