Design and Characterization of Printed Flexible Humidity Sensor

The development of humidity sensors is essential for applications in the environmental, agriculture, medical and semiconductor industries 1-2. The basic mechanism of humidity sensors is explained by a Grotthuss chain reaction, which is usually simplified as a proton-hopping process 3. This refers to a dynamic charge transfer process at a certain relative humidity (RH). The humidity sensors can be of different types such as capacitive type, resistive, colorimetric and surface acoustic wave 4-7. Capacitive type sensors are linear, require less complex circuitry and can operate for a wide range of humidity 8. A typical capacitive type humidity sensor uses hydrophilic films as sensing layers which are stable at higher humidity levels 9. Polymers that are being used for the fabrication of capacitive type humidity sensors include poly (2-hydroxyethyl methacrylate) (pHEMA) 9, cellulose acetate butyrate (CAB) 10, polyimide 11 and polymethyl methacrylate (PMMA) 12. In this work, a humidity sensor has been fabricated on glass and flexible substrates. The design of humidity sensor consists of printed pattern of a conductor such as silver and a spin coated dielectric such as polymer poly (2-hydroxyethyl methacrylate) (p-HEMA) and polyimide. The printing of conductor was done in an interdigitated pattern as shown in figure 1. The thickness of sensitive materials was controlled by varying the spin time from 10 to 30 seconds. A thick solution of sensitive materials was prepared by adding a 3 gm of polymer in 50 mL ethanol in the case of p-HEMA. Currently we are preparing one more set of samples using polyimide and we will present a comparison of two humidity sensors- one with p-HEMA and another with polyimide. A humidity sensor that employs interdigitated capacitors (IDC) printed with silver on a glass and flexible substrate was successfully fabricated using p-HEMA and polyimide at spin speed of 2000 rpm for 20 seconds. Our goal is to integrate the humidity sensor into a robotic arm to perform real-time object characterization in agricultural applications via comprehensive understanding of the operations of harvesting robots in various crop types and environmental conditions. References: 1 Laville C, Deletage JY, Pellet C. Humidity sensors for a pulmonary function diagnostic microsystem. Sens. Actuators, B 2001; 76 : 304-9. 2. Trupti M, Mulla IS, Sainkar SR, Mohanan KV, Shaikh KI, Patil AS, Vernekar SP. Humidity sensing properties of surface functionalised polyethylene and polypropylene films. Sens. actuators, B Chem. 2002; 81 :141-51. 3 D. Zhang, Z. Xu, Z. Yang, X. Song, High-performance flexible self-powered tin disulfide nanoflowers/reduced graphene oxide nanohybrid-based humidity sensor driven by triboelectric nanogenerator. Nano Energy 67 , 104251 (2020). https://doi. org/ 10. 1016/j. nanoen. 2019. 104251 4 Ducere V, Bernes A, Lacabanne C. A capacitive humidity sensor using cross-linked cellulose acetate butyrate. Sens.Actuators B: Chem. 2005; 106 :331-4. 5 Kim DU, Gong MS. Thick films of copper-titanate resistive humidity sensor. Sens. Actuators B: Chem. 2005; 110 :321-6. 6 Naydenova I, Jallapuram R, Toal V, Martin S. A visual indication of environmental humidity using a color changing hologram recorded in a self-developing photopolymer. Appl. Phys. Lett. 2008; 92 : 031109 1-3. 7 Li Y, Li P, Yang M, Lei S, Chen Y, Guo X. A surface acoustic wave humidity sensor based on electrosprayed silicon containing polyelectrolyte. Sens. Actuators B: Chem. 2010; 145 :516-20. 8 Harrey PM, Ramsey BJ, Evans PSA, Harrison DJ. Capacitive-type humidity sensors fabricated using the offset lithographic printing process. Sens. actuators, B Chem. 003; 87 :226-32. 9 A.S.G. Reddy, B.B. Narakathu, M.Z. Atashbar, M. Rebros, E. Rebrosova, M.K. Joyce, “Fully Printed Flexible Humidity Sensor,” Procedia Engineering 25 (2011) 120–123. 10 Rui Zhou, Jun Li, Hongwei Jiang, Hui Li, Yao Wang, Danick Briand, Malick Camara, Guofu Zhou, Nico F. de Rooij, “ Highly transparent humidity sensor with thin cellulose acetate butyrate and hydrophobic AF1600X vapor permeating layers fabricated by screen printing,” Sensors 73 :847-51. Figure 1