The problem is, just like a monitor, a tactile feedback device needs a certain resolution and refresh rate to create smooth sensations. The required resolution and refresh rate can be identified by studying the physiology and psychology of tactile perception. For example, a measurement called the “two-point threshold” determines the resolution of human tactile perception. The two-point threshold varies from less than 2 mm at the tip of the index finger to as much as 70 mm across the torso. This means that you would need over 1000x as many tactile actuators in a given area of the fingertip versus the same area on the torso! As you might imagine, it’s difficult to pack so many actuators into such a small space in a device that is still practical to wear.
To make things more challenging, each actuator needs to have a significant amount of displacement. The skin is not a rigid surface. If fact, parts of the skin deform by up to several centimeters when a significant amount of pressure is applied. This means that no matter how hard the actuator can push, it has to be able to move quite far to apply a meaningful pressure to the skin. In practice, this means the actuator at each pixel must create a lot of motion relative to its size.
For example, the kind of motor used to produce vibrations in cell phones is often used for basic haptic feedback. This kind of motor is several millimeters thick, and offers no more than a few tenths of a millimeter of displacement. This means the ratio between actuator thickness and displacement is less than 0.1 (i.e. it displaces less than 1 mm for every 10 mm of thickness). An ideal tactile actuator needs to be thin enough to be comfortably worn on the skin – no more than 2-3 mm – and needs to offer up to about 2 cm of displacement. This means the ratio between actuator thickness and displacement for an ideal tactile actuator is around 10, over 100x higher than the haptic motors commonly used in cell phones!