Rheological aspects of mucin-containing solutions and saliva substitutes.

In this study rheological properties of aqueous solutions of mucin, albumin and mucin-albumin have been investigated in search for saliva substitutes. They were compared with commercially available saliva substitutes on the one hand and natural human saliva on the other hand. For the latter a few measurements on saliva are reported here in addition to previously reported measurements done in our laboratory. Proteins absorb at the interface and saliva proteins do so strongly and rapidly. Therefore rheological measurements were carried out on the interface and on the bulk underneath the layer. In both cases the flow curve and the complex viscosity was determined. The results show that specific mucin-albumin solutions were rheologically similar to human whole saliva with respect to both bulk liquid and surface properties. The rheological properties of commercial saliva substitutes were essentially different from those of human saliva. It is concluded that mucin-albumin solutions have good perspectives as saliva substitutes.

Rheological properties of human saliva.

From measurements with a Couette-type viscometer provided with a guard ring it was shown that at the saliva-air interface a protein layer is adsorbed. Measurements of the surface shear modulus of this layer on saliva of 7 healthy subjects were performed at a frequency of about 70 Hz and a temperature of 25 degrees C. For a surface age of about 1.5 h the surface shear modulus and the surface viscosity were in the order of 1 Nm-1 and 10(-3) Nm-1 s, respectively. From ellipsometric measurements it was found that the thickness of the protein layer was approx. 100 nm and, using this value, it could be concluded that the shear modulus and the dynamic viscosity were in the order of 10(7) Pa and 10(4) Pa s, respectively. The layer appeared to be fragile. Even shear deformation amplitudes of 4 x 10(-5) are too high to assure linearity. The complex viscosity (eta = eta' - i eta") of the bulk liquid of human submandibular saliva below the absorbed layer was measured in the frequency range 70 Hz-200 kHz with 3 torsional resonators, each for a different frequency, and a Ni-tube resonator. It was concluded, that the real part of the complex viscosity (eta') decreases from 1.1 mPa s at 70 Hz to a value of 0.95 mPa s at high frequencies. Except at the lowest frequency (70 Hz), the value of eta" was too small to be detected.

Rheological properties of saliva substitutes containing mucin, carboxymethylcellulose or polyethylenoxide.

Apparent viscosities at different shear rates were measured for 3 types of saliva substitutes: (a) mucin-containing saliva; (b) substitutes based upon carboxymethylcellulose (CMC), and (c) solution of polyethylenoxide (PEO). The apparent viscosities were compared with those of human whole saliva. Human whole saliva and mucin-containing saliva substitutes appeared to be similar in their rheological properties. Both types of solution are viscoelastic solutions and adjust their apparent viscosities to their biological functions. Preparations containing CMC or PEO are non-Newtonian liquids. From this study it is concluded that mucin-containing saliva substitutes appear to be the best substitutes for natural saliva, as far as rheological properties are concerned.