Preparation Of Flexible Pressure Sensor And Its Application In Fingertip Pulse Monitoring

The pulse is a key biomedical signal containing various human physiological and pathological information highly related to cardiovascular diseases.Even at early stages when physical symptoms are often elusively observable,they can be diagnosed via the change of pulse wave,which is helpful for diagnosing or treating cardiovascular diseases on the early stage.Therefore,monitoring the pulse signal has become a focus of current research.In the past,Traditional Chinese Medicine（TCM）was a common method for people to understand the pulse signal,and judged the pathological conditions by sensing the pulse at radial artery.However,TCM is incapable of real-time recording of pulse wave,and thus the diagnosis accuracy is limited.Additionally,such an empirical diagnostic procedure may strongly depend on the experiences and emotion of the practitioner,leading to a skewed or problematic diagnosis,and thus diagnostic results among practitioners are often discrepant.Therefore,a more quantitative and objective method for pulse monitoring is needed.With the progress of science and technology,flexible pressure sensors have been developed and applied to real-time monitoring of pulse signals.This method allow recording objectively the pulse waveform of human body for doctors to accurately analyze the pathological situation.The radial artery is often used to feel the pulse or to monitor the pulse wave.However,the skin of radial artery at wrist usually appears to be concave,and it is difficult for the sensor to stabilize contact with the surface of skin at radial artery.At the same time,in the process of monitoring the pulse signal at the radial artery,the pulse signal is unstable with the hand bending.Therefore,appropriate locations need to be explored for rapidly and stably monitoring of pulse signals.The skin at fingertips of the human body is usually convex,and the movement of the hand does not affect the interface between the sensor and the skin.Therefore,fingertip is believed to be an ideal platform for stable pulse waveform detection in long-term,pulse signals at the fingertips were more stable than those at the radial artery.However,the intensity of fingertip pulse signal is too weak.and it is difficult to acquire accurate pulse signal.Therefore,a highly sensitive pressure sensor is required for monitoring fingertip pulse signal.In order to realize the highly sensitive sensing of pulse signal and the long-term and stable monitoring,an iontronic flexible sensor was prepared.The sensor uses ionic-liquid-activated fabric as the dielectric layer,and deposited a layer of gold（Au）with a thickness of 60-70 nm on the surface of the polyimide（CPI）as the electrode of sensor.Thus,a flexible capacitive pressure sensor with low cost,high sensitivity and wide linear range was prepared for pulse monitoring.The performance of the sensor is characterized in a series of ways.Our device exhibits a highly linear sensitivity of 13.5 kPa^-1over a wide pressure ranges up to 200 kPa,and a swift response time of 30 ms.The sensor shows response of the capacitance that decreases as the test frequency increases,which is a characteristic of ionic sensors.The sensor also exhibits a high mechanical stability over 5000 cyclic loading/unloading and 3000 cyclic bending/release.When the sensor was applied to pulse signal sensing,thanks to its high sensitivity,the sensor is capable of sensing signals from different skin areas of the body,such as the fingertips,neck,arms and instep.It was found that monitoring pulse signals at fingertip were more stable than those at the radial artery.Compared with the radial artery,we can quickly detect a more stable pulse signal at fingertip under the movement of the hands or other limbs.In addition,fingertip pulse from subjects of different genders and age were collected and analyzed,and the effect of respiratory and exercise state on fingertip pulse has been studied,which clearly suggests that the change of physical condition can be reflected in fingertip pulse waveform.The technique of monitoring fingertip pulse signal can provide valuable information for early diagnosis and analysis of cardiovascular diseases.Doctors can remotely analyze the physiological and pathological information of the human body based on the pulse waveform,so as to give relevant treatment suggestions to patients and alleviate the problem of long queues.At present,the technology is not yet mature,the specific situation needs to be judged and analyzed by doctors,and more research data are needed to help doctors analyzing the disease.