Abstract: A sensor is described for detecting the force applied to or by soft material, and that is thus able to measure the stiffness of a soft material. The sensor comprises a sensor body into which is supplied a fluid under pressure. At least one sensor members are provided that are arranged to project from the sensor body under the pressure exerted thereon by the fluid. A sensor member displacement detection system is also provided, that is preferably optically based using optical fibres to illuminate the sensor member and that measures the modulation of the light reflected from the sensor member as the member is displaced against the pressure of the fluid to detect and measure the displacement. From the measured displacement an estimate of the force being applied to the sensor member can be obtained. The sensor is of a small size suitable for use during MIS or catheterisation procedures. Preferably the sensor is constructed of non-metallic material such that it is MRI compatible.