This article presents a method for preparing and testing the piezoelectric properties and stability of cellular electret based on polypropylene (PP). Introducing 5% mineral filler as a mixture of crystalline silica, colloidal silica, and kaolin to isotactic polypropylene followed by a film stretching process resulted in the formation of a composite cellular structure. To manufacture electrets, the films were polarized at a constant electric field in the range from 100 V/lm to 125 V/lm, in a climatic chamber heated up to 80°C. The durability of the electrets was determined using thermostimulated discharge currents and approximate calculations of depolarization process activation energy. For electrets made of cellular films, the depolarization temperature T m at which the density of the discharge current assumes the highest value was $108°C and the activation energy was 6.25 eV. The response of the polarized composite film to mechanical stress expressed as the piezoelectric constant d 33 was about 3 times higher than for a-PP film of the prevailing atactic phase and poly(vinylidene fluoride) film without a cellular structure. In the range of stress of 1 kPa to 120 kPa it was 135 pC/N for lower stresses and 60 pC/N for higher stresses.

Purpose -This paper aims to present a prototype of the diagnostic system for the examination of the distribution of the force applied by foot to substrate during usual human moving. Presented system is competitive to other currently available devices, thanks to sensors reliability, user-friendly operation manner and design based on cheap parts. The results of examinations are transmitted by radiomodem. Its recording and visualization are possible on either personal or mobile computers. Design/methodology/approach -During selection of the sensors substrate, many polymeric electrets were examined. Polyvinylidene fluoride films were selected, because they have good charge uniformity across the surface, wide range of acceptable temperatures, linear relation between mechanical stress and output signal and high resistance for squeezing. The system measures the charge generated in film. Findings -The pressures are recorded in relation to maximum value; therefore, measuring system does not require calibration. The simultaneous recording of data from all eight sensors allows tracking the signal without distortion. Originality/value -An array of sensors is installed in the shoe insole. The measuring device is fixed to the outer surface of the shoe. Its weight is 75 g. The range of transmission is suitable for examination in the natural environment, outside traditional consulting room. Software is dedicated for analysis of the pressure distribution in every moment of the foot movement. The system is suitable for examination of flat feet, diabetic foot and recovery progress after injures.

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The paper presents a dynamic measurement method of the distribution of foot pressure exerted on the ground by a four-point shoe insole, developed by authors, which can be placed in any sport footwear. The value of pressure was measured on the heel, medial midfoot, metatarsal, and great toe by recording values of a generated voltage by sensors which were made of piezoelectric polymer PVDF film 110 µm thick with printed silver electrodes. As confirmed by scanning microscope studies, the foil applied in the sensors is semi-crystalline. The shoe measurement insert consists of two polyester films without piezoelectric properties between them, electroactive polymer sensors were placed. The films were glued together. To match the measuring circuit to the sensors used, two circuits were tested, a voltage measuring circuit with an input resistance of above 1012 Ω (open circuit), and a charge measuring circuit (shorted circuit). The charge measuring circuits with the RC high-pass filter, which attenuates the slow-changing pyroelectric signal was selected as it ensures the desired measurement accuracy. As presented in the paper, as PVDF sensors are very sensitive to any mechanical deformation, it is important to properly design the shoe insole to ensure its correct use during pressure distribution measurements. The measuring system developed by the authors, allows testing of foot pathology for any length of time in a dynamic way.

Parasitic harvesting of energy from environment is being investigated as a possible solution for powering electronic devices. One possible solution is to harvest energy generated from human walking, by placing a piezoelectric elements inside footwear. As the amount of available energy is extremely low, conversion effectiveness is of prime importance. This paper deals with a micropower source, designed to harvest energy from walking, built in a form of a shoe insole containing a piezoelectric material (43 mm thick polyethylene foil). Laboratory investigation of the piezoelectric properties is presented first, followed by a measurement result of an actual generator with 0?6 mW output power. Finally, a modification of the generator, by adding springy elements, is presented, resulting in ninefold increase in power output (to 5?6 mW).