Film-type sensor materials in measurement of physiological force and pressure variables

Abstract

Measurement of physiological signals reveals valuable information on the physiological state of a patient. In this thesis, new and unobtrusive ways to measure physiological force and pressure variables with film-type sensor materials are explored. The measured variables are divided into two application areas; the first is related to cardiopulmonary variables while the second one is concentrated on plantar pressure mapping. In the area of cardiopulmonary variables, the measurement of heart and respiration rates and heart sounds are studied. Pressure mapping, instead, provides information on the interface pressure distribution between a person and the surface the person is on. Here the plantar pressure distribution between a foot and ground or shoe is measured to find out regions of the plantar area where the pressure has high values. The high pressure values are linked with pressure ulcers.

Two film-type sensor materials are used here; polyvinylidenefluoride (PVDF) and ElectroMechanical Film (EMFi). The semicrystalline PVDF material has solid structure whereas the structure of the EMFi material is cellular. However, both materials generate a charge when they are mechanically deformed and thus, in principle, operate similarly. With normal measurement arrangement, PVDF and EMFi are not suitable for static measurements and only the change of an external force can be measured. Hence, the sensors made of these materials are useful especially in the measurements of physiological pulsatile signals. The sensors used in this thesis are constructed manually from commercial film materials. The entire measurement process is considered; from the design and construction of the sensors and measurement devices to the analysis of the measured data with Matlab® software.

In the measurements of cardiopulmonary variables, the sensor attachments are minimized and the measurement systems are designed to be unobtrusive and comfortable for the user. The sensors utilizing the PVDF and/or EMFi materials can be integrated into clothing or into daily life objects (e.g. a chair or a bed) to measure vital signals. This thesis suggests that the both sensor materials are suitable for such measurements even though some differences between the results obtained with the materials were found. In the area of the plantar pressure measurements, instead, a new sensor prototype for pressure measurements during gait is introduced. The developed sensor utilizes commercial PVDF material with silver ink metallization and it simultaneously measures both normal and shear stresses. Also, a sensor based on unmetallized PVDF material with printed electrodes is tested. Based on the promising results obtained with these sensor prototypes, the aim is to further develop a matrix version of the sensor for on-floor and also in-shoe plantar pressure measurements.