An experimental study about hydrogels for the fabrication of optical fiber humidity sensors

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Abstract

A study about the optical sensitivity of four different hydrogels to the humidity is presented. The investigated hydrogels were poly-hydroxyethyl methacrylate, poly-acrylamide, poly-N-vinyl pyrrolidinone and agarose. These hydrogels were deposited on optical fiber by means of direct polymerization on the optical fiber surface. In addition, these materials were examined with different light sources, temperature and relative humidity conditions. The conclusion deducted from the experiments is that increasing the pore size of the hydrogels, the sensitivity and response time of these materials with the humidity is remarkably improved.

Introduction

Hydrogels are materials formed by networks of cross-linked hydrophilic polymers that typically retain around 30% in weight of water. These water-containing gels are currently the subject of extensive research due to their possible use on diverse and very different applications, such as controlled drug release, ocular devices, soil additive to conserve water, wound dressings, food-thickening agents, implants or other applications that require the utilization of biocompatible materials [1]. In addition, some of these hydrogels have been used for the development of chemical sensors and, among other uses, these include ion selective membranes, immobilization matrix for the entrapment of the sensing indicators or even as the sensor materials themselves [2], [3], [4], [5], [6], [7]. The most of these sensors are based on conductimetric, capacitive or amperometric schemes, the rest of the works are related to optical sensing [6], [7]. On the other hand, in order to monitor the humidity, different optical sensing approaches have been proposed. Among others, some of these approaches involve the measurement of the refractive index of air by means of a remote fiberized low-coherence interferometric sensor [8], the entrapment of a fluorescent indicator on a gelatin matrix [9], the immobilization of a phosphorescent indicator on an organic or inorganic matrix [10], the use of an organic film coated onto the cladding of a bent fiber [11], the deposition of an organic thin film on optical fibers by electrostatic self-assembly [12], [13], the fabrication of a porous sol–gel fiber [14] or the fabrication of sensors based on plastic optical fiber with swelling polymers [15]. Most of these cited measurement schemes are based on sensing coatings deposited on optical fibers. The work presented here deals with the possible utilization of some hydrogels for the fabrication of optical humidity sensing coatings. Particularly, the hydrogels poly-hydroxyethyl methacrylate (poly-HEMA), poly-acrylamide and poly-N-vinyl pyrrolidinone (poly-N-VP) have been already used for the immobilization of indicators on optical fiber for the detection of pH, solved gases and calcium ions [16], [17], [18], [19], and also in an earlier work, we have experimentally demonstrated that the agarose hydrogel could be used for the fabrication of humidity sensors [20]. Here, a comparative study of these four hydrogels for the possible fabrication of optical fiber humidity sensors is presented for the first time.

Section snippets

Sensing mechanism

In earlier works, Culshaw and co-workers have used poly-ethylene oxide hydrogels for the development of optical fiber sensors for the detection of water ingress in bridge structures [6]. The sensors of these works are based on the swelling behavior of the hydrogel in the presence of water. When this happens, the expansion of the hydrogel can induce microbends in an optical fiber, that is, inducing losses of light that will be located along the fiber with optical time domain reflectometry

Sensors fabrication

The fabrication of the sensors involved several steps and two different deposition processes. The first deposition process was based on the boiling water bath method and the second deposition process was achieved by means of the photo-polymerization of the monomers. The former was used for the fabrication of agarose gel matrices, the latter was used for the fabrication of poly-HEMA, poly-N-VP and poly-acrylamide hydrogels.

Experimental set-up for the characterization of the sensors

In order to study the behavior of the sensors with the humidity and temperature, the optical fiber was introduced in a climatic chamber (model Challenge 250) from Angelantoni Industrie, Cimacolle, Italy. This climatic chamber is equipped with a humidification–dehumidification system and a cooling–heating system that can change the temperature and humidity in a controlled way by means of a PC and a configurable software (Winkratos). Both extremes of the optical fiber were kept outside the

Experimental results

The poly-acrylamide, poly-N-VP, poly-HEMA and agarose optical fiber devices were studied by using the same experimental set-up of Fig. 1. All these hydrogels were investigated with different pore sizes, light sources, temperature and relative humidity (RH) conditions. The pore size was controlled by means of varying the dilution of monomer following the generic rule that when the concentration of monomer decreases the pore size increases [24].

Conclusions

The hydrogels poly-hydroxyethyl methacrylate, poly-acrylamide, poly-N-vinyl pyrrolidinone and agarose were deposited on optical fiber in order to study their behavior with humidity. These hydrogels are suitable for their application as humidity sensors and as a generic rule, the bigger the pore size, the higher dynamic range and the shorter response time are obtained. Among these hydrogels, the hydrogel with less stable response was the poly-HEMA hydrogel and the hydrogel with best performances

Acknowledgements

The authors gratefully acknowledge the Fuentes Dutor Foundation and Colegio Oficial de Ingenieros Industriales de Navarra. This work was also supported in part by the Spanish CICYT Research Grant TIC2001-0877-C02-02 and Gobierno de Navarra Research Grants. The authors would also like to thank Dr. Eva Gonzalez for her suggestion in the utilization of agarose, Mr. David Galbarra for his assistance with the experiments and Dr. Mahesh Uttamlal of the Glasgow Caledonian University for his helpful

Francisco J. Arregui received his MS degree in electrical engineering from the Catholic University of Navarra (San Sebastian, Spain) in 1994, and PhD degree from the Public University of Navarra (Pamplona, Spain) in 2000. He has been a member of the CEIT Research Center (San Sebastian, Spain) for 2 years and has been involved in different projects with industry including medical instrumentation, monitoring of high power lines and communications hardware. Since 1995, he has been with the Public

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Francisco J. Arregui received his MS degree in electrical engineering from the Catholic University of Navarra (San Sebastian, Spain) in 1994, and PhD degree from the Public University of Navarra (Pamplona, Spain) in 2000. He has been a member of the CEIT Research Center (San Sebastian, Spain) for 2 years and has been involved in different projects with industry including medical instrumentation, monitoring of high power lines and communications hardware. Since 1995, he has been with the Public University of Navarra. During 1998 and 2000, he was a visiting scientist at the Fiber & Electro Optics Research Center, Virginia Polytechnic Institute and State University (Blacksburg, VA, USA). His main research interests include optical fiber sensors, sensor materials and nanostructured materials. He has served as a referee for the journals Optical Engineering, IEEE Photonics Technology Letters, IEEE Sensors Journal and Sensors and Actuators A. He is a member of IEEE and SPIE.

Zuri Ciaurriz received in 2001 her MS degree in electrical and electronic engineering from the Public University of Navarra (Pamplona, Spain). She has been working for Vodafone during 2000 and 2001. Her main research interest are optical fiber sensors, pH and humidity sensors.

Maria Oneca received her MS degree in biology from the Catholic University of Navarra (Pamplona, Spain) in 1992, Master degree in food engineering from the Polytechnic University of Valencia (Valencia, Spain) in 1994, and PhD degree from the Public University of Navarra (Pamplona, Spain) in 2002. She has been working in the Public University of Navarra for 8 years on food technology, microbiology, molecular biology, physicochemical studies and has been also involved in different projects with industry including microbiological, physicochemical and sensory control.

Ignacio R. Matı́as received his MS degree in electrical and electronic engineering from the Polytechnic University of Madrid (UPM), Spain, in 1992. He was involved in optical communications research project from 1990 to 1992 in the Alcatel SESA Research Center in Madrid. In 1993, he joined the optical fiber group in the Photonic Technology Department, UPM, where he received his PhD degree, specialty in optical fiber sensors. In 1996, he took up a lectureship at the Public University of Navarra (Pamplona, Spain). His research interest is in the areas of optical fiber sensors, passive optical devices and systems and optical networks for smart structures and buildings. He has co-authored over 80 journal and conference papers and 2 book chapters related to fiber optics. Presently, he is an associate professor of electronic technology at the Public University of Navarra. He is an IEEE member.

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