When using a prosthetic arm with a terminal device, grip performance is a key issue.
Usually, a bare steel hook such as the Hosmer model 5 works through just about every situation. That is just because that is how it is. A closer look reveals, however, that that view may be overly simplistic. If anything, it requires explanation.
Constraint 1: grip geometry and surface softness
The higher the grip surface friction against the object, the fewer fingers are necessary to hold an object. With a soft stump skin and a soft upper body (where one presses an object against), the count of grip fingers may be just, maybe, one (a stump works as a single although large finger, technically). But the surface softness - when working with exposed skin - is absolutely ideal. I can get superb form closure, thus reducing actual stump-object-pressure to a minimum. I do not even have to exert a lot of work that way.
So, we start by understanding why not wearing a prosthesis as unilateral below elbow amputee is so effective. With a soft stump skin, a soft upper arm skin and a soft upper body, no extra fingers are needed to hold even a pepper mill firmly or to hold a heavy box. Friction is maximized by having a sloppy skin and subcutaneous fat mold into all objects that we hold like that.
Secondly, a well-evolved hook shape will fit most everyday object geometries even without any (or just minimal) soft interfaces.Typical examples are Hosmer hooks or the Trautman hook derived new hooks by Toughware PRX (V2P, Retro, etc.) as well as the TRS Prehensor series.
Thirdly, for the stabilization of an ultra fragile hard object, there is no other way than a 5-7 finger device with a fully adaptive grip. Typical examples are a raw egg or a plastic cup.
Read more here [link].
Constraint 2: device design and soft surface modification
As we already know by now, any added soft layer will massively improve grip performance of a hard device. That reduces required actuation force. The softer and better adaptive the surface, the lower the actuation force. So everyone that is not into massive actuation forces necessarily has to work hard on their surface softness. "Work hard" on "soft surface", gotta love the wordplay here.
Now, the softer the cover or cover layers, the more efficient the device gets, grip wise. At the same time, the faster these layers will wear down. And the faster they wear down, the more often they need to be replaced.
The more often they need to be replaced, the cheaper they must be. Also, the more convenient the swap must be for the amputee. Ideally, the terminal device has a shape that is compliant with easy to find materials.
So grip, cover, shape and availability form a seriously constrained space.
Hosmer hook, silicone tubing
By far the simplest setup is a Hosmer hook and silicone tubing.
Buy a few dozens of meters of silicone tubing. Pull a short bit over your split hook claws or over just one claw. Wear them down over three days of hard work. Then, replace them.
Enjoy how easy, cheap, effective and fast that is.
V2P or TRS Prehensor, layered surface modification
A second path provides far more excessive and lush if not insane grip performances at very low actuation forces. This is the add-on of several soft layered material to a gripper These can be sheet rubber, double sided sticky tape or other materials.
Finally, they can be protected, covered and fixed in place by a single finger cut from a nitrile covered work glove that is pulled over one of the flat surfaces of the prosthetic gripper (V2P 1 [link]: V2P 2: [link], Prehensor 1: [link], Prehensor 2:[link]).
These materials are also easy and cheap to find. And they are easy to install.
However one requires a slightly more sophisticated materials collection compared to the Hosmer hook and silicone tube option outlined above.
Becker Hand: nitrile covered work gloves
The Becker Mechanical hand is lightning fast and extremely performant "as is".
A Becker hand can be maximized for grip performance by increasing the force of the springs. That can be done on both Imperial and Lock grip models. But grip performance can also be increased by using any work glove. Yes, you read that right. ** Any ** work glove.
I use regular Becker hand sizes. They all fit standard size eight gloves for normal human hands. With that, any hardware store or web store that has your glove of choice is your friend. These gloves slip on very easily. The manufacturer of the Becker hands imposes no restriction on the choice or make of gloves that is worn over these prosthetic hands.
Also, Centri makes PVC gloves that fit the description of "cosmetic" gloves. These fit the Becker hands as well.
iLimb Touch Bionics and grip performance
Touch Bionics is a manufacturer that sells overpriced prosthetic gloves. These may deteriorate even by themselves, or after a very short time under light usage.
Their hands are also excruciatingly weak, relating to their very low actuation force. The word "performance" is not the first word that comes to mind here.
So far, we have not seen a reliable work glove that is built for, and approved for, sustained use.
With that, the iLimb has to be still classified as problematic choice for real work.
Generally, "bionic" hands need useful grip geometries. To get thumbs to wildly oppose, they use awkward-looking thumb opposition shapes. That attempts to improve grip geometry at least somewhat.
That entails increasingly non-anatomical hand shapes. And that, of course, makes it impossible to wear standard work gloves.
Manufacturers also have to build their prosthetic hands with low weights to accommodate for the easily fatigued user. So grip actuation power is marginal. One effect is that these prosthetic hands are unable to simultaneously grip and deform sufficiently protective and thick gloves.
To avoid total disaster, manufacturers usually restrict users in their glove choice. They must use manufacturer issued very thin and thus fragile gloves. And as the series that are sold are exceedingly small, glove prices are extremely high. Plus, prosthetists that cash in around 30% of all materials sales need to live, too.
An ideal glove has maximal material strength as to perforation. It has so much soft padding underneath, that grip form closure replaces actuation force as main technical feature for a good grip. The material may be very thin and should deform easily, so as not to impede the hand motors. Then, such a glove technically can be said to be well made.
There is the possibility that this may be improved within the next two decades.
- With that, a steel hook that has a usefully well-evolved shape does it for the most part.
- Anything after that, concerning the added soft stuff mentioned above, needs to be fast, cheap, available and easy to swap. So, steel hook with silicone tubing (or so), a gripper with soft layered covers or a Becker hand with a padded glove.