Analyzing the Biological Entity of Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 86

biomolecules, but may be fundamental reforming of the particles. Obviously as shown
for silica or silver, nanoparticles may completely dissolve inside GIT. Furthermore,
the nanoparticles will not only get dissolved completely but also can be reformed later
on (inside or outside the GIT), but then they would be very different in all aspects such
as chemical composition, size, and size distribution as well as surface. The question
that is remaining now is whether this is something general or actually does apply only
for a very few selected types of nanomaterials. Most likely there should be also more
biopersistent nanoparticles. When considering that in vitro studies are performed to
predict in vivo outcomes and now taking into account that nanoparticles in vivo might
be changed completely, it becomes quite evident that especially for predicting out-
comes from oral studies the current design of in vitro studies might not be suitable, as
nanoparticles are added directly into the cell culture medium just in the way they have
been synthesized and not taking into account the (quite fundamental) changes that
may occur during the GIT passage. A possible approach to overcome this issue would
be to include something like the in vitro digestion model before actually adding the
nanoparticle into the in vitro test system. Thus, one is at least mimicking the complex
changes that could occur in vivo . Although the in vitro digestion protocols as such are
standardized, one still needs to agree on a harmonized approach for how to use them
in nano-safety testing. Only this would allow for being able to compare later on the
outcome from different studies.
However, it also becomes easily obvious when analyzing the available literature
that the oral route, both in terms of published toxicity studies as well as published
in situ characterization of nanoparticles, is by far the least intensive studied route of
uptake. Only a limited number of published oral studies do exist and actually even
fewer deal with in situ characterization of the nanomaterials under conditions rel-
evant for oral uptake. Many open questions remain and clearly much more research
is needed to understand what is going on with the nanomaterials inside the GIT, also
to significantly improve the quality and the relevance of in vitro studies for predict-
ing outcome of oral studies.
4.5 SUMMARY AND CONCLUSION
Although careful and complete material characterization as-synthesized has been
recognized since several years within the nanotoxicology field, the proper character-
ization inside the test medium or so to say the in situ characterization has long been
ignored completely. Meanwhile consensus has grown that actually the nanomaterials
need to be characterized under test conditions (“ in situ ”) as well. Only when under-
standing the properties of the nanomaterial in the test system, be it an in vitro or an
in vivo situation, one would finally be able to analyze and correlate the results from
the toxicity testing and thus ultimately derive structure-activity relationships.
We have provided here several lines of evidence for different uptake routes that
upon first interaction with the biological matrix, which is serum for i.v. injection,
lung lining fluid for inhalation, or gastric juices for oral uptake, the nanomateri-
als will change significantly with respect to size (e.g., aggregation/agglomeration),
adsorption of biomolecules (e.g., formation of a protein or lipid corona), or particle
integrity (e.g., dissolution). This in turn has implications for nanomaterial uptake,