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Hydrogels can absorb a significant amount of water and swell greatly. When the swelling is constrained, hydrogels exhibit inhomogeneous deformation, stress, and water concentration fields. This paper studies the swelling behavior of a bi-layered spherical hydrogel containing a hard core after imbibing sufficient water. By adopting a continuum field theory of hydrogels, the governing differential equation of the hydrogel radial deformation is derived, which is further solved numerically at proper mixed boundary conditions and inner field continuity conditions across the hydrogel interface. Effects of material properties and the presence of the hard core on the deformation, stress, and water concentration fields of the hydrogels are examined. It shows that the inner hard core can remarkably reduce the water concentration near the core surface and simultaneously induce large stresses. In addition, the water concentration in the bi-layered hydrogel heavily depends upon the stiffness of the individual hydrogel. The study offers a rational route to design and regulate hydrogels with tailorable swelling behavior for practical applications in drug delivery, leakage blocking, etc. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to determine the threshold electrostatic field to initiate electrohydrodynamic jetting from a polymer solution droplet wetting on a conductive wire. The study is crucial to understand the roles of the material and process parameters in wire-based needleless electrospinning for controllable mass production of continuous nanofibers. Two types of polymer solutions, i.e., polyacrylonitrile/N,N-dimethylformamide (PAN/DMF) and aqueous polyethyloxide solution (PEO/H2O), with the mass concentrations of 4%, 8%, 12%, and 16% for PAN and 1%, 2%, 4%, 6%, and 8% for PEO, respectively, were considered in the experiments. Taut thin copper wires with diameters of 0.254, 0.508, 1.016, and 1.524 mm (i.e., 0.01, 0.02, 0.04, and 0.06 in.) were utilized, respectively, as the positive electrodes. The effects of the polymer concentration and wire diameter on the threshold electrical voltage for jetting initiation and nanofiber diameter were examined. Given the droplet volume and spacing between the copper wire and fiber collector, experimental observations show that the threshold electrical voltage increases with increasing either polymer mass concentration or wire diameter. In addition, the effects of polymer mass concentration on the transient shear viscosity and surface tension of the PAN/DMF solutions were examined. It shows the positive correlation between the transient shear viscosity and polymer mass concentration, shear thinning of the polymer solution at a high shear rate, and nearly constant surface tension of the polymer solution in the range of the present PAN mass concentration. Furthermore, a simple electrostatic model is formulated to phenomenologically elucidate the experimental observations. The present study provides useful scaling laws for controllable scale-up nanofiber fabrication by means of a wire-based needleless electrospinning technique. [ABSTRACT FROM AUTHOR]