Single-layer microelectrodes fabrication
Extended Data Fig. 2 illustrates the fabrication process of single-layer microelectrodes. Below are the step-by-step details:
Step 1. Preparation of the polyimide substrate (Extended Data Fig. 2a). This work uses PI2611 (HD MicroSystems) as the backbone material. PI2611 is poured onto a blank glass substrate and then spin-coated at 1,500 rpm for 30 s. The glass substrate used to hold the printed hydrogel microactuators must have a thickness of less than 300 µm, as this is the working range of the TPP laser. For this purpose, we use a 180-µm-thick glass substrate for hydrogel printing. No specific thickness is required for the glass substrate used for moulded hydrogel fabrication.
Step 2. Curing the PI2611 polyimide substrate (Extended Data Fig. 2b). We place the spin-coated substrate on a hotplate and heat it from room temperature to 150 °C. We hold this temperature for 10 min and then increase it to 200 °C. The temperature ramp rate is 20 °C per minute. This temperature is maintained for 5 h to fully cure the polyimide.
Step 3. Photoresist coating (Extended Data Fig. 2c). We pour the positive photoresist AZ ECI 3012 (MicroChemicals GmbH) onto the polyimide substrate and spin-coat the photoresist for 30 s at 5,000 rpm.
Step 4. Soft baking (Extended Data Fig. 2d). We bake the positive photoresist at 90 °C for 90 s.
Step 5. Ultraviolet exposure (Extended Data Fig. 2e). We expose the substrate for 8 s using the MJB4 mask aligner (SUSS MicroTec). The ultraviolet density of this machine is 14.3 mJ cm−2 and the patterns are defined using a photomask.
Step 6. Post-exposure bake (Extended Data Fig. 2f). We bake the exposed substrate at 110 °C for 90 s.
Step 7. Development (Extended Data Fig. 2g). We develop the substrate in AZ 726 (MicroChemicals GmbH) developer for 60 s to reveal the patterns.
Step 8. Platinum sputtering (Extended Data Fig. 2h). We deposit a 150-nm-thick platinum (Pt) layer onto the substrate using sputtering.
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