Patterning multilayer microfluidic electrochemical devices by maskless laminar flow lithography

Abstract

Laminar flow lithography (LFL) is a fabrication technique that leverages the tendency of multiple fluids flowing in a microfluidic channel to remain confined to distinct parallel streams, enabling the substrate to be selectively etched to form patterned lines. The purpose of this investigation was to evaluate LFL as a fabrication technique for patterning multiple layers of parallel metal electrodes to produce microfluidic electrochemical cells. To understand how channel geometry impacts the location and width of the etched structures, finite element modelling (FEM) was used to simulate the profiles of the diffusive gradients that govern the extent and location of etching. LFL was then employed to fabricate two different electrochemical devices requiring parallel electrodes of different materials. First, a three layer Ag–Au–Ni thin film stack was patterned by LFL, yielding a three electrode device with electrically isolated and independently sized Ag, Au, and Ni electrodes within a cyclic-olefin copolymer (COC) microchannel. Second, a functional alkaline direct methanol fuel cell was fabricated by performing two sequential LFL processes to pattern a bi-metallic Ag–Ni film also deposited in a COC microchannel. The electrical characteristics of these Ag–Ni devices were then evaluated. Ag–Ni fuel cells produced a peak open circuit voltage of 185 mV and a peak power density of 12.0 ± 0.4 μW cm⁻² (equivalent to 696 ± 20 μW cm⁻³, in terms of chamber volume) while flowing 10 μL min⁻¹ of dilute methanol anolyte and dilute hydrogen peroxide catholyte solutions. Fabrication methods presented in this investigation are applicable to many aspects of microfluidics and can be adapted for other types of microfluidic devices where in situ etching is required.