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3D printing has made prosthetics accessible to many more people than had access to them before, especially in developing countries. However, the World Health Organization estimates that there are still about 40 million amputees in developing countries and that only about five percent of them have access to the prosthetics they need. While several organizations have formed with the purpose of providing 3D printed limbs to amputees, these limbs typically require additional hardware and assembly steps after the printing process has completed, making them less directly accessible.

In a paper entitled “Ten guidelines for the design of non-assembly mechanisms: The case of 3D printed prosthetic hands,” a group of researchers detail how they developed a prosthetic hand that can be 3D printed in one piece without the need for additional hardware or assembly. They list several functional requirements for a prosthetic hand:

  • Body-powered control
  • Adaptive grasp
  • Cosmetics
  • Low weight
  • Waterproof and dust-proof

The main design concept of the researchers’ prosthetic hand consists of four moving fingers driven by a single link and a static thumb. The fingers are connected to the palm by a single hinge joint allowing full rotational motion.

“The fingers are coupled together following a whipple tree configuration,” the researchers explain. “This enables motion differentials between the fingers, thereby allowing adaptive grasp. All moving fingers are pulled by a force transmission system consisting of a main driving link, the whipple tree configuration, and the connecting links of the fingers. The fingers are driven by a Bowden cable connected to the main driving link and restricted to follow a linear motion along with the excursion of the cable. Restoring forces are achieved by springs connected to the base of the fingers.”

They go on to define 10 guidelines for the design of a 3D printed non-assembly prosthetic hand:

  1. Design in terms of large mechanical play.
  2. Support moving parts perpendicular to their plane of interaction. Planes which require smooth surfaces (e.g. planes of interaction) should not be touched by the support material.
  3. Provide openings for removal of supports.
  4. Integrate the functionality of standard parts (e.g. bolts, springs, and washer) into the design.
  5. Reduce play by positioning the parts with high tolerances in parallel with the printing plane.
  6. Remove play by pretension or actuation force
  7. Avoid play by replacing rigid contact by compliant interfaces.
  8. Design the shape of various components according to the stress.
  9. Choose the material density (when 3D printing) according to the stress.
  10. Align the 3D printing plane with the dominant load.

Using an Ultimaker 3 and PLA material, the researchers 3D printed a prosthetic hand in one piece and tested its mechanical performance.

“The prosthetic device could achieve adaptive grasping even though it was driven by a single force signal,” the researchers state. “A counter force was delivered by the compliant behaviour of the leaf springs to return the fingers to an open state during passive conditions. A driving link was fabricated with the hand prosthesis in order to manually activate the device and perform several grasping patterns.”

The prosthetic allows for body-powered control, resembles a real human hand, and is lightweight and water- and dust-proof. The total cost for the material came to about $10. The activation force required for a pinch grasp, and the energy required for closing the hand, are significantly lower than other body-powered commercial prosthetic hands, according to the researchers. The effectiveness of the hand is promising, as is the fact that it was 3D printed without need for additional assembly or hardware.

This design could make a big difference in developing countries in particular. If only an inexpensive 3D printer and materials are required, it becomes much easier for people in these countries to fabricate their own prosthetics without relying on outside aid.

Authors of the paper include Juan Sebastian Cuellar, Gerwin Smit, Amir Zadpoor and Paul Breedveld.

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