That handful of potting soil may not look like much when you tamp it down around the tomato plant: clay, bark, perhaps recycled cardboard. It's the characteristics of soil—to retain water, to provide space for a plant's roots to obtain oxygen—that make it important. These characteristics affect not just plants but the microbes that live in soil, like those that break down nitrogen.
More space means room for more microbes, more soil components mean more chemical compounds for microbes to interact with. Changing these properties will impact the soil's microbial population. So, another way to think about soil's characteristics is as a means for intentionally influencing microbe behavior. This could benefit us in areas other than growing tomatoes, such as medicine and the creation of alternative fuels.
To assess this benefit, a team comprised of members from Chicago-area institutions has synthesized a soil-inspired material and quantified its features. They demonstrate that the new material could be advantageous to humans in a wide range of applications, from write-and-erase conductivity to encouraging therapeutic microbial behavior in cases of rodent digestive diseases.
To influence microbes as soil does, the new material would need to respond to physical forces, like compression, and be chemically diverse, to allow microbes to interact with it. The material would also need scaffolding to support open spaces (porosity) to promote microbe populations. To meet these requirements, the team chose a montmorillonite nanoclay as scaffolding for the new material. The team added granules of starch which, when subjected to heat, transform into spacers within the clay, creating pores, and acting as a scaffolding stabilizer. The team also incorporated a mixture of the metals gallium and indium; this mixture is liquid at room temperature and increases the material's chemical diversity.
To determine whether the material had sufficient porosity and chemical diversity, the team used multiple types of imaging to characterize it. They made a three-dimensional model using X-ray fluorescence imaging and X-ray ptychography on the 9-ID-B beamline at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory.
The model showed a scaffolding of nanoclay with numerous blank regions that suggest open space and the accumulation of the liquid-metal mixture on the interior surfaces of those open spaces. The team also applied laser writing to some samples to locally modify their electrical conductivity. The team used APS/CNM beamline 26-ID-C to take X-ray absorption near-edge structure data on the laser-written samples, which confirmed the presence of single metal atoms on the laser-written surfaces.
Next, the team investigated the benefits of the soil-inspired material for biological applications. The team chose Bacillus subtilis for testing because this soil microbe has applications relating to biofilms, biofuel and gut health. The team found that biofilms of Bacillus subtilis grow to a larger surface area on the laser-written samples of new material than samples without laser-writing. They concluded that this improvement occurs because of the availability of single metal atoms for chemical reactions produced when the soil-inspired material was laser-written. Additionally, cultures of Bacillus subtilis grew larger on the soil-inspired material than on the sheets of carbon microfibers (used in fuel cell applications) or hydrogels (used in medical applications).
The team also used the new material to influence microbes in situations relating to gut health. After confirming the material was benign to test mice, the team orally administered it to mice suffering from two different digestive diseases: tetracycline-induced dysbiosis (where use of a common antibiotic causes digestive side-effects) and dextran sulfate sodium-induced ulcerative colitis. In the tetracycline test, the mice who ingested the soil-inspired material showed more beneficial microbes than the control mice. In the colitis test, the mice who ingested the soil-inspired material showed less severe effects of the disease.
As surprising as it may be to find inspiration in a handful of soil, the team's results demonstrate potential for improvements ranging from gut health to patternable circuits, underscoring what we gain from paying close attention to the natural world.
– Mary Agner
See: Y. Lin1, X. Gao1, J. Yue1, Y. Fang1, J. Shi1, L. Meng1, C. Clayton1, X-X Zhang1, F. Shi2, J. Deng3, S. Chen3, Y. Jiang3, F. Marin3, J. Hu4, H-M Tsai1, Q. Tu4, E. Roth4, R. Bleher4, X. Chen4, P. Griffin1, Z. Cai3, A. Prominski1, T. W. Odom4, B. Tian1, “A Soil-Inspired Dynamically Responsive Chemical System for Microbial Modulation,” Nature Chemistry 15, 119–128 (October 24, 2022)
Author affiliations: 1University of Chicago; 2University of Illinois Chicago; 3Argonne National Laboratory; 4Northwestern University
This work was supported by the US Office of Naval Research (N000141612958), the National Science Foundation (NSF CMMI-1848613) and a Zhong Ziyi Educational Foundation Award. This work was partially supported by the University of Chicago Materials Research Science and Engineering Center, which is funded by the National Science Foundation under award no. DMR-2011854. This work used instruments in the Electron Microscopy Service (Research Resources Center, UIC). This work made use of the BioCryo facility of Northwestern University’s NUANCE Center, which has received support from the SHyNE Resource (NSF ECCS-2025633), the IIN and Northwestern’s MRSEC programme (NSF DMR-1720139). Use of the Advanced Photon Source and the Center for Nanoscale Materials, both US Department of Energy Office of Science User Facilities, was supported by the US Department of Energy, Office of Science, under contract no. DE-AC02-06CH11357. The BNP was obtained through an NIH ARRA S10 grant no. SP0007167, and S.C. also acknowledges the support of DOE grant no. PRJ1009594.
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