230 CHEMICAL SENSORS: TECHNOLOGIES. VOLUME 4: SOLID-STATE DEVICES Figure 5.1. Parallel-plate capacitor. In the other approach, researchers have developed capacitance-type chemical sensors and sensor ar- rays of silicon/polymer bimorphs operating either in resonance or static bending. These devices (referred to as bimorph sensors) are made of a flexible and a rigid electrode, in which the flexible electrode is covered by chemically sensitive material (usually a polymer) that is able to expand in volume in the pres- ence of analyte molecules. As a result, stress is induced on the flexible electrode as the overlying polymer swells, resulting in a change in the device resonance frequency and/or bending, which are then detected as a change in capacitance. In that case the previous simple expression is modified and the capacitance is given by