Development of a piezoelectric polymer film sensor for plantar normal and shear stress measurements

Abstract

We have developed a new sensor prototype for plantar pressure measurements during gait. The mechanical stress at the plantar surface has two components, pressure acting normal to the surface and shear stress acting tangential to the surface. The shear stress can be further divided into anterior–posterior (AP) and medial–lateral (ML) components. The developed sensor simultaneously measures both normal and shear stresses. It utilizes commercial polyvinylidenefluoride (PVDF) material and consists of four separate sensor elements. This paper presents the sensor development and calibration for each force components. A shaker providing dynamic excitation force was used in the calibration. Average sensitivities computed from the results were (12.6 ± 0.8) mV/N for the normal force, (223.9 ± 20.3) mV/N for the AP shear force and (55.2 ± 11.9) mV/N for the ML shear force. A preliminary plantar pressure measurement was also done. The results obtained were promising. However, the sensor developed here is the first prototype. In future, a matrix sensor based on this principle is planned to be constructed.

Introduction

A lot of research has been done with the measurement of plantar pressure distribution. The high mechanical stress between foot and shoe has been linked with pressure ulcers [1], [2]. The pressure ulcers, also known as pressure sores and decubitus ulcers, occur when the tissue is compressed under pressure. The most common cause for foot deformities and pressure ulcer formation in feet is diabetic neuropathy [3]. Abnormally high pressures occur due to the poor load distribution as a result of reduced sensitivity of the foot [4]. At particular risk are heavily loaded regions overlying bony prominences, such as under the metatarsal heads, where the majority of plantar neuropathic ulcers occur [5]. The reduction of plantar pressure on foot has become a primary focus on the prevention and treatment of pressure ulcers [2].

The mechanical stress at the plantar surface has two components, pressure acting normal to the surface and shear stress acting tangential to the surface [5]. Only normal stress is widely reported [1]. One of the main reasons, why the shear stress data is not obtained is the lack of a validated and commercially available shear stress sensor [6]. Shear stress at the skin interface has been shown to increase blood flow occlusion in the deeper tissues, generating stresses which are additional to those of normal force [1].

The shear stress can be further divided to anterior–posterior (AP) and medial–lateral (ML) components [7]. 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]. The shear stress is a vector addition of the AP and ML components [9].

During the last few decades, a variety of methods have been developed for the measurement of shear stress. Hosein and Lord measured the shear stress locally beneath the metatarsal heads and heel [1], [5]. The motion under the action of shear stress was detected in two orthogonal directions by magneto-resistive elements. Also, in-shoe plantar pressure was measured with commercial Tekscan F-Scan Gait Analysis System.

Perry et al. recorded the forefoot shear stress and pressure during the initiation of a gait [10]. They measured simultaneously all the three components of shear stress and pressure. The system consists of 16 transducers based on strain gauge technology; each with surface area measuring 2.5 cm × 2.5 cm. Heywood et al. utilized capacitive technology to measure the AP and ML components of shear stress [7]. The shear stress sensor consists of a central post and four parallel plates forming a capacitor between each face of the post and the parallel plate that it opposes. When a subject walks over the post, the forces on the plantar surface of the foot cause the post to be deflected and thus the capacitance to be altered. A commercial miniature pressure sensor was also used to measure the normal stress.

Mackey and Davis developed a 16 element sensor array to measure 3D stress [9]. The system has an optical basis. Wang et al. utilized a sensor consisting of an array of optical fibers lying in perpendicular rows and columns separated by elastomeric pads [6]. The measurement of normal and shear stresses with this method is based on intensity attenuation in fibers due to the physical deformation of two adjacent perpendicular fibers.

Pedotti et al. used a 200 μm thick polyvinylidenefluoride (PVDF) film poled in selected areas [11]. Sixteen circular disks, diameter 6 mm, were deposited onto the film by vacuum evaporation to provide the electrodes for pressure sensors. Razian and Pepper developed an in-shoe triaxial pressure transducer utilizing piezoelectric copolymer with the mixed composition of PVDF and trifluoroethylene (TrFE) [12].

The aim of this study was to develop a thin and flexible sensor for measuring the normal pressure and the AP and ML components of the shear stress. The developed sensor consists of four separate sensor elements implemented from commercial PVDF material and stacked together. The pressure and shear stress components are separated from the measured signals computationally. This study is concentrated on the sensor design and evaluation even though some preliminary results of plantar pressure measurement are also presented. The sensor introduced in this study is the first prototype. The goal is to further develop the sensor structure and design a matrix sensor to be used in in-sole measurements of plantar pressure distribution.