Skin-inspired materials that bend and flex have myriad potential applications for medical devices and consumer electronics. But the ability of these materials to degrade has not been paired with good electronic performance—until now. For the first time, scientists have engineered a stretchable, semiconducting material that is easily degraded in weak acid (ACS Cent. Sci. 2019, DOI: 10.1021/acscentsci.9b00850).
The new semiconductor was able to maintain its electronic performance even under 100% strain—the first time this has been demonstrated for a degradable semiconductor material. This material could be implemented in wearable electronics, medical implants, and environmental monitoring, says Stanford University material chemist Zhenan Bao, who co-led the new study with Stanford chemist Helen Tran.
Trying to optimize stretchiness, conductivity, and degradability in the same material is a massive challenge, Tran explains, because the traits needed to optimize these functionalities are at odds with one another.
Like many scientists before them, the group used a combination of two polymers to achieve the right combination of material properties: a super-stretchy elastomer and a semiconducting polymer—but not a typical one. Most semiconducting polymers have difficult-to-cleave carbon-carbon bonds. In order to enhance the degradability of their polymer, the researchers instead used an imine bond to attach the sidechains of the semiconducting material. The imine bond is more readily hydrolyzed in aqueous acidic conditions. By engineering the sidechains on the conducting polymer, the group was able to achieve the right mix of electronic mobility and stretchiness. The mobility of the semiconductor is on par with or surpasses that of other organic-based semiconductors, Bao says. This means the material might one day be used to make a stretchy pacemaker that could be wrapped around a heart, then break down in the body without need for a secondary surgery. Furthermore, the group’s work is the first to show that a degradable device can perform under strain.
When mixed together and spin-coated, the two polymers self-assemble into a sandwich-like formulation: an elastomer-rich region in between two regions of high density semiconducting polymer strands. The group showed that both polymers broke down in weak acid. The products of this degradation were not toxic to in vitro cultures of human cells. Further tuning of the molecular design of each polymer could optimize the speed of the device’s degradation based on the specific needs of each use, Bao says. In the future, this functionality may even allow scientists to recover valuable precursor molecules.
Other researchers have designed semiconductors with elastomers that can break down, but the Stanford group’s device is the first designed to be fully degradable. It’s early days for the work, Tran says, but she hopes the team’s approach to the problem “shifts how we think about designing stretchable materials.” Tran and the group are beginning to collaborate with surgeons and other medical professionals to understand the best applications for their semiconductor.
John Rogers, a materials scientist at Northwestern University, says that the study is a “really nice bit of polymer science” with “compelling application opportunities.” Although he thinks the current semiconductor’s electronic performance needs improvement before it’s suitable for most applications, he is excited by the framework it establishes for future designs.
Texas A&M University chemical engineer Jodie Lutkenhaus agrees that the work is an exciting advance for the field. She’s interested to see what sort of applications the group can develop for their technology. “There’s a whole playground out there for them to explore,” Lutkenhaus says.