Biomedical Engineering Reference
In-Depth Information
There have been numerous seminal reports and review articles detailing the scientific evidence
that nanomaterials have unique toxicological properties and are more toxic than their bulk materials
(Borm et al. 2006; Oberdörster et al. 2005). When considering nanomaterial toxicity, it is extremely
important to recognize that there are many different types of nanomaterials, which negate a generic
approach. Indeed, studies in both tissue cultures and laboratory animals have shown that seemingly
slight changes to the surface chemistry of nanomaterials can result in significant changes in their
tox icit y.
For example, fullerenes and carbon nanotubes (CNTs) that were chemically treated in different
ways showed remarkably different toxicities compared to their untreated counterparts (Carrero-
Sanchez et al. 2006; Sayes et al. 2004). As a result, the detailed characterization of nanomaterial
properties is now recognized as a critical component of quality nanotoxicology studies. Additionally,
it seems unlikely that future studies will provide generalizations that can describe the toxicity of all
nanomaterials (Borm et al. 2006).
The number of nanotoxicology studies being conducted is gradually increasing, but there are
many knowledge gaps that need to be filled before appropriate risk assessments and workplace
exposure standards can be established. Data concerning the effects of engineered nanomaterials in
humans are limited and, to date, the majority of studies have been conducted in rodent models and
tissue cultures using tumor cell lines. Nevertheless, the studies conducted so far have identified a
number of fundamental issues concerning the toxicity of nanomaterials. The most common finding
is that the surface chemistry, particle size, and surface area are all key determinants in the adverse
effects caused by particulate matter.
The exact cause of nanomaterial-induced inflammation has not yet been clarified, although a
number of hypotheses have emerged. The most prominent hypothesis is that cellular damage may be
due to the ability of some nanomaterials to produce reactive oxygen species (ROS), which can dam-
age cell membranes and proteins (Xia et al. 2006). Nanomaterials have the ability to adsorb many
different environmental contaminants to their surfaces due to their large surface areas and chemical
natures. Nanomaterial-induced toxicity and inflammation could be due to chemical “contaminants”
that are adsorbed to the surface of nanomaterials (e.g., bacteria-derived molecules, catalyst metals,
or combustion waste products from manufacturing processes). Furthermore, chemicals attached to
nanomaterials may be presented to receptors on the surface of immune cells, which could result in
inflammation (Becker et al. 2005; Vallhov et al. 2006). On the other hand, toxic chemicals adsorbed
to particles, such as metals and combustion waste products, may also be delivered into cells result-
ing in toxic effects (Penn et al. 2005). Some environmental research groups are using the theoretical
term “nanovectors,” which reflects this characteristic of nanomaterials as a portal of entry for the
cellular uptake of toxic moieties.
In general, these findings suggest that the mechanism of toxic action from nanomaterials may
depend on the processes used to produce engineered nanomaterials and the materials' ability to
adsorb chemicals to their surface. Therefore, it is possible that some processes used in the quality
fabrication of nanomaterials, such as using dust-free clean rooms and sterile cabinets, could also
reduce the toxicity of nanomaterials; however, this is yet to be proven. Currently, there is evidence
for all hypotheses but future research will demonstrate whether the mechanisms are operating inde-
pendently, synergistically, or if one mechanism is more damaging than the other.
13.2 GASTROINTESTINAL TRACT UPTAKE AND CLEARANCE OF NPS
As different tissues and organs have different compositions, structures, and functions, toxic
responses are mostly different once NPs enter different organs. The gastrointestinal tract (GI tract),
also known as the digestive tract, can uptake, transport, digest, and adsorb various substances such
as nutrients, water, and vitamins from food. On the other hand, the GI tract is also designed as a
barrier to restrain the entry of pathogens, undigested macromolecules, and toxins. As such, these
potential exposure routes are likely to be the first portal of entry for NPs invading the human body.
