Biomedical Engineering Reference
In-Depth Information
10.3.3.2 Polymers and Dendrimers
A range of polymers, either artificial or made from natural building blocks, is
being developed for drug delivery (e.g., copolymers, dendrimers) or as active
drugs (e.g., chitosan). A few of these NMs have been tested for ecotoxicity;
information available is summarized in Table 10.4.
10.3.4 Factors Affecting Toxicity
These factors can be divided into two types, those pertaining to the NPs
themselves, such as the presence of coatings or any other type of surface
modification, and those pertaining to the medium (water, soil, or sediment)
or to the ecological receptor itself. Surface coating or surface derivatization
has been used to modify inherently toxic materials (e.g., uncoated CdTe
quantum dots) and makes them usable in medical applications (as tested
in animals). Dunphy Guzmaän et al. [39] questioned whether the surface
coating reduces the bioavailability (thus toxicity) and limits the solubility
of some toxic NMs. In fact, such coatings may not be persistent after release
into the environment. For instance, in the case of toxic cadmium selenide NP,
the coating (e.g., ZnS) can be oxidized over time, leading to the subsequent
release of the toxic core materials.
Surface modifications are another method to alter the physico-chemical and
ultimately the toxicity of some NMs. Fullerene has been subjected to several
types of surface modifications, for example, partial oxidation (producing ful-
lerol) that improves water dispersibility and diminishes its toxicity to living
organisms, possibly via a lower affinity for hydrophobic lipid membrane sites
[41]. Whether the lower toxicity could be due to a reduced capacity to generate
reactive oxygen species (ROS) has been questioned because the production of
ROS (as superoxide and singlet oxygen production under UV light) is, in fact,
greater for fullerol than for fullerene [97]. It is suggested that NMs have to be
in the vicinity of the cells to induce toxicity through ROS generation.
Soil and the aquatic media, including sediments, contain several constitu-
ents such as clays and organic matter that may bind NM and change their
bioavailability compared with their suspension in pure water. These con-
stituents have high specific surface areas (300-500 m
2
g
-1
typically), and the
surface charge allows them to interact with charged particles [16]. In addi-
tion, uncharged hydrophobic NP can interact with hydrophobic domains
of natural organic matter such as humic and fulvic acids present in water,
soil, and sediments [16]. For example, the presence of natural organic matter
improved the stability of multiwalled NT suspensions in water [41,131]. As
mentioned earlier, most laboratory-based aquatic toxicity studies are using
simple or well-defined aquatic media that are not representative of diverse
natural waters. Similarly, conditions are optimized in some plant toxicity
studies, for instance using filter paper in aquatic test and response endpoints,
such as germination and root elongation, or effects on plant transpiration
