A device measures physical properties for a solution by detection of dimensional changes of sensitive, or sensitized, polymer reticulations caused by contact with the solution. The dimensional changes may be monitored by mechanical, optical or electronic means, and in a preferred arrangement the device takes the form of a field effect transistor. A semiconductor device has two regions (8,9) of n-doped silicon separated by a p-doped silicon region (10), and covered by a silicon insulating layer (7) which is in turn covered by a thin, swellable, polyacrylamide layer (1) connected to a gate electrode (6). A potential difference applied between the gate (6) and base (10) initiates a drain - source current flow through the semiconductor; application of an appropriate liquid to the polymer layer causes swelling and an increased separation of the gate from the semiconductor layer, with consequent change in the monitored current.