Unidirectional surface acoustic waves enhance fluid transport and reaction sensitivity in porous nitrocellulose...

While MHz-level surface acoustic waves (SAW) have shown promise for microfluidic actuation, a persistent challenge is using SAW to achieve high reaction sensitivity in highly acoustic-attenuating porous media, such as in lateral flow immunoassay (LFIA) membranes. Acoustic energy loss in these substrates can limit convective reactant transport and reaction rate, hindering visual signal intensity where it is relevant to the detection of molecules. Here, we explore the potential to enhance membrane-based reaction sensitivity by employing a unidirectional SAW, generated via a floating electrode unidirectional transducer (FEUDT), to enable precise acoustic streaming and improved directional control of reactant transport and chemical reactions in nitrocellulose LFIA membranes. Using pH-sensitive methylene blue and sodium hydroxide as a model colorimetric chemical reaction, we show that the unidirectional SAW can markedly improve reactant transport, mixing, and reaction visibility in the membrane through acoustic streaming. Particularly at low analyte concentrations, this approach offers mechanistic insight into pore-scale advection. Systematic variation of SAW intensity, reactant concentration, drop volume, and membrane thickness indicates tunable flow enhancement with minimal heating. The integration of FEUDT-based unidirectional SAW with porous LFIA platforms can provide improved directional control over chemical reactions in LFIA membranes, giving a versatile platform for sensitive chemical reactions and potentially enabling rapid, sensitive, and portable point-of-care diagnostic devices for biomarker detection.