3D-printed mini-actuators move small soft robots

Researchers have demonstrated miniature soft hydraulic actuators that can be used to control the deformation and motion of soft robots that are 0.8mm thick.

Soft hydraulic actuators can be used to control the deformation and motion of soft robots
Soft hydraulic actuators can be used to control the deformation and motion of soft robots - Jie Yin, NC State University

The researchers from North Carolina State University have also demonstrated that this technique works with shape memory materials, allowing users to repeatedly lock the soft robots into a desired shape and return to the original shape as needed. The team’s work is detailed in Advanced Materials.

In a statement, Jie Yin, corresponding author and an associate professor of mechanical and aerospace engineering at NC State, said: “Soft robotics holds promise for many applications, but it is challenging to design the actuators that drive the motion of soft robots on a small scale.

“Our approach makes use of commercially available multi-material 3D printing technologies and shape memory polymers to create soft actuators on a microscale that allow us to control very small soft robots, which allows for exceptional control and delicacy.”

The soft robots consist of two layers, the first a flexible polymer created using 3D printing technologies and incorporating a pattern of microfluidic channels. The second layer is a flexible shape memory polymer.

By pumping fluid into the microfluidic channels, users create hydraulic pressure that forces the soft robot to move and change shape. The pattern of microfluidic channels controls the motion and shape change of the soft robot. The amount of fluid being introduced, and how quickly it is introduced, controls how quickly the soft robot moves and the amount of force the soft robot exerts.

To ‘freeze’ the soft robot’s shape, the operator can apply moderate heat (64oC), and then let the robot cool briefly. According to NC state, this prevents the soft robot from reverting to its original shape, even after the liquid in the microfluidic channels is pumped out. If users want to return the soft robot to its original shape, they apply the heat again after pumping out the liquid, and the robot relaxes to its original configuration.

“A key factor here is fine-tuning the thickness of the shape memory layer relative to the layer that contains the microfluidic channels,” said Yinding Chi, co-lead author of the paper and a former PhD student at NC State. “You need the shape memory layer to be thin enough to bend when the actuator’s pressure is applied, but thick enough to get the soft robot to retain its shape even after the pressure is removed.”

To demonstrate the technique, the researchers created a soft robot gripper capable of picking up small objects. The researchers applied hydraulic pressure, causing the gripper to pinch closed on an object. By applying heat, the researchers were able to fix the gripper in its ‘closed’ position, even after releasing pressure from the hydraulic actuator. The gripper could then be moved – transporting the object it held – into a new position. Researchers then applied heat again, causing the gripper to release the object it had picked up.

“Because these soft robots are so thin, we can heat them up to 64oC quickly and easily using a small infrared light source – and they also cool very quickly,” said Haitao Qing, co-lead author of the paper and a PhD student at NC State. “So this entire series of operations only takes about two minutes, and the movement does not have to be a gripper that pinches: we’ve also demonstrated a gripper that was inspired by vines in nature. These grippers quickly wrap around an object and clasp it tightly, allowing for a secure grip.”

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