Great grip of a prosthetic terminal device usually benefits from soft gripper surfaces.
The more elaborate the gripper shape, the less likely you can fit any material - and the more standard the shape with regard to soft materials, the easier and cheaper. So, a Becker Mechanical Hand [link] tends to fit most standard work gloves from e.g. hardware stores [link], whereas an intricate and peculiar shape design like an iLimb or Michelangelo hand will most likely not fare too well with such (it can be done though, it just may be akward); really, though, they require rather costly original manufacturer covers [link] that seem to cost a fortune and may even tear up with little [link] or no [link] active use.
The softer that cover material, the better the grip, but the more often it will require replacement. Just like car or bicycle tires, or shoe soles, the material is abraded and used up as part of fulfilling its role - so a damaged soft cover is not per se a bad sign, if it served a lot of good grips before. Conversely, prosthetic manufacturers may opt for concessions or trade-offs, for compromise, using a semi-soft material that then lasts a bit longer, at the expense of less insane grip quality of course.
While I probably was the one to introduce the use of standard work gloves to prosthetic hand use, with many following that [link], the commonality of standard split hook users to employ silicone tubing by the meter, cut to length, that can be replaced easily, cheaply and at any time, is well known amongst users (or so it appeared to me), but still, no one publishes these "tricks" or "use recommendations".
Of course, how is any "bionic man" or "hook hater" [link], "researcher" or "developer" (or what they all are and do these days) to understand why their struggles [also see Cybathlon 2016 or 2020] (i.e., picking up things that I pick up with reliability and ease, see videos below) may not be necessarily mine.
And upon revision of some text basics and foundational aspects, this subject of how a split hook grasps so well occurred to me as one of the apparent greater unknowns. Because if we are not really careful here, sooner or later, we may find some dedicated "researcher" that writes up a paper saying "steel hooks cannot lift hard objects because the stuff always slips", when in fact, a proper way to wear a bare steel hook may be when manipulating soft deformable materials, but not hard objects. And how could anyone blame them, when proper use of a steel split hook was never written up properly - or so it seems! Because their focus may certainly anywhere, organizing freak show circuses or crappy control systems, but it seems not to be at all on grip mechanics, so it may fall to us users to point out that their focus, with priority, should be - in fact - on grip mechanics and nowhere else. And when a bum like me manages to grasp a tool by its edge or a needle by its tip in a single unreflected sweep, that should get any "bionic" individual thinking.
Standard steel hook with cut to length silicone tube bits on fingers:
This is a really simple thing to do, but go, go, do it with your device and see how this works out.
Also here, the performance of the hook is almost stellar - only because of well designed grip mechanics. Not because it looks so futuristic. That is why people that lack understanding do not see that this is, in fact, modern [link].