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Voluntary-Opening / Voluntary-Closing (VO/VC)Terminal Device

A novel hook prosthesis with bi-directional control

Employer: Shirley Ryan AbilityLab / RIC

Role: Research Engineer III

This was one of my primary projects during my tenure at RIC and was the lead design engineer on the project.

Traditional terminal devices come in two flavors based on the position of the tongs of the hook when the user is relaxed:

  1. Voluntary-opening (VO): the tongs of the hook are held closed by a pre-deteremined spring force

    • Pros: Once the user grasps an object they can relax and maintain grip of what they are manipulating

    • Cons: The device has a pre-determined grip strength which may not be that universal and the user must ovecome a spring force everytime they manipulate objects.

  2. Voluntary-closing (VC): the tongs of the hook are biased open by a spring

    • Pros: user can generate as much force as needed depending on the task

    • Cons: user must maintain that force during the entirity of their manipulation task and must overcome a spring force in addition to the force they desire to generate.

The solution, a device that can operate in both VO and VC modes through the toggle of a switch!

Key Design Features:

  • A single terminal device capable of both VO and VC modes

  • Same tong and "thumb" design as traditional devices

  • Approximately the same size as existing devices

  • Ability to switch modes via bump switch for bi-lateral limb deficiency or manipulation with one hand

Once the design was stable we fabricated several for a small take home study with prosthesis users. In order to monitor and encourage user behavior, we instrumented the device with a counter, data logger, and feedback LCD. Methods and results can be found in the publication linked below.

Shirley Ryan AbilityLab VO/VC Prosthesis Information Page

Publication: Design and evaluation of voluntary-opening and voluntary-closing prosthetic terminal device

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