Sensitively photoelastic biocompatible gelatin spheres for investigation of locomotion in granular media

We present a method for measuring forces in granular media experiments using photoelastic gelatin spheres, which is applicable for investigating the forces generated by organisms moving through noncohesive granular materials. We describe how to fabricate gelatin spheres with appropriate characteristics for high-sensitivity photoelastic measurements. We present a calibration methodology to relate photoelastic signal to force applied to the spheres, and evaluate the photoelastic performance of gelatin spheres as a function of gelatin concentration. The spheres can be used across a range of salinities, allowing investigation of freshwater and marine organisms. We show that photoelastic gelatin spheres can detect forces as small as 1 μN, and quantitatively measure forces with up to 60 μN precision. We provide a proof-of-principle experiment in which the forces exerted by an earthworm in a granular environment are measured.