Biomedical Engineering Reference
In-Depth Information
than their larger but chemically identical counterparts [63] and are having a potential to induce, for
example, the formation of ROS, thereby rendering them
a priori
harmful to biological systems. The
exposure of Degussa P25, a commercially available TiO
2
nanomaterial, at concentrations between
25 and 120 mg/L of cell culture medium to the cultured immortalized mouse microglia (BV
2
), rat
dopaminergic (DA), and primary cultures of embryonic rat striatum, found an immediate and pro-
longed release of ROS in BV
2
cells [64]. Naturally, this is an
in vitro
study without a relevance to risk
assessment, but it provides insight into the possible mechanisms of the effects of nanosized TiO
2
on
microglial cells. The agglomeration of particles though adds complexity to assessment of the true
effect. However, the effective surface area of the agglomerates may not be that different from that
of the primary particles forming the agglomerate. In an
in vivo
study, the exposure of commercial
TiO
2
NPs, SiO
2
NPs, and nanosized TiO
2
to BALB/c mice, generated a gas-to-particle conversion
process at 10 mg/m
3
, and to SiO
2
-coated TiO
2
NPs [65]. The inflammatory response induced by
nanosized TiO
2
clearly exceeded that induced by the otherwise corresponding coarse particles [66].
Interestingly, the only particle that induced a dramatic inflammatory response in these animals was
the SiO
2
-coated TiO
2
NPs [65]. These findings indicate that in addition to the size of the particles,
other features such as the surface chemistry potentially adds to the reactivity matters. Furthermore,
recent observations on the translocation of engineered nanomaterials through the circulation due to
their small size to any organ in the body have provided evidence that systemic effects due to exposure
to the engineered nanomaterial are also possible [67]. The issue of size of particles and the possible
harm to human health by nanoscale particles is further illustrated by a recent study that assessed risks
of cancer in rats subsequent to exposure to different types of titanium dioxide (fine TiO
2
< 2.5 μm;
ultrafine TiO
2
< 0.1 μm in diameter) [68]. They used the available rat dose-response data on these
materials. The results of the modeling suggested that the maximum likelihood estimate (MLE) was
much lower for the nanosized than for fine TiO
2
particles. At this stage, the reliability of any quantita-
tive health risk estimates is limited due to the small amount of data on nanosized TiO
2
. Nevertheless,
it is quite obvious that nanosized particles seem to have more potential than chemically identical but
larger particles to harm human health as the example of TiO
2
suggests.
19.7.1 c
haracterIzatIoNs
of
B
ehavIor
of
e
NgINeered
N
aNoMaterIals
One major uncertainty in the safety and risk evaluation of the engineered nanomaterial arises from
a lack of systematic knowledge about the physicochemical characteristics of the material arriving at
the receptor (in this context the receiving organ or tissue), be it the nose, the skin, or the mouth of
an exposed human. This is true for all forms of engineered nanomaterial, whether they exist in the
form of macroscopic solid objects, as powders, emulsions or suspensions, or as aerosols (i.e., in the
form of airborne particles). All these engineered nanomaterials are essentially constituted of NPs,
or at least nanostructured building blocks such as agglomerates, with the potential of being released
by some kind of mechanism into a transport chain from the “source” to the receptor (see the above
definition of a receptor in this context) [69].
An issue requiring special and thorough consideration is the possible exposure to products con-
taining the engineered nanomaterial during the entire life cycle of the products. Most products are
not likely to cause exposure as long as the engineered nanomaterials are embedded in the polymers
or other matrices in a given product [70]. The situation may change though, when a given mate-
rial needs further processing, becomes waste, or is recycled. A detailed discussion of this issue is
beyond the current presentation, but will require a thorough life-cycle analysis as the number and
volume of products containing the engineered nanomaterial rapidly increases [71].
The tendency of airborne-engineered nanomaterial to the agglomerate that becomes attached to
the ubiquitous background aerosol is of special importance because this may very rapidly change its
specific size-related characteristics. Such transformations can be modeled for given scenarios pro-
vided enough information is available about the system [62]. The changes in airborne-engineered
nanomaterial characteristics during transport have several consequences. For one, a change in
