On-chip biochemistry: Droplet generation and control

Digital microfluidics, the use of small sample droplets in oil filled channels, offers exciting possibilities in biochemistry. With the ability to form nano or picolitre droplets, there is potential for massive miniaturisation of high throughput screening (HTS) processes, where the full functionality of microtitre plate technology is replicated on a single chip for improved efficiency and substantially reduced costs. To implement such a system, the ability to create reagent mixtures on-chip in multiple permutations is required. For this goal, conventional droplet formation approaches are not well suited, where streams of hundereds of single-reagent droplets are created continuously, whereas applications such as HTS require the time-controlled creation of individual droplets. In this work, we examine the use of surface acoustic wave (SAW) excitation as a method to first generate single droplets on-demand, and then control their subsequent behaviour such that a platform for on-chip HTS can be developed. In addition, we examine methods of particle handling suitable for their introduction, in small numbers, into droplets, with the future potential for single cell encapsulation.