The need to optimize space in electronic devices has made flexible electronics an attractive option for manufacturing electronics. Techniques to fabricate flexible circuits have become more and more common and the processes increasingly more efficient. Printed electronics is a potential technique for manufacturing electronic patterns on flexible substrates. In particular, inkjet printing is an effective way to produce fine, thin, conductive structures without touching the substrate material. This study concentrated on dynamic bending analysis of inkjet-printed silver conductors on a polymer substrate. Because printed electronics is a relatively new manufacturing method, not much research is yet available on mechanical endurance of printed structures. By default, thin layers of inkjet-printed traces may just prove to have good tolerance against bending. However, factors such as adhesion between ink layer and substrate and the effect of the porous structure of sintered nanoparticle ink must be studied. This paper evaluates the capability of the inkjet technique on a flexible substrate and benchmarks the results on conventional flexible copper circuit boards. Measurements were made in real time of the resistance of conductors while bending the sample along two different radii. Results showed that printed conductors were superior in endurance over etched copper circuits.
3D printing is widely used for manufacturing complex non-functional parts, and recently, the fabrication of electronics has also attracted research attention. The commercialized process of fused-filament fabrication (FFF), which is still evolving,has been used in the preparation of basic electronic conductors and sensors but only a few studies of more complex structures with integrated circuits and passive components have been reported. Notably, the usage of FFF in wearable stretchable electronics has not been studied previously. We demonstrate that the combination of FFF printing and commonly used stretchable electronics materials and methods enables new wearable stretchable electronics. In this study, thermoplastics were extruded directly onto a stretchable substrate and their adhesion was measured using T-peel tests. The test results were further used in the fabrication of supports for meander-shaped screen-printed interconnects. The elongation of the interconnects with the supports were studied by tensile tests with simultaneous measurements of the electrical conductivity. The results were good, and the adhesion exceeded the constitution of the substrate when the filament and the substrate were of the same material type. The average bond strength was ∼2 N mm−1. Support structures placed close to the meander-shaped interconnects changed the interconnects’ deformation under elongation. The average maximum elongation of the interconnects was improved by ∼27% when the supports directed stresses away from the interconnects’ weak areas. Conversely, the results were ∼21% lower when the supports directed stresses towards the weak areas. This study demonstrates that it is possible to use direct 3D printing onto highly stretchable substrates. Currently, commercial FFF materials and methods can be used to manufacture supports, frames and other non-functional parts on wearable electronics substrates in a single process step. We believe that in the future, FFF will become a valuable tool in the manufacture of inexpensive and reliable wearable electronics.
The addition of fillers has been implemented in fused filament fabrication (FFF), and robust carbon fillers have been found to improve the mechanical, electrical, and thermal properties of 3D-printed matrices. However, in stretchable matrices, the use of fillers imposes significant challenges related to quality and durability. In this work, we show that long carbon staple fibers in the form of permeable carbon fiber cloth (CFC) can be placed into a stretchable thermoplastic polyurethane (TPU) matrix to improve the system. Four CFC sample series (nominally 53–159-µm-thick CFC layers) were prepared with a permeable and compliant thin CFC layer and a highly conductive and stiff thick CFC layer. The sample series was tested with single pull-up tests and cyclic tensile tests with 10,000 cycles and was further studied with digital image correlation (DIC) analyses. The results showed that embedded CFC layers in a TPU matrix can be used for stretchable 3D-printed electronics structures. Samples with a thin 53 µm CFC layer retained electrical properties at 50% cyclic tensile deformations, whereas the samples with a thick >150-µm CFC layer exhibited the lowest resistance (5 Ω/10 mm). Between those structures, the 106-µm-thick CFC layer exhibited balanced electromechanical properties, with resistance changes of 0.5% in the cyclic tests after the orientation of the samples. Furthermore, the suitability of the structure as a sensor was estimated.
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