Biocompatible and Ultra-Flexible Inorganic Strain Sensors Attached to Skin for Long-Term Vital Signs Monitoring

Wearable electronics have attracted much attention and are experiencing rapid growth in recent years. Such devices are expected to stay closer to the human body (i.e., attached to the skin) for better performance. Therefore, ultra-flexibility of such devices is necessary in order to make the sensor conform to the human body when the devices are used for healthcare monitoring. Here, we present a biocompatible and ultra-flexible strain sensor for pulse and body motion real-time and long-term measurement. The sensor, fabricated and integrated on a semi-permeable substrate with good biocompatibility and waterproofness, is mechanically invisible for the human. It owns good linearity (r2 = 0.997), good repeatability, low resistance (350 Ω), and short response time (less than 100 ms). The sensor is designed with the shear lag theory, obtaining greater measuring range but still with good linearity. The liquid transfer printing method is used for thin-film sensing part and soft substrate integration in order to avoid damage. The sensor shows better performance and higher precision in motion and pulse monitoring than other similar sensors. The in vitro experiments demonstrate that the sensor is more suitable for long-term health monitoring at medical grade.