The Ni layer provides the ABF with its magnetic properties and the Ti layer (autoxidized to titanium dioxide) increases biocompatibility. The surface of the formed ABFs was then coated with a 50 nm Ni film and a 5 nm Ti film. Operation inside Microfluidic Dropletįor the current experiments, ABFs (16 μm in length and 5 μm in diameter) were fabricated by 2PP-based 3D laser lithography. Specifically, we assess the ability of ABFs to perform a range of unit operations on soft objects within microfluidic environments and evaluate the operational challenges associated with their use. To demonstrate the feasibility of using ABFs to manipulate soft materials in the context of wide range of chemical and biological applications, we herein explore the three highly relevant scenarios incorporating soft microdroplets and cells. The ability to produce, manipulate and process soft objects in a high-throughput manner is an important feature of microfluidic systems, and their utility in applications such as digital PCR drug delivery systems, single cell analysis, nanomaterial synthesis, and the generation of artificial tissues is well documented. Over the past two decades, microfluidic systems (commonly termed Lab-on-a-Chip devices) have become increasingly popular platforms in which to perform a wide range of chemical and biological assays and process a diversity “soft” biological entities (e.g., microdroplets, gel microparticles, and cells). Accordingly, their potential utility in many real biological systems remains an unresolved issue. Nevertheless, to date, ABFs (with dimensions below 100 µm) have only been used to manipulate “hard” objects under idealized experimental environments, such as deionized water. In principle, ABFs can be used for manipulating microscopic objects to realize specific chemical and biological operations in both in vitro and in vivo environments. Although, the fabrication of ABFs has been described in detail elsewhere, it should be noted that adoption of two-photon polymerization (2PP) techniques for micro-/nanostructure fabrication has allowed the rapid realization of three-dimensional micro-/nanoscale structures in a variety of materials. Unsurprisingly, the development of control methods for ABF manipulations and the characterization of their swimming properties has been an active area of research, with recent efforts being focused on the addition of supplementary functional units (such as “micro-holders”, “micro-bars”, and “micro-rings”) for the manipulation and transport of microscale objects, developing magnetic composites exhibiting superior biocompatibility and the functionalization of ABF surfaces to furnish them with novel properties. Such an approach is especially advantageous since the swimmer can be powered remotely (without the need for an on-board fuel source) and manoeuvred in controllable and dynamic fashion ( Figure 1a). Put simply, ABFs move in fluid environments by translating rotational motion to translational motion under the application of low-strength rotating magnetic fields. In contrast to chemically-propelled microswimmers and artificial cilia constructed from electroactive polymers, magnetically actuated helical microswimmers rely on the application of magnetic fields for motion. Accordingly, such microswimmers are often termed artificial bacterial flagella (ABFs), and have been used to address the challenging issue of swimming within the low Reynolds number regimes typical on the microscale. The basic structure of a helical microswimmer is inspired by flagellated bacteria, where thin, whip like appendages protruding from the cell body are used to move the bacteria towards nutrients and other chemo-attractants. Indeed, their implementation and controlled actuation within in vivo systems has recently been reported for deep tissue analysis. Helical microswimmers represent a category of microrobotic tools recognized as highly promising solutions for biomedical applications including minimally invasive surgery and targeted drug delivery.
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