**1. Introduction**

Additive manufacturing (AM) (3D printing), the method of creating geometrically complex 3D parts in a layer-by-layer manner, enables the fabrication of complex parts with high topological freedoms within a single manufacturing step. For metals, macroscale parts are usually fabricated using the powder-based fusion (PBF) method, which involves a high-energy external heat source such as a laser or electron beam to selectively melt metallic powders and fuse them together according to a computationally programmed pattern [1,2]. For microscale and nanoscale structures, electrochemical additive manufacturing (ECAM), in which metal ions are deposited as metal atoms at given locations by an electrochemical process, has been suggested as a promising approach to achieve excellent mechanical and electrical properties with extremely high resolution [3,4]. However, there is a gap in terms of cost-effective AM approaches to print mesoscale structures in the sub-millimeter range.

The resolution of the PBF method is limited by the powder diameter, which is in the range of 20–100 μm [5]. Additionally, high residual stress induced by thermal deformation [6,7] is pronounced in PBF, which considerably impairs the printing precision. For ECAM, two approaches are mainly utilized:


As in MGED, the printing process is operated in air, and the printed structures will not be contaminated or destroyed by submersion [11]. Seol et al. [4] demonstrated geometrically complex microarchitectures can be printed using the MGED approach by modulating the applied potential in two steps with different amplitudes and durations. By utilizing a pulsed potential, Behroozfar et al. [3] successfully printed nanotwinned-metals, which exhibit superior mechanical and electrical properties. Kim et al. [13,14] developed an AFM-based nanofountain probe with the form of volcano tip, which is capable of delivering a meniscus at the sub-100 nm scale. This design is then successfully applied for multiple applications [15–18]. Besides, Falola et al. [19] introduced a wide range of elements that can be electrodeposited from aqueous electrolytes. Reiser et al. [20] printed bi-metal (Cu-Ag) pillars by controlling the electrohydrodynamic ejection of metal ions dissolved from the Cu or Ag anode. Chen et al. [21] developed a two-syringe system, one for copper and one for nickel, to fabricate thermally responsive Cu-Ni strips. Ambrosi et al. [22] demonstrated how the materials (metal copper and polyaniline) to be deposited within a single syringe can be precisely controlled by tuning the applied potential.

Although these methods demonstrate they are valid alternatives to conventional ink-based or laser-based AM technologies, the products are mainly limited to micro- or nanoscale applications. Furthermore, expensive piezo-driven positioning stages are usually required to maintain a stable meniscus [22]. Therefore, a more economically efficient metal printing technology to fabricate large-scale products is highly needed. The key problem is how to maintain a stable mesoscale meniscus, which cannot be obtained merely by the surface tension of the liquid. Wang et al. [23] modified a syringe extruder to print 2D patterns of Cu2O. A controlled volume of liquid electrolyte was ejected to form a meniscus between the print head and substrate. Kim et al. [24] introduced a suction nozzle at the print head to maintain a stable meniscus. Chen et al. [25] proposed an ECAM system based on low-cost stepper motor stages instead of expensive piezo-based stages. A nozzle with a diameter of 400 μm was employed to increase the size of the meniscus considerably and thus improve the deposition rates. In order to maintain a stable meniscus with such a large diameter, they proposed inserting a porous sponge at the tip of the nozzle to adjust the electrolyte flow rate. They further improved their approach by using porous electrospun nanofiber mats at the tip instead of a sponge to print Cu-Ni strips [21]. However, in their approach, the flow rate of the electrolyte solution was intrinsically controlled by the permeability of the porous material, which may vary during the printing process and hence cause certain difficulties in maintaining a stable meniscus. Moreover, precise control over insertion of the porous material at the desired nozzle position was also difficult. Nevertheless, they demonstrated that mesoscale meniscus-confined ECAM is possible.

In the present paper, we propose a novel ECAM system design based on the MGED approach for printing mesoscale metal objects. A microfluidic system referred to as the fountain pen feed system

was employed to maintain a stable mesoscale meniscus. With the proposed design, passive control of continuous electrolyte flow at the tip of the print head can be achieved without liquid leaking or breaking during the printing process, and thus the stability of the meniscus can be automatically adjusted by the fountain pen feed system. As there is no need for active control, the proposed fountain pen feed system is convenient to establish. Moreover, the proposed fountain pen feed system opens the door to controlling the profile of the meniscus with various designs of the microfluidic system. A fluid flow simulation performed on the commercial software Fluent was utilized to help to explain the mechanism of the fountain pen feed system. Two materials, copper and nickel, with various geometric shapes were attempted to print to demonstrate the feasibility of the proposed ECAM system. A printing process on a non-conductive substrate was also conducted to provide a possible application with the proposed design. The article concludes with a reiteration of the most salient points of the study.
