Nature provides abundant inspiration for biomimetic research, particularly in soft robotics. Mimicking biological locomotion strategies offers exciting possibilities for creating soft robots capable of navigating challenging environments. While various stimuli (electric fields, chemicals, heat, light, pressure) can control the morphology changes in soft active materials for locomotion, magnetic fields stand out due to their ability to penetrate diverse materials harmlessly and provide remote actuation. Numerous magnetically driven soft robots have been developed, inspired by various creatures (snakes, inchworms, birds, jellyfish, etc.). However, existing methods for manufacturing magnetically driven soft materials, including splicing pieces with different magnetic properties and additive manufacturing (4D printing), often have limitations. Advanced magnetic stimuli like rotational magnetic fields and robot-controlled electromagnets have been employed, but most existing magnetically driven soft actuators only achieve changes in shape, angles, or lengths, without significant internal volume changes. This limitation restricts their application as soft drivers, for example, in micropumps. Furthermore, mimicking the deformation of tissues like the heart and muscle, requiring surface expansion or compression, often leads to large elastic resistance, limiting deformation range or demanding strong magnetic fields. This study addresses these limitations by proposing a novel magnetic-driven folded diaphragm.

The literature review extensively covers existing bio-inspired soft robots and their actuation methods. The authors highlight the advantages of magnetic actuation for remote control and penetration capabilities. They review various bio-inspired designs, including snake-like, inchworm-like, bird-like, jellyfish-like, and other configurations. The existing limitations of magnetic soft actuators, including restricted deformation types (shape, angle, length changes) and the difficulty in achieving large internal volume changes are discussed. Existing manufacturing methods, like splicing and 4D printing, are mentioned along with the use of advanced magnetic stimuli like rotational fields and robot-controlled electromagnets. The lack of high-strength, large internal volume changes in existing designs is identified as a key limitation for applications like micro-pumps. The challenge of overcoming the large elastic resistance associated with surface expansion/compression in biomimetic actuators is also noted.

The researchers developed a novel magnetic-driven folded diaphragm fabricated using a one-piece mold and a simple manufacturing process. The diaphragm consists of segments made from a composite of neodymium–iron–boron (NdFeB) magnetic microparticles and silicone elastomer (Ecoflex 00-20). The composite material is poured into a mold and cured, then magnetized to achieve different radial magnetization directions in each segment. The magnetization process involved fixing the diaphragm's outer edge and propping each surface at an angle to the magnetization direction during exposure to a 3 or -3 T pulse magnetic field. The folded diaphragm's deformation properties were tested and compared with flat diaphragms using an electromagnetic system to generate magnetic fields (0-40 mT), a digital camera for deformation measurement, and image recognition technology. Elastic-resistance force tests were conducted using a TA ElectroForce test system with a load cell and displacement sensor. Diaphragm pump prototypes (single and double diaphragm) were constructed, integrating the folded diaphragm, magnetic sheets, PLA channels, and a pump body. The pump's performance (pressure and flow rate) was evaluated using a LabVIEW program, data acquisition board, linear power amplifier, and electromagnet, measuring pressure with manometers and flow rate with flow sensors. Biomimetic robots (crawling and swimming) were designed and tested using the folded diaphragm as the actuator. The crawling robot, inspired by earthworms, involved multiple segments with integrated magnetic sheets acting as bristles. Its locomotion performance (speed, maneuverability) was assessed in various environments. The swimming robot, mimicking a squid, used the diaphragm for jet propulsion. Its swimming capabilities (snorkeling, diving, horizontal swimming) were evaluated.

The folded diaphragm demonstrated superior deformation characteristics compared to flat diaphragms under the same magnetic field. The folded structure significantly reduced elastic-resistance force, enabling larger deformations (e.g., 146% larger deformation for a circular folded diaphragm compared to a flat one under 40 mT). The diaphragm achieved large 3D and bidirectional deformations due to the varying radial magnetization in each segment. The one-piece molding process simplified fabrication and allowed for customization of diaphragm shape. The diaphragm pump exhibited powerful output and rapid response with lightweight properties. The single diaphragm pump generated specific pressures of -167.3 ± 6.7 kPa kg⁻¹ under a 40 mT field, and the double diaphragm pump achieved flow rates of 178.1 ± 3.1 mL min⁻¹ under a ±10 mT, 10 Hz field. These values are superior to those reported for other magnetic micro-pumps. The biomimetic crawling robot demonstrated flexible locomotion, including climbing, turning, and traversing triangular slopes, exceeding the capabilities of other magnetic crawling robots. The robot crawled through a 150 mm channel at an average speed of 30 mm/s. The biomimetic swimming robot effectively mimicked squid-like movement, exhibiting snorkeling, diving, and horizontal swimming capabilities with a speed of 24 mm/s under a ±40 mT, 2 Hz field.

The results demonstrate the effectiveness of the proposed folded diaphragm as a powerful and versatile actuator for soft robots and micropumps. The ability to achieve large, bidirectional deformations with internal volume changes under a low magnetic field is a significant advancement compared to previous designs. The simple manufacturing method and customizable design make this approach attractive for various applications. The high performance of the diaphragm pump and the flexible, fast locomotion of the biomimetic robots showcase the potential of this technology. The successful integration of the diaphragm into different robotic systems highlights its adaptability and versatility. The use of a single, homogeneous magnetic field for actuation simplifies control and reduces complexity compared to systems requiring gradient or rotational fields.

This study successfully introduced a bio-inspired magnetic-driven folded diaphragm with excellent deformation properties under a low homogeneous magnetic field. Its application in diaphragm pumps and biomimetic robots demonstrates its potential for various applications in soft robotics and microfluidics. Future research could explore the use of different materials and magnetization patterns to further enhance performance. Investigating the scalability and integration of this technology into more complex soft robotic systems would also be beneficial.

The study primarily focused on specific robot designs and applications. Further research is needed to explore the wider applicability of the folded diaphragm in other robotic systems and tasks. The long-term durability and reliability of the diaphragm under repeated actuation need further investigation. The current experimental setup and testing parameters might not fully represent all possible real-world conditions.