Patent Publication Number: US-2011076638-A1

Title: Apparatus for cleaning teeth using a variable frequency ultrasound

Description:
This invention relates generally to devices for cleaning teeth using ultrasound, and more specifically concerns the combination of a bubble generator and an ultrasound source which vibrates the bubbles at or near their resonant frequency. 
     Gas bubbles in a liquid such as water results in a vigorous fluid flow when the bubbles are vibrated with ultrasound frequencies at or near the resonance frequency of the bubbles. Such a fluid flow directed toward teeth has the effect of disrupting and removing dental plaque from the teeth. Such a system is the subject of pending PCT patent application No. PCT/IB2006/054463, which is owned by the assignee of the present invention, the contents of which are hereby incorporated by reference. Such devices, however, use a single ultrasound frequency. The bubble generation for such systems must accordingly be quite precise, with the bubbles having a radius matched with the frequency of the ultrasound signal for maximum effect of the ultrasound signal. 
     In practice, such precise bubble generation is difficult to achieve, particularly in a mass-produced device, since the required precision requires additional expense. The lack of precision in bubble generation leads to bubbles having a range of sizes, which results in a decrease in efficiency of the device, because not all the bubbles can be effectively used for cleaning plaque with a single ultrasound frequency. In addition, the use of a single ultrasound frequency produces a stationary standing wave/interference pattern on the teeth, with the high intensity and the low intensity points of the ultrasound being always in the same position. This typically results in a particular biofilm removal pattern on the teeth, in which certain areas are not cleaned as well as other areas, leaving dental plaque on the teeth in those areas, which is undesirable. 
     Hence, it is desirable that a bubble generator/ultrasound system be able to effectively make use of a range of bubble sizes, while producing a more homogeneous cleaning of the teeth. 
     Accordingly, described and shown herein is an apparatus for cleaning biofilm from teeth, comprising: a source of gas bubbles in a liquid medium, the bubbles having a range of sizes associated with effective removal of bacteria in the biofilm, each gas bubble having a resonance frequency; and a source of ultrasound signals having a range of frequencies, the ultrasound frequency range including frequencies corresponding to the resonance frequencies of a majority of the air bubbles, wherein the ultrasound signals are applied to the flow of air bubbles/liquid, vibrating the bubbles so that upon reaching the biofilm, a cleansing action occurs. 
    
    
     
       Also described and shown herein is a toothbrush, comprising: a toothbrush handle portion; a toothbrush head portion extending from the body portion and having an extending cup-shaped portion; an ultrasound transducer mounted in the cup portion and operably connected to transmit ultrasound waves from the cup portion, focused on teeth surfaces; and a source of gas bubbles in a liquid medium, the bubbles having a size associated with effective removal of bacterial in the biofilm. 
         FIG. 1  is a block diagram of the bubble generation/ultrasound apparatus as shown and described herein. 
         FIG. 2  is a diagram showing a portion of the apparatus positioned in the interproximal area between two teeth. 
         FIG. 3  is a diagram showing the generation of the ultrasound signal. 
         FIGS. 4A and 4B  are diagrams showing a system for generating the bubble liquid mixture. 
         FIG. 5  is an elevational, partially cross-sectional view of a toothbrush embodiment. 
         FIG. 6  is an elevational, partially cross-sectional view of another embodiment. 
     
    
    
     The apparatus of  FIG. 1 , shown generally at  10 , when contained partially or completely in a teeth-cleaning device body/casing, including a handle and an extended head, is designed for cleaning teeth and is described in the context of that particular application. However, the principles of the apparatus can be used effectively in other applications, which are discussed below. The apparatus generally combines a bubble generator  14  with a piezoelectric transducer  16  and associated piezoelectric transducer drive electronics  18 , referred to as the drive electronics. 
     Apparatus  10  includes a nozzle/standoff member  12  which is designed to be positioned against the teeth, particularly the interproximal area, to provide a desired spacing between the piezoelectric transducer  16 , which produces a range of ultrasound frequencies, and the teeth, specifically to maintain the teeth at or near the focus of the transducer. For instance, at a 400 kHz center frequency of the ultrasound signal, the focus distance is 6.7 mm, for a flat, round transducer 10 mm in diameter. This size will provide good coverage for the teeth surfaces as well as the interproximal space. The range of transducer focus, for instance, for a frequency range of 300-500 kHz, will be 5.1-8.4 mm. The total height of the transducer  16  and standoff member should not be more than 20 mm, which is approximately the size of a regular toothbrush head. From the above, the standoff distance of member  12  will be in the range of 1-15 mm. 
     If the transducer  16 , including the body/casing, has a thickness of 5 mm, the standoff distance is preferably equal to the focus distance of the transducer at the lowest efficient frequency, which in the example above is 5.1 mm. In another example, when the ultrasound frequency varies over a range of 0.75 to 1.25 MHz, with a center frequency of 1 MHz, the focal distance of a flat, round transducer 10 mm in diameter will range from 12.6 mm to 21 mm. The preferred standoff distance is 12.6 mm. This distance can be decreased if the transducer has a non-flat design. 
     Bubble generator  14  in operation produces a stream of air bubbles in a liquid jet to nozzle member  12 . Bubble generator  14  produces bubbles of a range of sizes which are effective in removing dental plaque. In particular, the size of the bubbles will match the size of the bacteria, or colonies/clumps of bacteria, referred to as lumps, present in the biofilm on the teeth. Since the bacteria and/or the lumps have a range of sizes, the bubbles also will have a corresponding size range of bubbles, typically in the micron range. The piezoelectric transducer  16  is designed for broadband ultrasound generation, driven by the drive electronics  18 , as mentioned above. The piezoelectric transducer producing a range of frequencies has the advantage of matching the resonant frequencies of a range of bubble sizes, thereby producing effective resonant vibration of a range of bubble sizes as opposed to just one bubble size. This results in effective cleaning for a range of bacteria/bacteria lump sizes, as well as producing homogeneous cleaning effect of the teeth, including the interproximal areas. 
     More specifically, drive electronics  18  and piezoelectric transducer  16  will produce an ultrasound signal having a selected center frequency, with a particular bandwidth about that frequency. The center frequency can vary over a considerable range. At the low end, the center frequency could be 200 kHz, while at the high end, the center frequency could be 2 or even 4 MHz. A more preferred range is between 200 kHz and 2 MHz, while a preferred center frequency is approximately 1.0 MHz, although a 400 kHz center frequency has also produced good results. In the case of a 1 MHz center frequency, with a bandwidth of 50%, the range of ultrasound frequencies produced will be 750-1250 kHz, while a 50% bandwidth for a 400 kHz center frequency is 300 kHz-500 kHz. 
     Besides the range of ultrasound frequencies produced by the piezoelectric transducer/drive electronics combination about a selected center frequency, the drive signal produces bursts of ultrasound, instead of a continuous ultrasound signal.  FIG. 3  shows the ultrasound signal burst arrangement. The ultrasound signal will be off for a selected time T 1  and on for a selected time T 2 . The duty cycle of the ultrasonic signal is controlled by a first trigger control signal (trigger  1 ). In one embodiment, T 1  and T 2  are equal, each being approximately 1 second. T 1 /T 2 , however, can vary, typically within a range of 0.1 to 2. Time T 1  (the off time for the ultrasound), however, must be sufficient so that a fresh set of bubbles is present for the next ultrasound wave. Time T 1  will thus depend upon the velocity and the concentration of the bubbles, but will typically be between 10 ms and 1 second, most preferably 100 ms. This T 1  “pause” in the ultrasound is important to prevent agglomeration of bubbles, which tends to occur when the ultrasound signal is continuous or there is insufficient pause (off) time T 1 . Preventing agglomeration of bubbles is an important advantage of the present system using ultrasound signal bursts. 
     Time T 2  contains one or more ultrasound bursts. The frequency of the bursts, indicated at  24 - 24 , can be varied. In one example, the frequency of the bursts ranges between 25 and 600 Hz. This is referred to as the burst repetition frequency (BRF), controlled by a second trigger signal (trigger  2 ). The lowest possible BRF depends on the value of T 1 +T 2 , where BRF=1/T 1 +T 2 , where there is only one burst during T 2 . Preferably, the burst repetition frequency is within the range of 100-500 Hz. Most preferably the frequency is approximately 200 Hz. Within each burst, there are a number of individual ultrasound cycles  25  at one ultrasound frequency within the range of frequencies produced by the ultrasound device. In one example, the ultrasound signal frequency in one burst is 1 MHz. The number of cycles within each burst can vary, typically within the range of 50-5000, with a preferred value of approximately 1000. This results in an ultrasound signal pattern indicated at  26  in  FIG. 3 , comprising successive bursts of an ultrasound signal  30  at a selected ultrasound frequency when the ultrasound device is on (T 2 ), followed by a pause time (T 1 ), when the ultrasound device is off. 
     It should be understood, however, that the above-noted preferred values of T 1 , T 2 , BRF and the number of cycles per burst are merely illustrative, as the optimal settings are determined by the parameters of the actual flow, including the bubble concentration, bubble size distribution, bubble flow velocity, bubble liquid flow rate and bubble liquid velocity. 
     The frequency of the ultrasound signal within each time period T 2  can be the same, with the frequency changing for each successive time T 2 , or the frequency of the ultrasound signal can change within each time T 2 , i.e. in accordance with a pre-selected pattern, as the ultrasound frequency changes over the bandwidth of the ultrasound device. 
     The optimal frequency range of the ultrasound signal depends on several parameters, including several safety parameters. The lower end of the frequency range is limited by one such safety concern, determined as follows. The amplitude of the ultrasound signal needed for effective removal of biofilm is within the range 0.3-0.5 MPa, referred to peak rarefractional pressure. The peak rarefractional pressure is related to the mechanical index (MI) value associated with the ultrasound signal, which in turn is a good predictor of the likelihood of possible damage to the tissues, including teeth, gum and bones. The mechanical index is defined as follows: 
     
       
         
           
             MI 
             = 
             
               P 
               
                 f 
               
             
           
         
       
     
     In the use of diagnostic ultrasound, the FDA permits a maximum MI of 1.9. Using a pressure P of 0.5 MPa, which is at the upper end of effective pressure, the resulting lower limit of ultrasound frequency is 69 kHz in order to meet the FDA MI standard. 
     The intensity of the ultrasound signal is also limited by safety issues. For example, a 1.9 MI would limit the maximum peak rarefractional pressure at a 300 kHz ultrasound signal to 1.0 MPa. This value will change, depending on the actual ultrasound frequency. Further, the FDA maximum time averaged intensity, which takes into account duty cycle, is set at 0.72 W/cm 2 . The intensity I can be calculated from a value of P as follows: 
     
       
         
           
             I 
             = 
             
               
                 P 
                 2 
               
               
                 2 
                  
                 
                     
                 
                  
                 ρ 
                  
                 
                     
                 
                  
                 c 
               
             
           
         
       
     
     With a continuous wave of ultrasound, and a pressure of 1 MPa, the intensity is 34 W/cm 2 . Accordingly, the maximum duty cycle with those values would be 2.1%. Using 0.5 MPa, the intensity decreases to 8.4 W/cm 2 , which increases the maximum duty cycle value to 8.5%. Hence, duty cycle is important to accommodate safety concerns of pressure and intensity while still producing effective ultrasound action. 
     The duty cycle can be calculated from the ultrasound driving signal parameters shown in  FIG. 3 . The burst lengths are calculated from the number of cycles per burst divided by the ultrasound frequency. For example, with an ultrasound frequency of 400 kHz and 1000 cycles per burst, the burst length is 2.5 ms. The duty cycle during T 2  is determined by the burst length (t) and the burst repetition frequency, in particular BRF×(t)/100, in %. The total duty cycle of the system is T 2 /T 1 +T 2 ×BRF×(t)/100, in %. For a BRF of 200 Hz, T 1  of 0.2/s and T 2  of 0.03 s, the duty cycle of the system is 10%. 
     As indicated above, an important aspect of the present system is that the ultrasound generates a range of ultrasound frequencies, in the form of signal bursts of ultrasound, with the range of frequencies being associated with/corresponding to the range of bubble sizes produced by the bubble generator, which in turn is associated with the range of bacterial and/or bacterial lump sizes in the dental plaque biofilm on the teeth. 
     The bubble generator  14  is shown in more detail in  FIGS. 4A and 4B . In general, the bubble generator  14  mixes air and water to produce bubbles. As indicated above, it is important to prevent the agglomeration/aggregation of bubbles during the operation of the apparatus. Accordingly, bubbles are continually produced so that there is always a fresh set of bubbles directed toward the teeth. The rate of bubble agglomeration depends on the bubble flow velocity and concentration and the intensity and duty cycle of the ultrasound signal. In one example of flow velocity, when the bubble liquid is discharged from a nozzle 1 mm in diameter, a flow velocity of 28 cm/s is obtained from a flow rate of 13 ml/min. 
     The velocity of the bubble mixture is produced by a pump. A continuous flow centrifugal pump is generally preferred, as shown at  40  in  FIGS. 4A and 4B . Pump  40  includes a housing  42  and impeller  44  which produces a suction effect for the gas bubbles and liquid introduced into the pump, vigorously mixing them and then directing the resulting fluid bubble mixture to a discharge port  46  connected to the nozzle/standoff element  12 . Such centrifugal pumps are well known and commercially available. 
     The formation of the gas (preferably air) bubble/liquid mixture which moves to the impeller is shown in  FIG. 4B . This includes a body portion  50  which includes an opening for fluid from a reservoir  54  ( FIG. 1 ) and an air inlet  56  at a proximal end  59  of an air inlet tube  60 . The air inlet  56  is at atmospheric pressure P 0 . The pressure P 1  of the liquid in interior volume  58  surrounding air inlet tube  60  is larger than pressure P 0  by a factor which depends upon the height of the liquid level in interior volume  58 . The dimensions of interior volume  58  decrease as the interior volume approaches the distal end  61  of the air inlet tube  60 . The interior surface  63  of the body portion  50  is spaced a small distance from the distal end  61  of air inlet tube  60 . In the embodiment shown, there is a pressure drop between P 1  and pressure P 2  at outlet  62  of air inlet tube  60 , which is larger than the pressure generated by the height of the liquid in the interior volume  58 . The dimensions of interior surface  63  of body portion  50  are significant. For example, when outlet opening  62  is 0.3 mm, and the exterior diameter of the inlet tube  60  is 0.6 mm, then the diameter of the interior surface  63  at point  66  should be smaller than 0.67 mm. 
     The bubble/liquid mixture coming from through outlet  62  is sucked into the impeller, which thoroughly mixes the liquid and air. The resulting flow of bubbles/liquid is then directed into a connecting line  70  to the nozzle/standoff element  12 . A soap or a surface active substance (surfactant) can be added to the liquid from a container  72 . This reduces the surface tension of the fluid, increasing the number of small bubbles and the uniformity of the bubbles. One example of a suitable surfactant is sodium laurylsulphate, which may be added in an amount of 0.25 m %. This results in optimal surface tension and viscosity. Increasing the viscosity of the liquid increases the shear forces and may have a greater effect against the bacteria on the teeth. It should be understood that  FIGS. 4A and 4B  illustrate one embodiment for generating a bubble-liquid stream. Many other pumps or similar devices to mix liquid and gas could be used. One alternative way to create a fine bubble mixture is to suck up air and liquid with a pump and then pressurize the mixture in the pump. The air will dissolve in the liquid. When the pressurized air and liquid is released through the nozzle, air bubbles are formed due to the lowered pressure. It is even possible to use a pre-pressurized gas-liquid mixture, for example carbonated fluid-containing pressurized CO 2 . At the nozzle, bubbles will be generated that can be employed for dental plaque biofilm removal. 
     Typical bacteria in dental plaque biofilm are somewhat spherical in shape, with a radius of approximately 4 μm. Since the bacteria are typically very rigid, they may not break under the applied shear stress, particularly if the bubbles are smaller than the bacteria. Hence, the bubbles should typically be greater than the size of the bacteria. It has been found that the bacteria are usually organized in colonies. These colonies or lumps are typically easier to dislodge than the bacteria within the lumps. The colonies can vary between 5 μm and 25 μm in radius. Bubbles in this size range are thus most efficient in effectively and quickly removing bacteria from the teeth. 
     In operation, bubbles of a desired size are produced by the bubble generator in a continuing stream. The size of the bubbles may vary over a range of +/−30%, which permits the use of a relatively inexpensive bubble generator. A range of bubble size is important and the various bubble sizes, when energized by the ultrasound at their resonant frequencies, operate on a variety of bacteria colony sizes normally encountered in dental plaque. The bubbles are resonated by periodic bursts of ultrasound signals, with the ultrasound having a selected on/off pattern, which tends to prevent aggregation of the bubbles, thus increasing the effectiveness of the plaque removal. Using a range of ultrasound frequencies, besides the advantages of operating effectively on a range of bubble sizes, produces a varying interference pattern on the plaque, which produces a more homogeneous cleaning effect. 
     As discussed above, the apparatus of  FIGS. 1-4  is useful in effective removal of dental plaque bacteria. However, the system can be used for cleaning of other surfaces, including membranes and microchips as well as cleaning of biofilm infections in a variety of applications. The bubble size and the ultrasound frequency range must simply be matched to the size of the bacteria or other item to be removed. 
     Another embodiment of an oral cleaning device in the form of a toothbrush using gas bubbles and/or vibration of the toothbrush with an ultrasound signal is shown in  FIG. 5 . In this embodiment, the toothbrush/applicator  80  includes a handle portion  81  and a head portion  82 . The handle portion  81  includes piezoelectronics  84 , bubble generator  86  and a toothbrush drive circuit  88  for moving the toothbrush head in a selected motion. The toothbrush drive circuit may be used with the gas bubbles and ultrasound or just with ultrasound, or not at all. The toothbrush drive can be any of a number of different drive arrangements to vibrate the head portion  82 , which in  FIG. 5  is shown with bristles  83 . The bubble generator  84  and the piezoelectronics  86  are like that described above for the embodiment of  FIGS. 1-4 . They can be provided in a separate unit attached to toothbrush  80 , if desired. A water container  85  is connected to the bubble generator. 
     Handle portion  81  includes an elongated section  90  which extends to head portion  82 . A wire  91  or similar element carrying the piezoelectric drive signals from piezoelectronics  84  extends through elongated section  90 , as does a line  92  for the gas bubble/liquid mixture, from bubble generator  86 . Head portion  82  includes a curved surface  98  in which is disposed a cup member  100 . Cup member  100  is curved, for instance a prophy cup, which is shaped to focus, i.e. direct, ultrasound waves produced by piezoelectric transducers  102  and  104  positioned on or in the cup member  100  toward the teeth. Cup member  100  is preferably fabricated from a flexible, pliable material, such as rubber or other polymer elastomers. Additional ultrasound transducers can be provided so as to provide a ring of ultrasound transducers around the cup member. The ultrasound transducers are typically located near the middle of cup member  100 , as shown. 
     An opening  106  in the center of cup member  100  provides an exit for the gas bubble/liquid moving through line  92 . During operation, opening  106  serves as an outlet for the gas bubbles in the liquid medium, directed toward the target surface, e.g. teeth. The ultrasound waves produced by transducers  102  and  104  are focused toward the target surface by the shape of cup member  100 . The ultrasound waves vibrate the bubbles in the liquid medium, as discussed in detail above, producing the desired cleansing bubble action described above. The characteristics of the ultrasound signal discussed above with respect to the embodiments of  FIGS. 1-4 , including the various possible ranges of frequencies and center frequencies, on/off time and burst rate can also be used in this embodiment, although it should be understood that a single ultrasound frequency can also be used. This provides the good cleaning action with a range of bubble sizes described in detail above. 
     Bristles  83  are provided on the head portion  82  to provide a brushing action if desired, with a brushhead motion produced by driver circuit  88 . The vibrating action can be used with just the ultrasound or with the ultrasound and the gas bubbles. 
     In addition to the effect of the ultrasound waves acting on the bubbles, which in turn act on the dental plaque for cleaning plaque from the teeth, as discussed above, the gas bubble/liquid can be used to transport the ultrasound waves from the transducer to the teeth for direct action on the dental plaque. The gas bubble/liquid thus acts as a guide for the ultrasound waves. When the successive bursts of ultrasound energy in this arrangement are sufficiently long, a portion of each ultrasound burst will reach the surface of the teeth without much energy loss, producing a desired cleaning effect. 
     In this arrangement, when water, for instance, is used as a fluid for guiding the ultrasound waves, the fluid needs to be refreshed (replenished) as it escapes from the cup or other openings in the hollow member during operation. In another embodiment, two separate pumps  107 ,  108  can be used, as shown in  FIG. 6 , one for pumping a bubble/liquid through line  110 , while another pumps liquid without bubbles through line  112 . The bubble/liquid, for instance, can be released through the cup member  114  close to the surface of teeth, while the other liquid fills the cup to act as a transport for the ultrasonic waves. As an alternative, gel can be used to fill the cup for transport of the ultrasound. A gel may aid in plaque cleaning as well, since it will mix with the bubble/fluid and increase the viscosity thereof, thereby benefiting the shear forces associated with the cleaning. 
     With the two-liquid embodiment of  FIG. 6 , different and otherwise incompatible fluid chemistries can be used, which are mixed just before application to the teeth in the volume defined by the cup member. One example is for teeth bleaching. In the embodiment of  FIG. 6 , the bubble/liquid supplies bubbles at a sufficient rate to the surface of the biofilm to flush away previous clusters of bubbles, maintaining the site clear for ultrasound action. 
     Although a preferred embodiment of the invention has been disclosed here for the purposes of illustration, it should be understood that various changes, modifications and substitutions may be incorporated in the embodiment without departing from the spirit of the invention, which is defined by the claims which follow.