The demand for wearable devices is creating the need for flexible, printable sensors. Researchers at the Korea Institute of Science and Technology have developed a transfer-printing technology that uses hydrogel and nano ink to create high-performance sensors on flexible substrates of diverse shapes and structures.
The transfer printing works in a way similar to that of a tattoo sticker—sticking the tattoo sticker on the skin and then removing the paper backing leaves an image on the skin. The process creates a structure on one surface and then transfers it to another in a similar way. The process largely avoids the difficulties involved in creating devices directly on substrates that are thermally and/or chemically sensitive.
At the same time, the process developed by the KIST team can transfer print on topographic services with diverse features and textures, thus avoiding the limitations of being restricted to printing on flat surfaces.
Using the porous and hydrophilic nature of hydrogels, the KIST team inkjet-printed an aqueous solution-based nano ink onto a hydrogel layer, which was solidified onto a topographic surface. The surfactant and water in the nano ink passed quickly through the hydrogel's porous structure, leaving only the hydrophobic nanomaterial remaining on the surface—the particles are greater in length than the diameter of the holes in the hydrogel.
The amount of nano ink used for this printing process was very small, allowing for the rapid formation of electrodes. Moreover, the electrical performance of the electrodes was high due to the high levels of purity and uniformity of the resulting nanonetworks. Also, the hydrophobic nature of the nanomaterial allowed for easy transfer of the electrodes to diverse topographic surfaces.
The technology to transfer nanonetworks via a method that solidifies a moldable elastomeric fluid onto a hydrogel surface enables the easy creation of flexible electrodes, even on substrates with rough surfaces. The team transferred nanoelectrodes directly onto a glove (see photo) to create a modified sensor that can immediately detect finger movements. It also created a flexible, high-performance pressure sensor that can measure the pulse in the wrist.
Lead researcher Dr. Hyunjung Yi of the Korea Institute of Science and Technology said in a statement, "The outcome of this study is a new and easy method for creating flexible, high-performance sensors on surfaces with diverse characteristics and structures. We expect that this study will be utilized 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."