3D-printed mini-actuators drive tiny soft robots

North Carolina State University researchers develop sub-millimeter grippers with shape memory materials.

Researchers from North Carolina State University have demonstrated miniature soft hydraulic actuators that can be used to control the deformation and motion of soft robots that are less than a millimeter thick. The researchers have also demonstrated that this technique works with shape memory materials, allowing users to lock the soft robots into a desired shape and return to the original shape as needed.

“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,” said Jie Yin, corresponding author and associate professor of mechanical and aerospace engineering at NC State in a press release. “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.”

Designing a 3D-printed soft robot

The new technique relies on creating soft robots that consist of two layers. The first layer is a flexible polymer that is created using 3D printing technologies and incorporates a pattern of microfluidic channels – essentially very small tubes running through the material. The second layer is a flexible shape memory polymer. Altogether, the soft robot is only 0.8 millimeters thick.

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 – whether it bends, twists, or so on. In addition, 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.

If users wish to ‘freeze’ the soft robot’s shape, they can heat it to 64 C (147 F) and then let it cool briefly. 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 can apply the heat again after pumping out the liquid, causing the robot to relax 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,” explained Yinding Chi, co-lead author, in the same press release. “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.”

Demonstrating 3D-printed soft robots

To demonstrate the technique, the researchers created a soft robot “gripper,” capable of picking up small objects.

“Because these soft robots are so thin, we can heat them up to 64C 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 Ph.D. student at NC State. “So this entire series of operations only takes about two minutes. 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.”

The researchers believe their paper serves as a proof-of-concept for this new technique, with potential applications in small-scale soft robots, shape-shifting machines and biomedical engineering. The paper is published in the journal Advanced Materials.

Written by

Ian Wright

Ian is a senior editor at engineering.com, covering additive manufacturing and 3D printing, artificial intelligence, and advanced manufacturing. Ian holds bachelors and masters degrees in philosophy from McMaster University and spent six years pursuing a doctoral degree at York University before withdrawing in good standing.