**2. Materials and Methods**

### *2.1. Fabrication of Liquid Metal (LM) Microdroplets*

EGaIn (75.5 wt% Ga and 24.5 wt% In, Sigma-Aldrich, St. Louis, MO, USA) was used to prepare the microdroplets. It consists of Ga and In, and its melting point is 15.7 ◦C. EGaIn (500 mg) was placed in a 20 mL vial and was filled with ethanol (94.5%, Daejung, Korea). Hereafter, the vial was sonicated using an ultrasonic cleaner (80 W, 40 KHz) for 30 min. After ultrasonication, a suspension of LM microdroplets (<10 μm) was formed, as shown in Figure 1a.

### *2.2. Preparation of Fully and Incompletely Cured PDMS Substrates*

The fully-cured PDMS substrate was prepared by the following process: A PDMS (Dow Corning, Sylgard 184 A/B) mixture with a monomer and a curing agen<sup>t</sup> (at a ratio of 10:1) was prepared and poured onto a flat Petri dish (SPL, Gyeonggi, Korea). The bubbles arising from the vacuum chamber were removed after 1 h, post which it was cured in a convection oven for another 1 h at 80 ◦C. For an incompletely-cured PDMS substrate, a PDMS mixture with a monomer and a curing agen<sup>t</sup> (at a ratio of 11:1) was used. The mixture was poured on a flat Petri dish, and the height of the incompletely-cured PDMS substrate was 1 mm. It was then cured in an oven for 15 min at 80 ◦C after removing the bubbles.

### *2.3. Fabrication of Thin Conductive LM Film with Microdroplet Suspension*

A suspension of LM microdroplets formed by ultrasonication was dropped on a flat, fully-cured PDMS substrate and dried at room temperature for 24 h to avoid formation of cracks by rapid solvent evaporation. Subsequently, another flat, fully-cured PDMS substrate was placed onto the dried LM film, which was then pressed by a hydraulic press at 15 MPa for 1 s. After removing the pressure, the upper PDMS substrate was peeled o ff from the bottom substrate. Finally, thin conductive LM films were left on both the upper and bottom PDMS substrates.

**Figure 1.** (**a**) Preparation process of EGaIn microdroplets; (**b**) Illustration of the fabrication process of a flexible and transparent photodetector.

### *2.4. Laser-Engraved Conductive Patterns and Circuits*

The thin liquid metal (LM) film was patterned by a fiber laser marker (50 W, Dongil laser technology, Gyeonggi, Korea). The desired circuits and electrodes were fabricated by a subtractive method at a resolution of 20 μm. The scanning speed of the fiber laser marker was 600 mm/s, and the power intensity of the laser was 1% of its maximum power. A pattern with an area of 4 cm<sup>2</sup> could be engraved within 5 s by a fiber laser marker.

### *2.5. Fabrication of a Photodetector Based on Oxidized LM Film*

An incompletely-cured thin PDMS film was placed slightly on the surface of a gallium-based conductive thin film for conformal contact. The gallium oxide (<10 nm) film on the LM was attached to the incompletely-cured PDMS film, and it was exfoliated from the conductive film after the peeling off process. After cutting down a part of the transparent gallium oxide film on the PDMS film, it was then placed onto the laser-patterned EGaIn electrodes. Finally, the photodetector was fabricated with a transparent gallium oxide film and patterned conductive EGaIn electrodes.
