Biodegradable semiconductor addresses our growing e-waste problem
How many of your personal gadgets or home appliances have you replaced or thrown away in the last few years? While it might not get as much coverage as plastic bags, electronic waste is just as thorny an environmental problem. Our love affair with technology is leading to pollution, depletion of resources and a growing stockpile of discarded devices. But what if there was a way for our unwanted electronics to break down so we wouldn't even need to think about the environmental impact of disposing of them?
That's what Stanford's Zhenan Bao is trying to do. Her lab has come up with a prototype for an ultrathin, skin-like semiconductor that can fully degrade when it comes into contact with a weak acid such as vinegar.
No stranger to organic polymer-based electronics, she and her team have been pushing the envelope with all kinds of next-gen applications inspired by the human skin, from a stretchable electrode that could help prostheses users regain their sense of touch to paper-thin heart sensors and self-healing, pressure-sensitive plastic polymers. However, while they have managed to recreate its flexibility and ability to heal itself, one of skin's attributes has long eluded them: biodegradability.
"In my group, we have been trying to mimic the function of human skin to think about how to develop future electronic devices," says the chemical engineer, adding that creating a robust material that is a good conductor of electricity and can biodegrade was no easy feat, given the nature of traditional polymer chemistry. "We have been trying to think how we can achieve both great electronic property but also have the biodegradability."
While the idea of transient electronics is not new, recent developments have been of the silicon-based variety and such inorganic devices are "mechanically brittle and require high-vacuum, high-temperature, and generally high-cost manufacturing processes," note the authors in the study. Organic polymer electronics, on the other hand, can be produced at low temperatures and are biodegradable, which helps keep their carbon footprint low.
Eventually, the researchers at Bao's lab hit upon the idea of tweaking the polymer's structure so it would be able to break down easily. This was done using reversible imine bonds.
"We came up with an idea of making these molecules using a special type of chemical linkage that can retain the ability for the electron to smoothly transport along the molecule," explains Bao, adding that the chemical bond "is sensitive to weak acid – even weaker than pure vinegar."
That said, the polymer is just one part of the semiconductor. The team also developed a substrate from cellulose, a natural biodegradable polymer found in wood, paper and cotton. By altering its fibers, the researchers were able to achieve a transparent and flexible base layer that would allow them to mold the semiconductor to rough and smooth surfaces alike.
The final part comprises an electronic component made of iron. Usually, gold is used to make these components because it is a stable, biocompatible material that is commonly used in electronic implants. However, it is not dissolvable and cannot physically disintegrate, unlike iron, which also has the added advantage of being environmentally friendly and nontoxic to humans. Despite not having the stability of gold, the researchers found that the iron electrode was stable operating at less than 4 V..
In their experiments, the researchers found that when placed in a pH 4.6 buffer solution (by way of comparison, vinegar and gastric acid have a pH range of 2 to 3 and 1.5 to 3.5, respectively), the iron electrodes degraded rapidly within an hour. The other materials, such as the polymer and cellulose substrate, were gone by the end of 30 days.
While still a work in progress, this new semiconductor, which according to the paper is 40 times lighter than a sheet of office paper (80 g/m²), opens the door to a wide range of applications, say the researchers. It could, for example, be used in medical implants that can be absorbed by the human body, meaning that patients wouldn't need to undergo surgery to extract them. Lead author Ting Lei tells us that the iron electrodes would be encapsulated with the biodegradable polymers to prevent them from rusting in such an application, but more studies will have to be conducted before such implants make it into clinical trials.
Other potential uses include healthcare applications, such as in wearable trackers that can be worn on the skin to measure blood pressure, glucose levels or sweat content; or in secure electronics that can disintegrate in a controlled fashion, thus making it hard to trace them. On his part, Lei believes it could also be used to conduct surveys in remote regions. Airplanes or drones would drop these electronic sensors over forests or hard-to-reach locales so researchers can survey the landscape without having to spread or retrieve the devices themselves. Upon accomplishing their purpose, they would then disintegrate naturally without polluting the environment.
Whatever the application, given that the number of electronics is growing year on year, biodegradability of such devices is going to be an increasingly important feature.
"We currently … generate millions and billions of cell phones, [which are] hard to decompose," says Lei. "We hope we can develop some materials that can be decomposed so there is less waste."
The study has been published in Proceedings of the American Academy of Sciences.
Source: Stanford University