The complex shear time response of saliva in healthy individuals

Saliva plays a critical role in oral health, offering protection, aiding in digestion, and facilitating speech and swallowing. This study explores the biochemical composition of human saliva from healthy subjects, including total protein, glycoprotein, and calcium concentrations, in relation to its shear and time-dependent rheological properties. Eleven healthy, nonsmoking subjects were recruited, and salivary secretion rates were measured. Assays were used to determine concentrations of total protein, glycoproteins, and calcium, in addition to rheometry for evaluating the rheological properties of saliva. The results showed that unstimulated saliva, dominated by the mucins MUC5B and MUC7, displayed significantly higher viscosity and pronounced viscoelastic properties compared to stimulated saliva. Rheological analysis revealed saliva to be a viscoelastic material, exhibiting both elastic (solid-like) and viscous (liquid-like) responses. Shear thinning behavior was observed, where viscosity decreased with increasing shear rates, contributing to the fluid's ability to adapt to varying oral conditions. Furthermore, saliva exhibited thixotropy, a time-dependent material behavior characterized by structural breakdown under shear and recovery at rest. Calcium and glycoprotein levels were positively correlated with increased viscoelasticity, particularly with the storage modulus (G′), which reflects the ability of saliva to store elastic energy. These findings highlight the intricate relationship between the biochemical composition of saliva and its rheological properties, specifically its capacity for shear thinning, viscoelastic behavior, and time-dependent recovery, which are vital for its lubrication and protective functions in the oral cavity.