In nature, “mechanosensors” like those spider leg-hairs are perfectly tuned to focus only on the data that the spider needs for survival. They’ll pick up vibrations that indicate a bug is stuck in the web, for example, but won’t concern themselves with lower frequency vibrations that might just be the wind.
Inspired by this, the Purdue team set out to create mechanosensors that will ignore minor forces, and only signal the rest of the machine after that sensation hits a certain threshold. The trick to this is making the sensors out of a material that starts off stiff, but changes shape rapidly when an external force is applied to it. When its changed shape reaches a certain point, conductive particles inside the material come together and allow electricity to flow through. That in turn sends a signal to the rest of the machine, which responds as needed.