Figure 1 . (A) Concept of RMS cilia array; (B) Fabrication process of the RMS cilia array; Self-assembling process; (i) Add mixture of magnetic particles and silicone rubber on base layer, (ii) Add hexane to lower the silicone viscosity, (iii) Put the solution on the magnet to self-assemble the cilia, and (iv) Gain the final RMS cilia array and cut it into the required form; Reprogramming process; (v) Attach the RMS cilia array to a cylinder and freeze to hold place in the reprogramming process, (vi) Magnetize it in an impulse magnetizer to reprogram the magnetization direction of the cilia, and (vii) Channel fabrication process, and (C) SEM images and EDS analysis results of the RMS cilia
Moreover, the aspect ratio of the fabricated cilium is too small (approximately 5), which makes it difficult to mimic natural cilia with an aspect ratio of > 10.
To overcome the limitations of the reported artificial cilia array, we propose a reprogrammable magnetically actuated self-assembled (RMS) cilia array fabricated using a self-assembly method and reprogrammed by changing the magnetization direction of the cilia array as shown in Figure. 1A. Therefore, compared to the molding method used in previous studies, the cilia array can be easily developed and the shape and aspect ratio of natural cilia can be mimicked using the proposed method. In addition, we implemented various motions of the RMS cilia array in fluids using an electromagnetic actuation (EMA) system that can form a magnetic field of the desired waveform.
2. Results
2.1. Fabrication of Reprogrammable Magnetically Actuated Self-Assembled Cilia Array
The RMS cilia array was fabricated using the self-assembly method, where NdFeB powder and Ecoflex 00-30 silicone rubber were mainly used for stretchability and magnetic manipulation (Figure. 1B). The fabricated RMS cilia have a shape similar to the actual cilia and show a length of approximately 3 mm, a bottom diameter of approximately 0.3 mm, and an aspect ratio of approximately 10 (Figure. 1C). EDS analysis was performed to confirm the components constituting the cilia (Figure. 1C). Consequently, it was confirmed that C, O, and Si elements constituting the Ecoflex silicone were detected at a high ratio, and Nd, Fe, and B elements were detected at a low ratio because the NdFeB particles were trapped inside the Ecoflex. To confirm the possibility of magnetic-field actuation, the magnetization value of the cilium was measured, and the saturation magnetization value of the cilium was approximately 68,608 A/m (Figure. S1B ). In addition, the stress-strain graph was measured using a universal material testing machine, and Young’s modulus (E) of the cilium was estimated to be approximately 211 kPa (Figure. S1C).
2.2. Fundamental Motions of RMS Cilia Arrays
Due to the ferromagnetic properties of the NdFeB particles, the RMS cilia array’s magnetization direction can be reprogrammed using a magnetizer. In this experiment, the magnetization direction of the cilia array was selected, as shown in Figure. 2A(i) , from among the two magnetization directions for the RMS cilia array. To mimic the asymmetric motion of natural cilia, two types of magnetic fields (strike magnetic field and rotating magnetic field) were applied. When a strike magnetic field was applied to the cilium, an  effective stroke due to the quick change of the magnetic field direction and a recovery stroke due to the slow return of the magnetic field direction to the original state could be implemented (Figure. 2B(i)). On the other hand, when a rotating magnetic field was applied, the cilium could follow the magnetic field and rotate along a plane to achieve an effective stroke and when the magnetic field could not overcome the elastic force of the cilium anymore, the cilium could snap back and rotate sideways to make a recovery stroke (Figure. 2B(ii)).