Application of micro-bubbles on oral care

2.1 Micro-bubble generator and control variables

This study modified the Braun MD20 Oxyjet to develop our micro-bubble generator (Fig. 1). In this device, micro-bubbles were generated and channeled through catheter and nozzle into the ejection pores of the soft teeth-tray and entered the gap between the denture and teeth-tray to remove the dental bacteria on the denture. Fourteen holes were drilled on the bottom of the inner side of the teeth-tray, corresponding to the position of each tooth on the denture, to allow water jets to clean the tooth. The original 5 rotational speed settings of the MD20, measured by a contact tachometer as 2580 rpm, 3021 rpm, 3527 rpm, 4210 rpm and 5380 rpm, respectively, have been retained as the levels of the first control variable. Since studies on micro-bubble applications by both Legner [6] and Merkle and Deutch [7] speculated that nozzle diameter is a key factor affecting the dimension of micro-bubbles, so we took nozzle diameter as the second control variable. The nozzle heads were made from stainless steel by CNC turning (length 16 mm, outer diameter 6 mm, inner diameter 5 mm), and the Electrical Discharge Machining (EDM) was used to produce the demanded pore diameter of nozzles. Five nozzle diameters: 0.16 mm, 0.3 mm, 0.63 mm, 0.8 mm and 1.2 mm, were controlled as levels of this variable. The soft teeth-tray was a molded production by a dental material company and was constructed out of medical silicone with a hardness of 40 with 14 holds in total (each diameter of holes is 4.8 mm).

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The average (over 10 repeated measures) flow volume per second of the water stream ejected from the micro-bubble generator under each parameter combination was measured. The flow velocity of the water stream was calculated from the distance of the same micro-bubble in the subsequent two photos taken with a jet stream of 1,000 frames/sec for 1 second by a high-speed camera (Mage Speed HHC X2). The average diameter of the micro-bubbles was also measured from the photos. The second, third and fourth part of Table 1 shows the measured flow volume, flow velocity and micro-bubble diameter of ejected water stream under the 25 control variable combinations (5 rotational speeds × 5 nozzle diameters), respectively, when teeth-tray with 4.8 mm ejection pore diameter was adopted.

2.3 Preparation of denture and bacterial solution

To investigate the plaque removal effect of micro-bubble under each variable combination, we used microbial validation as the basis of quantifying cleaning efficacy. The culturing of bacterial solution followed the methods described by Oliveira Paranhos et al. [8] and Lee et al. [9]: bacterial strains were collected from patients with serious periodontal disease at a dental clinic. During collecting, a sterilized cotton swab was rubbed evenly around the oral cavity of the patient, and the cotton swab was immediately placed inside a sterilized test tube filled with Sabourand dextrose agar for cultivation. After 24 hours, the cultivated bacterial strains was transferred to a beaker with culture solution and the beaker was placed on an orbital shaker in the incubator for 48 hours in 37 ∘ C at a speed of 180 rpm for further cultivation.

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The dental plaque removal experiment in this study followed the procedure established by Oliveira Paranhos et al. [8] and Lee et al. [9] and was performed in a sterilized laminar flow. In each of the 25 dental plaque removal experiment conditions, firstly, the denture of upper teeth was sterilized and then immersed in a 250 ml square container with solution containing 8 × 10 10 cfu/ml bacterial strains for 30 minutes, and dried for another 60 minutes. The prepared denture was placed in the soft teeth-tray and cleaned for 3 minutes by the micro-bubble generator under the demanded experiment condition. The occlusal surface of the cleaned denture was printed onto the culture medium for 30 seconds. The culture medium then was placed at room temperature for 24 hours for counting the residual colonies (plaque) on it with a magnifying glass on a colony counter. The difference between the amount of residual colonies and the amount of colonies on the uncleansed denture was measured as plaque (colony) removal. The cleansing effect of each experiment condition was then calculated based on the study by Lee et al. [9], which was assessed by the percentage of plaque removal. In this study, the estimation of plaque removal was not directed at single species of strain, but counted for all types of colonies, as there were typically many different species of bacteria in a normal oral cavity of the real life situation. Due to the plane nature of the culture medium, only the colonies produced from the occlusal surface of the denture were calculated. The steps of preparing, cleaning and clean efficacy measuring of the denture are summarized in Fig. 2.