Characteristics of saliva films adsorbed onto different dental materials studied by QCM-D

The formation of salivary films onto different surfaces relevant in dental research like titania, hydroxyapatite, gold, zirconia, silica, and hydrophobized silica has been studied by means of QCM-D. Human whole saliva (HWS), and sterile filtered HWS (sHWS) both diluted in water to a final concentration of 25% (v/v) were used. Main differences between the salivary films formed from the two saliva types were observed with the help of ΔD vs Δf plots where sHWS samples showed an almost linear adsorption regime for most of the surfaces whereas most of the HWS samples had a marked multi-regime nature indicating that the former ones are homogenous and the later are heterogeneous supporting previous data on a multi-phase adsorption process. The films with highest shear elastic modulus, μ > 105 N m−2, shear viscosity, η ∼ 3 × 10−3 N s m−2, and lowest thickness (∼10 nm) were formed for both types of saliva onto hydroxyapatite and for sHWS on titania. Furthermore, the ratio between the loss, G″, and the storage modulus, G′, indicates that these films have a solid-like behavior (G″/G′ ≤ 0.5). In contrast, for the remaining surfaces the adsorbed films show higher d values and are also characterized by low μ ∼ 104 N m−2, η ∼ 10−3 N s m−2, and by high ratios, G″/G′ > 2, that indicate a fluid like behavior. These observations might be expected to have influence on the lubricating properties of the salivary films. The SDS induced elutability also indicates a different interaction strength and composition of the adsorbed films and is likely associated with the ease by which these surfaces can be cleaned. Our results suggest that, among the relevant materials, zirconia and titania would yield the more lubricious films whereas hydroxyapatite will be the most easily cleaned.