No matter how thoroughly you brush your teeth in the morning and evening, the battle against bacteria is almost certain to be lost. Within a few hours of brushing, it will return in force and rebuild a solid defence line on the surface of your teeth. From ancient times to now, humanity has essentially had only one way of dealing with it, whether it was using twigs and sticks or the first bristled toothbrushes, which date back to 10th century China. But even today’s most advanced electric brushes...