Granular Jamming as Controllable Stiffness Mechanism for Medical Devices

Endoscopic medical devices require high bending flexibility to navigate through tortuous channels while exhibiting some stiffness to exert force on tissues. The granular jamming is a solution which can be implemented at the tip or along the body of these devices to control their stiffness. In this work, the stiffness of sphere packings is studied experimentally and modeled by Discrete Element Method (DEM). The secant stiffness, at a medium level of strain, is evaluated by means of special vacuum assisted triaxial compression tests using polydisperse glass beads as granular material. A cycling method is performed during the experimental procedure to ensure the repeatability of the measurements by eliminating the initial experimental conditions and to be compared to the DEM results. The model has been calibrated by fitting the experimental curves and varying the contact stiffness of the particles, the contact friction angle, the grain size distribution and the confining stress. This numerical tool is used for forecasting the behavior outside the experimental conditions. Among all parameters, the pressure difference shows the largest effect on the stiffness change and can therefore be used as the stimulus for future controllable stiffness medical devices.