Bio-inspired magnetic-driven folded diaphragm for biomimetic robot

Nature offers numerous examples for biomimetic research, inspiring soft robotic systems capable of complex locomotion, morphology control, and autonomous dynamics. Soft active materials have been actuated by various stimuli (electric, chemical, thermal, light, pressure, magnetic fields), with magnetic fields particularly attractive due to their safe, remote, and penetrative actuation suitable for biomedical and synthetic environments. Despite advances in magnetically responsive soft actuators and robots (e.g., snake-, inchworm-, bird-, jellyfish-, spermatozoid-, eel-, tadpole-, and starfish-inspired systems), many existing magnetically driven actuators primarily enable changes in shape, angle, or length, but struggle to realize large inside-volume changes with high strength. This limitation hinders practical use in applications like micro-pumps and in mimicking tissues (e.g., heart, muscle) where surface expansion/compression induces large elastic resistance, restricting deformation range or necessitating strong magnetic fields. Therefore, this study proposes a one-piece-mold magnetic-driven folded diaphragm with radial magnetization that enables large 3D, bi-directional deformation and substantial inside-volume change under low, homogeneous magnetic fields. It is customizable in appearance and can be integrated as a soft driver into different untethered robotic systems. The work demonstrates its utility via a diaphragm pump with rapid response and soft robots (earthworm-like crawler and squid-inspired swimmer) actuated by a single homogeneous field.

Recent work in soft robotics spans bioinspired designs and diverse stimuli, including electric fields, chemical stimuli, heat, light, pressure, and magnetic fields, to drive morphology changes for locomotion or manipulation. Magnetic actuation is advantageous for remote, safe control through most materials and is a growing hotspot for soft robots. Fabrication strategies for magnetic soft materials include assembling segments with different magnetic properties and additive manufacturing (e.g., 4D printing) to program magnetization. Advanced magnetic stimuli, such as rotational fields and robot-controlled electromagnets, have been used to realize locomotion. However, most magnetic actuators focus on shape, angle, or length modulation and do not provide large inside-volume changes with high strength, limiting applications like pumping. Moreover, mimicking biological tissues that undergo surface extension/compression often encounters high elastic resistance, constraining deformation or requiring powerful magnetic fields, which limits broader application of magnetic soft robots. These gaps motivate the development of actuators that provide large 3D, bi-directional deformations and significant volumetric change at low field strengths with simple, homogeneous excitation.

Design and operational principle: Inspired by annelid (earthworm) locomotion based on segmental contraction and stretching, the authors designed a folded diaphragm comprising multiple segments made from a composite of hard-magnetic particles embedded in silicone rubber. After curing in a shape mold, the diaphragm is placed in a magnetization mold to impart radial magnetization with different directions in adjacent segments (Type+ and Type− configurations), generating magnetic torque-driven segment rotation opposed by elastic torque. Under an axial magnetic field, segments attempt to align with the field, producing 3D, bi-directional folding/unfolding and large cavity volume change. Material fabrication: The diaphragm composite consists of NdFeB microparticles (MQP-15-7, average diameter 5 μm, density 7.61 g/cm³) mixed with silicone elastomer (Ecoflex 00-20) at a 1:1 mass ratio. The mixture is stirred (3 min), poured into the mold, degassed in vacuum (4 min), and cured. Multiple samples (triangle, square, hexagon, circular) were fabricated; for each type, three samples were tested for three cycles (9 total tests) to assess deformation and elastic resistance. Magnetization: To achieve radial magnetization around the diaphragm center, the outer edge is fixed in an upper mold, and surfaces are propped at set angles via the lower mold. The assembly is exposed to a ±3 T pulse magnetic field (vertical) using a magnetizer (SYZ-700), generating the desired radial magnetization pattern. Deformation characterization: Folded diaphragms and flat diaphragms (same magnetization and bottom area) were tested under vertical magnetic fields from 0 to 40 mT (Dexing DXSBX-90 electromagnet). Deformation at the diaphragm tip was recorded by a Canon EOS M6K2 camera and quantified via image recognition. Each sample underwent three cycles; averages and standard deviations were reported. Elastic-resistance force testing: A TA ElectroForce system (ETR2000) with a 22 N load cell and integrated displacement sensor (±7.5 mm, 0.03% FS) measured elastic resistance. Diaphragms mounted in PLA frames were pressed at the center with a non-magnetic 1 mm rounded head from 0 to 10 mm in 1 mm increments, recording force-displacement across three cycles per sample. Diaphragm pump design and testing: A single-diaphragm pump incorporated one Type-I folded diaphragm, two magnetic sheets acting as check valves, PLA channels, and a PLA body; a double-diaphragm pump added a second folded diaphragm on the bottom. Under upward vertical fields, inlet opens and outlet closes while the cavity expands to suck fluid; under downward fields, the cavity compresses to expel fluid. Harmonic magnetic fields were generated via a LabVIEW-controlled setup (NI PCI-6229, AE Techron 7224 amplifier, DXSBX-80 electromagnet). Pressure tests used two KEG674A manometers (±5 kPa, 1% FS) at 1 Hz with amplitudes 5–40 mT. Ideal flow rate tests used two SARGO FS004-CV-A flow sensors (0–500 mL/min, 1% FS) across field amplitudes 5–40 mT and frequencies 1, 2, 5, 10 Hz. Unidirectional transfer was demonstrated with accumulator setups under ~±40 mT (single pump) and ~±20 mT (double pump) harmonic fields. Bionic crawling robot: A single-section robot (length 21.6 mm, diameter 22 mm) used two folded diaphragms (different magnetizations), magnetic sheets (for bristle-like anchoring), and wheels to reduce friction; head and tail segments were connected via a PLA connector. A double-section robot (length 27 mm) added Type+ and Type− diaphragms in series. Locomotion under horizontal harmonic fields was tested for crawling, climbing, turning, and negotiating a triangular slope. Speed dependence on field amplitude and frequency was evaluated, and long-distance crawling in a 150 mm pipe used a solenoid generator at 2 Hz, ±40 mT. Bionic swimming robot: A squid-inspired robot (length 29 mm, diameter 22 mm) integrated one Type+ folded diaphragm driving body cavity expansion/contraction, magnetic check valves (funnel-like), and a jet pipe within a shell. Swimming behaviors (snorkeling, diving, horizontal swimming) were tested under vertical or horizontal harmonic fields. A solenoid generator (65 mm OD, 35 mm ID, 140 mm length, 1500 turns) provided ±40 mT at 2 Hz for channel swimming (150 mm), driven by a power amplifier and monitored by a sensor.

- The folded diaphragm achieves large 3D, bi-directional deformation under low, homogeneous magnetic fields (up to 40 mT), enabled by reduced elastic resistance due to the folded geometry and radial magnetization of segments. - Compared to a flat magnetic diaphragm of equal area and magnetization, the circular folded diaphragm exhibits 146% larger deformation at 40 mT: 5.568 ± 0.118 mm (folded) vs 2.267 ± 0.14 mm (flat). Folded diaphragms consistently showed lower elastic-resistance force across 0–10 mm displacements. - Customizable geometries (triangle, square, hexagon, circular) were demonstrated with reproducible deformation across 9 test cycles per type; maximum reported standard deviations remained modest across cycles. - Diaphragm pumps driven by homogeneous fields achieved strong performance without magnetic field gradients: the double-diaphragm pump delivered 178.1 ± 3.1 mL min⁻¹ at a −10 to 10 mT harmonic field and 10 Hz; specific pressure of the single-diaphragm pump reached −167.3 ± 6.7 kPa kg⁻¹ at 40 mT (weight 3.01 g); specific flow rate reached −619.4 ± 748.1 mL min⁻¹ kg⁻¹ for the double-diaphragm pump (weight 2.94 g) under −15 to 15 mT at 5 Hz. The pumps operate with a single homogeneous magnetic stimulus and integrated magnetic check valves, enabling unidirectional transfer. - The earthworm-inspired crawling robot, actuated by a single homogeneous field, demonstrated crawling, climbing, turning, and traversing a triangular slope. In a 150 mm channel, it traversed in about 5 s under ±40 mT at 2 Hz (≈30 mm s⁻¹ average speed), with speed tunable by field amplitude and frequency and enhanced by stacking body segments. - The squid-inspired swimming robot achieved snorkeling, diving, and horizontal swimming under ±40 mT harmonic fields at 2 Hz, traversing a 150 mm channel in about 6.25 s (≈24 mm s⁻¹).

The work addresses a central challenge in magnetically actuated soft robotics: achieving large, controllable inside-volume changes with high strength under simple, low-intensity, homogeneous magnetic fields. By segmenting a one-piece diaphragm with programmed radial magnetization, the actuator harnesses magnetic torque to produce 3D, bi-directional folding with reduced elastic resistance relative to flat membranes. This design enables substantial cavity volume modulation, directly translating to high-performance pumping using only uniform fields, in contrast to conventional magnetic pumps requiring strong gradients. Integration into untethered biomimetic robots further validates functional versatility: earthworm-like crawling via coordinated extension/anchoring cycles and squid-like jet propulsion via periodic cavity expansion/contraction. The low-field, wireless, and homogenous excitation modality, along with straightforward one-piece molding and customizable geometries, enhances practicality and potential for deployment in constrained or aqueous environments where electrical actuation is undesirable. The observed strong deformation gains, rapid pump response with high specific outputs, and effective locomotion demonstrate that the folded diaphragm effectively bridges gaps identified in prior literature.

This study introduces a one-piece-mold magnetic folded diaphragm with radial magnetization that enables large 3D, bi-directional deformation and significant inside-volume change under low, homogeneous magnetic fields. The actuator is easily customizable and serves as a compact, powerful driver for soft systems. Demonstrations include: (i) lightweight diaphragm pumps with high specific pressure and flow, operating via a single homogeneous field and integrated magnetic check valves; (ii) an earthworm-inspired crawler capable of versatile terrain negotiation and rapid, tunable locomotion; and (iii) a squid-inspired swimmer achieving snorkeling, diving, and horizontal swimming with appreciable speed. These results highlight the actuator’s potential for soft actuation and biomimetic robotic applications where wireless, low-field magnetic control is advantageous. Future work could optimize magnetization patterns and segment geometries for greater efficiency, scale the concept across sizes, integrate sensing/feedback for closed-loop control, and explore biomedical and microfluidic applications.

The demonstrations were conducted under controlled laboratory conditions using specific electromagnet and solenoid setups to generate homogeneous harmonic fields; performance in complex, unstructured environments was not evaluated. The approach requires specialized magnetization tooling and strong pulse fields (±3 T) to program radial magnetization, which may limit ease of fabrication in some settings. Reported performance metrics (e.g., flow rate per unit mass) exhibit variability depending on field amplitude/frequency and device configuration, and some measurements show notable standard deviations across cycles. The study focuses on selected geometries and scales; generalization to significantly different sizes, loads, or long-term durability under repeated cycling was not extensively characterized.