INTRODUCTION - Nanoindentation with Biological Applications - page 15

2.2.2. Plant kingdom
In the plant kingdom, tissues are roughly divided by their being “woody”
(wood, bark, cork, cone) or “non-woody” (leaf, sisal, pitcher plant, fruit,
coir, seed, cotton, bast fiber, hemp, jute, flax, straw). All wood and
wood-like materials rely on cellulose for their mechanical stiffness and
strength. They are light, and, parallel to the grain, they are stiff, strong
and tough—as good per unit weight as many man-made materials, except
carbon fiber reinforced plastic (CFRP, a very strong, lightweight and
expensive fiber reinforced polymer used in engineering).
All tree-woods have broadly the same chemical composition: 40-50
wt% crystalline cellulose fibers in a matrix consisting of 20-25 wt%
partly-crystalline hemi-cellulose and another 25-30 wt% amorphous
lignin; in addition there is 0-10 wt% oily extractives that give wood its
color and smell. Three main features characterize the microstructure—
elongated cells that make up the bulk of the wood, called tracheids in
softwoods and fibers in hard woods; rays , radial channels connecting the
inner part of the trunk to the thin outer region or cambium; sap channels ,
which are enlarged cells with thin walls and large pore space but conduct
fluids up the tree, driven by osmotic pressure.
Palms are monocots, more closely related to grasses and ferns than to
trees. Bamboos are grasses, mineralized with bio-silica. Many trees have
a thin layer of cork just below the outer bark. Structurally it is low
density, polymeric closed-cell foam. Nutshells are made up of a network
of fiber tissues with a composition close to that of wood, but with an
approximately random 3-D fiber lay-up. Materials from the plant world
are briefly examined in Chapter 9 along with soft tissues, as
nanoindentation testing of these materials is a rapidly growing area of
current research.
2.3 . Biomimetic and derived materials
In addition to naturally-occurring materials, there are a number of quite
important materials that are naturally-derived. For example, engineered
wood is a composite made of real wood with a polymer binder, and
leather is animal skin that has been artificially treated with chemical