Biomedical Engineering Reference
In-Depth Information
wavelength generate coherently emitted SHG in forward as well as backward direction. Generally, the
combined contributions from individual dipole sources result in destructive interference and sup-
pression of the coherent back-scattered SHG whereas constructive interference is obtained for SHG-
emission in the forward direction. However, for sources much smaller than the excitation wavelength,
the backscattered SHG is not completely cancelled and a direct backward-emitted signal can still be
maintained. An additional incoherent backward-scattered SHG signal contribution is obtained in
dense media, such as the developed cellulose fiber matrices, due to multiple scattering of the forward-
directed SHG (Nadiarnykh et al. 2007). Similar observations have also been made for SHG generated
in collagen (Williams et al. 2005).
Figure 18.6 shows results from SHG microscopy measurements carried out to further characterize
the SHG process in biosynthesized cellulose. A comparison between excitation at 800 and 1064 nm is
shown in Figure 18.6a and 18.6b, and it can be seen that similar signal strengths are obtained. Cellulose
is reported to have electronic resonances in the ultraviolet wavelength regime and the index of refrac-
tion for the material has been shown to be similar in the visible and near-infrared wavelength regimes
(Nadiarnykh et al. 2007). Thus, similar SHG signals can be expected for the two near-infrared excita-
tion wavelengths. The back-scattered SHG signal is dependent on material bulk density as presented in
Figures 18.6c through 18.6e, showing SHG images measured in samples of different density, expressed
as weight percentage of the mass of cellulose relative to the total mass of cellulose and absorbed water.
A linear relation is obtained between the average SHG signal and cellulose sample density, Figure
18.6f, confirming a larger fraction of backscattered SHG signal for denser samples and that primarily
multiple-scattered forward-directed SHG signal is detected.
The SHG signal of individual fibers is dependent on their orientation relative to the laser polarization,
as demonstrated in Figures 18.6g and 18.6h measured in the same field of view using vertical and hori-
zontal laser polarization, respectively. Although the large-scale features of the sample appear similar in
the images, the fibers aligned with the laser polarization generate the strongest signal contribution to
the image. This can be seen comparing Figures 18.6g and 18.6h and also clearly in the images of Figures
18.6i and 18.6j, covering a smaller field of view.
18.3.2 tissue engineering Applications of Biosynthesized cellulose
The use of microbial-derived cellulose in medical industry has mainly been focused on liquid-loaded
pads, wound dressings, and other external applications (Czaja et al. 2007). Nevertheless, biosynthesized
cellulose has interesting properties in its wet, unmodified state. The high water content of around 99%
suggests that the material can be considered a hydrogel, which are known for their favorable biocompat-
ible properties, much due to little protein adsorption. The versatility of the material, allowing it to be
manufactured in various sizes and shapes, depending on product requirements, has made it of interest
to explore its use as an implant in biomedical applications, such as a bone graft material (Zaborowska
et al. 2010), blood vessel substitute (Bodin et al. 2007a, Klemm et al. 2001), cartilage replacement (Bodin
et al. 2007c), or as a hydrophilic coating of other biomaterials (Charpentier et al. 2006).
18.3.2.1 Biosynthetic Blood Vessels
The most common vascular graft material in use today is autologous veins and arteries. However, about
10-20% of the patients that need vascular bypass are lacking usable small-diameter (<5 mm) vessels
as graft material. Furthermore, purely synthetic grafts are not suitable replacements for small vessels
due to problems with occlusion and mechanical mismatch to the native vessel. Thus, there is a strong
demand for small replacement blood vessels with mechanical and surface properties similar to those
of native ones. The possibility to produce small-diameter cellulose tubes (see photo Figure 18.7a) with
designed tissue properties makes the material of high interest for tissue-engineered blood vessels. In
addition, the material has promising mechanical properties for use as a blood vessel, such as a high
tensile strength, ascribed to the super-molecular structure in which the microfibrils are tightly bound
