First author Hui Yue, then a postdoctoral research fellow at Westlake University’s engineering school in Hangzhou, said 3D structures could be printed in free form in hydrogel – unlike traditional 3D printing where objects are built layer by layer.
“We can build circuits in a 3D space without external support,” said Hui, who is now a postdoctoral research fellow at the University of Adelaide in Australia.
Hydrogel is a network of hydrophilic polymers that holds a large amount of water and is similar to biological tissue. To make electronic devices out of hydrogels, they need to contain stretchy, conductive and other functional materials.
In the study, the researchers created hydrogel electronics using hydrogel as a supporting matrix and an ink with silver, the most conductive metal. The end result can be stretched, twisted or compressed and will return to its original shape.
The team 3D-printed electrocardiography (ECG) electrodes – which record the electrical activity of the heart – and found the signals had a higher resolution than electrodes available in the market.
They also printed a millimetre-scale electrode to stimulate the sciatic nerve in mice and trigger movements in their hindlimbs. The hydrogel device caused motion at an angle of 77 degrees – far more than the 10-degree movement from conventional ionically conductive electrodes at the same voltage, according to the study.
“These outcomes highlight the superior electrical-stimulating capability of the printed Ag [silver]-hydrogel electrodes over the ones employing ionically conductive materials,” the researchers wrote.
This could be attributed to “the tight and conformal interfaces between the tissues and 3D printed electrodes, as well as the high conductivity of our Ag-hydrogel ink”.
The nervous system sends electrical signals to tell muscles to move. Hui said the technology could potentially help people who need external electrical stimulation for their nerves due to disease or accident.
The team made a hydrogel RFID device that can withstand stretching and be detected by a commercial RFID reader.
Hui said it could be used as a wearable device for pets or farm animals for identification, or for people who are ill or have a condition where they need their medical records to be easily read.
He said the next step was to test the durability, stability and safety of the hydrogel as an implant in animals over a long period of months and years.
“When the implant is removed we will check if it causes scarring or inflammation,” Hui said. “We hope to prove that hydrogel is more biocompatible than other materials. We will explore more of its potential applications while ensuring its safety.”