Modelling and testing of a sensor capable of determining the stiffness of biological tissues

In this paper, the design, fabrication, testing, and mathematical modelling of a new sensor for determining the stiffness of biological tissues are discussed. A tactile probe is constructed using polyvinylidene fluoride (PVDF) as the main element of the sensing system. The probe is composed of several parts that together can provide an estimate of the stiffness of a material that it touches. Two different elastic materials in the form of two cylinders with different moduli of elasticity make up the major structure of the sensor assembly. The central cylinder in the sensor has a greater stiffness than the peripheral cylinder. To test the sensor, it is mounted on a rigid base structure. Then the resulting probe is pressed against a rubber-like material to experimentally determine the Young's modulus of elasticity of the touched material. In addition to the experimental work (and for the first time), an analytical method is derived that can predict the modulus of elasticity of the touched object without needing to know the thickness of the object. Based on this approach, the stiffness of the sensed object is obtained by means of a closed-form formula. Additionally, finite element modelling is used to compute a more accurate estimate for the same parameter. The discrepancy between the two theoretical methods is proven to be negligible. Comparisons of the experimental data with the analytical and numerical approaches show a reasonably good correspondence between the methods. The practical implications of the designed system are considerable, since it provides a strong basis for detecting tumours embedded in various biological tissues.