To develop a droplet preparation device by structural design with advantages of small volume, low cost and high throughput, a group led by Prof. WU Wenming from the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences realized an off-chip vertical step emulsification (VSE) droplet preparation device based on step emulsification method, and it is applied to droplet digital PCR (ddPCR) detection.

The VSE device consisted of a perfluoroalkoxy (PFA) capillary and a 200μL centrifuge tube. It realized the flexible change of different nozzle sizes by changing the inner diameter of different capillary tubes, and the adjustable step height through the general screw mechanism to meet the requirements of preparing droplets with different sizes.

Using the controlled variable method, by changing the sample injection speed, the inner diameter of the capillary and the height of the step one by one, the change rule of the droplet diameter under different variables was measured. The droplet diameter is in the range of between 30μm and 120μm, the coefficient of variation (CV) is less than 5%.

Finally, the droplets prepared by the VSE device were applied to ddPCR reaction. The detection results were consistent with commercial instrument (Rainsure), and the method of preparing droplets using the VSE device was easier to operate, and the throughput of droplets was higher.

In summary, the VSE device greatly reduces the difficulty of fluid control compared with traditional droplet preparation methods, and reduces the difficulty of design and manufacture of a microfluidic system. However, in the integrated application of the ddPCR system, the operation process of transferring droplets cannot be avoided, and the existence of pollution cannot be completely avoided. Therefore, the in-depth research of a simpler and easier operation scheme to the integrated ddPCR system will be further studied.