INTRODUCTION - Nanoindentation with Biological Applications - page 7

sapiens (humans). These classification schemes are fundamentally cell-
based. Biological organisms can be unicellular or multicellular, where
the cell is the smallest individual unit of living matter, the presence of
which signals a difference between a living tissue and a nonliving entity.
Cells are typically micrometers in diameter, and the products of cells
range from molecules sized in the nanometers, to multimolecular
assemblies larger than the cells that produced them.
The subject of this work is biological materials, where it is
recognized that this could include non-living materials that were
manufactured by biological entities. In addition, materials per se can be
sub-units of an organism, and similar tissues (or materials) can exist
across many different species. As such, a slightly different taxonomy is
required to categorize the natural materials considered here, as opposed
to the organisms and species considered by classical Linnaen taxonomy.
This overall structure—starting with the kingdom designation from
traditional taxonomy—is shown in Fig. 1-4 . Within the individual
kingdoms, there are large structures called organs, which are found both
in animals (bones, kidneys, brain, skin) and in plants (root, leaf, flower).
In general organs are a high-order, complicated structure that is made up
of several individual tissues, where tissues contain cell types for
performing specific functions. Since the same tissues can be found in
multiple organs, organs are not considered as a separate level in the
taxonomy. There are four animal tissues (epithelium, nervous tissue,
muscle tissue, connective tissue) and three plant tissues (epidermis,
ground tissue, and vascular tissue). Tissues are themselves composed
of both cells and extracellular matrix (ECM). While many tissues
are primarily cellular, the connective tissues in animals and the
sclerenchyma in plants (part of the ground tissue) are dominantly
extracellular matrix and this is where the majority of structural and
mechanical functions are found.
Biological tissues are not continuous media, but are composites
consisting of living cells in the non-living ECM that is built and
remodeled by the cells. Biological materials are dynamic—there exists
great potential for changes in the tissue hierarchy with growth and
development or with disease because of the action of live cells. The