Direct Laser Writing and Wet Metallization of Bioinspired Artificial Bacterial Flagella

Nature has always been a major source of inspiration for technological development. In the last few years, micro- and nanoswimmers inspired by the shape and motion of microorganisms (like bacteria or protozoan) have been proposed. <i>Eschirichia coli</i>, a bacterium that moves at low Reynolds numbers by rotating its flagella can be taken as model for this class of microdevices. Taking inspiration from this microorganism, the first so-called artificial bacterial flagellas (ABFs) were fabricated by Nelson and co-workers in 2007 [1]. These devices resembled the helical shape of microorganisms’ flagella and were magnetic in order to allow actuation via the application of an external magnetic field. Following this pionieristic work, the production of mobile miniaturized small-scale devices and their control at low Reynolds numbers has attracted an increasing interest due to their great potential in a broad range of applications. ABFs are highly attractive since their control in high viscosity environments is highly precise when applying a low strength rotating magnetic field. This property is fundamental in the biomedical field, where less invasive surgery, cell manipulation and drug delivery would become possible thanks to their use [2].<br/><br/>Given the small dimensions of the devices, adequate production methodologies are required. One of the most interesting techniques for ABFs production is direct laser writing (DLW) [3], which allows the direct printing of devices at the microscale in a fast and reproducible way. However, DLW works with polymers and ABFs require the presence of a magnetic material to allow actuation by an external magnetic field. A possible technique to apply a magnetic layer on the surface of the devices is sputtering [4]. This technology, however, presents some intrinsic limitations: the well-known shadowing effect, its relatively high cost and the limited hard magnetic properties of the obtainable alloys. In an attempt to overcome the limitations of the sputtering approach, we investigated the possibility to apply functional coatings on DLW printed ABFs via wet metallization. This technique is less costly than vapor deposition approaches and able to yield uniform and conformal metallic layers. Furthermore, employing wet metallization, interesting hard magnetic properties can be obtained [5]. Another major advantage of the use of wet metallization is the possibility to deposit composites, which combine two or more functionalities in the same layer [6]. On the other hand, the application of metallic layers via electroless metallization on micrometric and delicate objects like ABFs presents significant challenges and it must be carefully investigated and optimized.<br/><br/>In this context, we initially fabricated micrometric ABFs templates using a DLW Nanoscribe setup. Subsequently, we deposited for the first time a semi-hard magnetic alloy (CoNiP) on such templates via room temperature metallization. Nanometric conformal layers of CoNiP were successfully deposited on the devices, whose morphology and swimming behavior were characterized. Finally, we also demonstrated the possibility to introduce multilayers on the surface of the ABFs by depositing a silver layer via Tollens reaction. By depositing a nanometric layer of Ag, we were able to provide a bacteria killing functionality to the devices.<br/><br/>[1] D. J. Bell et al., Proceedings 2007 IEEE international conference on robotics and automation, 1128 (2007)<br/>[2] X. Z. Chen et al., Adv. Mater. 30(15), 1705061 (2018)<br/>[3] J. Li et al., Chem. Soc. Rev. 50(4), 2794 (2021)<br/>[4] S. Kim et al., Sci. Rep. 6(1), 30713 (2016)<br/>[5] R. Bernasconi et al., Mater. Horiz., 5(4), 699 (2018)<br/>[6] R. Bernasconi et al., Addit. Manuf. 28, 127 (2019)