The influence of tiered layers of surface-grafted poly(ethylene glycol) onreceptor-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 adhesi on between a prototypical phospholipid-stabilized ultrasound contrast agent , the surface of which has been derivatized with a ligand molecule, and a g lass bead surface coated with the corresponding receptor. In particular, th e role of surface "architecture" (the size and density of surface-grafted m olecules) in mediating adhesion is examined, The ligand surface density on bubble shells is varied by changing the mole ratios of shell components [li pid: poly(ethylene glycol) (PEG)ylated surfactant stabilizer:ligand-lipid] during preparation. Two receptor-ligand systems are tested: avidin-biotin a nd antifluorescein-fluorescein. The central investigative method is a novel application of the micromanipulation technique in which an individual micr obubble and a glass bead are captured by separate pipets in an aqueous envi ronment and brought into adhesive contact with each other. Aspiration press ure applied by the bead pipet is incrementally increased until the level of force required to detach the bead from the bubble is exerted. The microman ipulation technique offers the advantage that a single adhesion event can b e observed under controlled conditions, and the force required to effect bu bble detachment is determined from pressure and system geometry. The streng th of adhesion is examined as a function of composition and structure of th e 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 s pacer longer than that of the PEG stabilizer, then adhesion succeeds; if th e ligand is not on an extended spacer, adhesion fails. Adhesion strength in creases and plateaus with increasing ligand-lipid concentration. Such pheno mena must be considered when engineering a targetable stabilized contrast a gent.