In nature, biological micro-organisms are sophisticated micromachines which have evolved for billions of years to adapt and survive. Flagellates are organisms with one or more whip-like appendage called flagella, whose morphology and motion determines their swimming speed, efficiency of foraging, and how stealth they are among predators. Cilia are thin-hair like structures with a diameter of hundreds of nanometers covering surfaces of tissues, micro-organisms, and animals (such as starfish larvae and sea sponges). The metachronal beating (non-synchronized) of cilia is an essential collective behavior that allows cilia arrays to transport fluids efficiently. When in contact with fish mucus, nematocysts of myxozoan parasites can automated discharge their capsules, anchor to host tissue, and release capsule content. These are just examples of a small fractions of what inspirations the nature has to offer to build artificial micromachines. They combine motion, sensing, decision-making, and metabolism in the length scale of micrometers. This level of integration is challenging for manmade devices to achieve, especially when it goes down to micrometer length scales with heterogeneous functional materials, complex three-dimensional (3D) geometries, and 3D programmable functionalization. 

I worked on 3D printing biocompatible soft materials at the micrometer scale and wirelessly-controlled microdevices with complex 3D architectures. Novel materials, mechanical design, and microfabrication techniques go hand in hand to enable the realization of bioinspired functional micromachines. I envision the development of this field will lead to breakthroughs in biomedicine, environmental remediation, agricultural production, to as visionary as space exploration and terrain forming.

3D-printed micrometer-scale wireless magnetic cilia with metachronal programmability
Zhang, S.*, Hu, X.*, Li, M.*, et al., Sitti, M., Science Advances 9, eadf9462 (2023). (*equal contribution) Link

Abstract: 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 with biocompatibility and metachronal programmability at the micrometer length scale. Each cilium is fabricated by direct laser printing a silk protein hydrogel beam affixed to a hard magnetic FePt Janus microparticle. The 3D-printed cilia show stable actuation performance, high temperature resistance, and high mechanical endurance. 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. 

Creating three-dimensional magnetic functional microdevices via molding-integrated direct laser writing
Liu, Z.*, Li, M.*, Dong, X., Ren, Z., Hu, W.,Sitti, M., Nature Communications 13, 2016 (2022). (*equal contribution) Link
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Abstract: Two-photon-polymerization (2PP)-based 3D printing is able to achieve sub-micron resolution to build complex 3D architectures. However, the limitation for printable-materials can hinder the development of functional devices. Materials that are non-transparent (e.g. most magnetic particles), reflective (e.g. metals), and/or photothermal are not suitable to be printed directly. To fill this gap, we propose a molding-integrated direct laser writing-based microfabrication approach in this study and showcase its advanced enabling capabilities with various proof-of-concept functional microdevice prototypes. Unique motions and functionalities, such as metachronal coordinated motion, fluid mixing, function reprogramming, geometrical reconfiguring, multiple degrees-of-freedom rotation, and wireless stiffness tuning are exemplary demonstrations of the versatility of this fabrication method. 

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Optomechanically actuated microcilia for locally reconfigurable surfaces
Li, M., Kim, T., Guidetti, G., Wang, Y., Omenetto, F. G., Advanced Materials 32, e2004147 (2020). Link

Abstract: Artificial microcilia structures have shown potential to incorporate actuators in various applications such as microfluidic devices and biomimetic micro-robots. Among the multiple possibilities to achieve cilia actuation, magnetic fields present an opportunity given their quick response and wireless operation, despite the difficulty in achieving localized actuation because of their continuous distribution. In this work, a high-aspect-ratio, elastomeric, magnetically responsive microcilia array is presented that allows for wireless, localized actuation through the combined use of light and magnetic fields. The microcilia array can move in response to an external magnetic field and can be locally actuated by targeted illumination of specific areas. The periodic pattern of the microcilia also diffracts light with varying diffraction efficiency as a function of the applied magnetic field, showing potential for wirelessly controlled adaptive optical elements.