No abstract

Growing advances in printed flexible electronics enable novel design and manufacturing processes of electrical circuits and components in aircraft and automotive industries, [1][2][3] as well as in consumer electronics for fabrication of printed passive components, semiconductor devices, and sensing elements. [4][5][6][7][8][9][10][11][12][13][14][15] In the healthcare domain, the impact of printed electronics becomes more profound and offers a wide range of applications from printed RF coils for medical imaging, [16] biochemical sensing, [17,18] vital sign monitoring [19][20][21][22][23][24][25] to assistive wearable devices. [26,27] At the industrial level, its low cost and high-throughput manufacturing process are the key motivators for transition from conventional solutions to printed technologies. In wearable applications, its capability to realize soft wearable sensors, which can comply with the dynamic deformations of human skin, is attractive.Despite the promising opportunities of printed electronics in different domains, there are concerns regarding the reliability of printed materials (i.e., conductive inks, isolators, and adhesives) and their interconnections to electronic components for longterm use in real-life applications. Over the past decade, there has been an increasing number of studies on the reliability of printed conductors, interconnections, and chip-on-flex assembly. As a fundamental test, the cyclic bending endurance of printed conductors was investigated to analyze the characteristics of conductive tracks on a flexible substrate. [28][29][30] In more complicated test protocols, the influence of environmental parameters (e.g., temperature, humidity, etc.) was included in the test procedure. [31] More advanced test setups and protocols were developed to evaluate the reliability of complex hybrid integration of chip-on-flex, flexto-flex assemblies, and roll-to-roll (R2R) printed circuits. [32][33][34] This work presents a comprehensive study on the reliability of hybrid integration of thinned bare die chip on soft and stretchable substrate. More specifically, the findings of this study are expected to provide fundamental insights on the failure mechanisms, their corresponding contributors, and ways to minimize them. This study is divided into three phases as shown in Figure 1A. In the screening phase, the electromechanical performance of five different ink variants is evaluated and the most suitable ink variants are selected to be used in combination with three different types of conductive adhesive for fabrication and assembly of the test device. In the last phase of the study, the fabricated test devices are tested through a cyclic strain test and a comprehensive analysis of failure mechanisms is performed. The importance of the failure analysis is recognized in refining of the design steps, material selection, and processing parameters for improved reliability.

Printed, flexible, and hybrid electronic technologies are advancing rapidly leading to remarkable developments in smart wearables, intelligent textiles, and health monitoring systems. Flexible electronics are typically fabricated on petroleum‐derived polymeric substrates. However, in the light of global environmental concerns regarding fossil raw materials, there is a need to drive the production of flexible electronics devices based on sustainable materials. Additionally, there is a need to reduce the quantity of electronic waste by developing material recovery and recycling technologies. Here, a fully biobased and biodegradable substrate tailored for printed flexible electronic applications is developed. Based on a nanocomposite of cellulose nanofibril (CNF) and hydroxyethyl cellulose (HEC), the substrate shows excellent mechanical and optical properties for printed flexible electronics applications. High‐resolution screen printing of conductive ink and typical electronics assembly processes are possible to realize on the substrate. An electrocardiograph (ECG) device is fabricated on the cellulosic substrate as a technology demonstrator and its performance is confirmed on human volunteers. Last, end‐of‐life scenarios are studied for printed electronic devices where device degradation and subsequent material recovery concepts are presented. This work demonstrates that sustainable plant‐derived materials can play a big role toward a green transition in the electronics industry.

The increasing need for wearable electrophysiological monitoring devices has motivated significant amount of research to develop electronic tattoos (e‐tattoo) enabling high comfortability and unobtrusive usage. Despite considerable advancements, challenges still remain that limit the device testing under clinical settings and consequently, the uptake to practical usage. A key challenge is the robust full signal pathway from the skin to the electronics device. Another factor limiting the concept testing with high number of test devices is the lack for scalable materials and processes comprising all manufacturing steps. The study presents a scalable manufacturing of self‐adhering tattoo electrodes and interconnects on transferable breathable polydimethylsiloxane (PDMS) substrate with the thickness of 30 µm. Transfer tattoo enables two‐sided signal pathway from the skin, in to the encapsulated conductor traces and provides exposed connection points to electronics. The e‐tattoos withstand repeated one thousand, 10% elongation cycles and the transfer substrate showed breathability comparable to commercial medical patches. The integration of flexible wireless ECG module with the skin‐mounted e‐tattoo is also demonstrated. The results enable the up‐scaled fabrication of robust e‐tattoos with integrated electronic modules that is prerequisite for the scientific studies relying on high number of test devices and industrial uptake of e‐tattoo concepts.