Are stretchable electronics the next evolution in wearable technology?


SOURCE: ALLABOUTCIRCUITS.COM
OCT 02, 2021

Example of a flexible PCB. Image used courtesy of PCBWay

Flexible PCBs add immense functionality, but what about stretchable electronics? New research is demonstrating just how malleable electronics design can be.

Printed circuit board (PCB) design has come a long way from the tape and cut methods of the past. Advanced capabilities have allowed designers to use ECAD software to develop FR-4 rigid PCBs with little difficulty.

Flexible PCBs and interconnects are common today and often used for interfacing hardware like laptop ‘motherboards’ to display screens or allow for foldable electronic systems.

In recent years, a discussion has arisen about the third type of circuit board and electronics device: stretchable electronics. Stretchable electronics are on the rise in research arenas, and with the popularity of wearable technology, they may be becoming a commercial product in the near future.

In this article, let's discuss what stretchable electronics are, the challenges and applications, and finally dive into recent research.

How Stretchable Electronics Differ From Flex & Rigid-Flex

There are several variations on the traditional PCB, including rigid-flex designs that incorporate interconnect copper on the flexible substrate between rigid PCB elements and full flexible PCBs with entire systems soldered onto the flex substrate.

An example of a flex PCB with system components on board.

An example of a flex PCB with system components on board. Image used courtesy of Millennium Circuits Limited

The IPC-6013D (2017 edition) is the IPC standard related to the performance of flexible and rigid-flex PCBs. However, unlike flexible PCBs, which are typically bendable in a single axis, stretchable electronics are an in-development design class that is stretchable, twistable, and bendable.

Structures like signal interconnects, substrates, sensor electrodes, and even energy storage surfaces can be stretched by various amounts of strain and still maintain their operational properties.

Overall, there are various challenges to overcome to accomplish these attributes and specific applications they can be applied.

Challenges & Potential Applications of Stretchable Electronics

A comprehensive paper published in 2019 by Wuhan University researcher Wei Wu overviewed the manufacturing materials, challenges, and progress of stretchable electronics, along with the various potential or in-progress applications for this technology.

Wu states that there are two principal methods for achieving elastic electronics, one of which is the use of inherently stretchable materials like predominant poly(dimethylsiloxane) (PDMS).

One way to achieve conductivity is by using metal nanowires, carbon nanomaterial, and polymers, which are embedded within the stretchable materials in various configurations, including "wavy structural configuration, island-interconnect, fractal design and traditional paper-cutting."

The paper also covers three potential areas of stretchable electronics: conductors and electrodes, energy storage, and the development of printed electronics like transistors and sensors, which are stretchable and self-healing.

Flexible Lithium-ion battery device in a twisted state (left) and exploded view (top right).

Flexible Lithium-ion battery device in a twisted state (a) and exploded view (b). Image (modified) used courtesy of Wei Wu and Xu et al

Since Wei Wu's paper was published, the ongoing research has led to several important realizations for stretchable electronics. Next, let's review three recent announcements which stretch the limits (pun intended) of modern electronics applications.

An MXene Supercapacitor Modelled After An Accordion?

The first recent news comes from the American Chemical Society (ACS), highlighting the development of a flexible supercapacitor made of wrinkled titanium carbide, an MXene nanomaterial.

The supercapacitor electrodes were developed by disintegrating the MXene powder in hydrofluoric acid to retrieve pure layers of titanium carbide nanosheets. This material was placed on an 800% strained polymer, as seen below (in still-frame), and then crumpled when the polymer returned to rest.

Two MXene crumpled electrodes and a polyvinyl(alcohol)-sulfuric acid gel electrolyte as a dielectric were used to form the supercapacitor.

MXene coated supercapacitor film, in the center, on the stretched surface.

MXene coated supercapacitor film, in the center, on the stretched surface. Image [video] used courtesy of ACS

The resulting crumpling created an accordion-type structure on the surface of the electrode, which allowed for repeated stretchability. The researchers state that their supercapacitor maintained up to 90% of its energy storage capacity after 1000 stretches, bends, or twists.

Structures of various thickness MXene films after 1000 stretch cycles.

Structures of various thickness MXene films after 1000 stretch cycles. Image used courtesy of Kong et al

MXene nanomaterials are shown to be very versatile. For instance, nanomaterials have also been used successfully as an EMI shielding material in fabrics.

Moving on from MXene nanomaterials, let's take a look at research using a type of conductive film to push the limits of rigid PCBs.

Can Deformable Conductive Films Replace Rigid PCBs?

Also, this month, a team of three researchers, led by Professor Unyong Jeong from the Pohang University of Science and Technology, introduced the idea of a 'deformable circuit board' on stretchable anisotropic conductive film (S-ACF).

The team developed this material by patterning metal particles in a copolymer called SEBS-g-MA. The researchers found that it can interface the resulting material with electronic components to create conductivity between subsystems or devices.

Example figures from Pohang University researchers.

Example figures from Pohang University researchers. Image used courtesy of POSTECH and Jeong et al

They also note that S-ACF material can be patterned in such a way as to decrease the unused metal particles and increase the polymer surface. This attribute is said to lead to improved stretchability when compared to conventional anisotropic films.

Stretchy Skin Electronics Use Conductive Nanomembranes

Lastly, let's look at the development of nanomembranes engineered for use in 'skin electronics.'

Professor Hyeon Taeghwan and researcher Kim Dae-Hyeong, from the Institute for Basic Sciences (IBS) in Seoul, have developed a method to fabricate a new material that is said to achieve a challenging set of performance criteria.

Skin electronics typically require four major parameters for functionality: metal-like conductivity, stretchability, ultrathin, and easily patternable.

They coined the method "float assembly," which considers the Marangoni effect used to advantage when dropping the liquid composite to spread the nanomaterials. The materials are then packed by dropping a surfactant.

The basic process for the construction of the nanomembrane.

The basic process for the construction of the nanomembrane. Image [modified] used courtesy of IBS

The key successes which are said to have resulted from this method include an ultrathin rubber film, at 250 nm thick, with a stretchability factor of 10x.

Overall, the stackability and bonding features of the material could provide a conductivity of 100,000 S/cm. The material could also allow for the use of photolithography for patterning.

All in all, the world of stretchable and flexible electronics keeps building up momentum. With each of these new advances in materials to create better electronics, it will be encouraging to see where this technology goes next, especially outside of the realm of research.