Light-controlled liquid metal strips bring new flexibility to soft robotics

Dec 05, 2023
Light-controlled liquid metal strips bring new flexibility to soft robotics
(Nanowerk News) Soft robotics has tantalized researchers with its promise of machines that can squeeze into tight spaces, handle fragile objects, and intuitively interact with humans. Yet realizing this potential has remained an elusive goal due to limitations in the materials and methods used to actuate such flexible devices.
Rigid components like motors restrict motion while soft actuators struggle balancing strength, speed, and longevity. Now scientists have developed bioinspired “protoplasmic” liquid metal strips that can be programmed to twist, bend, and flap with precision upon exposure to light. This breakthrough may finally unleash the versatility needed for soft robots to crawl, grasp, and flap their way into real-world applications.
The findings have been published in Advanced Functional Materials ("Tendril-Inspired Programmable Liquid Metal Photothermal Actuators for Soft Robots").

a) Schematic LM/PI film preparation diagram. b) Photograph and schematic diagram of the grapevine structure. c) Force analysis of PTFE tape contraction and bending in usage. d) The LM/PI/PTFE photothermal actuator is prepared by assembling PTFE tape and LM/PI film. e) Relationship between the radius of curvature and assembly angle of LM/PI/PTFE photothermal actuators. (Reprinted with permission from Wiley-VCH Verlag)
Further enhancing the breakthrough, the study highlights the exceptional properties and capabilities of the gallium-based liquid metal components. These materials uniquely balance high electrical and thermal conductivity with the fluidity of a low-viscosity liquid. Their ability to form an atomically thin oxide layer ensures non-toxicity and oxidation resistance, crucial for safe and durable application. This innovative integration into polymer films results in a composite that not only maintains the flexibility of the base material but also adds significant photothermal responsiveness.
Impressively, experimental results underscore the robustness and efficiency of these materials. The liquid metal-infused laminates have been demonstrated to lift objects 48 times their own weight, showcasing their remarkable strength.
Additionally, their endurance is evidenced by their consistent performance over 2000 movement cycles, equating to more than ten hours of continuous operation. This combination of strength and durability marks a significant advancement in the field of soft robotics, promising a new era of highly capable and versatile robotic systems.
The enduring challenge for soft robotics has been finding actuators that match the versatility of natural muscle. Materials like shape memory alloys and polymers regain their molded forms when heated but the temporary shapes lack permanence. Hydrogels and liquid crystal elastomers can create sustained configurations through swelling and molecular ordering but these transitions tend to be one-way. And attempts to integrate rigid components like motors and batteries nullify the key advantage of soft systems. The difficulty programming complex reversible motions without hard elements has stifled progress.
Fortunately, several recent advances have readied soft robotics for a new breakthrough attempt. Gallium-based liquid metals uniquely combine high electrical and thermal conductivity with the flowability of a low-viscosity fluid. Their unusual properties come from the facile formation of an atomically thin gallium oxide skin that bestows non-toxicity, inhibits further oxidation, and provides sufficient surface tension for shape retention.
Researchers recently discovered that this spontaneously forming surfactant allows incorporation of microscale liquid metal droplets into flexible polymer films. The resulting composites preserve the host’s elasticity while gaining photothermal capabilities. In other words, they can be trained to move in response to light.
The latest study demonstrates programming an incredible diversity of sustained, reversible configurations into liquid metal-embedded polymer strips. The team credits bioinspiration for their clever solution. They replicated the coiling tendrils of climbing plants by layering strips with an outer “protoplasmic” tape designed to contract and bend when stimulated. This bilayer structure mimics certain cellular activity that causes the vine’s tendrils to twist toward supports as it grows.
Through careful laminate fabrication and strategic laser heating, the researchers produced films capable of tightly curling into spirals, gently flexing open, or smoothly transitioning between states – sustaining each shape until prompted to transform again.
By printing intricate patterns and tuning laminate parameters like film orientations and component ratios, the team created a remarkable palette of preprogrammable motions. Their liquid metal solution empowers rapid prototyping of soft machines tailored to complex tasks like wriggling, walking, grasping, and swimming.
To demonstrate adaptability, the researchers constructed a claw-tipped arm that sequentially reaches out, closes to pick up an object, raises it, then sets it back down. Such deft manipulation remains impossible for conventional soft robots. These impressive capabilities emerge from the bioinspired combination of a shape-locking supportive strip and a protoplasm-like contracting layer attached together in purposeful arrangements.
This research provides an exciting blueprint for specialized soft robots that are both strong and nimble. The team confirmed their liquid metal laminates can lift objects 48 times their weight and operate reliably after 2000 movement cycles – over ten continuous hours. Such resilience arises from the high-performance polymeric component while the embedded liquid metal particles contribute sufficient heat and robust adhesion. Together these properties may finally break through the robustness limitations plaguing soft robotics.
The authors also formulated a computational model that accurately predicts the laminates’ bending in response to laser inputs. This simulation tool enables designing custom movements via software prototypes and previewing the necessary physical parameters before fabrication.
Through this streamlined approach, the researchers could rapidly conceive and iterate original adaptations like photo-directed water striders, inchworms, and flapping butterfly wings. More complex migrations, manipulations, and gestures can now be imagined and brought to reality.
This study highlights how synergy from understanding nature and advancing technology can drive innovation. The researchers emulated adaptive biological processes perfected through evolution to create dynamic liquid metal-based actuators using modern nanofabrication and computational methods.
The work promises customizable soft robots that can better handle the messiness of the real world. As the materials and fabrication technique provide high degrees of freedom at low complexity, newfound ease prototyping highly integrated responsive devices may follow.
These liquid metal “muscles” could one day power nimble search-and-rescue snakes that squeeze through cracks, gentle crop-picking grippers that prevent bruising, intuitive wearable trainers that guide human motions, and similar applications demanding adaptability. By learning from nature then recreating its ingenuity, this technology helps progress soft robotic solutions toward long-promised potential.

Source: Nanowerk (Note: Content may be edited for style and length)

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