W. Richard Bowen and Nidal Hilal 4

7.4 CONCLUSIONS 217
In using simple Hertz-based models, it is important to realise that they
may be valid only for small indentations, in the region of 5–10% of the total
sample depth. In the case of cells, this will result in the first 200–500 nm
of indentation giving a valid fit, but at deeper indentations the underlying
substrate will start to influence the data. To overcome this problem, there
has been some work on models that can accommodate indentation deeper
than 10% of the total sample thickness [80], although currently they are not
as well established as the conventional Hertz model.
Other considerations that should be taken into account when measuring
the mechanical properties of cells include the inhomogeneity and non-elastic
nature of a cell. For example, the indentation of cell nucleus and filopodia
is quite different as shown in Figure 7.10(b). Although it is assumed
that the cell is truly elastic for the purpose of data analysis, some of the
energy delivered during indentation will be dissipated due to the viscous
and slightly plastic nature of a cell. This effect can be seen in the velocity-
dependent hysteresis that can be observed in some experiments [78, 81].
Measurements may also vary between cells, or even on the same cell due
to the internal components of cell moving beneath the indenting tip.
Since Tao et al. first utilised AFM for quantitative measurement of local
elastic properties of a biological sample (cow tibia) [82], there has been
significant growth in the use of AFM for elasticity measurements of living
cells [83–86]. It has been found that the elasticity (or stiffness) of different
cell types can vary from 0.1 to 40 kPa [87], and cell elasticity might
function as a quantitative indicator during cell differentiation. Many studies
have also shown that cancer cells are substantially softer than normal
cells [88, 89], indicating that quantitative analysis of mechanical properties
could be used to differentiate between cancerous and normal cells
with similar appearances. The quantitative information of local cell elasticity
in nanoscale spatial resolution has also provided valuable insights
into cellular processes such as cell spreading [90] and cell migration [91].
Monitoring the change in elasticity while treating cells with drugs that
disrupt specific cytoskeletal structures (i.e. F-actin, tubulin and intermediate
filaments) reveals that the actin network mainly determines the elastic
properties of living cells [66]. Apart from the continuous contribution to
understanding fundamental cellular mechanisms, these developments are
also promising for drug testing studies [92].
7.4 ConCluSIonS
Recent developments in micro- and nanoengineering have created
many opportunities to investigate the complex processes in cellular biology.
Engineered surfaces with micro- or nanoscale features, either chemical