Nanosafety and food technology
NanoSafety A short overview of knowledge gaps, future aspects and regional actors in the field Per Gustavsson, Department of Biology, Section of functional zoology, Lund university
Table of contents
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Introduction
The aim of this report is to give an overview of the knowledge and the knowledge gaps that exist today concerning the safety of engineered nanoparticles (ENP). Nanosafety is by nature a truly interdisciplinary science which necessitates an understanding of both physics of particles and the biology of humans, cells, animals and plants. There is no official definition of what an ENP is, but it is generally accepted that an ENP is a man made particle having one dimension in the size range of 1-100 nm. Why is there a need to study ENPs and the effect of ENPs on living organisms? All life on earth has been exposed to nanosized particles some time during evolution, and still is. Nanosized particles can be formed under natural conditions such as during combustion, such as forest fires and volcanic eruptions (Oberdorster et al., 2005) and animals have developed defence mechanism to deal with such exposure. The human skin is relatively impermeable to particle penetration, the mucociliary elevator continuously clears particulate matter from our airways and that which cannot be cleared this way is engulfed and digested by macrophages. As ENPs are designed for a specific purpose and pose new structures that life on earth has not been exposed to during evolution, there is a possibility that these particles might pose a larger threat than unintentional anthropogenic nanosized particles. Risk is a function of exposure and hazard and when the exposure increases, so does the risk. As the use of nanotechnology and ENPs are estimated to increase dramatically in the coming decade, so will the exposure and also the risk. If the exposure cannot be reduced, the hazard could be an important target. Reducing the hazard by designing ENPs which are less toxic could be a solution. However, when assessing risk, it is of great importance to make as correct as possible estimations on the exposure. Early animal studies of carbon nanotube exposure used very high levels of carbon nanotubes as compared to levels which could be expected in an occupational setting (Lam et al., 2004). In order for nanotoxicological data to be credible, experimental levels of ENP exposure must be in parity with that found in a real environment. In 2005, the Environmental Defence Fund and DuPont teamed up to form the Nano Risk Framework, in order to investigate the risks of nanotechnology. Initiatives such as 4
these could benefit both society and commercial interests (PEN 19, 2010). The study of ENPs originates from studies on ultra-fine particles (Oberdorster et al., 2005) and using the knowledge of the toxicology of other particles, e.g. asbestos and graphite, could be beneficial for the novel field of nanosafety. After reviewing the literature on carbon nanotube safety and toxicity, it seems that the study of ultra-fine particles has been forgotten by some researchers. It has been suggested that it would take decades to make complete risk assessments on nanomaterials, as compared to that of regular chemicals. The many different forms of nanomaterials is one key factor. In order not to slow down the development of nanomaterials nor allow hazardous nanomaterials to reach the market, new strategies for risk assessement have been suggested (Grieger et al., 2010). The general public’s knowledge on nanotechnology and nanosafety is limited. Two surveys conducted on laymen in Switzerland, where the perceived risks and benefits of nanotechnology in food technology was examined showed that people perceived nanotechnology in food packaging as less problematic than nanotechnology in food. The trust of the interviewees and the naturalness of food was important factor that influenced their acceptance of nanotechnology in food technology (Siegrist et al., 2007; Siegrist et al., 2008). Research activities within nanosafety could in an extended perspective impact society by providing the general public with unbiased information on the safety of nanotechnologies and allow laymen to make informed decisions. 5
Types of ENPs and applications
The possible applications of ENPs are immense and equally diverse. Carbon nanotubes can be used to enforce polymeric materials such as plastics and are used in Li-ion batteries to improve their capacity. ZnO nanoparticles are used in sunscreens, TiO 2 nanoparticles are used in paints, cosmetics and sunscreens among other things. The use of ZnO and TiO 2 in sunscreens is nothing new, both substances have been used as a part of formulations since the 1990’s. An inventory of commercially available products which contain some sort of nanosized components can be found at http://www.nanotechproject.org/inventories/ a webpage maintained by the Woodrow Wilson International Center for Scholars Project on Emerging Nanotechnologies. The Project on Emerging Nanotechnologies regularly publishes reports on the status of nanotechnology and society. The most recent report published in November 2010 is called “Voluntary initiatives regulation and nanotechnology oversight: Charting a Path” (Fiorino, 2010). Chen et al. (2010)(Chen et al., 2010) recently showed in an experiment conducted at NIOSH facilities, that a commercial spray can product containing TiO 2 produced respirable nanoparticles. The intended usage of the spray was cleaning and sanitizing bathrooms. The nanoparticle fraction in the spray was 170 µg/m 3 which was equivalent to1.2x105 particles/cm 3 with a mean particle diameter of 75 nm. The researchers estimated the dose to 0.075 µg TiO 2 per m 2 alveolar epithelium and minute. Exposure levels for rats were determined to 0.03 µg. This was the first report to ever show that a commerical product can create respirable nanoparticles and would serve as a basis for further toxicological studies on respirable TiO 2 particles.
TiO
2
Titanium dioxide (TiO 2 ) nanoparticles is one of the most common type of nanoparticles produced on an industrial scale and used in a large number of products and applications. TiO 2 occurs in two different chrystalline forms known as rutile or anatase. TiO 2 nanoparticles are used in sunscreens and cosmetics, used as pigments in paints, as dirt 6
repellants on windows and in household applications and can be added as a filler in concrete (Lee et al., 2009). When used in sun screens the TiO 2 nanoparticles are usually coated in order to stabilize them and to prevent particle aggregation.
ZnO
Zinc oxide nanoparticles are used in sunscreens and cosmetics added to rubber in order to improve material properties and used as a pigment in paints. The use of ZnO nanoparticles in sunscreens and cosmetics was banned by the European parliament in 2009 (European Parliament and Council Regulation (EC) No 1223/2009) due to concerns and uncertainties on the toxicity of nanosized ZnO.
Cu and CuO
Copper oxide nanoparticles have applications in the construction industry and can be added to concrete to improve mechanical properties (Lee et al., 2009). Certain cosmetics used for tanning puropses reportedly contain copper nanoparticles as well as beverages sold as health supplements (The Project on Emerging Nanotechnologies, 2011).
FeO
Iron oxide nanoparticles have several potential uses, especially within experimental biomedicine. Magnetic nanoparticles made from FeO are under investigation by various companies with intention of being used as contrast agents or cancer treatment.
Ag
Silver nanoparticles and colloidal silver has been used since the antiquity as a remedy and as a health booster. The most prominent biological property of silver nanoparticles is their antimicrobial effects. Recent tecnological development has led to the use of Ag nanoparticles being used in various household products, such as refrigerators and washing machines, to reduce bacterial growth but also in toothpaste and textiles (Project on emerging nanotechnologies; ENRHES 2009). 7
Au
Solid gold and larger gold particles are biologically inert. When downsizing particles on the nanoscale, different effects occur. A great number of mechanistic studies on ENPs have been carried out using gold nanoparticles. Effects on macrophage uptake have been done using spherical and rod shaped Au ENPs, as well as studies on alveolar translocation of Au ENPs of different sizes.
SWCNT and MWCNT
Single walled carbon nanotubes are made up of graphene layers folded into a tube. They are commonly 0.8-1.2 nm in diameter and up to several micrometeres long, depending on the mode of synthesis. Multi wall carbon nanotubes are made up of multiple layers of graphene sheets folded in to tubes which are held to ether by van der Waals forces. The external diameter can vary from a few nm up to 150 nm and they can be up to several micrometers long (Gustavsson et al., 2011). Both the surface of SWCNT and MWCNTs are inherently hydrophobic, causing the tubes to form aggregates/agglomerats. An organism exposed to pristine carbon nanotubes will therefore most likely encounter bundles or aggregates of tubes instead of single tubes. Most research in the past, as well as more recent research has been focused on how to make carbon nanotubes water soluble and monodispersed. Addition of hydroxy- or carboxyl groups to the surface generally makes carbon nanotubes hydrophilic and thus water soluble. Carbon nanotubes can be modified with other functional groups in order to modify the properties of the tubes. Examples of such are addition of chelators or poly(ethylene glycol) (PEG) to bind metal ions or to decrease the half-life of carbon nanotubes intended for biomedical applications. Applications of carbon nanotubes include uses in composite materials in order to make the material stronger or electrically conductive, applications in electronics such as electrodes in lithium ion batteries and are also being investigated for applications in drug delivery (Gustavsson et al., 2011). 8
Fullerenes
Fullerenes is an allotrope of carbon which form a three-dimensional 60 carbon atom structure. The applications of fullerenes are within biomedicine and drug delivery, as reinforcers of polymer matrices and fullerenes functionalised with metals could function as catalysts (ENRHES 2009). 9
Physicochemical properites determining biological effects
Various physicochemical properties of ENPs have been suggested to influence their biological effects and toxicity. A short description and examples of these are given below and is intended as an overview only.
Geometry and particle size
High aspect ration nanoparticles (HARN) have been suggested to cause frustrated phagocytosis by macrophages and also to be retained in the pleural stomata. This is due to the fact that such particles are too long and stiff to be efficiently phagocytosed by macrophages (Donaldson et al., 2010). Parallells can be drawn to asbestos and silica particles which cause similar effects. This type of geometry can cause ENPs to have a longer biopersistency in the body, increasing the risk of negative effects on cells and organs. The geometry and size may also have implications for tissue biodistribution as well as elimination and excretion of ENPs. Carbon nanotube show some characteristics of HARNs, but have been shown to under certain circumstances be excreted through the kidneys. Multiwalled CNTs which were made water soluble by surface functionalization could be excreted through the kidneys and into the urine if the CNTs were well separated and aligned longitudinal to the filtration slits in the kidney glomeruli (Lacerda et al., 2008a; Lacerda et al., 2008b). PEG-ylated gold ENPs shaped either as spheres or rods were tested for their uptake into macrophages in vitro and in the organs of mice in vivo and assayed for their protein binding capacity. Macrophages were found to take up the spherical ENPs almost four-fold as compared to the rod shaped ENPs. The absence of serum proteins greatly influenced the uptake leading to a five-fold greater uptake of spherical gold ENPs. The spherical particles also bound BSA to a much higher degree as compared to the rod-shaped particles. As for uptake into the organs of mice both spherical and rod-shaped ENPS were taken up and accumulated in liver and spleen, while only the rod-shaped ENPs were significantly taken up in heart, lung and kidneys. The rod-shaped form of gold nanoparticles had a longer blood circulation time than their spherical counterpart. (Arnida et al., 2011). Similar effect of 10
length and shape were observed when four different kinds of MWCNTs were injected in the abdominal cavity of mice. Two types of the MWCNTs were short and flexible and formed smaller entangled aggregates, while the other two types were longer and more rigid and less tangled. After introduction to the abdominal cavity the response of the mesothelial tissue which lines the abdomen was investigated. It was found that the thickness of the tissue was much larger for those animals which had been treated with the longer and more rigid MWCNTs. The animals treated with the longer MWCNTs also exhibited granulomas while the other two groups did not (Poland et al., 2008). The authors of the report suggested that the shape and size of the MWCNTs were the reason for their non-degradability by phagocytising cells and suggested that certain types of MWCNTs could exhibit asbestos-like effects in biological systems. Yet another study on the effect of ENP shape examined the effect of length on TiO 2 particles. Nanospheres (60-200 nm diameter), short nanobelts (60-300 nm diameter, 0.8-4 µm long) and long nanobelts (60-300 nm diameter, 15-30 µm long) were compared for their effects on mice after airway exposure and on macrophages. First, all of the three studied materials induced reactive oxygen species by alveolar macrophages in comparable levels. The nanospheres and the short nanobelts were taken up into lysosomes, while the longer nanobelts were unable to elicit functional lysosomes and were observed free in the cytoplasm. Cytotoxicity was only seen for the longer nanobelts at higer concentrations and only the long type of nanobelts induced cytokine production. It would seem that changing the shape of a material could confer different biological response (Hamilton et al., 2009). Two gold nanoparticles of different sizes, one 1.4 nm and one 18 nm in diameter, were studied for their biodistribtion in rats after intratracheal instillation and intravenous injection. The smaller particle is known to be cytotoxic, while the larger particle is rather intert. Interestingly, the particles showed very different biodistributions. After intratracheal instillation, most of the 18 nm sized particles remained in the lungs, while a part of the smaller 1.4 nm sized particles were able to translocate from the lung and into the blood, liver, kidneys, skin and urine (Semmler-Behnke et al., 2008). These 11
results further show that the behaviour of ENPs is dependent on size and is not easily predictable. The size of a particle can not only affect its toxicity or protein binding, but also their clearance and translocation from the airways. Henning et al. (Henning et al., 2010) investigated the speed of mucociliary clearence of ENPs with different sizes and chemical compostitions. The experiments were conducted in an in vitro model using chicken embryo tracheas. It turned out that polystyrene particles ranging from 50 nm to 6000 nm were all transported with a similar speed, while particles made from various combinations of poly(lactic-co-glycolic acid) (PLGA) showed a high variety of speeds. The speeds were dependant on the chemical compostion of the ENPs.
Surface functionalisation
As important as the small size of nanoparticles is the functionalization of the surface. In order for nanoparticles to be compatible with various applications they often require surface functionalisations (Hirsch and Vostrowsky, 2005). As an example on how surface functionalization can affect biological response, Table 1 summarizes some functionalizations of carbon nanotubes and their effect on the circulatory half live. The carbon nanotubes used in the studies differ in length, diameter and aggregation state. However, it can be seen that certain modifications, especially PEG-ylation, has dramatic effects on the half-life of carbon nanotubes in plasma. A short half-life in plasma does not necessarily indicate that the nanotubes are eliminated and excreted, rather it indicates the rate of absorption by internal organs. 12
Table 1. The effect of surface modification/functionalization of carbon nanotubes (CNTs) on the circulation half-life in plasma. Adapted from Gustavsson et al. (2011).
Type of CNT Surface modification Half-time Administration Species Reference
MWCNT - 3 h i.v. Mouse Lacerda et al. 2008a MWCNT SWCNT glucosamine Hydroxyl 5.5 h 50 min i.p. i.v. Mouse Mouse Guo et al. 2007 Wang et al. 2008 SWCNT SWCNT SWCNT SWCNT SWCNT Pluronics F108 DTPA chitosan PEG PEG 1 h 3-3.5 h 3-4 h 15 h 15.4 h i.v. i.v. i.v. i.v. i.v. Rabbit Mouse Mouse Mouse Mouse Cherukuri et al. 2006 Singh et al. 2006 Kang et al. 2009 Liu et al. 2008 Yang et al. 2008
Metal contaminants and metal leakage
During the production of carbon nanotubes by the CVD method, metals are used as catalysts. The resulting carbon nanotubes therefore usually contain various levels of metal contaminants. The most common metals are Fe, Co, Mn, Ni. Other forms of carbon than CNTs are also usually found after synthesis such as organic carbon species. The CNTs are purified to a certain level depending on their application, e.g. electronics or medicals. Some authors contribute toxicity of CNTs to the presence of metal contaminants (Kagan et al., 2006). Such contaminants could potentially cause oxidative stress through the production of reactive oxygen species. Another physicochemical property of ENPs suggested to cause toxicity is leakage of metal ions from metal ENPs. A recent review on the subject suggest that a majority of the toxic effects of metal nanoparticles can be attributed to their level chemical stability, since stable particles show little cytotoxicity as compared to those who are more easily dissolved for instance (Auffan et al., 2009). As an example the release of copper ions from two kinds of copper oxide nanoparticles were examined in cell culture. One of the particles was made from copper oxide and 13
soluble under intracellular conditions, and the other made from copper and coated with carbon. The particles had similar hydrodynamic diametres but slightly different surface area per gram. The copper particles which were stabilized by a carbon layer were less toxic than the soluble copper particles. The difference was found to be due to dissolution of copper from the particles and treatment with copper ions at similar levels as that released from instable particles gave a similar toxicological profile (Studer et al., 2010).
Protein and biomolecule binding
As a particle becomes smaller and smaller, it will eventually end up in the size range of proteins and other biomolecules, that is the size will be on the nanoscale. This will lead to a different ability of the nanoparticle to interact with biomolecules as compared to a partilce of the bulk material (Lynch and Dawson, 2008). Such interactions with plasma proteins in the blood could lead to the exposure of novel epitopes that are normally encrypted inside the proteins. Many nanoparticles have in common that they can bind apolipoproteins,The binding of apolipoproteins could be a reason for translocation of nanoparticles into cells (Lynch and Dawson, 2008). Deng et al. (2009) observed that TiO 2 , SiO 2 and ZnO ENPs could bind human plasma proteins, with different binding profiles for each type of ENP. In addition TiO 2 ENPs with different shapes such as spheres, rods and tubes also bound plasma proteins in different manners. Generally, the spherical TiO 2 bound more proteins than the TiO 2 rods and tubes did (Deng et al., 2009). Binding of complement factors by carbon nanotubes from plasma has been suggested as a possible mechanism whereby nanotubes induce biological effects (Salvador-Morales et al., 2006; Hamad et al., 2008; Moghimi and Hunter, 2010).. Apparently few other proteins selectively bind to carbon nanotubes and include fibrinogen and apolipoproteins. ENPs made from metal oxides have been shown to interact with lung surfactant proteins (Schelh and Hohlfeld 2009; Schulze et al. 2010). The function of lung surfactants is to prevent the alveoli from collapsing by reducing the surface tension in the alveolar fluid. Inhalation is believed to be a major exposure route for ENPs and the interaction with lung surfactants could influence the biological activity of ENPs (Schelh and Hohlfeld 2009). In one study seven different metal oxide ENPs were compared, as well as carbon black. Large differences were seen in the 14
protein binding patterns between all ENPs, also when the particles were made from the same bulk material (Schulze et al. 2010). If ENPs are able to enter into cells they could potentially interact with enzyme function and disurb cellular homeostatis. Examples of ENP interaction with enzymes include acetylcholinesterase and butyrylcholinesterase. These two enzymes are important for neuronal signaling and control of skeletal muscle function. Two studies have shown that different ENPs including copper ENPs and carbon nanotubes, were able to adsorb and inhibit the enzymatic activity of both enzymes. ENPs of Al and AlO 3 and SiO were much less potent in adsorption and inhibition (Wang et al., 2010b; Wang et al., 2009).. The authors suggested that the release of metal ions from ENPs could affect enzyme function as well. Carbon nanotubes commonly have a filamentous like structure, much like the structure of DNA strands and actin filaments. It has been observed that single walled carbon nanotubes, which are the thinnest type of carbon nanotubes, are able to cause bundling of actin filaments and affect cell division negatively. (Holt et al. 2010).
Aggregation and solubility
A key factor determining the toxicity of nanoparticles is their bioavailability, which in turn is governed by the ability of the nanoparticles to aggregate and to dissolve in water. Carbon nanotubes for instance, are inherently hydrophobic which mean that they form aggregates upon contact with water. Functionalization of carbon nanotubes is done in order to examine toxic effects of single particles different dispersant agents are often used in cell culture experiments, e.g. detergents as Tween-80. Pauhlun (2010) presented a study on long-term effect of MWCNT exposure in rats. The results from the study were interpreted as giving the agglomeration state of the nanotubes more influence on their toxicity as compared to that of size and shape of individual tubes (Pauluhn, 2010). Structure-activity relationships Recent progress has been made in the determination of structure-activity relationship of engineered nanoparticles. Application of structure-activity relationship methodologies in nanosafety is still in its cradle, as compared to other research areas such as pharmaceutical development or chemometrics. The activity is not necessarily due to the 15
physical structure of the ENP, it could also include chemical composition, charge, aggregation state or levels of contaminants. Adapting and implementing these tools in nanosafety could help to clarify what properties makes an ENP toxic to organisms. In principle such relationships could be calculated from dose-response effects reported in litterature, but there is not enough data sets with enough quality to allow such comparisons (Puzyn et al., 2009). Fourches et al. (2010) used datasets from previous studies and constructed a QSAR model which had an external prediction of 73% and an R2 of 0.72 for regression modelling (Fourches et al., 2010). Models like these could be beneficial for the design of ENPs which have desired properties without toxic effects. Bello et al. (2009) did not construct any QSARs, but did investigate the potential of various ENPs in causing biological oxidative damage, based on the materials ferric reducing ability of serum in an in vitro assay. When examining the data they found that 93% of the oxidative damage in serum could be explained by a mathematical model which took into account the combination of two factors: specific surface area and the level of transition metal content (Bello et al., 2009). Lanone et al. (2009) examined the effects of 24 nanoparticles on two human cell lines using two cytotoxicity screening methods (MTT and Neutral Red). The most toxic nanoparticles according to their experiments were copper and zinc nanoparticles, while titanium and cerium particles were less toxic. No correlation between size or specific surface are and the toxocity of the particles was found (Lanone et al., 2009).
Summary and research outlooks
Looking at the examples described in the previous section, it is clear that there is not a single uniting factor that explains the toxicological properties of ENPs. Size, shape, chemical compostion or surface functionalization - neither of these factors alone can account for ENP toxicity. Carbon comes in many allotropes which all have different properties. A nanosized fragment of carbon is not likely to have the same biological properties as a fullerene or a single-walled carbon nanotubes. Taking carbon nanotubes as an example, it is getting more plausible that these ENPs should be regarded as distinct chemical entities. A future research niche could be systematic data-mining of 16
structure-activity or cause-and-effect relationships of ENPs, in order to find a useful predictor of biological activity. According to the literature, few studies on the subject of ”nano-QSAR” exist (Sayes and Ivanov, 2010)thus warranting further research in this area. 17
Mechanisms of toxicity
This chapter aims to give a brief overview on the mechanisms by which ENPs cause toxic effects. The exact mechanism are not clearly understood, and neither is the knowledge on which step occurs first. However, parallels can be drawn to the effects of ultrafine particles (Oberdorster et al., 2005).
Oxidative stress
The most commonly accepted theory on the toxic mechanisms of ENPs is about oxidative stress. Oxidative stress occurs when there is an imbalance between the antioxidant system inside the cell and the levels of reactive oxygen species (ROS). The antioxidants inside the cell quench the free radicals under normal conditions to keep the cellular homeostasis. However, when depleted of antioxidants or exposed to a substance or particle that can induce ROS, the cell will be affected. The ROS can interact with proteins and DNA which causes protein malfunction or DNA damage. This in turn causes cellular processes to dysfunction and leads to cytotoxicity. The ROS not only cause direct effects on the cellular machinery, but can also induce the expression of pro inflammatory proteins. Pro-inflammatory proteins cause the cells of the immune system to relocate from the circulation to the site where pro-inflammatory proteins are released. This is prominent for alveolar macrophages which are exposed to ROS-inducing particles and is a process that is believed to be involved in the response to inhaled carbon nanotubes (Mocan et al., 2010). The cytotoxicity of carbon nanotubes is commonly considered to be due to the formation of ROS, which cause oxidative stress and cellular malfunction. The reason for the formation of ROS is but metal contaminants from the production process have been suggested as a plausible cause. The fibre like structure has also been suggested to participate in the toxic response of macrophages, with analogies to asbestos and quartz frequently made (Donaldson et al., 2010). 18
Frustrated phagocytosis
Particles which have an aerodynamic diameter small enough to allow them to deposit in the alveoli will eventually encounter alveolar macrophages. The function of the macrophages is to clear particles from the lungs by chemical breakdown followed by clearance via the mucociliary elevator in the airways. If an instilled particle is too stiff and too long, the macrophages will be unable to engulf the particle totally, leading to the phenomena known as frustrated phagocytosis (Brown et al., 2007).A macrophage undergoing frustrated phagocytosis will produce pro-inflammatory cytokines which lead to recruitment of other immune cells such as neutrophils. These recruited immune cells can in turn potentially cause oxidative stress in the tissue. An additional toxic effect of carbon nanotubes is that of macrophage functional impairment. It has been showed in vitro and in vivo that exposure of macrophages to carbon nanotubes can affect their phagocytic activity. In vitro exposure of macrophages to SWCNTs can effectively impair their engulfment of apoptotic cells, which under normal conditions is an important function of macrophages (Witasp et al., 2009).In vivo airway exposure of mice to SWCNT followed the bacteria Listeria monocytogenes led to an attenuated macrophage phagocytosis of bacteria. This in turn led to a greater pulmonary inflammation and less clearance of bacteria in the lungs (Shvedova et al., 2008). These effects on macrophages indicate that ENP exposure can cause negative effects on other biological functions, without being directly cytotoxic on their own. It has been shown that MWCNTs can cause physical damage to the cell membrane of macrophages. Apart from causing cytotoxic effects in the cells, the carbon nanotubes were found to have adsorbed numerous proteins including the macrophage receptor MARCO. Experiments indicated that binding to MARCO could cause effects on the cell membrane, eventually leading to its disruption (Hirano et al., 2008). 19
Effects of ENPs on reproduction and fetal development
The knowledge on the effects of ENPs on reproduction is limited. A major task is to determine relevant doses for exposure in animal models. The following section summarizes some results from animal studies using various ENPs, including carbon nanotubes and TiO 2 . Repeated administration of carbon nanotubes to rats by intra-scrotal injections led to a reversible damage of the testis, but no effect on fertility or on health effects of offspring (Bai et al., 2010). Studies on developmental effects of carbon nanotubes on zebra fish embryos have been done. Low concentrations of MWCNTs caused offspring defects, while cell death, embryonal death and hatching delay were observed for higher concentrations (Asharani et al., 2008). In similar studies using concentrations within the same span no effects on embryonal development were seen, however, the fertility of the second generation of zebra fish was compromised by an unknown mechanism (Cheng et al., 2009; Cheng et al., 2007). The passage of ENPs from the mother to the fetus has been poorly studied. Some ex vivo studies on human placentas have been carried out. In one of the studies, human placentas were perfused with fluorescently labeled polystyrene beads. The diameter of the beads ranged from 50 to 500 nm. Particles with sizes from 50 up to 250 nm were taken up and were able to cross the placental barrier, however the viability of the placenta was not affected (Wick et al., 2010). In another study human placentas were perfused ex vivo with nanosized PEG-ylated gold particles with a diameter of 10-30 nm. The experiments did not reveal any transfer of gold nanoparticles from the maternal circulation to the fetus (Myllynen et al., 2008). The effect of TiO 2 nanoparticles on the development of pups was investigated by subcutaneous injection of TiO 2 on pregnant mice. The TiO 2 particles used were 20-100 nm in their pure form, but after dissolution into saline solution containing the detergent Tween-80 the majority of the particles were aggregated. The aggregates could be divided into either a 27 nm category or a 2,429 nm sized one. The exposure to these TiO 2 particles resulted in an increase in dopamine (DA) and its metabolites in pups from mothers who were injected with TiO 2 (Takahashi et al., 2010). Results like these indicate that certain kinds of ENPs could influence the development of the nervous 20
system, but it should be remembered that further results are needed and the likelihood of the exposure level be determined. Subcutaneous injection of 0.1 mg of TiO 2 nanoparticles (anatase, 25-70 nm in diameter) on pregnant mice led to significant effects on their offspring and prenatal-transfer of nanoparticles from the mothers to their pups. The male offspring were found to have TiO 2 nanoparticles in their testis and their brains and also a reduced sperm production. Cells with apoptotic biomarkers were seen in the olfactory bulb and brain (Takeda et al., 2009). It would thus seem that the effects of ENPs are dependent on material composition as well as size and agglomeration in their abilities to cause effects. 21
Routes of exposure
Airway exposure
The effects on human airway exposure to ENPs are difficult to assess, as there is limited data available. On the other hand, there is much literature available on exposure to ultra fine particles originating from combustion sources. Nickel is a metal which has been associated with negative health effects upon exposure. Addition of nickel nanoparticles significantly increases the compressive strength of concrete (Guskos et al., 2010). Thus it is plausible that humans will be exposed to nickel ENPs in the future. One case study on nickel nanoparticles exist. Post-mortem analysis was done 16 years after the time of decease on a male worker who died by adult respiratory distress syndrome in 1994 after exposure to an aerosol of nickel nanoparticles. Electron microscopic evaluation of lung tissue revealed <25 nm in diameter nickel nanoparticles in lung macrophages and his urine contained high levels of nickel. There was also tubular necrosis seen in the kidneys. The results indicate that the nanoparticles were able to translocate from the airways into the circulation. Assessment of the occupational hygiene showed that the concentration of particles where the subject was working was 382 mg/m 3 and that the subject would have inhaled approximately 1 g particles during a 90 min operation. The subject was reported to having removed his protective face mask during operation (Phillips et al., 2010).
Dermal exposure
The effects of ENPs on skin, both human and in animal models are unclear. ENPs, such as TiO 2 and ZnO particles, are used in cosmetics and in sun screen. Thus dermal exposure to certain ENPs is very likely to occur, but the extent of particle penetration seems judging from available literature to be dependent on particle type, size and exposure scenario. One of the most probable scenarios is exposure by sunscreens or cosmetics. 22
A literature survey by Newman et al. (2009) examined the effects of TiO 2 and ZnO sunscreens as reported in 15 peer-reviewed investigations (Newman et al., 2009). Experiments were carried out on human or pigs skin in vitro. For TiO 2 , none of the investigations showed any significant particle translocation below the stratum corneum, which is the outermost skin layer also known as the horny layer. ZnO nanoparticles were found to penetrate a little, at levels less than 1% of the applied dose. Combinations of TiO 2 and ZnO did not penetrate significantly either. Contrasting results have been seen when nude mice and pigs were topically exposed to TiO 2 particles ranging from 4 to 90 nm in size. Repeated exposure of pigs for 30 days showed that both 4 nm (hydrophobic anatase) and 60 nm (hydrophobic rutile) TiO 2 nanoparticles were able to cross the stratum corneum and recovered in the lower epidermis. A 60 day repeated exposure of nude mice to TiO 2 nanoparticles resulted in uptake of titanium in major internal organs, including brain. Pathological changes were also observed in organs, especially the skin and the liver which showed indications of oxidative stress (Wu et al., 2009). An Australian research team investigated whether or not zinc or zinc oxide could be released from a sunscreen preparation after dermal application. The researchers prepared two formulations, one containing 19 nm sized ZnO particles and one containing >100 nm sized ZnO particles. Both particle types contained radioactive 68Zn as a tracer. The subjects were then exposed to natural sunlight and were free to conduct activities during the day. After dermal exposure it was found that the majority of the Zn was not absorbed by the skin, as determined by measuring radioactivity in urine and blood. A higher proportion of Zn was seen in females exposed to the 19 nm sized particles, as compared to males receiving the same treatment and as compared to both genders treated with >100 nm sized particles. The absorbed dose was 0.001% of the dose applied, which is very low, but still indicative that metal ions can be absorbed. The authors were not able to determine if the absorbed Zn was taken up as free Zn ions or as ZnO particles (Gulson et al., 2010). 23
One study on human dermal exposure to carbon nanotubes was made in 2001(Huczko and Lange, 2001). In this study, 40 volunteers were subjected to a patch that had been soaked in a carbon soot containing a high proportion of CNTs. The results from this study were negative, with no visible irritation or skin changes. The main critique of this study is that the authors did not quantify the amount of CNTs in the soot, which makes comparison with other studies impossible. CNTs are known to induce reactive oxygen species and inflammation in animal models of skin irritation. In the human study no investigations were made on skin thickness, inflammatory markers or on oxidative stress. Quantum dots have been assayed for their ability to cross pig skin. In one study researchers found that QD621, a nail-shaped cadmium/selenide core quantum dot with a cadmium sulfide shell coated with PEG, was unable to cross isolated pig skin but induced cell death in cultured human keratinocytes. This could mean that if quantum dots cross injured skin, they could possibly exert toxic effects on humans (Zhang et al., 2008). The likelihood of skin exposure to quantum dots is however rather limited as compared to TiO 2 nanoparticles for instance where exposure is intentional.
Oral exposure
The effects of oral exposure of ENPs have not been well studied. Most articles within this area are related to drug delivery and few have examined the acute oral toxicity of ENPs. Yang et al. (2010) synthesised ultra-short SWCNTs and delivered them orally to mice in order to investigate their therapeutic potential for treatment of Alzheimers disease (Yang et al., 2010). They found that the ultra-short SWCNTs could translocate from the gastrointestinal tract to the blood stream and be taken up into neurons in the brain. Similar studies were carried out by Kolosnjaj-Tabi et al. (2010) where mice where fed oral doses up to 1000 mg/kg b.w. without exhibiting any adverse effects (Kolosnjaj-Tabi et al., 2010). In contrast to this, Folkmann et al. (2009) found oxidative damage to DNA in liver and lungs after a single oral dose of 0.064 mg/kg b.w. of SWCNTs (Folkmann et al., 2009). 24
TiO 2 ENPs have been thoroughly studied in dermal exposure experiments but not to the same extent after oral delivery. Wang et al. investigated the uptake of 25 and 80 nm sized TiO 2 following treatment with 5 g/kg in mice and found translocation to major organs two weeks after exposure (Wang et al., 2007). Liver inflammation and increased serum levels of biomarkers for cardiac injury were also observed. A reservation to the relevance of the study was raised by Johnston et al., as the dose of 5 g/kg b.w. is extremely high and would not likely reflect that of any human exposure to nanosized TiO 2 (Johnston et al., 2009). Repeated oral dosage during 14 days of silver nanoparticles on the size range of 22-71 nm at a dose of 1 mg/kg b.w. to mice led to translocation of silver nanoparticles to brain, lung, liver, kidney and other organs. Larger particles bearing a size of 323 nm did not translocate from the gut to organs under investigation. Silver nanoparticles with a diameter of 42 nm at repeated doses from 0.25 -1 mg /kg b.w. during 28 days led to negative effects on kidney and liver in mice (Park et al. 2010). A similar study for 28 days using Sprague-Dawley rats given an oral dose of 30, 300 or 1000 mg/kg b.w. of 60 nm sized silver nanoparticles indicated in contrast that doses over 300 mg/kg b.w. resulted in slight liver damage (Kim et al., 2008).
Summary and future research needs
There is a debate going on as whether or not ENPs are able to cross the dermal barrier. As it is unlikely that the majority of ENPs would be able to cross the barrier due to their size, it is possible that small effects of ENP exposure in the short term could have larger impact in the long term. The importance of conducting repeated exposure experiments should be stressed, simply giving a high single dose (bolus dose) does not mimic effects of long term chronic or subchronic exposure. Secondly, crossing of particles may not be the most important toxic impact; release of metal ions or contaminants which can migrate across the dermal barrier may have equally important effects. Local reactions in the skin such as oxidative stress could potentially also have effects, e.g. carbon nanotube treatment has been shown to cause oxidative stress in cultured skin (Murray et al., 2009). Such investigations are however scarce. 25
Human exposure
There is an immense gap in the knowledge of how ENPs affect human health. There are three major routes of exposure and uptake of ENPs. The first is inhalation, the second is by ingestion and the third is dermal uptake. A fourth, but less explored exposure route is uptake by the olfactory nerve (Oberdorster et al., 2005). Inhalation can be considered the major route of exposure, as ENPs are likely to become airborne. Ingestion of ENPs could occur if food is contaminated e.g from food packaging containing ENPs or by swallowing of ENP containing mucus from the mucociliary elevator. Dermal exposure is not likely to be a major route of exposure, as the stratum corneum of the epidermis is considered and effective diffusion barrier to particulate material. The sweat glands could however pose a port of entry in certain cases (Crosera et al., 2009).
Workplace emission of ENPs
Studies on the exposure of workers towards ENPs have been reported in a few cases. Measurements on carbon nanotubes exposure levels have been reported both for SWCNT and MWCNTs, both at manufacturing sites and in laboratories were carbon nanotubes were handled. Maynard et al. (2004) did some of the first measurements on workplace exposure to carbon nanotubes. They examined exposure to SWCNTs at four different production sites by personal sampling for 30 min. The exposure was found to be between 0.7 and 53µg/m 3 . Analysis of particle morphology showed a compact structure. Dermal exposure was assayed by examination of protective gloves used by the workers. Approximately 0.2-6 mg was deposited per glove (Maynard et al., 2004). Determining dermal exposure is important as it allows for an extrapolation to experimental dermal exposure in animal models or in cell culture.
Consumer exposure
Little attention has been put to quantify the consumer exposure to ENPs. The studies that do exist have examined effects of sun screens, looked at release of silver from silver nanoparticle treated garments and utensils and in one case the release of aerosolised TiO 2 from a bathroom cleaning agent. Release of silver from fabrics has been examined following washing of ENP endowed fabrics. This was done by first examining the 26
dissolution of silver ENPs used for fabric treatment, followed by actual washing of the fabrics. It was found that the released silver varied from 1.3 to 35% of the original silver in the fabric and was dependent on the type of binder system (Geranio et al., 2009). Chen et al (2010) showed in an experiment conducted at NIOSH facilities, that a commercial spray can product containing TiO 2 produced respirable nanoparticles (Chen et al., 2010). The intended usage of the spray was cleaning and sanitizing bathrooms. The nanoparticle fraction was 170 µg/m 3 , equivalent to1.2×105 particles/cm 3 , with a mean particle diameter of 75 nm. The researchers estimated the dose to 0.075 µg TiO 2 per m 2 alveolar epithelium and minute. Levels for rats were determined to 0.03 µg. This was one of the first reports to show that a commercial product can create respirable nanoparticles and would serve as a basis for further toxicological studies on respirable TiO 2 particles. Similar studies on Ag-nanoparticles released from consumer spray products were done by Hagendorfer et al. who compared pump spray and gas spray formulations in an experimental setup using a scanning mobility particle sizer in combination with TEM analysis. The pump spray did not cause any measurable release of Ag nanoparticles, while the gas driven spray caused significant release of Ag nanoparticles (Hagendorfer et al., 2010). Thus, different formulations in consumer products can cause different exposures. In order to better assess any risk associated with exposure to ENPs, the release of ENPs from composites, fabrics, paints etc should be determined. For consumer products, such as fabrics, very little is known. Kulthong et al. examined the presence, release and antibacterial effects of commercial and laboratory prepared nanosilver containing fabrics. The authors found that some fabrics claiming to contain nanosilver, contained no silver at all. The efficacy of fabrics to exert antibacterial effects varied from 0% to more than 99%. Using an artificial sweat preparation, as to mimic a real exposure situation, the release of silver was determined to vary from 0 mg/kg to 322 mg/kg of fabric, depending on pH and fabric (Kulthong et al., 2010). Friends of the Earth issued a consumer report in 2009 entitled “Manufactured Nanomaterials and Sunscreens: Top Reasons for Precaution”, where the concern for nanosized particles in consumer products was raised. However the references made in the report on nanoparticle translocation across the skin did not concern TiO 2 or ZnO nanoparticles, but rather quantum dots. This is an example which shows the problem in comparing biological 27
effects of nanosized particles without taking into account the differences in particle physicochemical properties (Friends of the Earth, 2009).
Biomarkers of exposure to ENPs
In order to assess if clinical signs of a patient are related to ENP exposure, the development of biomarker assays are of high relevance for nanosafety. Few studies have been conducted in this area at the moment. Higashisaka et al. examined the effects of commercial silica ENPs with diameters ranging from 30 to 1000 nm and one functionalized silica ENP bearing either carboxyl and amino groups. (Higashisaka et al., 2011). Particles were intravenously injected in mice and made a proteomic profiling of the plasma proteins. This resulted in significantly higher plasma levels of haptoglobulin for mice treated with 30 or 70 nm particles, while not for those treated with larger diameter particles. The levels were significantly higher up to 3 days after injection. The proteins c-reactive protein (CRP) and serum amyloid A (SAA) also increased in a similar manner. In contrast, he functionalized particles did not elicit a significant increase of haptoglobulin. Further, 30 nm particles which were intranasally instilled in mice led to significant increases in the plasma levels in all three proteins, while the larger 70 nm particles did not cause any effect. These results indicate that a combination of these three putative markers could be used for assessing exposure, but it should be borne in mind that other diseases and afflictions could cause similar increases. Expressed in renal carcinoma (ERC)/mesothelin is a protein that can be used as biomarker for mesotheliomas in humans. Sakamoto et al recently showed that it can also be used as a biomarker for mesothelial proliferative lesions in rats after intrascrotal injection of multi-walled carbon nanotubes (Sakamoto et al., 2010).
Summary and future aspects
It is clear that exposure to ENPs can occur in the working environment, but that protective clothing and devices are capable of limiting the personal exposure. Different operations during work will evidently lead to an extremely high exposure, such as packing and transfer of ENPs. 28
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Effects of long term exposure to ENPs
As could be expected, long term data on ENP exposure is scarce. Following a single intraperitoneal injection of carbon nanotubes, rats were studied for a period of two years. The aim of the study was to investigate the carcinogenicity of carbon nanotubes, e.g. if they can cause mesothelioma. The results from the study were negative but not conclusive as the length of the tubes might not have been sufficiently long in order to qualify for the fibre definition and was not comparable to that of cancerogenous asbestos fibres (Muller et al., 2009). Only two studies on repeated long-term pulmonary exposure to carbon nanotubes exist so far, one by Ma-Hock et al. (Ma-Hock et al., 2009) and one by Pauhlun (Pauluhn, 2010). Both studies investigated repeated exposure for MWCNTs on rats during a 90 day period. Both studies found inflammatory cells in the lungs and different degrees of lung fibrosis. However, neither of the studies found any systemic effects nor any signs of mesothelioma in the airways. Ma-Hock et al. (Ma Hock et al., 2009) used well dispersed MWCNTs, while the MWCNTs used by Pauhlun (Pauluhn, 2010) were never characterised as individual tubes but rather found as agglomerates both during aerosolisation and after recovery from the lungs of the experimental animals. Pauhlun (Pauluhn, 2010) concluded that the toxicity was not due to the fibre-like characteristics of single MWCNTs but instead due to particle overloading. The lowest concentration that gave a biological response was that of Ma Hock et al. where as little as 0.2-0.3 mg/kg b.w. caused an inflammatory response (Ma Hock et al., 2009) 30
New technologies for ENP detection
A major difficulty in nanotoxicology and nanosafety is to detect and image nanoparticles in biological matrices. A biological matrix may be a tissue from an animal or a plant, a sample of earth or a food specimen. Detection of nanoparticles is usually done by using electron microscopic techniques, such as TEM or SEM. Fluorescence microscopy, e.g. confocal microscopy, can be employed if the particles can be excited using an appropriate wavelength of light. Quantum dots can be detected using fluorescence microscopy and have found wide applications in bioimaging, either as free particles or used to label antibodies and other biomolecules. Other nanoparticles can be detected by labelling with a suitable fluorophore. One drawback of such labelling is that the addition of a fluorophore may influence the biological activity of a nanoparticle. The same is true if a nanoparticle is functionalized with a chelating molecule and then labelled with a radioactive isotope, e.g. In111 (Singh et al., 2006), which may cause the particle to behave differently from a pristine nanoparticle. Carbon nanotubes have been detected in animals by radioactive labelling, by TEM and by elemental analysis. As carbon nanotubes quite often contain detectable amounts of trace metals such as Ni and Co, the presence of these metals in tissues are indicative of the presence of carbon nanotubes. Using a combination of both TEM and elemental analysis (STEM-EDX) can allow more precise detection and validation of ENP presence in a biological matrix, e.g. carbon nanotubes instilled in a rat lung (Elgrabli et al., 2008). Another issue is to characterize interactions between nanoparticles and biomolecules in situ, that is directly in the tissue or biological matrix under investigation without any isolation steps. Techniques for doing this include soft x-ray microscopy and related synchrotron radiation techniques (Wang et al., 2010a). Pascolo et al (2011) used synchrotron soft x-ray imaging to study asbestos in a sample of human lung tissue (Pascolo et al., 2011). The method allowed the authors to chemically dissect the asbestos bodies found in the samples and they found that levels of silicon were highest in the asbestos particles, while the shell of the fibre and the closest surrounding tissue were higher in Fe, Mg and O. Based on these findings and other experiments, they could conclude that both Mg and Fe were part of the mechanisms which lead to 31
formation of asbestos bodies. Although the particles under investigation were not ENPs and not nanosized, the methodology describe could be implemented when investigating tissue reaction to ENPs and the mechanisms of how different ENPs induce toxicity on an elemental level. 32
Nanosafety and food technology
There has been an increased interest for applications of nanotechnology in the food industry in recent years. The applications of nanotechnology, i.e. nanoparticles, in food technology include food safety such as packaging, sensors for detection of pathogens or food spoilage, encapsulation of flavours or nutrients in nutraceutical applications, in novel additives and during food production (Bouwmeester et al., 2009; Das et al., 2009; Restuccia et al., 2010). The use of nanoparticles in the food industry is clearly not only an issue for occupational hygiene, but also an issue for consumer safety. Using nanoparticles in food applications has been judged to pose a likely risk for human exposure (Bouwmeester et al., 2009). In order to assess such risks for exposure, there is a need to develop techniques to find and characterize ENPs in food. Technologies for assessing nanoparticle contents in food include electron microscopy and x-ray microscopy used in conjunction with separation techniques such as field-flow fractionation and chromatography. These techniques are described with in-depth detail by Blasco and Picó (Blasco and Pico, 2011) which also includes an extensive list of products on the market which allegedly incorporates nanoparticle technology. Food applications of ENPs are also an important part of life cycle analysis for ENPs as nanotechnology does evidently come into many of the different steps in food productions. In agriculture ENPs can have applications on water and soil cleaning. In food processing nano-coatings and sieves can be used to prevent biofouling and to filter bacteria. Applications in packaging serve to prevent bacterial growth by nanostructured coatings for instance (Bouwmeester et al., 2009). Examples of coatings are silicate ENPs, composites, Ag and ZnO ENPs. Nanoparticles could also be used to improve bioavailability of substances of nutritional value, in a similar way to that of nanoparticle based drug delivery (Bouwmeester et al., 2009). Examples of ENPs used in or suggested for use in food applications are metal/metal-oxide ENPs are Ag, ZnO, Cu and TiO 2 , as well as nanosized lipid and polymer based delivery systems. The two latter types of nanoparticles are intended to degrade upon consumption and release factors 33
with nutritional benefits. Ag-nanoparticles have been incorporated into foams and used as filters to reduce bacteria (Jain and Pradeep, 2005). The toxicological effects of nanoparticles employed in food technology are not well characterized. The genotoxicity of chitosan-poly(methyl methacrylate) (PMMA) composite nanoparticles for use in food packaging films in order to improve food preservation was examined by de Lima et al (De Lima et al., 2010). Chitosan is accepted for use as an additive in food in some countries, including Japan, and has applications in the pharmaceutical industry (Illum, 1998). The particles used in the study were 60, 82 and 111 nm in diameter and concentrations ranged from 1.8, 18 and 180 mg/ml. No DNA damage was seen, but 82 and 111 nm particles reduced the mitotic index at the highest concentration. The smallest particles, with a diameter of 60 nm did not cause any significant effects on gentoxicity. These results suggest that if chitosan composite nanoparticles are released from food packaging, the genotoxic effects are limited (De Lima et al., 2010) However, studies such as these should be complemented with animal experiments. Chorianopoulos et al. applied nanosized TiO 2 particles to metal and glass surfaces as a means of photosterilization. They found that Listeria monocytogenes biofilms were significantly reduced on surfaces that had been coated with TiO 2 nanoparticles and then subjected to UV-A radiation. The authors proposed that such coatings could be used to improve food safety with an antimicrobial perspective (Chorianopoulos et al., 2011). The potential release of nanoparticles from such coatings should be studied from the nanosafety perspective, since release of TiO 2 nanoparticles could potentially influence human health. The biological effects of TiO 2 nanoparticles after oral intake are very limited and the studies that exist have mainly shown negative effects on internal organs (e.g. liver and kidneys) at doses of 1 or 5 g/kg body weight in rats and mice respectively (Bu et al., 2010; Wang et al., 2007) Treatment with lower doses such as 0.16 and 0.4 g/kg bodyweight led to increases in serum biomarkers of liver injury and cytotoxicity markers (Bu et al., 2010). 34
Other issues in food nanosafety that need to be addressed include whether or not ENPs used in food production and processing could carry unintended biomolecules with them on their passage from the gut lumen into the body (Das et al., 2009).
Summary, knowledge gaps and research needs
According to Bouwmeester et al. (Bouwmeester et al., 2009) important issues in food nanosafety include development of analytical tools for detection of ENPs, studies on oral uptake and behaviour of ENPs related to food production and identification of products on the market which may contain ENPs as well as setting up policies and regulations on this matter. As a consequence of the lack of knowledge, the European Union issued two regulations in 2004 and 2009 which address some of the problems of food nanosafety and food packaging specifically (Restuccia et al., 2010). Not all nanoparticles or nanosized materials used in food technology are associated with possible toxic risks. Nanoparticles made from the maize protein zein have found applications in food packaging and as carriers of flavouring and dietary supplements (Srinivas et al., 2010). 35
Environmental and ecotoxicogical aspects of nanosafety
The majority of research within nanotoxicology is focused on cytotoxicity and animal studies. The impact of nanomaterials and specifically engineered nanoparticles on the environment and populations needs to be assessed with similar effort. The antimicrobial effects of certain nanoparticles, e.g. silver ENPs, which are desirable when applied to kitchenware, utensils and to clothes may have totally undesirable effects on microbes in an ecosystem. The propagation and accumulation of ENPs through food chains/food webs are of an environmental concern. The effect on plants is largely unknown, a Web of Science search on “plant uptake of nanomaterials gives only 50 hits. Modelling of the possible adverse effects of three different kinds of nanoparticles was made by Muelller and Nowack (2008). They made risk assessments based on two scenarios one realistic and one high-exposure worst case scenario. Predictions were made for Ag NP, TiO 2 NP and carbon nanotubes (both SWCNTs and MWCNTs), based on litterature findings of production, use and flows of nanoparticles. They found that the risk quotient, that is the predicted environmental concentration in relation to the predicted no effect concentrations, for Ag NP and carbon nanotubes was less than one which suggest that there would be no reason to expect any adverse effects from these particles. On the other hand, values for TiO 2 NP indicated that the effects of such particles on the environment should be further studied. As this is only a model, the results must be interpreted with caution. This particular model was based on circumstances specific for Switzerland. This meant that flows from sewage treatment plants to landfills and soil was excluded, as waste sludge application to soil in Switzerland is forbidden and the sludge is incinerated.
Adsorption of biological materials to ENPs
Just as the adsorption of proteins to ENPs in animals or cell culture forms a protein corona which determines their effects, ENPs in the ecosystem will also associate with other materials. Carbon nanotubes are considered a reactive carbonaceous material and concerns have been raised on their impact on both humans and environment. Studies were made on oxidized carbon nanotubes which had been stored in fresh water for 2.5-7 36
years and in PBS or natural organic matter (NOM) for up to 120 days. The purpose of this treatment was to investigate how environmental parameters could affect toxicity. It was found that storage in fresh water did not affect the toxicity of the nanotubes, they were still as toxic as freshly pepared CNTs. In contrast, when nanotubes were stored in PBS or NOM, the toxicity was reduced to control levels (Panessa-Warren et al., 2009). Investigations like these provide useful information on how to reduce biological impact of ENPs. In a meta-analysis of available literature, Kahru and Dubourguier investigated the LC50 and EC50 (L(E)C50) values reported for ENPs in the available literature. The ENPs included the traditional spectra e.g. C60 fullerenes, carbon nanotubes, TiO 2 and Ag. They found 77 reported values and arranged the nanoparticles under investigation according to their median L(E)C50 on relevant species in representative food chains. From their analysis made on this basis they concluded that the most harmful ENPs would be nanosized Ag and ZnO with L(E)C50 less than 0.1 mg/l, followed by C60, CuO, SWCNT, MWCNT and TiO 2 in that order of decreasing harmfulness. Nanosized Ag and ZnO were considered as “extremely toxic” (Kahru and Dubourguier, 2010) This type of meta-analysis is useful as it provides a basis for assessment of possible ecological impact and ecotoxicity of different ENPs. However, it should be noted that merely using mass as a dose-metric when comparing the toxicity of different ENPs can give a false impression of the relative toxicity of particles. Much indicates that it is not that simple (Warheit, 2010). In order to handle the impact of ENPs on society and the environment in the near future, nanowaste management must be considered since waste is thought be an important source of environmental contamination and dissipation of ENPs (Musee, 2011). The environmental impact of nanowaste does not necessarily implicate that the ENP cause a direct effect on organisms and populations, there is also a possibility that the ENPs carry with them other pollutants that cause biological effects in an unexpected manner as compared to that of the pollutant only. 37
It has been shown that polycyclic aromatic hydrocarbons (PAH) can be absorbed by ENPs. Yang et al. (Yang et al., 2006) showed that SWCNTs, MWCNTs and fullerenes could adsorb PAHs including pyrene and naphtalene. Carbon nanotubes were potent adsorbers of PAHs which could lead to environmental effects not only depending on the inherent toxicity of the carbon nanotubes but also due to adsorbed pollutants. The same authors presented data in a subsequent publication in 2007, where they showed that adsorbed PAH could desorb from carbon nanotubes and fullerenes in water. The authors suggested that this could lead to the release of PAHs should an organism be exposed to carbon nanomaterials carrying PAH pollutants (Yang and Xing, 2007). The adsorption of substances such as pollutants to carbon nanomaterials does not necessarily have to be a negative event, and has been proposed to be something that could be exploited in order to remove pollutants from aqueous environments or from gas phase (Ren et al., 2010). The most prominent carbon nanomaterials described as having adsorbent properties are carbon nanotubes, which according to the review are able to bind divalent heavy metals including Cd and Pb and aromatic compounds.
Effects of ENPs on plants and crops
The impact of ENPs on plant growth is not well characterized. Future contamination of plants and crops with ENPs is a scenario with increasing probability, with ENPs released into the atmosphere or into ground water. Applications of nanomaterials and nanoparticles in agriculture are being found, e.g. as pesticides or as carriers of pesticides. A recent review of ENPs and their interactions can be found in Ma et al. (2010). A study was conducted on deposition and uptake of cerium dioxide (CeO 2 ) nanoparticles in maize plants. CeO 2 nanoparticles were generated using flame spray pyrolysis, which led to nanoparticles with a mean size of 37 nm and a surface of 110 m 2 /g. Exposure of maize plants was done by aerosol generation in a controlled environment or by irrigation of plants with water containing CeO 2 nanoparticles. The CeO 2 nanoparticles were deposited onto the leaves of the maize plants, and significantly more when plants were exposed to light. Significant amounts of CeO 2 nanoparticles were adsorbed to the leaves, as evident after washing the leaves. However, no translocation of nanoparticles was observed after aerosol exposure. The irrigation experiments indicated no uptake or accumulation of nanoparticles and most of the CeO 2 38
was detected on the surface of the soil. This suggested that the soil functioned as a filtration mechanism of nanoparticles, and all together the results implicated that plants were more resilient to nanoparticle exposure and uptake as compared to mammalian systems (Birbaum et al., 2010). When soybeans exposed to CeO 2 and ZnO nanoparticles (7 nm and 8 nm in size), slightly different results were seen. The effect on seed germination and growth, as well as uptake of nanoparticles was investigated. Neither of the particles affected germination but genotoxic effects of CeO 2 nanoparticles was observed. Elemental analysis of plant roots revealed that CeO 2 nanoparticles were taken up by the plants while ZnO nanoparticles were not (Lopez-Moreno et al., 2010). Tomato seed treated with carbon nanotubes during germination were observed to exhibit increased rate of germination and plant growth with increasing concentrations of carbon nanotubes, without any observable negative effects on plant integrity. The authors of the study suggested that the observed phenomena could be exploited in agricultural application. The proposed mechanism was that carbon nanotubes were able to penetrate the seeds and thus increased the uptake of water into the seeds (Khodakovskaya et al., 2009). Should nanoparticles like as carbon nanotubes find their way into agricultural and horticultural applications, the risk of human or animal exposure will surely increase and studying the effects of oral administration of ENPs will become more urgent.
Food chain transfer of ENPs and biomagnification
Judy et al. (Judy et al., 2011) showed in an experimental set up that gold nanoparticles in the size range of 5-15 nm were able to accumulate in a terrestrial food chain, from a primary producer to a primary consumer. Tobacco plants, Nicotiana tabacum, were seeded and after 4 weeks placed in test tubes with 100 mg/ml of gold nanoparticles. Hornworms, the larvae of Manduca sexta, were then allowed to grace on plants previously exposed to gold nanoparticles. Gold could be detected in dried tobacco leaves and in the gut of the hornworms. Calculations showed that total gold (bulk gold) accumulated in the hornworms relative to the tobacco leaves and that 10 and 15 nm gold particle treatments led to higher bulk gold accumulation in the worms. The total number of gold nanoparticles which accumulated in the hornworms did not depend on the particle size, and the number of particles was significantly higher than in the tobacco 39
leaves (Judy et al., 2011).Similar transfer has been shown for TiO 2 nanoparticles in a simple food chain too, although bioaccumulation of nanoparticles was not observed. Daphnia magna were exposed to 0.1 or 1 mg/ml of TiO 2 nanoparticles with a mean size of 21 nm. After exposure the D. magna were transferred to tanks with zebra fish, Danio rerio which had been trained to eat D. magna. Analysis of the zebra fish and the D. magna revealed that TiO 2 nanoparticles taken up and biomagnified in the D. magna and transferred to the zebra fish, without biomagnification (Zhu et al., 2010). The uptake of quantum dots (QD) was studied in an invertebrate food web using bacteria (E. Coli), ciliates (Tetrahymena pyriformis) and rotifers (Brachionus calyciflorus). The QDs were ellipsoid, with major axis of 12 nm and a minor axis of 6 nm and had a core made up from CdSe and a shell of ZnS. Two different surface chemistries were used, one carboxylated and one biotinylated. Accumulation of QDs in bacteria was not observed, probably due to the surface chemistry and hydrodynamic diameter of the dots. Ciliates were found to accumulate QDs with biotinylated QDs being twice as biopersitent as carboxylated QDs. Transfer of QDs to rotifers which preyed on ciliates was observed, but without any biomagnification (Holbrook et al., 2008). In another similar simplified food chain, Pseudomonas aeruginosa bacteria were exposed to CdSe QDs. QDs accumulated in the bacteria which were then preyed upon by the protozoan Tetrahymena termophila. Biomagnification of the QDs at a five-fold level were observed in the protozoan which had ingested bacteria containing QDs. In addition, it was observed that protozoans had difficulties in digesting QD laden bacteria which eventually resulted in reduced growth of protozoan. The cellular integrity of the protozoans remained intact however and the majority of the QDs found inside the protozoans were intact. The observations of reduced growth indicate that QD could impact ecosystems at an early producer/consumer level (Werlin et al., 2011). The biopersistency of ENP is a critical issue when assessing associated risks. Some ENPs are designed to degrade, e.g. particles intended for drug delivery while for particles intended for enhancing material properties degradability would not be expected to be a desired property. Kummerer et al. (Kummerer et al., 2011)investigated the degradability of various nanoparticles in an aqueous environment. They found that nanocrystals of starch and cellulose were more biodegradable than the corresponding 40
bulk material, while fullerenes and carbon nanotubes were not biodegradable at all. The stability of carbon nanotubes was studied by Liu et al (Liu et al., 2010) who found that surface functionalisation could influence bioperistency. Carbon nanotubes with carboxyl groups attached to their surface could be degrade over time in simulated phagolysosomes, while nonfunctionalized tubes were inert to such degradation. Similarly Kagan et al. showed that carbon nanotubes functionalized with antibody fragments could be degraded by neutrophiles, a cell type which normally serves important functions in clearing particulate materials from the lungs (Kagan et al., 2010).
Life cycle assessment of ENP
Life cycle assessment or life cycle analysis (LCA) is a technique by which the impact of a process or product can be assessed. The impact that can be evaluated include environmental, economical and societal. As an example, the energy expenditure of methods for producing TiO 2 nanoparticles has been examined (Grubb and Bakshi, 2010). A life cycle assessment of socks containing silver nanoparticles suggested that the user phase of the product, e.g. washing, had a stronger impact than that of the production phase (Meyer et al., 2011). Healy et al (2008) investigated the environmental impact of three different carbon nanotube production methods using LCA. When taking into account both energy expenditure and chemicals employed in the production, the method which had the least environmental impact under normal production condition was high-pressure carbon monoxide (HiPco), while if using best case yield conditions the arc technique had the lowest impact. Köhler et al (2008) investigated two case studies where carbon nanotubes were involved. One case was related to the application of carbon nanotubes in Li-ion batteries, which is a growing area where the use of carbon nanotubes is expected to grow, and the other related to the application of carbon nanotubes in textiles. The main purpose of this study was to investigate the points during a products life cycle where carbon nanotubes could potentially be released. They found that the knowledge on where release of carbon nanotubes is possible during a products life cycle is limited and urged further research in the area. For Li-ion batteries, the nanotubes are part of a matrix which is porous but the matrix is enclosed in casing which makes release during usage unlikely. For fabrics the nanotubes are bound more strongly to a polymer matrix but wear and tear, such as mechanical stress and exposure 41
to UV-light could lead to release of nanotubes during usage. The authors suggest that there is a potential for product and application developers to take into account the potential release points in the life cycle, especially since there are few products on the market (which was the case in 2008). Recycling of products is also mentioned as a possible risk of occupational exposure.
Summary and future research needs
There is evidence that nanoparticles can be transferred from producer to consumer in a food web/chain, albeit in very simplified models. Data for populations of e.g. mammals is not available. As the organisms used in these studies, bacteria, protozoa and hornworms, serve as food for other organism, such as mammals, it is very plausible that biomagnification on basal levels will lead to detrimental effects on higher levels in a food chain. The current understanding on the biological effects after consumption of nanoparticles is limited. All together, results such as this show that further studies on bioaccumulation/magnification are warranted. 42
Actors within nanosafety and related fields Sweden
Lund – Malmö
Governmental organisations Arbetsmiljöverket Malmö
Mats Ryderheim Lena Lindskog Monica Björk mats.ryderheim@av.se
lena.lindskog@av.se monica.bjork@av.se
Lund institute of technology (LTH) Dept of Fire Safety Engineering and Systems Safety /LUCRAM
The department of fire safety engineering at LTH are conducting highly competitive research within risk management. Not currently active within nanosafety at the moment, but could be of assistance with risk management and policies. http://www.lucram.lu.se/
Lund University Biomedial Polymer Technology, Dept of Experimental Medical Science
Researchers Henrik Kempe and Maria Kempe are developing magnetic nanoparticles of various kinds that could be used for treatment of thrombosis related to implanted stents. Their experience in particle synthesis and biocompatibility studies could be of synergetic effects to the NanoSafety cooperation. http://www.biomedicalpolymers.bmc.lu.se/Index.html
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MaxLab/ESS
The use of synchrotron radiation in nanotoxicological investigations has been suggested to be a new and emerging technology. Synchrotron radiation facilities are already present in Lund and Maxlab IV and ESS will be ready in the future. This technology is expensive and limited beam line time will be available. Thus, it could be of a strategic value to realise and assess the potential of this technology for nanosafety on an early scale. Synchrotron techniques could allow investigations of ENP interaction with biomolecules in situ in both cells and tissues.
Companies Genovis
Genovis have developed magnetic nanoparticles, called NIMT FeOdots. The particles can be used for drug delivery and can be functionalized with a variety of chemical entities. The particle size is 10 +/- 1 nm. Genovis currently markets their products in several countries. www.genovis.com
Camurus
Camurus focuses on drug delivery using FluidCrystal® nanoparticles, for oral, dermal and injection delivery. The nanoparticles are based on lipid membranes. www.camurus.com
Entomopharm
Entomopharm is a Danish biotechnology company based at the Bioincubator at IDEON/BMC. Entomopharm has developed models for drug transport over the blood brain barrier (BBB) using locusts as an experimental system. This technology could be useful for screening and modelling how nanoparticles interact with the BBB or with cell membranes. www.entomopharm.com
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SPAGO Imaging AB
Makes contrast agents for MRI detection of tumours by using nanoparticles based on gadolinium oxide. SPAGO Imaging is based in Lund. www.spagoimaging.se
Stockholm
Governmental organisations and projects The Institute of Environmental Medicine
Recruiting three post doc/ PhD students during Autumn 2010, in mathematical modelling of nanoparticle toxicokinetic behavior. www.ki.se
NANOMMUNE
An FP7 sponsored cooperation between researchers at KI, UU, KTH in Sweden, with European partners including EMPA in Switzerland and NIOSH in the US. The focus of this project, which will finish in 2011, was to elucidate the effects of nanomaterials on human health with the immunesystem in particular. Specific efforts were put on material characterization. Coordinated by Prof. Bengt Fadeel at KI. http://ki.projectcoordinator.net/~NANOMMUNE
The Swedish Defence Research Agency (FOI)
In 2009, FOI received a FORMAS grant in cooperation with Umeå University and Uppsala University for a project on the health risks on nanoparticles. The focus was on metal oxide particles. At this time, FOI had been given a total of 8 million SEK in grants to conduct research within this area. www.foi.se
45
The Swedish Chemical Inspectorate (Chi / KemI)
www.kemi.se
Companies Nanologica
Nanologica is a Stockholm based company which specializes in synthesizing nanoporous materials, which are less than 100 nm in size. The materials have industrial applications from pharma to photovoltaics. Nanologica participates in the FP7 sponsored EU project NanoSustain. http://www.nanologica.com/index.php
Kalmar
Kalmar University
Within the Department of Chemistry and Biology, the Nanosciences group conducts research within biology and nanoscience. According to their webpage they work with nanoparticles. http://www.kob.hik.se/nanoscience/research/index.php
Gothenburg
University of Gothenburg - Nanoparticles in Interactive Environments
According to the report The Nano Guide 2010 published by Nano Connect Scandinavia, this research platform was a collaboration between 2006-2010 between the Department of Physics and Chemistry at the University of Gothenburg. Interesting partners within this collaboration would be the Atmospheric science group, Gothenburg Atmospheric Science Centre, Marine Chemistry, Nano Toxicity Göteborg Science Centre for Molecular Skin Research Centre for Environment and Sustainability. The work has recently resulted in the FORMAS funded collaboration GU Nanosphere, from 2010-2015. The effort in this project is to give a total science perspective of the impact of nanoparticles on humans, the environment and society. 46
GU NanoSphere
http://www.cefos.gu.se/forskning/Samhallets_riskfragor/nanosphere/ Åsa Boholm Martin Hassellöv asa.boholm@cefos.gu.se
martin.hassellov@chem.gu.se
Chalmers Energi och Miljö
http://www.chalmers.se/ee/SV/forskning/forskargrupper/miljosystemanalys/forskning/p rojektbeskrivningar/nanorisksv Björn Sandén Sverker Molander Rickard Arvidsson bjorn.sanden@chalmers.se sverker.molander@chalmers.se rickard.arvidsson@chalmers.se
Companies IMEGO
Gothenburg Uni spinn-off. Manufactures magnetic nanoparticles, as well as other nanotechnology related materials and devices. http://www.imego.com/Expertise/Electromagnetic-sensors/Magnetic nanoparticles/index.aspx
Linköping
Companies Exova
Based in Linköping, develops composite materials in cooperation with SAAB among others and have been reported to use carbon nanotubes. www.exova.se
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Piteå
Companies Swerea SICOMP
A Swedish research institute located in Piteå. Specializes in polymeric fibre composites. Uses carbon nanotubes in some applications. http://www.swerea.se/sicomp/
Denmark
Copenhagen university / Nano-Science Center
Centre for Pharmaceutical Nanotechnology and Nanotoxicology
The centre is designated to examine the relationship between nanomaterials and biological substances by structure-activity assessments. http://nano.ku.dk/groups/cpnn/ Contact: Moein Moghimi, momo@farma.ku.dk
NanoPhysics
Employes x-ray methods to study semiconducting nanoparticles and nanowires, as well as medical imaging. Contact: Robert Feidenhans’l
Nanotoxicology
This group conducts research on the health effects of nanoparticles. Belongs to the Dept of Environmental Health and is located at the Institute of Public Health. Contact: Steffen Loft 48
DTU
CINF- Centre for individual nanoparticle functionality
Explore and understand fundamental relations between surface morphology and reactivity on the nanometre scale. Contact: Ib Chorkendorff
Nanotechnology and Risk
Studies potential environmental risks of engineered nanomaterials. Uses a laboratory based approach and one desk-based approach to analyze and suggest solutions to regulatory and management solutions from nanotechnology. The aim is to promote sustainable development and reduce any adverse effects.’ Contact: Anders Baun http://www.env.dtu.dk/English/Research/Research%20Themes/Environmental%20Che mistry%20and%20Microbiology/Nanotechnology%20,-a-,%20Risk.aspx
DTU-food
http://www.food.dtu.dk/Default.aspx?ID=23670 Ulla Birgitte Vogel ULBVO@food.dtu.dk Katrin Löschner Niels Hadrup Erik Huusfeld Larsen kals@food.dtu.dk nilha@food.dtu.dk ehlar@food.dtu.dk Current known activities at DTU: During december 2010, DTU is recruiting one PhD student in ”Life cycle assessment of Nanomaterials” and one post-doc for risk assessement of nanomaterials. 49
National Research Centre for the Working Environment
http://www.arbejdsmiljoforskning.dk/Aktuel%20forskning/Nanoteknologi.aspx?lang=en Håkan Wallin hwa@arbejdsmiljoforskning.dk Keld Alstrup Peder Wolkloff (KU) Gunnar D. Nielsen kaj@arbejdsmiljoforskning.dk pwo@arbejdsmiljoforskning.dk gdn@arbejdsmiljoforskning.dk
Aarhus University
INANO
iNANO is the interdisciplinary research center at Aarhus university, Denmark. Research on nanoethics is being made: http://inano.au.dk/research/research-areas/nanotoxicology-and-nanoethics/nanoethics/ Research is also made in the area of food safety and nanotechnology: http://inano.au.dk/research/research-areas/nanofood/ The School of public health at the University of Aarhus and iNANO are working within a collaboration with Chinese scientists in the FP7 sponsored programme SIDANO. The project aims to assess the risk of engineered nanotechnology materials. The strategy is to use in vitro and in vivo methods to look at the toxic response of engineered nanomaterials, validate in vitro systems, look at molecular pathways involved in the toxic response to nanomaterials etc.
Companies LM Windpower
www.lmwindpower.com
LM Windpower produces wind power plants. LM Windpower employs a composite material known as Hybtonite in some of their construction. Hybtonite is a composite containing approximately 0.5% carbon nanotubes and is produced by Amroy Europe Oy in Finland. 50
Identified knowledge gaps
The purpose of this list is to provide a short overview of areas of nanosafety were there are knowledge gaps that could potentially be exploited for further applications for funding. Determination of relevant levels of exposure – for implementation in animal and cell experiments Determining the relevant dose metrics of nanosized materials: size, shape, particle number, surface area, mass etc. Toxicity of ENPs after having adsorbed other substances, Bio-persistency – how long will an ENP be present in an ecosystem or in the body of an animal. Can they decompose? Aging of ENPs and the effect of time and environmental factors on their toxicity Bioaccumulation of ENPs Biotransformation of ENPs – the environmental fate and the effect of biochemical processes on the shape, size and function of ENP is relatively unknown Release of ENPs from composite materials – during processing, machining, wear or aging of materials. Toxicity of composite materials containing ENPs Risk and life cycle analysis of ENPs and ENP containing materials 51
Structure-activity relationships for ENPs Effects on skin and topic allergy Recycling and waste handling of ENP and ENP containing materials Lack of human data – very few case reports on human exposure Aerosol formation from commercial products containing ENPs Consumer exposure Biomarkers for ENP exposure Finding reliable reference materials when doing toxicity screenings, both negative and positive controls. Long term inhalation studies on animals Synchrotron radiation technology for assessing ENP interactions with biological systems and materials Translation of actual exposure levels to an in vitro or in vivo situation / Translation of in vitro results into in vivo models Characterization of ENPs after cellular uptake/interactions Reproductive effects 52
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