Contact-line bending energy controls phospholipid vesicle adhesion

Phospholipid bilayer bending energy is often discarded in the analysis of vesicle adhesion on the basis of a dimensionless parameter w=-Delta UR02/kappa b >> 1 (interaction energy Delta U, spherical radius R0, bending rigidity kappa b), considered a regime of strong adhesion. In this study, we propose a model by which bending energy in a singular proximity of the contact line balances the adhesion energy. This is developed for a regime in which the membrane correlation length xi is small compared with the vesicle radius R0, so a spherical cap with circular footprint presents an effective contact angle theta. Experiments are conducted in which the adhesion of 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) vesicles to hyaluronic acid hydrogel substrates is controlled by tuning the van der Waals attraction with systematic change in the hydrogel concentration. Theoretical interpretation of the data furnishes a dimensionless model parameter alpha approximate to 2-10 for contact angles theta approximate to 20-80 circle, beyond which vesicles collapse into discs. We show that the van der Waals interaction energy varies in the range -Delta U approximate to 0.14-0.68 mu J m-2 in response to varying the hydrogel concentration in a range cha approximate to 2-10%. The analysis provides a foundation for exploring vesicle-hydrogel interactions with electro-steric influences; these are poorly understood but pertinent in a wide variety of biological and technological applications.