Flexible Humidity Sensors Based on Nanocellulose for Wearable Electronics

Abstract

Problem. Humidity control is necessary for many areas of modern human life, including agriculture, food and automotive industries, electronics manufacturing, medicine and everyday life. A new trend in medical sensors is the use of flexible wearable sensors that can be attached to a person's body or the clothing adjacent to it. Such sensors are able to repeat the shape of the body and deform as needed, without breaking or causing discomfort to the patient. Today, nanocellulose (NC) is a rather promising humidity-sensitive material, which is characterized by high hydrophilicity and sufficient flexibility, and at the same time it is a biodegradable material.
 The purpose of the work. Development and research of flexible humidity sensors based on nanocellulose as a moisture-sensitive film, depending on the nature of the raw material and the method of NC manufacturing, as well as the configuration of the device electrodes.
 Research results. Three groups of humidity sensors were manufactured: capacitive sensors with electrodes in the form of expanded capacitor; resistive sensors made on the base of interdigital electrodes; resistive sensors made on the base of planar-parallel electrodes. In each configuration, 4 different nanocellulose hydrogels were used, made from reed or wheat straw by hydrolysis or TEMPO methods. The static and dynamic characteristics of humidity sensors were investigated, and conclusions were drawn regarding the optimal type of electrode configuration, raw materials and the method of NC synthesis. The largest sensitivity and the smallest hysteresis are demonstrated by the sensor made on the basis of an interdigital electrode and nanocellulose, synthesized by the TEMPO method - 0.164 (%RH)-1 and 1.5%, respectively. However, the high speed (response time - 6 s, recovery time - 10 s), short-term stability (deviation during device measuring under constant humidity for 1 h - 1.4%) and repeatability of results (deviation during cycling between different humidity levels - 1.6%) are observed for sensors, manufactured by the hydrolysis method, and for both types of sensors (resistive and capacitive device).
 Conclusions. It was established that the static parameters (response, sensitivity and reversibility) of humidity sensors depend mainly on the type of electrode configuration and the source material for the NC synthesis, while dynamic parameters (repeatability during cycling, short-term stability, response time and recovery time) depend on NC synthesis method. At the same time, it was shown that resistive sensors demonstrate significantly better parameters of sensitivity and hysteresis compared to capacitive devices. The obtained flexible humidity sensors can be used in wearable medical electronics.

Моделювання Впливу Конструктивних Параметрів На Характеристики Сенсорів Вологості На Основі Наноцелюлози

“…Зазвичай, у якості підкладок для гнучких сенсорів вологості використовують штучні полімери [15]- [20], однак такі прилади потребують утилізації після закінчення терміни їх служби. Тому на сьогодні ведуться роботи по розробці біорозкладних гнучких полімерів, серед яких вирізняється наноцелюлоза (НЦ).…”

“…Тому на сьогодні ведуться роботи по розробці біорозкладних гнучких полімерів, серед яких вирізняється наноцелюлоза (НЦ). На сьогодні існує два підходи у створенні таких сенсорів: drop casting approach, коли вологочутливий шар НЦ наноситься поверх електродної системи, нанесеної на синтетичну підкладку (зазвичай із PI, PET, PE, тощо [15]- [20]) та selfstanding approach [3], [6], [12], [21], [22], коли механічна підкладка із НЦ або її композиту використовується як механічний шар та вологочутливий шар одночасно. В межах останнього підходу можна отримати повністю біорозкладний сенсор, для створення якого достатньо на поверхню плівки НЦ нанести електроди.…”