A Nanomesh Pressure Sensor for Monitoring Finger Manipulation While Avoiding Interference with Human Sensations

An ultimate goal of biological information measurement is to monitor states of a living body in a non-invasive, continuous and accurate manner without disturbing the natural functions or activities of the living body. Because sensors in direct contact with biological tissues are inevitably exposed to physical disturbances caused by physical contact, considerable efforts have been made to minimize the effects of sensors. In temperature measurement, for example, it is necessary to reduce heat capacity or thermal conductance of sensors in order to suppress the effect of heat transfer from the object to the sensor. In addition, because our living body is soft and has a three-dimensional structure, it is required to use soft and/or flexible sensors to reduce the effects from modulus differences, and to achieve the stable contact to the curved objects. Recent development of soft and flexible pressure sensors has enabled to detect biological information such as human pulse or intraocular pressure, and human motions.<br/><br/>To continuously monitor biological information over a long period of time, sensors are required to exhibit a high mechanical durability. Soft and flexible pressure sensors allow an operation in bent or twisted states. The use of ultra-thin substrates and CNT/graphene nanofibers reduces a mechanical stress to sensors in the bent state, and results in an accurate detection of pressure with a bending radius of 80 μm. Recently, pressure sensors using porous PDMS or microstructured PDMS with Ag-NW coating have maintained a high pressure sensitivity (20 kPa^-1) without a damage under large pressure loads in MPa range. Furthermore, the use of rubber or textile gloves to protect the sensors significantly improves the durability of sensors, because such thick and durable materials prevent the sensors from causing mechanical damage to external forces.<br/>However, there still remain challenges to achieve practical durability of sensors while maintaining a natural skin sensation. When the thickness of the material applied to the skin is reduced, the skin can sense external changes more easily. However, thin materials are vulnerable, because the force required to penetrate and/or break the material, is proportional to the thickness of the material. Furthermore, thin materials are easily scraped off against surface frictions. In order to improve the frictional durability while maintaining the overall thickness thin, it is necessary to reduce a friction coefficient of the surface materials to reduce scraping, or to introduce rigid protective materials to minimize a plastic/rubber deformation of the surface materials. However, such approaches cause a difference with the surface of a normal finger and an influence on the sensation due to a difference in mechanical properties with the skin.<br/><br/>Here, we present extremely durable nanomesh pressure sensors that can be attached on human skin without affecting human senses. The nanomesh sensors are ultra-thin (10 μm) and lightweight, and all layers are composed of nanoporous structures. The sensors exhibit a high mechanical durability against lateral pressures such as shearing or friction, as well as vertical pressing. The performance of sensors can be maintained without a significant degradation or breaking, although the surface of sensor was directly rubbed while pressing with a high pressure of more than few hundreds kPa.