Mechanical (bottom)

The drive mechanism is unusual:  A nitinol wire (shape memory alloy) pulls alternately on two sides of a metal plate.  Each side has a finger that turns a plastic ratchet.  The center and each end of the wire has a brass contact that connects to the circuit board below.  As each half of the wire is energized, the plate tilts towards one ratchet or the other, turning the shaft by a small amount.  This is a very small and light stepper motor.  I call it a "reverse escapement" as it is similar to the escapement in a clock but transfers power in the opposite direction.

The cannula is inserted by rotating the shaft in the middle until the large spring is released, pushing the cannula out of the housing.  There is no carrier needle.