VC200C Children's and Women's Prosthetic Hand (Patent Application in Progress)
A low-cost, 3D-printed prosthetic hand for children and women in low-income countries
The Problem
The VC200 prosthetic hand is too large for women and children to wear and use comfortably.
The Design
Due to the complexity and size constraints of the mechanisms in the adult hand, designing a smaller hand was far more complicated than simply scaling down the parts.
Adaptive Grasp - Whipple Tree
In the adult hand, an adaptive grasp mechanism allows the fingers to articulate individually and conform around odd-shaped objects. Due to the small size of the children's hand, this mechanism couldn't be used and a new one had to be designed.
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An adaptation of a whipple tree mechanism was designed in SolidWorks to address this issue. As shown in the figures to the right, this mechanism allows the fingers to close in parallel or at an angle around an odd-shaped object. This mechanism also allows for a stronger pinch than the adaptive grasp in the adult hand.
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Pinch Alignment and Clearances
One small yet complex part of the VHP hand is the thumb coupler mechanism. The length and shape of the thumb coupler determines the rate of closure of the thumb which is important for different gestures. For example, the thumb and index finger must meet at a specific point during a pinch gesture or else they will slip past each other. In the VC200C, the adaptive grasp systems make it quite difficult to align the thumb with the index finger for a pinch gesture however, after numerous trials, this was successfully completed. Additionally, the thumb coupler must be positioned so that it does not invert when force is applied to the thumb. This was also successfully accomplished.
To allow the thumb to rotate from side to side, the thumb base connector shape had to be adapted. By using smooth, curved surfaces, points of stress were reduced.
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Palm Cover
Forces involved in the adaptive grasp motion try to push the system out of the palm. As well as protecting the inner components of the VC200C, the palm cover also holds the adaptive grasp system within the hand. As shown in the brown figure to the right, in the VC200, the palm cover is held in place by two screws directly into the palm and one hex nut and screw through the wrist.
Due to the small size of the VC200C, there was not enough room for the top screw to be used and a different securing system had to be designed. Many adaptations of a ‘hook’ were experimented with and a new secure system was designed without the need for any screws.
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Jigme - The First Recipient
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Palpasha - The Second Recipient
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