It included vibration motors in each of 3 fingers. As the finger pads were pushed against the skin, the swinging arms would bring them inwards and backwards, pinching and gripping the skin. When the fingers had withdrawn the vibration motors would turn on and then the whole assembly could slide backwards as the needle entered the body.
But with 93 parts it was complex. I managed to build a prototype with my Form 1 SLA rapid prototyping machine and spent a while testing it. I then discovered something unexpected. The analgesic effect persisted even when the vibration motors were not working. I had discovered that analgesia also worked from the mere tactile distraction effect, so I set about simplifying it to produce a variant without the motors. This reduced the part count to 75.
But the design had retained the complexities that were really only necessary for the powered variant. I resolved to start again with a clean sheet and try and simplify it further.