International Open Access Journal of Prevention and Research in Medicine
Director Prof. Francesco Tomei

The importance of nanotechnology is widely recognized in both biomedical and industrial applications, so the search for new nano materials with improved physical and chemical characteristics is rapidly growing, thus increasing the risk of exposure in the population.
Engineered nanoparticles, defined as particles having a different shape, but at least one dimension less than 100 nm, are constituents of many everyday products including, for example, sunscreens, cosmetics and some food packaging. This implies that an increasing number of people can come into contact with these nanoparticles in occupational settings and in the environment. It then becomes mandatory to assess what potential effects nanoparticles may have on biological systems. Although many nanoparticles may not represent a problem for the general population, they may instead be a problem for subgroups of susceptible individuals. In this context, our research was aimed at studying the effect that maternal exposure to silica nanoparticles (SiO₂ NPs) may have on the health of pregnant individuals, with particular attention to the possible harmful effects on the development of the placenta and fetus. To this end we have produced silica nanoparticles of three different sizes: small, medium and large. Each nanoparticle was in turn modified in two different ways, through the introduction of NH2 or COOH functional groups, in order to make their surface positively or negatively charged.
SiO₂ NPs were intravenously administered to pregnant mice, through the injection into the venous retro-bulbar eye plexus. Administration was performed at two different gestational stages. A group of females received the material 5.5 days after conception, when the placenta is still poorly formed, while a second group was exposed at 12.5 day of pregnancy, a time at which the placenta has completed development. The difference in the administration timing allowed us to evaluate the possible differences in susceptibility of the fetus depending on different stages of placental development. Our results have shown that the smallest SiO₂ NP have a high biocompatibility and do not interfere with the development of the embryo, or with placental development. In contrast, the NPs of medium and large size have demonstrated interference with the development of the fetus, leading to the onset of mild structural alterations and the appearance of a large number of identical twins, an extremely rare phenomenon in rodents, generally secondary to a mild teratogenic stimulus. Such an effect became apparent only after administration of high doses of  nano particles, showing also a relationship with the surface charge. In conclusion, these results suggest caution  in the exposure to SiO2 NP of medium and large size during pregnancy.

Silica nanoparticles are formed from atomic or molecular aggregates of spherical shape with a diameter between 5 and 100nm. Based on their physicochemical characteristics, they are used in the most disparate fields of application from industrial to biomedical. In the industrial sector the silicon nanoparticles are widely used in many products, such as toners for printers, cosmetics and packaging. With regard to their applications in biology, these nanoparticles have proven valuable for "drug" and DNA delivery "imaging", immobilization of enzymes, and development of filtering systems (1-5).
 As a result of the large number of possible applications, it is conceivable that, in the near future, the amount of silicon nanoparticles that will be released into the environment will grow dramatically, with a considerable increase of the possibility of human exposure. It is therefore of great importance to assess that this nanomaterial does not pose a risk to public health and the environment (6). Up to now a low number of studies have been conducted in order to investigate the biocompatibility of the nanoparticles of silicon. These studies have generally used in vitro models, which, while providing an indication of the effects, are difficult to extrapolate to human health. These studies, however, have given an indication that some physico-chemical characteristics, including size, shape, and the presence of functional groups that mediate the interaction with proteins and other biological molecules (7, 8, 9), may have influence on the toxicity of  silica nanoparticles.
Recently, it has been shown that nanoparticles of silicon, when administered intravenously to pregnant mice a few days before deliver, determine fetal resorptions and underdevelopment (10). This effect was associated with size and the presence of some functional groups. Alteration of the placenta, with cell damage and induction of apoptosis, was also observed . Although the effects reported were only observed after administration of a very high dose of the nanomaterial (800 micrograms per mouse), these results suggest that the possibility that silica nanoparticles  be embryotoxic should be studied further.
 

In this regard, we have conducted experiments in which female mice at 5 days of pregnancy were exposed intravenously to silica nanoparticles in three different sizes (small, medium and large) and two different functionalizations (NH2 and COOH). For each type of nanoparticle, we have used doses ranging from 0.1 and 30 m g / mouse. This dosage range was chosen on the basis of our previously published studies (11) where we used an experimental protocol to study the embryotoxic effect of carbon nanotubes.
 At the highest dose of 30 m g / mouse, carbon nanotubes were able to induce a high frequency of miscarriages and fetal
                                                          
 
malformations and placental damage. The presence of fetal malformations was still observed at the lowest concentration of 0.1 m g / mouse, although at a lower frequency. In similar experimental conditions, the silica nanoparticles showed a higher biocompatibility than carbon nanotubes; in fact, only at the concentration of 30 m g / mouse and only those of larger dimensions were associated with a small number of fetuses with structural malformations, associated with placental abnormalities. At lower concentrations, however, none of the silicon nanoparticles resulted in the appearance of structural abnormalities, nor to the fetus or placenta. While the dimensions are shown to be important in the induction of the harmful effect, the presence of NH2 or COOH functional groups did not play a key role, as no significant differences were observed among fetuses exposed to silica nanoparticles of the same size but with different functionalization.

Based on the results shown above, it may be concluded  that silica nanoparticles show a low potential embryotoxicity, and seem to be well tolerated during pregnancy, although it is clear that  their size and concentration play a key role.