Mouthguard-Type Wearable Sensor for Monitoring Salivary Turbidity to Assess Oral Hygiene

Abstract

Salivary turbidity is a promising indicator for evaluating oral hygiene. This study proposed a wearable mouthguard-type sensor for continuous and unconstrained measurement of salivary turbidity. The sensor evaluated turbidity by measuring the light transmittance of saliva with an LED and a phototransistor sealed inside a double-layered mouthguard. The sensor was also embedded with a Bluetooth wireless module, enabling the wireless measurement of turbidity. The mouthguard materials (polyethylene terephthalate-glycol and ethylene-vinyl acetate) and the wavelength of the LED (405 nm) were experimentally determined to achieve high sensitivity in salivary turbidity measurement. The turbidity quantification characteristic of the proposed sensor was evaluated using a turbidity standard solution, and the sensor was capable of turbidity quantification over a wide dynamic range of 1-4000 FTU (formazine turbidity unit), including reported salivary turbidity (400-800 FTU). In vitro turbidity measurement using a saliva sample showed 553 FTU, which is equivalent to the same sample measured with a spectrophotometer (576 FTU). Moreover, in vivo experiments also showed results equivalent to that measured with a spectrophotometer, and wireless measurement of salivary turbidity was realized using the mouthguard-type sensor. Based on these results, the proposed mouthguard-type sensor has promising potential for the unconstrained continuous evaluation of oral hygiene.

Keywords: mouthguard; optical sensing; oral hygiene; salivary turbidity; wearable sensor.

Figure 1

(a) Schematics of configuration of proposed MG-type salivary turbidity sensor and enlarged view of turbidity-sensing part; (b) circuit diagram and data transfer system of MG-type sensor; (c) photograph of fabricated MG-type turbidity sensor.

Figure 2

(a) OD spectrum of a saliva sample and FSS prepared to 553 FTU; (b) turbidity-dependent characteristics of OD for wavelengths of 405 and 660 nm on FSS.

Figure 3

(a) Transmittance spectrum of some commercial MG materials; (b) mechanical characteristics of MG materials (PETG and EVA) obtained from three-point bending test.

Figure 4

(a) Output currents of MG-type sensor to FSS for various turbidity levels; (b) identified correspondence between output OD of the MG-type sensor and turbidity level.

Figure 5

Result of in vitro measurements of saliva sample with microvolume spectrophotometer and MG-type sensor.

Figure 6

(a) Schematics of in vivo experiment of wireless turbidity measurement; (b) output current of MG-type sensor for tap water and saliva in the oral cavity.