Abstract: Aerosolization, the disintegration of liquids into micrometer-diameter aerosol droplets, is a critical process widely used in diverse fields ranging from respiratory drug delivery to sample delivery into mass spectrometers. Ultimately, the aerosol production and the delivery process should be stable for extended periods: the spatial and temporal position of the droplets should be precisely controlled after ejection. Moreover, the droplet size should be tuneable independently of the liquids being atomized
Currently, these cannot be accomplished with any commercially available device. We have developed PALM: a novel approach to aerosol production and delivery, and showed that droplet diameter can be controlled on-demand. PALM is actuated ultrasonically, similar to widely used mesh nebulizers and recently proposed surface acoustic wave nebulizers. However, it has several unique features which differentiate it from its competitors: The device comprises microfabricated, hydrophilic channels placed on top of a piezoelectric substrate. Upon actuation, a controlled volume of fluid can be drawn into the device through a rapid capillary driven flow. Consequent high frequency vibration of the well defined fluid/air interface results in a mist of micrometer-diameter droplets to be ejected.
The approach has two key advantages: Firstly, the liquid/air interface pins to the deep channel sidewalls, which are only several wavelengths apart considering MHz level actuation frequencies. Notably, the channel bounds the fluid to be atomized and ensures a well-defined vibrating fluid air interface, from which droplets are ejected. Secondly, the inlet facility, effectively a large reservoir at the end of the channel, ensures any liquid volume which is ejected is rapidly replenished through acoustically enhanced capillary action. As a result, throughout the atomization process, the width and depth of the liquid remain constant and the periodicity and amplitude of the capillary waves can be maintained over a narrow and long rectangular region.
We show an unprecedented robust operation enabling aerosol delivery of a wide range of fluids from arbitrary reservoirs. The mean droplet size can be tuned with up to a 100% fine particle fraction. This results in a universal platform and provides unique advantages for sample delivery whether directed to a patient or an analytical instrument.