Physical conjugation of (tri-) block copolymers to liposomes toward the construction of sterically stabilized vesicle systems

The physical conjugation of (tri-) block copolymer molecules to phospholipi d vesicle bilayers in order to construct sterically stabilized vesicles can be carried out in two different ways: by allowing the copolymer molecules to freely participate in the small unilamellar vesicle (SUV) formation proc ess along with the lipids or by adding the copolymer molecules to pre-forme d small unilamellar liposomes. Structurally and morphologically different c opolymer coated vesicle systems occur. The effect on the mean vesicle diame ter and the vesicle surface characteristics is monitored by dynamic light s cattering and laser Doppler electrophoresis techniques for a wide variety o f block copolymer molecules of the PEO-PPO-PEO type (PEO is poly(ethylene o xide); PPO poly(propylene oxide)). Systematic investigations as a function of copolymer added concentration and molecular structure were undertaken th roughout. The results indicate a dramatic increase in mean vesicle diameter when the polymer molecules are present during vesiculation, while in the c ase of copolymer addition to already formed liposomes the mean vesicle size follows a classic Langmuirian-type adsorption curve as a function of copol ymer concentration. The zeta-potential values obtained decrease in a very s imilar pattern irrespective of the way of addition for the large PF127 (PEO 99-PPO65-PEO99) molecule, illustrating the presence of polymer chains at th e vesicle surface. For the small, more hydrophobic L61 (PEO10-PPO16-PEO10) molecule, the reduced zeta-potential value is maintained only when the copo lymer molecules participate in bilayer formation, indicating absence of int eraction between the polymer and the lipids when added to preformed liposom es, due to the preferred copolymer tendency to aggregate into micelles sepa rate from the lipid bilayer particles (that eventually leads to phase separ ation). According to the molecular models proposed to describe the occurrin g lipid-copolymer interactions, addition of copolymer molecules after lipos omes have been formed leads to their adsorption onto the outer liposome sur face, its effectiveness being dependent on the influence that the hydrophil ic (PEO) and hydrophobic (PPO) blocks exert on the copolymer molecular beha viour. Copolymer-lipid coparticipation toward bilayer formation, at low add ed polymer concentrations, leads to PPO block protection by arranging along with the lipids as integral parts of the vesicle bilayer, hence anchoring the PEO chains that dangle in the aqueous solution onto the vesicles. Simpl e geometrical considerations are also included, reinforcing the theoretical feasibility of the described models. The latter type of physically conjuga ting polymer chains onto vesicle surfaces is proposed as an improved altern ative to the weak adsorption of amphiphilic molecules and the cumbersome ch emical modification of the lipid polar headgroups to confer steric protecti on to liposomal surfaces.