Sunwoo Sohn, Hyoryong Lee, Hyeonwoo Kee, Sukho Park*
Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk
Institute of Science and Technology (DGIST), Daegu 42988, Republic of
Korea
E-mail: shpark12@dgist.ac.kr
Keywords: reprogrammable artificial cilia; metachronal wave;
biomimetics; magnetic actuators; fluidic channel
Motile 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. Introduction
Cilia 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.