Miniaturized and untethered McKibben muscles based on photothermal-induced gas-liquid transformation

Pneumatic artificial muscles offer continuous movement and flexibility, but miniaturization is crucial for confined space operation. Existing designs often rely on external pumps and valves, hindering miniaturization and wireless operation. This research proposes a light-driven solution using photothermal-induced gas-liquid phase transformation to overcome these limitations. The goal is to develop a miniaturized, untethered pneumatic actuator with sufficient force and range of motion for practical applications. This approach leverages the advantages of pneumatic actuation (simple, rapid response) while addressing the challenges of miniaturization and wireless control prevalent in existing pneumatic artificial muscle designs. The miniaturization of these actuators is a significant step forward in advancing the capabilities of soft robotics in challenging environments such as search and rescue operations.

Existing artificial muscles utilize various actuation methods including pneumatic, electrical, and magnetic approaches. Responsive materials like liquid crystal polymers, hydrogels, and electroactive polymers are commonly employed. While pneumatic muscles offer advantages in terms of output and speed, their reliance on external pumps and valves limits miniaturization and wireless applications. Previous research has explored thin McKibben muscles, but limitations in energy sources and control valves hinder further size reduction. The use of liquid-gas phase transitions driven by external fields (magnetic, light) offers a promising alternative for wireless control, eliminating the need for external connections.

The researchers developed a novel fabrication method based on mold editing. A prepolymer mixture containing polyurethane acrylate (H810), carbon nanoparticles, and a low boiling point liquid (1,1,1,4,4,4-Hexafluorobutene) was poured into a transparent cylindrical mold and cured under UV light. The carbon nanoparticles blocked UV penetration, leaving a central cylindrical cavity filled with uncured prepolymer fluid. A PTFE fiber membrane was wrapped around the cured elastomer to constrain radial expansion and enhance axial elongation. This technique prevents leakage and improves sealing, allowing for the creation of miniature muscles (2 mm diameter × 5 mm length). The effects of carbon and liquid content on the muscle's performance were investigated. The mechanical properties (stress-strain curves, maximum elongation) were characterized using a universal testing machine. The actuators were tested in various application scenarios (walking device, in-tube crawling, gripper). The characterization included optical microscopy, infrared imaging, and force measurements.

The study successfully miniaturized the artificial muscle to 15.7 mm³ (2 mm diameter × 5 mm length), smaller than existing fiber-reinforced pneumatic actuators. The PTFE fiber membrane effectively constrained radial expansion, enhancing axial elongation. Optimization of carbon and liquid content yielded a maximum axial elongation of up to 60%. The artificial muscle demonstrated efficient expansion, elongation, and programmable bending under light irradiation. Application scenarios demonstrated the feasibility of using the muscle in a miniature walking device, an earthworm-like crawling device for navigating tubes, and a gripper for object manipulation. The walking device moved 3 mm after four cycles of light switching. The earthworm-like device achieved multiple displacements in a tube (14 mm inner diameter), with a total travel distance of 5 cm. The gripper successfully grasped and released a ping-pong ball under light control. The study found that the stability and maximum elongation of the muscle are affected by the adhesion between the PTFE film and the elastomer. The researchers suggest that applying prepolymer solution to the overlapping interface of the PTFE film before curing would enhance adhesion and improve stability. To address the issue of gas leakage, which contributes to a reduction in elongation over cycles, the researchers found that soaking the damaged actuators in the low boiling point liquid could restore the original driving effect.

The findings address the challenges of miniaturizing pneumatic artificial muscles and achieving wireless control. The light-driven approach successfully creates a compact and untethered actuator with significant potential for various applications. The demonstrated functionalities in walking, in-tube crawling, and object grasping highlight the versatility of this design. The miniaturized size of the actuator opens new possibilities for applications where space is limited, such as minimally invasive surgery or micro-robotics. The results significantly advance the field of soft robotics by providing a practical solution for creating small, efficient, and remotely controlled actuators.

This study successfully developed a miniaturized, light-driven artificial muscle based on a photothermal-induced gas-liquid phase transition. The actuator's small size, wireless operation, and demonstrated functionalities in diverse applications showcase its potential for various fields. Future work could focus on further miniaturization through microencapsulation techniques, exploring biocompatible materials for biomedical applications, and improving the actuator's response time and force output.

The study notes that the stability of the muscle is affected by the adhesion between the PTFE film and the elastomer. While soaking the actuators in the low boiling point liquid restored the original driving effect, gas leakage remains a potential issue that needs to be addressed for long-term durability and high-cycle applications. The current design's output force is relatively low; further optimization may be needed for certain applications requiring high force generation. The current fabrication method may not be easily scalable to high volume manufacturing; further development is needed to facilitate mass production of miniaturized actuators.