AbstractMotile cilia move in an asymmetric pattern and implement a metachronal wave (MCW) to facilitate fluid movement in a viscous environment. Studies have been conducted to mimic MCW movement of motile cilia, but the fabrication process was too complicating or there were difficulties in accurately mimicking the shape of the cilia. To overcome these limitations, we introduce a self-assembly method to fabricate a reprogrammable magnetically actuated self-assembled (RMS) cilia array that can be reprogrammed by changing the magnetization direction through additional magnetization. Using the RMS cilia array, a unilateral cilia array (UCA) channel and a bilateral cilia array (BCA) channel were constructed, and the motion and fluid flow of the RMS cilia array were analyzed by applying different magnetic fields (strike magnetic field and rotating magnetic field). When a rotating magnetic field was applied to the UCA channel, a distinct MCW appeared. In the BCA channel test, fluid pumping was observed when a strike magnetic field, whereas fluid mixing was observed when a rotating magnetic field was applied. Based on these results, it is expected that the proposed RMS cilia array and magnetic field actuation method can be applied to lab-on-a-chip or microfluidic channels for fluid mixing and pumping.1. IntroductionCilia are hair-like, microtubule-based structures that have various distributions with a length of approximately 3–200 µm and an aspect ratio ranging from 10 to 100, depending on the location where they are found, and are divided into primary cilia and motile cilia.[1–5] Motile cilia can move objects or mix fluids by moving mucus or body fluids in the human body.[6,7] Among these motile cilia, the cilia that are found in the fallopian tube of the female reproductive system help the movement of the ovary, and the cilia existing in the lungs mix settled dust and bacteria through mucociliary clearance and move them out of the body.[8–10] The environment in which motile cilia move is normally filled with fluid with a low Reynolds number. In such an environment, the viscous force is generally more dominant than the inertial force, and has a significant influence on the fluid flow.[11–13] To be helpful in this environment, the cilium moves in an asymmetrical pattern comprising an effective stroke and a recovery stroke, creating a net fluid flow. In an effective stroke, the cilium moves in an arc that is fully stretched, while in the recovery stroke, the cilium returns to the starting point in a bent state as if swinging, which increases the moving area of the cilium.[14–16] In addition, when several cilia gather to form a cilia array, they move in a sequential pattern that forms a wave called the metachronal wave (MCW), which helps move the fluid faster and more efficiently because of their asymmetrical motion.[17–20]Several studies have reported mimicking the asymmetric motion of cilia and the MCW motion to efficiently pump or mix fluids in microfluidic devices with low Reynolds numbers.[21–23] To mimic cilia motion, many actuation methods have been used; actuation via a magnetic field is the most used method among them.[24–27] In addition, diverse manufacturing methods exist for magnetically actuated artificial cilia, and, the fabrication method using self-assembly has the advantages of simplicity and capability to mimic the appearance of natural cilia. However, it is difficult to program the magnetization direction, thus limiting the implementation of the MCW of the cilia.[28–31]Nevertheless, studies have been conducted to imitate the metachronal wave of cilia by fabricating artificial cilia using the molding method, which facilitates reprogramming.[32,33] Nelson formed a cilia array using a molding method and then reprogrammed the cilia array, which moved the cilia array to form an MCW.[34] Sitti fabricated micro-cilia using a mold, magnetized each cilium independently, and attached them to form a cilia array with the desired arrangement that implements the MCW.[35] However, these studies actuated the cilia array using only a rotating external magnetic field, and because the cilia array was fabricated using the molding method, several complex steps were required for making the cilia array.