XIX IMEKO World Congress Fundamental and Applied Metrology September 611, 2009, Lisbon, Portugal A PVDF SENSOR WITH PRINTED ELECTRODES FOR NORMAL AND SHEAR STRESS MEASUREMENTS ON SOLE Satu Kärki 1 , Miika Kiiski 2 , Matti Mäntysalo 2 , Jukka Lekkala 1 1 Department of Automation Science and Engineering, Tampere University of Technology, Tampere, Finland, satu.karki@tut.fi, jukka.lekkala@tut.fi 2 Department of Electronics, Tampere University of Technology, Tampere, Finland, miika.kiiski@tut.fi, matti.mantysalo@tut.fi Abstract  We have tested a method of printing electrodes on the unmetalled polyvinylidenefluoride (PVDF) material to construct a matrix version of a sensor for normal and shear stress measurements on sole. A commercial PVDF material with silver ink metallization has previously been used to manufacture a single sensor prototype. With the metal-coated PVDF material, a matrix sensor is challenging to construct; the metallization should be removed from the certain areas of the material to form an electrode grid pattern or a number of identical separate sensors should be cut off from the material sheet. Hence, a new method is explored here. The sensor is manufactured from unmetalled PVDF material and an array of electrodes with desired size and shape is printed on the material surface. This study concentrates on the characteristics of single sensors manufactured with this method. Based on the results, the sensitivity seems to be decreased due to the thermal stress caused by the electrode printing process. In the normal force direction the sensor sensitivity was found to be about fifth and in shear force directions about tenth of the corresponding values measured with the sensor with commercial electrodes. The sensitivity in this case, however, is still adequate for stress measurements on sole. Keywords: plantar stress, PVDF, printed electrodes. 1. INTRODUCTION Measurement of plantar forces has indicated a relationship between the excessive mechanical stress and ulceration of the foot [1]. The pressure ulcers, also known as pressure sores or decubitus ulcers, occur when the tissue is compressed under pressure, e.g. due to a use of improper footwear. An early identification of individuals at risk of foot ulceration is one of the primary means to reduce the incidence of ulceration [2]. Foot ulceration is a common problem in people with diabetes mellitus and peripheral neuropathy [3]. The mechanical stress on the plantar surface has two components, pressure acting normal to the surface and shear stress acting tangential to the surface [4]. The shear stress can be further divided into anterior-posterior (AP) and medial-lateral (ML) stresses [7]; the shear stress is a vector addition of these two components [9]. The AP shear stress is the horizontal component in the movement direction and the ML shear stress the horizontal component perpendicular to the movement direction [8]. Only normal stress is widely reported [5]. The main reason is the lack of validated and commercially available shear stress sensors [6]. During the last few decades, however, a variety of methods has been developed for the measurement of shear stress, see e.g. references [2-7] and [9-10]. Shear stress on the skin interface has been shown to increase blood flow occlusion in the deeper tissues, generating stresses which are additional to those of normal stress [5]. The aim of this study is to further develop a piezoelectric polymer film sensor for plantar normal and shear stress measurements, previously reported in reference [10]. At the moment, only discrete measurements of plantar pressure can be done with the sensor. The discrete sensor requires a clinician to choose the appropriate location for optimal data collection [11]. With an array of sensors, instead, plantar pressure distribution over the entire plantar surface of the foot could be assessed simultaneously [11]. Hence, we aim to develop a matrix version of the sensor for on-floor and also in-shoe plantar pressure measurements. We have tested a method of printing electrodes directly on the polyvinylidenefluoride (PVDF) material to implement a matrix sensor. The single sensor prototype reported in reference [10] was constructed manually from commercial 28 ȝm thick PVDF material with silver ink metallization, manufactured by Measurement Specialties Inc. The sensor consists of four separate sensor elements stacked together. Each piece of PVDF material was cut off from the sheet and the wires were connected to the electrodes with tin-plated copper tape. The construction of an array of sensors with this method is time-consuming and challenging: a number of identical separate sensors should be cut off from the PVDF material sheet and manufactured manually, or alternatively, the metallization should be removed from the certain areas of the PVDF material sheet to form an electrode grid pattern. Hence, here the sensor elements are implemented from 28 ȝm thick unmetalled PVDF material. The electrodes are printed on the material sheet to form a grid pattern with desired array size and shape. The thermal stress caused by the printing process may affect the PVDF material properties, and thus, this 1765 ISBN 978-963-88410-0-1 © 2009 IMEKO