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Functional nanofibrous scaffolds produced by electrospinning have great potential in many biomedical applications, such as tissue engineering, wound dressing, enzyme immobilization and drug (gene) delivery. For a specific successful application, the chemical, physical and biological properties of electrospun scaffolds should be adjusted to match the environment by using a combination of multi-component compositions and fabrication techniques where electrospinning has often become a pivotal tool. The property of the nanofibrous scaffold can be further improved with innovative development in electrospinning processes, such as two-component electrospinning and in-situ mixing electrospinning. Post modifications of electrospun membranes also provide effective means to render the electrospun scaffolds with controlled anisotropy and porosity. In this article, we review the materials, techniques and post modification methods to functionalize electrospun nanofibrous scaffolds suitable for biomedical applications.

For a lamella-forming poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer (M h n PEO ) 8.7K and M h n PS ) 9.2K), the glass transition temperature of the PS blocks is 62 °C, and the melting temperature of the PEO crystals is around 51 °C when the sample is crystallized below 40 °C. The PEO blocks thus crystallize in a one-dimensionally confined lamellar space of 8.8 nm, as studied recently by one-dimensional small-angle X-ray scattering (SAXS) and transmission electron microscopy. In this report, the crystal orientation (the c-axis of the PEO crystals) within nanoscale confined lamellae has been investigated using combined two-dimensional SAXS and wide-angle X-ray scattering experiments. The c-axis orientation in the PEO crystals is observed for the first time to change from random to perpendicular, then to inclined, and finally to parallel to the lamellar surface normal, depending only on the crystallization temperature (T c ). Detailed crystallographic analyses indicate that the c-axis orientation at each T c corresponds to a uniform orientation rather than a mixture of different crystal orientations.

In-situ synchrotron small-angle X-ray scattering (SAXS) was used to follow orientation-induced crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear under isothermal conditions. The melt was subjected to a shear strain of 1428% at three different shear rates (10, 57, and 102 s-1) using a modified Linkam shear stage. The SAXS patterns showed strong meridional reflections due to the rapid development of oriented polymer crystallites within the melt. On the basis of the SAXS data, a schematic representation of nucleation and growth in orientation-induced crystallization of i-PP is proposed. During flow, orientation causes alignment of chain segments of polymer molecules and results in the formation of primary nuclei in the flow direction. These nuclei facilitate the growth of oriented crystal lamellae that align perpendicular to the flow direction. The half-time of crystallization was calculated from the time evolution profiles of the total scattered intensity. The crystallization kinetics was found to increase by 2 orders of magnitude as compared to quiescent crystallization. A method was used to deconvolute the total integrated scattered intensity into contributions arising from the isotropic and anisotropic components of the crystallized chains. The fraction of oriented crystallites was determined from the ratio of the scattered intensity due to the oriented (anisotropic) component to the total scattered intensity. At low shear rates (∼10 s-1) the oriented fraction in the polymer bulk was lower than at high shear rates (57 and 102 s-1). It was shown that only the polymer molecules above a “critical orientation molecular weight” (M*) could become oriented at a given shear rate (γ̇). The M* values at different shear rates were determined from the area fractions of the molecular weight distribution of the polymer. The observed dependence of M* on shear rate was fit to the relationship M* ∝ γ̇-α, with α being an exponent. Analysis of results suggests that the value of M* is sensitive at low shear rates (below 60 s-1) but not at high shear rates. Experimental results are shown to be in agreement with theoretical predictions having the α value of 0.15.

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▪ Abstract  Nanostructured fibrous materials have been made more readily available in large part owing to recent advances in electrospinning and related technologies, including the use of electrostatic or gas-blowing forces as well as a combination of both forces. The nonwoven structure has unique features, including interconnected pores and a very large surface-to-volume ratio, which enable such nanofibrous scaffolds to have many biomedical and industrial applications. The chemical composition of electrospun membranes can be adjusted through the use of different polymers, polymer blends, or nanocomposites made of organic or inorganic materials. In addition to the control of material composition, the processing flexibility in maneuvering physical parameters and structures, such as fiber diameter, mesh size, porosity, texture, and pattern formation, offers the capability to design electrospun scaffolds that can meet the demands of numerous practical applications. This review provides a selective description of the fabrication of nanofibrous membranes and applications with specific examples in anti-adhesion in surgery and ultrafiltration in water treatment.