Balancing stability and cellular interaction in surface-engineered small extracellular vesicles for pulmonary delivery

Small extracellular vesicles (sEVs) are emerging as promising nanocarriers for non-invasive pulmonary drug delivery, yet their effectiveness is limited by sequential barriers imposed by airway mucus and the pulmonary epithelium-endothelium interface. Here, we investigated how post-secretory surface engineering with exogenous phospholipids modulates sEVs transport across lung-relevant barriers. eGFP-CD81 engineered sEVs were post-secretory modified at their surface composition using different lipid-to-vesicle ratios of either the zwitterionic lipid 1,2- Dilauroyl-sn-glycero-3-phosphocholine (DLPC) or the PEGylated lipid 1,2-Distearoyl-sn-glycero-3-phosphoethanolamine (DSPE) -PEG(5000) - Azide (DPA). Both modifications enabled controlled tuning of surface charge and colloidal stability without altering vesicle size at optimized lipid-to-vesicle ratios, although excessive DPA induced aggregation. The physicochemical properties and transport behaviour of engineered sEVs were evaluated in a reconstituted mucin gel and a 3D pulmonary epithelial-endothelial co-culture model. In mucin-containing medium, lipid-engineered sEVs showed enhanced diffusion compared with unmodified vesicles, with DLPC at a 30,000:1 ratio providing the highest cumulative permeability, sustained apparent permeability coefficients, and more diffusive motion profiles, as confirmed by multiple particle tracking. In the 3D co-culture, the same modification achieved the greatest cumulative permeability and basolateral accumulation, indicating efficient transcellular passage, while DPA-functionalized vesicles displayed moderate permeability and predominant retention at the apical epithelial layer. Through fluorescently-labelling EVs, we confirmed that all formulations were internalized by apical cells, but only DLPC-engineered sEVs reached detectable levels in basolateral cells. Neither DLPC nor DPA affected cell viability or barrier morphology. Overall, our results identify DLPC at 30,000:1 as a lead formulation that balances mucus penetration and efficient crossing of the pulmonary barrier, while DPA is better suited for applications requiring strong epithelial engagement. Our findings highlight rational phospholipid engineering as a powerful approach to tailor sEVs-based nanomedicines for pulmonary delivery.