Off-menu materials science
April 27, 2023
Photo: David Sella/MIT Corporate Relations
A formerly self-described dyed-in-the-wool chemist who has gradually transitioned to research that sits at the interface of science and engineering, Associate Professor Robert Macfarlane and his Macfarlane Lab at MIT explore the chemical sciences that impact materials development and real-world applications. Considering his chosen line of research, he says, “I want to understand things from the level of a chemist, using the intuition of bonding and chemical interactions I gained from my chemistry education, and translate that molecular-level understanding into control over material structure across all length scales from micro- to macroscopic.” His work has implications for areas of impact including climate and sustainability, energy, health and medicine, manufacturing technologies, sensing and computing, simulation and data science, transportation, and infrastructure.
According to Macfarlane, one of the great limitations of industrial and applied research is a shortsighted view that equates “material design” with “material selection.” In other words, there is already a well-defined catalog of materials to consider when designing devices or architectures. Macfarlane’s hypothesis: Current devices and applications are stymied by the materials available. So, while many of his colleagues are focused on designing specific applications using just the materials that currently exist, Macfarlane and his lab prioritize making the materials that enable future development of those applications. He’s expanding the catalogue of materials from which both academics and industry can choose, building a new tool set to build better versions of the next solar cells, batteries, drug delivery vehicles, etc.
“One of the driving principles of our work,” he says, “is designing smart materials that can spontaneously organize into more complex, higher-order structures upon introduction of a pre-programmed stimulus.” Broadly speaking, he applies these principles to developing novel ways to assemble nanoparticles that are scalable and compositionally versatile. His materials may look like, behave like, and can be processed like plastics, but they are partially (or in some cases predominantly) composed of metals, ceramics, or semiconductors.
His work with one of these new building blocks, self-assembling nanocomposite tectons (NCTs), put the Macfarlane lab on the map. He points out that while nanoparticle self-assembly is a decades-old concept, the field has persistently struggled to develop scalable, cost-effective methods to implement the innovation. At best, most researchers in the field making scalable materials this way can develop 2D films (i.e., a material that coats a full square centimeter area, but is only a few micrometers thick). Nobody had succeeded in building large structures that were macroscopic in all three dimensions until Macfarlane and his lab stepped in. Their innovation uses more scalable, cost-effective components like synthetic polymers as nanoparticle coatings to drive the particle assembly process. The resulting materials derive their properties from the original nanoparticle, but “sprinkling on these decorative objects,” as Macfarlane explains it, allows the particles to spontaneously organize themselves. The key advances enabled by the polymer coatings they use include greater scalability, but also greater composition versatility and better processability — meaning they can not only make the materials, but also shape them into physical forms that are critical for industrial use.
Rather than reinventing the wheel for every potential device application or material, Macfarlane tunes his NCTs, imbuing them with particular properties — optical, electrical, or mechanical — enabling faster turnaround between envisioning or designing a new structure and beginning the process of fabricating it. As for potential applications, Macfarlane says, “The modular nature of NCTs provides multiple design handles to alter the composition, size, and thermodynamics of assembly to introduce new geometric arrangements and properties of the resulting material. As a result, these structures have potential application in the areas of plasmonics and photonics, heterogeneous catalysis, and energy storage.”
More recently, the Macfarlane group has begun exploring cross-linkable nanoparticles. Otherwise referred to as “the XNP concept,” it has gained significant traction with industry. These XNPs similarly consist of nanoparticles coated with polymers, but with a key addition — the polymers can be chemically cross-linked after they are molded into the appropriate physical form. This cross-linking switches the XNP building blocks from being soft and malleable (i.e., a toothpaste or “Silly Putty”-like consistency) to being rigid, like a traditional plastic. While such materials are commonplace in polymer development, the Macfarlane lab’s XNPs are able to make such materials while still remaining as much as 85 percent-by-weight (wt%) nanoparticle content. For comparison, similar materials typically have about 1-10 wt% nanoparticle.
This new XNP-enabled composition space enables combinations of properties that are otherwise nearly impossible to access. The work borrows similar ideas from NCTs in that XNPs are also nanoparticles coated in polymers, but applies to a wider range of materials and pushes the bar for scalability even higher as the specific polymers used are even easier to synthesize. Applications for this material might include protective coating for a battery or a micro electronic device that enables rapid heat dissipation to prevent device burnout. Other potential future applications include low dielectric materials required for 5G and 6G communications, scratch-resistant anti-reflection coatings for lenses and mirrors, or porous materials for gas separation and storage.
“There are a host of different things that we are thinking about in the optical, mechanical, chemical, and thermal spaces,” says Macfarlane. “The XNP concept has become an enabling technology for all sorts of different applications. And we've been talking with multiple industry partners, each of which has their own specific niche. One of the advantages is that the XNP approach enables a plug-and-play concept where we can change out the polymer, change out the particle, or change out the physical form of the object being made, but the XNP concept remains the same.”
Speaking of industry collaboration, Macfarlane notes a recent collaboration with a large adhesives company. “We were able to take some very simple constructs that we had been working with, and by sprinkling in a tiny amount of them to these adhesives, we kept the stickiness of the tape intact and increased the cohesive strength by factor of three. This is a very immediate, obvious real-world impact from something that we might not have even thought of if we hadn't been talking with industry.”
Going forward, Macfarlane says he and his lab intend to develop new materials with an eye toward scalability, sustainability, and versatility — using the templates that they have already developed and expanding them into the most impactful areas of application. “At the Macfarlane Lab, we don't build one-off materials or one-off devices,” he says. “We build platforms that allow a multitude of people to make a variety of applications, devices, and technologies. Industry doesn't always consider the limitations of the current materials-design catalogue. In my lab at MIT, we’re working to provide off-menu options to solve your real-world challenges.”