Dynamic multimodal holograms of conjugated organogels via dithering mask lithography.

Polymeric materials have been used to realize optical systems that, through periodic variations of their structural or optical properties, interact with light-generating holographic signals. Complex holographic systems can also be dynamically controlled through exposure to external stimuli, yet they usually contain only a single type of holographic mode. Here, we report a conjugated organogel that reversibly displays three modes of holograms in a single architecture. Using dithering mask lithography, we realized two-dimensional patterns with varying cross-linking densities on a conjugated polydiacetylene. In protic solvents, the organogel contracts anisotropically to develop optical and structural heterogeneities along the third dimension, displaying holograms in the form of three-dimensional full parallax signals, both in fluorescence and bright-field microscopy imaging. In aprotic solvents, these heterogeneities diminish as organogels expand, recovering the two-dimensional periodicity to display a third hologram mode based on iridescent structural colours. Our study presents a next-generation hologram manufacturing method for multilevel encryption technologies.

Superabsorbent Polymer Network Degradable by a Human Urinary Enzyme.

Owing to its superior water absorption capacity, superabsorbent polymer (SAP) based on a poly (acrylic acid) network is extensively used in industrial products such as diapers, wound dressing, or surgical pads. However, because SAP does not degrade naturally, a massive amount of non-degradable waste is discarded daily, posing serious environmental problems. Considering that diapers are the most widely used end-product of SAP, we created one that is degradable by a human urinary enzyme. We chose three enzyme candidates, all of which have substrates that were modified with polymerizable groups to be examined for cleavable crosslinkers of SAP. We found that the urokinase-type plasminogen activator (uPA) substrate, end-modified with acrylamide groups at sufficient distances from the enzymatic cleavage site, can be successfully used as a cleavable crosslinker of SAP. The resulting SAP slowly degraded over several days in the aqueous solution containing uPA at a physiological concentration found in human urine and became shapeless in ~30 days.

A Microfluidic Model Artery for Studying the Mechanobiology of Endothelial Cells.

Recent vascular mechanobiology studies find that endothelial cells (ECs) convert multiple mechanical forces into functional responses in a nonadditive way, suggesting that signaling pathways such as those regulating cytoskeleton may be shared among the processes of converting individual forces. However, previous in vitro EC-culture platforms are inherent with extraneous mechanical components, which may saturate or insufficiently activate the shared signaling pathways and accordingly, may misguide EC mechanobiological responses being investigated. Here, a more physiologically relevant model artery is reported that accurately reproduces most of the mechanical forces found in vivo, which can be individually varied in any combination to pathological levels to achieve diseased states. Arterial geometries of normal and diseased states are also realized. By mimicking mechanical microenvironments of early-stage atherosclerosis, it is demonstrated that the elevated levels of the different types of stress experienced by ECs strongly correlate with the disruption of barrier integrity, suggesting that boundaries of an initial lesion could be sites for efficient disease progression.

Network structure and enzymatic degradation of chitosan hydrogels determined by crosslinking methods.

Polysaccharides can be modified by reactive functional groups to enable chemical crosslinking. We studied how different methods of crosslinking methacrylate-functionalized chitosan affected the network structures and various properties relevant for utilization of the chemically crosslinked hydrogels in biomedical applications, including tissue engineering and delivery of therapeutic agents. Four chitosan hydrogels were made by either the free radical polymerization with varying initiation kinetics and an addition of chain transfer agents or the based-catalyzed Michael-type addition reaction. Four chitosan hydrogels having identical polymer fractions at equilibrium swelling exhibited marked differences in shear moduli, dextran diffusion rate, and especially enzymatic degradation behaviors. Hydrogels made by the free radical polymerization with no chain transfer agent were highly resistant to complete degradation by enzyme for an extended period. We inferred that such resistance originated from chain bundles characterized by densely branched networks of chitosan chains, which was determined by small-angle X-ray scattering analysis.

Synthetic hydrogels with stiffness gradients for durotaxis study and tissue engineering scaffolds.

Migration of cells along the right direction is of paramount importance in a number of in vivo circumstances such as immune response, embryonic developments, morphogenesis, and healing of wounds and scars. While it has been known for a while that spatial gradients in chemical cues guide the direction of cell migration, the significance of the gradient in mechanical cues, such as stiffness of extracellular matrices (ECMs), in directed migration of cells has only recently emerged. With advances in synthetic chemistry, micro-fabrication techniques, and methods to characterize mechanical properties at a length scale even smaller than a single cell, synthetic ECMs with spatially controlled stiffness have been created with variations in design parameters. Since then, the synthetic ECMs have served as platforms to study the migratory behaviors of cells in the presence of the stiffness gradient of ECM and also as scaffolds for the regeneration of tissues. In this review, we highlight recent studies in cell migration directed by the stiffness gradient, called durotaxis, and discuss the mechanisms of durotaxis. We also summarize general methods and design principles to create synthetic ECMs with the stiffness gradients and, finally, conclude by discussing current limitations and future directions of synthetic ECMs for the study of durotaxis and the scaffold for tissue engineering.