Polyelectrolyte scaling laws for microgel yielding near jamming.

Micro-scale hydrogel particles, known as microgels, are used in industry to control the rheology of numerous different products, and are also used in experimental research to study the origins of jamming and glassy behavior in soft-sphere model systems. At the macro-scale, the rheological behaviour of densely packed microgels has been thoroughly characterized; at the particle-scale, careful investigations of jamming, yielding, and glassy-dynamics have been performed through experiment, theory, and simulation. However, at low packing fractions near jamming, the connection between microgel yielding phenomena and the physics of their constituent polymer chains has not been made. Here we investigate whether basic polymer physics scaling laws predict macroscopic yielding behaviours in packed microgels. We measure the yield stress and cross-over shear-rate in several different anionic microgel systems prepared at packing fractions just above the jamming transition, and show that our data can be predicted from classic polyelectrolyte physics scaling laws. We find that diffusive relaxations of microgel deformation during particle re-arrangements can predict the shear-rate at which microgels yield, and the elastic stress associated with these particle deformations predict the yield stress.

3D In Vitro Platform for Cell and Explant Culture in Liquid-like Solids.

Existing 3D cell models and technologies have offered tools to elevate cell culture to a more physiologically relevant dimension. One mechanism to maintain cells cultured in 3D is by means of perfusion. However, existing perfusion technologies for cell culture require complex electronic components, intricate tubing networks, or specific laboratory protocols for each application. We have developed a cell culture platform that simply employs a pump-free suction device to enable controlled perfusion of cell culture media through a bed of granular microgels and removal of cell-secreted metabolic waste. We demonstrated the versatile application of the platform by culturing single cells and keeping tissue microexplants viable for an extended period. The human cardiomyocyte AC16 cell line cultured in our platform revealed rapid cellular spheroid formation after 48 h and ~90% viability by day 7. Notably, we were able to culture gut microexplants for more than 2 weeks as demonstrated by immunofluorescent viability assay and prolonged contractility.

Publisher's Note: Multicellular density fluctuations in epithelial monolayers [Phys. Rev. E 92, 032729 (2015)].

This corrects the article DOI: 10.1103/PhysRevE.92.032729.

Liquid-like Solids Support Cells in 3D.

The demands of tissue engineering have driven a tremendous amount of research effort in 3D tissue culture technology and, more recently, in 3D printing. The need to use 3D tissue culture techniques more broadly in all of cell biology is well-recognized, but the transition to 3D has been impeded by the convenience, effectiveness, and ubiquity of 2D culture materials, assays, and protocols, as well as the lack of 3D counterparts of these tools. Interestingly, progress and discoveries in 3D bioprinting research may provide the technical support needed to grow the practice of 3D culture. Here we investigate an integrated approach for 3D printing multicellular structures while using the same platform for 3D cell culture, experimentation, and assay development. We employ a liquid-like solid (LLS) material made from packed granular-scale microgels, which locally and temporarily fluidizes under the focused application of stress and spontaneously solidifies after the applied stress is removed. These rheological properties enable 3D printing of multicellular structures as well as the growth and expansion of cellular structures or dispersed cells. The transport properties of LLS allow molecular diffusion for the delivery of nutrients or small molecules for fluorescence-based assays. Here, we measure viability of 11 different cell types in the LLS medium, we 3D print numerous structures using several of these cell types, and we explore the transport properties in molecular time-release assays.

Kinetics of aqueous lubrication in the hydrophilic hydrogel Gemini interface.

The exquisite sliding interfaces in the human body share the common feature of hydrated dilute polymer mesh networks. These networks, especially when they constitute a sliding interface such as the pre-corneal tear film on the ocular interface, are described by the molecular weight of the polymer chains and a characteristic size of a minimum structural unit, the mesh size, ξ. In a Gemini interface where hydrophilic hydrogels are slid against each other, the aqueous lubrication behavior has been shown to be a function of sliding velocity, introducing a sliding timescale competing against the time scales of polymer fluctuation and relaxation at the surface. In this work, we examine two recent studies and postulate that when the Gemini interface slips faster than the single-chain relaxation time, chains must relax, suppressing the amplitude of the polymer chain thermal fluctuations.

Multicellular density fluctuations in epithelial monolayers.

Changes in cell size often accompany multicellular motion in tissue, and cell number density is known to strongly influence collective migration in monolayers. Density fluctuations in other forms of active matter have been explored extensively, but not the potential role of density fluctuations in collective cell migration. Here we investigate collective motion in cell monolayers, focusing on the divergent component of the migration velocity field to probe density fluctuations. We find spatial patterns of diverging and converging cell groups throughout the monolayers, which oscillate in time with a period of approximately 3-4 h. Simultaneous fluorescence measurements of a cytosol dye within the cells show that fluid passes between groups of cells, facilitating these oscillations in cell density. Our findings reveal that cell-cell interactions in monolayers may be mediated by intercellular fluid flow.