Flexo-printed piezoelectric PVDF pressure sensors

The main focus in this thesis was to study the suitability of flexographic printing and new electrode materials in the manufacture of flexible piezoelectric pressure sensors. There are numerous application where flexible and economical pressure sensors can be used. Such are process controll in industry and measurement of vital functions and pressure stress in health care. Flexography would enable economical and efficient mass-production of such sensors. Function of the pressure sensors fabricated in this work is based on piezoelectricity of the uniaxially stretched and poled polyvinylidene fluoride (PVDF) that is used as substrate in sensors. When piezoelectric PVDF is exposed to mechanical stress, electric charges of opposite signs (voltage difference) form on opposite sides of the substrate. If conducting electrodes are fabricated on surface of the substrate, formed charges can be conducted to measuring unit that analyses the magnitude of the charge. Because formed charge is proportional to the force applied on PVDF, force or pressure applied on sensor can be solved. Because piezoelectric PVDF cannot resist high temperatures, no inks that require high sintering temperatures can be used as electrode materials. Manufacture of metal electrodes by sputtering or evaporation is slow and expensive. In this work carbon nanotubes and conducting poly(3,4-ethylenedioxythiophene) polymer are studied as alternative electrode materials. Electrodes are fabricated by using RK Flexiproof 100 flexo-printer or CX202 Motorized bar coater. For lower price and better availability also non-piezoelectric substrates were used to evaluate printing process and electrode materials. Fabricated samples were electrically characterized. Main focus was in sheet resistance of the electrodes and sensitivity of the sensor elements. Obtained sheet resistance values varied a lot. Highest values were up to over four orders of magnitude larger than those of metal electrodes. Despite the high sheet resistance, fabricated samples showed sensitivities comparable to reference samples. According to the results of this work, studied new electrode materials are most likely suitable to be used in flexible pressure sensors