Biomedical Engineering Reference
In-Depth Information
the transit of these NPs down the gut and brings them closer to the absorp-
tion sites [8].
Many researchers have attempted to improve the GIT absorption rate by
increasing the complexities of the NPs using different approaches called func-
tionalization, consisting of a group change regarding the surface properties,
and subsequently increasing the propensity for absorption. The functional-
ization approach may help raise the absorption rate, but it may also increase
the size of the NPs, which reduces absorption based on our observations.
Equilibrium needs to be reached because when the chemical surface of the
NP is altered, the possible interaction of the NPs with the target organ and the
toxicity of the NPs also change. In this scenario, even if the absorption rates
increase, the new NPs must be characterized. Other techniques have been pro-
posed, for example, coating NPs with molecules or peptides such as lectins,
invasin, or internalin fragments. Even the results from such manipulation of
the surface properties are often clear
in vitro
: the adhesion to the cell increases,
and there is some evidence that absorption is enhanced [8]. The goal is to reach
the target organ, cells (e.g., M cells, normal epithelial cells), or tissues. Thus,
a lack of clarity of the outcome might result from the chemical and physical
instabilities of the coated particles. The increased adhesion from the interac-
tion of different receptors may lead to enhanced uptake into cells, but this does
not necessarily translate into increased transport through and out of the cell.
In the situation in which the epithelium itself is not the target, movement
of particles through the M cells or epithelial cells is only the first part of this
voyage, which involves passage through the mesenteric lymph, filtration in
the lymph nodes, and transfer to the blood and perhaps extravasation [8].
In a study by Carr and colleagues [11], the researchers said that estimation
between the uptake and the volume of NP is not straightforward. Other fac-
tors such as maceration may overestimate the uptake into the tissue. This
observation was clearly demonstrated by microscopy showing that in the
small intestine, most NPs are luminal or on the surface [12]. In this study, the
researchers did not find an important difference between particles; however,
the sizes of the particles were >1 μm.
In a histological study by Desai and colleagues [13], the researchers demon-
strated that NPs of 100 nm were diffused throughout the submucosal layers
as opposed to the larger sizes, which were predominantly found in the tissue
surface. Jani and colleagues [14] exposed rat GIT mucosa to different sizes of
NP (i.e., 50 nm, 100 nm, 300 nm, 500 nm, 1 μm, and 3 μm) every day for 10 days.
At 48 h postexposure, the researchers measured the amount of NPs in dif-
ferent regions. Several authors discovered that NPs <100 nm accumulated
in GIT tissue; however, if the sizes were greater than this cutoff, then the
accumulation rate was significantly lower [13,14]. In another study, colloidal
gold was administered in water for 7 days to mice; the results showed a large
distribution across several organs, including the kidney, spleen, liver, blood,
and brain [15]. Even after 12 h of postexposure, NPs were still measured in
the GIT, which suggests sequestration in cells or at the surfaces.
