From short electrospun nanofibers to ultralight aerogels with tunable pore structure

Nanofiber production by electrospinning has made great progress within the past two decades. Yet, recently the research area was revolutionized by a novel post-processing approach. By cutting the endless and intertwined nanofibers into short pieces, it is now possible to reassemble them into interconnected 3D structures. Such highly porous structures are built from dispersed short nanofibers by freeze-casting. This solid templating process controls the structures’ ultimate properties and architecture in terms of primary and secondary pores below 5 µm and between 10 and 300 µm, respectively. The objective of this review is to provide insight into this young field of research, in particular highlighting the processing steps, materials and current applications, from scaffolds for tissue engineering, acoustics, sensors and catalyst supports to filtration

Efficient dye adsorption by highly porous nanofiber aerogels

Electrospun nanofiber membranes are frequently used in adsorption processes thanks to their high specific surface area, tailored surface functionality, and fiber uniformity. However, they are still facing challenges such as low mechanical stability and unfavorable mass transport properties. In this study, an ultra-light and robust 3D nanofiber aerogel (NFA) or nanofiber sponge with tunable porosity and flexibility was synthesized from short pullulan/polyvinyl alcohol/polyacrylic acid nanofibers using a freeze casting process followed by thermal crosslinking. We demonstrate time the application of such NFAs in batch and continuous adsorption systems and compare their performance with flat nanofiber membranes (NFM). The NFAs proved to be promising adsorbents for cationic dyes due to their high adsorption capacity (383 mg/g) and their reusability. Langmuir isotherm was a suitable model for describing the adsorption process. The endothermic system followed a pseudo second order kinetic model and intra-fiber adsorption is found to be involved in the adsorption process. Dye adsorption by 3D NFAs was four times faster than for the respective flat NFMs and when used in a continuous process as a deep-bed filter, the pressure drop through the NFA was reduced by a factor of 40 while maintaining equal adsorption performance as for the NFM

Surface enriched nanofiber mats for efficient adsorption of Cr(VI) inspired by nature

Adsorption is a surface process. By evolution, nature has created design principles such as scaffolds that allow to carrying surface bound agents at high density. We used a nanofibrous pullulan/poly(vinyl alcohol)/poly(acrylic acid) (Pul/PVA/PAA) support to carry surface active PAMAM dendrimer similar to spores attached to mushroom gills. A monolayer of ceria (CeO2) nanoparticles served as the linker between PAMAM and the nanofiber. The nanocomposite was a highly effective Cr(VI) adsorbent and the maximum adsorption capacity qmax = 847 mg g-1 is the highest reported value for the same kind of materials so far. The materials was characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Fourier transform-infrared spectroscopy (FTIR), zeta potential and multipoint BET method to measure the specific surface area. Removal of Cr(VI) from aqueous media was tested under different batch and fixed bed column operation conditions such as pH, temperature and competing ions. Thermodynamic properties were determined based on a modified Langmuir adsorption isotherm and the adsorption kinetic was investigated. Positive entropy of adsorption and an endothermic adsorption process was found, while the rate-limiting step was pseudo second order which is associated with a chemisorption process. The nanocomposite was reusable and up to 95% of the adsorbed Cr(IV) ions were recovered by alkyne washing

Nanofiber-based aerogels

Nanofiber-based aerogels or sponges are made from preformed polymeric nanofibers. They are very porous, ultralight and have a large internal surface as classical aerogels. But their network of interconnected fibers renders them also elastic and mechanically resilient. Moreover, they show a hierarchic architecture with minor primary pores between tangled nanofibers and major cell-like pores. Nanofiber aerogels can be tailored to many applications due to flexibility in the choice of polymer together with the possibility to chemically modify the surface of the fibers. Possible applications include filtration, thermal insulation, support for catalysts, or scaffolds for tissue engineering. Mostly, synthetic polymers such as PAN and PVA have been used as fiber materials or their blends with biopolymers such as pullulan and gelatin

Tailoring the microstructure of ultra-light nanofiber aerogels by solid templating and their application as wound dressing materials

Hydro active wound dressings provide a favorable wound-healing environment and promote wound closure. Currently, synthetic polymers are the material of choice for such dressings. However, capillary forces could also be envisioned as the operating principle. Therefore, we developed a bio-based material from nanofibers with tunable hierarchical pore structure. This ultralight nanofiber aerogels showed high water absorbency and it formed a mechanically stable hydrogel-like material. The range of applications of this versatile material can be extended to filtration, catalysis, or tissue engineering

Improved fiber diameter determination of nanofibers through image analysis using a hierarchical scaling approach

The determination of nanofiber diameters from SEM images is the standard procedure in characterizing the fiber morphology of electrospun materials. Typically, fiber diameter determination is done manually – a time consuming step including subjective factors of the analyst. An automated approach should save time and reduce the subjective part of the operator. Recently several algorithms have been suggested which use image pre-processing such as picture segmentation or edge detection followed by fiber detection algorithms such as radon, Hough, thinning based centerline determination and Euclidian distance transformation including corrective steps such as intersection correction and post operations such as fitting. None of the procedures is perfect and they will all strongly depend on the fiber orientation and fiber density of the analyzed SEM image. On developing our own algorithms we found that the field of view with respect to the number of fibers is in particular crucial when fiber orientation based algorithms such as Hough and radon. By selecting the appropriate field of view, a robust algorithm was developed and the results were compared with manually analyzed SEM images and results from the recent open source tool DiameterJ

Cover picture : tailoring pore structure of ultralight electrospun sponges by solid templating

Zugehöriger Artikel: https://digitalcollection.zhaw.ch/handle/11475/2062The cover picture shows the processing of nanofibers into ultralight 3D sponges or aerogels by solid templating. These sponges exhibit a hierarchical porous structure, where the major pores are replica of the growing crystals while minor pores are formed from entangled nanofibers. By rigorously controlling crystal growth the ultimate pore structure is created. Changing the size of the microstructure by one order of magnitude allowed us to change the macroscopic properties of the nanofiber based sponges by more than two orders of magnitude as exemplified for air permeability and particle adsorption. Based on the solid templating technology, we envision custom designed materials as tissue scaffolds, wound dressings, catalyst supports, suited for liquid adsorption or used in chromatographic applications