3D-Printed Actuators Can Move, Lock Soft Robots into New Shapes

Andrew Corselli

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 repeatedly 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 Corresponding Author Jie Yin, Associate Professor of Mechanical and Aerospace Engineering. “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 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 apply moderate heat (64 °C, or 147 °F), and then let the robot 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 simply 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 Co-Lead Author Yinding Chi, 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 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 64 °C quickly and easily using a small infrared light source — and they also cool very quickly,” said Co-Lead Author Haitao Qing, 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,” added 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.”

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Qing.

Tech Briefs: What was the biggest technical challenge you faced while developing these miniature soft hydraulic actuators?

Qing: The biggest technical challenge was how to 3D print the microfluid channel in millimeter-scale soft actuators in one single run without using supporting materials. The challenge resides in the facile fabrication of microfluidic channels embedded in thin-layered soft elastomers in terms of both high resolution and complexity in networked channels. The traditional molding method is limited to the fabrication of millimeter-scale channels.

Tech Briefs: Can you explain in simple terms how everything works?

Qing: In this work, we propose fully 3D-printed, thin-plate-like miniature soft hydraulic actuators (MSHAs) with shape memory effect (SME) for delicate and noninvasive manipulation, programable shape morphing, shape locking, and shape recovery. The miniature soft actuators have submillimeter thickness and are composed of a soft elastomer layer with embedded microfluidic channels and a strain-limit stiff shape memory polymer (SMP) layer.

The MSHAs are directly printed in a single print run via the PolyJet printing method using commercial multi-material 3D printers (e.g., Stratasys Objet 260 Connex 3 3D Printer). The microfluidic channel is directly printed by replacing solid support materials with non-curing fluid in high resolution. This eliminates the time-consuming and challenging step of dissolving support materials inside the confined spaces of microfluidic channels in conventional 3D printing.

Tech Briefs: How did this work come about? What was the catalyst for your project?

Qing: Although fluidic actuation holds great promise, several challenges still limit its widespread application in miniature soft robotics and morphing materials, particularly in terms of fabrication and actuation. As a researcher in soft robotics, my goal has been to address these challenges and enhance actuation performance in this field.

Tech Briefs: What are your next steps? Do you have plans for further research/work/etc.?

Qing: We aim to apply this technology across various fields, including not just engineering but also medicine, food, and animal science. I am committed to continuing my work on addressing the challenges in soft robotics. Our goal is to make soft robots more versatile, practical, and robust for a wide range of applications.

Tech Briefs: Do you have any updates you can share?

Qing: We are currently focused on extending the operational lifespan of the miniature soft actuators reported in this article. We are collaborating with partners from other departments to apply our reported miniature soft actuators across various fields.

Tech Briefs: Do you have any advice for researchers aiming to bring their ideas to fruition (broadly speaking)?

Qing: I want to share two important suggestions here. First, start by clearly defining the problem you want to solve. Break down your long-term vision into smaller, manageable goals to help you stay focused and track your progress. Second, stay informed about the latest research, technologies, and trends in your field. This will not only inspire new ideas but also ensure that your work remains relevant.

---