Safe and sustainable by design strategies for multicomponent nanostructured materials: Learnings from industrially relevant case studies

The continuous improvement of the functionality of materials by modulating their physicochemical properties by design is key to innovation and market growth. Novel materials with an enhanced function are referred to as ‘advanced’, and they are often nano enabled and/or consisting of multiple components. Despite their improved performance compared to a conventional alternative, new functionalities also may induce new characteristics in hazard and exposure profiles, challenging the existing materials risk assessment frameworks in balancing performance, safety, and environmental impact.

In the context of the European Union Chemicals Strategy for Sustainability, the Safe and Sustainable by Design (SSbD) concept developed. SSbD recommends the assessment of chemicals and materials from early innovation phases, when options are many and budget is low. To support innovators during early screening phases, a combination of New Approach Methodologies (NAMs), guided by Integrated Approaches to Testing and Assessment (IATA), can be of value for informed decision making, bringing the advantage of increasing the screening speed for a set of materials, avoiding unnecessary animal testing. In this thesis a series of selected methodologies were developed and were shown to be suitable for early screening phases i.e., from ideation to lab phase. Moreover, SSbD screening is demonstrated on the example of one case study, oxide-perovskites for automotive catalysts, with the scope of bridging scientific findings and decision making. Three case studies, all being advanced nano enabled materials consisting of multiple components, were chosen for testing the methodologies developed and/or adapted in this thesis: oxide-perovskites for automotive catalysts, quantum dots for LED screens and inorganic aerogel mats for buildings insulation.

With respect to human safety, the inhalation route of exposure was on focus in the thesis, given its importance when dealing with nanomaterials. Two NAMs from an inhalation IATA were selected as potentially useful SSbD screening tools and their applicability to multicomponent nanostructured materials was verified. The NAMs are dynamic dissolution in lung simulant conditions and bioreactivity by abiotic assays. Transformation studies after exposure with lung simulant fluids were also performed for all oxide perovskites, using the two previously mentioned NAMs and a combination of analytical techniques. Oxide-perovskites fate and effects in the lungs were found to be dependent on their multicomponent character which ultimately influence their electronic properties. Moreover, oxide-perovskites transformation by in chemico NAMs was found to be in good agreement with the studies performed in vivo, further confirming the utility of NAMs in a SSbD context.

In addition, dustiness measurements were performed so to allow for an exposure estimation for workers in occupational settings. For materials in powder form, measurements were performed by following the small rotating drum method; for materials not in powder form, like the inorganic aerogel mats, occupationally relevant mechanical treatments were simulated, and the aerosol released was measured. Simulation included cutting using an insulation knife, cutting using a circular saw and sanding. Despite inorganic aerogel mats superior thermal insulation capacity, the ranking in number of particles released was higher compared to a conventional marketed alternative. Moreover, the number of particles released was found to be dependent on the mechanical treatment, whereas SEM-EDX analysis indicated that particles generated by sanding or circular sawing shared similar size and morphology, despite the different coatings and manufacturing processes of the mats.

Regarding the environmental safety dimension, methods to measure dissolution and dispersion stability in aquatic media were developed and/or tested. For dispersible materials with medium to slow dissolution kinetics, the dispersion loading method offered higher precision in the rate constant determination. When dealing with fast dissolving materials, loading the material as a powder allows to deposit the material in a single layer on the filter, preventing material’s dissolution/agglomeration in the time between dispersion and injection. Several factors affecting nanomaterials transformation were investigated on the example of the oxide-perovskite case study. These include the influence of natural organic matter on dissolution and dispersion stability, the effect of pH, ionic strength, and water hardness. All perovskites were characterized by low stability in dispersion. The addition of natural organic matter slightly improved their stability whereas high water hardness resulted in higher instability. A combination of NAMs to evaluate dissolution and dispersion stability, was found to be an alternative way to account for environmental safety in aquatic systems at early innovation stages, avoiding higher tier testing. The same NAMs were also useful to inform ecotoxicity testing, by differentiating particles from solutes and by estimating the particles size in a dispersion.

Lastly, concepts of similarities were explored for the comparison of NAMs outcomes for different materials in a group. Pairwise analysis conducted property-by-property enables comparison and ranking of several SSbD versions and allows the development of targeted design principles. Similarity matrices were scaled for a biologically relevant range, defined by controls for a NAM, and representative test materials were used for method calibration and read across.

Sustainability along the life cycle was addressed by a qualitative approach, identifying positive and negative impacts on the targets of the UN Sustainable Development Goals (SDGs) for the case studies under analysis.

In the final demonstration chapter on SSbD screening, important trade-offs between safety, sustainability and performance were identified on the example of oxide-perovskites for automotive catalysts. A potential improvement in functionality, linked to a positively impacted SDG target, had to be balanced with a potential safety or circularity concern, linked to a negatively impacted SDG target. Specifically for the oxide-perovskites, SDG 11.6 (improve air quality to reduce environmental impact of cities) and SDG 12.2 (sustainable management and use of natural resources) were found to be respectively positively and negatively impacted. A decision on a more favourable oxide-perovskite version is made by balancing all SSbD dimensions and may vary depending on the experts evaluating the case and by the economic situation of a company. If a project gets funded, expert judgement combined with stakeholders’ input may be a way to take a transparent decision.

In this respect, this thesis provides guidance to informed decision making at early innovation phases, on the example of case study materials, contributing to the transfer, development and testing of alternative methods, further positioning suitable tools in a SSbD context. The collected knowledge brings together regulatory advice, industry practice and academia recommendations, and should be most of interest for small to medium enterprises and start-ups, as they often do not have the possibility to gather several experts to take informed decisions.

In a pre-regulatory context like it is SSbD, the final choice lays with innovators, therefore expert driven guidance is needed to help them making the most balanced decision as early as possible in the innovation process, preventing future issues for human health and the environment, and avoiding regrettable substitutions.