Physical constrains for prosthetic grip mechanisms [theory and practice]

Obviously, prosthetic grips are a real and - for amputees, not so much for industrial robots - moderately to unsatisfyingly solved problem. From my current view point, this is understandable: there are physical constraints, and working with them requires clear understanding of them.

For obtaining a good grip, you have two options [1]: either grasp the object with entirely undeformable hard fingers - then (in a 3D environment) you need about seven fingers that press against the object adaptively to actually achieve full form closure (Formschluss), or do with less fingers than seven, but then have their surfaces deformable to approximate form closure and make up for that approximation deficiency with friction. I already exploited the second option by manufacturing and using deformable V2P covers with great success and it is from using a whole range of options that I now type up this article.

In particular, I use/d the following:

  • Otto Bock hand with PVC glove (myoelectric)
  • Otto Bock voluntary opening hand with PVC glove (body powered)
  • Otto Bock voluntary opening hand with *work rubber glove (body powered)
  • Otto Bock voluntary opening hand with cosmetic silicone hand (body powered)
  • Otto Bock metal hooks of various shapes and makes, including *MovoHook 2Grip (body powered)
  • Otto Bock MovoHook 2Grip with silicone claw covers
  • Hosmer aluminium hook 555 (nitrile covers on claw insides) (body powered)
  • Hosmer steel work hook (body powered)
  • Becker Lock Grip and Imperial hands  (body powered)
  • Becker Lock Grip and Imperial hands with PVC glove (body powered)
  • Becker Lock Grip and Imperial hands with *nitrile covered working gloves (body powered)
  • *V2P Prehensor (body powered)
  • *V2P Prehensor with *different shore A values for factory issued claw covers (body powered)
  • *V2P Prehensor with *multilayered customized claw covers (body powered)
  • Cosmetic arm

The options marked with * have seen my personal suggestions or influence or were released after I had submitted such to the manufacturer. It may become apparent from the list alone that prosthetic grip quality is an interest that I pursue.

It is well known that mother nature equipped the average human with just about five fingers for one hand, and with ten fingers for two hands, all well padded. Even if completely slippery, any suitably sized object can always be held with two hands including ten fingers, or, if larger, with two hands in conjunction with torso and arms - and any suitably sized object that is not completely slippery can be held with one hand.

So any prosthetic gripper that has less than seven fingers - i.e., all that I know of - will either have to have deformable surfaces to make up for approximated form closure - or, if it has hard surfaces, exhibit frequent failure. However, deformable surface materials decay relatively rapidly under continuous usage and thus also will exhibit frequent / rapid failure due to wear down, whereas hard surfaces may stay in better shape for longer but will fail grips due to geometrical reasons.

In fact, either way, you will experience failure of grip, grasp, due to geometrical grip failure or due to material decay. In most instances, you will experience failure for at least both reasons if not more - and to no surprise, disappointment is a pre-programmed experience unless a very clear understanding about this particular constraint is verbalized, defined, brought forward and addressed very specifically.

Trends in shape design of prosthetic grippers

There are no design trends in prosthetic grippers. All improvements that are notable were made decades ago and then simply copied.

We have a practical benefit of these today, but as far as I see, there is not one single new prosthetic gripper that came out in the last two years has a significantly better shape design than previous models except maybe the V2P Prehensor. The V2P Prehensor however draws its enormous potential from combining powerful features of old hook models that were already improved earlier - Trautman hook shape and variable grip strength sliders are not really new. New "bionic" prosthetic hands such as Michelangelo, iLimb or BeBionic all fail to provide us with a grip design that is in any way improved - if anything, they have a thumb and fingers that adaptively grip. However, the Becker hand already uses this principle and the new myoelectric hands are not going much further than that. Recent design ideas include tentacles as well - but they are neither available as prototypes or have they been tested.

So in terms of modernizing shape design of prosthetic grippers, these are the latest validated design innovations that can be purchased and worn:

  • V2P Prehensor (~2009)
  • Hosmer hooks (decades ago)
  • TRS Grip Prehensor (about a decade ago)
  • Becker hands (decades ago)

This also means that if any new grip shape is to come about, we are safe to charge ahead ourselves. For decades, no one built us new or better grippers - I am not saying the current ones always fail - but if there is need for improvement, we need to go ahead both with analysis and construction ourselves.

Trends in deformable surfaces for prosthetic grippers

As it appears, the current status of prosthetic grip covers is as follows:

  • Otto Bock hooks can be equipped with factory issued silicone covers that are relatively hard
  • Hosmer hooks may feature nitrile inlays that are rather hard
  • V2P Prehensors can be equipped with factory issue claw covers of various stiffness
  • Prosthetic hands usually can be equipped with factor issue PVC or silicone gloves

In terms of self made options, there are the following practical options I see:

  • Otto Bock hands are hard to equip with work gloves due to their strange shape
  • Becker hands are relatively easy to equip with work gloves
  • V2P Prehensors are super to equip with various claw coats / coverings

Having established that, there are two basic options to be chosen from:

  • a relatively hard cover - advantage: longer durability - disadvantage: grip not so good
  • a relatively soft cover - advantage: better grip - disadvantage: durability not so good

Of course I want to have a first class prosthetic part on my arm and so I will opt for a good grip. That means I go for a soft cover. And a really soft cover with a really good grip in turn means that I will have to replace it really often, like, every two or three weeks. That then means I will go for a hand or hook that allows for cheap and readily available materials - rather than expensive parts that are hard to round up or that I run out of ever so quickly.

So, in essence, I will either wear the Becker hand without or with any choice of supermarket or mail order working gloves. Or I will wear the V2P Prehensor that can be equipped with super strong gripping materials from any hardware store. Changing these materials is affordable, can be done frequently and maximizes grip.

People believe they can work around these serious constraints but it turns out they can't. I can't, no one can. Grips work like that, or they don't work at all. If I cannot maximize grip with soft surfaces I either have to use more closing force, which then means I have to use greater cable force, straining whatever gets damaged by heavy cable operation or let go of that type of usage and strain my (overworked) non-disabled arm. Things all relate here.

What to develop now?

  • Hands or hooks with better gripper shapes
  • Shapes must be either perfect as hard surfaces or at least near perfect - or so they can be fitted with any cheap and easy to replace standard material for deformable covers

Any prosthetic hand that fits into a baseball, racing or golf glove is better than a prosthetic hand that does not. Any prosthetic hand that fits into any type of mass produced work glove is better than a hand that does not. Any hook that can be equipped with single glove digits is better than hooks that have different shapes.

And now we see why the Krukenberg operation is such a powerful option. The Krukenberg operation will turn a forearm stump into an only two-fingered gripper - but optimally, it features a soft deformable surface that - as it is skin - renews itself. Without surprise it is reported to be a very satisfying experience. In essence, a Krukenberg pincer has a definite potential to work around clear constraints that are imposed on the physics of grip.

Current prosthetic innovations miss both of these aspects. The introduction of different grips - pinch, lateral, power grip - does not do anything for the grip itself: you still need a perfect surface geometry, and you still need easy to replace deformable surface materials. If Otto Bock Michelangelo hand, iLimb or iLimb Pulse hand or BeBionic hand gloves cost 3000 dollars a piece, I would definitely not call that easy to replace. Not the slightest bit. A 3$ glove is what I call easy to replace.

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[1] Xanthippi Markenscoff, Luqun Ni, Christos H. Papadimitriou (1990) The Geometry of Grasping. The International Journal of Robotics Research 9:61.

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Physical constrains for prosthetic grip mechanisms [theory and practice]; published January 3, 2011, 00:18; URL: https://www.swisswuff.ch/tech/?p=380.

BibTeX: @MISC{schweitzer_wolf_1594172190, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Physical constrains for prosthetic grip mechanisms [theory and practice]}}, month = {January},year = {2011}, url = {https://www.swisswuff.ch/tech/?p=380}}