IEEE SENSORS JOURNAL, VOL. 14, NO. 3, MARCH 2014 623 Investigation of Humidity Sensor Effect in Silver Nanoparticle Ink Sensors Printed on Paper Henrik Andersson, Anatoliy Manuilskiy, Jinlan Gao, Cecilia Lidenmark, Johan Sidén, Member, IEEE, Sven Forsberg, Tomas Unander, and Hans-Erik Nilsson Abstract—Thin inkjet-printed tracks of silver nanoparticles have previously been observed to show a non-reversible decrease in resistance when exposed to a high degree of relative humidity and thus providing sensor functionality with a memory effect. This paper provides a more in-depth explanation of the observed humidity sensor effect that originates from inkjet-printed silver nanoparticle sensors on a paper substrate. It is shown that the geometry of the sensor has a large effect on the sensor’s initial resistance, and therefore also on the sensor’s resistive dynamic range. The importance of the sensor geometry is believed to be due to the amount of solvent from the ink interacting with the coating of the paper substrate, which in turn enables the diffusion of salts from the paper coating into the ink and thus affecting the silver ink. Index Terms— Humidity sensor, ink jet, silver nanoparticle ink. I. I NTRODUCTION P RINTED electronics is increasingly popular and there is a huge interest in this technology and its applications. It is also of significance to develop suitable manufacturing processes for the mass production of printed circuits on flexible substrates. The most commonly used flexible substrates are plastics, such as polypropylene or polyimide. However, there is a growing interest in using paper as a substrate for printed electronics [1]–[8]. The use of paper as a substrate offers several benefits, namely that it can be combined with existing graphical printing processes and that it is already used in the packaging industry and it is a renewable resource. Previously, inkjet-printed humidity sensors with a memory function have been manufactured on paper substrates and described in [3]. A stepwise and non-reversible decrease in resistance was correlated to the variations in relative humidity. Fig. 1, Pattern 1, presents the sensor geometry used in [3], Manuscript received May 17, 2013; revised September 18, 2013; accepted September 26, 2013. Date of publication September 30, 2013; date of current version January 7, 2014. The associate editor coordinating the review of this paper and approving it for publication was Prof. Sang-Seok Lee. H. Andersson, A. Manuilskiy, J. Gao, J. Sidén, and H.-E. Nilsson are with the Electronics Design Department, Sundsvall 851 70, Sweden (e-mail: henrik. andersson@miun.se; anatoliy.manuilskiy@miun.se; jinlan.gao@miun.se; johan.siden@miun.se; hans-erik.nilsson@miun.se). C. Lidenmark and S. Forsberg are with the Department of Applied Science and Design, Mid Sweden University, Sundsvall 851 70, Sweden (e-mail: cecilia.lidenmark@miun.se; sven.forsberg@miun.se). T. Unander was with the SCA Research and Development Centre, Sundsvall 851 21, Sweden. He is now with Dewire AB, Sundsvall 851 85, Sweden (e-mail: tomas.unander@dewire.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2013.2284033 Fig. 1. Sensor test patterns with sensor element printed close to the top of sensor contacts (Pattern A) and in the middle of the sensor contacts (Pattern B ). Top figures show the original sensor design. which consists of a 125 μm long structure that is printed between two contact pads. The humidity-induced sintering of the Silver NanoParticles (SNP) was speculated to originate from the softening of the Polyvinylpyrrolidone (PVP) shells by the humidity and by chemical contributions from the paper substrate [3]. During the previous work, variations in the initial resistance and sensor effect were observed for the sensor pattern printed on two different paper substrates [3]. Further studies presented in this article showed large dif- ferences in both the initial resistivity as well as the humidity sensitivity of the sensor which was dependent on the geometry of the sensor patterns, in addition to the previously observed substrate influence. The objective of this work is therefore to investigate how the geometry of ink-jet printed patterns as well as the paper substrate type may affect the resistivity as well as the humidity sensor effect. The observed high resistance and humidity sensor effect on the printed structures had previously only been obtained on certain substrates which is why the the hypothesis is that the humidity sensor functionality is due to specific chemical contributions from the substrate. The chemical content of the coating, such as sodium chloride (NaCl), can be brought into contact with the ink in relation to the amount of solvent that is present in the coating on which the structure is printed. The hypothesis is that solvent saturation of the sub- strate coating is responsible for the observed variations in 1530-437X © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.