The best sensors could be made of Silly Putty (and graphene)
Together, they make a material that is so sensitive it can detect the footsteps of a small spider.

Jonathan Coleman with G-Putty, and his son Oisin with Silly Putty.
Researchers have combined two unlikely bedfellows to create surprisingly sensitive slectrochemical sensors. Silly Putty (or polysilicone polymer) and graphene combine to make a material that is extremely sensitive to impact, graphene researchers from Trinity College Dublin and The University of Manchester have discovered.
Mounted to the chest and neck of a human test subject, the G-Putty, as the team is calling the material, was able to monitor breathing, pulse and blood pressure.
"When we added the graphene to the silly putty, it caused it to conduct electricity, but in a very unusual way. The electrical resistance of the G-putty was very sensitive to deformation with the resistance increasing sharply on even the slightest strain or impact. Unusually, the resistance slowly returned close to its original value as the putty self-healed over time," explained research leader Jonathan Coleman of Trinity College Dublin in a statement.
Robert Young of The University of Manchester added, "We have now developed a new high-performance sensing material, 'G-putty', that can monitor deformation, pressure and impact at a level of sensitivity that is so precise that it allows even the footsteps of small spiders to be monitored. It will have many future applications in sensors, particularly in the field of healthcare."