Miniaturizing Soft Robots
Continuum robots, with their flexible backbones, are well-suited for navigating tight, complex spaces, making them valuable tools in areas like medical procedures and industrial inspection. One particularly promising type is the soft everting, or “vine,” robot, which extends from its tip. This growth-based movement reduces contact with the environment and naturally creates a “follow-the-leader” path. Still, steering through tight, winding spaces remains a challenge.
Existing steering approaches have notable drawbacks. Distributed steering, which uses pneumatic actuators or tendons, tends to deform the entire robot body, making precise control difficult and limiting how small the robot can be made. On the other hand, localized steering methods often rely on rigid, electromechanical components at the tip, which undermines the softness and flexibility that make these robots so appealing in the first place.
This paper addresses those limitations by introducing a fully soft, millimeter-scale everting robot. The key innovation is a functionalized skin embedded with LCE actuators that contract when heated. This design allows the robot to bend sharply at specific points along its length, without using any rigid parts, solving the core challenges of miniaturization and flexibility.
Importantly, the authors also show that the robot’s steering behavior is theoretically scale-independent under ideal isometric scaling, meaning the design can be adapted across different size ranges without losing performance.
Fabrication and Operation
Fabrication began with the robot’s body, which was made from thin thermoplastic polyurethane (TPU) films. These films were layered, heat-pressed, and then rolled into a cylindrical shape. A key innovation in the process was the use of water-soluble tape to form internal pockets within the robot’s skin.
Flexible heaters constructed from the conductive polymer PEDOT:PSS and connected with fine copper wires were placed between the TPU layers. Once the tube was fully formed, the water-soluble tape was dissolved, leaving behind empty pockets where artificial muscles made from LCEs were inserted and glued in place.
The LCE actuators were synthesized using a carefully controlled chemical process that produced a material capable of significant contraction when heated. The heaters were fabricated separately by drop-casting and curing the polymer solution onto glass slides, then cutting them to size for integration.
To understand and predict how the robot would bend, the researchers developed a theoretical model that captured the interaction between the contracting force of the LCE actuators and the internal pressure of the inflated body. This model allowed them to calculate the expected steering angle based on applied pressure and temperature.
For testing, the team built an experimental setup that controlled internal pressure and independently powered each heater. They used photographic tracking to measure the robot’s bending in response to actuation.
In some of the demonstrations - like steering through winding 2D paths or a model of the human aortic arch - the researchers added extra components to better show what the robot could do in realistic scenarios. These included motorized spools to manage how much the robot extended, catheters to help guide it through tight curves, and a small camera to simulate inspection tasks. These additions weren’t essential for the robot’s basic function but helped highlight its potential uses in real-world environments.
Experimental Results
LCEs were selected as the robot’s artificial muscles due to their significant contraction when heated, offering high strain and work density compared to other technologies like shape memory alloys. The robot was steered by selectively heating these LCE strips using flexible heaters, creating localized bending along its body.
The researchers explored two main control inputs. The first involved adjusting the robot’s internal pressure, which changes the stiffness of the body and has a roughly linear effect on the bending angle. The second control input was temperature, which influences the LCE’s contraction force in a nonlinear way. A third, hybrid approach combined both pressure and temperature control for finer steering.
To better understand how the robot’s performance scales with design changes, the team developed a theoretical model. They then ran experiments to characterize steering behavior across various parameters: internal pressure, temperature, LCE thickness, and robot diameters ranging from 3 to 7 millimeters.
One key finding was a trade-off between size and performance. Smaller robots achieved tighter bends but needed higher internal pressure to grow. In other words, while miniaturization increased bending curvature, it also made tip extension more demanding, highlighting a practical constraint in scaling down.
To demonstrate real-world feasibility, the team used a robot equipped with six independently controlled actuators to navigate a complex, tortuous path. They also showcased two practical use cases: one in a medical setting, where the robot steered through a model of the human aortic arch for catheter delivery, and another in industrial inspection, where it navigated a jet engine model using a tip-mounted camera. Both scenarios emphasized the robot’s ability to perform precise, controlled tasks in highly confined environments.
Toward Medical-Ready Designs
This work introduced small, steerable soft robots that grow from their tip and can be precisely controlled using integrated LCE actuators and flexible heaters embedded in the robot’s skin. These robots demonstrated strong potential for tasks in both medical and industrial inspection settings.
While current LCEs activate at around 65 °C, which is too hot for direct use in the human body, the researchers noted that this actuation temperature can be lowered to safer levels (around 35–45 °C) through chemical adjustments to the LCE formulation. This makes future medical applications more feasible.
Looking ahead, the team plans to explore several improvements: incorporating sensors for closed-loop control, adding shape-locking mechanisms to maintain stability in open environments, and pushing for even smaller designs. The functionalized skin approach developed here offers a promising path not only for growing robots but also for other soft robotic systems such as wearables and soft grippers.
Journal Reference
Kim, S., Dong, G., Mangan, A., Agrawal, D., Cai, S., & Morimoto, T. K. (2025). LCE-integrated soft skin for millimeter-scale steerable soft everting robots. Science Advances, 11(42). DOI:10.1126/sciadv.adw8636. https://www.science.org/doi/10.1126/sciadv.adw8636
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