Abstract: In various aspects, the present disclosure provides an example autonomous microsystem for immersion into a fluid. The autonomous microsystem includes electronics, a power source, and a packaging system that surrounds the electronics and the power source. The electronics can be configured to sense and record one or more environmental conditions. The packaging system may include a deformable shell that defines an internal space and a plurality of filler particles disposed in the internal space and configured to control a density of the autonomous microsystem in relation to the fluid. The filler particles may comprise a low-density material having a bulk density greater than or equal to about 100 kg/m3 and less than or equal to about 1,000 kg/m3 and have a packing density greater than or equal to about 1011/m3 and less than or equal to about 1021/m3.