A Dual-Layer Orthogonal-Layout Thin-Film Force Sensor for Digital Orthodontic Functional Appliances

Measuring the force exerted on oral tissue is crucial for orthodontic treatment. Without proper tools and methods, orthodontists must rely on their clinical experience to estimate the force, which limits the efficiency and accuracy of diagnosis and treatment. Previous works using flexible force sensor arrays and wireless technology are power-hungry and require either complex wiring or batteries for power supply. However, human oral nerves are highly sensitive to foreign objects, so these solutions are not suitable for clinical practice. This work presents a thin-film force sensor based on carbon paste on a flexible printed circuit (FPC) substrate with an area of 24 x 4 mm(2). The sensor is embedded in functional appliances and can be read out through a miniature connector by a handheld device that uses frequency-domain signal processing. The device is powered by a 360-mAh battery and can last for 15 h with a form factor of 86 x 40 x 21 mm(3). The device achieves a force resolution of 10 mN with an update rate of 10 Hz. Six samples are characterized and after two-point calibration, this system achieves a force accuracy of +1.3/-0.6 N. Our measurement results also show that the sensor is robust against a liquid environment. A dual-layer orthogonal-layout method is proposed to check the alignment of the contact surfaces.