A wooden mechanism inspired by the spine and ribcage. 

My first attempt had a few problems. The mechanism relied on gravity to reset the pieces into position as the cam rotated but the weight of the cam follower wasn't enough to reset itself each time it rose up. I also used wooden dowels to support and drive the mechanism and had to use doubled up eight inch wooden components to make the quarter inch parts I originally wanted. At this thickness the wood warped too much and the wooden dowels weren't straight or strong enough.

On my second attempt I remodeled the driving mechanism to rely solely on rotational force. Along with this I took advantage of the facilities at Brown University to fabricate my parts at the quarter inch thickness I wanted to use. I also replaced the wooden dowels with steel rods, greatly reducing the warpage issues I previously encountered. Finally this time I made sure to wax every single component before assembly to make sure there would be as little friction as possible.

The mechanism is able to be constructed in multiple configurations depending on when each gear is engaged to the drive shaft.