NANOFIBROUS STRUCTURES

Nature is an intriguing inspiration for designing several artifificial functional structures on the microand nanoscales. The capture silk of the cribellate spider Uloborus walckenaerius has a unique fifiber structure with periodic spindle-knots separated by nanofifibril joints, facilitating continuous condensation and directional collection of water drops around the spindle-knots. Spider webs are a beautiful inspiration for the textile and composite industry. Some spider webs are made up of short irregular strands of randomly arranged fifibers (cobweb), whereas others are regular (orb web). The side-by-side fifiber structure, which is composed of the orthocortex and paracortex adhering to each other, leads to the self-crimping properties of wool fifibers. Natural fifibrous structures on gecko feet enable them to stick and move on a smooth surface. The feet have a hierarchical hyperbranched-type fifibrous structure with numerous thin fifibers (10 times thinner than human hair); each thin fifiber further splits into several other fifibers of approximately 100 nm. Many attempts have been made to mimic this structure for dry adhesion. The hollow fifibers of polar bear fur, made of keratin, provide warmth. They also act as fifiber-optic transmitters that allow the capture of incident sunlight, and the heat is transferred to the black skin. Plants and trees have also inspired humankind for decades. It isinteresting to see the wide variation of fifiber morphologies in different plants. In fact, nature hasbeautifully oriented or structured arrangements of fifibers; for example, in the leaf sheaths of the coconut palm the fifibers in alternating sheets are oriented nearly perpendicular to one another, in a kind of woven structure. There are several other inspiring examples in nature regarding fifiber reinforced composites; the best examples are wood and bamboo. The plant cell walls consist of stiff cellulosic fifibrils embedded in a moisture-sensitive matrix consisting of hemicelluloses, pectin, and lignin. The anisotropic deformation (actuation) of the cell wall takes place by the absorption and desorption of moisture by the matrix. The orientation and stiffness of the cellulosic fifibrils control the effificiency and direction of bending. These types of nano- and mesostructures are the basis for artifificial actuators.

Lotus leaf and silver ragwort leaf water strider legs and duck feathers have numerous micrometer and nanometer structures providing superhydrophobicity. This inspired several of us to make water-repellent, 93self-cleaning textiles and membranes. The nanoscale structures of duck and penguin feathers are also responsible for thermal insulation.

The nanofifibrous network structure in the natural extracellular matrix (ECM) provides optimum cell binding and proliferation of cells. Hierarchical tendons and bones are made up of collagen protein. Each collagen molecule consists of three self-assembled polypeptide chains, supercoiled around a central axis to form a triple helix, which further self-assembles in both lateral and longitudinal directions into fifibrillar nanofifibers. In tendons and ligaments, multiple fifibrils make up collagen fifiber.

Thus, bioinspired artifificial fifibers and corresponding membranes, featuring precise variations in material characteristics and morphologies, are highly promising materials for water collection, smart textiles and surfaces, reinforcements in composites, actuation, fifilters, oilewater separation, etc.

In the following we will describe several artifificially made nanofifibrous structures mainly inspired from nature. There are several bottom-up and top-down approaches for making nanofifibrous structures. We will provide an insight into these methods, with the main focus on structures made by the technique of electrospinning.

Hierarchical structure of collagen in which three peptide chains arrange in the form of a triple helix, which further self-assembles into nanofifibrils and microfifibers. 

Paper link：https://www.sciencedirect.com/book/9780323512701/electrospinning-nanofabrication-and-applications