High-Resolution Patterning and Efficient Fabricating of Liquid Metal Microelectrodes Using PNIPAM Sacrificial Layer

Microelectrodes play a crucial role in microfluidic chips. However, electrodes with micron-sized geometries lead to undesired impedance increases and processing difficulties. This study introduces a method for preparing low-resistance and low-cost liquid metal microelectrodes ($\mu \mathrm{LMEs}$), which leverages the distinct phase transition properties of liquid metal (LM) gallium (Ga) and Poly-N-Isopropylacrylamide (PNIPAM), along with the reversible bonding between PNIPAM and polydimethylsiloxane (PDMS). PNIPAM is spin-coated as a sacrificial layer on silanized glass and heated to dehydration. As it hydrates and swells in the water bath, Ga/PDMS can be easily peeled off, forming a precision surface-embedded $\mu \mathrm{LME}$. The resistance of the $\mu \mathrm{LME}$ with a thickness of 25 $\mu m$ was only 9.3% and 0.077% of the 100nm thin film Au and indium tin oxide (ITO) film microelectrode with the same plane size. Hydration and swelling of the sacrificial layer ensured the fabrication with high resolutions down to 5 $\mu m$ and an acute angle of 15°. The electroosmotic flow tests show that the $\mu \mathrm{LME}$ effectively reduces the operating voltage compared to conventional planar Au or ITO microelectrodes. These features make it a promising candidate for electrification requirements in microfluidic devices.