Retaining the Dose: How Cast Material and Particle Properties Affect Nasal Powder Rebound

The site of nasal deposition is critical when targeting specific areas of the nose and is studied in-vitro using nasal casts. Reliable information demands that particles remain where they deposit; however, dry powder formulations routinely ricochet from the cast surface. This study provides the first systematic data set quantifying that rebound. Lactose and colloidal microcrystalline cellulose (cMCC) powders (dmax ≤ 150 µm) were released from controlled heights or propelled by compressed air to impact aluminium, steel or silicone rubber plates at mean velocities of 0.2, 0.5 and 5 ms-1. High-speed imaging (20 kHz) captured every trajectory, allowing calculation of the percentage of particles that bounce (BP) and normal coefficient of restitution (COR).
On aluminium at 0.5 ms-1, all lactose particles rebounded (BP=100%, COR=0.62). cMCC exhibited a reduced BP (62.5%) and COR of 0.45 (-27% relative to lactose (p < 0.001). On steel, the COR of lactose was reduced (0.53), while silicone rubber suppressed rebound still further (BP=23.6%, COR=0.33). Increasing impact speed to 5 ms-1 diminished COR by an additional 11–12% on both metals and halved the rebound on silicone rubber. Cohesive aggregates (≤ 75 µm) returned a 26% lower COR than free-flowing 150–212 µm particles under otherwise identical conditions.
These results show that particle material, size distribution and nasal cast material must be considered simultaneously when designing test methodologies and delivery devices, because each independently alters the energy dissipation that determines whether a particle is retained on a surface after deposition and may affect the requirement for coating.