(4ej) Macromolecule-Mediated Ion Transport for Advanced Materials

I am currently a postdoc in Joanna Aizenberg’s group at Harvard. I obtained my Ph.D. in Chemical Engineering from the University of Michigan in Michael Mayer’s group, with whom I moved to the Adolphe Merkle Institute in Switzerland during my studies. My doctoral work focused on engineering the permeability of lipid and polymer membranes in various functional systems. This culminated in the design of a hydrogel-based electric power source inspired by the specialized organs of the electric eel, which was published in Nature, attracted the attention of media outlets such as The Atlantic, and served as a foundational entry into the then-nascent field of hydrogel ionotronics. We have recently published a follow-up in Advanced Materials in which we used paper as a scaffold for hydrogel synthesis, stabilizing a scheme with sufficient output to power small devices. My postdoctoral research has focused on patterning fast, exothermic crystal growth within hydrogels; we have recently submitted a manuscript for publication that shows how the resulting dynamic heat maps can be used to actuate downstream thermoresponsive processes. While in the Aizenberg group, I have additionally gained experience with using polymer-conjugated photoswitches to impart light-responsivity to polymer systems.

In sum, my core expertise is using polymers to engineer energy transduction processes within materials, and I frequently look to living organisms for inspiration while doing so. Additionally, both my graduate and postdoctoral research experiences have been partially funded by personal fellowships I applied for and received.

Research Interests:

The initial projects in my group will fall under two broad themes, macromolecule-mediated crystal engineering and autonomous ionotronics, which leverage my expertise. We will attempt to both answer fundamental scientific questions and develop practical applications of our knowledge in response to grand challenges in sustainability, materials fabrication, and biology.

Macromolecule-mediated crystal engineering: Crystals are ubiquitous in both natural (e.g. ice, minerals, bone) and engineered materials (e.g. silicon wafers, quantum dots, polyethylene), featuring Ångstrom-scale order that propagates over lengths ranging from the nanoscale to the macroscale. Crystallinity imparts materials with unique optical, mechanical, and electrical properties that are often highly desirable in engineered applications. Crystal formation is also often undesired, as in the case of atherosclerotic plaques, petroleum solidification, and ice formation in many contexts. Accordingly, many industries deploy additives in order to encourage, mitigate, or shape crystal growth.

My group will use polymers that undergo large chemical and morphological changes in response to externally-applied stimuli in order to exert an unprecedented level of control over crystallization processes. We will apply these polymers to crystal formation across a range of length and time scales, from the fast, exothermic solidification of supercooled “phase change materials” for thermal energy storage to the slower growth of shaped crystalline and composite materials for structural and optical applications. Over the course of our efforts, we will seek to answer fundamental questions that linger over the mechanisms of crystal formation in the presence of polymers and porous media. Eventually, I hope to apply the knowledge we gain during this project to model biomineralization in systems such as atherosclerotic plaques and bones, which involve macromolecule-templated crystallization processes in vivo. Given the ubiquity of both crystalline materials and additives in industrial crystallization processes, I anticipate that this project will likely be a fruitful source of intellectual property.

Autonomous ionotronics: Ionotronic systems are electrical circuits that use ions instead of or in addition to electrons as charge carriers. The potential of such systems to perform complex and useful operations is demonstrated superbly by excitable tissues in higher organisms, which include neurons, muscles, and the specialized organs of electric fish – the human brain is a three-pound aqueous supercomputer whose signals are made of salt. The past five years have seen significant advances in the development of synthetic ionotronic equivalents to traditional circuit elements, including cables, transistors, power sources (my own work), sensors, and actuators. These devices cannot compete with metal- and semiconductor-based circuitry based on performance alone, but they possess two key advantages: First, they can be made of soft, aqueous, transparent, benign materials well-suited for deployment in (for example) biomedical contexts. Second, as ionotronic circuits are typically made of liquid and/or gel media, they can leverage solution-phase chemistry.

I am interested in developing ionotronic systems that couple sensing to actuation in order to create autonomous units capable of functioning without external connections. A barrier to this is the fact that current ionotronic actuators are mostly based on dielectric elastomers and require operating potentials in the kilovolt range, while voltages produced by ionotronic circuit elements are generally significantly more modest. Accordingly, my group will work to develop electrochemical “actuators” that can be incorporated into ionotronic systems to produce chemical effects on an environment using sub-volt potentials. The resulting systems may enable the development of nonlinear behaviors such as feedback and logic in fully soft, polymer-based materials.

Teaching Interests:

I am well-equipped to teach core chemical engineering coursework to undergraduate and graduate students, including heat and mass transfer, thermodynamics, fluid mechanics, and research methods. I would additionally be excited to teach more specialized graduate classes such as polymers, nanotechnology, biomaterials, and imaging. Further, I have two ideas for new courses based on my research interests. The first would be a design course focused on bioinspired materials in which students would learn to navigate the biology literature, review case studies of bioinspired technology, and propose solutions to real engineering problems based on evolutionary adaptations in living organisms. The second would deal with transport in excitable tissues and would involve studying the action of neurons and other electrically active cell types as electrochemical engineering problems, additionally offering a historical perspective on the discoveries in this field.

I have recently taken several pedagogical classes, where I learned about a growing consensus that teaching with active learning techniques leads to better and more equitable outcomes than traditional lectures. I am excited to move beyond the traditional “sage on a stage” model toward an engaged, supportive classroom by building frequent opportunities for students to immediately reflect on and apply course material, as well as to give and receive feedback so I can course-correct if necessary. Further, I intend to employ techniques such as backward design and scaffolding to make sure that all assessments and classroom activities are well-aligned with the learning goals of the course.

Selected publications:

Schroeder, T. B. H.; Aizenberg, J. “Patterned crystal growth and heat wave generation in hydrogels.” Submitted to Nature Materials.

Schroeder, T. B. H.; Guha, A.; Lamoureux, A.; VanRenterghem, G.; Sept, D.; Shtein, M.; Yang, J.; Mayer, M. “An electric-eel-inspired soft power source from stacked hydrogels.” Nature 2017, 552 (7684), 214–218. https://doi.org/10.1038/nature24670.

Guha, A.; Kalkus, T. J.; Schroeder, T. B. H.; Willis, O. G.; Rader, C.; Ianiro, A.; Mayer, M. “Powering electronic devices from salt gradients in AA battery-sized stacks of hydrogel-infused paper.” Advanced Materials 2021, 2101757. https://doi.org/10.1002/adma.202101757.

Schroeder, T. B. H.; Houghtaling, J.; Wilts, B. D.; Mayer, M. “It’s not a bug, it’s a feature: functional materials in insects.” Advanced Materials 2018, 30, 1705322. https://doi.org/10.1002/adma.201705322.

Schroeder, T. B. H.; Leriche, G.; Koyanagi, T.; Johnson, M. A.; Haengel, K. N.; Eggenberger, O. M.; Wang, C. L.; Kim, Y. H.; Diraviyam, K.; Sept, D.; Yang, J., Mayer, M. “Effects of lipid tethering in extremophile-inspired membranes on H+/OH− Flux at room temperature. Biophysical Journal 2016, 110 (11), 2430–2440. https://doi.org/10.1016/j.bpj.2016.04.044.