Transitioning from impinging jets to single-jet impaction: investigation of an alternative nozzle design for the Respimat® Soft Mist Inhaler.

With a metered volume of 15 µL per actuation, the Respimat® Soft Mist Inhaler is well-suited for use in inhalation therapies with potent and soluble drugs. To expand its therapeutic range, an alternative nozzle concept was explored that replaces the twin impinging jets with a single jet impacting a baffle perpendicularly. Maximizing the impaction angle enables a more efficient droplet generation. This may allow for a higher liquid volume delivered per actuation at lower pressure. As a result, less potent or less soluble drugs-potentially including biologics-might be administered. Additionally, the enlarged nozzle orifice is expected to be less prone to clogging than narrow twin microchannels. This reduced clogging risk may allow the use of suspension formulations. An initial design of experiments identified key parameters influencing aerosol performance, mainly droplet size and residue of non-atomized liquid. While inhalable droplets (∼5 µm) were generated, high residual values indicated inefficient atomization. Computational fluid dynamics simulations revealed that oversized baffle diameters compromise atomization efficiency by decelerating the radially spreading liquid film on the baffle surface, leading to droplet accumulation. In silico droplet formation was observed via: (i) early detachment from the advancing liquid front, and (ii) disintegration of the liquid sheet at the baffle edge. A follow‑up DoE using smaller baffle diameters substantially reduced residue while maintaining inhalable droplet sizes. This alternative nozzle concept therefore presents a promising approach to broaden the therapeutic scope of the Respimat Soft Mist Inhaler by increasing the delivered volume per actuation.