Digital
polymerase chain reaction (PCR) plays important roles in
the detection and quantification of nucleic acid targets, while there
still remain challenges including high cost, complex operation, and
low integration of the instrumental system. Here, in this work, a
novel microfluidic chip based on co-flow step emulsification is proposed
for droplet digital PCR (ddPCR), which can achieve droplet generation,
droplet array self-assembly, PCR amplification, and fluorescence detection
on a single device. With the combination of single-layer lithography
and punching operation, a step microstructure was constructed and
it served as the key element to develop a Laplace pressure gradient
at the Rayleigh–Plateau instability interface so as to achieve
droplet generation. It is demonstrated that the fabrication of step
microstructure is low cost, easy-to-operate, and reliable. In addition,
the single droplet volume can be adjusted flexibly due to the co-flow
design; thus, the ddPCR chip can get an ultrahigh upper limit of quantification
to deal with DNA templates with high concentrations. Furthermore,
the volume fraction of the resulting droplets in this ddPCR chip can
be up to 72% and it results in closely spaced droplet arrays, makes
the best of CCD camera for fluorescence detections, and is beneficial
for the minimization of a ddPCR system. The quantitative capability
of the ddPCR chip was evaluated by measuring template DNA at concentrations
from 20 to 50 000 copies/μL. Owing to the characteristics
of low cost, easy operation, excellent quantitative capability, and
minimization, the proposed ddPCR chip meets the requirements of DNA
molecule quantification and is expected to be applied in the point-of-care
testing field.
As a platform to mix the bioagents (i.e. serum, urine), we take advantage of the alternating current electrothermal (ACET) effect which is quite suitable for rapid pumping/mixing of high conductive biomicrofluids. Here we demonstrate the concept of a high-efficient mixing microfluidic chip as a basic unit to provide rapid mixing for lab-on-a-chip applications. As an active mixer, two streams are introduced into a ring-shape microchamber by a passive flow rate regulator, and then the microfluids in the chamber are actuated by a nonuniform electric field with a phase shift of 180°. It shows perfect mixing performance by arranging four arc-electrodes around the ring-shape microchamber subsequently. Taking the Joule heating and conductivity/permittivity changes into consideration, a temperature dependent fully coupled numerical model is presented. Then, the effects of applied voltages on mixing performance and temperature rise are provided to get an optimized design for ACET mixer. Moreover, the arrangement of the electrode array is analyzed to show the effects of electrode patterns on the swirls and mixing efficiencies. Since all the electrodes here are located along a ring-shape central microchamber, the ring-shape micromixer is quite suitable to function as a compact element modular for integrated microfluidic chips.
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