The influence of tiered layers of surface-grafted poly(ethylene glycol) on receptor-ligand-mediated adhesion between phospholipid monolayer-stabilized microbubbles and coated class beads

The goal of the current study is to measure the strength of specific adhesion between a prototypical phospholipid-stabilized ultrasound contrast agent, the surface of which has been derivatized with a ligand molecule, and a glass bead surface coated with the corresponding receptor. In particular, the role of surface architecture (the size and density of surface-grafted molecules) in mediating adhesion is examined, The ligand surface density on bubble shells is varied by changing the mole ratios of shell components [lipid: poly(ethylene glycol) (PEG)ylated surfactant stabilizer:ligand-lipid] during preparation. Two receptor-ligand systems are tested: avidin-biotin and antifluorescein-fluorescein. The central investigative method is a novel application of the micromanipulation technique in which an individual microbubble and a glass bead are captured by separate pipets in an aqueous environment and brought into adhesive contact with each other. Aspiration pressure applied by the bead pipet is incrementally increased until the level of force required to detach the bead from the bubble is exerted. The micromanipulation technique offers the advantage that a single adhesion event can be observed under controlled conditions, and the force required to effect bubble detachment is determined from pressure and system geometry. The strength of adhesion is examined as a function of composition and structure of the lipid shell and the receptor-ligand pair. The success of adhesion between surfaces is dependent on the availability of ligand proximal to the steric barrier of surface PEG. If the ligand is attached to the shell via a PEG spacer longer than that of the PEG stabilizer, then adhesion succeeds; if the ligand is not on an extended spacer, adhesion fails. Adhesion strength increases and plateaus with increasing ligand-lipid concentration. Such phenomena must be considered when engineering a targetable stabilized contrast agent.