Inkjet-Assisted Electroforming of Magnetically Guidable Devices for Interfacial Microfluidic Manipulation
Authors: Bernasconi Roberto , Carniani Davide , Kim Minsoo , Pané Salvador , Magagnin Luca 
Published: August 2024

In the last few decades, inkjet printing (IJP) evolved into a versatile manufacturing technique able to find application in a great number of industrial fields (e.g. electronics, display production, flexible devices). Thanks to its scalability and efficiency, IJP proved to be one of the most promising technologies to pattern materials on a wide variety of rigid and flexible substrates. Recently, it also found application 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 IJP, 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 IJP in microfabrication, and microrobots production in particular, require the presence of conductive or magnetic layers to allow actuation or sensing. By directly employing IJP, however, it is difficult to obtain highly conductive and mechanically stable layers. From this point of view, the variety and the performances of IJP printed conductive and magnetic materials are in many cases limited. To expand IJP applicability to micromanufacturing, we developed a novel approach to indirectly pattern bulk metallic microstructures by combining IJP printing and electroforming. Initially, a layer of SU-8 photoresist is IJP printed [4] on a conductive substrate to form a negative of the final pattern. The positive pattern is then growth by means of electrodeposition inside this negative SU-8 pattern. SU-8 is finally 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.
In the present work, the technique described is applied to the microfabrication of untethered functional microdevices inspired to the shape and behavior of water-striders (Gerridae) [5]. These insects exploit water surface tension to walk on the surface of shallow lakes and ponds. Taking inspiration from Gerridae, micromachined devices able to move on the air-water interface were developed. A stack of three metallic layers (Cu, NiFe, Cu) was electrodeposited inside a SU-8 pattern. The substrate was subsequently dissolved, releasing thus the electroformed microdevices. These were actuated by applying a controlled magnetic field gradient and they proved able to manipulate droplets of fluids floating at the air-water interface. Moreover, the possibility to carry out microreactions by merging droplets in a controlled way was demonstrated as well.
[1] R. Bernasconi et al., Micromachines 14, 2082 (2023)
[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] R. Bernasconi et al., Polymer 185, 121933 (2019)
[5] R. Bernasconi et al., ACS Appl. Mater. Interfaces 15, 2396-2408 (2023)