Intra-tracheal photoacoustic endoscopy for in vivo tracking of inhaled nano-adjuvants

Nano-adjuvants as key vaccine components can more efficiently activate systemic immunity via inhalation. However, assessing the dynamic process of nano-adjuvant translocation across tracheal layers in vivo using conventional methods remains challenging. In this study, an intra-tracheal photoacoustic endoscopy (IT-PAE) system equipped with a photoacoustic coaxial catheter was developed to characterize the in vivo transport of nano-adjuvants. The system exhibits high resolution (with an axial resolution of 55 µm and a transverse resolution of 11.3 µm) and a large imaging field (∼16 mm). A well-known nano-adjuvant aluminum hydroxide oxide (AlOOH) was labeled with indocyanine green (ICG) by using bovine serum albumin (BSA) as a stabilizer. The resulting nano-adjuvant [AlOOH@ICG@BSA (AIB)] demonstrated strong optical absorption and excellent biocompatibility, enabling aerosolized delivery into the rabbit trachea. Ex vivo experiments confirmed that IT-PAE could visualize AIB diffusion within tracheal phantoms and tissue samples and allow quantitative evaluation of transport speed. In vivo IT-PAE results revealed that AIB could traverse all tracheal layers within ∼100 min, with cartilage tissue significantly influencing its translocation velocity.