WO2013026829A1

WO2013026829A1 - Electrokinetic membranes

Info

Publication number: WO2013026829A1
Application number: PCT/EP2012/066202
Authority: WO
Inventors: Trond Heldal
Original assignee: Osmotex Ag
Priority date: 2011-08-19
Filing date: 2012-08-20
Publication date: 2013-02-28
Also published as: GB2494043A; GB2494043B; GB201214803D0

Abstract

An electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, and the porous layer having coatings which at least partly form said pore walls. An electroosmotic liquid transport membrane, the membrane being flexible and having a porous layer with pores defined by pore walls made of inorganic material, and the porous layer being coated on opposite surfaces thereof with conductive coatings which are porous. An electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, and optionally wherein the porous layer has a web like structure, a foamy structure, or a structure with cylindrical pores.

Description

Electrokinetic Membranes

The present invention relates to electrokinetic, e.g. electroosmotic, liquid transport membranes, textile products comprising a membrane, methods of transporting liquid across an electrokinetic, e.g. electroosmotic, liquid transport membrane, and methods of making an electrokinetic, e.g. electroosmotic, liquid transport membrane.

It is well known that standard waterproof textiles typically transport only 0.1 to 0.5 litres of water per square metre and hour, while human perspiration rates are often 1 -2 litres per hour during vigorous activity. This creates challenges especially in foul weather clothing and in protective clothing such as fire-fighter or military uniforms, and can lead to reduced concentration and performance of wearers, in extreme cases hypo- or hyperthermia.

One solution is described in publications EP 0993328 and WO

2009/024779, where the liquid transport is aided by an electric field. By placing two porous conductive layers on each side of a porous membrane and applying a voltage difference between said layers, a water transport up to 100 litres per square metre and hour has been shown. The mechanism for this transport is

electroosmosis, which involves a small electric current through the porous structure.

A requirement for textile materials is that they should be flexible. In WO

2009/024779 an example textile has a porous layer made of polyvinylidene fluoride (PVDF) membrane, and on each surface of the porous layer there is a respective laminated polyimamide and steel filament textile layer. Other known materials suitable for forming the porous layer include polymer fibres which may be woven or non-woven. However, whilst such polymer fibres may be widely used in textile products, they may not be ideally suited to providing a porous structure for electroosmotic transport.

Viewed from a first aspect the invention provides an electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, and the porous layer having coatings which at least partly form said pore walls.

Since the membrane is flexible it is well suited for textile applications. By using coatings to at least partly form the pore walls of the porous layer, the pore size may be reduced and this can advantageously improve the electroosmotic pumping effect. Electroosmosis is a near wall effect and so liquid in the middle part of a large pore may be relatively unaffected by the application of an electric field across the pore layer. By reducing the pore size, the whole of the pore cross section can be brought within the scope of the electroosmotic near wall effect. Large pores may exist in certain porous materials, such as a glass fibre fleece, and the first aspect of the invention means that such materials, with larger pores, may be modified for use as an electroosmotic liquid transport membrane. If the membrane is to be used as part of a textile product, a smaller pore size is also beneficial in improving the dryness or waterproofness of the textile product, e.g. clothing.

The primary purpose of providing pore wall forming coatings is thus to provide a reduced pore size. This makes possible the use of materials as the porous layer which would not previously have been thought of as feasible for electroosmotic transport due to their large pore size, for example a glass fibre fleece.

Preferably the pore size formed by the pore wall coatings is less than or equal to 500 nanometres, 300 nanometres, or even 200 nanometres. The target pore size selected will depend on the desired electroosmotic pumping pressure and the desired waterproofness, which can be measured in terms of the resistance of the membrane to water leakage under a given water column pressure. The electroosmotic pumping pressure for a given voltage is inversely proportional to the square of the pore size, so that for example a tenfold reduction in pore size gives a pumping pressure which is 100 times greater. The pore size may be smaller than 100 nanometres, for example. The pore size may be in a range of 10 to 200 nanometres, or 50 to 150 nanometres.

The pore walls may be formed of the same material that forms the rest of the porous layer. Thus the porous layer may be made entirely of the same material. Preferably, the porous layer comprises a base material which is coated to form the pore walls. Such coating can serve to reduce the pore size, with the advantages discussed above. The base material and the coating material may be different from each other. Thus a secondary purpose of providing the coatings may be to use a material with better electroosmotic properties than those of the base material. The base material and the coating material may be the same as each other. In this case, the porous layer will be made entirely of the same material, but made by a process including a coating step to reduce the pore size. Examples of pore wall coating materials are silica or metal oxides e.g.

titanium oxide, which are materials having a suitable surface charge for

electroosmosis. The surface charge should be significant, preferably at least 50 mV. The material should have a stable surface charge over the expected pH interval for the use to which the membrane is to be put.

In some embodiments pore wall forming coatings are applied across the whole thickness of the porous layer. Thus a reduced pore size, compared to an embodiment where the pore walls are not formed by coatings, may be provided substantially throughout the porous layer. This can provide a high level of waterproofness, as well as a high electroosmotic pumping pressure. However, it may not always be practicable to effect coating across the full thickness of the porous layer, and in some embodiments it may not be necessary. Preferably the coatings extend over at least 10% of the thickness of the porous layer. In certain embodiments, the coatings extend over at least 50% of the thickness of the porous layer.

The invention also provides a method of making a flexible electroosmotic liquid transport membrane, the method comprising providing a porous layer having pores and applying coatings thereto so as to reduce the size of at least some of the pores. Further, the invention provides a method of making a flexible electroosmotic transport membrane comprising coating a base material (e.g. glass fibres) of the porous layer to form the pore walls.

Various pore wall coating methods may be used. The porous layer may be coated by physical or chemical vapour deposition. A preferred method is sputter deposition. Alternatively a sol-gel process may be used.

In preferred embodiments, the thickness of a pore wall forming coating is at least 10% of the pore size as it would have been if the porous layer were uncoated. In general, the coating will extend substantially entirely around the periphery of a pore. Preferably, the pore size (e.g. diameter) of the pores of the porous layer is reduced by at least 20% by applying pore wall forming coatings to the porous layer.

The thickness of a pore wall forming coating may be at least 25% or 30% or

35% or 40% of the pore size as it would have been if the porous layer were uncoated. The pore size may be reduced by at least 50% or 60% or 70% or 80%. In a preferred embodiment, the pore size is reduced from 700 nanometres to 100 nanometres, i.e. by about 86%. The relatively large reductions in pore size reflect the primary purpose of providing pore wall forming coatings, i.e. to reduce the pore size and hence improve the electroosmotic pumping effect. Of course, the material of the coating can also be selected with such improvement in mind.

As discussed above, it is known to use polymers to form the porous layer of an electroosmotic membrane. We have now found that when using a porous layer made of polymers, electrochemical processes can lead to slow breakdown of the polymer and hence the slow degradation of the membrane, when the membrane is subject to an electric field.

Viewed from a second aspect the invention provides an electroosmotic liquid transport membrane, the membrane being flexible and having a porous layer with pores defined by pore walls made of inorganic material, and the porous layer being coated on opposite surfaces thereof with conductive layers which are porous and which are in the form of conductive coatings.

By using an inorganic material to define the pore walls of the porous layer the problem which we have identified concerning the slow degradation of the membrane is avoided. This is particularly beneficial when the membrane is to be used in a textile application, such as clothing, but may also be of benefit in other applications.

The inorganic material may be a metal salt, preferably a metal oxide, more preferably titanium dioxide. The inorganic material may be glass or a silicon oxide, preferably silica or borosilicate. Also of interest are siloxanes, such as

polydimethylsiloxane (PDMS).

Viewed from a third aspect the invention provides an electroosmotic liquid transport membrane, the membrane being flexible and having a porous layer with pores defined by pore walls made of a material comprising:

a siloxane, preferably polydimethylsiloxane (PDMS); or

a metal salt, preferably a metal oxide, more preferably titanium dioxide; or glass or a silicon oxide, preferably silica or borosilicate;

and the porous layer being coated on opposite surfaces thereof with conductive layers which are porous and which are in the form of conductive coatings.

The use of these materials to form the pore walls can avoid any appreciable membrane degradation.

In certain embodiments the pore walls of the porous layer of the second and third aspects of the invention are at least partly formed by coatings. The pore wall forming coatings are discussed elsewhere in this specification, and the discussion is applicable to the second and third aspects of the invention.

In certain embodiments of the first aspect of the invention the first and second conductive layers are conductive coatings coated on opposite surfaces of the porous layer, i.e. opposed conductive coatings. The membrane of the second and third aspects of the invention has opposed conductive coatings.

The opposed conductive coatings add very little to the weight of the porous layer, so that the membrane can be extremely light in weight.

The use of conductive coatings to apply the electric field, rather than a conductive woven textile or other laminated layer, means that the overall thickness of the membrane may not be significantly increased and so flow resistance across the membrane can be minimised. This may lead to a high liquid transport capacity, for example in excess of 200 litres per square metre and hour.

The pore size, or "mesh size", of the conductive coatings may equal the pore size of the porous layer. A way of achieving this is by applying the coating evenly over the porous layer surface, for example. The mesh size of conductive textiles on the other hand is typically tens of micrometres, much bigger than a typical pore size of the porous layer, creating an uneven electric field and hence current distribution.

By using conductive coatings rather than textile layers, a consistent and even spacing between the conductive coatings may be obtained. In contrast, lamination of conductive textiles onto a porous layer results in conductive elements (e.g. steel filament yarn) from the conductive textiles on each side being pressed into the porous layer, thus creating spots where the conductive elements are in close proximity and thus high electric fields and currents. Without lamination the distance may also vary widely.

Even in the case where deformation, pressure or other impact results in points of closer proximity between the conductive layers, coatings (as opposed to thicker filaments of conductive textiles) will not be able to carry a large current over a small surface area.

Thus, the use of conductive coatings can reduce any risk of current concentrations. This can reduce the likelihood of electrochemical processes causing undesirable chemical degradation of the membrane.

It has surprisingly been found that a very high increase of the electroosmotic pumping pressure can be obtained when conductive coatings are used rather than electrically conductive textile layers (for example with steel filament or embroidered metal etc) or metal gauze / mesh, carbon meshes and similar free standing porous conductive layers. This effect is particularly pronounced when the membrane is smooth, as the conductivity and electric field distribution is good. It is preferable for the effective surface area of the membrane to be less than 5 times the geometrical membrane area and more preferably less than 2 times the geometrical membrane area.

The effective surface area of the membrane is the actual surface area of the membrane taking into account the additional area caused by roughness

(protrusions) on the surface.

Track etched membranes are especially suitable given that the surfaces are relatively smooth.

The resistivity of the coating is preferably smaller than 100 Ohms per square and more preferably smaller than 10 Ohms per square.

It has been observed that using a 20 micron thick track etched membrane of polycarbonate with a pore size of 100 nm, together with a 100 mesh steel mesh on both sides as electrodes, it was necessary to apply 8.5 V across the membrane to pump against a 10 cm water head, 65 V against a 30 cm water head and it was not possible to pump against 50 cm water head as the voltages required were so high that severe side effects occurred before a sufficient voltage was reached. In contrast, when using the same membrane (a 20 micron thick track etched membrane of polycarbonate with a pore size of 100 nm) which had been sputter coated with 50 nm of gold on each side and using the same steel meshes as contacts, it was possible to pump against a 10 cm water head with only 2.2 V, against a 30 cm at 2.8 V and against 50 cm at 3.2 V.

This beneficial decrease in the required voltage is achieved because the conductive coating has a better voltage and current distribution than simply using a steel mesh and results in every pore experiencing the same field strength. When the distribution is less even, some pores will experience a low field which effectively creates a leakage and thus reduces the pressure difference between the opposite sides of the membrane. A typical steel mesh has a mesh size of several micrometers or more, whereas porous electroosmotic membranes according to a preferred embodiment of the present invention have pore sizes which are a fraction of a micrometer. In addition, the distance between membrane and electrodes will vary when using separate electrode layers as opposed to coatings. By using conductive coatings the membrane can be produced at low cost since there is no additional laminating step. The membrane with the opposed conductive coatings can be cut to any size and shape, or perforated, without causing short circuiting between the opposite conductive layers. In particular, short-circuiting is avoided because the coatings will generally be very thin compared to the porous layer positioned between the conductive coatings. This is an improvement over laminated electroosmotic liquid transport membranes, where there is a risk when cutting or perforating that the two conductive laminated layers will be brought into contact with each other to cause a short circuit.

Rather than having any laminating step, the membrane may be

manufactured by roll-to-roll coating processes, i.e. by coating the porous layer as it is unrolled from a roll and then rolling the coated product onto another roll.

The invention also provides a method of making a flexible electroosmotic transport membrane comprising applying conductive coatings to opposite surfaces of a porous layer. The porous layer is preferably in accordance with the first, second or third aspects of the invention. The conductive coatings may be applied by physical or chemical vapour deposition. A preferred coating method is sputter deposition. An alternative method of applying the coatings is by a sol-gel process.

Physical or chemical vapour deposition of the conductive coatings, or application of the conductive coatings by a sol-gel process, are preferred to spray coating or printing with conductive ink.

The coatings are preferably metal coatings. The metals can be noble, such as gold or platinum, or non-noble, such as titanium, tantalum, chromium or nickel (e.g. nanocrystalline nickel electrodes). The metal could also be an oxidized metal, e.g. tantalum covered with tantalum oxide. Further, it could be a mixture of metals (noble and/or non-noble) and optionally also metal oxides. A non-metallic conductor such as graphene or conductive polymers (e.g. doped polyaniline or pedot (poly(3,4- ethylenedioxythiophene)) may also be used.

The conductive coatings may each have a thickness less than or equal to 500 nanometres, preferably 200 or 100 or 80 or 60 or 50 or 40 or 30 or 25 or 20 or 15 or 10 or 5 nanometres. In one preferred embodiment, the thickness of the conductive layers is about 100 nanometres. In another preferred embodiment, the thickness of the conductive layers is about 20 nanometres. A thickness between 5 and 100 nanometres is also of interest Such small thicknesses are easily achieved by using conductive coatings rather than laminated layers as the conductive layers. Typical conductive textile laminates have a thickness of 20 micrometres, i.e. 20,000 nanometres.

The thickness of the conductive coatings is preferably small compared to the thickness of the membrane. Each conductive layer may have a thickness less than or equal to 1/5, or 1/10, or 1 /100, or 1/1000, of the thickness of the membrane.

In certain embodiments, the membrane has a thickness of less than 1 millimetre, preferably less than 600 or 400 or 200 or 100 or 60 or 50 micrometres. The thickness may be 400 to 500, or 30 to 50, or 10 to 20 micrometres.

The membrane of any aspect of the invention is preferably of a size suitable for making a textile product or for an industrial use. It may be in generally planar form, for example in the form of a web, sheet or the like. It may have a minimum dimension measured in any dimension in the plane of at least 20 cm.

The porous layer of the membrane of any aspect of the invention described herein may have a structure which is generally monolithic apart from the pores, for example a one piece material formed with pores.

The pores may have a well ordered structure. They may be equal in size. In preferred embodiments, the porous layer comprises fibres. The fibres may be woven or non-woven. The pores between adjacent fibres provide the necessary porous structure for electroosmosis to take place when an electric field is applied.

In the context of a porous layer comprising fibres, such as a fleece e.g. a glass fibre fleece, the pore size can be understood as the diameter of the smallest particles which would be retained by the porous layer if it were used as a filter.

The porous layer may comprise glass fibres. The glass fibres may be woven. Preferably the porous layer comprises non-woven glass fibres, e.g. a glass fibre fleece.

Glass fibres also have the advantage of being stable even at high temperatures, for example above 400°C, compared to known polymers which can melt at considerably lower temperatures. Thus glass fibres used as the porous layer of the membrane are beneficial in protective clothing such as military or fire fighting clothing, where high temperatures may be encountered.

One example of a suitable porous layer is a Whatman glass microfiber binder free filter, grade GF/F. This product has a borosilicate glass structure. Such a product, normally provided for filtration of particles down to 700 nanometres in diameter, has been found as a suitable porous layer which may be coated on opposite surfaces thereof with conductive coatings.

Where glass fibres are used, the pore walls may be formed by the raw glass fibres. Preferably, however, the glass fibres are coated to increase their effective diameter and to reduce the pore size.

Viewed from a fourth aspect the invention provides an electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls.

In certain preferred embodiments, the porous layer has a web like structure, a foamy structure, or a structure with cylindrical pores.

In the case where the porous layer has a web like structure, the pore walls may be formed by arms which interconnect at nodes. Thus for example a node may have at least three arms extending therefrom. The web structure may comprise fibres, for example drawn out fibres, i.e. fibres which are created by stretching the porous layer.

In the case where the porous layer has a foamy structure, there may be a plurality of pores which extend in different directions from each other. Thus for example some pores may extend in the direction of the thickness of the porous layer, other pores may extend in a direction normal to the thickness, and other pores may extend in a direction intermediate between the thickness direction and the direction normal to the thickness direction. Individual pores may extend in more than one direction.

Whether the porous layer has a web like or a foamy structure, the arrangement of the pores may be substantially disordered.

In the case where the porous layer has a structure with cylindrical pores, the pores may be elongate and may extend in the direction of the thickness of the porous layer. The arrangement of the pores may be well ordered.

It is preferable for the effective surface area of the membrane to be less than 5 times the geometrical membrane area and more preferably less than 2 times the geometrical membrane area.

In certain embodiments of the fourth aspect of the invention the first and second conductive layers may be conductive coatings coated on opposite surfaces of the porous layer, i.e. opposed conductive coatings. The various advantages and preferred features of such conductive coatings as discussed elsewhere herein are applicable also to the fourth aspect of the invention.

In certain embodiments of the fourth aspect of the invention the pore walls of the porous layer are at least partly formed by coatings. The various advantages and preferred features of such pore wall forming coatings as discussed elsewhere herein are applicable also to the fourth aspect of the invention.

The porous layer may be polymeric. When a polymer membrane is used as the porous layer, this may have:

A) Web like structure

o Expanded membranes like ePTFE (expanded polytetrafluoroethylene)

(e.g. Gore Tex (trade mark)), other expanded membranes like ePC (expanded polycarbonate), ePE (expanded polyethylene), ePP (expanded polypropylene)

o Fibers drawn out

- Pore size from 20 nanometres to 5 micrometres

- Porosity from low to high (<1 % to more than 90%)

B) Foamy structure

o Typical for phase inversion membranes, or those made by a sol-gel process

Made by Pall, Millipore, others

- C) Cylindrical pores

o Typical for track-etched membranes.

It has been found that almost any porous organic membrane can be used as the porous layer of the electroosmotic liquid transport membrane. These include polymeric membranes with cylindrical pores, like for instance track etched membranes

- polymeric membranes with e.g. web like structure

expanded polymeric membranes (e.g. ePTFE or GoreTex), or polymeric membranes produced by other processes but with the same web structure as expanded membranes

polymeric membranes with e.g. foamy like structure

- polymeric membranes produced by phase inversion, or a sol-gel process - polymeric membranes produced by other processes to form a foamy structure.

A preferred property is that the porous layer surface be smooth, preferably so that the effective surface area is less than 100 or 10 or 5 or 2 times the porous layer surface area as determined by its footprint (i.e. its geometrical membrane area). This property makes it easier to deposit electrodes directly onto the porous layer, as conductive coatings, and makes it possible to obtain good conductivity with a thin electrode layer.

The following description corresponds most closely to the cylindrical pore version, which we have found to provide the best performance of tested

membranes, but the described features are not necessarily limited thereto.

In certain embodiments, the porous layer will have a specific set of properties. The pore size should preferably vary by less than 100% from a nominal pore size in the direction perpendicular to the membrane surfaces through 50% of the porous layer thickness, more preferably through 90% or more of its thickness. As electroosmotic pressure increases with decreasing pore size, intermittently larger pore size reduces the pumping pressure compared to that obtained by the nominal pore size, and can further result in "dead volumes" from which water is not fully removed electroosmotically. The latter may lead to higher residual humidity and a wet and heavy membrane in the worst case. For these reasons it is preferable to provide a porous layer in which the pore size has little variation from the desired nominal pore size, and in which, if there are variations, these are preferably confined to the minimal possible extent in the porous layer thickness direction.

In other embodiments, the porous layer has a web-like structure, where the pore size variance is higher. These membranes could have a mean pore size bigger than the ideal pore size, but as the pore size is not uniform, there is a significant proportion of pores which are smaller that the average pore size, and which are particularly adapted to transport liquid by a electrokinetic, preferably electroosmotic, process.

Another preferred property is pores which are substantially directed normally to the membrane surface, with the porosity measured in the direction parallel to the surface being lower than, and preferably lower than 10% of, the porosity measured in the direction normal to the surface direction. This will also lead to less residual humidity in the membranes, as there will be fewer pore wall surfaces parallel to the membrane surfaces where the electric field is hence substantially normal to the pore wall surface and thus no significant electroosmosis is obtained.

The pore size is preferably smaller than 5 micrometres, more preferably less 500 nanometres and yet more preferably less than 250 nanometres. The small pore size increases the EO pumping pressure, which is inversely proportional to the squared pore size, according to the Smoluchowsky equation. It further reduces the passive water leakage e.g. due to rainfall in textile applications.

A further preferred property is that the porous layer surface be smooth, preferably so that the effective surface area is less than two times the porous layer surface area as determined by its footprint. This property makes it easier to deposit electrodes directly onto the porous layer, as conductive coatings, and makes it possible to obtain good conductivity with a thin electrode layer.

It is further a preferred property of the porous layer that the porosity be between 0.5 and 50%, more preferably between 3 and 30% and most preferably between 6 and 15%. This helps decrease the passive pressure driven transport (leakage), and also contributes to making the surface smoother.

The porous layer thickness is preferably between 5 and 200 micrometres, more preferably between 10 and 50 micrometres.

Especially preferred embodiments are obtained where the pores of the porous layer are shaped as cylinders with circular cross section. A type of membrane for use as the porous layer which fulfils this and other preferred properties especially well are track-etched membranes.

Typically, porous layers as described are polymer membranes. It is possible but not necessary to have the polymer membranes with said properties coated at least partially by an inorganic layer such as Si0₂, in order to render the material more stable and durable, and to enhance the surface charge and thus the electroosmotic performance.

It is further preferred to use conductive layers (e.g. electrodes) with a mesh size smaller than 3 times the pore diameter and more preferably smaller than or equal to the pore diameter. A preferred embodiment includes conductive layers which are coated onto both porous layer surfaces by means of vapour deposition or another technique. For example, 20 to 50 nanometre thick gold coatings applied by sputter deposition may be adopted.

A very large increase in performance was obtained by changing from a standard polymer or glass fiber filter porous layer to a metal coated porous layer with properties as described above. Using as a porous layer a standard porous membrane made by phase inversion, such as Pall "Supor", with a nominal pore size of 200 nanometres, and using as conductive layers steel 100 mesh electrodes (Bopp, Switzerland), a potential of 9 Volt was needed to create a water head (pressure) of 10 cm. A glass fiber porous layer with nominal pore size 700 nanometres (Whatman / GE) needed approximately 20 V to obtain the same pressure. Each of the two membranes retained an amount of water similar to its own weight after EO pumping, i.e. their weight was doubled by having been wetted by the water.

In comparison, using as the porous layer a track-etched membrane with cylindrical pores with diameter 400 nanometres, porosity approximately 10%, thickness 20 micrometers (from Oxyphen, Switzerland) and coated with 20 nanometres gold on each side, gave a water head of 50 cm at 4 Volt. Using a similar track-etched membrane as the porous layer but with pore size 200 nanometres the same pressure was obtained at 2.9 V. Further, these track etched membranes retained only 1 -5 % of their weight in water after EO pumping, and thus were water repellent and had a dry touch, as opposed to standard porous membranes made by phase inversion or consisting of fibers.

Similarly, the passive pressure driven transport was strongly reduced for the membranes using track etched membranes as the porous layer. Importantly, efficient EO pumping, with flow rates in excess of 10 litre / m² hour, was obtained at only 1.5V for the metal coated polymer membranes, whilst standard membranes would require at least 2.5 V to give a significant EO transport. The lower voltage leads to reduced problems with electrolysis of the liquid transported, which could result in potential harmful products as well as gas bubbles blocking the flow path, and would further lead to higher energy consumption. The ability to use only 1.5 V resulted in a power consumption of typically 1 to 5 W per square meter, whilst for standard membranes the figure is typically at least 10 W.

The track etched polymer membranes were coated with a thin layer Si0₂ by Plasma Enhanced Chemical Vapor Deposition, before electrodes were applied by gold coating. For pure water a similar performance was obtained without first applying the Si0₂ coating, however the durability as well as the performance with liquid having different pH values and salt concentrations will be better with the Si0₂ (or other inorganic coating), due to the mechanical and chemical stability of such coatings, as well as the stability of the electrochemical potentials for certain coatings like Si0₂ and Ti0₂. In order to bring about electroosmotic liquid transport, it is necessary to apply an electric field across the membrane. In a preferred embodiment of any aspect of the invention described herein, the conductive layers are connected to an electric power source such that, in use, a voltage can be applied across the porous layer to effect flow of liquid across the membrane. Thus the invention may comprise the membrane in combination with an electric power source.

Viewed from a fifth aspect the present invention provides an electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, wherein in a direction perpendicular to the membrane surface at least 50% of the pore lengths have a dimension less than 2 times a nominal pore size.

In a preferred embodiment at least 99% of the pore lengths have a dimension less than 2 times a nominal pore size.

Also, it is preferable for less than 5 % of the porosity to be constituted by pores with a dimension more than 10 times a nominal pore size in a direction parallel to the membrane surface. More preferably, less than 1 % of the porosity is constituted by pores with a dimension more than 10 times a nominal pore size in a direction parallel to the membrane surface and even more preferably less than 0.1 %.

Viewed from a sixth aspect the present invention provides an electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, wherein in a direction parallel to the membrane surface less than 5 % of the porosity is constituted by pores with a dimension more than 10 times a nominal pore size.

In a preferred embodiment, less than 1 % of the porosity is constituted by pores with a dimension more than 10 times a nominal pore size in a direction parallel to the membrane surface and more preferably 0.1 %.

It is preferable that at least 50% of the pore lengths have a dimension less than 2 times a nominal pore size in a direction perpendicular to the membrane surface and more preferably at least 99%.

The membrane of the fifth and sixth embodiment preferably has at least 50% of the porosity constituted by pores with a dimension less than 2 times a nominal pore size in a direction parallel to the membrane surface, more preferably at least 90% and yet more preferably at least 99%.

The dimension may for example be a pore diameter.

In an aspect, the present invention provides an electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, wherein the pore size distribution above a nominal pore size in both a direction parallel to the membrane surface and a direction parallel to the membrane surface is low.

In other words, the number of pore sizes which are greater than a nominal pore size is low i.e. less than 5% and more preferably less than 1 % of the pores.

Larger pores, even when subject to the same field strength as smaller pores generate a smaller electroosmotic pressure. This creates leakage points which reduces the overall pressure. For a given electric field the pressure is proportional to the minus second power of the pore size. As a result only a small fraction of the pores should significantly deviate from a target (or nominal) size which achieves a target (or desired) pressure.

Given this beneficial reason for having a narrow size distribution of pore size, the above size distribution features and preferable features of the fifth and sixth aspects can also apply to anyone of the first to fourth aspects of the invention.

The membrane of any aspect of the invention described herein may be provided with first and second electrical contacts, connected respectively to the first and second conductive layers. These electrical contacts may be used for connection to an electric power source, so that a voltage can be applied across the porous layer to effect flow of liquid across the membrane.

The membrane of any aspect of the invention described herein may be used for various applications. Preferably the membrane is used as part of a textile product.

The invention also provides a method of making a textile product, comprising attaching a fabric layer to a flexible electroosmotic transport membrane.

The textile product may for example be clothing, seating (e.g. for a vehicle such as an automobile, aircraft or train), or a mattress. In preferred embodiments, a textile product comprises an electroosmotic liquid transport membrane as discussed herein, and a fabric layer. The fabric layer may provide strength to the textile product. Thus the membrane may be integrated as part of a textile product. In certain embodiments, the membrane may be provided between fabric layers.

The fabric layer or layers is (are) preferably made of woven material. An example of a textile product is a waterproof jacket, which may incorporate an electroosmotic liquid transport membrane as described herein for the removal of perspiration away from the body.

Other applications of the membrane of any aspect of the invention described herein include a range of industrial applications, such as dewatering, humidity control or filtering. Whilst the membrane itself is flexible, it may be attached to a rigid substrate for use in some applications. As it is flexible it can be supplied on a roll or the like ready for attachment.

According to one method of making a preferred embodiment of the membrane, a porous layer comprising glass fibres is first subjected to a process in which the pores between the fibres are reduced. This is done by coating the fibres such that the pore walls defining the pores in the porous layer are then at least partly formed by the coatings (pore forming coatings). After that, the first and second conductive layers are formed by applying conductive coatings to the fibres on the outside surfaces of the porous layers. The fibres at the surface of the porous layer may thus have two coatings; a first pore forming coating which is then overlayed by a conductive coating. Away from the surface of the porous layer, into the thickness thereof, only the pore forming coatings will be provided.

The invention in its various aspects is described herein with reference to electroosmotic liquid transport membranes. The invention may however extend to any membrane or related technology where an electric field is used to produce a liquid displacement. Examples are electrophoresis or electric sonic amplitude. The invention may extend to any electrokinetic liquid transport membranes.

Certain preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which :

Figure 1 is a schematic view of an electroosmotic liquid transport membrane in a circuit;

Figure 2 shows a membrane having a glass fiber porous layer and conductive layers in the form of coatings;

Figure 3 is an enlarged schematic view of glass fibres located in the porous layer; and Figure 4 is an enlarged schematic view of glass fibres in one of the conductive layers.

In Figure 1 , there is shown a porous layer 1 where the liquid transport is to be induced by an electric field and current, a conductive coating 2 (first electrode), and a conductive coating 3 (second electrode). A is a point where current is measured, and V is a voltage source.

In this embodiment, the conductive coatings are formed of gold and have a thickness of about 100 nanometres.

Figure 2 shows a membrane having a glass fiber porous layer 1 , a first conductive layer 2 in the form of metal coated glass fibres, and a second conductive layer 3 in the form of metal coated glass fibres.

Figure 3 is a view to a larger scale of the interior of the porous layer, showing two adjacent fibres. Each fibre 4 is coated by a coating 5, thereby effectively increasing the size of the fiber and reducing the size of the spaces or pores between the fibres. The pore size created by the uncoated fibres is shown as P_u, whilst the smaller pore size created by the coated fibres is shown as P_c. In the case of the coated fibres, the coatings 5 form pore walls 7. In practice, in the case of a preferred glass fibre fleece where the fibres extend in multiple directions, some fibres will be touching and some will be further apart, and Figure 3 is an idealised example serving to show how in general the pore size is reduced by the coatings 5.

Figure 4 shows a view of the fibres in the conductive layers. These fibres 4 have been first coated with pore size reducing coatings 5, and have then been coated with a metal coating 6 to provide electrical conductivity.

Claims

1 . An electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, and the porous layer having coatings which at least partly form said pore walls.

2. An electroosmotic liquid transport membrane, the membrane being flexible and having a porous layer with pores defined by pore walls made of inorganic material, and the porous layer being coated on opposite surfaces thereof with conductive layers which are porous and which are in the form of conductive coatings.

3. An electroosmotic liquid transport membrane, the membrane being flexible and having a porous layer with pores defined by pore walls made of a material comprising:

a siloxane, preferably polydimethylsiloxane (PDMS); or

4. An electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, and optionally wherein the porous layer has a web like structure, a foamy structure, or a structure with cylindrical pores.

5. A membrane as claimed in claim 4, wherein the porous layer has a web like structure, and wherein the pore walls are formed by arms which interconnect at nodes.

6. A membrane as claimed in claim 4, wherein the porous layer has a foamy structure in which a plurality of pores extend in different directions from each other.

7. A membrane as claimed in claim 4, wherein the porous layer has a structure with cylindrical pores in which the pores are elongate and extend in the direction of the thickness of the porous layer.

8. A membrane as claimed in any of claims 4 to 7, wherein the effective surface area of the membrane is less than 5 times the geometrical membrane area and preferably less than 2 times the geometrical membrane area.

9. An electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, wherein in a direction perpendicular to the membrane

I- surface at least 50% of the pore lengths have a dimension less than 2 times a nominal pore size.

10. A membrane as claimed in claim 9, wherein in a direction parallel to the membrane surface less than 5 % of the porosity is constituted by pores with a dimension more than 10 times a nominal pore size.

1 1. An electroosmotic liquid transport membrane, the membrane being flexible and comprising first and second conductive layers which are porous, a porous layer positioned between the first and second conductive layers and having pores defined by pore walls, wherein in a direction parallel to the membrane surface less than 5 % of the porosity is constituted by pores with a dimension more than 10 times a nominal pore size.

12. A membrane as claimed in claim 1 1 , wherein in a direction perpendicular to the membrane surface at least 50% of the pore lengths have a dimension less than 2 times a nominal pore size.

13. A membrane as claimed in any of claims 9 to 12, wherein in a direction parallel to the membrane surface at least 50 % of the porosity is constituted by pores with a dimension less than 2 times a nominal pore size.

14. A membrane as claimed in any of claims 2 to 13, wherein the pore walls are at least partly formed by coatings.

15. A membrane as claimed in claim 1 or 14, wherein the pore wall forming coatings extend over at least 10 per cent of the thickness of the porous layer, preferably 50 per cent, more preferably 1 00 per cent.

16. A membrane as claimed in claim 1 , 14 or 15, wherein the thickness of a pore wall forming coating is at least 10% of the pore size as it would have been if the porous layer were uncoated.

17. A membrane as claimed in any preceding claim, wherein the pore size of the pores is less than or equal to 300 nanometres, preferably 200 nanometres, more preferably 100 nanometres.

18. A membrane as claimed in claim 1 or any of claims 4 to 17, wherein the first and second conductive layers are conductive coatings coated on opposite surfaces of the porous layer.

19. A membrane as claimed in any preceding claim, wherein the conductive layers have a thickness less than or equal to 500 nanometres, preferably 200 nanometres.

20. A membrane as claimed in any preceding claim, wherein the conductive layers have a thickness less than or equal to 1/5, preferably 1 /100, of the thickness of the membrane.

21 . A membrane as claimed in any preceding claim, wherein the porous layer comprises glass fibres.

22. A membrane as claimed in any preceding claim, wherein the conductive layers or coatings are connected to an electric power source such that, in use, a voltage can be applied across the porous layer to effect flow of liquid across the membrane.

23. A membrane as claimed in any of claims 1 to 21 , further comprising first and second electrical contacts, connected respectively to the opposite conductive layers or coatings and for connection to an electric power source, such that, in use, a voltage can be applied across the porous layer to effect flow of liquid across the membrane.

24. A textile product comprising a membrane as claimed in any preceding claim, and a fabric layer to provide strength to the textile product.

25. A textile product as claimed in claim 24, wherein the textile product is clothing, seating, or a mattress.

26. A method of transporting liquid across an electroosmotic liquid transport membrane as claimed in any of claims 1 to 23, or across a membrane of a textile product as claimed in claim 24 or 25, the method comprising applying a voltage across the porous layer to effect liquid transport by electroosmosis.

27. A method of making a flexible electroosmotic liquid transport membrane, the method comprising providing a porous layer having pores and applying coatings thereto so as to reduce the size of at least some of the pores.

28. A method as claimed in claim 27, wherein the pore size of the pores of the porous layer is reduced by at least 20% by applying pore wall forming coatings to the porous layer.

29. A method as claimed in claim 27 or 28, wherein the coatings are applied by physical or chemical vapour deposition, preferably by sputter deposition.

30. A method as claimed in claim 27 or 28, wherein the coatings are applied by a sol-gel process.

31. A method of making an electroosmotic liquid transport membrane as claimed in any of claims 1 to 23, the method comprising providing a porous layer having pores and applying conductive coatings to the porous layer to form the conductive layers.

32. A method as claimed in claim 31 , wherein the conductive coatings are applied by physical or chemical vapour deposition, preferably by sputter deposition.

33. A method as claimed in claim 31 , wherein the conductive coatings are applied by a sol-gel process.

34. A method of making a textile product, comprising attaching a fabric layer to a flexible electroosmotic transport membrane as claimed in any of claims 1 to 23.