Upconverting microgauges reveal intraluminal force dynamics <i>in vivo</i>.

The forces generated by action potentials in muscle cells shuttle blood, food, and waste products throughout the body's luminal structures. While non-invasive electrophysiological techniques exist,^1-3 most mechanosensitive tools cannot access luminal structures non-invasively.^4-6 Here, we create non-toxic, ingestible mechanosensors to enable the quantitative study of luminal forces and apply them to study feeding in living <i>Caenorhabditis elegans</i> roundworms. These optical "microgauges" comprise upconverting NaY_0.8Yb_0.18Er_0.02F₄@NaYF₄ nanoparticles (UCNPs) embedded in polystyrene microspheres. Combining optical microscopy and atomic force microscopy to study microgauges <i>in vitro</i>, 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 <i>C. elegans</i> 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 within the pressure range used to lyse the worm's bacterial food.^7,8 Microgauges have the potential to enable quantitative studies that investigate how neuromuscular stresses are affected by aging, genetic mutations, and drug treatments in this and other luminal organs.