Particle sorting is an important step in many biological and biomedical techniques. One sorting technique that has received a lot of attention is based on acoustophoresis. Manipulating cells with acoustic forces allows for a sorting method based on physical properties in a non-invasive, label-free, and biocompatible manner. To date, acoustic sorting devices have been developed using either an interdigitated transducer (IDT) or a lead zirconate titanate (PZT) as an acoustic source. IDTs generate surface acoustic waves (SAW) that can sort particles with high precision, however only low throughput is possible. PZT, on the other hand, generates bulk acoustic waves (BAW) that allows for a simple device architecture with high throughput. Nevertheless, the current research done on BAW sorting devices also shows the disadvantage of frequency restrictions.

An attractive alternative to PZT in acoustic sorting devices that has yet to be researched, are the capacitive Micromachined Ultrasonic Transducers (CMUT). CMUTs are well-known for their broad bandwidth, batch production and the possibility for complex designs and integration with electronic circuits.
In this work, we present proof-of-concept devices to examine that CMUTs can be used as an acoustic source for particle sorting. Two different acoustic-microfluidic device interfaces were considered, namely the sidewall and the top area of the CMUT. Additionally, the presence of an acoustic pressure within the silicon substrate of the CMUT's die was investigated.

Experiments show that CMUTs generate an acoustic pressure within the silicon substrate of the CMUT's die. This acoustic pressure increased 6.4 times when a phase alignment approach was used compared to when no phase alignment approach was used. Following these findings, acoustic-based sorting devices were assembled to perform particle alignment experiments. It was shown that continuous particle sorting could be demonstrated efficiently for both interfaces by using a pressure field based on standing waves. Additionally, structures integrated into the microfluidic devices, such as a vacuum horn, have the potential to amplify the acoustic signal. These findings indicate that CMUT-based microfluidic devices could provide a promising method for sheathless particle sorting.