Stretchable electronics could be created from hydrogels

Scientists from Russia and the US have proposed a method for the fabrication of highly transparent, electrically conductive, stretchable tough hydrogels modified by single-walled carbon nanotubes (SWCNTs), with their study published in the journal ACS Applied Materials & Interfaces.

Hydrogels are soft materials that have made possible or improved on an array of modern technologies, such as tissue engineering, drug delivery, biomedical devices, stretchable/bio-integrated electronics and soft robotics. Hydrogels that have similar physiological and mechanical properties to human skin are ideal materials for effective bio-integration of such electronics devices.

Electrically conductive hydrogels (ECHs) are attracting much interest in the field of biomaterial science due to their unique properties. However, the effective incorporation of conductive materials in the matrices of hydrogels for improved conductivity remains a great challenge.

SWCNTs, meanwhile, are a family of materials exhibiting exceptional thermal, electronic and mechanical properties, and therefore have been utilised as nanofillers of nanocomposite hydrogels. Now, scientists from the Skolkovo Institute of Science and Technology (Skoltech) and MIT have used a one-step technique to facilitate the dry transfer of SWCNTs on hydrogels, thereby making it possible to avoid problems associated with SWCNT agglomeration, and the removal of surfactants, while also simplifying the whole fabrication process.

The researchers demonstrated two ways of fabricating SWCNT/hydrogel structures, the first of which is based on a simple transfer of the SWCNTs from a filter to the as-prepared hydrogel surface. Based on the performed characterisation, the first approach can be used for SWCNT/hydrogel structures utilisation as strain-sensitive material; the team observed stable behaviour during 5000 stretching/releasing cycles.

The second approach of structure fabrication, based on the pre-stretching of the hydrogel before the SWCNT film is deposited, makes it possible to overcome low conductivity at high strains and ensure high transparency. Moreover, it can be utilised for applications where the stable performance of the electrodes during stretching is needed without the alteration of the electrical properties.

“In this work, we report new transparent, stretchable, conductive and biocompatible hydrogels modified by SWCNT films to create passive electrodes and active sensors for wearable and skin-like electronics,” said Skoltech PhD student Evgenia Gilshteyn, the first author of the paper. “We introduce here a one-step, universal and applicable method for SWCNT/hydrogel structure fabrication, able to withstand intrinsic stretching of up to 100% strain. Our method of SWCNT film patterning makes it possible to create large-area electronic circuits, as well as a variety of wearable devices, including electronic skins.”

“Using the proposed approach, we created mechanically robust, highly stretchable, biocompatible, conductive and transparent SWCNT/hydrogel structures and demonstrated their applications as finger-mounted joint motion sensors and electrocardiographic electrodes,” added Skoltech Professor Albert Nasibulin, corresponding author on the paper. “The advantages of the proposed structures in terms of conductivity, stretchability, transparency and applicability for electronic circuit creation are evident and discussed in our research paper.”

Image caption: SWCNT/hydrogel-based patterned circuit pictured in three ways: attached to human skin, relaxed and stretched by 50%. Image credit: Evgenia Gilshteyn/Skoltech.