Biomedical Engineering Reference
In-Depth Information
flux in hydroponics setting. These model systems were used because of their
“simplicity” and ease of certain measurements (e.g., elongation of plant roots);
however, toxicity tests conducted for soil or sediments as substrate would be
more reliable and environmentally relevant under field conditions. In fact, few
available data regarding the exposure of organisms (plants, invertebrates) in
soil or sediments suggest that the toxicity of various NPs is strongly reduced
compared with those in aquatic media. Apart from direct binding to medium
constituents, aggregation (or agglomeration) may occur, that leads to a reduc-
tion of bioavailability and therefore ecotoxicity, even if NP aggregates (or
agglomerates) can be toxic by itself [24,39]. As described earlier for nano-TiO
2
,
factors that reduce the transport of particles (such as aggregation caused by
variations in ionic strength or pH, binding to larger particles) may reduce the
toxicity as well, at least for live organisms in the water column [16].
Lastly, it is noteworthy to highlight the bacteria-NP interactions, that
bacteria might modify the environmental fate of NMs, that is, through the
production of proteins that could change their aggregation state and con-
sequently their transport in the environment [138]. It is also reported that
bacteria may influence the chemical degradation of NPs, or the stability of
NMs coated with biodegradable coatings [138]. This ability of bacteria may
be shared by other ecological receptors such as
Daphnia magna
[128].
10.3.5 Adequacy of Current Ecotoxicity Testing Methods
While novel physico-chemical properties of engineered NMs may induce
different toxicity mechanisms (as described below), one wonders whether
the short-term ecotoxicity assays are adequate for the detection of all modes
of toxicity. According to a review by Stern and McNeil [139], the available
toxicity data demonstrate a lack of size-specific mechanisms [i.e., toxicologi-
cal profile shared by all NMs], with an overall picture of “material-specific
rather than nanogeneralized risk.” Until now, the toxicological properties of
NMs are known to be similar to that of ultrafine particles [25]. Some aspects
of testing methods are still unknown. For instance, using the deionized
water and harsh NM suspension methods may not be realistic in compari-
son with NM dispersion and suspension in natural waters because they vary
significantly with respect to the water chemistry and the reactivity of NMs
[35]. The use of sonication as a suspension method is questionable, as it leads
to unpredictable results (enhanced or reduced activity) according to the test
material [35,124]. The physico-chemical characterization of the test materi-
als is thus an important issue (for more details, refer to the website:
h t t p : //
characterizationmatters.org/
)
, as toxicity results of identical materials have
been shown to be contradictory. Insufficient chemical characterization may
explain, at least in part, some of those discrepancies [140].
Assay sensitivity should also be considered for the adequacy of the eco-
toxicity test. Stampoulis et al. [56] indicated that standard root germination
and elongation assays might not be sufficiently sensitive to detect the toxicity
