Hydrogel microparticles from lithographic processes: Novel materials for fundamental and applied colloid science

In recent years, there has been a surge in methods to synthesize geometrically and chemically complex microparticles. Analogous to atoms, the concept of a “periodic table” of particles has emerged and continues to be expanded upon. Complementing the natural intellectual curiosity that drives the creation of increasingly intricate particles is the pull from applications that take advantage of such high-value materials. Complex particles are now being used in fields ranging from diagnostics and catalysis, to self-assembly and rheology, where material composition and microstructure are closely linked with particle function. This is especially true of polymer hydrogels, which offer an attractive and broad class of base materials for synthesis. Lithography affords the ability to engineer particle properties a priori and leads to the production of homogenous ensembles of particles. This review summarizes recent advances in synthesizing hydrogel microparticles using lithographic processes and highlights a number of emerging applications. We discuss advantages and limitations of current strategies, and conclude with an outlook on future trends in the field.National Science Foundation (U.S.) (Grant DMR-1006147)Novartis-MIT Center for Continuous ManufacturingNational Institute for Biomedical Imaging and Bioengineering (U.S.) (Grant R21EB008814

Homogeneous percolation versus arrested phase separation in attractively-driven nanoemulsion colloidal gels

We elucidate mechanisms for colloidal gelation of attractive nanoemulsions depending on the volume fraction (ϕ) of the colloid. Combining detailed neutron scattering, cryo-transmission electron microscopy and rheological measurements, we demonstrate that gelation proceeds by either of two distinct pathways. For ϕ sufficiently lower than 0.23, gels exhibit homogeneous fractal microstructure, with a broad gel transition resulting from the formation and subsequent percolation of droplet–droplet clusters. In these cases, the gel point measured by rheology corresponds precisely to arrest of the fractal microstructure, and the nonlinear rheology of the gel is characterized by a single yielding process. By contrast, gelation for ϕ sufficiently higher than 0.23 is characterized by an abrupt transition from dispersed droplets to dense clusters with significant long-range correlations well-described by a model for phase separation. The latter phenomenon manifests itself as micron-scale “pores” within the droplet network, and the nonlinear rheology is characterized by a broad yielding transition. Our studies reinforce the similarity of nanoemulsions to solid particulates, and identify important qualitative differences between the microstructure and viscoelastic properties of colloidal gels formed by homogeneous percolation and those formed by phase separation.United States. Army Research Office (Institute for Collaborative Biotechnologies. Grant W911NF- 09-0001)National Science Foundation (U.S.) (Grants CMMI-1120724 and DMR-1006147

A microfluidic model of human brain (μHuB) for assessment of blood brain barrier

Microfluidic cellular models, commonly referred to as “organs‐on‐chips,” continue to advance the field of bioengineering via the development of accurate and higher throughput models, captivating the essence of living human organs. This class of models can mimic key in vivo features, including shear stresses and cellular architectures, in ways that cannot be realized by traditional two‐dimensional in vitro models. Despite such progress, current organ‐on‐a‐chip models are often overly complex, require highly specialized setups and equipment, and lack the ability to easily ascertain temporal and spatial differences in the transport kinetics of compounds translocating across cellular barriers. To address this challenge, we report the development of a three‐dimensional human blood brain barrier (BBB) microfluidic model (μHuB) using human cerebral microvascular endothelial cells (hCMEC/D3) and primary human astrocytes within a commercially available microfluidic platform. Within μHuB, hCMEC/D3 monolayers withstood physiologically relevant shear stresses (2.73 dyn/cm2) over a period of 24 hr and formed a complete inner lumen, resembling in vivo blood capillaries. Monolayers within μHuB expressed phenotypical tight junction markers (Claudin‐5 and ZO‐1), which increased expression after the presence of hemodynamic‐like shear stress. Negligible cell injury was observed when the monolayers were cultured statically, conditioned to shear stress, and subjected to nonfluorescent dextran (70 kDa) transport studies. μHuB experienced size‐selective permeability of 10 and 70 kDa dextrans similar to other BBB models. However, with the ability to probe temporal and spatial evolution of solute distribution, μHuBs possess the ability to capture the true variability in permeability across a cellular monolayer over time and allow for evaluation of the full breadth of permeabilities that would otherwise be lost using traditional end‐point sampling techniques. Overall, the μHuB platform provides a simplified, easy‐to‐use model to further investigate the complexities of the human BBB in real‐time and can be readily adapted to incorporate additional cell types of the neurovascular unit and beyond.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149762/1/btm210126_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149762/2/btm210126.pd

Deformabilidad en hormigones con agregados reciclados

Diversos trabajos han demostrado la factibilidad de elaborar hormigones con agregados reciclados, sin embargo sobre algunos aspectos poco explorados aún existen informaciones contrapuestas. Entre ellos se destacan el comportamiento diferido del hormigón (contracción y fluencia) y la capacidad de deformación en tracción (extensibilidad). Estas propiedades afectan directamente el grado de fisuración que puede tener una estructura de hormigón, lo que adquiere una significativa relevancia en la práctica, al considerar su vida en servicio. En este trabajo se presenta un estudio de la deformabilidad de hormigones que contienen 50 o 100 % de agregado grueso obtenido a partir de la trituración de losas de pavimento. Los resultados se comparan con los de otros dos hormigones elaborados con idénticas proporciones de materiales componentes variando el tipo de agregado grueso, piedra partida granítica o piedra partida cuarcítica. Se evaluaron la resistencia a tracción, la extensibilidad en flexión bajo cargas rápidas, y la fluencia en compresión. Bajo cargas de corta duración se encontró una deformabilidad creciente en el hormigón con mayor contenido de agregados reciclados, tanto en flexotracción como en compresión; también se midieron mayores valores de contracción libre y de fluencia. A partir de los resultados surge que es posible estimar la deformabilidad del hormigón con agregados reciclados siguiendo criterios similares a los aplicados en hormigones con agregados naturales; las diferencias de deformabilidad se pueden justificar considerando la menor rigidez del agregado reciclado.Different works have demonstrated the feasibility of elaborating concrete with recycled aggregates, nevertheless there is still opposite information about some aspects that have not been widely studied. The differed behaviour (shrinkage and creep) and the deformation capacity in tension (extensibility) of concrete are among them. These properties have a direct effect over the degree of cracking that can have a concrete structure so, considering its service life, they acquire a significant relevance in practice. This paper presents a study on the deformability of concretes that contain 50 or 100 % of coarse aggregate obtained from crushed pavement slabs. The results are compared with those obtained on other two concretes prepared with the same mixture proportions varying only the type of coarse aggregate, granitic crushed stone or quartzitic crushed stone. The tensile strength, extensibility in flexure under rapid rate of loading and creep in compression were evaluated. It was found that under short term loads the deformability of concrete increases with the content of recycled aggregates, both in flexure and in compression, grater values of free shrinkage and creep were also measured. From the obtained results it appears that the deformability of concrete with recycled aggregates can be estimated following the same criteria applied to concrete with natural aggregate, the differences in deformability can be justified considering the lower stiffness of the recycled aggregate

Molecular-scale substrate anisotropy and crowding drive long-range nematic order of cell monolayers

The ability of cells to reorganize in response to external stimuli is important in areas ranging from morphogenesis to tissue engineering. Elongated cells can co-align due to steric effects, forming states with local order. We show that molecular-scale substrate anisotropy can direct cell organization, resulting in the emergence of nematic order on tissue scales. To quantitatively examine the disorder-order transition, we developed a high-throughput imaging platform to analyze velocity and orientational correlations for several thousand cells over days. The establishment of global, seemingly long-ranged order is facilitated by enhanced cell division along the substrate's nematic axis, and associated extensile stresses that restructure the cells' actomyosin networks. Our work, which connects to a class of systems known as active dry nematics, provides a new understanding of the dynamics of cellular remodeling and organization in weakly interacting cell collectives. This enables data-driven discovery of cell-cell interactions and points to strategies for tissue engineering.Comment: 29 pages, 7 figure