Biomedical Engineering Reference
In-Depth Information
lipid bilayer patches. 21 Use of the QCM-D permitted to confirm
the formation of lipid bilayers on silica and of lipid monolayers on
gold-supported alkanethiol monolayers by vesicle fusion, and the
adsorption of intact vesicles on oxidized gold. 22
Atomic force microscopy (AFM) has provided further direct
evidence that silica can indeed be covered by isolated vesicles that
remain stable for days. 23 Conversely, when adsorbing vesicles at
low surface density on mica, they initially remain intact but rup-
ture individually over a time interval ranging from minutes to
hours. 16 The particularly smooth surface of mica imparts to vesi-
cles and bilayer patches a certain lateral mobility, not to be con-
fused with that of single lipid molecules. Thus, laterally mobile
bilayer patches on mica tend to reshape into circular patches to
minimize their line tension. Conversely, bilayer patches on silica
frequently retain a strong irregular shape, providing evidence for
lack of mobility. A satisfactory lateral mobility accelerates the
formation of a complete lipid bilayer coating. In fact, the edges of
lipid bilayer patches are thermodynamically unstable, and tend to
interact with adjacent lipid material, e.g., by rupturing surface-
bound vesicles or by coalescing with other bilayer patches. 24
Initial adsorption of vesicles on hydrophobic surfaces is ener-
getically disfavored, due to the presence of the hydrophilic polar
heads on the outer surface of the vesicular membrane. Therefore,
the vesicular membrane must split to allow its inner hydrophobic
tails to get in contact with the hydrophobic surface. A possible
pathway for vesicle fusion involves vesicle splitting, unrolling and
spreading on the hydrophobic surface, as shown in Fig. 8a . The
kinetics of vesicle fusion on the hydrophobic surface of gold-
supported alkanethiol self-assembled monolayers was followed by
SPR. 25 In the initial stage, the adsorbed layer thickness d increases
linearly with the square root of time t , denoting control by vesicle
diffusion to the surface according to Fick's first law. In a second
stage, d increases roughly linearly with log t . Finally, the time de-
pendence of d becomes typical of an adsorption process on an al-
most fully occupied surface. The curve of the surface coverage by
vesicles against time was also monitored by SPR at different vesi-
cle concentrations; 12 it was fitted to an equation practically identi-
cal with that derived for an electrode process controlled by diffu-
sion and by a heterogeneous electron transfer step 26 . The resulting
kinetic constant was ascribed to some surface reorganization of the
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