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The Natural Dexterous Hand [new product]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - The Natural Dexterous Hand [new product]; published June 5, 2011, 15:22; URL: https://www.swisswuff.ch/tech/?p=427.

BibTeX: @MISC{schweitzer_wolf_1571444391, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - The Natural Dexterous Hand [new product]}}, month = {June},year = {2011}, url = {https://www.swisswuff.ch/tech/?p=427}}


Via CNN word gets around of a new body powered prosthetic hand:

In today's Big I we looked at Mark Stark's prosthetic hand - a device he made in his basement to help a friend. It was recently awarded a spot in Popular Science magazine's 2011 Invention Awards.

As it turns out this device got a 2011 PopSci Inventions Award:

Although its minimalist plastic assembly is nearly as light and inexpensive as a common steel hook, it looks and moves like a high-end electronic hand.

This statement is interesting as prosthetic hand development has undertaken interesting innovations in the last 100 to 200 years, not all of which were systematically documented. Nevertheless, there is a distinct development that the above cited remark requires to be examined in closer detail: how a prosthetic hand looks, and how a prosthetic hand grips. Because after Ballif's first strap operated hand (1812), the clenched fist appearance was recognized as appearing as threatening gesture and with subsequent developments such as by Van Peetersen (1844), extended fingers were introduced not only to avoid this appearance but also to offer a pincer grip to the user [link]. So in fact, practically all subsequent prosthetic hands were built to contain a non-threatening appearance. Except the Natural Dexterous Hand.

Natural appearance always was requirement along with function and always hard to achieve. The best current mechanical hands for body powered arms thus are the Becker hand (with adaptive grip, pincer precision grip and very good stability) and the Monestier hand (superb elegance and great function). If we are going towards current functional developments, hook devices such as the V2P, Hosmer-Dorrance hooks, Otto Bock Movohook 2Grip and TRS options come to mind before anything else. Inasmuch as electronic hands are concerned, almost all currently released new models - Otto Bock's Michelangelo, Touch Bionics' iLimb, RSL Steeper BeBionic, the Vincent hand - appear to aim at a far more natural resting position, at a natural 'at ease' appearance than anything previous despite their unimpressive two-electrode restrictions (mostly with options to open, close and switch program).

Here however, we have a clenched fist hand.

Photo of the inventor (C) Copyright PopSci:

Diagram of hand mechanism (C) Copyright PopSci:

Photo of hand (C) Copyright PopSci:

At the heart of that hand are Exacto Springs of the Exacto Spring Corporation: so rather obviously, clenched fists are what these springs do to 14-joint-containing prosthetic hands.

(C) Exacto Spring Corporation

Brushing floor:

This is really nice. However, any Otto Bock hand, V2P Prehensor, Becker hand, hook or other device does the same.

Getting gallon of milk:

Not bad. However, hook, Becker hand, myo hand or others also allow to get that milk out of the fridge. Most notably, I wear an aluminium Hosmer 555 hook around the house with an awful amount of rubbers on it. That lifts all of these and more.

Tying shoes:

Alright, where is the precicion pincer grip? Tying shoes is now ubiquitous for arm amputees - one handed, with hook, prosthetic hand - no one will have to walk with untied shoes these days. So, come on.

One-handed catch:

Great catch. Even though I found that catching a ball is hard to do with my prosthetic - not so much because the Natural Dexterous Hand really is the only body powered hand that can do it, but also, because I do not like to wear down my cables so fast. We did play some catch stuff years back and I went through two cable setups in a day or so. I really wore them down fast then - just as sealed roads and tires are consumables so are my cables. Just for information - if you do not specifically optimize for that you may be in for a surprise. So just because the terminal device allows for fast ball games does not mean the rest of the arm is built for that. - I now do have a high definition cable control in many aspects, but these parts are results of peculiar cooperations and developments. They don't lie around on the street. Other than that, take it easy with these one-handed catches.

The inventor explains the hand himself:

You see how the hand, similar to a Becker hand, wraps around a glass.

Representative and contact:

http://www.edisonnation.com/prosthetic-hand

Mark Stark’s Natural Dexterous Hand is currently represented by Edison Nation, the idea-to-shelf product developer and social community of inventor-entrepreneurs. Companies interested in licensing may contact Ken Paulus, vice president of business development, at kenneth.paulus@edisonnation.com.

Claims being made:

The Natural Dexterous Hand (Patent US 7,655,051) is the first low cost body-powered prosthetic hand that looks and moves like a normal hand. When using this prosthetic device, the user gains new day-to-day capabilities that extends from picking up a glass of wine, an egg, pouring a glass of milk and catching a ball.

The Natural Dexterous Hand is unique in the industry as all five fingers flex at 14 individual joints providing a healthy human hand-like motion without the cost associated with electronic mechanisms. This prosthetic hand is versatile, lightweight and interchangeable with hook devices, using the standard harness cable and threaded attachments.

I am not sure I would support all of these claims. A healthy human hand or normal hand contains constrained joints that are extremely relevant to appearance and function that the Natural Dexterous Hand does not contain - metacarpal joints, carpal joints and wrist. The latest Otto Bock innovation is the Michelangelo hand which features one particular new function - a flexible wrist unit. While that takes up space it surely adds to that hand appearing a bit more towards normal - not entirely normal, no , far from it, but a bit more like it. So, the Natural Dexterous hand does not at all look, or move, like a normal hand. It moves, and looks, like a 14-jointed prosthetic clenched fist that contains a considerable number of these curled up Exacto springs. That's what it is. And that's also what it looks like. No magic, really.

Similar designs:

  • The UTAH / M.I.T. dextrous hand [link] [pdf] - Interestingly this prosthetic hand has been named 'dexterous' already in 1996. The finger and hand setup shows interesting similarities.
  • The BECKER hand is an All American product that has also been developed by a designer-entrepreneur and that nowadays is built and sold by his son, John Becker. It comes in two varieties - BECKER Imperial and BECKER Lock Grip. As opposed to the Stark hand, the Becker hand is definitely sturdy, refined and improved over decades, it has attained cult status, it actually looks fashionable and great, it combines adaptive and pincer precision grip and it locks so you can carry a bag and the hand won't open. Also, with the BECKER Imperial hand, you can vary the spring force by turning a screw which is a feature the Stark hand lacks.
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