(LDraw file available soon, my Brickshelf account is under maintenance apparently)
This design works with both with pneumatics and linear accumulators. The tubing will be a bit tight for the pneumatic version, but if you are worried you can just remove the axle which is in the way. It might also work with the small pneumatic piston which would create a very compact design however I don’t have one of those so I can’t try it out.
In the design each finger is a section of its own and every section is completely the same, just rotated 90 degrees each. I like it as it is something I rarely see in Technic models; mirroring is quite common however rotating is much less seen.