The humble amino acid glycine – the simplest of all amino acids – is everywhere. The human body produces it, and it helps make up our skin, bones, and ligaments. It’s found in meats like beef and has even been discovered in comets returned from outer space.

Now, Stanford researchers are considering how glycine might enable the engineering of next-generation materials, like bioelectronics and environmental sensors, that could biodegrade when their job is done. A new paper in Advanced Functional Materials introduces a novel material designed by a team at the School of Engineering that addresses stubborn hurdles on the path toward glycine-based, electrified medical devices.

“Glycine is really interesting because it’s so abundant,” said Vivian Feig, an assistant professor of mechanical engineering in the School of Engineering and senior author of the paper. “We know how to process it, our bodies know how to break it down, and it’s cheap and really accessible.”

Feig’s lab focuses on developing dynamic functional materials for biomedical use, such as novel materials that break down over time in the body or can be introduced via minimally invasive methods.

“When we are making new materials, we think about the entire functional lifetime for a device, as well as practical constraints about how it will be deployed,” said Feig. “We try to use very well-established and safe building blocks combined with innovative approaches for engineering the material in order to give familiar materials exceptional or novel properties to open up use cases that haven’t existed before.”

This work is part of their investigations into leveraging glycine’s piezoelectric properties – its ability to produce energy from mechanical stress. Piezoelectric devices are used in household products like smoke alarms and watches, environmental monitors, and medical devices like ultrasound imaging and diagnostic sensors. Scientists already understood that glycine could make an exceptionally good piezoelectric material. But it also has important drawbacks; namely, when it comes in contact with water, it disintegrates within seconds, plus the crystalline form the material naturally prefers is not piezoelectric at all.

“Nature and our bodies can dispose of it. But how do we harness its potential?” said Feig. “We had to solve a lot of different problems.”

Confronting glycine’s challenges

The researchers came up with various solutions for glycine’s faults. First, to keep glycine in a desirable piezoelectric form, Feig and her co-authors created a novel engineering process that applies an electric field as glycine crystallizes under nanoconfinement. This process isolates particles in exceedingly small spaces and, in this case, aligns glycine crystals by polarity, allowing them to keep their piezoelectric properties. They also added molecular stabilizers to the brew to keep glycine from transitioning into a crystal phase that would negate its piezoelectricity, while keeping the material flexible enough for potential use in the human body.

To consider how they might develop the material, the team borrowed engineering strategies from Feig’s past research in delayed drug delivery. Many drugs readily dissolve in water just like glycine, explained Feig, so scientists and pharmaceutical companies encapsulate them in polymers that degrade more slowly.

The degradation of glycine is part of what makes it a desirable material, but only so long as that process is slow and controllable. Using a polymer matrix, the researchers slowed the disintegration of glycine to the point that, after 60 days, their mixture maintained piezoelectric performance. With further testing, the team expects its piezoelectric properties could last even longer.

A ‘huge leap’ forward

The team’s progress on the material’s stability and crystal formation represents “a huge leap,” said Zhuomin Zhang, a postdoc in Feig’s lab and the first author of the paper.

“When I think about what we achieved, I feel very excited because we addressed one of the biggest challenges limiting the use of glycine-based piezoelectric materials,” said Zhang. “This work moves the field one step closer toward practical biomedical applications, where long-term stability in physiological environments is essential.”

Next, the team hopes to test the material’s stability in more extreme environments: at human body temperature, for example, and in different pH environments representing conditions like the digestive tract. The researchers are exploring collaborations with the School of Medicine, where researchers are interested in biodegradable sensors that could replace current monitoring devices used after surgical procedures or for implants. They will also continue tinkering to increase stability and piezoelectricity, pushing the material’s capabilities further.

“Now we have a really good platform to be able to adjust and re-optimize for these different contexts,” said Feig.