Biological cells generate intricate structures by sculpting their membrane from within to actively sense and respond to external stimuli or to explore their environment1-4. Several pathogenic bacteria also provide examples of how localized forces strongly deform cell membranes from inside, leading to the invasion of neighbouring healthy mammalian cells5. Giant unilamellar vesicles have been successfully used as a minimal model system with which to mimic biological cells6-11, but the realization of a minimal system with localized active internal forces that can strongly deform lipid membranes from within and lead to dramatic shape changes remains challenging. Here we present a combined experimental and simulation study that demonstrates how self-propelled particles enclosed in giant unilamellar vesicles can induce a plethora of non-equilibrium shapes and active membrane fluctuations. Using confocal microscopy, in the experiments we explore the membrane response to local forces exerted by self-phoretic Janus microswimmers. To quantify dynamic membrane changes, we perform Langevin dynamics simulations of active Brownian particles enclosed in thin membrane shells modelled by dynamically triangulated surfaces. The most pronounced shape changes are observed at low and moderate particle loadings, with the formation of tether-like protrusions and highly branched, dendritic structures, whereas at high volume fractions globally deformed vesicle shapes are observed. The resulting state diagram predicts the conditions under which local internal forces generate various membrane shapes. A controlled realization of such distorted vesicle morphologies could improve the design of artificial systems such as small-scale soft robots and synthetic cells.

中文翻译：

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活性粒子在巨大的脂质囊泡中引起大的形状变形

生物细胞通过从内部雕刻它们的膜来产生复杂的结构，以主动感知和响应外部刺激或探索它们的环境1-4。几种病原菌还提供了局部力如何从内部强烈地使细胞膜变形，从而导致邻近健康哺乳动物细胞入侵的例子。巨大的单层囊泡已成功用作模拟生物细胞的最小模型系统 6-11，但实现具有局部主动内力的最小系统可以使脂质膜从内部强烈变形并导致剧烈的形状变化仍然具有挑战性。在这里，我们提出了一项联合实验和模拟研究，展示了封闭在巨大单层囊泡中的自推进粒子如何诱导大量非平衡形状和活跃的膜波动。使用共聚焦显微镜，在实验中，我们探索了膜对自泳 Janus 微型游泳者施加的局部力的反应。为了量化动态膜变化，我们对封闭在由动态三角表面建模的薄膜壳中的活性布朗粒子进行朗之万动力学模拟。在低和中等颗粒负载下观察到最明显的形状变化，形成系绳状突起和高度分支的树枝状结构，而在高体积分数下观察到全局变形的囊泡形状。由此产生的状态图预测了局部内力产生各种膜形状的条件。这种扭曲的囊泡形态的受控实现可以改进人工系统的设计，例如小型软机器人和合成细胞。