ROLLA, Mo. — Electronic components that can be elongated, twisted, or stretched—known as "stretchable" electronics—could soon power everything from medical wearables to in-vehicle systems. And researchers at Missouri University of Science and Technology are pioneering a manufacturing approach that may finally bring this technology to the mainstream.

Writing in the January 2017 edition of the journal Micromachines, the Missouri S&T research team, led by Dr. Heng Pan, assistant professor of mechanical and aerospace engineering, assessed the current state of stretchable electronics and proposed a solution to one of the field's most persistent challenges: the fundamental mismatch between flexible surfaces and rigid electronic conductors.

The Rigid-Flex Challenge

At the heart of stretchable electronics is the substrate—typically an elastomer, a polymer with high elasticity that can be bent, stretched, buckled, and twisted repeatedly with little impact on its performance. However, the electronic conductors that must be built onto or embedded into this flexible surface are often brittle and rigid.

"Unique designs and stretching mechanics have been proposed to harmonize the mismatches and integrate materials with widely different properties as one unique system," the research team wrote

Direct Aerosol Printing: A New Approach

To overcome this rigid-flex barrier, Pan and his colleagues turned to additive manufacturing—a process that builds three-dimensional objects layer by layer, similar to 3D printing but using metals, ceramics, or other functional materials.

At Missouri S&T, they are testing a specific approach Pan calls "direct aerosol printing". The process involves spraying a conductive material and integrating it with a stretchable substrate.

"With the development of additive manufacturing, direct writing techniques are showing up as an alternative to the traditional subtractive patterning methods," the researchers explained. Traditional subtractive approaches include photolithography, which is commonly used to manufacture semiconductors.

Pan and his team believe additive manufacturing offers a more economical path to creating these new devices. "Direct printing, as an additive manufacturing method, would satisfy such requirements and offer low cost and high speed in both prototyping and manufacturing," they wrote. "It might be a solution for cost-effective and scalable fabrication of stretchable electronics".

A Prototype That Sticks

The team has already created a working prototype of a stretchable electronic device that can adhere to the face. "The biggest benefit of these electronics is that they can be completely wearable, and they can completely form to any kind of motion," Pan told R&D Magazine. "They can be mounted on face, for example, and could detect any small motion from your face".

Potential applications extend far beyond facial sensors. These stretchable conductors could one day replace the rigid, brittle circuit boards that power today's electronic devices. They could be used as wearable sensors that adhere to the skin to monitor heart rate or brain activity, as sensors embedded in clothing, or even as thin solar panels that could be plastered onto curved surfaces.

"We see a lot of benefit. We think this can be the future of electronic development," Pan said.

Challenges Ahead

Despite the promise, significant hurdles remain before stretchable electronics become widely adopted. All materials needed for each device must be printable, requiring the development of inks and printable materials with all the necessary properties for each electronic function.

"There are also integration challenges, such as varying temperature requirements among different materials," Pan noted. One of the team's biggest current focuses is developing an effective, long-lasting stretchable battery. "The energy device is a very critical component in order for this to be realistic," Pan said. "We are intensely working on the battery".

The researchers also emphasize the need to ensure that stretchable electronics and the malleable surfaces they are built upon perform and age well together.

A Flexible Future

Despite these challenges, Pan remains optimistic. "Additive manufacturing has the benefit that it can easily change from one material to the other and integrate all the different materials together in one print," he said. "We believe the additive technique has a very strong advantage in the creation of electronics".

Once perfected, the technology will need to be scaled up for commercial production—a process that 3D printing inherently streamlines. Ultimately, the team envisions devices that are not only low-cost to create but also biodegradable.

"There is a lot of potential to this related to human-computer interaction," Pan said.

The research was published in Micromachines (2017, 8(1), 7) under the title "Materials, Mechanics, and Patterning Techniques for Elastomer-Based Stretchable Conductors".