Polyethylene Glycol Architecture Governs Interfacial Thermodynamics and Drug Adsorption in Anionic Lipid Microbubbles

Polyethylene glycol (PEG)-stabilized lipid microbubbles (MBs) are promising ultrasound-responsive drug delivery platforms; however, their optimization remains largely empirical and fails to resolve the interfacial mechanisms governing drug adsorption and stability. Here, we established a thermodynamics-guided framework to determine how PEG grafting density, spanning mushroom to brush conformational regimes, regulates drug adsorption in anionic lipid MBs. Using doxorubicin (DOX) as a model drug, loading was quantified across macroscopic, mesoscopic, and microscopic descriptors, while adsorption energetics were evaluated through apparent Gibbs free energy (ΔGapp), enthalpic (ΔHapp) and entropic (ΔSapp) contributions. Surface-normalized loading (DOXmolecules/μm2) emerged as the most predictive descriptor of intrinsic apparent adsorption energetics, minimizing geometric and population-dependent variability across formulations. Increasing anionic lipid and PEG content progressively shifted adsorption from thermodynamically unfavorable to favorable regimes, accompanied by increasingly negative ΔGapp, interactions–SA values (from 4.6 ± 0.2 to −1.3 ± 0.9 kJ/mol). Mushroom architectures were dominated by hydrophobic interactions (56.0 ± 4.9%), intermediate regimes exhibited the strongest electrostatic contributions (77.1 ± 21.0%), whereas brush architectures promoted DOX–DOX π–π-dominated adsorption (78.9 ± 5.0%). Temperature-dependent studies revealed predominantly ΔHapp,SA-driven adsorption, with brush architectures exhibiting the strongest thermodynamic stabilization (ΔHapp,SA= −27.8 ± 4.0 kJ/mol). Correspondingly, brush MBs achieved up to 6.2 ± 1.5 × 106 DOXmolecules/μm2 without aggregation, representing ∼7.8-fold and ∼3.9-fold higher surface loading than mushroom and intermediate architectures, respectively. Together, these findings establish PEG architecture as a dominant regulator of adsorption energetics and loading behavior, providing predictive and rational design principles for engineering ultrasound-responsive MB drug delivery systems.