Cytocompatibility and Toxicity of Functional Coatings Engineered at Cell Surfaces - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 99

cytocompatibility issues of various approaches used for cell surface func-
tionalization. The impact of various physicochemical properties of nano-
materials including the chemical compositions, surface charge and
chemistry, dimensions and shape on cytotoxicity will be reviewed. The effect
of cell type on their capability to tolerate chemical and physical stresses will
be presented.

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6.1 Methods for Evaluation of Cell Toxicity
Cell viability is an important parameter in cell and tissue engineering as well
as in culture studies to evaluate survival of cells. At the same time, viability
may not be a sucient indicator of biosynthetic capacity of modified cells; a
cell may tolerate the treatment but still develop a high stress affecting cell
productive functions. Currently, there are a number of assays available to
determine cell viability and its functional capacities in culture or in three-
dimensional scaffolds. Thus, cell viability and function may be inferred from
(1) their ability to divide and proliferate, (2) their morphological integrity and
metabolic activity, or (3) the cell response to a certain physicochemical trigger.

6.1.1 Colorimetric Assays
In vitro colorimetric assays provide a simple, reproducible, sensitive and
affordable method to assess cell survival or proliferation in the process of
cell surface modification. Dyes are often used to assess the viability of
functionalized cells using untreated cells as a control group. Fluorescence-
based techniques provide alternative methods for viability assays and allow
rapid and direct measurements. Here, we will describe the advantages and
limitations of several colorimetric probes that are routinely used to assess
the viability (cell counting) and the proliferation of cells in order to deter-
mine the cytotoxicity potential of a cell treatment and its effect on cell
viability, growth, and response. 1 The colorimetric assays can be divided
into those that usually probe (1) changes in membrane integrity based on
the exclusion of certain dyes or the uptake and retention of others, or (2)
metabolic activity of cells.

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6.1.1.1 Probing Cell Membrane Integrity
Viable cells have intact membranes and are distinguished by their ability to
exclude dyes that easily penetrate dead or damaged cells. Staining of dead
cells with propidium iodide has been performed on most cell types. Propi-
dium iodide (PI) is a small molecule of

1 nm size and is normally excluded
from cells with an intact plasma membrane; however, it can penetrate cells
in which the plasma membrane has been disrupted. The PI stains nucleic
acids resulting in bright red fluorescence from the dead cells and is com-
monly used as a marker of dead cells. Trypan Blue is another widely used
assay to probe the cell membrane integrity and identify dead cells. In the

B