Researchers Advance Stretchable Electronics Toward Commercialization

Soft stretchable electronics (SSEs) are a hotbed of research in wearables and health-monitoring technology. One challenge for SSEs, however, has been the inability to develop economic and scalable manufacturing processes for the technology.

This week, researchers from Carnegie Mellon University (CMU) published a paper describing a new method for mass-producing SSEs. In this article, we’ll take a look at the current state of SSEs, its existing challenges, and the new research from CMU.

 

In this application, battery-less soft sensor patches are applied to a silicon wafer post-fabrication. Image courtesy of Carnegie Mellon University

 

The Challenges of Soft Stretchable Electronics

There are currently many techniques for developing SSEs, but amongst these, liquid metal (LM) architectures are one of the most promising.

LM-based SSEs generally work by depositing liquid metals onto a flexible electronic substrate, where the LM can either serve as a physical interconnect between microchips or be deposited to form different components such as strain gauges. Compared to other forms of SSEs, LMs are popular because they flow freely inside a channel, exhibit high compliance, and retain their electrical conductivity even under large deformations.

 

Liquid metals like gallium have many emerging use cases, including soft electronics. Image courtesy of Knowable Magazine

 

One of the most popular forms of LM is the gallium-based alloy eutectic gallium–indium (EGaIn). EGaIn is particularly popular because of its unique combination of stability, negligible toxicity, and extremely high electrical conductivity (3.4 × 106 S m−1 at 298 K). 

Yet, despite EGaIn’s popularity, researchers have yet to devise a way to mass produce the technology. One of the main challenges is that when EGaIn is exposed to air, a gallium-oxide “skin” forms around the LM. This skin makes it difficult to achieve uniformity and continuity in the deposited metal. So far, no one has solved this challenge in a scalable way.

 

Carnegie Mellon Finds a Way to Mass Produce SSEs

Now, researchers at Carnegie Mellon University have published a new paper that describes a means to commercialize LM-based SSEs.

The new technique combines selective metal alloy wetting and controlled dip coating of EGaIn. The metal-alloy wetting process deposits the liquid metal into the desired circuit layout, while the dip-coating process dissolves the oxide skin that appears when EGaIn is exposed to air without maintaining the selective deposition of the LM. 

 

The proposed fabrication process. Image courtesy of Advanced Materials Technologies

 

Here, thin copper traces are lithographically patterned onto an elastomer surface as a wetting layer. Dip coating then enables the selective deposition of EGaIn onto the circuit layout. The dip coating is achieved using a custom-built machine that dips the wafers into a bath consisting of aqueous sodium hydroxide (NaOH) solution at the top surface, followed by the EGaIn. The NaOH removes the oxide skin, while the EGaIn is deposited as desired. 

Using this technique, the team successfully fabricated EGaIn-based LM devices. The study showed that the researchers could successfully fabricate many LM capacitors with a high level of geometric and electrical reproducibility. 

 

SSEs May Make Their Mark in Wearables

The presented technique can be integrated into the standard microfabrication flow for microelectronics, which means that it may be easily scalable and affordable. The CMU researchers are eyeing applications like wearable technologies and health-monitoring devices that can easily conform to the skin for improved accuracy and performance.