Functional Chemical Motor Coatings for Modular Powering of Self-Propelled Particles

Amphibious Soft Robots Based on Programmable Actuators Fabricated by Brushing Chinese Ink on Paper

Soft robots based on actuators that can work in both on-ground and on-water situations are environmentally adaptable and can accomplish tasks in complex environments. However, most current amphibious actuators need external stimuli to move on water and require complex preparation processes. Herein, amphibious Ink-paper/polyethylene programmable actuators and robots are proposed, which are fabricated by rapidly brushing Chinese ink on paper. The actuator can bend on the ground and move autonomously on the water. On one hand, the actuator shows a maximum bending curvature of 2.66 cm-1 under near-infrared light, and the actuation performance can be programmed by ink concentration. Moreover, actuators with pen-brushed information can be shape-programmed for dynamic information display. On the other hand, the actuator can autonomously move on the water by using Chinese ink as Marangoni fuel. The maximum moving velocity is 4.73 cm s-1. When the ink is saturated in the water, the actuator can further be driven by an infrared laser. Finally, three soft robots with diverse programmable amphibious motions are designed. Both the crawling/bending motion on the ground and autonomous linear/rotary movement on the water can be programmed by altering actuator structures. This research will provide new inspirations for next-generation amphibious actuators and soft robots.

High-Performance MXene Hydrogel for Self-Propelled Marangoni Swimmers and Water-Enabled Electricity Generator

Developing multifunctional materials that integrate self-propulsion and self-power generation is a significant challenge. This study introduces a high-performance MXene-chitosan composite hydrogel (CM) that successfully combines these functionalities. Utilizing Schiff base bond and hydrogen bond interactions, the CM hydrogel, composed of chitosan, vanillin, and MXene, achieves exceptional self-propulsion on water driven by Marangoni forces. The hydrogel demonstrates rapid movement, extended operation, and controllable trajectories. Notably, the CM hydrogel also exhibits superior degradability, recyclability, and repeatability. Furthermore, the nano-confined channels within the hydrogel play a crucial role in enhancing its water-enabled electricity generation (WEG) performance. By efficiently adsorbing water molecules and selectively transporting cations through these channels, the hydrogel can generate electricity from water molecules and cations more efficiently. As a result, the CM-WEG achieves a stable open-circuit voltage of up to 0.83 V and a short-circuit current of 0.107 mA on seawater, with further improvements in K2CO3-containing water, reaching 1.26 V and 0.922 mA. Leveraging its unique combination of self-propulsion and WEG functionalities, the CM hydrogel is successfully used for cargo delivery while simultaneously powering electronic devices. This research represents a significant step toward the development of self-powered, autonomous soft robotics, opening new research directions in the field.

A Miniaturized, Fuel-Free, Self-Propelled, Bio-Inspired Soft Actuator for Copper Ion Removal

We present a novel miniaturized, gear-shaped, fuel-free actuator capable of autonomously propelling itself in an aquatic environment to absorb heavy metals, such as copper ions. While hydrogel-based absorbents are promising solutions for cationic pollutant remediation, their stationary nature limits their effectiveness in areas where contaminants are unevenly distributed. To address this, we developed a bio-inspired soft actuator that mimics natural propulsion mechanisms. The Marangoni effect, driven by its inherent chemical properties, demonstrated a self-propelled motion without requiring external fuel. The proof-of-concept actuator generated a plane motion lasting up to 2 h and swept over an area approximately 400 times bigger than its size. By harnessing the chemical and optical properties of the hydrogel, we efficiently removed and quantitatively analyzed copper ions through a colorimetric method. This innovative integration of self-propelled movement and efficient copper ion absorption underscores its potential for advancing miniaturized devices in environmental remediation, paving the way for more active and efficient pollutant removal systems in challenging aquatic environments.

Using Footpad Sculpturing to Enhance the Maneuverability and Speed of a Robotic Marangoni Surfer

From insects to arachnids to bacteria, the surfaces of lakes and ponds are teaming with life. Many modes of locomotion are employed by these organisms to navigate along the air-water interface, including the use of lipid-laden excretions that can locally change the surface tension of the water and induce a Marangoni flow. In this paper, we improved the speed and maneuverability of a miniature remote-controlled robot that mimics insect locomotion using an onboard tank of isopropyl alcohol and a series of servomotors to control both the rate and location of alcohol release to both propel and steer the robot across the water. Here, we studied the effect of a series of design changes to the foam rubber footpads, which float the robot and are integral in efficiently converting the alcohol-induced surface tension gradients into propulsive forces and effective maneuvering. Two designs were studied: a two-footpad design and a single-footpad design. In the case of two footpads, the gap between the two footpads was varied to investigate its impact on straight-line speed, propulsion efficiency, and maneuverability. An optimal design was found with a small but finite gap between the two pads of 7.5 mm. In the second design, a single footpad without a central gap was studied. This footpad had a rectangular cut-out in the rear to capture the alcohol. Footpads with wider and shallower cut-outs were found to optimize efficiency. This observation was reinforced by the predictions of a simple theoretical mechanical model. Overall, the optimized single-footpad robot outperformed the two-footpad robot, producing a 30% improvement in speed and a 400% improvement in maneuverability.

A Multi-engine Marangoni Rotor with Controlled Motion for Mini-Generator Application

Marangoni rotors are smart devices that are capable of self-propulsive motions based on the Marangoni effect, namely interfacial flows caused by a gradient of surface tension. Owing to the features of untethered motions and coupled complexity with fluid, these Marangoni devices are attractive for both theoretical study and applications in biomimicking, cargo delivery, energy conversion, etc. However, the controllability of Marangoni motions dependent on concentration gradients remains to be improved, including the motion lifetime, direction, and trajectories. The challenge lies in the flexible loading and adjustments of surfactant fuels. Herein, we design a multi-engine device in a six-arm shape with multiple fuel positions allowing for motion control and propose a strategy of diluting the surfactant fuel to prolong the motion lifetime. The resulting motion lifetime has been extended from 140 to 360 s by 143% compared with conventional surfactant fuels. The motion trajectories could be facilely adjusted by changing both the fuel number and positions, leading to diverse rotation patterns. By integrating with a coil and a magnet, we obtained a system of mini-generators based on the Marangoni rotor. Compared with the single-engine case, the output of the multi-engine rotor was increased by 2 magnitudes owing to increased kinetic energy. The design of the above Marangoni rotor has addressed the problems of concentration-gradient-driven Marangoni devices and enriched their applications in harvesting energy from the environment.