a sensor for chemical species or biological species or radiation presenting to test fluid a polymer composition comprises polymer and conductive filler metal , alloy or reduced metal oxide and having a first level of electrical conductance when quiescent and being convertible to a second level of conductance by change of stress applied by stretching or compression or electric field , in which the polymer composition is characterized by at least one of the features in the form of particles at least 90 % w / w held on a 100 mesh sieve ; and / or comprising a permeable body extending across a channel of fluid flow ; and / or affording in - and - out diffusion of test fluid and / or mechanically coupled to a workpiece of polymer swellable by a constituent of test fluid .