CELLS AND MEMBRANES - Nanoindentation with Biological Applications - page 319

and the value is nearly three orders of magnitude lower than that of
their natural counterparts when collagen is utilized as the scaffold. 1,2
Hence, a reliable and efficient method is necessary to characterize
membrane properties for providing feedback to optimize the factors and
procedures in tissue construction. Nanoindention has been regarded
as one of the viable methods. It is obvious that such mechanical
measurement is nontrivial and in fact quite challenging due to the small
sample size and thickness and the unusual mechanical properties such
as low modulus and viscoelasticity. However, recent advances in
indentation instrumentation and the various modes of testing geometry
(their details are addressed later) have facilitated the mechanical char-
acterization of biological cells and biomembranes. Several new models
which have recently been developed for quantitative data analysis are
also presented in this chapter.
1.1. Cell structure
All living organisms are composed of cells. According to their structure,
cells can be categorized as eukaryotic and prokaryotic cells. A eukaryotic
cell is a cell with a nucleus which hosts the cell's chromosomes, while
prokaryotic cells have no nuclei. Animals, plants, protists, and fungi are
eukaryotes, while Eubacteria and Archaea are common prokaryotes.
Generally eukaryotic cells comprise at least three major structural
domains: (i) a plasma membrane to identify the boundaries of the cell
and to retain its contents, (ii) membrane-bounded organelles, a nucleus to
house its DNA, and (iii) a cytoplasm with its cytoskeleton of tubules and
filaments. In addition, plant cells usually possess a rigid cell wall, while
animal cells are encapsulated by a compliant plasma membrane coated
with glycolproteins. 3 Most biologists agree that cell walls are essential to
maintain the cell integrity. In fact, the membrane is not simply a lipid
bilayer studded with protein as many biologists suggest, but “a material
with a very small thickness, in comparison with its radii of curvature,
which separates two adjacent liquid-like domains and supports the stress
created by the embedding medium”. 4