*2.2. Device Fabrication*

As illustrated in Figure 1a, the electrochemical devices were fabricated via standard microfabrication techniques [33]. Initially, a liquid crystal polymer (LCP) sheet (ULTRALAM 3850, 100 μm) with copper cladding (18 μm) purchased from Rogers Corporation, USA, was cut into a 4-inch wafer size and used as a substrate for the devices. The cladding layer was removed by dipping the sheet in the copper etchant for 45 min, during which agitation was provided in order to facilitate the etching process. The sheet was then ultrasonically cleaned with acetone and deionized water to remove both organic and inorganic impurities. After drying with nitrogen gas, the sheet was thermally attached to a 4-inch silicon wafer using a photoresist (AZ 9260, 10 μm) as an adhesion layer. Thereafter, another layer of the photoresist (5 μm) was spin-coated on top of the LCP sheet and baked on a hotplate at 110 ◦C for 4 min. Upon the exposure of the photoresist under ultraviolet (UV, 365 nm, i-line) light followed by developing in the photoresist developer solution (AZ 400K), the patterns of the electrodes, connection lines, and contact pads were formed. After photolithography, a layer of chromium/gold (Cr/Au, 50/300 nm) was deposited using a magnetron sputtering system, in which the Cr layer was used to improve adhesion between the Au layer and the substrate. The remaining photoresist together with the deposited material was completely removed through the lift-off process by dipping the wafer in acetone solution for 12 h. This Au layer served as both the working and counter electrodes for the electrochemical devices. By repeating the photolithography-deposition procedure with a different photomask, the reference electrodes of the electrochemical devices were constructed by a combined layer of silver/silver chloride (Ag/AgCl, 150/250 nm).

**Figure 1.** (**a**) Schematic drawing to illustrate the structure of the HAMA hydrogel-modified electrochemical device. (**b**) Photograph of the HAMA hydrogel-modified electrochemical device after packaging and modification, in which the cap on two sides of the polycarbonate mold is removed in order to make the swollen hydrogel visible. A transparent HAMA hydrogel layer is successfully formed on top of the electrochemical device inside the mold, which can be seen in the region marked by the dashed red rectangle.
