Membrane-Based Nanotechnology and Drug Delivery - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 132

Fig. 6.2 Polymeric nanoparticle interactions with biological membranes are presented here. In
panel I: AFM observation of DMPC-supported lipid bilayers (a,c,e) before and after incubation
with b PLL, d PEI, and f DEAE-DEX, respectively, are presented. Panel II shows the chemical
structures of a PLL, b PEI, and c DEAE-DEX. Panel III shows the LDH leakage out of a KB and
b Rat2 cells as a result of exposure to the various polymeric nanoparticles at 37 ◦ C for 3h. Polyca-
tionic polymers are observed here to induce the enzyme leakage whereas charge neutral polymers
such as PEG and PVA behave neutrally. This figure with description has been taken with the pub-
lisher's permission from [ 11 ]

The geometry of vesicles can be predicted by considering the free energy asso-
ciated with the molecular interaction coupled with geometric constraints imparted
by the choice of lipid. Although entropy favors structures with small aggregation
numbers, packing constrains of the double-chained lipids energetically resist the for-
mation of arbitrarily small structures below a critical radius RC [ 9 , 10 ]. In these
references, Israelachvili et al. proposed to combine thermodynamic and geometric
principles to derive physical properties of self-assembled lipid vesicles such as their
size distribution. They used the “opposing forces” model to describe the interaction
between the lipid molecules, each occupying a surface area at the hydrocarbon-water
interface of a lipid layer. In this theoretical approach, properties including vesicle
size distributions and bilayer elasticity emerge from a unified theory that links ther-
modynamics, interaction free energy, and molecular geometry. Using an analogous
but slightly modified protocol, Mecke's group [ 17 ] have proposed that the inclusion
of a dendrimer into the vesicle (see Fig. 6.4 ) causes a change in the surface free
energy of lipid molecules per unit area which is given by the following equation

M φ
4 π r 2

γ = (6.6)