Cable mechanisms consist of a tension(/compression) bearing cable running through a compression(/tension) bearing channel. This mechanism can be used to transmit force over relatively long distances with light and flexible material. Notable examples include Bowden type 3D printer extruders, bicycle brakes, early car throttles etc.
While experimenting with sphincter type channels, I discovered that to get symmetric contraction of the mechanism it is important to apply force through both ends of the cable as shown below.
I used this mechanism to make Cookie Monster, an evil cookie jar that doesn’t let you take cookies. If you reach in and grab a cookie, it mouth of the jar traps your hand and won’t let go until you drop the cookie back in the jar.
I want to make a robust version of it, keep a delicious cookie or amazing toy inside and exhibit it in a children’s museum. I can just imagine kids trying all sorts of ways to get at the toy and failing. Ahhh.. it’ll be fun to watch that! If you’re someone who can make this happen, I want to get in touch with you!
I also experimented with developing a lightweight upper body exoskeleton to help me do pull-ups. But I gave up on the idea when I realized that a robust mechanism would require at least some metal machining. If you want to develop the concept further, feel free to reach out. I’d be happy to share thoughts and experience.