DIRECT OBSERVATION OF ABRASIVE PARTICLES AND TOOTHBRUSH FILAMENTS DURING SIMULATED TOOTH CLEANING R. LEWIS, R. S. DWYER-JOYCE Department of Mechanical Engineering, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK; e-mail: roger.lewis@sheffield.ac.uk M. J. PICKLES Unilever Research Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, UK SUMMARY Most people clean their teeth using a toothpaste, consisting of abrasive particles in a carrier fluid, and a filament based toothbrush to remove plaque and stain. In order to optimise cleaning efficiency it is desirable to understand how the toothbrush filaments, abrasive particles and fluid interact in a teeth cleaning contact. The following describes work carried out to develop optical apparatus to enable the visualisation of a simulated teeth cleaning contact. Studies have been carried out using the apparatus to investigate particle entrainment into the contact and the effect of parameters such as brushing load and speed, particle size, fluid viscosity and filament configuration. These have shown how particles are trapped at the tips of toothbrush filaments. There is some motion of particles around the filaments and exchange of trapped particles especially at low brush loads and high sliding speeds. Keywords: teeth, cleaning, abrasive particles 1 INTRODUCTION An essential function of a toothpaste is to clean the teeth, by removing accumulated deposits of plaque and stain. In order to optimise cleaning efficiency it is desirable to understand how toothbrush filaments, abrasive particles and fluid interact in a tooth cleaning contact. This involves understanding how particles are entrained into and trapped in the cleaning contact, how particles are loaded against a tooth and how they cause material removal from the deposited layers on the tooth surface. Knowledge of how particles and toothbrush filaments interact may also provide an improved measure for assessing the performance of a toothbrush. Addy [1] has suggested that there is a need for standardisation in toothbrush assessment methods. The objectives of this work were to devise a means of observing abrasive cleaning contacts using optical apparatus. This was then used to study particle entrainment into the contact and the effect of parameters such as brushing load and speed, particle size, fluid viscosity and filament configuration. The effects of filament shape and load on a trapped particle were investigated as well as the process by which an abrasive particle removes material. 2 VISUALISATION APPARATUS Simple optical apparatus was developed to enable the visualisation of a simulated teeth cleaning contact (shown in Figure 1). A toothbrush head is loaded against a rotating glass disc using a hydraulic piston. The toothbrush head is located in a clip attached to the hydraulic actuator. Load is varied either by using the actuator or by varying the displacement of the toothbrush head relative to the glass disc (and hence the filament deflection) using spacers. The fluid/abrasive particle mixture is applied either to the brush head prior to loading against the glass disc or fed into the contact as the disc is rotating. Rotating Glass Disc Microscope Light Source Liquid/Particle Mixture Applied to Filaments Before Attachment of Glass Disc Toothbrush Head Clip to Hold Toothbrush Head Figure 1: Optical Apparatus Set-up for the Study of Particle Interaction with Toothbrush Filaments The contact region was observed using a positionable microscope attached to the rig. Image capture was by CCD camera or short duration flash photography. 3 EXPERIMENTAL DETAILS In order to study particle entrainment and the effect of fluid viscosity, brushing load and speed, tests were run on the optical apparatus employing a standard toothbrush consisting of equi-spaced tufts of filaments of equal length, loaded against the glass disc. Test parameters used are shown in Table 1. Loads and brushing speeds were based on reported measurements taken during in vivo experiments [2, 3]. Large (200-300 µm) coloured silica particles were used
in the initial tests because they were easy to observe during motion. A later test used more common 20-40 µm particles. The fluid used was a glycerol/water combination. These were mixed to achieve a viscosity similar to that in the mouth during teeth cleaning (i.e. that of a saliva/toothpaste slurry). Particle/fluid mixtures were made up at 1% concentration by mass. The particle/fluid mixture was applied to the filaments prior to the glass disc being placed on the apparatus. Video recordings were taken of each test. Subsequent tests, to study the effect of filament configuration, length, angle and stiffness on particle entrainment, were run on the optical apparatus using several different toothbrush head designs. A load of 360 g was used for the tests, with brushing speeds of 30 and 150 cm/s, a particle size of 200-300 µm, a particle concentration of 1% by mass and a fluid viscosity of 1.3 Pa.s. 4 EFFECT OF BRUSHING PARAMETERS On loading against the glass disc, it was found that the filament tufts all deflected in the same direction, pointing in the direction of sliding (see Figures 2-5). With a load of 360 g (1 mm filament deflection) there were gaps present around the tufts of filaments through which fluid and particles could pass (see Figure 2). Particles suspended in the fluid were sometimes seen to get caught in the tufts, but otherwise passed straight through. Particles entrained in the tufts at the start remained so. They were trapped by the ends of the filaments (see Figure 3). Only a few of the filaments were seen to have particles trapped at their tips. With a load of 980 g (3 mm filament deflection) there were no gaps between tufts through which particles could pass as the ends of the tufts spread more (see Figure 4). There were pockets of fluid, however, in which particles were seen to circulate. While circulating some became entrained in the ends of the filaments. Most particles were wedged under the bend in the filaments rather than being trapped at the filament tips (see Figure 5). Figure 2: Filaments Loaded Against a Glass Disc at 360g Glass Surface Brush Figure 3: Particle Trapping in Lightly Loaded Filaments Load (g) 360 980 360 980 360 Fluid Viscosity (Pa.s) 1.3 1.3 0.8 0.8 1.3 Particle Conc. (% mass) 1 1 1 1 1 Brushing Speed (cm/s) 3/15 3/15 3/15 3/15 3/15 Particle Size (µm) 200-300 200-300 200-300 200-300 20-40 Table 1: Fluid, Particle and Brushing Test Parameters
the filaments. As a result there were few filament tufts of filaments in which particles could become entrained and particles were able to force their way through those that did form as they were so small and loose. Where the longest filaments were in contact with the glass disc, particles were entrained (see Figure 8). Again, increasing the velocity increased the movement of particles through the contact area. Figure 4: Filaments Loaded Against a Glass Disc at 980g Glass Surface Brush Figure 6: Particle Exchange Process Occurring at a Filament Tip Figure 5: Particle Trapping in Heavily Loaded Filaments Decreasing the viscosity appeared to have little effect on particle entrainment (over the limited range of viscosities tested). With a velocity of 30cm/s there was little or no movement of particles entrained in the filaments. Increasing the velocity to 150cm/s, however, led to particles forcing their way through the filaments and either being carried away in the fluid or becoming reentrained in another tuft of filaments. It could be seen on the video recording that the 20-40µm particles were being trapped at the filament tips, as shown in Figure 6. A particle exchange process appeared to occur. Trapped particles would remain for a short time at a filament tip before they were dislodged and replaced by new particles. Two or more particles were seen to be trapped at some filament tips. 5 FILAMENT CONFIGURATION 5.1 Filament Length Equi-spaced tufts of straight filaments of graduated lengths were used to establish the effect of varying filament length (see Figure 7). On loading the filaments splayed in all directions due to the graduated length of Figure 7: Equi-spaced Tufts of Filaments of Graduated Length Figure 8: Filaments of Varying Length Under Load with Particles Entrained 5.2 Filament Angle Tufts of angled filaments pointing in opposite directions were used to establish the effect of filament angle on particle entrainment (see Figure 9). On loading the fila-,
ment tufts deflected in whichever direction they were angled on the brush head. Particles entrained at the ends of these tufts remained so (see Figure 10). Increasing the velocity did not cause any movement of particles to occur. 5.4 Brush Head Flexibility Tests were also run using a toothbrush with a flexible head design. The brush head consists of stiff and flexible regions intended to allow the filaments to adapt to the contours of the teeth. On loading particles were observed to move out of the filament tufts initially, especially in the flexible region of the head. More particles were entrained in the tufts of filaments in the stiff region of the head (see Figure 12). Increasing the velocity increased the particle movement. These results agree with those observed for the stiff and flexible filaments. Figure 9: Tufts of Angle Filaments Pointing in Opposite s Figure 12: Filaments in Flexible and Stiff Brush Head Regions Under Load with Particles Entrained Figure 10: Angled Filaments Under Load with Particles Entrained 5.3 Filament Stiffness A large tuft of stiff bristles was used to investigate whether bristle stiffness played a role in particle entrainment. It was found that particles trapped on loading remained lodged in their original positions (see Figure 11). The tuft of filaments hardly splayed out at all under load preventing the entry of particles into the tuft. Increasing the velocity had no effect. More flexible bristles, under the same applied load, deflected more and splayed out. Particles were therefore able to pass through the bristle tips and less particle trapping occurred (see Figure 11). 6 DISCUSSION The particle entrainment process in a model teeth cleaning contact occurs in the following manner; Particles, suspended in fluid, approach the contact region, as they pass into the contact they may become trapped by the brush filaments. Where and how the particles are trapped depends largely on the applied load to the filaments, and hence the degree of filament deflection, and the particle size. Increasing load increases filament deflection and affects filament dispersal within the contact. This generally has the effect of ensuring most particles in the contact remain there. However, the particles tend to be trapped under the bends of the filaments rather than at the tips. Small particles are less likely to be retained as they are able to pass though the gaps between filament tips where larger particles may become trapped (see Figure 13). They are, however, more likely to be trapped under an individual filament tip. Stiff Filaments Flexible Filaments Figure 13: Particles Passing Between Filament Tips Figure 11: Particle Entrainment with Stiff and Flexible Filaments Whether the particles are likely to be trapped and how long they remain so depends on the bristle stiffness, and hence degree of splay on loading, filament configuration and the brushing speed. The direction the filaments point in, the number of filaments in a tuft, the spacing of the tufts and the way
the filaments splay when deflected all have an influence on entrainment of particles. Tufts with tightly packed stiff filaments which deflected together on loading were more effective at trapping particles than tufts with more flexible filaments that splayed out on loading as they present more of a barrier to particle entry and exit from a contact. Raising the velocity increases the movement of particles as a result of increased fluid drag forces. Particles that would previously have been trapped under filament tips are able to force their way through filaments at higher velocities and pass through the abrasive contact. In these tests viscosity appeared to have little effect on entrainment, but only a small range of viscosities were used in testing. Further analysis using a larger range of viscosities may be more revealing. For all the tests carried out using the optical apparatus the toothbrush head was loaded against the glass disc as shown in Figure 14 (i.e. initial brush configuration perpendicular to the surface). Different brushing techniques, however, may cause the brush head to be used in different orientations, as shown in Figure 15. Clearly this will influence the effect the filament configuration has on entrainment and filament deflection will certainly not be uniform in the cleaning contact. Figure 14: Orientation of a Toothbrush Head in the Optical Apparatus In principle, it would be possible to study the effect of both rough surfaces and toothbrush head orientation using the optical apparatus. 7 CONCLUSIONS Optical apparatus has been used to visualise a model teeth cleaning contact. Using the apparatus it is possible to vary parameters such as brushing load and speed. Visualisation tests run using the apparatus to study particle entrainment into the cleaning contact, varying brushing load and speed, fluid viscosity and toothbrush design, have shown that: There are two distinct particle trapping mechanisms depending on the brushing load. At low load the particles are trapped under the filament tips whereas at higher loads most particles are wedged under the bend in the filaments. Large particles tend to get trapped at the wedge between the filaments and the counterface, whilst smaller particles can be trapped under filament tips. Fluid viscosity did not have a big effect on particle entrainment or residence time (for the viscosities tested). At higher brushing velocities particles can pass through the filaments as a result of higher fluid drag forces. Some brush designs were more effective at trapping abrasive particles than others, in the apparatus described. Typically where filament tufts deformed and splayed uniformly against the counterface good particle trapping was observed. 8 ACKNOWLEDGEMENTS R. Lewis and R.S. Dwyer-Joyce acknowledge the financial support of Unilever Research. 9 REFERENCES Figure 15: Alternative Toothbrush Orientation The tests carried out using the optical apparatus employed a flat counterface. In reality the tooth surface is full of grooves and crevices. This will obviously affect particle entrainment to a similar degree as toothbrush head orientation. [1] Addy, M.: Measuring Success in Toothbrush Design - An Opinion and Debate of the Concepts. International Dental Journal, 48 (Supplement 1) (1998), 509-518. [2] Allen, C.R., Hunsley, N.K., MacGregor, I.D.M.: Development of a Force-Sensing Toothbrush Instrument using PIC Micro-Controller Technology for Dental Hygiene. Mechatronics, 6 (1996) 2, 125-140. [3] Phaneuf, E.A., Harrington, J.H., Dale, P.P., Shklar, G.: Automatic Toothbrush: A New Reciprocating Action. Journal of the American Dental Association, 65 (1962), 12-25.