Hollow microgels are fascinating model systems at the crossover between polymer vesicles, emulsions, and colloids as they deform, interpenetrate, and eventually shrink at higher volume fraction or when subjected to an external stress. Here, we introduce a system consisting of microgels with a micrometer-sized cavity enabling a straightforward characterization in situ using fluorescence microscopy techniques. Similarly to elastic capsules, these systems are found to reversibly buckle above a critical osmotic pressure, conversely to smaller hollow microgels, which were previously reported to deswell at high volume fraction. Simulations performed on monomer-resolved in silico hollow microgels confirm the buckling transition and show that the presented microgels can be described with a thin shell model theory. When brought to an interface, these microgels, that we define as microgel capsules, strongly deform and we thus propose to utilize them to locally probe interfacial properties within a theoretical framework adapted from the Johnson–Kendall–Roberts (JKR) theory. Besides their capability to sense their environment and to address fundamental questions on the elasticity and permeability of microgel systems, microgel capsules can be further envisioned as model systems mimicking anisotropic responsive biological systems such as red blood and epithelial cells thanks to the possibility offered by microgels to be synthesized with custom-designed properties.
Cellular engulfment and uptake of macromolecular assemblies or nanoparticles via endocytosis can be associated to both healthy and disease-related biological processes as well as delivery of drug nanoparticles and potential nanotoxicity of pollutants. Depending on the physical and chemical properties of the system, the adsorbed particles may remain at the membrane surface, become wrapped by the membrane, or translocate across the membrane through an endocytosis-like process. In this paper, we address the question of how the wrapping of colloidal particles by lipid membranes can be controlled by the shape of the particles, the particle–membrane adhesion energy, the membrane phase behavior, and the membrane-bending rigidity. We use a model system composed of soft core–shell microgel particles with spherical and ellipsoidal shapes, together with phospholipid membranes with varying composition. Confocal microscopy data clearly demonstrate how tuning of these basic properties of particles and membranes can be used to direct wrapping and membrane deformation and the organization of the particles at the membrane. The deep-wrapped states are more favorable for ellipsoidal than for spherical microgel particles of similar volume. Theoretical calculations for fixed adhesion strength predict the opposite behavior—wrapping becomes more difficult with increasing aspect ratio. The comparison with the experiments implies that the microgel adhesion strength must increase with increasing particle stretching. Considering the versatility offered by microgels systems to be synthesized with different shapes, functionalizations, and mechanical properties, the present findings further inspire future studies involving nanoparticle–membrane interactions relevant for the design of novel biomaterials and therapeutic applications.
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