A team of researchers at the Wyss Institute for Biologically Inspired Engineering and the John A. Paulson School of Engineering and Applied Sciences (SEAS) at Harvard University has created battery-free folding robots that are capable of complex, repeatable movements powered and controlled through a wireless magnetic field. The system requires only basic, passive electronic components on the robot to deliver an electric current, and the structure of the robot itself takes care of the rest.
The robots are flat and thin plastic tetrahedrons, with the three outer triangles connected to the central triangle by hinges, and a small circuit on the central triangle. Attached to the hinges are coils made of a type of metal called shape-memory alloy (SMA) that can recover its original shape after deformation by being heated to a certain temperature. When the robot’s hinges lie flat, the SMA coils are stretched out in their “deformed” state. When an electric current is passed through the circuit and the coils heat up, they spring back to their original relaxed state, contracting like tiny muscles and folding the robots’ outer triangles in toward the center. When the current stops, the SMA coils are stretched back out due to the stiffness of the flexure hinge, thus lowering the outer triangles back down.
The power that creates the electrical current needed for the robots’ movement is delivered wirelessly using electromagnetic power transmission, the same technology inside wireless charging pads that recharge the batteries in cell phones and other small electronics. An external coil with its own power source generates a magnetic field, which induces a current in the robot’s circuits that heats the SMA coils and produces the folding. In order to control which coils contract, the team built a resonator into each coil unit and tuned it to respond only to a very specific electromagnetic frequency. By changing the frequency of the external magnetic field, they were able to induce each SMA coil to contract independently from the others.
Just like the muscles in the human body, the SMA coils can only contract and relax, so the robot’s origami “joints” translate those contractions into specific movements. To demonstrate this capability, the team built a small robotic arm capable of bending to the left and right, as well as opening and closing a gripper around an object. The arm is constructed with a special origami-like pattern to permit it to bend when force is applied, and two SMA coils deliver that force when activated while a third coil pulls the gripper open. By changing the frequency of the magnetic field generated by the external coil, the team was able to control the robot’s bending and gripping motions independently.
There are many applications for this kind of minimalist robotic technology. For example, rather than having an endoscope assist a doctor with surgery, a patient could swallow a micro-robot that could move around and perform simple tasks, like holding tissue or filming, powered by a coil outside their body. The researchers built a variety of robots ranging from a quarter-sized flat tetrahedral robot to a hand-sized ship robot made of folded paper.