In origami, a traditional Japanese art, a simple sheet of paper is transformed into intricate, 3D shapes through a very specific pattern of folds, crimps and creases. Folding robots based on that principle have emerged as a sensational new frontier of robotic design, but usually require a wired connection to a power source or onboard batteries, making them clunkier and bulkier than their paper inspiration and restricting their functionality.
Researchers at the Wyss Institute for Biologically Inspired Engineering and the John A. Paulson School of Engineering and Applied Sciences (SEAS) at Harvard University have built battery-free folding robots that can perform complex, repeatable movements powered and manipulated through a wireless magnetic field.
Like origami, one of the main points of our design is simplicity. This system requires only basic, passive electronic components on the robot to deliver an electric current – the structure of the robot itself takes care of the rest.
Je-sung Koh, Ph.D., Co-Author, Postdoctoral Fellow at the Wyss Institute and SEAS, and Assistant Professor at Ajou University, South Korea
The details of this research have been reported in Science Robotics.
The research team’s robots are thin and flat (similar to the paper-based inspiration) plastic tetrahedrons, with the three outer triangles linked to the core triangle by hinges and a small circuit on the core triangle. Attached to the hinges are coils made from a type of metal called shape-memory alloy (SMA) that can return to its original shape after deformation by being heated to a specific temperature.
The SMA coils are stretched out in their “deformed” state when the robot’s hinges lie flat; and when an electric current is transmitted through the circuit and the coils heat up, they spring back to their initial, relaxed state, contracting like small muscles and folding the robots’ outer triangles in toward the center. When the current is stopped, the SMA coils are stretched back out because of the stiffness of the flexure hinge, thus dropping the outer triangles back down.
The power that generates the electrical current required for the robots’ movement is supplied wirelessly using electromagnetic power transmission, the same technology found within wireless charging pads that recharge the batteries in cell phones and other small electronic gadgets. An external coil with its own power source produces a magnetic field, which induces a current in the circuits in the robot, thereby heating the SMA coils and resulting in folding. So as to control which coils contract, the team constructed a resonator into each coil unit and tweaked it to respond only to a very particular electromagnetic frequency. By altering the frequency of the external magnetic field, they were able to induce each SMA coil to contract individually from the others.
Not only are our robots’ folding motions repeatable, we can control when and where they happen, which enables more complex movements.
Mustafa Boyvat, Ph.D., Lead Author and Postdoctoral Fellow at the Wyss Institute and SEAS
Similar to the muscles in the human body, the SMA coils can only contract and relax: it is the structure of the robot’s body – the origami “joints” – that translates those contractions into particular movements. To exhibit this capability, the team constructed a small robotic arm that can bend to the left and right, as well as can open and close a gripper around an object.
The arm is built with a special origami-like pattern to allow it to bend when force is applied and two SMA coils convey that force when triggered while a third coil pulls the gripper open. By altering the frequency of the magnetic field produced by the external coil, the team was able to regulate the robot’s bending and gripping motions individually.
There are a number of applications for this kind of simple robotic technology; for instance, instead of having an uncomfortable endoscope sent down the throat to assist a doctor with surgery, a patient could just swallow a micro-robot that could move around and perform simple functions, like filming or holding tissue, powered by a coil outside their body. Using a much bigger source coil – on the order of yards in diameter – could enable wireless, battery-free communication between many “smart” objects around an entire home.
The team constructed a range of different robots – from a quarter-sized flat tetrahedral robot to a hand-sized ship robot made of folded paper – to demonstrate that their technology can house various circuit designs and effectively scale for devices big and small. “There is still room for miniaturization. We don’t think we went to the limit of how small these can be, and we’re excited to further develop our designs for biomedical applications,” Boyvat says.
When people make micro-robots, the question is always asked, ‘How can you put a battery on a robot that small?’ This technology gives a great answer to that question by turning it on its head: you don’t need to put a battery on it, you can power it in a different way.
Rob Wood, Ph.D., Corresponding Author, Core Faculty Member at the Wyss Institute who Co-Leads its Bioinspired Robotics Platform and the Charles River Professor of Engineering and Applied Sciences at SEAS
“Medical devices today are commonly limited by the size of the batteries that power them, whereas these remotely powered origami robots can break through that size barrier and potentially offer entirely new, minimally invasive approaches for medicine and surgery in the future,” says Wyss Founding Director Donald Ingber, who is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital, as well as a Professor of Bioengineering at Harvard’s School of Engineering and Applied Sciences.
This research received support from the National Science Foundation award No. CCF-1138967, the ARL DURIP program award No. W911NF-13-1-0311, and the Early Postdoc Mobility Fellowship from the Swiss National Science Foundation.
Wireless, battery-free folding robots are powered by electromagnetic fields, enabling them to move without bulky batteries. (Credit: Wyss Institute at Harvard University)