Fundamentals and biomicrofluidic applications of surface acoustic wave atomization

Surface acoustic wave (SAW) atomization is attractive for generating micron or submicron aerosols in a rapid and controllable fashion. SAW atomizers are small, light weight and low cost, providing advantages in many applications, especially for pulmonary drug delivery and biomicrofluidics. Pulmonary drug delivery administration has very strict requirements on aerosol size distribution to obtain optimal drug efficacy: the aerodynamic diameter of the aerosol generated is required to be in the range of 1–5 μm. Therefore, it is crucial to control the sizes of the aerosols generated by SAW. Whilst it is widely believed that the aerosol size can be controlled by the driving frequency, the experimental results, show a rather weak frequency dependence, especially when the driving frequency is above 10 MHz. Fundamental studies were therefore carried out to determine the underlying mechanism associated with the destabilization of the liquid interface leading towards atomization with the objective of elucidating this apparent contradiction. The investigation supports the notion that the drop sizes appear to be governed by the capillary vibration frequency given by a balance between the acoustic forcing and capillary stress, not the driving frequency as previously claimed. Depending on the parent drop’s aspect ratio, this capillary vibration frequency is either specified by the capillary-viscous resonant frequency of the drop or the capillary-inertial resonant frequency of the thin liquid film. With a fundamental understanding of the underlying mechanisms that govern SAW atomization it is possible to design a pulmonary drug delivery system. Whilst the platform is generic, a model drug, namely, the short-acting b2-agonist salbutamol, has been chosen to determine the feasibility of the device. It is shown that the SAW atomization technique is capable of generating aerosol droplets of salbutamol dissolved in an ethanol-octanol precursor with a mean diameter of 4.55 ± 0.2 μm, within the desired range for aerosols of the drug to be deposited deep in the lung for efficacious dosing. The approach confirmed was to work through the high 70%–80% lung dose efficiencies obtained using a twin-stage impinger in-vitro lung model, significantly larger than the 20%–30% lung dose efficiencies obtained with conventional metered dose inhalers. In addition, it is also found that the SAW is capable of transporting biomolecules and solvents through paper—a concept known as paper microfluidics—which as recently gained considerable attention. Moreover, the SAW atomization can be used to extract these biomolecules and solvents from the paper for subsequent analysis. Studies have been taken to demonstrate the transport and extraction of protein molecules and yeast cells using the SAW with insignificant degradation of either the protein or the cells