Jason P. Killgore

Transition metal dichalcogenides like molybdenum disulfide (MoS₂) are compelling for next-generation electronic devices. In this work, we investigate the impact of electronic stress on MoS₂ to illustrate that observational and phenomenological information on multiple devices can be useful to describe changes in the device, and caution against the rationalization of paltry results as representative or correlative to device behavior. Here, we stress MoS₂ by applying a sustained 20 V DC bias to study the material’s response. Post-stress electronic characterization revealed nonuniform shifts in current–voltage (I–V) behavior alongside microscale changes. Complementary mechanical, spectroscopic, and scanning microwave impedance measurements showed that stress-induced features locally modulate stiffness, surface potential, Raman intensity, and charge carrier density. We correlated I–V behavior with morphological features (wrinkles, tears, folds, height) and device-level geometry (MoS₂ overlap with electrodes, channel area, contact length) on 50 test structures across five chips to move beyond anecdotal conclusions. We found no universal correlations before DC stress. However, device-level geometry was correlated with I–V behavior after DC stress, suggesting that electrode contacts play a more dominant role than morphology in determining performance. Delamination and thinning induced by DC stress led to localized reductions in charge carrier density within the affected regions. Further, delamination and thinning appear to map to I–V device performance in a few samples, but the correlation is lost when a larger sample size is considered. This suggests significant sample-to-sample variability in surface electronic states of the test structures. We also discuss how environmental factors introduced during fabrication may contribute to the observed heterogeneous device response. Progress will require high-resolution, multimodal analysis across many samples constructed under controlled, clean conditions. By building data sets that capture variability, we can better identify the true drivers of performance.

Vat photopolymerization is a high‐resolution and high‐throughput technology used in many biomedical applications. However, achieving geometric precision in printed devices with features spanning orders of magnitude in length scale is non‐trivial. Here, a new characterization tool combining fast, high‐resolution optical coherence tomography imaging with a high‐powered digital light processing projector enables real time measurements of photopolymer curing. The direct, quantitative measurement of hydrogel working curves (the relationship between cure depth and light exposure) shows that the critical energy for gelation ( E _c ) exhibits extreme size dependence, demanding a rethinking of gray‐scaling light intensity for achieving predictable voxel formation at high resolutions. This in situ method also enables measurement of size‐dependent working curves and dead zone thicknesses using oxygen permeable window materials, which is impossible via ex situ methods. Generally, size sensitivity is amplified at low irradiance, high dye‐loading, and in the presence of oxygen permeable windows. Despite the extreme size sensitivity, calibrating the light exposure to the size dependent E _c allows a 3x improvement in layer‐growth uniformity compared to a naïve approach. Overall, these results highlight the challenges in high‐resolution printing of hydrogels and provide a framework to measure and account for size dependence.

Suspension bath bioprinting, whereby bioinks are extruded into a yield stress bath with rapid recovery from shearing, has enabled the printing of low viscosity bioinks into constructs with high geometric complexity. Previous studies have often relied upon external stabilisation of the suspension bath (e.g. collagen) in order to culture soft materials without loss of printed structure. Here, we report a systematic investigation of suspension bath properties that support the printing, fusion, and culture of spheroid-based bioinks without added stabilisation. Specifically, agarose fluid gels of varied polymer concentrations and dilutions were produced and characterised morphologically and rheologically. Juvenile bovine chondrocytes or mesenchymal stromal cells (MSCs) were formed into spheroids of ∼150 µm in diameter and investigated within agarose suspension baths either for their fusion in hanging drop cultures or as jammed bioinks. MSC spheroids were also printed when mixed with hydrogel microparticles to demonstrate additional versatility to the approach. Suspension baths of lower polymer concentrations and increased dilution enabled faster spheroid fusion; however, the most heavily diluted suspension bath was unable to maintain print fidelity. Other formulations supported the printing, fusion, and culture of spheroid-based inks, either as simple lines or more complex patterns. These findings help to inform the design of suspension baths for bioprinting and culture.

Contrast-enhancing agents are commonplace in medical X-ray Computed Tomography. However, their use in laboratory-scale micro–X-ray Computed Tomography (µXCT) for morphological measurements of tissue-like or tissue-scaffold applications (e.g., 3D-printed artificial organs) is limited. We demonstrate enhanced contrast and enable print quality measurement with µXCT via a colloidal suspension of nano-silica spheres (c-SiO₂). Contrast is enhanced by the higher attenuation coefficient of the silica than hydrogel, which provides for 3D image analysis. We tested several c-SiO₂ dilutions and found an optimal selection that balances viscosity and contrast. We further demonstrate the technique by measuring a prototype dialyzer-like device. Graphical abstract

In the writing of holographic photopolymers, the addition of a third-stage cure to the typical polyurethane matrix and acrylate writing monomer steps is used here to modify the ultimate thermomechanical properties of the final holographic photopolymer. Inclusion of a thermally latent, low-refractive-index epoxide homopolymerization increases the T_g from a value of -22 °C during the writing step to a final T_g of 101 °C after the epoxide cure. Critically, the diffraction grating structure is retained with high fidelity, an index contrast of 0.0057, and a diffraction efficiency of 89% achieved in these materials. Ultimately, the 3-stage design and final glassy nature of these materials promote thermal and dimensional stability of the final holographic material.

Analytical ultracentrifugation (AUC) is applied to the characterization of as‐dispersed graphene nanoplatelet dispersions and differential sedimentation separated daughter dispersions. The liquid‐phase characterization of AUC is demonstrated to resolve both the broad sedimentation coefficient distributions of as‐dispersed samples and changes in daughter dispersions determined by a protocol of applied differential sedimentation process steps. Comparison is made to measurements on deposited samples by scanning electron microscopy and atomic force microscopy. The value of AUC to rapidly monitor changes in the sedimentation distribution of each particle population is demonstrated to allow tailoring of the differential sedimentation protocol to produce significantly narrower population distributions. This rapid characterization is particularly important for technologies in which dispersed nanoparticles cannot be removed from a solvent solution for microscopy analysis.

Polymer-based dielectrics are struggling to keep pace with the increasing demands of modern electronics. This lag in dielectric performance has spurred significant interest in the production of advanced dielectrics via novel chemistries and processing techniques. Polyelectrolyte complexes (PECs) have recently shown great promise as dielectric insulation, but processing challenges presented by these ionically bound networks limit their use to conformal thin films. Recent advances have enabled the additive manufacturing of PECs with vat photopolymerization, allowing the creation of a polyelectrolyte complex of arbitrary shape. Herein, multiple polyelectrolyte resin formulations, comprised of polyethylenimine and methacrylic acid (with varying amounts of 2-hydroxyethyl methacrylate and/or N,N-dimethylacrylamide), are investigated for the production of additively manufactured dielectric insulators. These dielectrics not only possess high dielectric breakdown strengths (>300 kV/mm), but their dielectric behavior can also be readily tailored through resin formulation and post-processing conditions. The presented vat photopolymerization of PECs allows for the creation of bulk dielectrics with arbitrary geometry, while the novel chemistry provides a practical route forward to produce dielectrics with precisely tailored properties for specific applications.

Incorporation of polymer chain entanglements within a single network can synergistically improve stiffness and toughness, yet attaining such dense entanglements through vat photopolymerization additive manufacturing [e.g., digital light processing (DLP)] remains elusive. We report a facile strategy that combines light and dark polymerization to allow constituent polymer chains to densely entangle as they form within printed structures. This generalizable approach reaches high monomer conversion at room temperature without the need for additional stimuli, such as light or heat after printing, and enables additive manufacturing of highly entangled hydrogels and elastomers that exhibit fourfold- to sevenfold-higher extension energies in comparison to that of traditional DLP. We used this method to print high-resolution and multimaterial structures with features such as spatially programmed adhesion to wet tissues.