Think of a temperature sensor – and a traditional glass thermometer, or perhaps a small thermistor would probably spring to mind. However, a new class of temperature sensors is rapidly emerging that offers spatial resolution together with a thin-film format.
This new class of temperature sensors is made from a solution-processable semiconductor that is coated between conductive rows and columns in a passive matrix architecture. Both the temperature sensing semiconducting layer and the conductors can be printed onto flexible substrates such as PET, enabling low-cost production of a wide range of shapes and sizes.
Why now
With the ability to provide spatially resolved temperature resolution at a low cost, it might seem surprising that printed temperature sensors are not already widespread. IDTechEx identify two main reasons.
Firstly, existing temperature sensors are cheap, well understood, and very small. While conventional inorganic thermistors or platinum resistive temperature detectors (RTDs) are certainly not flexible, they are generally so small that incorporating them into a component or device doesn’t impose any form factor constraints.
Secondly, thermal diffusion is quite slow. This means that thermal gradients are usually very gentle, certainly when compared to spatial variation in force or light intensity for example. As such, introducing measurement with fine spatial resolution is unnecessary in many cases, since a few thermistors or RTDs embedded in a thermally conductive layer provide equivalent insights.
However, while both of these factors remain true, emerging application areas mean that a combination of spatially resolved temperature measurement, low-cost roll-to-roll (R2R) production, and flexible thin-film format mean that demand for printed temperature sensors is forecast to grow substantially over the next decade, with an increasing number of companies developing this technology.
Battery monitoring for electric vehicles
A forthcoming increase in electric vehicle (EV) adoption is almost unarguable, with multiple governments legislating to ban the sale of internal combustion engines for vehicles at various points in the future. This technological transition is already creating huge opportunities for battery manufacturers.
Thermal management is extremely important for batteries, as anyone who has noticed their smartphone battery life plummet when subjected to heat or cold will attest. Batteries work best within a narrow temperature range, while hot spots can provide an early indication of malfunctions.
Keeping track of battery temperature to adjust heating or cooling as required of course requires temperature sensors. Printed temperature sensors are well suited to this purpose as they are lightweight, thin, cheap to produce in high volumes, offer good thermal contact with pouch cells, and can be laminated together with thin-film heaters to produce an integrated thermal management solution.
Healthcare applications
Printed temperature sensors are also highly promising for healthcare applications. The key attribute here is conformality since printed thin-film sensors can adapt to the curvature of the skin. Indeed, there is growing interest in continuous healthcare monitoring, which will provide extensive opportunities for many types of printed sensors. It involves the continuous tracking of various parameters such as heart rate and temperature to enable remote patient monitoring.
A specific healthcare application currently being explored for printed temperature sensors includes wound monitoring since the healing process is associated with increased blood flow and thus slightly elevated temperatures. The spatial resolution available with printed temperature sensors thus enables the extent of the wound to be tracked over time.
