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Issue 8, 2017
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Power-free, digital and programmable dispensing of picoliter droplets using a Digit Chip

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Abstract

There is a growing need for power-free methods to manipulate small volumes of liquids and thereby enable use of diagnostic assays in resource-limited settings. Most existing self-powered devices provide analog manipulation of fluids using paper, capillary or pressure-driven pumps. These strategies are well-suited to manipulating larger micro- and milliliter-scale volumes at constant flow rates; however, they fail to enable the manipulation of nanoliter and picoliter volumes required in assays using droplets, capillary sampling (e.g. finger prick), or expensive reagents. Here we report a device, termed the Digit Chip, that provides programmable and power-free digital manipulation of sub-nanoliter volumes. The device consists of a user-friendly button interface and a series of chambers connected by capillary valves that serve as digitization elements. Via a button press, the user dispenses and actuates ultra-small, quantitatively-programmed volumes. The device geometry is optimized using design models and experiments and precisely dispenses volumes as low as 21 pL with 97% accuracy. The volume dispensed can be tuned in 10 discrete steps across one order-of-magnitude with 98% accuracy. As a proof-of-principle that nanoliter-scale reagents can be precisely actuated and combined on-chip, we deploy the device to construct a precise concentration gradient with 10 discrete concentrations. Additionally, we apply this device alongside an inexpensive smartphone-based fluorescence imaging platform to perform a titration of E. coli with ampicillin. We observe the onset of bacterial death at a concentration of 5 μg mL−1, increasing to a maximum at 50 μg mL−1. These results establish the utility of the Digit Chip for diagnostic applications in low-resource environments.

Graphical abstract: Power-free, digital and programmable dispensing of picoliter droplets using a Digit Chip

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Publication details

The article was received on 26 Feb 2017, accepted on 14 Mar 2017 and first published on 15 Mar 2017


Article type: Paper
DOI: 10.1039/C7LC00199A
Citation: Lab Chip, 2017,17, 1505-1514
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    Power-free, digital and programmable dispensing of picoliter droplets using a Digit Chip

    A. Mepham, J. D. Besant, A. W. Weinstein, I. B. Burgess, E. H. Sargent and S. O. Kelley, Lab Chip, 2017, 17, 1505
    DOI: 10.1039/C7LC00199A

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