Upconverting microgauges reveal intraluminal force dynamics in vivo

The forces generated by action potentials in muscle cells shuttle blood, food and waste products throughout the luminal structures of the body. Although non-invasive electrophysiological techniques exist^1-3, most mechanosensors cannot access luminal structures non-invasively^4-6. Here we introduce non-toxic ingestible mechanosensors to enable the quantitative study of luminal forces and apply them to study feeding in living Caenorhabditis elegans roundworms. These optical 'microgauges' comprise upconverting NaY_0.8Yb_0.18Er_0.02F₄@NaYF₄ nanoparticles embedded in polystyrene microspheres. Combining optical microscopy and atomic force microscopy to study microgauges in vitro, we show that force evokes a linear and hysteresis-free change in the ratio of emitted red to green light. With fluorescence imaging and non-invasive electrophysiology, we show that adult C. elegans generate bite forces during feeding on the order of 10 µN and that the temporal pattern of force generation is aligned with muscle activity in the feeding organ. Moreover, the bite force we measure corresponds to Hertzian contact stresses in the pressure range used to lyse the bacterial food of the worm^7,8. Microgauges have the potential to enable quantitative studies that investigate how neuromuscular stresses are affected by ageing, genetic mutations and drug treatments in this organ and other luminal organs.