Controllable Fabrication of Highly Uniform Sub-10 nm Nanoparticles from Spontaneous Confined Nanoemulsification.

Sub-10 nm nanoparticles are known to exhibit extraordinary size-dependent properties for wide applications. Many approaches have been developed for synthesizing sub-10 nm inorganic nanoparticles, but the fabrication of sub-10 nm polymeric nanoparticles is still challenging. Here, a scalable, spontaneous confined nanoemulsification strategy that produces uniform sub-10 nm nanodroplets for template synthesis of sub-10 nm polymeric nanoparticles is proposed. This strategy introduces a high-concentration interfacial reaction to create overpopulated surfactants that are insoluble at the droplet surface. These overpopulated surfactants act as barriers, resulting in highly accumulated surfactants inside the droplet via a confined reaction. These surfactants exhibit significantly changed packing geometry, solubility, and interfacial activity to enhance the molecular-level impact on interfacial instability for creating sub-10 nm nanoemulsions via self-burst nanoemulsification. Using the nanodroplets as templates, the fabrication of uniform sub-10 nm polymeric nanoparticles, as small as 3.5 nm, made from biocompatible polymers and capable of efficient drug encapsulation is demonstrated. This work opens up brand-new opportunities to easily create sub-10 nm nanoemulsions and advanced ultrasmall functional nanoparticles.

Size-induced motion mode transitions in collective cell invasion toward free spaces.

Collective cell migration plays a vital role in various physiological and pathological processes, such as embryonic development and tumor metastasis. Recent experiments have shown that different from isolated cells, the moving cell groups exhibit rich emerging motion modes in response to external geometrical constraints. By considering the interactions between neighboring cells and internal biomechanical processes of each cell (i.e., cell sociality and cell individuality), we develop an active vertex model to investigate the emerging modes of collective cell migration in microchannels. Single-cell polarization is propelled by continuous protrusion of its leading edge and retraction of the rear. We here introduce the contribution of continuous protrusions and retractions of lamellipodia, named the protrusion alignment mechanism, to the cell individuality. Using the present model, it is found that altering the width of channels can trigger the motion mode transitions of cell groups. When cells move in narrow channels, the protrusion alignment mechanism brings neighboring groups of coordinated cells into conflicts and in turn induces the caterpillar-like motion mode. As the channel width increases, local swirls spanning the channel in width first appear as long as the channel width is smaller than the intrinsic correlation length of cell groups. Then, only local swirls with a maximum diameter of the intrinsic correlation length are formed, when the channel is sufficiently wider. These rich dynamical modes of collective cells originate from the competition between cell individuality and sociality. In addition, the velocity of the cell sheet invading free spaces varies with the channel size-induced transitions of migration modes. Our predictions are in broad agreement with many experiments and may shed light on the spatiotemporal dynamics of active matter.

Cell chirality regulates coherent angular motion on small circular substrates.

Collective cell migration occurs in a wide range of physiological and pathological processes, such as wound healing and tumor metastasis. Experiments showed that many types of cells confined in circular islands can perform coherent angular rotation, yet the underlying mechanisms remain unclear. Here we propose a biomechanical model, including the membrane, microtubules, and nucleus, to study the spatiotemporal evolutions of small cell clusters in confined space. We show that cells can spontaneously transfer from "radial pattern" to "chiral pattern" due to fluctuations. For a pair of cells with identical chiral orientation, the cluster rotates in the opposite direction of the chiral orientation, and the fluctuations can reverse the cluster's rotational direction. Interestingly, during the persistent rotation, each cell rotates around its own centroid while it is revolving around the island center and shows a constant side to the island center, as tidal locking in astronomy. Furthermore, for a few more cells, coherent angular rotation also appears, and the emergence of a central cell can accelerate the cluster rotation. These findings shed light on collective cell migration in life processes and help to understand the spatiotemporal dynamics of active matter.

Anisotropic power-law viscoelasticity of living cells is dominated by cytoskeletal network structure.

During physiological and pathological processes, cells experience significant morphological alterations with the re-arrangement of cytoskeletal filaments, resulting in anisotropic viscoelasticity. Here, a structure-based cell model is proposed to study the anisotropic viscoelastic mechanical behaviors of living cells. We investigate how cell shape affects its creep responses in longitudinal and perpendicular directions. It is shown that cells exhibit power-law rheological behavior in both longitudinal and perpendicular directions under step stress, with a more solid-like behavior along the longitudinal direction. We reveal that the cell volume and cytoskeletal filament orientation, which have been neglected in most existing models, play a critical role in regulating cellular anisotropic viscoelasticity. The stiffness of the cell in both directions increases linearly with increasing its aspect ratio, due to the decrease of cell volume. Moreover, the increase in the cell's aspect ratio produces the aggregation of cytoskeletal filaments along the longitudinal direction, resulting in higher stiffness in this direction. It is also shown that the increase in cell's aspect ratio corresponds to a process of cellular ordering, which can be quantitatively characterized by the orientational entropy of cytoskeletal filaments. In addition, we present a simple yet robust method to establish the relationship between cell's aspect ratio and cell volume, thus providing a theoretical framework to capture the anisotropic viscoelastic behavior of cells. This study suggests that the structure-based cell models may be further developed to investigate cellular rheological responses to external mechanical stimuli and may be extended to the tissue scale. STATEMENT OF SIGNIFICANCE: The viscoelastic behaviors of cells hold significant importance in comprehending the roles of mechanical forces in embryo development, invasion, and metastasis of cancer cells. Here, a structure-based cell model is proposed to study the anisotropic viscoelastic mechanical behaviors of living cells. Our study highlights the crucial role of previously neglected factors, such as cell volume and cytoskeletal filament orientation, in regulating cellular anisotropic viscoelasticity. We further propose an orientational entropy of cytoskeletal filaments to quantitatively characterize the ordering process of cells with increasing aspect ratios. Moreover, we derived the analytical interrelationships between cell aspect ratio, cell stiffness, cell volume, and cytoskeletal fiber orientation. This study provides a theoretical framework to describe the anisotropic viscoelastic mechanical behavior of cells.