Capacitance of Flexible Polymer/Graphene Microstructures with High Mechanical Strength

Carbon-modified fibrous structures with high biocompatibility have attracted much attention due to their low cost, sustainability, abundance, and excellent electrical properties. However, some carbon-based materials possess low specific capacitance and electrochemical performance, which pose significant challenges in developing electronic microdevices. In this study, we report a microfluidic-based technique of manufacturing alginate hollow microfibers incorporated by water dispersed modified graphene (bovine serum albumin-graphene). These architectures successfully exhibited enhanced conductivity ∼20 times higher than alginate hollow microfibers without any significant change in the inner dimension of the hollow region (220.0 ± 10.0 μm) compared with pure alginate hollow microfibers. In the presence of graphene, higher specific surface permeability, active ion adsorption sites, and shorter pathways were created. These continuous ion transport networks resulted in improved electrochemical performance. The desired electrochemical properties of the microfibers make alginate/graphene hollow fibers an excellent choice for further use in the development of flexible capacitors with the potential to be used in smart health electronics.

High-resolution fabrication of nanopatterns by multistep iterative miniaturization of hot-embossed prestressed polymer films and constrained shrinking

The fabrication of nanostructures and nanopatterns is of crucial importance in microelectronics, nanofluidics, and the manufacture of biomedical devices and biosensors. However, the creation of nanopatterns by means of conventional nanofabrication techniques such as electron beam lithography is expensive and time-consuming. Here, we develop a multistep miniaturization approach using prestressed polymer films to generate nanopatterns from microscale patterns without the need of complex nanolithography methods. Prestressed polymer films have been used as a miniaturization technique to fabricate features with a smaller size than the initial imprinted features. However, the height of the imprinted features is significantly reduced after the thermal shrinking of the prestressed films due to the shape memory effect of the polymer, and as a result, the topographical features tend to disappear after shrinking. We have developed a miniaturization approach that controls the material flow and maintains the shrunken patterns by applying mechanical constraints during the shrinking process. The combination of hot embossing and constrained shrinking makes it possible to reduce the size of the initial imprinted features even to the nanoscale. The developed multistep miniaturization approach allows using the shrunken pattern as a master for a subsequent miniaturization cycle. Well-defined patterns as small as 100 nm are fabricated, showing a 10-fold reduction in size from the original master. The developed approach also allows the transfer of the shrunken polymeric patterns to a silicon substrate, which can be used as a functional substrate for many applications or directly as a master for nanoimprint lithography.

Multi-step proportional miniaturization to sub-micron dimensions using pre-stressed polymer films

The ability to define patterns and fabricate structures at the nanoscale in a scalable manner is crucial not only in integrated circuit fabrication but also in fabrication of nanofluidic devices as well as in nano and micromechanical systems. Top down fabrication at the nanoscale often involves fabrication of a master using a direct write method and then its replication using a variety of methods such as by hot embossing, nanoimprint lithography, or soft lithography. Nevertheless, fabrication of the master is a time consuming and expensive process. One interesting approach is to define patterns at larger dimensions on pre-stressed films using methods such as xurography or lithography which are scalable and heat them to de-stress and shrink which can reduce the size proportionally. Although attractive, suitable fabrication processes that can perform iterative shrinking of patterns over several cycles and into the nanoscale have not been demonstrated. Here, we demonstrate a fabrication process that is capable of accurately producing patterns and features over several cycles of miniaturization and shrinking to achieve resolution in the order of 100 s of nanometers. In this approach, a pattern transfer method is developed by combining soft imprint lithography followed by reactive ion etching, both of which are scalable processes, to transfer the original patterns into a shrinkable polymer film. The patterned shrinkable film is heated to allow thermal shrinking. As a result, the pattern size was decreased by 60% of the original size in a single cycle. This reduced pattern was used as the master for the next cycle and three cycles of miniaturization was demonstrated. Sub-micron patterns of 750 nm were generated by the multi-step miniaturization method, showing approximately 20× reduction in size of the original patterns. Finally, these patterns are transferred into features on a silicon substrate to demonstrate its application in semiconductor microfabrication or its use as a master template for microsystems applications.

Advances in Functional Polymer Nanofibers: From Spinning Fabrication Techniques to Recent Biomedical Applications

Functional polymeric micro-/nanofibers have emerged as promising materials for the construction of structures potentially useful in biomedical fields. Among all kinds of technologies to produce polymer fibers, spinning methods have gained considerable attention. Herein, we provide a recent review on advances in the design of micro- and nanofibrous platforms via spinning techniques for biomedical applications. Specifically, we emphasize electrospinning, solution blow spinning, centrifugal spinning, and microfluidic spinning approaches. We first introduce the fundamentals of these spinning methods and then highlight the potential biomedical applications of such micro- and nanostructured fibers for drug delivery, tissue engineering, regenerative medicine, disease modeling, and sensing/biosensing. Finally, we outline the current challenges and future perspectives of spinning techniques for the practical applications of polymer fibers in the biomedical field.

Multifunctional Micro/Nanoscale Fibers Based on Microfluidic Spinning Technology

Superfine multifunctional micro/nanoscale fibrous materials with high surface area and ordered structure have attracted intensive attention for widespread applications in recent years. Microfluidic spinning technology (MST) has emerged as a powerful and versatile platform because of its various advantages such as high surface-area-to-volume ratio, effective heat transfer, and enhanced reaction rate. The resultant well-defined micro/nanoscale fibers exhibit controllable compositions, advanced structures, and new physical/chemical properties. The latest developments and achievements in microfluidic spun fiber materials are summarized in terms of the underlying preparation principles, geometric configurations, and functionalization. Variously architected structures and shapes by MST, including cylindrical, grooved, flat, anisotropic, hollow, core-shell, Janus, heterogeneous, helical, and knotted fibers, are emphasized. In particular, fiber-spinning chemistry in MST for achieving functionalization of fiber materials by in situ chemical reactions inside fibers is introduced. Additionally, the applications of the fabricated functional fibers are highlighted in sensors, microactuators, photoelectric devices, flexible electronics, tissue engineering, drug delivery, and water collection. Finally, recent progress, challenges, and future perspectives are discussed.

Photo-Cross-Linked Poly(ethylene glycol) Diacrylate Hydrogels: Spherical Microparticles to Bow Tie-Shaped Microfibers

Bow tie-shaped fibers and spherical microparticles with controlled dimensions and shapes were fabricated with poly(ethylene glycol) diacrylate hydrogel utilizing hydrodynamic shear principles and a photopolymerization strategy under a microfluidic regime. Decreasing the flow rate ratio between the core and sheath fluids from 25 (50:2) to 1.25 (100:80) resulted in increasing the particles size and reducing the production rate by 357 and 86%, respectively. The width of the fibers increased by a factor of 1.4 when the flow rate ratio was reduced from 2.5 to 1 due to the decrease of the shear force at the fluid/fluid interface. The stress at break and Young's modulus of the fibers were enhanced by 32 and 63%, respectively, when the sheath-to-core flow rate ratio decreased from 100:40 to 100:80. The fiber fabrication was simulated using the finite element method, and the numerical and experimental results were in agreement. Adult hippocampal stem/progenitor cells and bone-marrow-derived multipotent mesenchymal stromal cells were seeded onto the fibrous scaffolds in vitro, and cellular adhesion, proliferation, and differentiation were investigated. Microgrooves on the fibers' surface were shown to positively affect cell adhesion when compared to flat fibers and planar controls.

Characterization of Chlorella vulgaris and Chlorella protothecoides using multi-pixel photon counters in a 3D focusing optofluidic system

A new type of multi-pixel photon counter was employed to miniaturize the device, lower its power consumption, and make it insensitive to magnetic fields.

Polycaprolactone Microfibrous Scaffolds to Navigate Neural Stem Cells

Fibrous scaffolds have shown promise in tissue engineering due to their ability to improve cell alignment and migration. In this paper, poly(ε-caprolactone) (PCL) fibers are fabricated in different sizes using a microfluidic platform. By using this approach, we demonstrated considerable flexibility in ability to control the size of the fibers. It was shown that the average diameter of the fibers was obtained in the range of 2.6-36.5 μm by selecting the PCL solution flow rate from 1 to 5 μL min^-1 and the sheath flow rate from 20 to 400 μL min^-1 in the microfluidic channel. The microfibers were used to create 3D microenvironments in order to investigate growth and differentiation of adult hippocampal stem/progenitor cells (AHPCs) in vitro. The results indicated that the 3D topography of the PCL substrates, along with chemical (extracellular matrix) guidance cues supported the adhesion, survival, and differentiation of the AHPCs. Additionally, it was found that the cell deviation angle for 44-66% of cells on different types of fibers was less than 10°. This reveals the functionality of PCL fibrous scaffolds for cell alignment important in applications such as reconnecting serious nerve injuries and guiding the direction of axon growth as well as regenerating blood vessels, tendons, and muscle tissue.

Designing highly structured polycaprolactone fibers using microfluidics

Microfibers are becoming increasingly important for biomedical applications such as regenerative medicine and tissue engineering. We have used a microfluidic approach to create polycaprolactone (PCL) microfibers in a controlled manner. Through the variations of the sheath fluid flow rate and PCL concentration in the core solution, the morphology of the microfibers and their cross-sections can be tuned. The microfibers were made using PCL concentrations of 2%, 5%, and 8% in the core fluid with a wide range of sheath-to-core flow rate ratios from 120:5µL/min to 10:5µL/min, respectively. The results revealed that the mechanical properties of the PCL microfibers made using microfluidic approach were significantly improved compared to the PCL microfibers made by other fiber fabrication methods. Additionally, it was demonstrated that by decreasing the flow rate ratio and increasing the PCL concentration, the size of the microfiber could be increased. Varying the sheath-to-core flow rate ratios from 40:5 to 10:5, the tensile stress at break, the tensile strain at break, and the Young׳s modulus were enhanced from 24.51MPa to 77.07MPa, 567% to 1420%, and 247.25MPa to 539.70MPa, respectively. The porosity and roughness of microfiber decreased when the PCL concentration increased from 2% to 8%, whereas changing the flow rate ratio did not have considerable impact on the microfiber roughness.

Mechanical and physical properties of poly(vinyl alcohol) microfibers fabricated by a microfluidic approach

A microfluidic platform was used to fabricate polyvinyl alcohol microfibers with various morphology and mechanical properties.