  Biological agents possess bodies that are mostly of soft tissues. Researchers
have resorted to soft bodies to investigate Artificial Life (ALife)-related
questions; similarly, a new era of soft-bodied robots has just begun.
Nevertheless, because of their infinite degrees of freedom, soft bodies pose
unique challenges in terms of simulation, control, and optimization. Here we
propose a novel soft-bodied agents formalism, namely Pressure-based Soft Agents
(PSAs): they are bodies of gas enveloped by a chain of springs and masses, with
pressure pushing on the masses from inside the body. Pressure endows the agents
with structure, while springs and masses simulate softness and allow the agents
to assume a large gamut of shapes. Actuation takes place by changing the length
of springs or modulating global pressure. We optimize the controller of PSAs
for a locomotion task on hilly terrain and an escape task from a cage; the
latter is particularly suitable for soft-bodied agents, as it requires the
agent to contort itself to squeeze through a small aperture. Our results
suggest that PSAs are indeed effective at those tasks and that controlling
pressure is fundamental for shape-changing. Looking forward, we envision PSAs
to play a role in the modeling of soft-bodied agents, including soft robots and
biological cells. Videos of evolved agents are available at
https://pressuresoftagents.github.io.
