Biomedical Engineering Reference
In-Depth Information
20.3.5 Optimization of Deposition Parameters
The deposition parameters of these monolayers depend upon the polarity of
the polymeric substrates as well as the composition of the lipids. To optimize
the deposition parameters, binary PTC/Chol (1:0.7) compositions at different
dipping and withdrawal speeds of the polymer substrate from the air/water inter-
face, were studied with the help of the LB trough. The deposition was done at
30 mN/m in all the studies. In natural membranes, the surface pressure is above
20 mN/m. For an equivalent distribution and packing of the lipids in the model
monolayer systems, the surface pressure is expected to be at 30-35 mN/m. Studies
at varying dipping speed did not show any difference in the transfer ratio with the
speed. It indicates the strong hydrophobic interaction between the monolayer
and the polymer fi lms. However, the second monolayer deposition during
withdrawal of the polymer substrate has been varying with the speed (Figure
20.4 (I)).
At 5 mm and 100 mm/min withdrawal speed, there has been no second mono-
layer deposition. This ensures a single hydrophilic face exposed monolayer on the
polymer substrate. This is schematically represented in Figure 20.4(II) and may
be due to the close packing of the deposited monolayer caused by the condensing
effect of cholesterol.
The compositions of the monolayer were varied at this juncture to determine
the effect of composition on the interaction and packing of the monolayers.
Further deposition of various other PTC/Chol/GalC combinations was done at
constant deposition parameters (5 mm/min dipping as well as withdrawal speed)
at ambient temperature. Figure 20.4(II) shows the composition effect on transfer
ratio of the monolayer to the polymer substrates, from the air/water interface. The
deposition of the PTC monolayer on this hydrophobic polymer surface showed a
tight packed second monolayer deposition during withdrawal (Figure 20.4IIB).
This ensures the exposure of the nonpolar face of the monolayer by burying the
polar group inside. For the combination of (PTC : Chol) (1 : 0.7) only a single
monolayer deposition is seen on the polymer surface, exposing the hydrophilic
phase outside as shown in Figure 20.4IIC. This surface resembles the natural
cell membrane where the polar groups are exposed to the aqueous phase. This is
due to the condensing effect of cholesterol, which imparts close packing of the
deposited monolayer. When the cholesterol concentration is reduced to half
(PTC : Chol) (1 : 0.35) to its original concentration, the interaction between the
two monolayers was increased during deposition. This will give a loosely packed
second monolayer on the surface that is inversely oriented exposing the hydro-
phobic face to the aqueous phase (Figure 20.4IID). From this it is evident that the
concentration of cholesterol plays an important role in the second monolayer
deposition.
The introduction of the Galactocerebroside into the monolayer (PTC : Chol :
GalC) (1 : 0.35 : 0.125) further improved the transfer ratio during dipping as well
as withdrawal to give a more optimized deposition under reduced cholesterol
concentrations (Figure 20.4IIE). Therefore the authors chose this monolayer
Search WWH ::
Custom Search