After reading a scientific article about grip strength, after reflecting about my current experience, I figured it was time for an update on grip strength.
Based on a very nice article of Markenscoff et al. (1990), it becomes clear that to securely grip an object in three-dimensional space one requires a rather large number of fingers if there is no friction at all.
This means that if there are less than an optimal number of fingers - such as less than seven in three-dimensional space or less than four in two-dimensional space - the only way to secure an object is by friction.
That means that for any hand or hook with two, three or five fingers there is no way around optimizing friction. But once you do that you can actually try the minimum of fingers such as two in the case of a hook.
Now, some hooks have silicone covers (Otto Bock offers these) and some others contain Nitrile covers or parts. Those are interesting attempts to improve grip a bit. However, there is a real and distinct difference between attempting to improve grip a bit and seriously optimizing it. This difference can be shown and measured. The difference is huge.
Actually, it's not hard. One just has to sit down and do it. - Based on engineering common sense on how to best increase friction, I then cut up some of my rubber mats (red snips), placed them onto the claws (that already are built to keep such rubbers in place) and fixed them there using nitrile glove fingers that I cut up and taped into place. I already had a really cool hand ordering the rubber rolls (good Shore grade for grips) and having these in stock here now allows me to play with the stuff.
Native V2P Prehensor claw shows metal surface with rubber dots to keep covers in place:
Rubber mat being cut up:
V2P Prehensor claw getting a rubber cover (I used two for each claw for starters):
Nitrile glove fingers [see this article on how to cut up abd mount these] keeping two rubber mat covers in place on each claw:
This also contains a few elements of surprise but then, this is a proof of concept, not a pathway to help you copy it :) I mean, where are we. Do your own experiments!
Comparison of grip strength
The following sequence uses the exact same rubber / tension setting for the V2P Prehensor. For some reason my computer reverses the video image so it appears mirrored. Please read the number scale as mirrored. Sorry about that.
First test - 95 Shore A rubber cover pulls around 10 Newton:
Second test - 80 Shore A rubber cover pulls around 30 Newton:
Third test - multi layered customized claw cover pulls over 50 Newton:
Super tweaked secretly enhanced multi layered claw covers pull almost 100 Newton simply by massive squeeze:
I found that for better grip strength, some type of outer rippled and inner deformable surface construction was already used in e.g. baseball glove designs of the eighties. Also, Jacques Monestier patented a similar design for his famous mechanic hand.
So by all means, these should be open source these days. I guess that must be why nobody manufactures prosthetic hooks or hands like that.
At any rate, that was just a start using stuff I had sitting around my work bench.
Using fingerbrush under running water:
 Xanthippi Markenscoff, Luqun Ni, Christos H. Papadimitriou (1990) The Geometry of Grasping. The International Journal of Robotics Research 9:61.