3D Printed Prosthetics with Little Assembly Show Promise for Greater Use in Developing Countries

Statistics regarding amputees, discussed in the recently published ‘Functional evaluation of a non-assembly 3D-printed hand prosthesis’ are disturbing, as nearly 40 million people are in need of prosthetics today and only five to fifteen percent of them are granted the proper access and affordability necessary for replacement limbs.

While some modicum of healthcare may be accessible for patients in big cities of developing countries, those who live in more isolated areas may find transportation challenging—not to mention expenses all around. For those who attain healthcare, the chances of them returning for follow-up care are slim.

3D printing has been shown to offer positive benefits for healthcare in rural areas, from offering diagnostic devices often in the form of 3D printed smartphone attachments, for testing and treating of parasitic infections, cancer, malaria, and more. As for prosthetics, organizations like e-NABLE have also been behind the design and fabrication of countless prosthetic devices, often for children in need.

Relatively easy to master and affordable to purchase, 3D printers offer undeniable potential for developing countries; however, complex assembly could be an issue—leaving the researchers to design a prosthetic that can be fully printed. One of the most realistic scenarios involves the completion of 3D prints of prosthetics by servicers, allowing delivery to the end-user through local organized networks.

For this study, the researchers designed a prosthesis with the following features:

• Body powered control
• Cosmetic appearance
• Lightweight structure
• Water and dirt resistance

Design of our 3D printed prosthetic hand. The palm in the left picture is translucent to show the inner mechanisms (from top to bottom: leaf springs, whippletree mechanisms, and driving link), and the right picture shows the 3D-printed prosthetic hand without the palm.

Each hand prosthetic is made up of four fingers and a thumb, with the fingers connected to the palm and then joined via a whippletree design.

“All fingers in motion are activated by a force transmission scheme which is made out of a main driving link, the whippletree arrangement, and the links that connect the four fingers,” explain the researchers.  “The hand is actuated by a Bowden cable attached to the main driving link that is allowed to go on a linear motion following the movement of the cable.”

The leaf spring configuration consists of 3D printed plastic sheets that both bend and pull, driven by activation of the fingers.

“When the fingers are activated, the pulling forces drive the leaf springs to unbend and deform to a straight configuration,” explain the researchers. “As the leaf springs return and recover from the deformation, spring-like behavior is provided, combining actuation and a return spring in one non-assembly 3D printed element.

“In order to achieve non-assembly fabrication of the concept, a number of design guidelines were used using the advantages of 3D printing for design versatility while circumventing many of its shortcomings.”

The experimental setup for the leaf spring ultimate strength test. The leaf spring is in its neutral configuration (left). The 3D-printed sample is under tensile and bending loading conditions during the experiment. Note that the leaf spring is bent to a straight configuration, corresponding with a 90 flexion of the finger (right).

Customized samples were printed on an Ultimaker 3, using PLA, testing the leaf spring and movement of the hand in five different experiments. The designs were tested using both the Box and Blocks Test (BBT) and the Southampton Hand Assessment Procedure (SHAP) test as 20 healthy students were enlisted from the Delft University of Technology to participate in the testing portion of the study, simulating use of the prosthetic.

Our 3D printed prosthetic hand attached to the simulator and a figure-of-nine shoulder harness. The cable tension that is delivered by the harness and activates the prosthetic hand is depicted (left).

Our 3D printed prosthetic hand attached to the simulator and used by the participating subjects (left) for the Box and Blocks test (top right) and the SHAP test (bottom right). Gaffer tape strips were put over the thumb, the index, and middle fingers to increase grip (right).

The hand was fabricated with almost no assembly required—aside from the removal of supports and one ‘snap-fit step.’

“The hand achieves adaptive grasping even though it is made out of only two parts,” explained the researchers. “We argue that such a design and fabrication approach can increase accessibility of hand prostheses since easily accessible low-cost equipment (an Ultimaker 3 3D printer) was used together with low-cost material (PLA).”

There were some challenges, in the end, however, as the material for the leaf spring was deemed ‘not suitable’ for durability and reliability over the long term.

“… the 3D-printed hand evaluated in this study shows comparable performance with just a small friction enhancement on the finger pads. Yet, grasping and functionality can be improved by increasing the pinch force. Moreover, it is worth noting that most ADLs require low grip forces,” concluded the researchers.

“The non-assembly design achieved a comparable level of functionality with respect to other BP alternatives. Taking into consideration that most ADLs require low gripping forces and adding an increased accessibility provided by the advantages of the nonassembly and 3D printing approach, we consider this prosthetic hand a valuable option for people with arm defects in developing countries,” concluded the researchers.

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