Biomedical Engineering Reference
In-Depth Information
accuracy at low absolute mass content. As a result, it may be difficult to accu-
rately determine nanoparticle doses. Air-to-liquid interfaces have been devel-
oped for controlled transfer of nanoparticle aerosols to
in vitro
assays [33].
Also, a reproducible transfer from suspended particles to dry, substrated
particles suffers from incomplete dispersion and uncontrolled suspension
concentration. In addition, evaporation-related convection and capillary
forces can induce reagglomeration on the substrate. Resulting agglomerated
material, such as highly entangled nanotubes or large particle agglomerates,
may not be accessible to microscopic or individual analysis and thus dete-
riorate the representativeness of the analyzed sample fraction by systematic
omission. Likewise, sedimenting fractions may escape from ensemble-
averaging characterization of suspensions and thus will bias property dis-
tributions. Also, orientation effects and incorrect viewing angle may bias
particle shape and fiber length determination. Sample preparation for TEM,
especially preparation of tissue samples containing nanoparticles, requires
profound experience and great care to preserve relevant details.
Owing to these complications, toxicological testing of nanoparticles must
be based on very carefully designed tests with respect to particle selection,
characterization, preparation, dosing, and tracing. Whenever possible, par-
ticle concentrations and property distributions should be determined under
conditions as relevant as possible to the specific testing conditions. It has to
be considered that dynamic interactions in test media will generally affect
the agglomerate size. Double-layer expressing salts, pH-dependent surface
charge, and proteins such as albumin can induce agglomeration or dispersed
nanoparticles. The dynamics of adsorption and agglomeration can be stud-
ied experimentally by light or x-ray scattering (SLS, DLS, SAXS).
The use of dispersants may help study the effect of individualized nanopar-
ticles. However, individualized nanoparticles are a limiting case and perhaps
a worst-case scenario that does not necessarily reflect naturally occurring
agglomeration. In addition, interference effects of the dispersants with the
assay have to be studied carefully.
The particle concentration “as dosed” and “as exposed” may differ signifi-
cantly. Which dose unit is relevant for toxicity testing is still under controver-
sial discussion. Up to now, the most frequently used unit for concentration
is the mass of nanomaterial per testing medium volume. However, also the
dosed particle number concentration, surface area of nanoparticles, and even
the number of nanoparticles per area of lung tissue or culture dish may be
suited to assess the significance of nanoparticle effects. Therefore, both mass
concentration and particle size distributions must be determined in order to
allow for dose unit conversions and comparability of different studies.
2.4.4 Importance of Reference Materials
A reliable nanotoxicological understanding of fundamental structure-
response relations can only be obtained from systematic studies under
