A 1.3 micrometer thick elastic conductor for wearable and implantable devices

In recent years, engineers have worked to develop ever more sophisticated and smaller electronic components that could power the devices of the future. This includes thin and stretchy components that could be easily worn on the skin or implanted inside the human body.

Researchers from RIKEN, Nanyang Technological University, National University of Singapore, University of Tokyo and other institutes in Japan, Singapore and China have recently achieved a new elastic electrical conductor of a thickness of 1.3 micrometers. This conductor, presented in an article published in Natural electronicscould advance the development of wearable and implantable sensors.

“Ultra-thin electronic devices can form a conformal interface with curved surfaces, are not perceptible to humans when worn, and do not induce strong foreign body rejection (FBR) when implanted in animals,” said Zhi Jiang, one of the researchers who conducted the study. , said TechXplore.

“Previously, ultra-thin electronic devices were built on plastic films, such as polyimide, parylene and SU8. However, in wearable and implantable applications, devices can experience deformations with human skin and certain organs (e.g. , heart, muscles and nerves), so they must be stretchy.”

The primary goal of recent work by Jiang and his colleagues was to create a stretchable material that could support the stable operation of ultrathin wearable and implantable electronic devices for long periods of time. To do this, they created a 1.2 μm thick film using an FDA-approved elastomer called Polydimethylsiloxane (PDMS). They then used this film as an alternative to the plastic films typically used to create biocompatible electronic devices.

“Previously, all PDMS-Au conductors used thick PDMS films (ranging from tens to hundreds of micrometers in thickness), which had poor interfaces with textured human skin and small organs (sciatic nerves and muscle bundles ),” Jiang said.

“In addition, human skin needs to breathe at all times, and the gas permeability of thick PDMS films is not high enough to allow this. By reducing the thickness to 1.2 μm, ultra-thin PDMS films have showed high gas permeability and skin breathability.”

To create their elastic conductor, the researchers first created a 1.2 μm thick PDMS film using a technique known as spin coating. They then transferred this ultra-thin film onto a 100 μm thick PDMS coated glass and thermally evaporated a 50 nm thick layer of gold (Au).

“Using a shadow mask, we could pattern our conductors and form multi-channel electrode arrays with high resolution (100 μm),” Jiang said. “Next, using another ultra-thin PDMS film as an encapsulation layer, we selectively exposed small-area Au as electrode sites. Encapsulation occurred by the tight bonding of two PDMS films after the O₂ plasma treatment.”

Due to its unique microcracked design, the team’s PDMS-Au material was found to be highly stretchable, far more than plastic films tested in the past. Remarkably, its fabrication process is also highly compatible with existing microelectronic fabrication methods, which could facilitate its large-scale production.

“By comparing electrodes of different thickness for interfaces on skin and nerves, we demonstrated that a transparent interface can contribute to both the processes of delivering electrical stimuli and recording electrical signals,” said Jiang. “The interface between electronic devices and tissue proves to be extremely important for nerves for the first time. Our study and the phenomenon we discovered should therefore be insightful for the creation of other device/tissue interfaces.”

The strategy of growing gold microcracks on PDMS used by Jiang and his colleagues resulted in a material that could be stretched by up to 300%, while retaining its conductive abilities. In the future, this strategy could be used by other researchers to design alternative stretchable materials based on microcracks.

Moreover, the new conductor presented in this recent article could be used to create more reliable portable and implantable microelectronic devices. The team has already used it to make breathable, water-resistant electrodes that can be applied to human skin, as well as 3-µm-thick sensors capable of detecting mechanical forces and implantable nerve electrodes.

“We are currently exploring two main research directions,” Jian added. “The first aims to further improve the performance of our ultrathin elastic conductors from a device engineering perspective. Second, we are working with biologists to explore the potential of our ultrathin elastic electronic devices as a powerful tool for understanding biological phenomena.

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