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Here, we present a protocol for electrotaxis of large epithelial cell sheets without compromising the integrity of cell epithelia in a high-throughput customized directed current electrotaxis chamber. We describe the fabrication and use of polydimethylsiloxane stencils to control the size and shape of human keratinocyte cell sheets. We detail cell tracking, cell sheet contour assay, and particle image velocimetry to reveal the spatial and temporal motility dynamics of cell sheets. This approach is applicable to other collective cell migration studies. For complete details on the use and execution of this protocol, please refer to Zhang et al. (2022).1.

Recent research has elucidated mechanochemical pathways of single cell polarization, but much less is known about collective motility initiation in adhesive cell groups. We used galvanotactic assays of zebrafish keratocyte cell groups, pharmacological perturbations, electric field switches, particle imaging velocimetry, and cell tracking to show that large cell groups initiate motility in minutes toward the cathode. Interestingly, while PI3K-inhibited single cells are biased toward the anode, inhibiting PI3K does not affect the cathode-directed cell group migration. We observed that control groups had the fastest cathode-migrating cell at the front, while the front cells in PI3K-inhibited groups were the slowest. Both control and PI3K-inhibited groups rapidly repolarized when the electric field direction was reversed, and the group migration continued after the electric field was switched off. Inhibiting myosin disrupted the cohesiveness of keratocyte groups and abolished the collective directionality and ability to switch direction when the electric field is reversed. Our data are consistent with a model according to which cells in the group sense the electric field individually and mechanical integration of the cells results in coherent group motility.

Directional migration initiated at the wound edge leads epithelia to migrate in wound healing. How such coherent migration is achieved is not well understood. Here, we used electric fields to induce robust migration of sheets of human keratinocytes and developed an in silico model to characterize initiation and propagation of epithelial collective migration. Electric fields initiate an increase in migration directionality and speed at the leading edge. The increases propagate across the epithelial sheets, resulting in directional migration of cell sheets as coherent units. Both the experimental and in silico models demonstrated vector-like integration of the electric and default directional cues at free edge in space and time. The resultant collective migration is consistent in experiments and modeling, both qualitatively and quantitatively. The keratinocyte model thus faithfully reflects key features of epithelial migration as a coherent tissue in vivo, e.g. that leading cells lead, and that epithelium maintains cell-cell junction.

A microfluidics-based three-dimensional skin-on-chip (SoC) model is developed in this study to enable quantitative studies of transendothelial and transepithelial migration of human T lymphocytes in mimicked skin inflammatory microenvironments and to test new drug candidates. The keys results include 1) CCL20-dependent T cell transmigration is significantly inhibited by an engineered CCL20 locked dimer (CCL20LD), supporting the potential immunotherapeutic use of CCL20LD for treating skin diseases such as psoriasis; 2) transepithelial migration of T cells in response to a CXCL12 gradient mimicking T cell egress from the skin is significantly reduced by a sphingosine-1-phosphate (S1P) background, suggesting the role of S1P for T cell retention in inflamed skin tissues; and 3) T cell transmigration is induced by inflammatory cytokine stimulated epithelial cells in the SoC model. Collectively, the developed SoC model recreates a dynamic multi-cellular micro-environment that enables quantitative studies of T cell transmigration at a single cell level in response to physiological cutaneous inflammatory mediators and potential drugs.

Organic solar cells (OSCs) have achieved remarkable power conversion efficiencies (PCEs) of over 19 % in the past few years due to the rapid development of non‐fullerene acceptors (NFAs). However, the operational stability remains a great challenge that inhibits their commercialization. Recently, oligomeric NFAs (ONFAs) have attracted great attention, which not only can deliver excellent device performance, but also improve the thermal‐/photo‐ stability of OSCs. This is attributed to the suppressed molecular diffusion of ONFAs associated with their high glass‐transition temperature (Tg) and improved thermodynamic properties of ONFAs. Herein, we focus on investigating the correction between the ONFA chemical structure, material properties, device performance, and stability. In addition, we also try to point out the challenges in synthesizing ONFAs and provide potential directions for future ONFA designs. [ABSTRACT FROM AUTHOR]

When a constraint is removed, confluent cells migrate directionally into the available space. How the migration directionality and speed increase are initiated at the leading edge and propagate into neighboring cells are not well understood. Using a quantitative visualization technique-Particle Image Velocimetry (PIV)-we revealed that migration directionality and speed had strikingly different dynamics. Migration directionality increases as a wave propagating from the leading edge into the cell sheet, while the increase in cell migration speed is maintained only at the leading edge. The overall directionality steadily increases with time as cells migrate into the cell-free space, but migration speed remains largely the same. A particle-based compass (PBC) model suggests cellular interplay (which depends on cell-cell distance) and migration speed are sufficient to capture the dynamics of migration directionality revealed experimentally. Extracellular Ca2+ regulated both migration speed and directionality, but in a significantly different way, suggested by the correlation between directionality and speed only in some dynamic ranges. Our experimental and modeling results reveal distinct directionality and speed dynamics in collective migration, and these factors can be regulated by extracellular Ca2+ through cellular interplay. Quantitative visualization using PIV and our PBC model thus provide a powerful approach to dissect the mechanisms of collective cell migration.

Se‐functionalized small molecule acceptors (SMAs) exhibit unique advantages in constructing materials with near‐infrared absorption, but their photovoltaic performance lags behind that of S‐containing analogs in organic solar cells (OSCs). Herein, two new Se‐containing SMAs, namely Se‐EH and Se‐EHp, are designed and synthesized by regulating bifurcation site of outer alkyl chain, which enables Se‐EH and Se‐EHp to form different 3D crystal frameworks from CH1007. Se‐EH displays tighter π–π stacking and denser packing framework with smaller‐sized pore structure induced by larger steric hindrance effect of outer alkyl chain branched at 2‐position, and a higher dielectric constant of PM6:Se‐EH active layer can be obtained. OSCs based on PM6:Se‐EH achieved very high PCEs of 18.58% in binary and 19.03% in ternary devices with a high FF approaching 80% for Se‐containing SMAs. A more significant alkyl chain steric hindrance effect in Se‐EH adjusts the molecular crystallization to form a favorable nanofiber interpenetrating network with an appropriate domain size to reduce rate of sub‐ns recombination and promote balanced transport of carriers. This work provides references for further design and development of highly efficient Se‐functionalized SMAs. [ABSTRACT FROM AUTHOR]

Typical bulk‐heterojunction organic photovoltaics (OPV) suffer from instability due to the slowly diffusing small‐molecule acceptors (SMAs) under operational conditions. The originally well‐phase‐separated morphology will undergo SMA self‐aggregation to form over‐purified domains that will deteriorate the OPV performance. This issue can be partially alleviated in sequentially processed OPV cells with a planar‐mixed heterojunction absorber, which is intrinsically more robust due to larger domain sizes and a lower degree of mixing. To further enhance the morphological stability, a strategy is developed to inhibit the disordered molecular diffusion that leads to the loss of crystallinity of SMA domains. Two SMAs, C9‐4F and Y6‐OBO, are incorporated as guest acceptors to form an alloy‐like acceptor with the host acceptor BTP‐eC9. These alloy‐like acceptors improved the crystallinity of the acceptor phase by promoting more ordered molecular packing, effectively restraining the diffusion‐caused loss of SMA crystallinity upon illumination. This approach helps maintain favorable gradients and ultimately ensures OPV morphological stability. Consequently, the derived devices exhibited considerably enhanced stability, with the T80 lifetime of C9‐4F‐based devices reaching ≈790 h and a remarkable ≈1010 h for Y6‐OBO‐based devices, significantly surpassing the ≈450 h observed from binary devices. [ABSTRACT FROM AUTHOR]