Two types of IDT manufacturing approaches have been used to date: patterning IDTs directly onto piezoelectric substrates exposed to air, commonly lithium niobate wafers; or embedding conductive metal liquids into channels shaped in the form of IDTs within PDMSbased microfluidic devices constructed on a piezoelectric substrate. SAW-producing microfluidic units with patterned IDTs open to the air have already been established for several droplet-based microfluidics applications such as droplet merging , production , injection , splitting and sorting . In contrast, embedded IDTs have so far been implemented for droplet production and micro-mixing within a single aqueous droplet . In the case of a continuous aqueous flow, the embedded IDTs were used for the separation of cells and mixing of two separated liquid flows .