Spontaneous Wrinkle Formation on Hydrogel Surfaces Using Photoinitiator Diffusion from Oil-Water Interface

Patterning wrinkles on three-dimensional curved or enclosed surfaces can be challenging due to difficulties in application of uniform films and stresses on such structures. In this study, we demonstrate a simple one-step wrinkle-formation method on various hydrogel structures utilizing the oil-water interfaces. By diffusion of the photoinitiator from the oil phase to the prepolymer solution in water through the interface, a characteristic cross-linking gradient is set up in the hydrogel. Then, after photopolymerization, we observe diverse patterns of wrinkles upon changing the concentration of the hydrogel or photoinitiator. As the wrinkle formation via photoinitiator diffusion through the interface requires only UV exposure for polymerization, while taking advantage of the oil-water interfacial tension, wrinkles can be developed easily on various curved structures. In addition, we illustrate the formation of wrinkles on surfaces underneath another layer of polymer or on completely enclosed surfaces, which is difficult with conventional methods. We expect that our results will lead to production of novel microstructures and provide a platform for studying the morphogenesis of wrinkles found in nature such as in curved substrates and multilayers

Fabrication of poly(dimethylsiloxane) membrane with well-defined through-holes for three-dimensional microfluidic networks

We report a simple method for the fabrication of a poly(dimethylsiloxane) (PDMS) membrane with through-holes by blowing a residual prepolymer away from a photoresist (PR)-patterned Si wafer. The fabrication method for the perforated polymer membrane is crucial to achieve both complicated three-dimensional microfluidic devices and polymer sieve sheets. This method has several advantages over the previous methods in that we can repeatedly make the well-defined holes on the PDMS membranes even if the excess prepolymer remains on the PR mold after spincoating at a relatively low rpm. In addition, the desired pattern can be selectively perforated from the whole wafer even if the mold is fabricated by single-step lithography.clos

Symmetrically pulsating bubbles swim in an anisotropic fluid by nematodynamics

Abstract Swimming in low-Reynolds-number fluids requires the breaking of time-reversal symmetry and centrosymmetry. Microswimmers, often with asymmetric shapes, exhibit nonreciprocal motions or exploit nonequilibrium processes to propel. The role of the surrounding fluid has also attracted attention because viscoelastic, non-Newtonian, and anisotropic properties of fluids matter in propulsion efficiency and navigation. Here, we experimentally demonstrate that anisotropic fluids, nematic liquid crystals (NLC), can make a pulsating spherical bubble swim despite its centrosymmetric shape and time-symmetric motion. The NLC breaks the centrosymmetry by a deformed nematic director field with a topological defect accompanying the bubble. The nematodynamics renders the nonreciprocity in the pulsation-induced fluid flow. We also report speed enhancement by confinement and the propulsion of another symmetry-broken bubble dressed by a bent disclination. Our experiments and theory propose another possible mechanism of moving bodies in complex fluids by spatiotemporal symmetry breaking

Observations of different oscillation modes and control of droplet sequence in alternating droplet generation

Controlling the sequence of microdroplet generation is useful for quantitative manipulation of multiple fluids, but little understanding is available for how the sequence can be determined based upon the geometry of a microchannel and the fluid properties. In this work, we study the design of a double Tjunction microchannel, and observe that there are three modes of oscillations of the droplet generation: i) one by one alternate generation, ii) synchronization, and iii) fluctuations. We also report that if the fluids from the two branches have asymmetric properties (e.g. viscosity), we can change the ratios of the droplets in sequence

One-step preparation of magnetic Janus particles using controlled phase separation of polymer blends and nanoparticles

We present a simple method with the aid of a microfluidic droplet-generation technique to fabricate magnetic Janus particles by utilizing a solvent evaporation-induced phase separation and preferential partitioning of magnetic nanoparticles in the polymer blends. Non-aqueous emulsion droplets of the polymer blends and nanoparticles solution are produced to become Janus particles after the evaporation of the solvent. The stabilizing polymer of the nanoparticles, which is compatible only with one of the polymer blends to be phase-separated, plays a key role in the anisotropic positioning of the nanoparticles in the Janus particles. Using this phase separation-based method and microfluidics, excellent control over the size, size distribution, and morphology of the particles is achieved. Especially, we could control the morphology of the Janus particles easily by varying the volume ratio of the polymers. However, with an analysis of the shapes of resulting Janus particles, we found that non-equilibrium aspects of the evaporation-induced phase separation play a major role in determining the particle morphology. We expect that the versatility of this method in the choice of polymer blends and functional nanoparticles will enable the fabrication of colloids with various functionality and desired morphologyclose111

Cell migration in microengineered tumor environments

Recent advances in microengineered cell migration platforms are discussed critically with a focus on how cell migration is influenced by engineered tumor microenvironments, the medical relevance being to understand how tumor microenvironments may promote or suppress the progression of cancer. We first introduce key findings in cancer cell migration under the influence of the physical environment, which is systematically controlled by microengineering technology, followed by multi-cues of physico-chemical factors, which represent the complexity of the tumor environment. Recognizing that cancer cells constantly communicate not only with each other but also with tumor-associated cells such as vascular, fibroblast, and immune cells, and also with non-cellular components, it follows that cell motility in tumor microenvironments, especially metastasis via the invasion of cancer cells into the extracellular matrix and other tissues, is closely related to the malignancy of cancer-related mortality. Medical relevance of forefront research realized in microfabricated devices, such as single cell sorting based on the analysis of cell migration behavior, may assist personalized theragnostics based on the cell migration phenotype. Furthermore, we urge development of theory and numerical understanding of single or collective cell migration in microengineered platforms to gain new insights in cancer metastasis and in therapeutic strategies