# Investigating the Reliability and Dynamic Response of Fully 3D-Printed Thermistors
###### Abstract  
This paper investigates the measurement capability, dynamic response, and mechanical reliability of all 3D-printed multi-material thermistors. The thermistor design consisted of three main components: a polycarbonate (PC) substrate, a silver (Ag) electrode pair, and a poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) thermosensitive layer. The thermistors were fabricated using two manufacturing techniques: fused deposition modeling (FDM) for the substrate and micro-dispensing for the Ag and PEDOT:PSS films. Two designs with different sensing areas, D1 (90 mm\({}^{2}\)) and D2 (54 mm\({}^{2}\)), were fabricated. As the indicator of measurement capability, the highest thermal indexes were recorded as 905.64 and 813.03 K for D1 and D2 thermistors, respectively. Thermistors exhibited comparable performance, with normalized resistance variations ranging from 0.96 to 1 for temperature changes between 25 and 45 \({}^{\circ}\)C. The sensing area influenced both measurement capability and dynamic performance, where larger sensing areas enhanced measurement capability but extended the time required to complete dynamic cycles, around 400 s for D1 versus 350 s for D2. Adhesion tests revealed a strong bonding between the PEDOT:PSS and Ag layer with less than 5% material removal. However, the adhesion of the PEDOT:PSS to the PC substrate was weak, with over 65% material removal. Morphological analysis indicated that the poor adhesion was caused by suboptimal surface properties of the 3D-printed substrate, even resulting in gaps between these two surfaces. This study demonstrates that our all 3D-printed multi-material thermistors can match reported measurement performance with an acceptable dynamic performance while highlighting the need to improve 3D-printed substrate surface properties to enhance the performance of such multi-material structures. FDM-substrates; hybrid printing; multi-material manufacture; PEDOT:PSS; silver film; 3D-printed sensor; thermistor +
Footnote †: journal: Applied Sciences  
deposit functional materials onto these substrates. This, therefore, has opened new opportunities for manufacturing smart multi-material structures such as interconnects, sensors, and circuits fabricated and integrated within AM-manufactured housings [8; 9]. Among different AM technologies, fused deposition modeling (FDM) is widely preferred for the fabrication of the substrate or base material for such multi-material 3D-printed structures due to its advantages, including material availability, ease of use, and seamless integration with different PE platforms [10; 11; 12]. Such an approach has been demonstrated by different researchers. FDM-fabricated polyetherimide substrate was employed in CubeSat subsystems for aerospace applications [13]. A multi-material circuitry was created by directly writing functional inks onto FDM-manufactured acrylonitrile butadiene styrene substrate [14]. In another study, a 3D electric circuit was developed using an automated hybrid direct-write 3D printing process that employs a dual-FDM system [15]. A free-form functional structure was created by embedding electrical components within FDM-fabricated housing by [PERSON] et al. [16]. [PERSON] and [PERSON] [17] explored the feasibility of embedding electrical parts using copper-based conductive polymer composites into FDM-fabricated polyactic acid parts. [PERSON] et al. [18] used vacuum-forming technology to shape FDM-manufactured sheets to be used as substrates. In another study, the implementation of fusible alloys for creating conductive paths inside 3D-printed polymer structures was also demonstrated [19]. [PERSON] et al. [9] comprehensively evaluated the effect of substrate performance on FDM-fabricated parts made from various materials, in terms of the mechanical and electrical performance of printed films. Similarly, the influence of process parameters on the electrical conductivity of the printed paths was also assessed [20].