Fig. 1. Spring-like flow sensor. (a) Schematic of femtosecond laser two-photon direct writing fabrication system; (b) 3D spring-like structure with diameter of 16 μm and pitch of 10 μm; (c) physical picture of chip

Fig. 2. Microscopic images of spring-like structures with diameter of 16 μm and pitch of 8 μm but different numbers of turns. (a) 2; (b) 3; (c) 4; (d) 5

Fig. 3. Microscopic images of elongation states of spring-like flow sensor under different flow rates. (a) 0 nL·min-1; (b) 2.02 nL·min-1; (c) 3.36 nL·min-1; (d) 5.38 nL·min-1

Fig. 4. Effect of multiple flow-rate cycles on elongation rate of spring-like flow sensor

Fig. 5. Thickness of spring-like structure versus exposure energy of femtosecond laser

Fig. 6. Flow rates monitored by spring-like flow sensor under different exposure powers. (a) 50 mW; (b) 60 mW; (c) 80 mW; (d) 90 mW