Ultralight nanofiber aerogels (NFAs) or nanofiber sponges are a truly three-dimensional derivative of the intrinsically flat electrospun nanofiber mats or membranes (NFMs). Here we investigated the potential of such materials for particle or aerosol filtration because particle filtration is a major application of NFMs. Ultralight NFAs were synthesized from electrospun nanofibers using a solid-templating technique. These materials had a tunable hierarchical cellular open-pore structure. We observed high filtration efficiencies of up to 99.999% at the most penetrating particle size. By tailoring the porosity of the NFAs through the processing parameters, we were able to adjust the number of permeated particles by a factor of 1000 and the pressure drop by a factor of 9. These NFAs acted as a deep-bed filter, and they were capable of handling high dust loadings without any indication of performance loss or an increase in the pressure drop. When the face velocity was increased from 0.75 to 6 cm s, the filtration efficiency remained high within a factor of 1.1-10. Both characteristics were in contrast to the behavior of two commercial NFM particle filters, which showed significant increases in the pressure drop with the filtration time as well as a susceptibility against high face velocities by a factor of 105.

Freeze-casted nanofiber based sponges or aerogels exhibit a hierarchical porous structure. Pore formation is only partially understood. Therefore, we studied the underlying solid templating mechanism. We were able to tailor the secondary pore size between 9.5 and 123 µm while retaining the smaller primary pores known from electrospun nanofiber membranes. To understand the effect of microstructure on the sponges' bulk properties, mass flow through the pores and interaction with the sponges' internal surface were investigated. By solely altering the sponges' microstructure we indeed found tunability in permeability by a factor 7 and in filtration efficiency by a factor of 220. Hence, pore architecture of nanofiber based sponges is a key element for their performance. The selected pullulan/PVA polymer blends and aqueous electrospinning conditions are benign and allow the facile adaptation of these ultralight highly porous sponges for a large number of applications.

The 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. More information can be found in the Communication by Adlhart et al. (DOI: 10.1002/slct.201601084) (Design: F. Deuber, ZHAW, Waedenswil).