Integration of sensing/acoustofluidic functions and modulation of surface acoustic wave fields on printed circuit board

In this study, we utilised prototyping printed circuit boards (PCBs) as a surface acoustic wave (SAW) platform to integrate sensing and acoustofluidic functions and study acoustic wavefield modulation. The PCB is a sandwich structure with a woven glass fibre array encapsulated in an epoxy matrix between top/bottom copper layers. On top of this structure, we fabricated SAW devices by sputtering a piezoelectric thin film (using zinc oxide as an example) followed by patterning metal interdigital electrodes. We demonstrated that the thickness of top copper layer on the PCB, relative to the acoustic wavelength, plays a key role in achieving actuation and acoustofluidic functions. On PCB substrates with copper layers of 35 and 135 μm thick, we achieved multiple SAW sensing functions, with SAW wavelengths of 220 and 100 μm, respectively. However, efficient acoustofluidic actuation (e.g., droplet streaming and transportation) was only achieved on the SAW device with a 135 μm thick copper layer but not on that with the copper layer of 35 μm thick, because of epoxy's damping of acoustic energy. Additionally, we observed an intriguing phenomenon of SAW field modulation, and the composite structure of glass fibre array and the epoxy matrix led to spatially differentiated attenuation of acoustic energy. The weakened attenuation in the glass fibres and enhanced attenuation in the epoxy matrix resulted in striped patterns of SAW fields on the substrate, parallel to the glass fibres. This means that the SAW field geometries can be flexibly modulated by designing structured substrates to meet diverse application needs. The multi-functional integration and modulation of SAW field geometry, combined with mature integrated circuit process, make PCB-based SAW devices a promising platform for developing future lab-on-a-chip systems.