Fast Optical Humidity Sensor Based on Hydrogel Thin Film Expansion for Harsh Environment

• Main Authors: Anton Buchberger, Sebastian Peterka, Anna Maria Coclite, Alexander Bergmann
• Format: Article
• Language: English
• Published: MDPI AG 2019-02-01
• Series: Sensors
• Subjects: humidity measurement; polymer; hydrogel; thin film; initial chemical vapor deposition; laser interference; spectral reflectance; Flory–Huggins theory; interaction parameter
• Online Access: https://www.mdpi.com/1424-8220/19/5/999
• ISSN: 1424-8220

With the application of a recently developed deposition method called initiated chemical vapor deposition (iCVD), responsive hydrogel thin films in the order of a few hundred nanometers were created. When in contact with humid air, the hydrogel layer increases its thickness considerably. The measurement of the thickness change was realized interferometrically with a laser and a broadband light source in two different implementations. The relative change in thickness with respect to humidity can be described with the Flory&#8315;Huggins theory. The required Flory&#8315;Huggins interaction parameter was determined for the actual hydrogel composition. The setup was designed without electric components in the vicinity of the active sensor layer and is therefore applicable in harsh environments such as explosive or corrosive ones. The implemented sensor prototype delivered reproducible relative humidity (<inline-formula> <math display="inline"> <semantics> <mrow> <mi>R</mi> <mi>H</mi> </mrow> </semantics> </math> </inline-formula>) values and the achieved response time for an abrupt change of the humidity <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>&#964;</mi> <mn>63</mn> </msub> <mo>&#8804;</mo> <mn>2.5</mn> </mrow> </semantics> </math> </inline-formula> s was about three times faster compared to one of the fastest commercially available sensors on the market.