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In this paper, we design, fabricate, and test a single-heater microinjector, whose ejected-droplet volume is adjusted by a digital combination of multiple current paths connected to a single microheater. The novel aspect of the present method includes using the single microheater having multiple current paths to achieve multilevel droplet volume adjustment. In the design process, we design four pairs of current I/O interconnection lines connected to the microheater. We numerically estimate the actually heated area whenever we vary the combination of 4-b current path through the single microheater. On the basis of the numerical and theoretical estimation results, we design the droplet-volume-adjustable microinjector having a rectangular (R)- and a circular (C)-shape single microheater. In the experimental study, we measure the sizes of the generated bubbles, as well as the volumes and velocities of the ejected droplets, according to the digital current-path combination. In the bubble generation test, we use the 1-kHz 15.0-V 3-mus pulsewidth electrical signal and DI water at room temperature. The measured input power is varied from 8.7 to 24.9 muW for the R type and from 8.1 to 43.8 muW for the C type as the current path is changed. The projected area of the generated bubble is varied from 440 to 1,360 mum2 for the R type and from 800 to 3300 mum2 for the C type at six levels, respectively. Under the same experimental condition, we measure the ejected-droplet volumes and velocities. It is found that the ejected-droplet volumes are varied from 9.4 plusmn 0.7 to 20.7 plusmn 1.8 pL at three levels for the R type and from 7.4 plusmn 0.8 to 27.4 plusmn 2.0 pL at five levels for the C type, respectively, while the ejected-droplet velocities are varied from 0.8 plusmn 0.01 to 1.7 plusmn 0.01 m/s for the R type and from 0.5 plusmn 0.02 to 2.8 plusmn 0.03 m/s for the C type, respectively.