Hundred-micron droplet ejection by focused ultrasound for genomic applications

Droplet-based microfluidic (DMF) technology is widely used in genetic analysis. Among multiple droplet generation techniques, acoustic droplet ejection (ADE) is a non-contact, high-precision method that utilizes the energy of focused ultrasound. The ejection of a single droplet of controlled size, the performance of multi-droplet ejections and the effects of the ultrasound energy on nucleic acid molecules in the droplets, are the main research interests in droplet-based genomics. In this work, a focused ultrasonic transducer with a frequency of 20 MHz was used, and we achieved droplet ejection successfully. Various experiments were carried out to study ADE and the beneficial results were obtained, including the characterization of ADE conditions, the relationship between droplet volume and parameters (including power, duration and distance), the energy required for ADE, and the effects of ultrasound energy on DNA strands, etc. It was first noticed that the superior limit of the droplet size could be increased by adjusting the height of the transducer. In addition, multi-droplet ejections at the optimal delay time were realized with good linearity. Above all, we demonstrated that the nucleic acid molecules remained their integrity when the exciting power was the maximum and the exciting duration was the longest in our system. In summary, ADE was proved to have an important role and a vast prospect in DMF technology for genomics-related applications.