The first design I printed was in PLA plastic and uses a Live hinge design.

This is a quite simple design, simply a cylinder with cut-outs to allow the hinge to bend, A piece of 22gauge wire is used to pull the finger closed.

A mix of the spring from the hinges, and the rigidity of the wire is used to push the finger back open again.

This robot will primarily be used for pushing with straight fingers or grabbing objects and so high torque opening is not a high priority. but it is something to keep in mind for later prototypes.