Biomedical Engineering Reference
In-Depth Information
to different intracellular fates and biological responses (Chou 2011).
The most common pathway is receptor mediated endocytosis. It
has been shown that the cellular uptake of GNP is size and shape
dependent, and an optimum size exists (~50nm) for maximum
GNP cellular uptake (Chithrani 2006). For detailed discussion
of the intracellular delivery of NPs, one can refer to the extensive
review of Chou et al. (Chou 2011) and the references therein.
temperature have been modeled or characterized for a laser beam
irradiating the monolayer cells with GNR (Huang 2010). The cell
viabilities at different positions were then predicted and compared
with experimental measurement. It is still unclear whether the
presence of GNPs in cells affects the thermal injury kinetics.
On the other hand, if a pulsed laser is used with a pulse dura-
tion that is shorter than or comparable with the thermal equi-
librium time of the GNP or cell, local injury can be achieved
(selective nanophotothermolysis) (Zharov 2005a, Zharov 2006,
Anderson 1983). For example, in a mixed-cell suspension
Kalambur et al. showed that a nanosecond pulsed laser can
selectively destroy malignant tumor cells loaded with NPs while
leaving normal cells intact (without NPs) (Kalambur 2007). This
implies highly localized effects such as bubble formation around
the NPs, which are suggested to form and then collapse thereby
destroying cell membranes by cavitation as discussed next
(Zharov 2005a, Zharov 2006, Anderson 1983).
18.6.2.2 Cellular thermal Injury (Kinetics)
Cell injury after heating is considered a kinetic process depend-
ing on both the temperature and time (Sapareto 1978) and
protein denaturation is highly correlated to thermally induced
cell death (Lepock 1993, He 2004). However, the mechanism
and kinetics of thermally induced cell death and protein dena-
turation at high temperatures above 50°C and especially near
boiling, such as that achievable under pulsed laser irradiation,
is an area of active investigation (He 2003, He 2009, Yan 2010).
According to the Arrhenius kinetic model, the heat induced cell
injury (Ω) or survival fraction (
S
) can be described as
18.6.2.3 Nonthermal Injury (Mechanical
and Chemical)
If the absorbed energy is high enough, phase change (i.e., boil-
ing) occurs. The ensuing bubble generation and cavitation can
induce mechanical stress and lead to selective killing of cancer
cells. A schematic of the pressure-temperature water phase dia-
gram showing the relative position of these and other heating
events is shown in Figure 18.7.
Lin and coworkers showed that the immuno-targeted GNPs
(30 nm) induce selective cell injury under pulsed laser irradia-
tion (Pitsillides 2003). Detailed heat transfer analysis showed
that bubble generation and cavitation damage was responsible
for cell killing. Recently, Lapotko and coworkers showed that
the bubbles generated by GNPs can be fine-tuned for imaging
and therapeutic purposes (Lapotko 2009, Lukianova-Hleb 2010).
Small photothermal bubbles serve as sensitive imaging contrast
agents for target cells without significant damage to the cell,
while bigger bubbles lead to disruption of cellular structure and
cell death.
The mechanical effects on the cell strongly depend on the
localization of GNPs in the cell. If GNPs are targeted and bound
to the cell membrane, increased cell membrane permeability can
be achieved, and this could be used to deliver foreign molecules,
such as drugs, proteins, and genes, to the selectively targeted
cell (Pitsillides 2003). Further increasing the laser energy will
compromise the integrity of the cell membrane and lead to cell
death. Tong et al. showed that it requires much less energy to
lyse the cell with membrane-bound gold nanorods than when
these nanorods are in the cytoplasm (Tong 2007). Zharov et al.
showed that the microbubbles overlapping on the cell membrane
synergistically enhance the photothermolysis of targeted cancer
cells (Zharov 2005b). These studies speak to the ability of laser
heating to selectively heat GNPs at the subcellular level (i.e., no
bulk heating) with consequent cell destruction.
Recently, it was shown that the presence of GNP under low
power laser irradiation also increases the intracellular reactive
oxygen species (ROS) level, which causes damage to the endosomal
E
RT
()
−
1
τ
=
∫
Ω=
In
Ae
dt
(18.17)
S
where
A
is the frequency factor (
s
−1
), Δ
E
is the activation energy
(J mol
−1
),
R
is the universal gas constant,
T(t)
is the temperature
history the cell experiences,
t
is the time, and τ is the duration of
the heating. The kinetic parameters,
A
and Δ
E
, are cell type (and
thus tissue) and assay dependent (Bhowmick 2000, He 2009, He
2004). The thermal injury of cell (or tissue) accumulates faster at
higher temperatures. According to the clinically used thermal
dose model (TID), a derivative of the Arrhenius model, it takes
roughly half the time to induce the same injury for every degree
increase in temperature. Not surprisingly, the TID model and
the Arrhenius model predict similar injury in various urologic
cell and tissue systems (i.e., kidney, BPH, and prostate tumor)
between 43.5 and 50°C. However, the TID model is less accu-
rate than the Arrhenius model at higher temperatures (He 2009).
For instance, TID predicts a longer time to accumulate injury at
temperatures above 50°C than the Arrhenius model (i.e., 25%
longer for kidney and BPH). Computational and experimental
studies to explain how thermal injury kinetics scale at higher
temperatures (i.e., near boiling) is of continuing interest in the
field of thermal injury and directly relevant to laser tissue and
laser GNP therapies (Yan 2010).
Using CW laser, it has been shown that laser treatment is
selective for cells that have taken up GNPs versus cells without
GNPs (Huang 2006, El-Sayed 2006, Hirsch 2003, Loo 2005). For
example, using antibody-conjugated gold nanorods, Huang et al.
showed that the treatment of malignant oral epithelial cells after
incubation with gold nanorods requires about half the energy
versus cells that did not take up nanorods (Huang 2006). This
CW laser treatment usually takes several minutes, which corre-
sponds to a steady state macroscopic temperature increase instead
of just local subcellular heating. The spatiotemporal changes of
