3D-Printed Micrometer-Scale Wireless Magnetic Cilia with Metachronal Programmability

Biological cilia play essential roles in self-propulsion, food capture, and cell transportation by performing coordinated metachronal motions. Experimental studies to emulate the biological cilia metachronal coordination are challenging at the micrometer length scale because of current limitations in fabrication methods and materials. We report on the creation of wirelessly actuated magnetic artificial cilia (MAC) with biocompatibility and metachronal programmability at the micrometer length scale. Each cilium is fabricated by 3D nano-printing a proteinous hydrogel beam affixed to a hard magnetic microparticle. Specifically, we used hard magnetic FePt Janus microparticles (FePt JMPs) and silk fibroin to 3D print biocompatible and programmable MAC arrays at a length scale close to biological cilia using two-photon polymerization in combination with magnetic actuation controls.<br/>Magnetic, mechanical, and biological properties of the fabricated cilia were systematically investigated, showing that our cilia were mechanically tunable and robust, biocompatible, and biodegradable. Programmable metachronal coordination can be achieved by programming the orientation of the identically magnetized FePt Janus microparticles, which enables the generation of versatile microfluidic patterns. Upon applying a uniform rotating magnetic field of as small as 3 mT, each cilium performed a whip-like reciprocal motion consisting of a slow forward stroke and a fast backward stroke. The difference in the orientation between neighboring FePt JMPs endowed the MAC array with heterogeneous magnetization directions, initiating a metachronal wave within the MAC array when subjected to a uniform rotating magnetic field.<br/>We showcased the design and fabrication of MAC arrays to generate translational flows and locally circulating flows. This work could provide an experimental platform for biocompatible and programmable microcilia arrays with a large design space for future potential applications in microfluidic devices and cilia-inspired biomedical soft microrobots.