A new phase-changing material built from wax and foam that’s capable of switching between hard and soft states could allow surgical robots to shape-shift and move through the body to reach a particular point without damaging any of the organs or vessels along the way, say engineers at Massachusetts Institute of Technology, Cambridge.

The material, which was developed by mechanical engineers at MIT, along with researchers at the Max Planck Institute for Dynamics and Self-Organization, and Stony Brook University, was inspired by the ability of an octopus to squeeze through tight spaces and then expand again to move around a given area.

But, when the material is used to create a soft robot that’s needed to perform meaningful tasks, it needs to be able to exert a reasonable amount of pressure on its surroundings, and not just deform. In addition, controlling a very soft structure is more difficult to predict how the material will move, and what shapes it will form, than it is with a rigid robot.

The researchers decided that the best way to build a deformable robot would be to develop a material that can switch between a soft and hard state. To do so, they coated a foam structure in wax, which can change from hard to pliable with moderate heating.

They found that this could be accomplished by running a wire along the coated foam struts and then applying a current to melt the surrounding wax. Turning off the current allows the material to cool down and return to its rigid state. In addition to switching the material to its soft state, heating the wax in this way also repairs any damage sustained, making the material self-healing.

To build the material, the researchers simply placed the polyurethane foam in a bath of melted wax. They then squeezed the foam to encourage it to soak up the wax.

In order to study the properties of the material in more detail, they then used a 3D printer to build a second version of the foam lattice structure, to allow them to carefully control the position of each of the struts and pores.