The 2021 Schoenborn Graduate Research Symposium was Outstanding!

The Schoenborn Graduate Research Symposium is a showcase for the talent and accomplishments of our graduate students, featuring formal talks and poster presentations.

The Symposium is divided into two sections. During the first section, senior-level PhD candidates make formal presentations about their research and the associated results. During the second section, mid-level PhD students make informal poster presentations about their research. Faculty judges select first-, second-, and third-place winners of the formal presentations and graduate student voters select the poster presentation winners.

The Symposium program also includes presentations of the Vivian T. Stannett Fellow Award and the Praxair Exceptional Teaching Assistant Award.

The 2021 winners are:

Formal Presentations

First Place – Rachel Nye (Parsons)

Background: Atomic and molecular layer deposition are layer-by-layer thin film synthesis techniques used prevalently in microelectronics and energy storage due to their sub-nanometer thickness and conformality control. Recent research has focused on exploiting chemical differences on the starting surface to selectively deposit material on one desired region without affecting an adjacent region. This area-selective deposition (ASD) enables a bottom-up, self-aligning nanopattern, where higher selectivity indicates a better quality and higher resolution pattern. High selectivity processes are essential for reducing the burden on expensive and complex electronic manufacturing steps, such as lithography. [1] Consider titanium oxide (TiO2), which is useful in lithography and solar cells due to its high etch resistance and refractive index. [2] While several nanometers of selective TiO2 patterning is possible, further improvement towards commercial requirements is hindered by incomplete understanding on how pattern loss occurs. Herein we demonstrate how deepening this understanding enables improved TiO2 selectivity.

Results: Baseline selectivity is determined by comparing TiO2 growth on silicon oxide (SiO2) with and without a chemical passivation layer. In this standard process, the desired growth region (non-passivated SiO2) has 3.8 nm TiO2 while the non-desired growth region (passivated SiO2) is limited to <0.2 nm, corresponding to 90% selectivity. Undesired TiO2 particles on the passivated surface were analyzed with scanning electron microscopy (SEM) to gain insight on how and when these particles formed, e.g. via defect sites, adsorption, or diffusion of TiOx species. Particle size distributions reveal a broad range of particle sizes and a high concentration of small particles even after depositing relatively thick films, when larger particles would be expected. This indicates that undesired particle nucleation sites are generated during processing, which is further supported by kinetic modeling. To reduce the impact of nucleation site generation, we develop a defect mitigation strategy to periodically etch small particles and then re-passivate the surface. The resultant three-step [passivation + deposition + etch] supercycle process is evaluated on 90 nm wide 3D patterns of passivated pillars (no growth desired) and non-passivated trenches (growth is desired). This supercycle process results in significant reduction of undesired TiO2 content and pattern roughness on the passivated pillars, while still enabling substantial (7 nm) TiO2 deposition on the desired growth surface. These results correspond to a ~200% increase in selectivity compared to the baseline process.

Conclusions: We leverage insight from mechanistic analysis on TiO2 patterning to develop a strategy that significantly improves selectivity. This process is promising for enhancing lithography and other nanoelectronic material applications. Overall, this work demonstrates the importance of mechanistic understanding towards advancing thin film applications.

References:

  1. Parsons, Gregory N., and Rob D. Clark (2020). Area-Selective Deposition: Fundamentals, Applications, and Future Outlook. Chem. Mater. 32: 4920–4953.
  2. Soethoudt, Job, et. al. (2020). Insight into Selective Surface Reactions of Dimethylamino-trimethylsilane for Area-Selective Deposition of Metal, Nitride, and Oxide. J. Phys. Chem. C. 124: 7163–7173.
Second Place – Adam Bachmann (Dickey)

Background: Origami, the Japanese art of paper folding, allows complex 3D systems to be produced from flat starting materials, such as robots [1]. Origami folding allows for compact storage [2] and can endow new functionality such as reconfigurability [3] or improved mechanical properties [4]. Despite these advantages, origami 3D electronics remain sparse owing to the manual folding steps required [5]. Self-folding methods have been developed [1] but these methods rely on responsive polymers that are poor electrical conductors. Metal self-folding techniques have been developed [6] but only laser forming works in ambient conditions at multiple length scales. Laser forming is the process of using photothermally produced stresses to generate plastic deformations in a substrate. To date, laser forming has not been demonstrated on metal/polymer bilayers such as flex PCBs.

Results: Successful laser cutting and self-folding of flexible printed circuit boards (PCBs) was demonstrated and the folding process was characterized. The high reflectivity of copper (~99%) was modulated by two methods: using the laser to first drive surface oxidation or using a nickel/gold coating as an absorbing layer. This allows low-power settings to be used when folding and prevents cutting through the thin copper traces. Further testing revealed the forces generated during laser forming are enough to lift surface mount electronic devices. A common timing circuit was then fabricated, populated with surface mount devices, cut, and laser self-folding while maintaining electrical connectivity.

Conclusions: Flex PCBs can be cut and self-folded using a single, low-cost (~$10k) laser system. Lasers are already used in PCB manufacturing and this work demonstrates another potential function—self-folding. Thus, this study paves the way for rapid prototyping of 3D electronics.

References:

  1. Felton, Samuel, Michael Tolley, Erik Demaine, Daniela Rus, and Robert Wood (2014). A method for building self-folding machines, Science, 345: 644-646.
  2. Zirbel, Shannon A., Robert J. Lang, Mark W. Thomson, Deborah A. Sigel, Phillip E. Walkemeyer, Brian P. Trease, Spencer P. Magleby, and Larry L. Howell (2013). Accommodating Thickness in Origami-Based Deployable Arrays. J. Mech. Des., 135: 111005.
  3. Liu, Xueli, Sun Yao, Benjamin S. Cook, Manos M. Tentzeris, and Stravos V. Georgakopoulos (2015). An Origami Reconfigurable Axial-Mode Bifilar Helical Antenna, IEEE Trans. Antennas Propag., 63: 5897-5903.
  4. Hubbard, Amber M., Jay Kalpesh Patel, Sigurd Wagner, Chih-Hao Chang, Jan Genzer, and Michael D. Dickey (2021). Stiff or Extensible in Seconds: Light-Induced Corregations in Thin Polymer Sheets, Adv. Mater. Technol., 6: 2000789.
  5. Sterman, Yoav, Erik D. Demaine, and Neri Oxman (2013). PCB Origami: A Material-Based Design Approach to Computer-Aided Foldable Electronic Devices. J. Mech. Des., 135: 114502
  6. Lazarus, Nathan, Gabriel L. Smith, and Michael D. Dickey (2019). Self-Folding Metal Origami, Adv. Intell. Syst., 1: 1900059
Third Place (Tie) – James Crosby (Kelly)

Background: Commercialization of lignocellulosic biofuels and chemicals has been limited by the inability of current industrial microbial hosts to access and utilize all fermentable sugars without expensive pretreatment. Although conversion of the cellulose portion of plant biomass by pretreatment and engineering has been successful in many hosts, the pentose-rich and often cross-linked hemicellulose fraction remains both a barrier to cellulose access and the major portion of unconverted carbohydrates. The extremely thermophilic bacterium Caldicellulosiruptor bescii (Topt = 78 °C) can potentially overcome these barriers since it is capable of deconstructing lignocellulose without pretreatment and simultaneously fermenting hexose and pentose sugars [1]. While the cellulolytic capabilities have been thoroughly examined, few studies have comprehensively investigate hemicellulose deconstruction and metabolism in C. bescii.

Results: A comprehensive analysis of hemicellulose deconstruction and metabolism was conducted for C. bescii. Genomic and regulatory reconstruction of C. bescii reveals 65 genes responsible for hemicellulose metabolism, controlled by 9 separate regulators. A single transcriptional regulator, XynR, controls 28 of these genes 11 of which are hemicellulolytic glycoside hydrolases (GHs). To assess the role of the GHs in xylan metabolism, full length proteins were recombinantly expressed and biochemically characterized. Of the 11 GHs, 5 were found to have endo-β-xylanase activity, although branched substrates were more difficult to hydrolyse. The remaining 6 enzymes are debranching enzymes or exo-β-xylanases, which improve access and activity of the endo-xylanases. However, only one of the debranching enzymes is predicted to be extracellular, suggesting that C. bescii primarily uptakes oligosaccharides and hemicellulose crosslinking may remain a barrier for C. bescii. To supplement the biochemical activity, continuous cultures and transcriptomics were employed to study xylose metabolism in C. bescii. At a growth rate of 0.2 hr-1, xylose consumption was approximately 1.5-fold lower than glucose consumption, despite similar growth and fermentation yields. This suggests that xylose may be used in an assimilatory role, moreso than glucose. Transcriptional comparisons on glucose versus xylose show differential regulation of only 22 genes, which includes monosaccharide transporters and xylose isomerase, the first step in xylose metabolism. Interestingly, the rate limiting step of xylose metabolism, xylulose kinase, is not upregulated on xylose, but is up-regulated on xylooligosaccharides. Overall, this suggests that C. bescii maintains a broad inventory of enzymatic activity and tightly controls pentose metabolism.

Conclusions: The combination of enzymatic studies and continuous culture elucidate the mechanisms for hemicellulose hydrolysis and conversion in C. bescii. Improving hemicellulose decrosslinking and sugar turnover rate are two ongoing efforts to modify xylose metabolic regulation to further develop C. bescii as a biofuel platform organism.

References:

  1. Crosby, J.R., L. T. Laemthong, A.M. Lewis, C.T. Straub, M.W.W. Adams, and R.M. Kelly. Extreme thermophiles as emerging metabolic engineering platforms. 2019. Curr. Opin. Biotechnol. 59:55-64.
  2. Rodionov, D.A., I.A. Rodionova, V.A. Rodionov, A.A. Arzamasov, K. Zhang, G.M. Rubinstein,T.N.N. Tanwee, R.G. Bing, J.R. Crosby, I. Nookaew, M. Basen, S.D. Brown, C.M. Wilson, D.M. Klingeman, F.L. Poole, II, Y.Zhang, R.M. Kelly, and M.W. W. Adams. 2021. Transcriptional regulation of plant biomass degradation and carbohydrate utilization genes in the extreme thermophile Caldicellulosiruptor bescii. mSystems (in press)
Third Place (Tie) – Victoria Karakis (Rao and San Miguel)

Background: Many pregnancy complications are often the result of improper placental development. Specifically, impaired differentiation of trophoblast cells (TBs) of the placenta can result in complications like preeclampsia, miscarriage, and placenta accreta [1]. Due to a lack of appropriate models to study placental development, mechanisms involved in trophoblast differentiation remain poorly understood. Current in vitro trophoblast models exploit artificial signaling activation or inhibition to induce differentiation which prevent mechanistic studies and insights into extracellular cues that regulate trophoblast differentiation [2]. Here, we develop novel, chemically defined in vitro culture systems for the maintenance of human trophoblast stem cells (hTSCs) and for terminal trophoblast differentiation to extravillous trophoblast (EVT) and syncytiotrophoblast (STB). We then use these models to uncover mechanisms involved in trophoblast development.

Results: hTSCs derived from primary placenta or from human pluripotent stem cells (hPSCs) were cultured in the previously published trophoblast stem cell medium (TSCM) [1,2]. Upon passaging cells into a defined trophoblast differentiation medium (TDM), multinucleate STB formed that gained expression of appropriate STB markers and lost expression of TB markers. When TDM was supplemented with laminin-1, however, EVTs formed that expressed appropriate EVT markers and were able to spontaneously transition from an epithelial to mesenchymal cell type, similar to EVTs in vivo. Interestingly, when we differentiate to STB and EVT in the presence of the protein kinase C inhibitor, STB form but EVT differentiation is impaired. hTSCs cultured in TSCM were compared to hTSCs cultured in a chemically defined trophoblast stem cell medium (DTM). hTSCs cultured in DTM could be maintained for multiple passages and expressed similar TB markers as hTSCs cultured in TSCM. Interestingly, however, hTSCs cultured in DTM gained expression of a more primitive TB marker, CDX2. These cells also retained the ability to differentiate to EVT and STB under similar defined differentiation conditions.

Conclusions: We have developed a chemically defined trophoblast differentiation medium (TDM) where the addition of a single factor, laminin-1, switches the terminal trophoblast differentiation fate from STB to EVT. Using this model, we were able to determine that PKC signaling plays an important role in the formation of EVT. We have also developed a chemically defined trophoblast stem cell medium (DTM) where cells gain expression of a more primitive TB marker. This signifies that TBs of a later developmental stage can revert to an earlier developmental stage cell type.

References:

  1. Mischler, A., Karakis, V., Mahinthakumar, J., Carberry, C., San Miguel, A., Rager, J., … & Rao, B. M. (2021). Two distinct trophectoderm lineage stem cells from human pluripotent stem cells. J. of Biol. Chem., 296.
  2. Okae, H., Toh, H., Sato, T., Hiura, H., Takahashi, S., Shirane, K., … & Arima, T. (2018). Derivation of human trophoblast stem cells. Cell stem cell, 22(1), 50-63.

Poster Presentations

First Place – Mohammad Shamsi (Dickey and Spontak)

Background: Due to their toughness and extensibility, thermoplastic elastomers (TPEs) are beneficial for a number of applications such as medical devices, automotive parts, and consumer goods. Poly[styrene-b-(ethylene-co-butylene)-b-styrene (SEBS) is a TPE consisting of rigid S endblocks and a soft EB midblock. Because of the thermodynamic incompatibility between the covalently-linked blocks, these copolymers are capable of spontaneously self-organizing into a variety of nanoscale morphologies such as spheres, cylinders and lamellae, as well as more complex ones, that regulate copolymer properties [1]. One approach by which to control the mechanical properties of TPEs is through the physical incorporation of a low-volatility midblock-selective oil. Such materials, collectively known as TPE gels (TPEGs), display highly adjustable properties that make them attractive in a plethora of applications. Most of the TPEs employed for this purpose derive from styrenic TPEs with a polyolefin midblock that can be selectively swollen with an aliphatic oil. In most instances, TPEGs are processed as cast films or extrudates [2]. Here, we consider the preparation of highly tunable TPEG microfibers via electrospinning. Electrospun nano/microfibers can be used to fabricate nonwovens and are appealing because they can be easily generated at ambient temperature. As highly porous mats, these materials are suitable for filtration and medical products. In the specific case of electrospinning TPEGs, process parameters and the oil loading level affect morphology and, in turn, the properties of the microfibers. Our goal here is to produce highly stretchable nonwovens from styrenic TPEGs.
Results: A series of commercial SEBS copolymers possessing different molecular weights and S content have been first used to establish their structure-property-processing relationships during electrospinning. Prior to electrospinning TPEGs, we first discern the electrospinning solution concentration at which TPE fibers can be formed. These observations are quantitatively related to the critical chain overlap concentration, c*, which was experimentally ascertained for each copolymer examined here. A plot of specific viscosity versus concentration (c) normalized relative to c* clearly differentiates three solution regimes: dilute (c/c*<1), semidilute unentangled (1<c/c*<3) and semidilute entangled (c/c*>3). Uniform copolymer fibers are acquired at c/c*~4 for all the copolymers varying in molecular weight. The effect of chain length on electrospinning is effectively eliminated once the concentration is normalized with respect to c*, and the fiber diameter is found to universally scale as (c/c*) 2.5.

Conclusions: This work is anticipated to enable fabrication of highly stretchable nonwovens that can improve comfort in current products and introduce potentially new applications in the not-too-distant future.

References:

  1. Matsen, M. W. Effect of Architecture on the Phase Behavior of AB-Type Block Copolymer Melts. Macromolecules (2012), 45, 2161–2165.
  2. Armstrong DP, Mineart KP, Lee B, Spontak RJ. Olefinic Thermoplastic Elastomer Gels: Combining Polymer Crystallization and Microphase Separation in a Selective Solvent. ACS Macro Lett. (2016) Nov 15;5(11):1273–7.
Second Place – Fazel Bateni (Abolhasani)

Background: Currently, hot-injection colloidal synthetic routes using batch reactors are used to prepare and study metal cation-doped perovskite quantum dots (PQDs). The high-temperature synthetic methods using batch reactors suffer from high overall energy costs, non-uniform heat distribution, and self-annealing of lead (Pb) halide PQDs.[1] Moreover, the manual flask-based colloidal synthesis technique results in high reagent consumption, high waste generation, and batch-to-batch inconsistency of products between syntheses, users, and different reactors.[1,2] In contrast, room-temperature post-synthetic metal cation doping is an effective synthetic strategy which prevents self-annealing phenomena and provides a higher level of process control, and thus facile synthesis of cation-doped Pb halide PQDs. Compared to batch reactors, droplet-based microfluidic synthesis strategies have been considered as reliable and precise tools for accelerated formulation discovery, fundamental studies, and large-scale manufacturing of high-quality colloidal PQDs.[1,2] Specifically, the reduced characteristic length scale of microfluidic reactors enables intensified and tunable mass and heat transfer rates that are challenging or in some cases impossible to achieve in batch reactors.[1]

Results: A modular flow chemistry platform was developed and utilized for accelerated in-flow studies of metal cation doping of a model all-inorganic PQDs, cesium lead chloride (CsPbCl3). The dynamics of doping the Mn2+ ions into the host CsPbCl3 QDs was monitored through a temporal in-situ spectral characterization enabled by a fully automated modular microfluidic platform. The Mn2+ doping level (Mn-to-exciton peak area ratio) and the emission color were tuned in-flow through a continuous flow dilution strategy. Different inflow-adjusted concentrations of Mn precursor justified different doping contents, and therefore provided controlled color tunability. Furthermore, we systematically studied the effect of dopant concentration (i.e., manganese chloride, MnCl2) and ligand composition on the extent and kinetics of the in-flow cation doping process.[3]

Conclusions: The developed modular microfluidic platform provides access to early-stage reaction time of colloidal PQDs to unravel the fundamental mechanism of an ultrafast cation doping process. The results of this study open new opportunities for on-demand continuous nanomanufacturing of high-quality cation-doped Pb halide PQDs for direct applications in next- generation photonic devices.

References:

  1. K. Abdel-Latif, F. Bateni, S. Crouse, M. Abolhasani, “Flow Synthesis of Metal Halide Perovskite Quantum Dots: From Rapid Parameter Space Mapping to AI-Guided Modular Manufacturing”, Matter 2020, 3 (4), 1053– 1086.
  2. Z. S. Campbell, F. Bateni, A. A. Volk, K. Abdel‐Latif, M. Abolhasani, “Microfluidic Synthesis of Semiconductor Materials: Toward Accelerated Materials Development in Flow”, Part. Part. Syst. Charact. 2020, 37
    (12), 2000256.
  3. F. Bateni, R. W. Epps, K. Abdel-latif, R. Dargis, S. Han, A. A. Volk, M. Ramezani, T. Cai, O. Chen, M. Abolhasani, “Ultrafast Cation Doping of Perovskite Quantum Dots in Flow”, Matter. 2021, 4 (7), 2429-2447.
Third Place – Prottasha Sarker (Khan)

Background: Hydrogels are porous three-dimensional structures composed of polymeric cross-linked networks that has the provision for sufficient water and nutrient flow for cell proliferation to stimulate the regeneration of defective tissues. Compared to surgical scaffold implantation, injectable hydrogels can be easily applied by minimal invasive techniques to form a self-standing hydrogel. Naturally derived polymer, collagen, has been widely employed as injectable hydrogel since it inherits the structural and functional cursors to accelerate tissue formation, however it shows poor rheological properties. The use of biodegradable tannic acid particles provides a useful approach to improve the rheology of these systems while its inherent antibacterial and anticarcinogenic nature adds to gel functionality. Polyphenolic tannic acid particles could potentially interact with collagen through their hydroxyl and carboxyl groups allowing us to modulate the rheology.
Results: In this study, we examined how liquid as well as particulate tannic acids impact the rheological properties of collagen-based hydrogels. Liquid and tannic acid particles of different shapes are synthesized and incorporated into collagen in this regard. While samples exhibit solution-like features at storage conditions (4°C), they transform into gels under physiological temperatures (37°C). The sol-gel transition in terms of temperature and time is monitored as a function of tannic acid concentration and morphology. Interestingly, particulate tannic acid incorporation exhibits the largest increase in elastic modulus with shape playing a secondary factor. Measurement of yield stress using the elastic stress method also reveals a similar trend. These results are interpreted in terms of the underlying interactions of tannic acid with collagen, as examined using other analytic approaches.
Conclusions: The use of morphology-controlled tannic acid particles provides a facile approach to control the rheological characteristics of collagen hydrogel. In particular, changes in microstructure and cross-linking kinetics leads to hydrogels with enhanced modulus and yield stress while still preserving injectability. Results from this work could lead to the development of a new functional hybrid hydrogel system for use as injectables in tissue regeneration technology.

View the complete Symposium program here

Dr. Ahmed Abdala (Ph.D., 2003), Professr of Chemical Engineering at Texas A&M University, Qatar delivered the Keynote talk, “Development and Applications of Graphene-based Nanocomposites” virtually from Qatar.

Ms. Begum Yagci won the Linde Exceptional Teaching Assistant Award.

The Vivian T. Stannett Graduate Award for Outstanding Early Publication is named in honor of Professor Vivian T. Stannett (1917-2002), who was a CBE faculty member and an internationally renowned polymer scientist, research leader and member of the National Academy of Engineering. Doctoral students in their first through fourth year of study with a first-authored, peer-reviewed publication based upon work done at NC State are eligible to compete for the Award.

The 2020-21 Stannett Award winners are:

First-place: Mr. Kevin Lin (Keung) for his paper “Dynamic and scalable DNA-based information storage,” Nature Communications, 11, 2981 (2020).

Honorable Mention: Mr. Zachary Campbell (Abolhasani) for his paper “Continuous Synthesis of Monodisperse Yolk-Shell Titania Microspheres,” Chemistry of Materials, 30 (24), 8948 (2018).

Congratulations to the Schoenborn competition participants and winners. Well done to all!

We also gratefully acknowledge a gift from Linde plc for logistical support of the Symposium and from Dr. Russ and Susie O’Dell for establishing the Edward M. Schoenborn Graduate Student Fund endowment, which funds cash prizes for the Symposium’s oral and poster award winners.

Professor Saad Khan Student explaining poster Linde Excellence Award winner
Dr. Russ O'Dell Student explaining poster Audience viewing presentation 2