A Pressure-Controlled Optical Platform for Buckling, Gas Dissolution, and Shell Remodeling in Lipid-Coated Microbubbles.

Lipid-coated microbubbles are widely used as ultrasound contrast agents, yet their structural response to prolonged or transient ambient-pressure changes remains less directly characterized than their behavior under acoustic excitation. In this study, a custom pressure-controlled optical platform was developed to visualize the compression, dissolution, buckling, and destabilization of bare air bubbles and air-filled microbubbles prepared from a 5:5 DSPC:PEG40St feed. Bright-field microscopy was used to monitor gas-core size and morphology, while 0.5 mol % FITC-DHPE labeling enabled fluorescence visualization of shell reorganization and residual shell material. Bare air bubbles exhibited size-dependent behavior such that larger bubbles underwent partially reversible compression, whereas smaller bubbles shrank more rapidly and disappeared. Their contraction at elevated pressure exceeded that predicted by isothermal compression of a closed gas bubble, indicating substantial pressure-enhanced gas dissolution. Lipid-coated microbubbles exhibited localized wrinkling, crumpling, and asymmetric buckling at low and moderate overpressures, often followed by recovery of a smoother spherical morphology after pressure release. Repeated pressure cycling nevertheless caused progressive reductions in baseline diameter, demonstrating cumulative gas loss despite apparent morphological recovery. Sustained or higher pressures resulted in extensive gas-core dissolution, irreversible shell folding, and eventual disappearance of optically detectable bubbles. Fluorescent shell-derived structures persisted after gas-core loss as partial rings, filaments, bright aggregates, and surface-associated residues, indicating shell collapse and redistribution rather than immediate loss of coating material. Sequential pressure pulses further demonstrated that destabilization depended on pressure amplitude and loading history. Marked bubble-to-bubble variability under identical conditions was consistent with heterogeneous shell composition and domain organization. These findings demonstrate that quasi-static pressure microscopy provides a complementary, ultrasound-free approach for investigating the transition from reversible shell buckling to irreversible gas dissolution and shell collapse in DSPC:PEG40St microbubbles.