Biomedical Engineering Reference
In-Depth Information
where
v
= average settling velocity (m s
-1
)
g
= gravity acceleration (9.8 m s
-2
)
ρ
p
= density of a particle (kg m
-3
)
ρ
m
= density of medium (kg m
-3
)
Accordingly, a smaller particle travels in the medium and reaches the tar-
get predominantly by diffusional movement and larger and heavier particles
settle to the target monolayer by gravitational settling. An image showing
how a particle reaches the target cell is shown in Figure 8.2. In general, par-
ticles of ca. 50 nm take more time than particles of other sizes to reach the
cell monolayer under the usual culture condition because both gravitational
settling and diffusional movement are less effective for particles of this size
to reach the target cell [2].
A nanoparticle has a large specific surface area and is usually coated with
a larger amount of proteins than larger particles are in the culture system.
Thus, understanding the particle-protein interaction or the “corona” forma-
tion on the nanoparticles is important to investigate the effects of nanopar-
ticles in
in vitro
culture systems. For example, human serum albumin forms
a 3.3-nm-thick monolayer on quantum dots and the residential time of albu-
min on the surface is approximately 100 s [3]. Culture media usually contain
biomolecules that are adsorbed on and desorbed from the surface of par-
ticles. Thus, the “corona” can be an important determinant for the reactivity
of particles with the cells.
Unless nanoparticles are labeled with fluorescence or radioisotopes, quan-
titative measurement of cellular uptake of nanoparticles is usually difficult.
However, as most of nanoparticles are not soluble and refractory, an optical
method such as turbidimetry is possible for quantitative measurement of
nanoparticles. After lysis of the cells, carbon nanotubes taken up by bron-
chial epithelial cells were evaluated by turbidimetry (optical density = 640 nm).
This simple method enables measurement of <1 μg of multiwalled carbon
nanotubes (MWCNTs) per well of a common six-well culture dish [4].
FIGURE 8.2
Gravitational settling (left large particle) and diffusional movement (right small particle) to
reach to target cell.