Nanofiber hierarchical structures

Techniques to produce multiscale biomaterial scaffolds with designer geometries are the need of the hour to provide improved biomimetic properties for functional tissue replacements. While micrometer fibers generate an open pore stnicture, nanofibers support cell adhesion and facilitate cell-cell interactions. This was further proven by cell penetration studies, which showed superior ingrowth of cells into hierarchical structures. Mixed bimodal scaffolds of two different polymers are another promising approach, because they exhibit hierarchical pore/ surface systems and combine the beneficial properties of both polymers at two different scales. Vaiious 3D micro- and nanoscale multiscale scaffolds have been fabricated through various techniques and were found to have the potential to essentially recreate natural bone, cardiac, neural, and vascular tissues. 

Rang, H., Ma,L, Li, C., 2012. Polyaniline-Mn02 coaxial nanofiber with hierarchical structure for high-performance supercapacitors. J. Mater. Chem. 22,16939-16942. 

Liu, Z.Q., Chen, D.Y., Wu, X., Li, N., Qiao, S.Z., 2015a. Poiypyrroie sheii 3D-Ni metai core structured eiectrodes for high-performance supercapacitors, Chem. Eur. J. 21, 4614-4621. Copyright 2015, Wiiey-VCH. (E,F) Reproduced with permission from reference Jiang, FI, Ma, J., Li, C., 2012. Poiyaniiine-Mn02 coaxiai nanofiber with hierarchical structure for high-performance supercapacitors. J. Mater. Chem. 22, 16939. Copyright 2012, Royal Society of Chemistry. 

Inspired by the hierarchical structures that enable bone function, Deng et al. recently developed a mechanically competent 3D scaffold mimicking the bone marrow cavity and the lamellar structure of bone by orienting electrospun polyphosphazene-polyester blend nanofibers in a concentric manner with an open central cavity (Figure 11.9b and c) [66]. The 3D biomimetic scaffold exhibited mechanical characteristic similar to native bone. Compressive modulus of the scaffold was found to be within the range of human trabecular bone. When tuned to have desired properties, the concentric open macrostructures of nanofibers that structurally and mechanically mimic the native bone can be a potential scaffold design for accelerated bone healing. 

The authors developed a unique form of i-glucan association, nematic ordered cellulose (NOC) that is molecularly ordered, yet noncrystalline. NOC has unique characteristics in particular, its surface properties provide with a function of tracks or scaffolds for regulated movements and fiber production of Acetobacter xylinum (=Gluconacetobacter xylinus), which produces cellulose ribbon-like nanofibers with 40-60 nm in width and moves due to the inverse force of the secretion of the fibers (Kondo et al. 2002). This review attempts to reveal the exclusive superstructure-property relationship in order to extend the usage of this nematic-ordered cellulose film as a functional template. In addition, this describes the other carbohydrate polymers with a variety of hierarchical nematic-ordered states at various scales, the so-called nano/micro hierarchical structures, which would allow development of new functional-ordered scaffolds. 

Over time, a number of possibilities have been explored to produce cellulose-based nanometric fillers. In particular, microfibrillated cellulose (MFC) (Section 6.2.1) is formed by fibrous cellulose structures with the length of several tens of microns and generally a few tens of nanometers thick, as it consists of aggregates of microfibrils, natmally occurring as an effect of the hierarchical structure of cellulose in plants [3]. In other words, MFC is formed by long, flexible and entangled cellulose nanofibers, where both amorphous and crystalline phases are present [4]. 

Recently, hierarchical nanostructured ZnO has been fabricated through electrospinning technique to enhance the photocatalytic performance. For example, Kanjwal and co-workers prepared ZnO nanobranches by electrospinning of the colloidal solution containing zinc nanoparticles, zinc acetate dehydrate, and PVA, followed by the calcination process and the hydrothermal treatment [29]. The photocatalytic activity of the hierarchical structure of ZnO is much higher than that of traditional electrospun ZnO nanofibers for the degradation of MB under UV light irradiation. 

Recently, with the developed technique, the hydrothermal synthesis has been established as an efficient way to fabricate multifunctional materials with different morphologies. Thanks to the advantages of hydrothermal synthesis, various kinds of micro-Znanofibers with versatile secondary hierarchical structures can be obtained easily. Chang designed a facile and effective strategy that combines electrospinning method with hydrothermal process to prepare firecracker-shaped ZnO/polyimide (PI) hybrid nanofibers (Fig. 18.14) [54], Firstly, PI nanofibers were prepared. In this case, the polyamic acid (PAA) solution produced from pyromellitic dianhydride... 

Nevertheless, formation of lithium dendrites and deteriorated battery performance still exist owing to uneven current distribution derived from the relatively large pore size of electrospun membranes. To construct nanofibrous separators with controllable pore structure, Zhai et al. prepared hierarchically structured Si02 nanoparticles coated polyetherimidepolyurethane (PEI-PU) composite nanofibrous membranes by combining the advantages of electrospun nanofibers with surface... 

Yuan ZY, Zhou W, Su BL (2002) Hierarchical interlinked structure of titanium oxide nanofibers. Chem Commun 1202-1203... 

A number of research groups have taken up this challenge and have developed rationally designed peptides adopting coiled coil structures that self-assemble into more complex nanostmctures. As a dominating and perhaps the most practical form of nanostructures, nanofiber assembly serves to illustrate the hierarchical molecular self-assembly possible in these systems. 

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