Dana Mitra

In this work, we present an approach to stimulate and control the heating effect across a glass substrate using inkjet-printed heaters, while simultaneously monitoring the surface thermal behavior through printed thermochromic layers with optical sensing capabilities. The thermochromic coating provides a direct optical indication of sudden temperature increase, as it exhibits an irreversible color change upon surpassing a defined threshold temperature. This method allows the straightforward identification of critical thermal events and localized instabilities without the need for complex measurement systems. For localized heating, silver meander structures with a line width of about 500 µm were manufactured on glass, resulting to an electrical resistance ranging between 170 -260 Ω. Prototype devices were developed by inkjet printing the heating elements on one side of the substrate and the thermochromic layer on the other side. Electrical characterization demonstrates that the partial or localized color change occurred at sub-threshold power inputs, while a complete and uniform color transitions was achieved at a higher input power levels ranging between 44 -46 W, leading to a peak surface temperature of 270 °C and above, enabling full activation of the thermochromic layer. This approach may be extended to conventional electronic components by integrating a suitable printed thermochromic layer directly onto their surfaces, offering a simple and low-cost method for thermal monitoring, overheat protection and failure prevention. The combination of functional printing with thermochromic detection thus holds promise for applications in quality assurance, safety monitoring, and the development of robust next generation electronics systems.

The implementation of the laser sintering for inkjet-printed nanoparticles and metal organic decomposition (MOD) inks on a flexible polymeric film has been analyzed in detail. A novel approach by implementing, next to a commonly 3.2 mm diameter spot laser optic, a line laser optic with a laser beam area of 2 mm × 80 mm, demonstrates the high potential of selective laser sintering to proceed towards a fast and efficient sintering methodology in printed electronics. In this work, a multiplicity of laser parameters, primary the laser speed and the laser power, have been altered systematically to identify an optimal process window for each ink and to convert the dried and non-conductive patterns into conductive and functional silver structures. For each ink, as well as for the two laser optics, a suitable laser parameter set has been found, where a conductivity without any damage to the substrate or silver layer could be achieved. In doing so, the margin of the laser speed for both optics is ranging in between 50 mm/s and 100 mm/s, which is compatible with common inkjet printing speeds and facilitates an in-line laser sintering approach. Considering the laser power, the typical parameter range for the spot laser lays in between 10 W and 50 W, whereas for the line optics the full laser power of 200 W had to be applied. One of the nanoparticle silver inks exhibits, especially for the line laser optic, a conductivity of up to 2.22 × 10⁷ S‧m⁻¹, corresponding to 36% of bulk silver within a few seconds of sintering duration. Both laser sintering approaches together present a remarkable facility to use the laser either as a digital tool for sintering of defined areas by means of a spot beam or to efficiently sinter larger areas by means of a line beam. With this, the utilization of a laser sintering methodology was successfully validated as a promising approach for converting a variety of inkjet-printed silver patterns on a flexible polymeric substrate into functionalized conductive silver layers for applications in the field of printed electronics.

Robot-guided inkjet printing technology offers a new way for the digital and additive deposition of low-viscous inks to be made directly onto arbitrary surfaces and, thus, enables the production of individualized printed electronics on large-scale objects. When compared to conventional flatbed printing, the distance between the nozzle plate and the object’s surface varies and needs to be considered in order to match the accuracy requirements needed for the positioning of single drops. Knowledge about applicable distance limits and the influence of tunable print parameters is crucial for improving the print process and results. This study discusses the sources of errors in the inkjet printing process onto 3D objects and presents extensive results about position accuracy in relation to jetting distance for different parameter sets of functional inks, drop volumes, and piezo voltages. Additionally, an efficient novel method was applied to determine the drop position accuracy of inkjet droplets in relation to the jetting distance. The method relies on cylinder geometry for the object and an inkjet head that is guided by a six-axis robot manipulator along the cylinder’s axis. For the determination of drop placement accuracy, the position of single dots on the surface was compared to a model which considered the cylinder radii, drop velocity, and the movement speed of the guided inkjet printhead. The method and the extensive research results can be utilized for the prediction of achievable drop placement accuracy and the prior definition of distance limits.

Herein, the reliability and potential of inkjet‐printed flexible heaters are demonstrated by a set of resilience tests, showing never before performed high‐temperature application. The flexible heaters can operate up to 400 °C and failures are not caused by the printed conductor, but due to the temperature sensitivity of the base substrate only. Furthermore, these heaters show an accurate repetition rate with fluctuations <3% (2 °C within ten up–down cycles between 20 and 150 °C) and great long‐lasting capability (200 h on‐time operation at 150–180 °C). The bending strength has been proven by 10 000 cycles with resistance change less than 5%. Additionally, the adaptability and adjustability of defined temperature zones with the help of design rules for the layout definition are illustrated. These inkjet‐printed heaters demonstrate a high potential of usage in high‐temperature and flexible applications with the prerequisite of high stability and reliability.

This article reports on developments in the manufacturing of heating elements by means of digital inkjet printing technology. The area coverage of the meander lines ranges from 34 lpi (lines per inch) to 51 and 102 lpi, which mainly influences the temperature distribution and homogeneity. Furthermore, the line width of the meander lines is varied between 250, 500, and 1000 μm. All heating elements are deposited by single‐pass printing of a nanoparticle silver ink with subsequent thermal sintering on a standard flexible polymer film, to demonstrate that inkjet printing allows the manufacturing of printed thin devices also on low‐cost material. The implementation of various designs allows the control of the temperature distribution and heat development. The printed structures are evaluated regarding their optical and electrical characteristics and their thermal performance is assessed using an infrared camera. This research has succeeded in developing bendable printed heaters, which reach homogeneous average temperature of 100 °C over an area of approximately 15 cm² at a power supply of 12 V. The feasibility of the inkjet printed heaters is demonstrated by a long‐term test over several days with negligible fluctuations in the area temperature and highest stability in the resistance.