Inkjet Assisted Electroforming of Bioinspired Untethered Microrobots

Abstract

In the last few decades, inkjet (IJ) printing evolved from a mere graphical and visual tool to a versatile methodology able to find application in a great number of industrial fields (e.g. electronics, display production, flexible electronics). The technology is based on the controlled emission of droplets from a nozzle. Such droplets, which contain the material of interest in the form of a solution or a dispersion, are deposited on a substrate, allowing thus controlled patterning. Thanks to its flexibility and efficiency, IJ proved to be one of the most promising technologies to pattern materials on a wide variety of rigid and flexible substrates. Recently, IJ found application also in the field of microfabrication. It was employed, for example, to manufacture microelectromechanical systems (MEMS) [1] or microelectronic components and sensors [2]. Another possible application of IJ, still not developed in the existing literature, is the production of magnetically guidable microrobots [3]. These can find application in a wide variety of fields, including for example micromanipulation, cell transport and drug delivery applications.Many applications of IJ in microfabrication, and microrobots production in particular, require the presence of conductive or magnetic layers to allow actuation or sensing. By employing IJ, however, it is difficult to obtain highly conductive and mechanically stable layers. From this point of view, the variety and the performances of IJ printed conductive and magnetic materials are in many cases limited. Metals IJ printing, for example, is normally based on the use of nanoparticle suspensions or on reactive IJ deposition [4]. Both methods, however, cannot outperform bulk metallic layers. To expand IJ applicability to micromanufacturing, we developed a novel approach to indirectly pattern bulk metallic microstructures by combining IJ printing and electroforming. The technique requires the presence of a conductive substrate. Initially, a layer of SU-8 photoresist is IJ printed [5] on the substrate to form a negative of the final pattern. Subsequently, the positive pattern is growth by mean of electrodeposition inside the negative SU-8 pattern. SU-8 is then removed, leaving an indirectly printed metal pattern. It is also possible to dissolve the substrate in a suitable aggressive solution, leaving thus freestanding electroformed planar structures. Moreover, this latter approach can be employed to transfer metal patterns on nonconductive substrates [6].We applied the approach described to the microfabrication of untethered functional microdevices inspired to the shape and behavior of water-striders (Gerridae). These insects exploit water surface tension to walk on the surface of shallow lakes and ponds. They are characterized by long legs, useful to distribute their weight on the water. Taking inspiration from Gerridae, we developed micromachined devices able to run on the air-water interface (insert in Figure 1). Initially, their negative pattern was printed on an aluminum substrate. Then, different metallic layers (Cu, NiFe and Cu) were sequentially electrodeposited inside. Copper was deposited to provide mechanical stability, while NiFe was deposited to allow magnetic actuation of the final devices. Subsequently, SU-8 was stripped from the surface and the aluminum substrate was dissolved in a NaOH solution. As a result, freestanding electroformed microdevices were obtained. They were morphologically characterized by mean of SEM and AFM. Finally, they were actuated by applying a controlled magnetic field gradient (Figure 1). The high level of precision achieved during the actuation tests performed suggests possible applications in the field of micromanipulation at the air-water interface.[1] G. K. Lau et al., Micromachines 8, 194 (2017)[2] N. K. Mani et al., “Bioelectrochemical Interface Engineering”, chapter 19, John Wiley & Sons (2020)[3] S. Palagi et al. Nat. Rev. Mater. 3(6), 113 (2018)[4] P. Calvert, “Reactive Inkjet Printing: A Chemical Synthesis Tool”, chapter 10, RSC publishing (2017)[5] R. Bernasconi et al., Polymer 185, 121933 (2019)[6] R. Bernasconi et al., IEEE Sensors 20(23), 14024 (2020)Figure 1. Visual appearance and speed vs. applied magnetic field gradient relationship for Cu/NiFe/Cu electroformed water striders. Figure 1