ABSTRACT
III. Polymer Steric Interactions: Polymer Extension Length and Polymer Compressibility ............................................................................................................ 38 A. Mushrooms ............................................................................................................ 39
1. Mushrooms, Low Compression ....................................................................... 41 2. Mushrooms, High Compression ...................................................................... 41 3. Interdigitated Mushrooms, Low Compression ................................................ 42 4. Interdigitated Mushrooms, High Compression ............................................... 43
B. Brushes .................................................................................................................. 43
IV. Phase Behavior of the Polymer-Lipid/Lipid Mixture in Aqueous Medium and Maximum Concentration of Polymer-Lipids in the Bilayer ............................... 44 A. Phase Transition, Determined by the Material Properties of the
Self-Assembling Polymer/Lipid/Lipid System (the Minimum Energy Requirement) ......................................................................................................... 45
V. Discussion .................................................................................................................... 47
References ............................................................................................................................. 48
The behavior of solvated polymers that are adsorbed or grafted at the interface between biofluids and biomaterials is of interest to polymer physicists and bioengineers alike because of the unique repulsive properties that these polymers possess. For example, a strategy based on the conformation of water-soluble polymers grafted to lipid bilayers is leading to a more effective intravenous liposome drug delivery system. 1 7 As described elsewhere in this volume, drugs and conventional liposomes are rapidly cleared from the bloodstream by the reticuloendothelial system and other nonspecific mechanisms.8 However, this clearance of liposomes can be significantly decreased by incorporating polyethylene glycol-linked lipids (molecular weights 2000 and 5000 Da). This leads to a substantial increase in the blood circulation time of these so-called "Stealth" liposomes. * It is believed that the mechanism of stabilization is a physical one: t,l,4 ,5 the polymer creates a steric barrier to enhance the repulsive
For in vivo applications, the polymer-grafted liposomes are formed by spontaneous rehydration of a polymer-lipid/lipid mixture (for instance, distearoyl phosphatidyl-ethanolamine-polyethyleneglycol/distearoyl phosphatidylcholine) in which the concentrations ofpolymer-lipids in the lipid mixture are less than 15 M%.9•10 To characterize these so-formed polymer-grafted bilayers we use several different experimental methods that give information about the physical properties of the polymer-lipid/lipid system (i.e., X-ray diffraction, micropipette manipulations, nuclear magnetic resonance (NMR), calorimetry, electron and optical microscopy.4•9•10 Of these the most powerful so far has been X-ray diffraction (see Chapter 7, Mclntosh et al.). As discussed in Reference 4 and Chapter 7, the X-ray diffraction method gives information about: ( 1) the extension length of the polymer away from the lipid surface, (2) the repulsive pressure with which the grafted polymer opposes mutual surface compression, and (3) structural information about the lipid phase and indication of phase separation if it occurs.
