Transfer printing and hydrogels key to new wearable electronics

Korean engineers devise method for constructing high-performance sensors on three-dimensional and stretchy surfaces

wearable electronics
Electrodes formed on top of a hydrogel are transferred onto a PET film. Image: KIST

Increasing demand for electronic devices that conform to the skin is posing difficulties for the medical sector. Both wearables and sensor-based electronics that can be printed onto adhesive patches are proving valuable for gathering data about the human body, whether for wearers to monitor their own health or for clinicians to keep tracks on patients at home or in hospitals. But most of the methods for constructing electronic devices and the connections necessary for them to work are suited to printing onto flat and rigid surfaces, rather than clothing-like substrates or plasters.

At the Korea Institute of Science and Technology’s (KIST) Post-Silicon Semiconductor Institute, Hyunjung Yi and her colleagues have developed a method for producing high-performance sensors on flexible substrates of diverse shapes and structures by using a combination of hydrogels and nano ink in a transfer-printing process.

Transfer-printing is a popular method for assembling electronic devices, as the devices are assembled onto a transfer mould surface and then printed onto their final substrate. It avoids the problems inherent in assembling onto the difficult shaped and flexible surfaces needed for wearables. The KIST method takes advantage of the highly-flexible nature of hydrogels to form the shapes of the sensors required, while the connections and sensors themselves are formed from the conductive aqueous solution-based nano ink.

The method, detailed in the American Chemical Society publication Nano Letters, begins with forming a layer of hydrogel based on a water-soluble seaweed-derived material in a mould. They then produced an aqueous solution of single-walled carbon nanotubes with surfactants to act as an ink. Using an inkjet printer, they deposited this ink into a “designed serpentine shape” on the surface of the hydrogel. The surfactant and the water in the ink passed through the porous surface of the hydrogel, leaving only the hydrophobic nanotubes (which are longer than the size of the pores) on the surface, leaving the required electrode pattern. The nano network electrodes, which do not stick to the gel, could then be transferred onto the wearable substrate

Only a very small amount of ink was needed to produce effective electrodes, making their formation a fast process. Moreover, the team claims, the electrodes’ electrical performance was outstanding, due to the high levels of purity and uniformity of the resulting networks formed the gel surface. The electrodes worked well even if the substrate was rough, they added.

To test the technique, Yi’s team created nano electrodes which they transferred onto a glove to create sensors that could immediately detect finger movements. Similar electrode configurations were made to make a flexible, high-performance pressure sensor that can measure a pulse in the wrist.

“The outcome of this study is a new and easy method for creating flexible, high-performance sensors on surfaces with diverse characteristics and structures,” Yi said. “We expect that this study will be utilised in the many areas that require the application of high-performance materials onto flexible and/or non-traditional substrates, including digital healthcare, intelligent human-machine interfaces, medical engineering, and next-generation electrical materials.”