Fibrous network materials are ubiquitous in nature and used in a wide range of industrial, engineering and medical applications. The many reasons for this broad use include their light-weight properties and their porous structure which allows fluids and particles to enter, interact, pass or be retained. A major limitation of these materials is that once they have been produced, their structure is typically static so that porosity and pore size, and thus all the associated physical properties, can hardly be altered on-demand at a later stage. Another limitation, that applies, e.g., to classic electrospinning, is that the production process itself limits the achievable pore sizes so that either complex process modifications or secondary treatments are required to generate larger pores.
Empa researchers present auxetic fibre networks with particular microstructure: they counter-intuitively increase their thickness on demand when stretched along a certain direction and thereby multiply their volume. The volume gain entails a change of dimensions, porosity, pore size and pore shape, all of which affect the transport properties through the network. Electrospinning can be used to produce such a network from a wide range of base materials.
Domaschke S, Morel A, Fortunato G, Ehret AE, Random auxetic from buckling fibre networks. Nat. Commun. 10(2019):4863.
Most materials contract laterally when stretched along one direction. Strikingly, the here described sheet-like network structures feature the unique characteristic of expanding out-of-plane to an enormous extent when moderately stretched along an in-plane direction. The effect is very pronounced and can lead to a multiplication of thickness and volume, with according changes of pore size and overall porosity. While this pronounced auxetic effect is due to the network structure, and little affected by the fibre material properties, the latter have a strong impact on the reversibility of the stretch expansion, i.e., on the extent of expansion that remains after load removal. Electrospinning, being simple and cost-efficient, is the preferred process for producing a wide range of such fibre networks as large sheets of non-wovens from a variety of materials.
The pronounced changes in dimension and volume on-demand, that rapidly generate a bulky piece out of a thin strip of material, call for many fields of application ().
Potential medical and filtration applications include
tissue engineering scaffolds with enhanced infiltration of cells
space-saving absorbent materials activated by stretching
materials with on-demand release of entrapped particles or embedded drugs
filter materials insertable into small or poorly accessible lumina
adjustable filters with changeable permeability
The dimensional changes themselves can be employed to fill gaps by expanding strips of material after placing them in the space to be filled. The increase in porosity is beneficial for infiltration of cells, and may be used to overcome current limitations in the use of electrospun networks as scaffolds for tissue engineering. Activation of this porosity gain through stretching only when required suggests the use of these networks as space-saving absorbent materials. The controllable change in pore size may also serve to release drugs from the network, either by unleashing entrapped particles, or through changes in the hydrodynamic and diffusive properties that lead to an increased interaction with the surrounding fluid. Filter products are one of the most common application of fibrous non-wovens. The change of porosity on demand influences the filter properties such as permeability and the critical size of particles that can pass to the filtrate, and thus suggest adjustable filters for specific needs. The large change of overall dimensions may furthermore serve to deploy filters in small lumina.