Wearable electronics that can be seamlessly integrated into clothing, onto skin, or inside the body, can enable a variety of novel applications within healthcare monitoring, biosensing, biomedical devices and the internet of things. Seamless integration requires matching of the mechanical properties of the electronics to clothing, skin, and tissues, i.e., the electronics need to be soft, flexible, and stretchable. One approach to achieve this is to make all or most components of a device stretchable in themselves by developing functional intrinsically stretchable composites. Such composites are typically based on a filler, which provides electronic or other functionality, and an elastomer matrix, which provides the mechanical properties of the composites. Manufacturing of intrinsically stretchable electronics is challenging and often involve time consuming and tedious fabrication procedures of low throughput, based on chemically harmful monomers and solvents. An alternative approach, printing of electronics, has experienced a boom in the past decade, recently even for stretchable applications. However, despite its appeal, stretchable printed electronic products have yet to reach the consumer market in larger numbers. Screen printing is a versatile printing method that is cost-effective, scalable, can be tailored to use harmless solvents with little waste, and can be made environmentally friendly by careful choice of materials. Furthermore, some applications of stretchable technology – such as implants and on-skin electronics – require conductors that are stable under humid, corrosive, or polluted conditions, which puts even more weight into choices of ink components.In paper I, we protected readily available conducting silver flakes through a thin coating with gold in a low-toxicity water-based process and demonstrated its use in inks for screen printed corrosion-resistant stretchable conductors. The novel silver-gold flake ink was used to fabricate a functional stretchable near-field communication device. Papers II and III both concern entirely screen printed and inherently stretchable devices, utilizing novel stretchable inks in combination with commercial inks to print vertical stacks. Two electrochemical devices – electrochromic displays and organic electrochemical transistors – were printed and tested under stretched conditions to push the limits of how screen printing can be used in applications for thin and stretchable wearable technology. The results show that the devices can retain electrical function even under practically high strains of 50 % (display) and 100 % (transistor). Finally, in paper IV, we investigate the operational principle of gold nanowire- based stretchable composites and find that interactions on the nano-and microscale differ between composites using the same filler but different elastomers. This study sheds light on the importance of the type of elastomer chosen for composites, as this heavily influences the composite’s electrical performance under strain.
Altogether, the studies presented in this thesis provide knowledge, materials, and processes that in the long run can contribute to more effective devices within healthcare and other wearable electronics applications.
