Extracellular vesicles (EVs) have demonstrated unique advantages in serving as nanocarriers for drug delivery, yet the cargo encapsulation efficiency is far from expectation, especially for hydrophilic chemotherapeutic drugs. Besides, the intrinsic heterogeneity of EVs renders it difficult to evaluate drug encapsulation behaviour. Inspired by the active drug loading strategy of liposomal nanomedicines, here we report the development of a method, named “Sonication and Extrusion‐assisted Active Loading” (SEAL), for effective and stable drug encapsulation of EVs. Using doxorubicin‐loaded milk‐derived EVs (Dox‐mEVs) as the model system, sonication was applied to temporarily permeabilize the membrane, facilitating the influx of ammonium sulfate solution into the lumen to establish the transmembrane ion gradient essential for active loading. Along with extrusion to downsize large mEVs, homogenize particle size and reshape the nonspherical or multilamellar vesicles, SEAL showed around 10‐fold enhancement of drug encapsulation efficiency compared with passive loading. Single‐particle analysis by nano‐flow cytometry was further employed to reveal the heterogeneous encapsulation behaviour of Dox‐mEVs which would otherwise be overlooked by bulk‐based approaches. Correlation analysis between doxorubicin auto‐fluorescence and the fluorescence of a lipophilic dye DiD suggested that only the lipid‐enclosed particles were actively loadable. Meanwhile, immunofluorescence analysis revealed that more than 85% of the casein positive particles was doxorubicin free. These findings further inspired the development of the lipid‐probe‐ and immuno‐mediated magnetic isolation techniques to selectively remove the contaminants of non‐lipid enclosed particles and casein assemblies, respectively. Finally, the intracellular assessments confirmed the superior performance of SEAL‐prepared mEV formulations, and demonstrated the impact of encapsulation heterogeneity on therapeutic outcome. The as‐developed cargo‐loading approach and nano‐flow cytometry‐based characterization method will provide an instructive insight in the development of EV‐based delivery systems.

Food-derived extracellular vesicles (FDEVs) have attracted increasing attention as potential delivery vehicles for therapeutic agents due to their desirable features such as excellent biocompatibility, easy accessibility and cost effectiveness. However, the intrinsic targeting capability of FDEVs is unsatisfactory compared to artificial nanoparticles or other source-derived EVs, which calls for efficient surface engineering strategies to equip them with specific ligands. Here we report a general and mild modification method via reduction of disulfide groups to maleimide reactive thiols. Taking milk-derived EVs (mEVs) as a model system, we demonstrated the feasibility for tethering various ligands on the surface without compromising the vesicular structures. Building an ultra-sensitive nano-flow cytometer (nFCM), the heterogeneous nature of the functionalized samples was revealed, and a magnetic separation approach was proposed accordingly to remove the as-observed non-EV particles. The cellular uptake and cytotoxicity experiments provided direct evidence showing an enhanced cell targeting and cargo delivery capability of the ligand conjugated mEVs. In addition, the in vivo imaging further proved the applicability of transferrin conjugation for increased tumor enrichment of mEVs. Collectively, this general and mild ligand conjugation method enables an efficient surface functionalization of FDEVs, which is of vital importance for enhanced targeting delivery.

Surface functionalization is an effective approach for enhancing the cancer‐targeting efficiency of extracellular vesicles (EVs). However, the lack of direct comparisons between functionalization strategies has hindered the rational design of EV delivery systems. To address this gap, we developed a nano‐flow cytometry‐based methodology to quantitatively evaluate ligand conjugation and its relationship to targeting efficiency across three representative post‑production EV engineering strategies: lipid modification, protein modification, and membrane insertion. All three strategies achieved high conjugation efficiencies (>90%) with ligand densities ranging from several to tens of ligands per 100 nm ² under optimized conditions. Beyond ligand density, functionalization strategies resulted in varying degrees of ligand clustering and reduced accessibility of endogenous cell‐binding proteins on EVs, such as MFGE8, leading to differences in targeting performance. For milk‐derived EVs, lipid modification achieved the highest ligand density, the most uniform conjugation, and minimal disruption to surface protein accessibility, yielding superior cancer‐targeting efficiency. These findings highlight the importance of precise quantification of ligand conjugation performance and provide a robust methodology for optimizing surface functionalization to advance EV‐based drug delivery.