Chip-Based Optoacoustic Single-Cell Detection in Flow Using Point-Source Optimized Surface Acoustic Wave Transducers

Sensitive measurement of the optoacoustic (OA, also photoacoustic) properties of cells in flow is highly desirable, as it provides information about the optical absorption properties of cellular compounds. Hence, optoacoustic spectral characteristics can deliver information about the cell state or disease parameters, but can also be used for high-throughput cell sorting by intrinsic properties without additional fluorescence labeling. The current implementation of optoacoustic measurements of cells in a microfluidic context typically relies on piezoelectric (ultrasound) transducers attached to the microfluidic chip, whereby the transducer records the ultrasound signal originating from absorbing species in cells when excited by laser pulses. The arrangement of the transducer outside of the microfluidic chip leads to the challenge of signal integration over a larger area and coupling interlayer effects resulting in attenuation and a reduction of sensitivity. Moreover, the placement of the bulky transducer outside of the chip prevents the exploitation of the full advantages of microfluidics. As a solution, we demonstrate the use of point-source optimized interdigital transducers (pIDTs) directly fabricated on the surface of the microfluidic chip for the detection of surface acoustic waves (SAW) from single cells in continuous flow. The SAW is excited by bulk acoustic waves originating from the optoacoustic effect of absorbing species inside the cells illuminated by laser light. The use of these highly focused pIDTs and on-chip lithographically fabricated hard-wall microchannels allows the detection of SAW with a spatial resolution on the order of the cell diameter directly on-chip, offering the possibility of miniaturization, parallelization, and cheap mass production.