In a unique approach to develop a “green” solution for in-situ monitoring, low-cost inkjet-printed temperature sensors on paper substrate were fully integrated into natural fiber-reinforced lightweight components for which structural health monitoring is becoming increasingly important. The results showed that the sensors remained functional after the vacuum infusion process; furthermore, the integration of the sensors improved the mechanical integrity and stability of the lightweight parts, as demonstrated by tensile testing. To verify the qualification of the printed sensors for the target application, the samples were exposed to varying temperature and humidity conditions inside of a climate chamber. The sensors showed linear temperature dependence in the temperature range of interest (−20 to 60 °C) with a TCR ranging from 1.576 × 10⁻³ K⁻¹ to 1.713 × 10⁻³ K⁻¹. Furthermore, the results from the tests in humid environments indicated that the used paper-based sensors could be made almost insensitive to changes in ambient humidity by embedding them into fiber-reinforced lightweight materials. This study demonstrates the feasibility of fully integrating paper-based printed sensors into lightweight components, which paves the way towards integration of other highly relevant sensing devices, such as strain and humidity sensors, for structural health monitoring of smart, sustainable, and environmentally compatible lightweight composite materials.

With the growing significance of printed sensors on the electronics market, new demands on quality and reproducibility have arisen. While most printing processes on standard substrates (e.g., Polyethylene terephthalate (PET)) are well-defined, the printing on substrates with rather porous, fibrous and rough surfaces (e.g., uncoated paper) contains new challenges. Especially in the case of inkjet-printing and other deposition techniques that require low-viscous nanoparticle inks the solvents and deposition materials might be absorbed, inhibiting the formation of homogeneous conductive layers. As part of this work, the sheet resistance of sintered inkjet-printed conductive silver (Ag-) nanoparticle cross structures on two different, commercially available, uncoated paper substrates using Van-der-Pauw’s method is evaluated. The results are compared to the conductivity of well-studied, white heat stabilised and treated PET foil. While the sheet resistance on PET substrate is highly reproducible and the variations are solely process-dependent, the sheet resistance on uncoated paper depends more on the substrate properties themselves. The results indicate that the achievable conductivity as well as the reproducibility decrease with increasing substrate porosity and fibrousness.

Different conductive bonding strategies for the hybrid integration of flexible, inkjet-printed electronics are investigated. The focus of the present work lies on providing a practical guide comprising standard techniques that are inexpensive, easily implementable and frequently used. A sample set consisting of identical conductive test structures on different paper and plastic substrates was prepared using silver (Ag) nanoparticle ink. The sintered specimens were electrically contacted using soldering, adhesive bonding and crimping. Electrical and mechanical characterization before and after exposing the samples to harsh environmental conditions was performed to evaluate the reliability of the bonding methods. Resistance measurements were done before and after connecting the specimens. Afterwards, 85 °C/85% damp-heat tests and tensile tests were applied. Adhesive bonding appears to be the most suitable and versatile method, as it shows adequate stability on all specimen substrates, especially after exposure to a 85 °C/85% damp-heat test. During exposure to mechanical tensile testing, adhesive bonding proved to be the most stable, and forces up to 12 N could be exerted until breakage of the connection. As a drawback, adhesive bonding showed the highest increase in electrical resistance among the different bonding strategies.

Fan-out wafer-level packaging (FOWLP) is an interesting platform for Microelectromechanical systems (MEMS) sensor packaging. Employing FOWLP for MEMS sensor packaging has some unique challenges, while some originate merely from the fabrication of redistribution layers (RDL). For instance, it is crucial to protect the delicate structures and fragile membranes during RDL formation. Thus, additive manufacturing (AM) for RDL formation seems to be an auspicious approach, as those challenges are conquered by principle. In this study, by exploiting the benefits of AM, RDLs for fan-out packaging of capacitive micromachined ultrasound transducers (CMUT) were realized via drop-on-demand inkjet printing technology. The long-term reliability of the printed tracks was assessed via temperature cycling tests. The effects of multilayering and implementation of an insulating ramp on the reliability of the conductive tracks were identified. Packaging-induced stresses on CMUT dies were further investigated via laser-Doppler vibrometry (LDV) measurements and the corresponding resonance frequency shift. Conclusively, the bottlenecks of the inkjet-printed RDLs for FOWLP were discussed in detail.

The growing demand for sustainable and efficient environmental monitoring systems has driven the development of innovative sensor technologies. This study presents a hybrid ultra high frequency radio-frequency identification (UHF RFID) sensor tag fabricated on uncoated paper substrate, which constitutes approximately 87% of the tag's mass thereby making the sensor tag more sustainable and eco-friendly. The sensor tag integrates an AS3213C.4 RFID chip together with an antenna, interdigitated capacitor as a humidity sensor, pads and interconnects. Temperature sensing is facilitated by the RFID chip's internal temperature sensor, while humidity is monitored through changes in the printed capacitor. All structures except for the chip were screen printed using a conductive silver ink. The silver layer exhibited a thickness of 5.6 μm and a sheet resistance of 56.4 mΩ/sq, sufficient for wireless communication over a distance of 2 meters. The sensor was wirelessly interrogated using a Kathrein antenna and reader system, with data retrieved via commercial software. Temperature tests demonstrated accurate readings from 26°C to 80°C, aligning with the chip's specifications of -40 to 125°C, with a precision of 1°C in the range of 10 to 50°C. Humidity measurements in a climate chamber, conducted between 15% and 55% relative humidity, showed an average sensitivity of 0.45% per % humidity change. Hysteresis effects of 7.4% were observed due to the moisture absorption and structural changes of the paper substrate. This work highlights the potential of paper based sensor tags for sustainable environmental monitoring, aligning with the principles of Industry 4.0 and the Internet of Things (IoT), while addressing the growing challenge of electronic waste.

In this work a versatile approach for the creation of laser-induced graphene (LIG) onto virtually any kind of substrate is demonstrated. The process is based on preconditioning the substrate with a layer of polyimide sealing resin and subsequently laser scribing the LIG structure onto it. By optimizing the laser process, the resulting LIG is firmly attached to the substrate and the unutilized resin is entirely removed. Most substrates are not amenable for LIG fabrication, including glass or metals. Using this process, LIG strain, temperature as well as humidity sensors have been fabricated on various substrates. The process features the same advantages of easy and fast patterning which LIG is well known for and enables sensor fabrication directly on the substrate.