It sounds like something gadget-inventor “Q” from the James Bond movies might have devised: a prototype material for an ultrathin, skin-like semiconductor that degrades when it comes into contact with a weak acid like vinegar.
A research team at Stanford University has developed just such a material.
"In my group, we have been trying to mimic the function of human skin to think about how to develop future electronic devices," says Zhenan Bao, a chemical engineering professor whose team created the biodegradable polymer.
Bao previously created a stretchable electrode modeled on human skin. That material could bend and twist in ways that allowed it to interface with the skin or brain. But material’s inability to degrade limited its application for implantable devices and as important, at least to Bao, created more waste.
Bao described how skin is stretchable, self-healable and also biodegradable, an attractive list of characteristics for electronics. “We have achieved the first two [flexible and self-healing], so the biodegradability was something we wanted to tackle.”
SMALLER CARBON FOOTPRINTS
In their paper detailing their findings, the researchers noted that recent advances in silicon-based electronics have produced inorganic devices that are "mechanically brittle and require high-vacuum, high-temperature, and generally high-cost manufacturing processes."
However, organic polymer electronics can be produced at low temperatures, and their ability to decompose leaves a small carbon footprint, an attractive feature considering in 2017 almost 50 million tons of electronic waste was tossed, 20% more than in 2015.
Troubled by this mounting waste, Bao and her team began rethinking electronics. In addition to the polymer, the team developed a degradable electronic circuit and a new biodegradable substrate material for mounting the electrical components. This substrate supports the electrical components, flexing and molding to rough and smooth surfaces alike. When the electronic device becomes obsolete, it can biodegrade into nontoxic components.
Bao said that creating a robust material that is both a good electrical conductor and biodegradable was a challenge, considering traditional polymer chemistry.
To achieve the material’s biodegradability, the team found that by tweaking the chemical structure of the flexible material, it would break apart under mild stressors.
"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."
The result was a material that could carry an electronic signal but break down without requiring extreme measures.
In addition to the biodegradable polymer, the team developed a new type of electrical component and a substrate material that attaches to the entire electronic component.
Electronic components typically are gold based. But for this device, the team made components from iron due to the metal’s environmentally friendly and nontoxic properties.
The researchers also created the substrate, which carries the electronic circuit and the polymer, from cellulose, a natural biodegradable polymer found in wood, paper and cotton. Bao’s team altered the cellulose fibers so the “paper” is transparent and flexible, yet still able to easily break down. The thin film substrate allows the electronics to be worn on the skin or even implanted inside the body.
IMPLANT APPLICATIONS ON THE HORIZON?
The semiconductor clearly remains a work in progress, the authors note. The combination of a biodegradable conductive polymer and substrate makes the electronic device useful in a variety of settings, ranging from wearable electronics to large-scale environmental surveys with sensor dusts.
“We envision these soft patches that are very thin and conformable to the skin that can measure blood pressure, glucose value, sweat content,” Bao says.
A person could wear a specifically designed patch for a day or week, and then download the data. According to Bao, this short-term use of disposable electronics seems a perfect fit for a degradable, flexible design.
The biodegradable substrate, polymers and iron electrodes make the entire component compatible with insertion into the human body, as well. The polymer breaks down to product concentrations much lower than the published acceptable levels found in drinking water.
Although the polymer was found to be biocompatible, Bao said that more studies are needed before implants become a regular occurrence.
Biodegradable polymers have been commercially available for over 20 years, but the materials are still considered very early in their product life cycle, according to BCC Research. The global biodegradable polymer market is expected to reach 5.6 billion pounds by 2021, up from 2.4 billion pounds in 2016, demonstrating an 18% CAGR through the end year.
Details of the Stanford study were published in Proceedings of the National Academy of Sciences.