Developing Bendable and Entirely Flexible Electronics with A New Class of Films

The future of electronics is flexible. There will be products like foldable phones, tablets, and paper-thin displays that can be rolled up, and wearable sensors to monitor health that conform to the human body's contours.

To develop these new bendable products, designers now can use existing materials such as thin films and plastics to supplant today's stiff circuit boards and bulky electronic components.

But they will soon have a new choice. Research conducted at the University of Wisconsin–Madison has uncovered a way to exploit wood to form the basis of a functional microwave amplifier. The research was summarized in a paper published in the journal Nature Communications.

In it, the University of Wisconsin-Madison's Professor Zhenqiang Ma describes how he and his team built a working microwave amplifier on a substrate made of wood-based cellulose nanofibril paper.

 

Previous Forays Into Flexible Circuitry

Previously, flexible microwave amplifiers have been constructed using rigid semiconductor-based ICs that had been thinned down and mounted on flexible substrates. But that methodology proved to be a cost-prohibitive approach. 

The amplifier chip built by Professor Ma and his team is based on breaking down wood fiber into nanoscale fibrils, tiny, slender fibers. Then, the fibrils are recombined to form a strong, transparent, and flexible film. As a bonus, the film is biodegradable.

 

Researchers at UW-Madison have built a functional microwave amplifier circuit on a flexible substrate made of wood fiber. Image credited to the University of Wisconsin-Madison.
 

Saving on the Cost of Gallium Nitride

Gallium nitride is said to be the highest performing microwave transistor material. But it is quite expensive. Ma's microwave amplifier avoids the expense of laying the entire wood-based substrate with the costly material.

Professor Ma said, "We use a tiny bead of the expensive substance, 500 microns by 500 microns, and the rest is wood. Compared to the gallium nitride, the cost of the wood is essentially nothing. The outcome is an amplifier that works very well." 

The resulting flexible circuit operates at frequencies of 5 GHz and above and provides an output of 10 milliwatts. The cellulose nanofibril substrate has proven to be equally compatible with microwave components as the more common polyethylene substrates. 

 

Biodegradable Components

With the global problem of the disposal of broken or obsolete electronic devices, the fact that circuitry based on this new process is biodegradable is a significant extra benefit. Since it is composed primarily of paper, the device can be disposed of by merely burning it when it becomes obsolete. Afterward, there is nothing left of the device but paper ash.

While this project focused on a microwave device, it applies to other sorts of flexible circuitry. As Professor Ma points out, "If you look at your own house, there are probably lots of old electronics," Ma says. "There are TVs, computer monitors, phones — all full of electronic circuits. You can't do much with them, and that generates a lot of waste. We can replace a lot of existing components with these types of circuits."