Current progress and future perspectives in toxicology of nanomaterials.
The novel and exciting properties that emerge within materials when developed at the nanoscale (1-100nm) is driving the exploitation of engineered nanomaterials in diverse products. From their incorporation within sunscreens to their use as drug delivery devices and for environmental remediation human and environmental exposure to nanomaterials will inevitably increase.
Accordingly, nanomaterials are expected to become part of our daily lives (and probably are already), and whilst their use is associated with many financial and societal benefits, there is currently uncertainty surrounding the potential risks they may exhibit. The introduction of novel properties into nanomaterials, that are often not apparent in larger forms of the same material, leads to their desirability and exploitation. However, it also prompts concern regarding the potential toxicity of nanomaterials as it is not possible to predict nanomaterial toxicity based on knowledge of how their larger ‘bulk’ counterparts (ie the same constituent material but not on the nanoscale) behave.
An increase in the deliberate and unintentional exposure of humans and the environment to nanomaterials makes it particularly pertinent to understand the consequences of exposure before their more widespread use. A lack in confidence regarding the safe production and use of nanomaterials due to perceived or real safety concerns could prevent nanotechnology reaching its full potential. A greater understanding of nanomaterial risk will therefore allow for the responsible, safe and sustainable development of nanotechnology.
In order to understand nanomaterial risk, it is essential to understand both exposure to nanomaterials and the hazards they pose to human health and the environment. Research activity in the area of nanotoxicology is constantly expanding, and it is appropriate to reflect on what progress has been made. Within this article a detailed account of the research priorities and knowledge gaps within the area of nanotoxicology will not be provided. Instead, an overview of the common strategies used to ascertain the risks posed by nanomaterials will be presented that will discuss in more general terms issues that need to be resolved in the short and long term.
Physico-chemical characterisation
At the heart of nanotoxicology studies is understanding how the physico-chemical properties of nanomaterials relate to their toxicity. Nanomaterials share the commonality of being ‘small’. The small size of nanomaterials has prompted concern surrounding their potential toxicity, however other physical and chemical properties are key in defining their behaviour and these include, but are not limited to; surface area, morphology, charge, agglomeration and surface chemistry (figure 1). Due to the diversity of nanomaterials in use and production, it is difficult to make generalisations regarding the risks they pose to human health and the environment. It is therefore essential to understand the relationship between nanomaterial physico-chemical properties and their toxicity. This can promote the generation of structure activity relationships to predict nanomaterial safety which can enable a safe-by-design approach to be adopted for nanomaterials in the future, as well as informing the development of appropriate regulatory and control measures for nanomaterials, where appropriate. In order to achieve this an extensive characterisation of nanomaterials is required and must be conducted in parallel to hazard investigations. This is a challenging task as obtaining information on the physico-chemical properties of nanomaterials in their ‘as produced’ and ‘as tested’ (i.e. in biological media) forms can be restricted by methodological, financial, and time limitations. It is also very challenging to simulate nanomaterial fate and behaviour along their life cycle, particularly considering their forms when incorporated in real-life products.
Exposure Assessment
Exposure information for nanomaterials is urgently required for risk characterisation purposes. There is a lack of suitable methodologies and devices to monitor nanomaterial levels throughout their life cycle. Within occupational, environment and consumer settings a limited number of measurement studies have been attempted. In order to help address this knowledge gap, computational modelling studies that enable a prediction of nanomaterial exposure have been utilised, but the many assumptions associated with their development leads to uncertainties regarding the relevancy of the data. In this respect, knowledge of the products that contain nanomaterials, and information regarding the production and usage levels of nanomaterials are essential, but comprehensive data are lacking. When combined with measurement data this approach could lead to the development of more reliable, validated, predictive models.
Knowledge of the fate of nanomaterials in humans and the environment is fundamental to exposure assessments and understanding the hazards posed by nanomaterials. As a result there has been an expansion in the number of studies concerned with evaluating nanomaterial uptake at an organism, tissue/organ and cell level (figure 2). For example, determining where nanomaterials distribute to in the body following exposure can direct appropriate hazard investigations.
This is vital as it is evident that nanomaterials are not restricted to their site of entry and that they accumulate within different secondary target sites (such as the liver), and the consequences of this require attention (figure 3). Within the environment, understanding where nanomaterials accumulate can help focus hazard investigations to relevant organisms (e.g. water column vs sediment dwelling organisms; potential uptake via food chain, figure 4).
However the inability to detect and quantify nanomaterials in humans, animals and complex environmental matrices impedes investigation of nanomaterial fate. Furthermore, exposure studies also have to take into account that nanomaterials may be transformed throughout their life cycle (e.g. due to interaction with biological molecules) which makes their identification and quantification more challenging. However, modifications to nanomaterials throughout their life cycle must be considered when designing hazard investigations.
There has been much debate regarding the best dose metrics to use to express and quantify exposure to nanomaterials. Currently it is evident that toxicity is related to nanomaterial size, surface area and surface properties and that expression of dose on a mass basis alone may not be the only means of quantifying exposure. It is essential that nanomaterial exposure is quantified in a toxicology-relevant way. Reaching a consensus on the most appropriate dose metric to use is some way off and will require further study of the importance of physico-chemical properties of nanomaterials to their toxicity.
Researchers are often criticised for testing nanomaterial doses within hazard investigations that are not reflective of real-life exposures. However without sufficient exposure assessment data it is not possible to design appropriate hazard investigations. Instead, dose response experiments can be conducted to assess the relationship between nanomaterial dose and biological effects, and such studies are likely to encompass real-life exposures. Knowledge of exposure levels to nanomaterials will also help within the interpretation of hazard data and perform risk assessments.
Hazard Assessment
From a historical perspective, information on the toxicity of pathogenic particles and fibres (such as quartz and asbestos) has proved extremely useful in providing the foundations for nanotoxicology research. However, there is a huge diversity of nanomaterials available and it is often difficult to find commonalities between different investigations that assess nanomaterial toxicity. The evaluation of the human hazards posed by nanomaterials is achieved through a number of approaches, including epidemiology, controlled human exposure studies, as well as using in vivo, in vitro and in silico models. From an environmental perspective, studies have focussed on addressing the response of a number of model organisms to nanomaterials (such as Daphnia Magna). The experimental design is critical to all approaches. This is important as many hazard studies have been conducted but their relevance is questionable. For example, all hazard studies should characterise the physical and chemical properties of the nanomaterials investigated, the doses selected should be justified, physiologically and environmentally relevant dispersion protocols should be utilised to prepare nanomaterials for hazard assessment, and appropriate models should be selected to assess toxicity (e.g. have relevant target organisms and cells been selected within in vivo and in vitro studies?). There is a lack of standardised methods to assess nanomaterial safety but research is ongoing to inform the best testing strategy to adopt, and the applicability of conventional testing guidelines.
Forward Look
The small size of nanomaterials results in the introduction of novel properties that can impart both beneficial and potential detrimental outcomes for human health and the environment. Balancing the risks and benefits posed by nanomaterials is essential within their safe and responsible development. There is a general assumption that research into the toxicity of nanomaterials is lagging behind the desire to exploit them. However, whilst standardised approaches for assessing nanomaterial safety are urgently required, exposure assessment studies, hazard investigations and risk assessments have been initiated in order to support the safe integration of nanotechnology into society.
Nanotoxicology evolved from studies concerned with investigating the pathogenicity of particles and fibres (such as asbestos, and ultrafine particles within particulate air pollution). Such historical knowledge cannot be neglected but must be exploited within all aspects of nanotoxiology. Existing studies demonstrate that nanomaterials may be hazardous to human health and the environment, but that this is very much dependent on the physical and chemical properties of the nanomaterial under investigation. Concern regarding nanomaterial safety may therefore not be unfounded but it is clear that there is a need to discriminate between nanomaterials that elicit a hazard and those that are relatively innocuous, and in which context these may occur.
Therefore, nanotoxicology studies need to identify the attributes of nanomaterials that are responsible for any observed effects. Ideally, predictions could be made regarding nanomaterial hazard using structure-activity relationships. However, whist trends are emerging regarding the importance of particular characteristics to nanomaterial toxicity (such as particle size, surface area and morphology), it is not currently feasible to predict nanomaterial safety using in silico models, but this objective should be considered as a goal for the longer term. In the short term, it is necessary to support the development of standardised tests that can reliably and accurately predict nanomaterial safety. As part of this, the development of animal alternatives should be promoted. Whilst in vitro models cannot be used exclusively they could be used to screen nanomaterial safety (ideally within high throughput systems), and can help select and prioritise nanomaterials that require further testing. In order to introduce exposure limits to ensure the safe use and production of nanomaterials risk assessments are required.
Currently, it is not possible to use conventional risk assessment approaches due to a lack of suitable data, however alternative strategies for risk assessment are being considered. In all aspects it is evident that there must be flexibility in the approach taken as knowledge evolves. Of interest in this context is the current work developed by the European FP7 funded project ITSNANO which concerned with developing an intelligent testing strategy for assessment of nanomaterial safety, in consultation with diverse stakeholders (http://www.its-nano.eu/).
A multi-disciplinary approach is an inherent part of nanomaterial safety assessment. It is critical to conduct exposure assessments and physico-chemical characterisation studies alongside hazard investigations and integrate these data within risk characterisation and assessment. This process will allow a thorough analysis of which nanomaterial properties of may be most influential to their toxicity, as well as the identification of innovative ways of investigating nanomaterial safety. Communication is also required between academics, industry, regulators and the public, who share the common goal of identifying any risks posed by nanomaterials to ensure their safe development. It is therefore essential that innovation is supported, and not stifled, but that the risks posed by nanomaterials are considered from nanomaterial design to exploitation.
