Just found this intriguing, found it in Industryweek from 1999:

Gillette uses miniature cameras to get at the core interactions between its products and their users.

While Gillette Co., Boston, is actively observing and videotaping men and women shaving and brushing teeth, it also uses microcameras to observe the interaction of brush and tooth, blade and whisker.

In the development of the premium-priced Oral B Cross Action toothbrush, for instance, a miniature high-speed video camera was placed inside a model of a mouth that included one transparent tooth. Examining some 50 prototypes of bristle design, Gillette videotaped the action of the bristles through the clear tooth as the brush moved across the teeth. This showed that cleaning took place most effectively when the bristles changed direction as the brush moved across the teeth and that angling the bristles in a crisscross fashion increased this effect. That led to the design just now being marketed at premium prices.

I was not aware of such "In-Mouth Ethnography".