Fabrication, Magnetization, and Characterization of Reprogrammable Magnetically Actuated Self-Assembled Cilia (RMS Cilia)
To fabricate the RMS cilia array, a mixture of NdFeB powder with an average size of 5 μm, EcoFlex 00-30 silicone rubber, and hexane at a 1:3:1 weight ratio was prepared using a planetary centrifugal mixer (ARM-310, THINKY). The NdFeB powder was used after being magnetized at 1.5 T in an impulse magnetizer (IM-30, ASC Scientific) to improve its magnetization value. Then, an Ecoflex layer with a thickness of approximately 0.5 mm was made in a 35 mm ∅ Petri dish, and 1 mL of the prepared mixture was poured on it (Figure. 1B(i)). Subsequently, 3 mL of hexane was added to the Petri dish, and the Petri dish was fixed at a distance of 30 mm using a spacer on a permanent magnet of 50 mm × 50 mm × 25 mm (Figure. 1B(ii), (iii)). After being placed in an oven at 60 °C for 2 h, the cilia array layer was separated from the Petri dish to obtain an RMS cilia array.
Second, to reprogram the magnetization direction of the fabricated RMS cilia array, the surface of the cilia array was treated with O2 plasma (approximately 4 min) to make it hydrophilic, and the specimen was prepared by cutting it into a size of 20 mm × 5 mm (Figure. 1B(iv)). Then, using a 3D printer (DP200, Sindoh) to make a cylinder jig with a diameter of 9.5 mm and a height of 15 mm, the prepared cilia array layer specimen was wrapped on the cylinder jig, fixed with an instant adhesive, and placed in a 20 mm × 20 mm × 20 mm cube. To fix the posture of the cilia during the reprogramming process, the cubes were filled with water and placed in a deep freezer (-80 °C, 20 min) (Figure. 1B(v)). Subsequently, each cilium was magnetized to a magnitude of 1.5 T so that the magnetization angle was sequentially distributed from -120° to 120° (Figure. 1B(vi)). Finally, the frozen water in the cube was melted at room temperature, and the specimen was separated from the cylinder jig to obtain the final RMS cilia array.
Finally, the characteristics of the fabricated RMS cilia array were measured. First, shape imaging and EDS analysis of the fabricated cilia array were conducted using an ultra-high resolution FE-SEM (SU8230, Hitachi). The hysteresis loop of one cilium was measured using a vibration sample magnetometer (7404, Lake Shore Cryotonics), where the remnant flux density indicates the final magnetization value of the magnetized cilium. Finally, an experiment was performed to estimate the elastic modulus of cilia. A sample with the same weight composition (NdFeB:Ecoflex = 1:3) and a similar aspect ratio (10) with a diameter of 2 cm and a length of 20 cm was prepared, and the stress-strain graph of the prepared sample was measured using a universal testing machine (SFM-100kN, United Calibration). The elastic modulus of the cilium was estimated based on the measurements of the prepared sample.