Bio-based silver conductive ink for flexible printed electronics, and in-mold electronics

Abstract

The development of environmentally friendly electronic circuits offers a sustainable alternative to conventional printed circuit board (PCB) manufacturing, which relies on chemically intensive and polluting processes. Screen printing of conductive inks is an effective and widely used method for fabricating flexible printed electronics (FPEs), and particularly well-suited for in-mold electronics (IMEs) applications. Screen printing of electronic circuits can be carried out on nearly all types of substrates, including recyclable and biodegradable polymers, thereby reducing the environmental impact. This paper presents the formulation of a silver conductive ink with stretchable properties, using poly(lactic acid) as the binder and Cyrene as the solvent, which are both biobased and biodegradable. Inks were formulated using silver flakes of different sizes (2.2 µm–5.6 µm) at a fixed content of 60%. Rheological behavior of the inks was measured to determine their printability, and resolution of printed patterns was tested using a scanning electron microscope. Thermal curing and photonic curing were used on the inks to study their electrical characteristics as functions of curing temperature and conditions. Among the latter tested formulations, the ink with a mixture of fine and large particles featured the best performance with an electrical resistivity of 34.77 ± 3.29 μΩ · cm after heat treatment at 120 °C and 9.39 ± 0.17 μΩ cm after photonic curing. This ink was also highly stretchable, with bending cycle tests showing an increase of electrical resistance only 8% after 1000 cycles, and the electrical continuity still intact even after 5000 cycles. Thermoforming tests confirmed electrical continuity at strains exceeding 30%. Demonstrators in FPEs and IME highlight the potential of these inks. This work shows that the developed ink exhibits electrical and stretchability properties equivalent to or even better than those of inks from previous research or commercial products, while reducing the environmental impact and enhancing user safety.