Temperature Sensors by Inkjet Printing Compatible With Flexible Substrates: A Review

During the past decade, microelectronics incorporated inkjet printing technology as a versatile tool for industrial applications, as it combines high printing quality and resolution along with low cost compared to conventional microelectronics techniques that require high cost and complex equipment. In addition, inherently, inkjet printing requires no photolithography steps. In this review article, we present temperature sensors that have been manufactured by inkjet printing. Based on the main active sensing material, the research studies are classified into four different types, i.e., metal- and metal oxide-based, carbon-based, polymer-based, and composite that combine the properties of multiple active materials. It is demonstrated that silver ink has been, by far, the most popular material for metal-based temperature sensors with a temperature coefficient of resistance (TCR) of around <inline-formula> <tex-math notation="LaTeX">$2\times 10^{-{3}}\,\,^{\circ} \text{C}^{-1}$ </tex-math></inline-formula>. Silver sensors as well as almost all metal-, carbon-, and polymer-based sensors were either resistance temperature detectors (RTDs) or thermistors. Regarding polymer and carbon-based sensors, it was found that, in some cases, they outperformed metal-based sensors in terms of TCR, although fabrication using such materials had a less predictable result with a TCR ranging from <inline-formula> <tex-math notation="LaTeX">$- 16\times 10^{-{3}}\,\,^{\circ} \text{C}^{-1}$ </tex-math></inline-formula> to <inline-formula> <tex-math notation="LaTeX">$2\times 10^{-{3}}\,\,^{\circ} \text{C}^{-1}$ </tex-math></inline-formula>. Finally, in the case of composite materials as temperature sensors, several combinations of active materials exhibited interesting results and yielded a variety of sensing technologies such as thermocouples and radio frequency-based.