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<p>Human saliva is a complex biological fluid whose rheological and interfacial characteristics are crucial for oral lubrication, bolus formation, swallowing, and sensory perception. Despite its importance, the degree of intra-individual variability in saliva structure and rheological behavior under controlled sampling conditions remains inadequately defined. In this study, unstimulated whole human saliva collected from a single donor over five consecutive days under standardized routine was examined in its uncentrifuged (UN – UWHS) and centrifuged state (C – UWHS). Particle size analysis revealed pronounced day-to-day variability, while centrifugation consistently reduced the contribution of larger populations. Extensional rheometry showed strong fluctuations in filament thinning dynamics, with relaxation times ranging from 0.2 to 6.2&#xa0;s in native saliva and decreasing to 0.002–0.04&#xa0;s after centrifugation. In contrast, shear viscosity exhibited lower variability, although native saliva consistently displayed higher viscosity and more pronounced shear-thinning behaviour. Size exclusion chromatography (SEC) and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed multiple macromolecular populations, while confocal microscopy indicated that saliva foam is stabilized by a protein-rich interfacial layer. Overall, the results demonstrate that saliva is a dynamic viscoelastic colloid with substantial intra-individual variability even under controlled sampling conditions. These findings are relevant for improving experimental reproducibility in saliva rheology studies and for guiding the development of biomimetic saliva substitutes for xerostomia.</p> Graphical Abstract <p></p>

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Rheological aspects of the day-to-day variability of human saliva

  • 期刊论文

  • 发表日期：

    2026年6月17日

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  • Ioannis Velopoulos,
  • Maria Dimopoulou,
  • Jianshe Chen,
  • Christos Ritzoulis

摘要

Human saliva is a complex biological fluid whose rheological and interfacial characteristics are crucial for oral lubrication, bolus formation, swallowing, and sensory perception. Despite its importance, the degree of intra-individual variability in saliva structure and rheological behavior under controlled sampling conditions remains inadequately defined. In this study, unstimulated whole human saliva collected from a single donor over five consecutive days under standardized routine was examined in its uncentrifuged (UN – UWHS) and centrifuged state (C – UWHS). Particle size analysis revealed pronounced day-to-day variability, while centrifugation consistently reduced the contribution of larger populations. Extensional rheometry showed strong fluctuations in filament thinning dynamics, with relaxation times ranging from 0.2 to 6.2 s in native saliva and decreasing to 0.002–0.04 s after centrifugation. In contrast, shear viscosity exhibited lower variability, although native saliva consistently displayed higher viscosity and more pronounced shear-thinning behaviour. Size exclusion chromatography (SEC) and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed multiple macromolecular populations, while confocal microscopy indicated that saliva foam is stabilized by a protein-rich interfacial layer. Overall, the results demonstrate that saliva is a dynamic viscoelastic colloid with substantial intra-individual variability even under controlled sampling conditions. These findings are relevant for improving experimental reproducibility in saliva rheology studies and for guiding the development of biomimetic saliva substitutes for xerostomia.

Graphical Abstract