Aerogels are not regulated as nanomaterials, but can be assessed by tiered testing and grouping strategies for nanomaterials.

Johannes G Keller,Robert Landsiedel,Antje Vennemann,Kai Werle,Wendel Wohlleben,Sibylle Gröters,Martin Wiemann

doi:10.1039/d1na00044f

Johannes G Keller, Robert Landsiedel + Show 5 more

Open Access

https://doi.org/10.1039/d1na00044f

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Abstract

Aerogels contribute to an increasing number of novel applications due to many unique properties, such as high porosity and low density. They outperform most other insulation materials, and some are also useful as carriers in food or pharma applications. Aerogels are not nanomaterials by the REACH definition but retain properties of nanoscale structures. Here we applied a testing strategy in three tiers. In Tier 1, we examined a panel of 19 aerogels (functionalized chitosan, alginate, pyrolyzed carbon, silicate, cellulose, polyurethane) for their biosolubility, and oxidative potential. Biosolubility was very limited except for some alginate and silicate aerogels. Oxidative potential, as by the ferric reduction ability of human serum (FRAS), was very low except for one chitosan and pyrolyzed carbon, both of which were <10% of the positive control Mn2O3. Five aerogels were further subjected to the Tier 2 alveolar macrophage assay, which revealed no in vitro cytotoxicity, except for silicate and polyurethane that induced increases in tumor necrosis factor α. Insufficiently similar aerogels were excluded from a candidate group, and a worst case identified. In the Tier 3 in vivo instillation, polyurethane (0.3 to 2.4 mg) elicited dose-dependent but reversible enzyme changes in lung lavage fluid on day 3, but no significant inflammatory effects. Overall, the results show a very low inherent toxicity of aerogels and support a categorization based on similarities in Tier 1 and Tier 2. This exemplifies how nanosafety concepts and methods developed on particles can be applied to specific concerns on advanced materials that contain or release nanostructures.

Highlights

Aerogels are unusual materials that combine macroscopic external and nanosized internal structures with high speci c surface area, low density and high porosity
The testing strategy complies with the recent GRACIOUS framework for the grouping of nanomaterials,[40] and the speci c tests comply with the tiered selection of methods in GRACIOUS inhalation grouping, with the nanoGRAVUR grouping framework,[41] and with the tiered DF4nanoGrouping,[42] which were tested in case studies including silica and organic nanomaterials.[41,43]
All of these tiered frameworks were previously applied to materials with external dimensions in the nanometer range only, whereas here we apply them to open-pore internal nanostructures

Summary

Introduction

Aerogels are unusual materials that combine macroscopic external and nanosized internal structures with high speci c surface area, low density and high porosity. The development of appropriate methods to characterize exposure and hazard of aerogel beads is challenging because they retain properties of the nanoscale only by their internal structure. Aerogels are exempted from the need to report to national nanomaterials product inventories such as those in France, Belgium, the USA or Canada.[17,18] producers cannot neglect that, the inherent toxicity of most materials that can be formulated as an aerogel is low, an increased bioactivity may result from their large inner surface area, possibly fostering a high surface reactivity and/or a high dissolution.[19] Toxicity inherent to nanostructures is not expected in general,[20] but the hazard assessment of aerogels needs to consider the composition of inhalable or ingestible fragments, and the modulation of their potency by the large surface area

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