An Ultraminiature Pressure Sensor Integrated into Acupuncture Needle Tip for In Vivo Monitoring of Local Muscle Response

Real-time monitoring of local muscle response during acupuncture could reveal mechanisms underlying acupuncture and provide objective feedback to support more precise needle control. However, the small dimensions of acupuncture needles impose stringent constraints on sensor miniaturization and packaging. This study presents an ultraminiature pressure sensor integrated into the tip of a 450-μm-diameter acupuncture needle. The sensor measures 200 × 1250 × 50 μm3 and incorporates an elongated pressure-sensitive diaphragm. Wafer-level AuSn solder deposition and thermocompression soldering connect the chip to an ultrafine wire ribbon routed through the 230-μm-diameter needle lumen. A 1.3-μm-thick Parylene C coating provides electrical insulation. After coating, the sensor exhibited a sensitivity of 40.3 μV/kPa at a 3.3 V excitation, retaining 78.4% of its uncoated sensitivity. Characterization over 90–150 kPa absolute and 25–45 °C yielded a maximum nonlinearity of approximately 0.73% of full scale (FS). The coated sensor exhibited a maximum hysteresis of 2.52% FS. In vivo experiments at the Zusanli acupoint in rats recorded pressure variations during thrusting–lifting manipulation. These results demonstrate the feasibility of monitoring local muscle response during acupuncture and their potential to provide quantitative feedback for needle manipulation.