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=
.
2nm for
profile 9
, it is much larger than
for the profile extracted in Fig.
4.5
. This is due to the smaller
Regarding the fit result of
d
625
φ
=
.
0
268, which
only gives a path length
L
m for the X-rays to travel through the film.
Regarding
profile 10
, which is depicted in Fig.
4.7
c, we obtain a dominant contri-
bution of the central maximum. Compared to this the higher order oscillations only
have little intensity. To match this strong decay the fit algorithm increases the Full
Width at Half Maximum (FWHM) of the Lorentzian up to
=
24
.
559
µ
σ
=
1
.
614, while it is
only
578 in
profile 9
. Consequently, the intensity is strongly decreased, which
can only be compensated by an increase in the thickness. A value of
d
σ
=
0
.
2nm
is unexpectedly high for a profile at a position so close to the bilayer part of the
film. However, when
profile 10
is fitted with the
=
217
.
value of
profile 9
, we obtain a
reasonable thickness of 78.4nm. Except for the central maximum, the higher order
fringes are fitted much more accurately with this constraint. The schematic repre-
sentation in Fig.
4.8
a gives a possible explanation for the intensity observed after
the membrane has thinned (see Fig.
4.8
b). We consider it as a “focusing” effect
of the Plateau-Gibbs border. The impinging X-rays are reflected at the PGB-water
interface, to be guided to the contact region of the two monolayers where they are
concentrated. The resulting image at the detector can be considered as a superim-
position of the diffraction pattern and the central intensity peak. The dominance
of the latter is the reason why the thinned state of the lipid bilayer, which will
show intensity variations of only 0.1%, cannot be observed in any of the diffraction
patterns.
σ
4.4 Summary and Outlook
Direct imaging of lipid bilayers in both their native and reconstituted environments
is a significant challenge with wide ranging implications. The work described in
this chapter demonstrates a first step towards imaging lipid bilayers reconstituted
in microfluidic channels. The wide applicability of the techniques of microfluidics
in a variety of interdisciplinary studies makes such an investigation vital. Our data
shows that it is possible to image an unthinned lipid membrane with resolutions
down to
200nm in one dimension. It should be pointed out that the reconstituted
bilayers did not undergo any beam damage during the experiments. This is remark-
able in spite of the high photon flux in the focused beam experiments, with a flux
density of
∼
10
19
photons
mm
2
s
. Further, the Plateau-Gibbs border limits the the resolu-
tion of a native bilayer membrane using the current simple theoretical model of
the lipid bilayer in such a setting. However, a modification to the model to include
the possible focussing effect of the PGB might lead to a more detailed extraction
of the relevant structural information from the images. An interesting possibility
in the future would be to incorporate giant unilamellar vesicles (GUVs), as a sol-
vent free membrane system within microfluidic channels for high resolution studies
of the interactions and fusion of lipid bilayers. The control and precision offered
∼
