Page 384 - Biosystems Engineering

Evaluation of Transgenic Wood for Pr oductivity & Quality 361

nm
5

3
4

2 3
μm

2
1
1

0 0
0 1 2 3 4 5
μm
FIGURE 12.5 AFM topography of two indent marks on Keranji (Dialium spp.)
cell wall after nanoindentation.

By using the nanoindentation technique, Gindl et al. (2002) exam-
ined possible differences in the behavior of developing and fully lig-
nified spruce tracheid cell walls under axial compressive loading,
which could not be investigated by other techniques. The average
–1
lignin content of developing tracheids was 0.10 g.g , as compared
with 0.21 g.g in mature tracheids. The modulus of elasticity of devel-
–1
oping cells was, on average, 22 percent lower than that measured in
mature, fully lignified cells. For longitudinal hardness, a large differ-
ence of 26 percent was observed.

Atomic Force Microscopy
Atomic force microscopy (AFM) has been used increasingly to char-
acterize biological samples. AFM has a high resolution and measure-
ment accuracy, far surpassing the capabilities of both contact and tap-
ping modes (www.parkafm.com). The resolution of the tapping mode
is not as high as that of the noncontact AFM because the very sharp
end of the tip is extremely fragile and is blunted instantaneously
when it forcefully establishes contact with the sample. This problem
is more severe in the case of imaging soft materials such as thermo-
plastic polymers. On the other hand, a constant tip–sample distance
of a few nanometers is maintained in the true noncontact AFM with-
out damaging the tip or sample to obtain a high resolution.
One obvious advantage to using AFM is that it is possible to
obtain sample topography (height image), elasticity (phase image),