A team of engineers at Princeton University in New Jersey have designed and built soft-rigid hybrid robots that move and shift without the need for motors or external pneumatic controls, combining a printable polymer, called a liquid crystal elastomer, with flexible electronics and folding techniques based on the art of origami.
With their ability to shapeshift and manipulate delicate objects, soft robots could work as medical implants, deliver drugs inside the body and help explore dangerous environments. But the squishy machines are often limited by rigid mechanical parts or external systems that provide power or help them move.
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To get around this, the team used a 3D printer to create a reconfigurable soft robot that was able to repeatedly move without noticeable degradation. As a demonstration, they built a robot in the shape of a classic origami crane that flaps its wings when powered with electricity.
The crane moves without a motor. Instead, the robot’s motion relies on targeted heating in the polymer to control the wing flapping. The experiment also demonstrated that the robot can precisely and repeatedly move and return to its original shape without wear or distortion with real-time programmable sequences, a key feature for future applications.
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The system starts with a molten polymer printed into patterned zones using Professor Emily Davidson’s customised 3D printer. Despite its flexible nature, the polymer the researchers use is a liquid crystal elastomer, which means its internal molecules have an ordered structure.
Davidson’s group are experts in controlling the structure of liquid crystal elastomers through molecular design, and controlling the nanostructure (in this case, the orientation) of polymers through printing, both of which were crucial to this project. The researchers programmed the printer to vary the internal orientation of the molecular structure of the polymer as it prints. Each of the patterned zones in the printed material features consistent molecular alignment. By stacking these zones and joining them in different ways, the researchers were able to create hinges in the material that bend in pre-programmed ways when the material is heated.
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As part of the printing, the researchers also added flexible electronics into the hinges in the material. The printed circuit boards’ flexible structure allowed the researchers to embed them directly in the printing material rather than apply the circuits in a separate step. This simplifies fabrication and allows for greater consistency and functional integration of the circuit into the robot.
Davidson noted that a critical advance in the current work was the integration of 3D-printed liquid crystal elastomers with printed circuit boards that could be commercially manufactured. The ability to co-design the liquid crystal elastomer hinges and the flexible printed circuit boards to drive actuation made the fabrication and control of these soft-rigid soft robots feasible.
Once embedded, these circuit boards allow the researchers to heat extremely specific areas of the polymer structure and perform closed loop control using embedded temperature sensors. This heating takes advantage of the carefully structured polymer, causing the material to contract in ways that the engineers program into the polymer printing. These contractions trigger folding along hinges. To ensure the material folds only at the hinges, the researchers added light fibreglass panels to the flexible printed circuit boards in between the polymer hinges.
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The researchers used mathematics derived from origami patterns to control the robots’ motion based on systems of folding and unfolding. Glaucio Paulino’s research team has pioneered the use of origami to design medical implants, construction components and robotics.
He said that an important feature of the design is that the software used to control the robot uses embedded temperature sensors in the origami to compensate for small errors that creep into the system as the robot repeatedly changes shape – the ability to correct these errors is key to soft robots’ durability.
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The research has been published in Advanced Functional Materials.