Flexible sensor simplified respiratory monitoring

Chinese scientists have developed a compact and flexible sensor for monitoring the breathing at the expense of temperature difference between inhaled and exhaled air. It automatically processes the signal and determines respiratory failure. This sensor is suitable for monitoring breathing through the nose and through the mouth, and it can be used both during physical exertion and during sleep. Article published in the journal Advanced Electronic Materials.

Regular sleep apnea (stop breathing during sleep for more than ten seconds) leads to daytime fatigue and increases the risk of high blood pressure, heart attack, stroke, or liver problems. For medical diagnosis of this disease requires monitoring of breathing and spend it now in two ways. The first allows you to record breathing using sensors directly on the face: they measure temperature, humidity and pressure change. This approach is implemented in the polysomnogram, which measures several different physiological parameters during sleep, but brings some discomfort to the patient.

The second approach is to measure movement of the chest or abdomen of a patient during respiration, and they are already in commercial use (e.g., Spire). For example, last year American engineers have developed special pajamas to sleep, which monitors heart rate, breathing, and posture of a sleeping person.

However, both mechanical methods of monitoring are often subject to errors caused by the random movements of the person. That is why scientists are trying to create compact and accurate sensor that would measure a person’s breath and not interfere. Xue Feng (Xue Feng) from Tsinghua University have developed a flexible device for monitoring breathing, working through different temperatures of the inhaled and exhaled air. The device consists of a temperature sensor, located under the nostrils of the user, and the integrated circuit on the crest of the nose, the position of the sensor does not restrict movement of the user, and measuring it, they had no control.

A sensor to translate the temperature difference into the cycles of breathing, the researchers developed a thermal model to predict the breathing pattern. They suggested that the speed of the respiratory flow is sinusoidal, and the flow temperature has a stepwise dependence. After the simulation of breathing under different conditions, the researchers noticed that the flow rate is changing exponentially, while the average temperature (average between maximum and minimum) remains constant. The response time of the sensor is directly proportional to the coefficient of convection heat transfer, which in turn depends on the surface area of the sensor — so it had to be done as little as possible.They also found that the amplitude of the detector signal increases linearly with the temperature of the exhaled air, in contrast to the temperature of the inhaled air. This system, however, will not work if the ambient temperature is the same as body temperature.

The structure of the device, the scientists created in the following way: on a substrate with a polydimethylsiloxane and a polyimide applied with a thin layers of chromium (ten micrometers) and gold (150 micrometers). Using photolithography and wet etching, they gave the metal layer a desired shape, and then reactive ion etching was set up in the form of a polyimide in the form of metal. In the next step, the scientists moved the metal layers and a layer of polyimide on a substrate with adhesive tape and a cured polydimethylsiloxane, connected with conductive contacts, and inflicted upon him a semi-permeable layer with a protective base.

After removal of the layer of polydimethylsiloxane and the protective bases of the sensor was ready, and due to the adhesion semi-permeable surface, which is easy to apply on any lot. This temperature sensor changes resistance, which is indirectly measured with a voltmeter. The scheme has transformed the analog data in numeric and transmit them via Bluetooth to the app on the smartphone. Nutrition was carried out using a small lithium-ion battery.

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