Biomedical Engineering Reference
In-Depth Information
(a)
(b)
HO
HO
OH
HO
HO
OH
HO
O
O
OH
O
O
O
HO
O
O
n
HO
OH
OH
HO
HO
HO
HO
OH
HO
HO
OH
HO
HO
O
O
OH
O
O
O
O
O
n
Microfibril
HO
Sub-elementary
fibril
Ribbon
assembly
OH
OH
HO
HO
FIgurE 18.4
(a) The molecular structure of cellulose where linear polymer chains containing
n
glucose mol-
ecules are attached by hydrogen bonds (thin dashed lines). (b) Cellulose synthesis by
G. xylinus
. Glucan chains are
synthesized into sub-elementary fibrils at specific sites on the bacteria surface. The fundamental fibrils are then
combined into microfibrils and further into fiber ribbon assemblies having diameters of typically 200 nm. (The
illustration in (b) has been adapted from Hirai, A. and F. Horii 1999.
ICR Annual Report
6:28-29.)
using SHG microscopy (Brown et al. 2003) have been followed by a more detailed characterization of the
forward- and backward-scattered components of the SHG signal from cellulose matrices (Nadiarnykh
et al. 2007). Furthermore, the synthesis and structure of cellulose scaffolds intended as blood vessel
replacements have been investigated by SHG microscopy (Brackmann et al. 2010a).
18.3.1 SHG Microscopy on Biosynthesized cellulose
Figure 18.5 shows SHG images measured on cellulose synthesized by
G. xylinus
. The image in Figure
18.5a, measured during the first hour of the synthesis process, is an overlay of SHG and CARS micros-
copy images showing bacteria, visualized by CARS, in red color and an initial network of a few cel-
lulose fibers in blue. Figure 18.5b shows an SHG image of a developed cellulose fiber network obtained
after 7 days of growth, the fiber ribbon assemblies can be clearly distinguished, and a volume repre-
sentation showing the fiber arrangement in three dimensions is shown in Figure 18.5c. Both cellulose
and bacteria were monitored under native conditions without labeling or specific sample preparation
for microscopy. For the initially produced cellulose tissue having low density of fibers (cf. Figure 18.5a),
the generated SHG signal can be monitored using forward detection geometry, whereas the developed
compact material shown in Figures 18.5b and 18.5c requires detection of the backscattered SHG signal
in epi-mode. Nadiarnykh et al. have investigated backward scattered SHG from cellulose in detail and
identified two signal contributions. Primarily SHG-active structures much smaller than the excitation
FIgurE 18.5
(
See color insert.
) (a) Overlay SHG/CARS microscopy image showing fibers (blue) and bacteria
(red) monitored during the first hour of cellulose production by SHG and CARS microscopy, respectively. (b) SHG
microscopy image showing a developed cellulose matrix after 7 days of growth with clearly distinguishable fibers
(c) volume representation generated from a stack of SHG images, showing the fiber matrix in three dimensions
(volume depth 5 μm).
