Three-dimensional electrospun nanofibrous materials: Fabrication, properties, and applications

Abstract

Electrospun nanofibers, which are at the forefront of advanced fibrous materials, combine the robust mechanical strength, low density, fine flexibility, extremely high aspect ratio and ease of scalable synthesis from various materials, have been widely applied in the fields including electronics, environmental remediation, safety protections, and tissue engineering. Despite their outstanding potential, the major problem associated with electrospun nanofibers is their anisotropic lamellar deposition character, with the resultant nanofibers usually assembling into close-packed membranes (with thicknesses smaller than 100 mm) rather than into bulk three- dimensional (3D) nanofibrous materials, which have restricted their widely applications. Thus, the construction of 3D nanofibrous materials with stable structures has become a key challenge for electrospun nanofibers. Currently, researchers have successfully fabricated various 3D nanofibrous materials based on different approaches. Herein, this review summarizes the recent representative literatures on 3D construction of electrospun nanofibers by using layer-by-layer stacking, liquid- or template-assisted collection, and condition optimization. In addition, the mechanism and latest development of each 3D construction method are briefly analyzed and reviewed. However, the principle of the aforementioned methods remains the direct deposition of nanofibers; thus, the anisotropic lamellar deposition problem of electrospun fibers has not been solved. Moreover, most of these products are not real 3D-structured aerogels but rather stacks of membranes or fluffy cotton-like nanofiber deposits, which exhibit poor mechanical strength with no elastic resilience. Therefore, we subsequently summarized a novel strategy for creating fibrous, isotropically bonded elastic reconstructed aerogels with a hierarchical cellular structure and superelasticity by combining electrospun nanofibers and the fibrous freeze-shaping technique. The premise for this design is that for the first time, the intrinsically lamellar deposited electrospun nanofibers are reconstructed into 3D fibrous bulk aerogels with tunable densities and desirable shapes on a large scale. Furthermore, this fibrous aerogels exhibit the integrated properties of extremely low density (minimum of 0.12 mg/cm3), super recyclable compressibility and multifunctionality of combining the thermal insulation, sound absorption, emulsion separation and elasticity- responsive electric conduction, all originating from the synergistic effect of hierarchical cellular fibrous networks and well-bonded nanofibers. Afterwards, a brief outlook on the future development is indicated for the construction of 3D electrospun nanofibrous materials.