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Sonic electric toothbrush by Oral B and Braun

Electric toothbrushes

by Chris Woodford. Last updated: July 21, 2020.

Do your friends reach for the sunglasses when you smile? If not, maybe it's time you invested in an electric toothbrush. These battery powered teeth scrubbers move their heads back and forth up to 7000 times a minute, shifting twice as much plaque (rotting, sugary food debris) as ordinary, manual toothbrushes. That's called good oral hygiene and it means you won't have to go to the dentist quite so often. The cleaner you keep your teeth (and the gums that support them), the longer they'll last you. Just remember, once your adult teeth are gone, you don't get replacements! Ever wondered how an electric toothbrush works? Let's take a closer look!

Photo: The manufacturers would love us all to buy expensive sonic toothbrushes like this, but do they get your teeth as clean as ordinary electric toothbrushes and manual brushing?

Contents

  1. What's inside an electric toothbrush?
  2. What's the difference between a sonic toothbrush and an ordinary electric one?
  3. Can you replace the batteries in an electric toothbrush?
  4. Why don't new brushes last as long?
  5. Find out more

What's inside an electric toothbrush?

Take an old electric toothbrush apart and it's easy to see how it works. I had one that didn't work anymore, so I had a quick look inside before taking it to the recycling center. Here's what I found:

Photo showing component parts of an electronic toothbrush taken to pieces

The first thing you notice is the removable brush. This is the bit that does all the hard work. It has a semi-rotating head at the top so, as the mechanism inside the brush handle rotates back and forth the brush turns from side to side. Directly underneath the brush there's a cam and gear unit, which looks like this (left photo):

How the cam unit in an electric toothbrush works Animation showing how an electric toothbrush head moves back and forth.

The cam and gear unit is the clever part of an electric toothbrush. It converts the high-speed rotary (spinning) motion of the brush's electric motor into reciprocating (back-and-forth) brushing motion that cleans your teeth more effectively. The cam and gear works a bit like this very simplified animation (right). The red motor is always rotating in the same direction at high speed, but the blue and black cam and crank convert that rotation into lower-speed, back-and-forth vibration. So, as the blue cam turns, the black crank pulls first one way and then the other, moving the brush head rapidly from side to side.

The cam and gear unit is connected to a gear built into the top of the motor, so the motor drives it directly. Underneath the motor, there's a rechargeable battery. Attached to the inner plastic case, there's a simple electric circuit board that controls the on/off switch on the outer case. The outer case is made of tough plastic and the on-off switch is set into it in a piece of thin, very flexible rubber. The purpose of the outer case is to keep water and toothpaste away from the circuit, motor, and battery—which would quickly rust if you got them wet. (If you're curious to know how an electric toothbrush can recharge itself standing on a plastic base, take a look at our article on induction chargers.) And that's pretty much all there is to it—a bit of clever, effective engineering technology that keeps your teeth in tip-top shape!

Early 1937 electric toothbrust patent by Tomlinson I. Moseley.

Artwork: Early electric toothbrushes didn't use this reciprocating action. Here's the oldest electric toothbrush invention I've found, filed by Tomlinson I. Moseley in December 1937 and granted in April 1940. The electric motor (blue) drives a rack (yellow) and pinion (red) gear that makes the brush head rotate continually in the same direction. Just like in a modern toothbrush, the electrical parts are sealed inside a waterproof casing (brown), which Moseley suggested might be made from Bakelite (a tough plastic popular in those days). This invention was filed long before cordless, battery-powered appliances and induction chargers became popular, so power is supplied in a more conventional (and more dangerous) way via an electric cable (labelled 15, on the extreme right of both pictures). Artwork from US Patent 2,196,667: Electric toothbrush courtesy of US Patent and Trademark Office.

What's the difference between a sonic toothbrush and an ordinary electric one?

Sonic and ordinary electric toothbrush side by side

Photo: The head from a sonic toothbrush (seen in the lower part of the photo) is just like an ordinary manual toothbrush. Unlike a standard electric toothbrush, it doesn't contain a rotating mechanism. The whole brush head vibrates—and this can make sonic toothbrushes a little more tricky to use at first.

Crudely speaking, sonic toothbrushes work just like ordinary ones: they move back and forth over the surface of your teeth at high speed, scrubbing away the plaque. The most obvious difference is the speed of the movement: sonic brushes typically vibrate at about 260Hz (260 times per second), which translates into 31,000 brush strokes per minute—about 10 times faster than ordinary electric toothbrushes. As we saw up above, in a normal electric toothbrush, the very top part of the brush rotates back and forth thanks to a little cam unit just above the motor. The rest of the brush head is stationary. You can see a conventional electric brush head (actually a special one designed for cleaning the spaces in between your teeth) in the top of this photo. The removable brush head contains a little spring mechanism that lets the brush part turn back and forth, but the rest of the head remains static. With sonic toothbrushes, the entire brush head vibrates very quickly—and the manufacturers claim this produces a different type of cleaning action that makes them more effective.

According to the marketing spiel, because a sonic toothbrush buzzes over your teeth at much higher speed than a normal electric toothbrush, it not only brushes away the plaque but also creates waves of turbulence in the toothpaste and water in your mouth. Tiny bubbles form in the space between the toothbrush and the teeth, and these help to shift more plaque than brushing alone. So, when you're using a sonic toothbrush, it helps if you have quite a bit of fluid near your toothbrush. Then you can actually feel an energetic, tingling sensation around your teeth as the acoustic pressure waves created by the brush do their work. At least, that's what the makers claim...

Why are they called "sonic" toothbrushes?

The bubbling cleaning action sounds similar to what happens with ultrasonic cleaning, where ultrasound (higher frequency sound than humans can hear) is used to generate pressure waves and imploding bubbles (technically called cavitation) that blast dirt off things like industrial machine parts that can be hard to clean any other way. However, as far as I can see, the resemblance between ultrasonic cleaning and what happens with a sonic toothbrush is utterly superficial. Indeed, a 2004 study of five different sonic toothbrushes by British dental researchers concluded that "cavitational activity does not occur around powered toothbrushes. Operating the toothbrushes for periods up to 20 min resulted in no cavitational activity being detected." That's hardly surprising. Ultrasonic cleaning typically uses a much higher frequency (20,000–40,000Hz) where the cavitation effect is much more pronounced. In other words, "sonic" is quite misleading—and little better than a marketing term. Ordinary (rotating head) electric toothbrushes could also be described as "sonic" inasmuch as they make a noise when they're vibrating. In both cases, it's actually the vibrating action of the brush that's doing the cleaning.

Do sonic toothbrushes work and are they worth paying for?

""People are certainly fooling themselves, with the majority of [electric] brushes at least, if they think they are more effective."

Professor Bill Shaw, of the University Dental Hospital in Manchester, quoted by BBC News, 2003.

Don't trust the "customer reviews" expressed on websites selling electric toothbrushes. It looks to me like quite a few of these are submitted by people who work for the toothbrush makers. Even some of the scientific papers published in this area are attributed to authors who work for brush companies—and even though the science may be perfectly legitimate, treat them with caution and be sure to read any "conflict of interest" statements.

My personal experience of the three common types of brushing—manual, rotating, and sonic—aligns with the findings from recent scientific studies. Electric brushes seem to get my teeth cleaner than normal brushing, and I definitely prefer my sonic brush to a rotating one, but I suspect effectiveness is often a matter of preference: if you like using a brush, you will use it more often and perhaps for longer, and that will very likely translate into cleaner teeth. My own opinion, for what it's worth, is that if you use an ordinary electric brush properly, and floss your teeth as well, there's no benefit to be gained from going sonic, especially given the considerable extra cost of a sonic brush.

What does the science say?

A few years ago, two British scientific studies (in 2003 and 2004) found that ordinary (rotating-oscillating) electric toothbrushes were the only electric brushes that outperform manual brushing. Some more recent studies (in 2015 and 2017) have demonstrated that sonic brushes can also perform better than manual ones, and this was confirmed by a 2017 meta-study.

How are sonic toothbrushes different inside?

Take apart a sonic brush and you'll find it's very similar to one with a rotating head, though the circuit board is typically bigger and more complex. In the sonic brush shown below, the circuit controls the brush speed (with two different speeds available at the flick of a button), a timer that beeps every 30 seconds and after two full minutes (to encourage rigorous brushing), and the battery recharging process.

A Braun sonic toothbrush with its outer case removed showing the component parts inside. Side view A Braun sonic toothbrush with its outer case removed showing the component parts inside. Front view.

Photo: Two photos showing the component parts of a typical sonic brush. Note the bigger and more complex circuit in this brush compared to the one up above. This one even has a microchip inside!

How do ultrasonic toothbrushes work?

If you really want ultrasonic cleaning, you need an ultrasonic toothbrush that vibrates about 100–1000 times faster than conventional rotating or sonic toothbrushes to produce a genuine cavitational cleaning effect. Ultrasonic brushes work in a quite different way from rotating and sonic toothbrushes: they have no electric motor inside:

The components inside an ultrasonic electric toothbrush.

Artwork: How a genuine ultrasonic toothbrush works. A battery (1) powers an electronic circuit (2) that produces a high-frequency DC current. Through connecting wires (3), this drives a piezoelectric transducer (4) that converts electricity into high-speed vibration. The transducer expands and contracts at very high speed, producing sound waves in your mouth that create gentle cavitation (5) around your teeth, loosening plaque that can be brushed aside (6).

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Can you replace the batteries in an electric toothbrush?

In theory, you never need to worry about the battery in your brush. Treat it kindly (let it discharge fully once in a while, don't leave it sitting on the charger forever, and don't overcharge it) and you may well get 10 years of life out of it (as I did for both of mine) before it stops working. Even with the very kindest treatment, however, a rechargeable battery won't last forever. You'll notice it doesn't stay charged as long or perhaps drive the motor as powerfully as it once did. When that happens, you have a choice: either throw out a perfectly good brush (and all the mechanism inside) or try to replace the battery yourself. If you're throwing the brush away, ideally, if you feel it's safe to do so, take it apart and try to recycle as many of the components as you can. Check the instructions to find out how. If you don't feel like doing this, don't toss your brush in the trash: dispose of it sensibly in the electrical recycling.

Replacing the battery in a Braun Oral-B electric toothbrush.

Photo: Top: To replace the battery in this brush, I had to desolder the connections for the charging coil (red) to get that out of the way. Then I could desolder the battery connections (blue), remove the old battery, and solder a new battery in its place. It only took five minutes to make this repair, but it is a little fiddly. Bottom: The battery is similar to a standard one, but comes with special metal ribbon connections attached for soldering it in place.

If the brush no longer works anyway, and you're reasonably confident at soldering, why not try a repair? Find out the make and model number and search online (and on YouTube) for simple repair videos. It's relatively easy to open the plastic brush case (usually by twisting the bottom, sometimes with the help of the power charger unit), which makes the whole mechanism (you can see it in the photo above) pop out. Once you've got to that point, you'll see that (annoyingly) the battery is soldered in place. Usually, you have to desolder the two contacts of the battery charging coil at the base to remove that temporarily. Once you've done that, you can remove the battery by desoldering its two metal ribbon contacts. It's a bit of a fiddly job to remove the old solder and clean up the circuit board ready for a replacement, but take your time and be patient; it won't take more than a few minutes. (Use a heat sink if you can to avoid damaging the coil and the battery.) Get an identical replacement battery (they're easy to find online) and run the disassembly process in reverse: solder the new battery in place and then replace the charger coil. Test that everything still works, put the thing back together (making sure it's still watertight), and your brush should be as good as new!

What sort of battery do you need? The batteries in electric toothbrushes are fairly standard and roughly the same size as normal AA ones, though they have flat contacts soldered onto them. Usually, they're rated at about 1.2 volts (some brushes have two 1.2 volt batteries in series) and are either nickel-cadmium (NiCd) or nickel metal hydride (NiMH); a few newer brushes use lithium-ion batteries. It's best to remove your old battery and find an identical replacement (same voltage, charge capacity, and physical size) if you can.

Why don't new brushes last as long?

Newer brushes tend to be more sophisticated than older ones. They have LED lights, buzzers, timer alarms, and so on, and all those things really eat up the power. My simple rotary brush gives about two weeks of battery life and I let it run down completely each time before recharging. My Sonic brush (which, you can see from above, had a much more elaborate circuit attached) obviously drew much more power: even though it had a similar battery, it only lasted a few days before needing a recharge. If you think about it, this is quite a cunning trick by the brush makers: if the battery needs recharging more often, the whole brush will need replacing much sooner (because rechargeable batteries can only be charged up so many times), which means you'll buy another one more quickly than before. If you find the battery life of new brushes disappointing, that's another good reason for trying to repair and replace the battery in your old brush when it gets to the end of its days.

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Text copyright © Chris Woodford 2007, 2020. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007/2020) Electric toothbrushes. Retrieved from https://www.explainthatstuff.com/electrictoothbrush.html. [Accessed (Insert date here)]

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