A bioinspired monolayer gel with efficient omnidirectional moisture-driven actuation for humidity sensing

Area of Science:

• Materials Science
• Polymer Chemistry
• Soft Robotics

Background:

• Moisture-responsive actuators are crucial for energy harvesting and soft robotics.
• Existing designs often require humidity gradients or bilayer structures.
• Achieving motion in monolayer actuators under uniform moisture is a significant challenge.

Purpose of the Study:

• To develop a novel monolayer actuator capable of mechanical bending under uniform moisture exposure.
• To investigate the fabrication and actuation mechanism of this new material.
• To demonstrate its potential applications in biomimetics, soft robotics, and smart devices.

Main Methods:

• Fabrication of a monolayer actuator using branched polyethylenimine (BPEI) and polyethene glycol diacrylate (PEGDA) via in situ copolymerization and casting-evaporation.
• Covalent application of an octadecyl acrylate (ODA) coating on one side of the gel.
• Systematic study of bending response by varying PEGDA content, hydrophobic coating, and temperature.

Main Results:

• The actuator exhibits unidirectional bending under uniform moisture or in water.
• A maximum bending angle of 140° was achieved.
• The actuation mechanism involves surface amine protonation leading to surface shrinkage, not swelling.
• Demonstrated proof-of-concept applications in biomimetics and soft robotics, and as a contactless electrical switch.

Conclusions:

• A novel monolayer moisture-responsive actuator was successfully developed, overcoming previous limitations.
• The unique actuation mechanism based on surface shrinkage offers new design principles for moisture-driven devices.
• The actuator shows significant promise for advanced smart and adaptive systems, including soft robotics and contactless switching.