A spray based method for biofilm removal

Biofilm growth on human teeth is the cause of oral diseases such as caries (tooth decay), gingivitis (inflammation of the gums) and periodontitis (inflammation of the tooth bone). In this thesis, a water based cleaning method is designed for removal of oral biofilms, or dental plaque. The first part of the work was done in the context of a two-year post-graduate study at the Stan Ackermans Institute of the Technische Universiteit Eindhoven. Five water-based cleaning concepts were evaluated on efficacy, safety and ease of use. The efficacy of the concepts was tested on real plaque in the mouth, and on artificial plaque layers. The latter consisted of a mixture of Poly(VinylAlcohol)-SBQ and latex particles in water (PVA), which was gelated by ultraviolet light. If it was technically impossible or if it was not safe to test the concepts in the mouth, the concept was only tested on artificial plaque. Different series of preliminary experiments on the removal of PVA layers were performed with water jets, water jets with air bubbles, spray jets with and without abrasive particles and ultrasound. Spray jets with or without abrasive particles performed better than water jets with and without air bubbles and better than ultrasound. Some further experiments were done to characterize the air assisted water sprays for different air pressures and water flow rates. Droplet diameters between 5 and 200 micrometers and droplet velocities between 10 and 100 meters per second were found for supply pressures in between 1 and 2 bar. The effect of the adjustable parameters of the spray, i.e. the air pressure and the water flow rate, on the cleaning efficacy was determined. Since the concept of a spray jet (without abrasive particles) was proven to be effective for plaque removal on real human teeth, it was decided to investigate the effect of spray parameters on cleaning efficacy in detail. In this way, the exact mechanism of plaque removal could be better understood, which was essential to optimize the system parameters. As a first step, an improved substitute for dental plaque was developed. The model comprised a biofilm build from Streptococcus mutans bacteria, naturally present in human plaque, that was grown on glass plates under favorable conditions. The biofilm was characterized mechanically by a micro-indentation device, in which a small indenter was pressed into the biofilm. The process was visually observed with a confocal microscope. The visual observations and the force-displacement curves of the indenter showed that the biofilm is a porous visco-elastic solid that has a tangential elasticity modulus ranging from 1-15 kPa at a strain of 10%. Second, different sprays with known size-velocity distributions were used to perform laboratory experiments in which a biofilm was exposed for a short time to the spray. Now, the cleaning efficacy of the spray could directly be linked to the parameters of the spray. In order to improve the experimental conditions, experiments were repeated with pencil jets, which have reasonably uniform droplets both in size and in velocity. The relationship between biofilm removal rate and droplet parameters was determined. It was found that two phases can be distinguished in the removal process. In the penetration phase, the droplets gradually remove biofilm until the substrate. In the subsequent growth phase, the existing cleaned area increases in time. It was found that the growth rate scales with the cube of the droplet velocity and with the square of the droplet diameter. It was concluded that biofilm is removed more efficiently during the growth phase than during the penetration phase. For a complete overview, numerical simulations were done of impacting droplets on solid surfaces which were either dry or covered by a thin water film. The temporal behavior of the pressure and the shear stress on the solid surface was determined as a function of the droplet’s initial velocity and diameter. The pressures on the surface scale with the stagnation pressure, while the shear stresses scale with the square root of the velocity and the square root of the diameter. It was established that the presence of a water film on the solid surface strongly decreases the magnitude of the stresses involved and quantitative estimates for these reducing effects of water films were found. For cleaning of a certain area with a given volume of water in a given time, regime diagrams were constructed for the penetration phase and for the growth phase. The efficacy of biofilm removal with monodisperse droplet streams was given as a function of the droplet velocity and the droplet diameter.