TECHNICAL FIELD

The present invention describes a fibre network, comprising a stack of fibre network layers each comprising a randomly or stochastically distributed at least one fibre with fibre thickness distributed around a mean fibre diameter, fixed at a multiplicity of cross-links along the at least one fibre, forming a multiplicity of fibre segments with average fibre segment length, produced by a random production process, wherein the fibre network shows auxetic behaviour, use of such fibre networks and a manipulation method of a fibre mat, comprising a stack of fibre network layers of randomly distributed at least one fibre with fibre thickness distributed around a mean fibre diameter, which is fixed at a multiplicity of cross-links along the at least one fibre, forming fibre segments with an average fibre segment length, produced by an electrospinning process or melting process showing auxetic behaviour, allowing control of stable pore size, pore shape, overall porosity, mat thickness or mat volume.

STATE OF THE ART

Fibre meshes and related applications are known, e.g ., produced by electrospinning, wherein volume, pore size and porosity can be changed. The retention rate, as well as the air and water permeability, as main filter properties, are influenced by the size and distribution of pores within the network of the fibre mesh. The here interesting meshes comprise multiple layers of fibres, stacked in a z-direction . The elastic or elastoplastic fibres are connected at multiple bonding points in a perpendicular x-y plane, building a grid structure in 3D. The fibre- thickness varies in particular between lOnm and 10 prn, especially if the meshes are produced by electrospinning . Electrospinning presents one referred manufacturing technique, since it allows creating fibre meshes with fibres and pores of length-scales relevant for a variety of technical applications. Electrospinning is a simple, cost-efficient and versatile method to produce advanced materials consisting of ultrathin fibres from a range of materials. The total thicknesses of the final as-spun (i.e. without further treatment) mats are in the range of 10 prn to 1 mm. Large scale electrospinning is possible on the meter range as in-plane dimension, however, the in- plane dimension for the proposed applications is most preferred in the mm to 50 cm range.

Applications are, for example, filters, textiles and scaffolds for tissue engineering, Fang, J., et al., Applications of electrospun nanofibers. Chinese Science Bulletin, 2008. 53(15) : p. 2265-2286. Pore size and pore shape play an important role in all of these mentioned applications, and several approaches have been reported to modify and control these parameters. Controlled porosity is also an important property of breathable textiles, as larger pore size leads to increased breathability. The quality and success of scaffold structures in tissue engineering are assessed by the cell seeding efficiency and subsequent cell spreading and proliferation in the scaffold . Cell infiltration is facilitated by high and interconnected porosity, and one of the main challenges for electrospun materials in biomedical applications is, indeed, the lack of colonialization due to small pore dimensions.

Since pore size generally increases with fibre diameter, larger pores can be obtained by changing electrospinning parameters that affect the latter, e.g. the viscosity of the solution, the applied voltage and the distance between syringe and collector, Joshi, V.S., et al., Macroporosity enhances vascularization of electrospun scaffolds. Journal of Surgical Research, 2013. 183(1) : p. 18-26. Due to this interdependence, obtaining scaffolds with nanoscale fibres and large pores is challenging, Vaquette, C. and J.J. Cooper-White, Increasing electrospun scaffold pore size with tailored collectors for improved cell penetration. Acta Biomaterialia, 2011. 7(6) : p. 2544-2557. Therefore, different techniques have been suggested to address this problem and to increase pore size during or after the electrospinning process.

One strategy to obtain larger pores is based on modified, usually non- continuous collectors. For example, patterned collectors were used by Vaquette & Copper-White and metal wire meshes were applied as collectors. The obtained pore-size is increased with larger space between the steel wires. A similar principle was used by Zhu et al . Electrospun fibrous mats with high porosity as potential scaffolds for skin tissue engineering. Biomacromolecules, 2008. 9(7) : p. 1795-1801, who collected fibres on a slowly rotating frame cylinder with metal struts, and thereby obtained larger pores compared to a plate collector or higher rotation speeds.

Blakeney et al., Cell infiltration and growth in a low density, uncompressed three-dimensional electrospun nanofibrous scaffold. Biomaterials, 2011. 32(6) : p. 1583-1590, produced highly porous scaffolds by collecting fibres within a non-conductive hemispherical dish equipped with conductive needle-shaped metal probes. Similarly, Bowlin, G., Enhanced porosity without compromising structural integrity: the nemesis of electrospun scaffolding. Journal of Tissue Science & Engineering, 2011. 2 : p. 103e, used a hollow mandrel as collector with a distribution of holes to allow pressurized air to expel through these holes preventing compaction of fibres during their deposition and disrupting the mesh. Other authors used a liquid bath to collect fibres. In a method termed "hydrospinning" [Tzezana, R., E. Zussman, and S. Levenberg, A Layered Ultra-Porous Scaffold for Tissue Engineering, Created via a Hydrospinning Method. Tissue Engineering Part C-Methods, 2008. 14(4) : p. 281-288.], fibres are spun onto an aqueous bath containing NaCI, and are collected in fixed intervals on a glass slide to form a stack of unconnected fibre sheets. After desiccation in vacuum, the evacuating water increased the porosity drastically compared to scaffolds obtained by classical electrospinning.

Similarly, Coburn et al. [Coburn, J., et al., Biomimetics of the extracellular matrix: an integrated three-dimensional fiber-hydrogel composite for cartilage tissue engineering . Smart Structures and Systems, 2011. 7(3) : p. 213-222] spun onto a 9 : 1 ethanol/water solution, froze and vacuum dried fibres to obtain scaffolds with high porosity.

Lee et al. [Lee, J. B., et al., Highly porous electrospun nanofibers enhanced by ultrasonication for improved cellular infiltration. Tissue Eng Part A, 2011. 17(21-22) : p. 2695-702], for example, applied ultrasonication to fibre meshes that were pre-wetted with ethanol and immersed in distilled water. Shim et al . [Shim, I. K., et al ., Novel three- dimensional scaffolds of poly((L)-lactic acid) microfibers using electrospinning and mechanical expansion : fabrication and bone regeneration. Journal of Biomedical Materials Research Part B-Applied Biomaterials, 2010. 95b(l) : p. 150-160] applied a metal comb to brush flat PLLA networks into three-dimensional mats with high porosity. Another common possibility to modify the pore size is to add sacrificial material during the spinning process which can be removed afterwards to generate void spaces. Several authors investigated this method for sodium chloride (NaCI) particles deposited during electrospinning and leached out later. Sacrificial fibres can be added by use of multi-jet electrospinning, which has been studied intensively. Ice as a sacrificial material is used in low-temperature or cryogenic electrospinning techniques. During electrospinning under humidity control onto a cooled mandrel ice crystals form between the depositing fibres, which leave large pores when removed. Simonet et al. [Simonet, M ., et al ., Ultraporous 3D polymer meshes by low-temperature electrospinning : Use of ice crystals as a removable void template. Polymer Engineering and Science, 2007. 47(12) : p. 2020-2026], for example, report an achievable four-fold increase in porosity. Ki et al. [Ki, C.S., et al., Development of 3-D nanofibrous fibroin scaffold with high porosity by electrospinning : implications for bone regeneration. Biotechnology Letters, 2008. 30(3) : p. 405-410] combined the use of NaCI as porogen with a dispersion of fibre obtained by spinning into a liquid bath, which was then stabilised and lyophilised to obtain nanofibrous fibroin foams with large pores.

All above reported methods adjust the pore size within the manufacturing process or a subsequent complex and/or expensive following treatment step, wherein the pore size and shape of the material is fixed for further applications.

As an exception, Kerr-Phillips et al . [Kerr-Phillips, T. E., et al ., Electrospun rubber fibre mats with electrochemically controllable pore sizes. Journal of Materials Chemistry B, 2015. 3(20) : p. 4249-4258] proposed electroactive fibre mats by swelling of rubbery electrospun fibre networks in EDOT, which was polymerized to PEDOT by oxidation. By immersion of the electroactive mats in an electrolyte, the pore size could be modified reversibly by control of the electrical field . Pore size variations of 5% in phosphate buffered saline and 25% in lithium bis- trifluoromethanesulfonimide were shown. The restriction of that method is that the material has to be immersed in an electrolyte and an electric field needs to be applied which complicates its use in most applications.

Another method that allows changing the microstructure and, with this, pore size and porosity, is based on the use of shape memory polymers to produce fibres. Such networks were suggested as supporting sleeves to stabilize bone-defects for example in WO2014205306 Al; the bone defects in these applications were filled by shape memory foams that expand to their programmed shape after heating .

The controllability of porosity, pore size and pore volume with existing techniques so far is largely restricted to the manufacturing process, an immediate post-treatment by using chemicals or energy, or it additionally requires electroactive properties of the fibres, which clearly implies restrictions on fibre material and the range of applications.

DESCRIPTION OF THE INVENTION

The object of the present invention is to generate a fibre mat or fibrous fabric with increased volume and porosity on demand, comprising a stack of fibre network layers produced by an electrospinning or other processes with fibre thickness distributed around a mean fibre diameter, which is fixed at a multiplicity of crosslinks along the at least one fibre, forming fibre segments with average fibre segment length, which can be produced more simplified and in a more cost-efficient way. The fibre mats should be produced without use of toxic chemicals, ultrasound, brush technique, application of external electric fields, and methods with high energy consumption. Another object of the invention was to enable various uses, claims 7 to 12, of such improved fibre mats, due to the novel manipulation method of the fibre mats, as claimed in claims 13 to 17. The disclosed method is based on stretch-expansion of fibrous networks to increase their thickness, volume, porosity and pore-size, to adapt the fibrous networks to related applications. The method and related applications apply to a variety of non-woven meshes and fibrous mats produced in various processes, but especially produced by electro-spinning .

The method proposed here mainly makes use of an auxetic effect that astonishingly occurs in electrospun networks and other fibrous materials with similar aspect ratios between fibre diameter and length of the fibre segments, as length between bonding points of the fibres. The presence of this particular auxetic effect as an intrinsic property of electrospun networks has not been reported before.

Auxetic behaviour, i.e. an expansion of material in a direction perpendicular to the axis of elongation, can be elicited by structuring sheets or layers of a material, including electrospun mats, on a larger scale.

In addition to applications as filter devices, scaffolds for tissue engineering and drug release devices, electrospun networks and other fibre network materials with on-demand porosity and volume change could improve or replace other solutions in several applications.

It can be used as filler material to fill gaps; the mentioned filling of bone defects with an expandable shape memory foam as known from WO2014205306. The increased volume taken by the stretch-expanded mesh here can be used to occlude a lumen or provide a filter within the lumen to collect solid particles. In cardiovascular surgery, these operations are performed by occlusion devices and embolic filters, respectively. The former are typically realized by a deployable frame, unfolded by a mechanism, and covered by some sort of thin material layer that acts as an occluding membrane. In patent applications WO2011153304 and US 20150257763 Al, for example, electrospun meshes are used to provide this function. Embolic filters need to be permeable for blood while retaining particles such as emboli. In patent applications US2004153117A1, US2004153118A1, US2004153119A1 such filters are described, where electrospun non-wovens were anticipated as one filter material (among many others) that could be used to reach desired pore sizes of 30 to 500 prn. The auxetic, stretch-expandable fibre networks disclosed herein may represent a simple and cost- efficient alternative with comparable filter properties.

Different methods, e.g . catheters and actuators were described to place and activate the filter, but none of them made use of a self- expanding mechanisms of the non-woven material itself. Finally, the characteristic to change pore size and porosity by application of mechanical loads entails the use as filters with modifiable filter properties.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred exemplary embodiment of the subject matter of the invention is described below in conjunction with the attached drawings.

Figure 1 a) shows a top view and cross-sections of unexpanded fibre mesh, while b) shows a top view and cross-sections of an expanded fibre mesh, in form of finite element (FE) models of electrospun networks, while c) shows two cross-sections of fibre meshes after uniaxial extension with lateral contraction load . d) shows a schematic view of a fibre section before extension, while

e) shows a schematic view of the buckled fibre section after the extension process.

Figure 2 shows a diagram with normalised out-of-plane dimension versus in-plane orientation angle φ of a fibre after the stretch expansion. Figure 3 Influence of segment length (normalised by fibre diameter) on (a) thickness increase (auxetic effect) and computed volume change (b) of stretch-expandable electrospun material at 10 % applied extension in x- direction computed with two different measures of volume.

Figures 4a) and 4b) are showing photographs illustrating the thickness change of an electrospun PLLA sample before and after uniaxial tensile stretching .

Figures 5 showing sketches illustrating the application of stretch expandable fibrous mesh, to fill gaps and cavities (a,b) or as deployable filters within a lumen (c). Figures 6 showing sketches illustrating the application of stretch expandable fibrous mesh, as porous structure with changeable permeability (a), filter with adjustable selectivity (b,c), or as on-demand drug-release material. Figures 7 showing schematic drawings of methods to overcome limitations related to the reduction of width (y-direction) upon extension in x-direction.

a) Placement of curved, rolled or folded structures.

b) Stepwise placement and expansion of strips one after each other.

c) Sketch of a multi-layered structure to compensate the reduced width of single layers.

DESCRIPTION

Fibre networks, fibre meshes or fibrous mats and related applications are disclosed that are produced, e.g., by electrospinning or melt spinning of polymer, and for which astonishingly thickness, volume, pore size and porosity can be changed on demand by simple mechanical stimulus. The fibre network comprises at least one fibre network layer, but to reach a higher volume expansion effect, the fibre network should comprise a multiplicity of fibre network layers in a stack of such layer. Each fibre network layer can consist of one or more fibres. Even a multiplicity of fibre network layers can comprise at least one fibre. The different fibre network layers can be entangled, overlap or be connected at different points with each other or are not interconnected. The fibre network abdicates periodic structures like periodic lattices or repetitive unit cells and comprises solely randomly or stochastically non-periodic and non-regular three-dimensional fibrous structures formed by at least one fibre forming the fibre network with multiplicity of cross-links along the fibre or fibres.

The total pore volume v _p is defined as the total volume of the network minus the volume of the fibres.

Porosity p is defined as total pore volume v _p by total volume v of a fibrous network.

V V

We show here that the pore size, porosity, overall thickness and overall volume of fibrous meshes produced such that the fibre segments between interaction points are beyond a critical length-to- diameter ratio, can be changed on demand by application of a tension in one direction. A cross-link is understood as a point where one fibre interacts with one or several other fibres or parts of the same fibre in a way that at least some of the displacement and rotation degrees of freedom of the first fibre are partly or entirely coupled to the degrees of freedom of the other fibres. At cross-links the fibres or parts of the fibre are thus fixed . The fixation may be permanent or temporary for the time at which stretch expansion occurs. The length of the fibre segments between two cross-links is defined by l _s . Extension (of the fibre mat) is understood as an increase of length by application of an external loading, such as an applied force or prescribed displacement at the boundaries. Expansion (of the fibre mat) is understood as an increase of thickness and overall volume. The stretch of a fibre mat is defined as the ratio between new and original length of an original fibre mat and an expanded fibre mat, wherein the tensile stretch occurs in direction of elongation.

Pore size, volume, porosity and thickness of fibrous networks and non- wovens, made of different materials, can be adapted, wherein the most critical parameter, that characterises stretch expandable fibre networks is the ratio (aspect ratio) between fibre segment length l _s , i.e. the length of the fibre segments between two cross-links, and the fibre diameter d. In order to obtain a significant increase in total volume of the network, and thereby pore volume and porosity, the network needs to contain fibre segments with an aspect ratio of l _s / d ≥ 5.

Due to the fact, that the networks are statistical, there will always be segments with lower ratios, but here the majority of segments have to have a ratio l _s f d > 5. Flat and dense fibre mats thereby turn into more spatial structures with large void space. The underlying mechanism is a structural rather than material property, and does hence impose little restrictions on the fibre material. This is particularly important in terms of biomedical applications, where this method to expand fibre meshes and increase porosity can be applied to materials that have already been approved for biomedical use.

Fig . la) shows a fibre mesh in original shape, unexpanded shape in a top view in z-direction and two cross-sections. After a stretch- expansion the fibre mat is shaped as depicted in Fig. lb) view in z- direction and cross-sections. A network stretched in x-direction is depicted . Here, the thickness d of the fibres is homogeneous, but fibres may also have a distribution in diameter d and change their diameter d slightly, at least when extended.

Due to the specific mechanism in stretch-expandable materials the shape of a single fibre, along the fibre after the expansion, depends strongly on its in-plane orientation. While fibres inclined towards the direction of expansion (Fig . lb) elongate and keep their in-plane structure, fibres inclined towards the direction that is perpendicular to the axis of loading are compressed due to the global lateral contraction of the network. Some fibres, respectively some fibre segments buckle in the out-of-plane z-direction. The angle φ defines the initial angle of a fibre relative to the direction of extension.

The achievable increase in thickness and volume of an expanded fibre mesh depends on the aspect ratio Is/d . For fibre networks that contain fibre segments with an aspect ratio of l _s J ά ≥ 5 , a thickness increase (in z-direction) of at least 40% and a volume increase by at least 50% can be achieved for 10% extension. A thickness and volume increase could be found after extension of 1% and more. Numerical simulations of the process show the clear dependence of the fibres' out-of-plane dimension after expansion (the distance between the lowest and highest z-coordinate of a fibre) on their in-plane orientation relative to the direction of expansion denoted by the angle Φ-

This leads to a characteristic structure of the stretch-expanded fibre mesh as shown in the sketch (Figure lc), that distinguishes the network from others whose porosity has been increased by other methods.

Fig. lc) shows a few example fibres in y-z and x-z cross-sectional views. The buckling of fibres respectively fibre segments in z-direction can be seen, leading to a special distribution of the buckled fibres. Due to the stretch expansion, all fibres stay damagefree. Stretch expansion leaves the network integrity and all fibres largely undamaged, avoids disruption of crosslinks at least partly breakage of fibres and elicits only marginal changes in fibre diameter and cross section. Due to the buckling of the fibre segments of different fibre layers, the fibre density decreases from a fibrous network core in -z and +z direction, which is visible for the outermost regions. Due to the buckling a core with higher fibre density forms, whereas at the lower and upper boundaries (-z and +z direction) the fibre density is lower. The buckling leads to an increase of the pores and of the total fibre mesh volume. Because the buckling cannot be reached by prior art methods, the shape of fibres in the resulting mesh and the structures of the resulting mesh are unique. In Figure Id) fibres of an electrospun network are depicted, wherein between two cross-links along a fibre, a fibre segment s is depicted, showing the fibre segment length Is. This fibre segment s lies substantially in the x-y-plane, while the angle φ, between the later extension direction x and fibre segment direction is shown. After the extension process of the fibre mat in x-direction, due to an external loading, the buckling, leading to a buckled fibre segment bs is shown, while the elongation of the former fibre segment is shown in dotted lines. Networks with long fibre segments s display substantial volume increase already for remarkably small longitudinal extensions.

To show the dependency between buckling and orientation of the fibres respectively fibre segments, Fig . 2 shows simulated values of angle φ versus the normalised out-of-plane dimension of the fibres.

We found :

• an auxetic effect in random fibrous materials such as electrospun networks, caused by out-of-plane fibre deflection due to buckling, allowing for stretch-expansion of these materials;

• the corresponding process of increasing thickness, volume and porosity of, e.g., electrospun membranes and, potentially, other non- wovens with adequate ratios between fibre segment length and diameter;

· the application of stretch-expandable, e.g ., electrospun networks to fill gaps and cavities

• the application of stretch-expandable, e.g ., electrospun networks as occlusion devices, e.g . for application in vascular surgery;

• the application of stretch-expandable, e.g ., electrospun networks as part of embolic filter devices

• the application of stretch-expandable, e.g ., electrospun networks to control flux and the dwell-time of a fluid travelling through the network;

• the application of stretch-expandable, e.g . electrospun, networks as filters and devices for separation of gas/liquid, gas/solid, liquid/solid, gas/liquid/solid mixtures with filter/separation properties adjustable by mechanical loads; • the application of stretch-expandable fibrous, e.g. electrospun, networks to create absorbent materials

• the use of stretch-expanded networks as scaffold for tissue engineering applications with beneficial properties for improved cell infiltration and proliferation due to increased porosity and pore size;

• structures consisting of the arrangement of several stretch- expandable parts or layers to realize a specific three-dimensional shape after expansion. The occurrence of the effect and the underlying basic mechanisms that enable this method were identified from FE models of electrospun networks. Fibre positions and orientations as well as fibre shapes and diameters used in these simulations mimic the real structure of electrospun materials. The model allows to investigate the response of representative volume elements of the material to macroscopic loads applied at the boundary.

Uniaxial extension with lateral contraction that leads to a macroscopically uniaxial state of stress revealed lateral contraction within the plane and a marked expansion in the out-of-plane (z) direction.

Lateral contraction, thickening and overall volume expansion entail a change in pore size, pore shape and overall porosity of the structure. Moreover, numerical investigations showed that a negative Poisson's ratio can be modified by changing the initial porosity or, equivalently, the free length (segment length) between crosslinks/interaction points/ bonding points of the network. Higher mean free lengths Is between crosslinks lead to an increased auxetic effect (Fig. 3a) and higher volume gain (Fig. 3b). The latter was determined by forming hulls around the deformed fibre network and computing the enclosed volume. Volume changes were estimated based on the convex hull (estimate of upper bound) and, as a more conservative measure, a compact boundary tightly enveloping the fibres (estimate of lower bound).

Both measures confirm that a doubling of the volume for a moderate longitudinal extension of 10% is easily achievable.

Depending on the mechanical properties of the single fibres both irreversibly and reversibly stretch-expandable materials are conceivable. Using a polymer with a small elastic range and hence, plastic deformations within the regime of stretch expansion leads to an irreversible increase of volume. The use of elastic polymers without a dedicated plastic zone allows for reversibility of the stretch expansion, and recovery of the initial, unexpanded configuration by unloading . Experiments confirmed the effects observed in computer simulations both in terms of the expansion caused by longitudinal stretching, and also as regards the dependence of the effect on the segment length. Samples of electrospun poly-(L)-lactide mats with high porosity (around 95%) and consequently a high segment length were used to perform uniaxial tension tests. Other preferred materials are thermoplastics in general, for example thermoplastic polyolefins, polyurethane or aliphatic polyamides like Nylon.

The thickness of the mats before and after stretching differed by a factor of 3-4, as depicted in Figures 4a and 4b. Here an electrospun PLLA sample was measured. Conversely, tests on denser types of mats with much lower porosity and consequently lower mean segment lengths Is showed no change in thickness visible by eye. Fibre networks can be achieved, wherein the bending stiffness of the fibre network can be adjusted on demand reversibly or irreversibly by a change of the network thickness. With the here described fibre networks or mats, stretch-expansion of the fibre networks due to out-of-plane deflection of fibres towards a z- direction, perpendicular to the axis of loading, due to buckling of fibre segments, can be reached . While fibre thickness d stays unchanged or almost unchanged, in order to reach a fibre mat expanded in z- direction with a ratio between the relative increase of thickness of the fibre mat in z-direction and the relative increase of length of the fibre mat preferably higher than 25 more preferably higher than 5 or even more preferably higher than 2 is possible.

Several applications will make use of the invention. With all these advantages, it should be considered that the production of electrospun material is a low-cost process, leading to low manufacturing costs. Possible applications are: i) Filling of gaps and cavities

Stretch expandable fibrous materials, such as electrospun networks, can be used to fill gaps and longish cavities by placing the unexpanded strip in place and expand it by longitudinal extension (Fig . 5).

Thereby, the strip can either be placed entirely within the lumen of the cavity (Fig. 5a) or protrude, so that after expansion, the material is locked within the gap (Fig. 5b). Potential biomedical applications are self-locking wound covers or swabs, e.g . to be placed between teeth in dental medicine. ii) Deployable filters and occlusion devices

When placed into a stream and deployed by axial extension, the expanded material might provide a barrier for particles but provides fluid flow through the porous structure (5c). This could be used for embolic filters that collect particles from the blood stream within artery, e.g ., during an up-stream surgery. Furthermore, hydrophobicity/-philicity of the fibre material may be used to affect liquid penetration in the latter case, in order to realise occlusion of a vessel in case of a vascular accident. Typically such devices consist of multi-part structures comprising coils, threads and tubes, cf. e.g ., EP2575637, US2015257763. iii) Adaptable throttle/duration of dwell time of fluid in the network Upon expansion of the material, the porosity increases, and entails an increase of permeability. At a given constant fluid pressure, this leads to higher fluid flux through the fibre mesh (Fig. 6a). On the other hand, the fluid has to travel a longer distance to pass the expanded material; this may affect the effective dwell time of the fluid within the network material and may thus be used to control interactions, e.g ., if the network material carries catalysts. iv) Nano- and microfilters, and semipermeable membranes with adjustable selectivity/separation properties

The increase of porosity and pore size with expansion will affect filter properties. The material's filter efficiency for particles of a certain size can thus be changed by expansion. By this means, the filter can be activated/deactivated for all particles (Fig. 6b), or its cut-off value can be changed to larger particles (Fig. 6c). The possibility to increase porosity on demand might be beneficial for back washing of filters: Temporarily increasing pore size and raising the fluid flux during flow reversal will not only disrupt the filter cake but the higher fluid velocities generally lead to higher pressures and increased shear stress on material clogging the filter. Likewise, the retention of liquid in a liquid/gas mixture could be controlled by the applied mechanical load, e.g . for applications to breathable textiles. v) Materials with on-demand drug release The increase of porosity and pore size with extension will affect the rate by which particles or microcapsules entrapped in the pores will be released. In addition to the direct effect, i.e. the release by opening pores, the higher porosity will affect the hydrodynamic conditions in a liquid environment and thus additionally favour the release of particles by hydrodynamic forces. Increased flux through the porous network would also affect the rate by which drugs embedded into the electrospun fibres will be released. vi) Scaffolds for tissue engineering

Direct use of the volume expansion effect can be made for scaffolds in tissue engineering applications, where low porosity and small pore sizes are a main restriction for cellular infiltration and propagation into electrospun networks. Compared to other methods such as cryoelectrospinning, spinning onto liquids and needles, or ultrasonication, this method allows to transform standard electrospun mats (with segment-to-diameter ratios in an appropriate range) into more three-dimensional structures with larger pores. vii) Absorbent materials

The increase of porosity may lead to a higher uptake of liquid . The on- demand activation of this property by expansion allows a dense packing of the absorbent material, e.g. for transport purposes. We have here shown by numerical simulations and confirmed by experiments that fibre networks with appropriate initial porosity can be stretch-expanded when subjected to load cases that allow for lateral contraction. Since the porosity increases with overall volume of the mesh, this simple mechanism allows changing volume and porosity at any time on demand by simple extension of the material. The computations and experiments suggest an achievable gain in volume by factors of above 1.5, in particular greater than 2 and between 2 and 4. The proposed method to change pore size, pore shape and overall volume of fibrous meshes on demand requires an extension of the fibre mesh in one direction that leads to a decrease of the lateral in- plane dimension and thus causes buckling of fibre segments.

Preferred materials of the fibres are biodegradable and bioactive thermoplastic aliphatic polyester like Poly-L-Lactid (PLLA), Poly-D- Lactid (PDLA) or Poly-(L-co-D/L-Lactid) (PLDLLA).

For optimum pore size adaptation the following are observed :

(i) It requires that the ends of the mesh sample are accessible during the application and can be displaced in order to increase the length albeit by a few percent.

At least one end must be accessible if the other end is fixed .

(ii) The effect is favoured by sample shapes with high aspect ratio, i.e. a high ratio between length and width, which possibly restricts the shapes that can be used in applications.

(iii) While length, thickness and the overall mesh volume increase, the (in-plane) width of the sample is reduced .

All limitations, but particularly (iii) may be circumvented by the following methods:

Unexpanded material may be placed in a curved/rolled state (Fig . 7a), or gaps may be filled step-wise, such that strips are positioned and expanded consecutively (Fig. 7b). Finally, multilayered structures of network strips that overlap before extension, and fit into each other afterwards, may allow obtaining an overall increase of volume at a nearly constant width. A sketch of an example structure is given in Fig. 7c. Spinning is a manufacturing process for creating polymer fibres. It is a specialized form of extrusion that uses a spinneret to form multiple continuous filaments. There are many types of spinning : wet, dry, dry jet-wet, melt, gel, and electrospinning. Electrospinning uses an electrical charge to draw very fine (typically on the micro or nano scale) fibres from a liquid - either a polymer solution or a polymer melt. Electrospinning shares characteristics of both electrospraying and conventional solution dry spinning of fibres. The process does not require the use of coagulation chemistry or high temperatures to produce solid threads from solution. This makes the process particularly suited to the production of fibres using large and complex molecules. Melt electrospinning is also practiced; this method ensures that no solvent can be carried over into the final product.