Nature is full of biological materials with the ability to change properties as needed. The trunks of many trees, for example, stand firm and strong yet can bend in a wind without breaking. Octopuses can change the color and texture of their skin as well as the shape of their bodies. Pine cones can open and shut their scales to time the release of seeds. Natural phenomena like these have inspired Meredith N. Silberstein, Mechanical and Aerospace Engineering, to think outside the box in her quest to create synthetic polymers with targeted functionality.
“A lot of what my group does is centered around a new field called mechanochemistry,” she says. “I think of it as mechanics driving chemistry. The core science idea is that we can break chemical bonds on purpose to change how a material behaves. A lot of mechanochemical concepts are bio-inspired because we’re trying to make materials that do more, adding multifunctionality by embedding it into the chemistry of the polymers.”
Materials That Switch States, Heal Themselves
In one project, Silberstein and Xinyue (Joy) Zhang, PhD ’22 Materials Science and Engineering, are working with the United States Office of Naval Research to develop an antifouling material that can be applied to ship keels. The material would be able to switch between two states: hydrophobic (repelling water) and hydrophilic (mixing with water). Hydrophobic materials are good for forcing organisms such as barnacles and seaweed to release their hold, whereas hydrophilic materials tend to stop organisms from growing in the first place.
“We’re trying to create materials whose surface energy changes as you pull on them, so that they switch between the two states,” Silberstein says. “Other people have done similar concepts with surface texturing, but we are building this into the chemistry of the material itself.”
In another project, the Silberstein lab seeks to strengthen material by increasing its dissipation mechanisms. Breaking chemical bonds lets the energy put into a material disperse instead of being stored, Silberstein explains. An example is the way cars crumple in order to dissipate the force of an impact. But the researchers want to move beyond the dissipation of energy as the end of the line. Instead, they want to create new materials that can reform themselves.
“A lot of mechanochemical concepts are bio-inspired because we’re trying to make materials that do more, adding multifunctionality by embedding it into the chemistry of the polymers.”
“This work is based on charged interactions within polymers—what we call metal coordination bonds,” Silberstein says. “In a lot of the concepts we’re working with, dissipation will happen and then the bonds will reconnect. You can keep pulling and breaking and reconnecting. There are a variety of chemistries you can do this with, but our system is special because of the amount of tailoring we can do on these bonds. We can modulate their environment, and that will change how easily they break and how easily they reform, which is critical if you want to have full control over the mechanical properties of your material.”
Electric Fields to Drive Changes in Mechanical Properties
Working with metal coordination bonds, or charged ions, led the researchers to wonder whether they could use electric fields to modulate the polymers to change their mechanical properties or their ability to heal, Silberstein says. “This is also a very bio-inspired concept,” she points out. “In biology, in human cells, healing and property modulation is based on small electric fields. Neurons, for example, are an ionic charge firing.”
Silberstein and her colleagues wanted to create a material that can dynamically change its properties in response to electric field intensity. Such a material might be useful for many applications, including as a fuel cell membrane, which sits between two rigid substrates (the cathode and anode) and conducts ion transfer. “The membrane is responsible for keeping the fuel cell system together,” Silberstein explains. “If it breaks, then you have short circuiting and fires and other things we don’t want.”
Instead of breaking, the material would change as the electric field changes. “For example, the material could become stiffer so that it can resist whatever is driving the increased electric field,” Silberstein says. “Or it could have the ability to flow more easily so that it can fix itself if it becomes torn. The electric field would actually drive the material’s healing, similar to how cells in our bodies are directed by electric fields to go to the right places to repair an injury.”
Recently, Silberstein looked at how the human body depends mostly on ionic-based transport and wondered if the ionic materials she and her lab were developing for other uses could also have bio-like conductivity. “In the synthetic world there are people who look at electronic-like components based on ionic transport, but they’re all mimicking electronics,” she says. “My idea is to stop doing that and explore, instead, what’s inherently good about stretchable materials that have ionic capabilities.” Such materials could give excellent ability to interact directly with biology—for instance, allowing for direct interfacing with the brain.
Silberstein pitched the idea to the Defense Advanced Research Projects Agency (DARPA) and won the DARPA Young Faculty Award to research bio-inspired circuits. The award comes with $500 thousand for the first two years of research and an option for another $500 thousand in year three.
Modeling the Physics of Materials
Along with developing new materials, the Silberstein lab also creates models—sets of equations that describe the physics of a material. Together with Michael R. Buche, PhD ’21 Theoretical and Applied Mechanics, Silberstein is currently working on a model describing the changes mechanical properties go through as bonds break. The researchers are attempting to create the model both for force-dependent bonds that are irreversible and for those that can be reversed.
Silberstein was drawn to polymers almost from the start, as a freshman undergraduate. “Their properties are defined by their chemical composition and structure, but they’re still very complicated,” she says. “I like the puzzle of connecting how things like time and length scales add up to overall behavior. I also like to look at things that are seemingly different and see their commonalities, like with the DARPA project. But what really drives my research now is being able to use the model we create to change and redesign the material to the properties we want. That’s exciting.”