A photoelastic substrate technique for dynamic measurements of forces exerted by moving organisms

The forces produced by small moving organisms have been quantitatively measured by having the organisms move on a polymeric substrate which transduces the applied stress into an optical birefringence signal (photoelastic effect). The optical signal can be rigorously interpreted to give static force measurements, and by calibrating the substrate empirically, dynamic measurements are obtained. The technique measures stresses, forces applied to an area, so it is not possible to determine ultimate sensitivity of the technique for measurements of forces without regard to area. The technique is especially useful when small forces are exerted over very small areas as, for example, may be the situation with moving tissue cells. The technique is noninvasive, requires minimal equipment, and is easily performed on microscopes adapted for polarized light measurements. Gelatin has the highest sensitivity and adaptability as a photoelastic substrate.