Metachronal waves in magnetic micro-robotic paddles for artificial cilia

Matthew T Bryan,Aleksandra Pac,Elizabeth L Martin,Andrew D Gilbert,Feodor Y Ogrin

doi:10.1038/s43246-021-00120-7

Matthew T Bryan, Aleksandra Pac + Show 3 more

Open Access

https://doi.org/10.1038/s43246-021-00120-7

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Abstract

Biological cilia generate fluid movement within viscosity-dominated environments using beating motions that break time-reversal symmetry. This creates a metachronal wave, which enhances flow efficiency. Artificially mimicking this behaviour could improve microfluidic point-of-care devices, since viscosity-dominated fluid dynamics impede fluid flow and mixing of reagents, limiting potential for multiplexing diagnostic tests. However, current biomimicry schemes require either variation in the hydrodynamic response across a cilia array or a complex magnetic anisotropy configuration to synchronise the actuation sequence with the driving field. Here, we show that simple modifications to the structural design introduce phase differences between individual actuators, leading to the spontaneous formation of metachronal waves. This generates flow speeds of up to 16 μm/s as far as 675 μm above the actuator plane. By introducing metachronal waves through lithographic structuring, large scale manufacture becomes feasible. Additionally, by demonstrating that metachronal waves emerge from non-uniformity in internal structural mechanics, we offer fresh insight into the mechanics of cilia coordination.

Highlights

Biological cilia generate fluid movement within viscosity-dominated environments using beating motions that break time-reversal symmetry
We demonstrate that spatially varying the axle width of paddle-based magnetoelastic micro-robots enables metachronal waves to emerge spontaneously as the paddles rotate between bistable states defined by the field direction
To establish the mechanical properties of the micro-robots, they were attached across a polydimethylsiloxane (PDMS) frame of 5 mm inner diameter and imaged in water under a uniaxial magnetic field

Summary

Introduction

Biological cilia generate fluid movement within viscosity-dominated environments using beating motions that break time-reversal symmetry This creates a metachronal wave, which enhances flow efficiency. We show that simple modifications to the structural design introduce phase differences between individual actuators, leading to the spontaneous formation of metachronal waves This generates flow speeds of up to 16 μm/s as far as 675 μm above the actuator plane. By far the most studied mechanism is magnetic actuation, in which columns typically formed from superparamagnetic particles[15,16] or a polymer/magnetic particle composite[14,17,28,29] are distorted by the presence of a magnetic field While all these schemes mimic cilia in appearance, only a few are capable of producing metachronal waves as they actuate all cilia with the same phase. We demonstrate that spatially varying the axle width of paddle-based magnetoelastic micro-robots enables metachronal waves to emerge spontaneously as the paddles rotate between bistable states defined by the field direction

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