Dermal Absorption of Nanomaterials.

This report and accompanying literature database forms part of a series of projects regarding nanomaterials in Denmark (“Better control of nano”) commissioned by the Danish Environmental Protection Agency (EPA). The report constitutes a literature study to gather and evaluate the existing knowledge on the systemic absorption of nanomaterials via dermal exposure with the intended aim of describing the current state of the art as well as gaps in knowledge on dermal absorption of nanomaterials in relation to consumer exposure. There exists concerns that whilst the properties of nano-scale materials have enabled numerous industrial applications and the consumer market already offers increasing numbers of products containing nanomaterials, these may result in skin exposure (intentionally or unintentionally) and knowledge concerning passage of nanoparticles through the skin and subsequent effects is limited. The approach to collating and analysing the current state of knowledge involved the development of a systematic search strategy based on key search terms and phrases of relevance to dermal absorption of nanomaterials. The search and subsequent stepwise review of identified studies took into account guidance provided in the Cochrane Handbook for Systematic Reviews of Interventions as well as currently considered best practice in reviewing nanotoxicological literature. The identified studies and their appraisal culminated in the formation of an MS Access Database which accompanies this report. This knowledge base has been used to evaluate the role of various physicochemical properties on nanoparticle absorption into the skin as summarised under the following broad physicochemical properties, concerning the role of: • Size • Composition • Surface Chemistry • Shape One of the key challenges in assessing the literature on the physicochemical properties influencing dermal penetration/absorption of nanomaterials is that it is difficult to draw conclusions due to either: i) limitations in the reporting of physicochemical data, and/or, ii) the alteration of multiple experimental parameters in a non-systematic way. The issue of a lack of information on nanoparticle physicochemical properties is common, yet the most challenging aspect is the alteration of multiple experimental parameters whereby multiple characteristics such as shape, charge, coating, size can all be changed. This means that little meaningful comparison of results can be made within a single experimental study, let alone between studies. Despite such challenges, some key conclusions can be drawn. Whilst there are many conflicting results, on balance the literature seems to suggest that absorption of particles in the nano-range through the skin is possible although occurs to a very low degree and that the level of penetration, depending on chemistry and experimental conditions, may be greater than for larger particles. The 12 Dermal Absorption of Nanomaterials role of size is considered a critical component of dermal absorption but this in itself does not seem to guarantee absorption or lack of as other properties can also influence dermal absorption markedly. In addition, particle size is not necessarily a constant parameter as agglomeration of particles can occur over time and also in relation to experimental conditions (e.g. presence of surfactants within particle vehicle formulation). However, whilst this issue of agglomeration has been suggested as being important (as well as an important experimental variable), agglomeration state is often not reported within studies. Although particle composition has been shown to have little effect on dermal penetration/absorption of nanoparticles, composition (either in terms of the bulk or as a contaminant such as iron) should be considered in relation to dermal toxicity. Another challenging issue noted in relation to nanoparticles is determining whether absorption (when detected) occurs as a solid particle or as soluble fraction (e.g. metal ions). This may have important connotations in terms of systemic availability, distribution as well as metabolism and excretion. Surface chemistry, due to its prominence in forming the interface between a nanoparticle and the biological environment, is seen as a key group of physicochemical properties dictating dermal interaction and has been indicated by several studies to be an important factor influencing the ability of nanoparticles to penetrate into the skin, with surface charge (through the modification of surface coating/functionalisation) being the most investigated aspect. However, elucidating a clear relationship between the many aspects of surface chemistry and dermal absorption/penetration is difficult as the level of characterisation of the surface chemistry of the nanoparticles studied is often poor and in many cases only a qualitative indication is provided. In addition, such issues are further confounded by the multiple alterations of surface properties within the same experiment, meaning that it is not possible to predict the effect of one single property on dermal absorption. However whilst the results are complex and inconclusive, there does appear to be a slight tendency towards greater uptake of positively charged particles although there are conflicting studies in relation to this. Shape has been seen as a key physicochemical property influencing toxicity in relation to inhalation exposure to nanomaterials, yet knowledge of its role in dermal absorption is limited. Whilst there are examples of non-spherical nanoparticles being evaluated within the literature, there is no study which critically evaluates in a systematic way the role of particle shape/length on absorption. Indeed those non-spherical particles which have been evaluated do not meet the criteria of what is conventionally considered to be a fibre (i.e. length greater than 5 µm) and there is also an absence of other shaped particles such as platelets (e.g. graphene) being evaluated. In addition to appraising the role of key physicochemical properties on dermal absorption of nanomaterials, this report also considered the test methods (in vitro, in vivo and in silico) used in the evaluation of absorption. There exist many cross-cutting issues such as sample preparation, vehicle and species employed, as well as particle detection and quantification that are common to both in vitro and in vivo test methods. These issues are discussed in the report, and add to difficulties in comparing results of differing studies due to non-harmonised approaches which, to a certain extent, may be enhanced by current test guidelines. The use of in silico approaches such as quantitative structure activity relationships (QSAR) to predict dermal absorption is apparent for Dermal Absorption of Nanomaterials 13 conventional chemicals yet is still very much within its infancy for nanomaterials. However, due to the challenges that the vast array and diversity of nanomaterials brings, this is considered to be an important area for further development. Other key gaps are described in relation to the physiochemical properties, test methods and detection methods related to dermal absorption leading to report recommendations on methods and endpoints to assess dermal absorption for nanoparticles. Such recommendations focus around the need for more robust and harmonised testing approaches (guidelines) for determining dermal absorption of nanomaterials and the need for a systematic approach to alteration of key physicochemical characteristics to understand their relative role in dermal absorption. This also involves recommendations surrounding sample preparation, dermal models, nanoparticle vehicles, dose, duration and particle detection and quantification. The report concludes on recommendations for relevant and priority candidate nanomaterials and how this may relate more to priority candidate physicochemical properties rather than a single or group of nanomaterials