Methods: A pseudo-ternary diagram containing biocompatible components rice bran oil(RBO), 1:1 lecithin/propylene glycol mix(SM) and water(W) was characterised. Flow rheological and viscoelastic properties were determined at 37°C with steady shear (0-50s-1) and oscillatory rheometry (0.1-10Hz, 5Pa stress amplitude) respectively, using a rotational rheometer with parallel-plate and 40mm solvent-trap steel plate geometry. Storage(G’) and loss(G”) modulus were used to identify the relative liquid-like to solid-like behaviour (tan δ, G”/G’). Results were compared with a commercial polymer-based saliva substitute (OralBalance, Biotene, USA) and human whole saliva collected from a single donor.
Results: Formulations containing over 30% SM were semi-solids, exhibiting pseudoplastic behaviour with a yield stress that increased as a function of SM concentration and was highest when RBO:W=1. A positive spike in tanδ occurred in some formulations (Table 1), suggesting a temporary change in droplet microstructure. Frequency-dependent regions were identified at 20-30% SM concentrations where liquid-like properties (tanδ>1) dominated at frequencies under 5Hz and solid-like properties (tanδ<1) dominated at higher frequencies. Although saliva had a lower G’ and G” at all frequencies, tanδ was similar and less than one at frequencies above 5.2Hz. The commercial substitute had higher G’ and G” than the tested formulations and tanδ<1, independent of frequency.
Table 1. Peak tanδ (magnitude from baseline) and frequency for selected compositions of RBO:SM:W |
||
RBO:SM:W |
Frequency (Hz) |
Tanδ (magnitude) |
5:4:1 |
3.73 |
0.46(0.17) |
4:4:2 |
4.72 |
0.17(0.06) |
3:4:3 |
5.05 |
0.26(0.14) |
2:4:4 |
3.40 |
0.34(0.19) |
1:4:5 |
2.41 |
0.49(0.32) |
Conclusion: Rheological properties changed as a function of frequency. Solid-like properties dominated at frequencies over 5Hz in a manner analogous to natural saliva. The viscous behaviour observed at lower frequencies may improve lubrication of the oral cavity whereas elasticity at higher frequencies may improve retention during mastication and deglutition.