Three-axis capacitive force sensor with liquid metal electrodes for endoscopic palpation

Endoscopic palpation is a promising technology for detecting tumours that are too small to be detected by CT and magnetic resonance imaging, are not located on tissue surfaces, and cannot be observed using endoscopes. This method uses a small force sensor mounted on the tip of an endoscope. It is desirable that the sensor can scan tissue surfaces and continuously measure the stiffness of organs. Prior work developed a ballpoint-pen-like capacitive force sensor and carried out proof of principle experiments; however, the sensor was too large to be mounted on an endoscope. This study designed three-dimensional (3D) microfluidic channels encapsulating liquid metal to develop a sensor that is small enough to be mounted on an endoscope. Eight polydimethyl siloxane layers with channel structures were assembled and filled with Galinstan, a commercial liquid metal, to form 3D electrodes. The sensor was experimentally characterised and verified to be applicable to endoscopic palpation.