Three 3D-printed soft, flexible scaffolds: those on the outside are maintained in rigid, bent positions via cooled, rigid wax coatings, while the one in the middle is uncoated and remains compliant (here, it collapses under a wrench).Researchers have developed a phase-changing material made from wax and foam that could lead to a new generation of low-cost robots able to switch between hard and soft states to move through small gaps. The innovative material could find uses in building deformable surgical robots that can pass through the body without causing any damage, or squeeze through the rubble of buildings looking for survivors during search-and-rescue operations.
With growing interest in soft robotics and shape-shifting systems, much research is going into the most effective way to achieve components of variable strength and stiffness. Since many existing robotic systems are comprised of rigid components, which limit their movement, the team wanted to develop components that allow robots to better conform to the environment and achieve significant changes in shape and volume to improve their capabilities.
To produce their material, the scientists from MIT, the Max Planck Institute and Stony Brook University, in collaboration with a robotics company, coated low-cost polyurethane foam in melted wax. The wax coating can change from a hard outer shell to a soft and flexible surface with moderate heating, in this case by running a wire along the coated foam before applying a current to heat and melt the surrounding wax. On turning current off, the material cools down and returns to its rigid state. The material is also self-healing – if the coating becomes fractured, it can be heated and then cooled so that the structure returns to its original configuration.
The research "demonstrated how a simple wax–foam composite composed of widely available and inexpensive components can exhibit large volume, shape, strength and stiffness changes.”Nadia Cheng
The study, reported in Macromolecular Materials and Engineering [Cheng et al. Macromol. Mater. Eng. (2014) DOI: 10.1002/mame.201400017], used 3D printing to make another version of the foam lattice structure to control the position of each strut and pore, and better investigate the properties of the material. On testing, it was shown that the printed lattice was more amenable to analysis than the foam, although the latter would still be effective in low-cost applications. Researcher Nadia Cheng said this demonstrated “how a simple wax–foam composite composed of widely available and inexpensive components can exhibit large volume, shape, strength and stiffness changes.”
The team think there could be many applications for practical wax–foam composites, and plan to explore other unconventional materials for robotics, such as those that switch states when a magnetic or electric field is applied. It may also be possible to optimize the wax–foam combination to exhibit favorable wetting and thermal properties.