Novel hand and hook prostheses with bi-directional control.

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.

Traditionally, body powered hand prostheses suffered several design flaws.

1. Terrible gear ratios, making the input to output forces exhausting for users

2. Limited grapsing patterns (i.e. one fixed grasping pattern)

3. A single direction of actuation. Imagine if you our hand was biased open and the only control authority you had was to close it, to a predetermined grasping pattern, with a fraction of the output force compared to the effort you exerted to generate the motion.

For the reasons listed above, body-powered hand prostheses where not popular and often abandoned for hooks despite the more anatomical cosmesis. Stemming from my work with the VO/VC hook and the RIC Arm, I combined many of the mechanical design innovations to create the first of its kind body-powered hand prosthesis.

Key Design Features:

• Nearly 1:1 force ratio

• Bi-directional control. Users had access to two modes of actuation, they could either use the actuation lever to close the hand or open the hand.

• Multi-articulating fingers and positional thumb. This allowed for multiple grasping patterns and more functionality for users.

• Independently locking fingers. This allowed users to move digits "out of the way". Prosthesis lack the proprioception of anatomical digits, users must use visual feedback to interact with their surroundings. It can be frustrating when the bulk of the device makes it difficult to obtain the appropriate visual feedback for fine motor tasks.

Shirley Ryan AbilityLab VO/VC Prosthesis Information Page

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