Those patterns function as distortion gauges. Metal electric lines are extended by the force of a finger pressing the screen, changing electric resistance. And the force is detected by sensing the change.
Most of the sensing areas are spiral patterns. But, in some sensing areas such as the corners and edges of the pressure sensor, there were other patterns such as meandering and folded patterns. The direction of the force applied to the sensing area differs between the corners and edges of the pressure sensor and the central area of the sensor. To address this issue, Apple probably used different patterns for the corners and edges and the central area.
Preparation for future?
The sensing areas where patterns such as a spiral pattern are formed are scattered on the surface of the pressure sensor with some space in between. It is greatly different from the conventional capacitive pressure sensor in which electrodes are arranged with no space in between.
When a force is applied to space between sensing areas, the strength of the force is probably estimated based on data collected by the surrounding sensing areas. However, it is probably difficult to estimate the strength with the same accuracy as in the case where a sensing area is directly pressed.
Still, by using a display to show icons right above sensing areas, it becomes possible to make use of the high resolution. At this point, applications using the 3D Touch function supporting more than three levels of pressing force have not been announced yet. But the employment of the resistance method is a preparation for the future, Touch Panel Laboratories Co Ltd said.