The development of miniature soft actuators at millimeter or smaller scales represents a significant advancement in robotics technology. These small-scale actuators offer a wide range of potential applications, from delicate grasping to precise drug delivery and medical diagnosis. However, designing actuators for motion control in small-scale soft robots has been challenging.
To address this challenge, North Carolina State University researchers have successfully demonstrated the use of miniature soft hydraulic actuators to control the deformation and motion of soft robots that are less than a millimeter thick.
What’s more, this innovative technique works seamlessly with shape memory materials, enabling users to repeatedly lock the soft robots into a desired shape and revert to the original shape as required. This breakthrough has the potential to revolutionize the field of soft robotics and open up new possibilities for applications in various industries.
“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,” says Jie Yin, corresponding author of a paper on the work and an associate professor of mechanical and aerospace engineering at NC State. “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 innovative method involves using soft robots comprising two layers. The first layer is a flexible polymer created using advanced 3D printing techniques and contains a network of microfluidic channels – extremely small tubes embedded in the material. The second layer consists of a flexible shape memory polymer. This results in an incredibly thin, soft robot, measuring only 0.8 millimeters.
By injecting fluid into the microfluidic channels, users can generate hydraulic pressure, causing the soft robot to move and alter its shape. The arrangement of microfluidic channels governs the movement and shape transformation of the soft robot, determining whether it bends, twists, and so forth. Moreover, the rate and volume of fluid injection determine the speed and force exerted by the soft robot.
If you ever need to ‘freeze’ the soft robot‘s shape, simply apply moderate heat (64C or 147F) and allow the robot to cool briefly. This will retain the robot’s shape, even after removing the liquid from the microfluidic channels. To revert the soft robot to its original shape, just reapply the heat after removing the liquid, and the robot will 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,” says Yinding Chi, co-lead author of the paper and a former Ph.D. 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 developed a revolutionary soft robot “gripper” capable of securely picking up and transporting small objects. Cleverly utilizing hydraulic pressure and heat enabled the gripper to maintain a firm hold on the object even after releasing the pressure.
The gripper could then be moved into a new position. Researchers then applied heat again, causing the gripper to release the object it had picked up. This breakthrough technology promises significant advancements in robotics and automation.
“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,” says Haitao Qing, co-lead author of the paper and a Ph.D. 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,” says Qing. “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.
“This paper serves as a proof-of-concept for this new technique, and we’re excited about potential applications for this class of miniature soft actuators in small-scale soft robots, shape-shifting machines, and biomedical engineering.”
Journal reference:
- Haitao Qing, Yinding Chi, Yaoye Hong, Yao Zhao, Fangjie Qi, Yanbin Li, Jie Yin. Fully 3D-printed Miniature Soft Hydraulic Actuators with Shape Memory Effect for Morphing and Manipulation. Advanced Materials, 2024; DOI: 10.1002/adma.202402517