Synthetic mineral fibers to predict the inflammatory effects of natural fibers

Status:

completed 12/2025

Aims:

The chemical synthesis of defined fibers with discrete dimensions will, for the first time, enable targeted investigation of specific fiber properties in a cell culture model. This will allow conclusions to be drawn that facilitate toxicological and occupational health risk assessment of inorganic fibers. Specifically, the study aims to determine whether the under- or exceeding of certain diameters – i.e., a pronounced needle-like shape – as well as high rigidity, promote the pro-inflammatory effects of fibers, assuming a consistent chemical composition. It is hypothesized that very thin fibers in the lower nanometer range may induce minimal inflammatory responses due to a lack of rigidity, whereas relatively thick fibers (in the micrometer range) may exhibit reduced effects due to their less needle-like morphology.

Activities/Methods:

Using various synthesis approaches (microemulsion, polyol, and hydrothermal synthesis), oxide-based mineral fibers (silica and titanium dioxide) with defined lengths and controlled variations in diameter were synthesized. These were thoroughly characterized using techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), elemental analysis, and inductively coupled plasma mass spectrometry (ICP-MS). Fiber uptake by cells was analyzed via SEM and confocal laser scanning microscopy in both human monocytic macrophages (THP-1 cells) and rat alveolar macrophages (NR8383 cells).

Cytotoxic effects were assessed using established toxicity assays. The pro-inflammatory effects were systematically studied using an established in vitro co-culture model that simulates fiber-induced recruitment of inflammatory cells to the lung, consisting of lung macrophages and a model cell line for neutrophilic granulocytes. In addition to the PICMA assay (Particle-Induced Cell Migration Assay), gene expression analysis was conducted. RNA (Ribonucleic acid) was isolated from both exposed and non-exposed NR8383 (rat alveolar macrophages) cells, followed by quantification and quality assessment. At the protein level, expression of chemotaxis and inflammation biomarkers was quantified from cell supernatants using ELISA (Enzyme-linked Immunosorbent Assay).

Results:

It was demonstrated that the synthesis of long microfibers composed of silica and titanium dioxide is feasible. However, doping with foreign ions such as aluminum or iron was unsuccessful, preventing the generation of synthetic asbestos-like fibers. Nevertheless, the synthesized fibers exhibited clear pro-inflammatory effects dependent on their length and morphology (e.g., straight versus curly). These effects correlated with those induced by known pro-inflammatory materials (asbestos and carbon nanotubes), supporting the applicability of the PICMA assay for in vitro estimation of subtoxic biological responses. Thus, the project goals were fully achieved, even though the chemical system was changed (titanium dioxide instead of silicate). Ultimately, fiber morphology proved to be more critical than chemical composition, provided the fibers are insoluble. However, it also became evident that the synthesized fibers did not reach the level of biological activity observed with crocidolite asbestos fibers. Comparative electron microscopy revealed that crocidolite asbestos causes membrane damage by piercing cell membranes with split fiber ends – a needle-like mechanism that was not replicated by the synthetic silica and titanium dioxide fibers. Therefore, beyond fiber length and diameter (or their ratio), the needle-like morphology plays a decisive role in fiber-induced effects.

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