Researchers from the McKelvey School of Engineering at Washington University in St Louis have developed ink pens that allow individuals to handwrite flexible, stretchable optoelectronic devices capable of emitting and detecting light on everyday materials such as paper, textiles, rubber, plastics and 3D objects.
The simple and versatile fabrication approach enables anyone to make a custom light-emitting diode (LED) or photodetector without the need for any specialised training or bulky equipment. The technology builds on earlier work by Wang and Junyi Zhao, a doctoral candidate in Wang’s lab, in which they demonstrated a novel way to fabricate stretchable LEDs with a simple inkjet printer.
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‘Handwriting custom devices was a clear next step after the printer,’ said Chuan Wang, an associate professor of electrical and systems engineering. ‘We had the inks already, so it was a natural transition to take the technology we had already developed and modify it to work in regular ballpoint pens, where it could be cheap and accessible to all.’
The team’s environmentally friendly, innovative handwriting approach empowers individuals to create multicolour LEDs and photodetectors in mere minutes.
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The technology harnesses the simplicity of a ballpoint pen, filled with specially designed inks made of conductive polymers, metal nanowires and crystalline materials called perovskites to generate a wide spectrum of emission colours. By writing layer upon layer with these functional inks, much like using multicoloured pens, a variety of functional devices can be created cheaply, easily and quickly.
Though the team had already developed printable ink, translating it for standard ballpoint pens that are feasible for handwriting on everyday materials required a few tweaks to control wettability and improve writability. Most importantly, Zhao had to make sure that the ink could be applied to porous and fibrous substrates, including paper and textiles, without running or mixing. Beyond avoiding the aesthetic disappointment of a smudged design, the layers must remain discrete to ensure functional, high-performance optoelectronic devices.
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‘The translation from printer to ballpoint pen might look simple, but it’s actually a bit trickier than just loading ink,’ Zhao said. ‘Our ink is specially formulated so the pens are universal, meaning they’ll work on almost all substrates. Each single layer of the device is designed to be intrinsically elastic so it will survive deformation and can be bent, stretched and twisted without impacting device performance. For example, LEDs drawn on a glove could tolerate deformations from repeated fist grasping and releasing, and LEDs drawn on a rubber balloon could survive inflation-deflation cycles over and over.’
The creation of ink pens that work on all substrates overcomes critical limitations of traditional LED fabrication – particularly the requirement of flat, smooth substrates and cost-prohibitive clean-room fabrication equipment – and opens the door for next-generation wearable electronics to permeate daily life in an unprecedented way.
Wang envisions future applications for the team’s breakthrough that are limited only by the user’s imagination. Immediate uses range from educational purposes and science popularisation to electronic packaging and clothing to medical sensors and bandages.
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‘Cheap, customisable LEDs open up opportunities for hands-on education, more vivid textiles like light-up clothing or greeting cards, and smart packaging,’ Wang said. ‘One area we’re really excited about is medical applications. Handwritten light emitters and detectors allow more patient-specific flexibility in creating wearable biomedical sensors and bandages that could have photodetectors and infrared LEDs drawn onto them for measuring pulse oximetry or to speed wound healing.’
The accessibility and flexibility of this method could democratise electronic manufacturing, allowing the possibility of customised, stretchable electronic devices to become part of the fabric of everyday life, the researchers said.
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The research has been published in Nature Photonics.