Water Vapor Plasma Bonding for Ultrathin and Flexible Electronics


Scientists at RIKEN research institute in Japan have developed a new method to bind gold electrodes to each other within flexible electronics. The technique, which does not require adhesives or high temperatures that can damage delicate electronic components, allows for extremely thin and flexible electronics and could lead to new types of medical wearables. The team’s method relies on exposing tiny gold electrodes to water vapor plasma before binding them together. The plasma generates hydroxyl groups that help to bind the gold surfaces together, and the process can take place at room temperature.

Conformability test on a curved surface (radius: 0.5 mm). Ultra-thin films bonded using (top) water-vapor plasma assisted bonding or (bottom) standard adhesive.

Wearables offer enormous opportunities for medical care, with continuous health monitoring allowing clinicians to keep an eye on their patients’ conditions over time to catch adverse events before they lead to long-term complications. However, many monitoring modalities require contact with the skin, and this often means that the electrical components within a wearable device need to be flexible and thin, but also robust enough to withstand repeated movement.

One issue involves connecting tiny electrodes within the flexible thin films that form the basis of many wearables. Conventional methods, which typically require adhesives or high temperatures, can damage the delicate electronic components or impair the flexibility of the wearable.

To address this, these researchers have employed a gentler technique, although they stumbled upon it by accident. Their method involves attaching gold electrodes to an ultrathin polymer sheet and then exposing them to water vapor plasma for 40 seconds. The researchers then press the polymer sheets together so that the electrodes touch at the areas where they are intended to bond. After 12 hours the electrodes are bonded, and the flexible film can be used as desired.     

“This is the first demonstration of ultra-thin, flexible gold electronics fabricated without any adhesive,” said Kenjiro Fukuda, a researcher involved in the study. “Using this new direct bond technology, we were able to fabricate an integrated system of flexible organic solar cells and organic LEDs.”    

(A) Evaporated gold surfaces on 2 ?m thick parylene substrates were exposed to water vapor plasma. (B) Bonding of water vapor plasma treated-gold was achieved by overlapping the two substrates and storing them in ambient air for a few seconds to several hours without any applied pressure or heat.

So far, the Japanese researchers have created thin films containing a variety of electrical components, including LEDs, and put them through the wringer by twisting them around a stick and crumpling them up, without failure.  

“We expect this new method to become a flexible wiring and mounting technology for next-generation wearable electronics that can be attached to clothes and skin,” said Fukuda. “The next step is to develop this technology for use with cheaper metals, such as copper or aluminum.”

See one of the films in the video below.

Study in Science Advances: Direct gold bonding for flexible integrated electronics

Flashbacks: Washable and Flexible Batteries for Wearable Medical Devices; Flexible Transistors for Body-Worn and Implantable Medical Devices; Printing Custom Flexible Electronics Directly Onto Skin, Bandages, Medical Devices; Hydrogels with Flexible Electronics Herald New Medical Possibilities


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