## 5 Conclusions  
This study investigated the rheological properties of human laryngeal mucus from the vocal folds. It is the first time that mucus was aspirated directly from the vocal folds and characterized rheologically. Due to the limited sample volumes, particle-tracking microrheology (PTM) was applied and the viscoelastic characteristics of 19 mucus samples were evaluated. Oscillatory shear rheology (OSR) was applied and evaluated for five additional mucus samples. The results can be summarized as follows:  
1. All investigated human laryngeal mucus samples presented as viscoelastic solids, at rest with varying rigidity, independent of the measuring method. 2. PTM led to a preliminary subdivision of the samples into three groups with different rigidity. Seven of the 19 investigated samples (group a) revealed consistent solid-like character, which is typical for gels. Their absolute storage and loss modulus were in the range of native human airway mucus investigated before [14; 15; 16]. For the other samples (group b, group c), a transition from solid-like to liquid-like character was  
Figure 6: Comparison of the mean viscoelasticity of all OSR measurements and the mean of PTM measurements, group b.  
found. The transition happened for group b samples at high frequencies, characterizing the samples predominantly as viscoelastic-solids. For group c mucus samples, the crossover was found at low frequencies, characterizing the samples predominantly as viscoelastic-liquids. We assume that this is caused by a looser mucin network, as a consequence of a varying hydration level of the samples. The hydration affects the concentration of the main gel-building component of mucus, the mucins [18; 20], and according network-building factors as pH, salt and surfactant concentration [21]. 3. A correspondence between the groups and the demographic data of the patients belonging to the samples was not found for gender, age and larynx status but was found regarding the smoking behavior. The mucus samples with the lowest rigidity (group c) were only assigned to current smokers. 4. OSR revealed consistent gel characteristics for four of the five investigated mucus samples. The rigidity of the gels varied. The absolute viscoelastic properties were in the range of PTM and could be assigned to group b mucus samples. The findings in this study reveal varying viscoelastic characteristics of human laryngeal mucus from the vocal folds and create a basis for the design of artificial mucus with natural viscoelastic properties. It can be expected that the characteristic properties of laryngeal mucus are governed by scaling laws similar to these known in harvested bronchial epithelial mucus [20] or in living cells [31; 32], which facilitates the creation of highly adaptable and well controllable substitutes. Although artificial mucus was already found to have an impact on the oscillatory behavior of the vocal folds [7], this study more firmly establishes a foundation for future ex-vivo larynx experiments with realistic viscoelastic conditions of mucus. This will permit the examination of the influence of mucus characteristics on laryngeal dynamics, and ultimately on clinical populations with functional dysphonia. Finally, the presented results may serve as a basis for artificial mucus fabrication for patients suffering from oral dehydration [33; 34]. Conceptualization, O.W., M.S. and M.D. ; methodology, G.P., O.W. and M.S. ; software, G.P. and D.B. ; validation, G.P., O.W., H.C., M.S., D.A.B., S.K. and M.D. ; formal analysis, G.P. ; investigation, G.P., A.-O.G., S.K.M., H.C. and N.H.; resources, D.B., A.-O.G., S.K.M., W.P. and M.D. ; data curation, G.P. ; writing--original draft preparation, G.P. ; writing--review and editing, O.W., D.B., A.-O.G., S.K.M., H.C., N.H., M.S., D.A.B., S.K. and M.D. ; visualization, G.P. and M.S. ; supervision, O.W. and M.D. ; project administration, M.D. ; funding acquisition, M.D. All authors have read and agreed to the published version of the manuscript.