The development and characterisation of 3D-printed multi-material thermistor

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This paper introduces a multi-material, low-cost, and highly sensitive thermistor concept developed for body temperature monitoring. The designed thermistor utilises poly(3,4-ethylenedioxythophene):poly(4-styrenesulfonate) (PEDOT:PSS) for the temperature sensing layer, silver (Ag) for the contact electrodes, and polycarbonate (PC) as the substrate. In contrast to traditional printed electronics substrates used in thermistor development, the utilisation of Material Extrusion via Fused Deposition Modelling (FDM) for the manufacture of PC substrates is the distinct feature of the work. For the manufacture of multi-material thermistors, two different Additive Manufacturing (AM) methods, FDM for substrates and micro dispensing for PEDOT:PSS sensing layer and Ag electrodes, were employed. Two thermistors with varying sensing areas, namely D1 and D2, were designed and fabricated. The thermistors demonstrated a high degree of linearity and repeatability within the temperature range of 25–45°C with a viable hysteresis maximum around 3 %. The measurement sensitivity of thermistors was assessed based on Temperature Coefficient of Resistance (TCR) values, which were –0.68 ±0.057 % and –0.49 ±0.078 % per °C for the D1 design and –0.53 ±0.078 % and –0.40 ±0.069 % per °C for the D2 design, during heating and cooling, respectively. Comparable average response and recovery times to those reported in the literature were also acquired, which were 31.47 ±1.02 and 54.42 ±0.70 seconds for the D1 design and 27.38 ±0.96 and 48.45 ±1.69 seconds for the D2 design, during heating and cooling, respectively. It was evident that the sensing area had an impact on thermistor TCR as well as response and recovery times, where higher TCR and faster response and recovery times were recorded with the small sensing area. It was also observed that humidity had a significant impact on thermistor reliability, exhibiting a normalised resistance change of ∼12 % and ∼9 % of the initial measurement at 50 % and 75 % RH, respectively. When compared to PEDOT:PSS-based thermistors reported in the literature, the obtained results in this research have demonstrated that our multi-material thermistor concept is a promising candidate for the low-cost and highly sensitive multi-material thermistor that can be manufactured in a fully additive manner using AM technologies.