An Ultra-Thin Piezoresistive Stress Sensor for Measurement of Tooth Orthodontic Force in Invisible Aligners

A silicon piezoresistive stress sensor is developed to measure the three components of the orthodontic forces of a tooth in invisible aligners. The stress sensor chip consists of multiple piezoresistor rosettes, and is fixed on the tooth surface to measure the orthodontic force induced stresses. An inverse problem algorithm is established to reconstruct the orthodontic forces of the tooth from the measured stresses. A key thinning process is developed using chemical-mechanical polishing to thin the sensor chip from 650 to 100, such that it can be accommodated to the limited space between the aligner and the tooth, without much influence on the orthodontic force. The ultra-thin thickness enables the sensor chip to be compliance with tooth deformation, and improves the stress measurement sensitivity as much as 40 times, making it capable of measuring small orthodontic force. The sensor design, fabrication, orthodontic stress measurement, and force reconstruction are given in details, and the experimental results demonstrate the feasibility of using miniaturized and ultrathin sensors to determine the in vitro orthodontic forces.