In the near future, a trip to the doctor’s office might include a nurse injecting a small sensor into your arm to monitor cardiac function or implanting an electrical device to speed up bone healing. These devices will be piezoelectric—or capable of producing electrical signals from mechanical stress produced by muscle stretching, breathing, blood flow and small body movements. With no batteries required, they will simply dissolve in the body when their mission is complete.
The potential benefits of these transient implantable electromechanical devices are enormous. However, there are several big sticking points: Currently, the vast majority of piezoelectric materials are rigid, brittle and some of them even contain toxic materials, making them unsuitable for implantation in the human body.
There are many natural materials that display piezoelectric properties, including silk, collagen, bone, cellulose and amino acids, but manipulating those large biomolecules at scale with the aligned orientation needed to function correctly has proved difficult.
Engineers at the University of Wisconsin-Madison, however, have developed a new technique for large-scale synthesis of the promising biodegradable and bioresorbable piezoelectric amino acid gamma glycine. The research appears in the July 16, 2021, issue of the journal Science.
Xudong Wang, a professor of materials science and engineering at UW-Madison, and his students including study co-first authors Fan Yang and Jun Li, have worked for a long time developing high-performance piezoelectric materials with a focus on flexibility and biocompatibility.
“The challenge we overcame in this work was showing a technique that enables glycine molecules to self-assemble over a very large area, on the wafer scale film structure, with the same orientation,” says Wang. “We’re able to show uniformly high piezoelectric response across the entire film.”
To assemble the heterostructural film, Wang and his team mixed the glycine with polyvinyl alcohol using water as a solvent. Since the polyvinyl alcohol has a weaker interaction with water molecules than the glycine, it precipitates out of the water more quickly, forming a sandwich with the crystallized glycine in the middle. The polyvinyl alcohol, which selectively bonds with the hydroxy groups on the glycine, spontaneously oriented the glycine molecules into a well-aligned crystal sheet when it precipitated to the surface. The result is a stable, uniform piezoelectric biofilm that is both flexible and bioresorbable.
The team implanted samples of the film in rats and showed that the material was completely absorbed after one day without inducing any inflammation or immune response. They also demonstrated that the material was able to produce voltages similar to other flexible piezoelectric materials when it was implanted under the skin in the thigh and chest area of the animals.
Glycine is the simplest amino acid and provides the best material platform for researchers to understand the crystallization and alignment of polarized biomolecules. Wang believes this synthesis knowledge could lead to many other useful biomaterials. “Hopefully, this discovery will lead to a scalable manufacturing process for making piezoelectric films from a range of sustainable biomaterials,” he says.
Many researchers will be interested in applying the new technique to produce films for biomedical projects, including Wang. In recent years, he’s developed designs for implantable devices that help with weight-loss, monitoring damaged arteries and accelerating bone healing, among other projects that could benefit from large-scale synthesis of these piezoelectric films.
Xudong Wang is a Grainger Institute for Engineering professor and Energy & Sustainability Thrust Lead for the Grainger Institute for Engineering at UW-Madison. Other UW-Madison researchers include Yin Long, Ziyi Zhang, Linfeng Wang, Jiajie Sui, Yutao Dong, Yizhan Wang of the Department of Materials Science and Engineering; Rachel Taylor and Timothy Hacker of the Cardiovascular Research Center; and Dalong Ni and Weibo Cai of the Department of Radiology and Medical Physics.
Ping Wang of Southwest Jiaotong University and the Key Laboratory of High-speed Railway Engineering in Chengdu, China also contributed.
The authors acknowledge support from the National Institutes of Health under Award Numbers R01EB021336, R21EB027857 and P30CA014520.
Author: Jason Daley