ARTIFICIAL GILL

Description

The invention relates generally to an artificial gill. More particularly, but not exclusively, the invention relates to an artificial gill for enabling a user to breathe in an underwater environment.

BACKGROUND

Environmental changes caused by human activity include sea level rises and partial or total flooding of coastal cities. There is also ever-decreasing space for building on land to accommodate a growing global population.

A number of countries and cities are preparing for such potentially drastic changes (and also to take advantage of marine environments more generally) by developing aquatic living environments on the surface of or in water. For example, the Netherlands is experimenting with a city built on water (“A Dutch Architect Offshores the Future of Housing”, www.nytimes.com). Amphibious hotels have been built in resort beaches (“Underwater hotel room opens off the cost of Zanzibar”, www.dezeen.com). The Japanese firm Shimizu Corp has goals of building floating cities in the Tokyo Bay and in the equatorial zone during this century (“The Botanical Future City Concept of a Plant-like City”, wwww.shimz.co.jp).

In such potentially submerged urban areas, the surrounding water can be exploited. At water depths of approximately 10 m or less, there is enough oxygen to be harvested for human consumption, there is enough light permeation through the water to enable humans to see, and the water pressure is bearable for repeated diving by humans.

Currently, humans can breathe underwater for short periods of time using SCUBA (Self-contained underwater breathing apparatus) technology. SCUBA divers carry one or more tanks of compressed air for breathing. However, SCUBA divers are limited by the amount of compressed air they are able to carry and have to return to the water's surface before they run out of breathable air.

Rebreathers are an alternative diving technology to SCUBA technology. Rebreathers differ from SCUBA by recycling the breath of the diver. Exhaled breath is ‘scrubbed’ of CO₂ by removing carbon dioxide as the exhaled breath flows past a CO₂ absorbent material, for example sodium hydroxide. However, the presence of such a chemical scrubber can be dangerous to the diver, especially if the chemical granules leak or come into contact with water.

In addition, due to the limited lifespan and single-use nature of CO₂ absorbent materials, in between every dive the used granules must be replace by disassembling and reassembling the rebreather equipment. Frequent handling of these dangerous chemicals increases the likelihood of potentially of fatal accidents. This creates a major barrier to existing divers switching to rebreather diving systems.

The present invention has been devised with the foregoing in mind, and aims to provide an artificial gill which can be used by humans to breathe in an underwater environment.

SUMMARY OF INVENTION

According to a first aspect of the invention, there is provided an artificial gill for enabling a user to breathe when in an underwater environment. The artificial gill comprises a membrane configured to allow permeation of gas through the membrane and prevent permeation of water through the membrane. The membrane may be or comprise polymethyl pentene, PMP. The artificial gill comprises a gas reservoir at least partially enclosed by a first or inner surface of the membrane for providing gas for a user to breathe. The artificial gill comprises a water flow device configured to direct a flow of water over a second or outer, opposite surface of the membrane.

Providing a liquid-impermeable gas exchange membrane comprising PMP in combination with a water flow device to direct water across the membrane may provide a viable, practical device enabling a user to breathe the contents of the gas reservoir for prolonged or indefinite periods of time. The membrane may enable CO₂ to be removed from the gas reservoir as well as the uptake of O₂ into the gas reservoir.

The membrane comprising PMP may significantly improve the gas permeability of the membrane (for example compared to conventional gas exchange membrane materials such as polypropylene), thus increasing the rate of gaseous exchange across the membrane. That may enable the membrane to have a reduced surface area compared with membranes which do not comprise PMP, and/or may enable the membrane to be made as thin as possible to maximise gas exchange while maintaining mechanical integrity. PMP also inherently has a low surface energy, meaning PMP is strongly hydrophobic, making it particularly suitable for use in an underwater environment. PMP is also a thermoplastic polymer, which may enable easy processing of the material.

The PMP may be or comprise a PMP polymer, for example 4-methyl-1-pentene polymer. The PMP polymer may be or comprise a copolymer of 4-methyl-1-pentene with one or more α-olefins. The one or more α-olefins may each have or comprise between 2 and 20 carbon atoms.

The water flow device may ensure that a fresh supply of water is substantially continuously passing over the membrane, which may maintain CO₂ and O₂ concentration gradients across the membrane, thereby improving gaseous exchange.

The membrane may be configured to enable gaseous exchange between the gas reservoir and water in contact with the second surface of the membrane.

The membrane may be configured to enable gaseous exchange at a sufficient rate such that, in use, a concentration of CO₂ in the gas reservoir does not exceed substantially 0.05% by volume. That may prevent the gas in the gas reservoir being breathed by a user from reaching high CO₂ levels which can be dangerous for the user to inhale.

The membrane may be configured to enable gaseous exchange at a sufficient rate such that, in use, a concentration of O₂ in the gas reservoir is substantially 17.75% or greater by volume. An advised minimum safe concentration of O₂ is 19.5% (US Occupational Safety and Health Administration). The membrane may therefore be configured to replenish at least 50% of the O₂ consumed by a user whilst breathing.

The second surface of the membrane may comprise a non-porous skin. The non-porous skin may be non-permeable to liquids, such as water. The non-porous skin may ensure that the membrane enables gaseous exchange without being at risk of liquid ingress into the air reservoir and/or membrane damage from liquid penetration. That may improve performance of the artificial gill in an underwater environment and prolong a working life of the membrane.

The membrane may comprise a plurality of fibres. The fibres may be hollow fibres. The first surface of the membrane may comprise inner surfaces of the hollow fibres. The hollow fibres may be configured to allow gas within the gas reservoir to flow through hollow centres of the hollow fibres. Having a plurality of fibres may increase the surface area of the membrane (without significantly increasing an overall size of the artificial gill), thereby increasing the rate of gaseous exchange through the membrane.

The second surface of the membrane may comprise the outer surfaces of the hollow fibres. The water flow device may be configured to direct the flow water over the outer surfaces of the hollow fibres.

The fibres may be arranged into bundles. The fibres may be twisted, coiled, folded or woven into bundles. The fibres may be knitted or woven, for example cross-knitted, into mats. These fibre arrangements may increase the surface area of the membrane whilst minimizing or reducing an overall size of the membrane (for example, compared to a planar or sheet membrane).

The fibres may comprise a microstructured texture. The microstructured texture may be configured to increase surface area and/or increase turbulence in water passing over the second surface of the membrane. Increasing the surface area and/or turbulence may increase the rate of gaseous exchange across the membrane.

The water flow device may comprise a pump. The water flow device may comprise a diver propulsion vehicle (DPV). Using a DPV as the water flow device enables a user to move around more easily in an underwater environment without requiring additional components, whilst simultaneously providing a fresh supply of water to the membrane.

The gas reservoir may be in direct fluid communication with facial respiratory features of a user. That may enable a user to breathe regularly whilst using the artificial gill.

The membrane may be configured to provide sufficient gaseous exchange to remove at least 320 cm³ of CO₂ per minute from the gas reservoir and/or add at least 140 cm³ of O₂ per minute to the gas reservoir.

The gas reservoir may be or comprise a substantially closed circuit or loop through which gas for the user to breathe is configured to flow or circulate.

The water flow device may be configured to direct a flow of water over the second surface of the membrane in a substantially opposite direction to a flow of gas through the gas reservoir. That may provide counter-current flow to maximise a concentration gradient across the membrane, which in turn may improve gaseous exchange across the membrane.

According to a second aspect of the invention, there is provided an artificial gill for enabling a user to breathe when in an underwater environment. The artificial gill comprises a membrane configured to allow permeation of gas through the membrane and prevent permeation of water through the membrane. The membrane may be made from, or comprise, a hydrophobic, gas-permeable membrane. The artificial gill comprises a gas reservoir at least partially enclosed by a first surface of the membrane for providing gas for a user to breathe.

The artificial gill may comprise a water flow device configured to direct a flow of water over a second, opposite surface of the membrane,

The second surface of the membrane may comprise a non-porous skin.

The membrane may be or comprise polymethylpentene, PMP.

Optional features of any of the above aspects may be combined with the features of any other aspect, in any combination. For example, features described in connection with the artificial gill of the first aspect may have corresponding features definable with respect to the artificial gill of the second aspect, and vice versa, and these embodiments are specifically envisaged. Features which are described in the context or separate aspects and embodiments of the invention may be used together and/or be interchangeable wherever possible. Similarly, where features are, for brevity, described in the context of a single embodiment, those features may also be provided separately or in any suitable sub-combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a side view of an artificial gill in accordance with the present invention in an underwater environment;

FIG. 2 shows partial cross-sectional views of a polymethylpentene (PMP) membrane and a polypropylene (PP) membrane:

FIG. 3 shows an embodiment of an artificial gill configured to be worn by a user:

FIG. 4 shows a side view of an artificial gill in accordance with the present invention in an underwater environment, in which the membrane is formed from a plurality of fibres:

FIG. 5 shows the fibres of the membrane of FIG. 4 formed into a mat and potted in a container:

FIG. 6 shows a side view of an artificial gill in an underwater environment with an enlarged cross-sectional view of the membrane:

FIG. 7 shows a side view of an artificial gill with a respirator in an underwater environment:

FIG. 8 shows an experimental set-up used to investigate rates of gaseous exchange through a membrane in accordance with the present invention:

FIG. 9 shows the change in concentration of CO₂ and O₂ in the air reservoir as a function of time for the experimental set-up shown in FIG. 8:

FIG. 10 shows the change concentration of CO₂ and O₂ in the air reservoir as a function of time for the experimental set-up shown in FIG. 8 at different water flow rates:

FIG. 11 shows another experimental set-up used to investigate rate of gaseous exchange through a membrane in accordance with the present invention:

FIG. 12 shows the change in concentration of CO₂ and O₂ in an air reservoir as a function of time for the experimental set-up shown in FIG. 11:

FIG. 13 shows a magnified portion of FIG. 12 showing the change in CO₂ concentration between approximately 0 minutes and approximately 2 minutes after activation of the water pump; and

FIGS. 14A and 14B show a substantially linear relationship between flow rate and predicted required membrane surface for CO₂ removal and O₂ addition based on data obtained using the experimental set-up shown in FIG. 8.

DETAILED DESCRIPTION

FIG. 1 shows an artificial gill 10 for enabling a user to breathe when in an underwater environment 5, in accordance with an embodiment of the present invention.

The artificial gill 10 comprises a membrane 15. The membrane 15 is configured such that air or other gases can permeate through the membrane 15. The membrane 15 is configured to prevent the permeation of water or other liquids through the membrane 15. The membrane 15 is made from a highly hydrophobic material. In the example shown, the membrane 15 is made from or comprises polymethyl pentene (PMP). In other examples, the membrane 15 can be made from other gas permeable materials, such as polypropylene (PP) or polytetrafluoroethylene (PTFE).

PMP materials may comprise a non-porous surface “skin”, whereas other materials, such as PP, typically comprise a porous surface. FIG. 2 shows a comparison between partial cross-sections of a PMP membrane 500 and a PP membrane 600.

In the example shown, the PMP membrane 500 comprises a porous main body 520 and a non-porous skin 530. The non-porous skin 530 may have a thickness of between substantially 50 nm and substantially 2 μm, or between substantially 100 nm and substantially 1 μm. Both the porous main body 520 and the non-porous skin 530 are gas permeable. The non-porous skin 530 is non-permeable to liquids, such as water. A first surface 505 of the PMP membrane is defined by the porous main body 520, and an opposing second surface 510 of the PMP membrane 500 is defined by the non-porous skin 530. In use, the first surface 505 is exposed to an air reservoir and the second surface 510 is exposed to a liquid. The non-porous skin 530 ensures that the membrane 500 enables gaseous exchange without being at risk of damage from liquid exposure. The diffusion coefficient for PMP is much higher for many gases than other materials such as PP. The skin 530 can therefore be provided to prevent liquid penetration without compromising the gas transfer properties of PMP. Alternatively, both the first surface 505 and the second surface 510 of the PMP membrane 500 which is exposed to a liquid may be defined by or comprise a non-porous skin to prevent liquid permeation. Alternatively, the PMP membrane 500 may not comprise a skin, and may rely on the highly hydrophobic properties of PMP to prevent or inhibit liquid penetration.

By contrast, the PP membrane 600 does not comprise a skin. Instead, both the first side 605 and the second side 610 of the membrane 600 are defined by or comprise the porous core 620 alone. PP has a lower diffusion coefficient for many gases (including CO₂ and O₂) than PMP, so the PP membrane 600 is required to be porous throughout. Providing a skin on the PP membrane 600 would reduce or inhibit the ability of the PP membrane 600 to support gas diffusion across it. While the PP membrane 600 allows gaseous exchange, there is risk of damage, as liquids, such as water, may enter the pores and pass through the membrane 600. The presence of liquid in the pores reduces the gas permeability of the porous core 620.

Returning to FIG. 1, the artificial gill 10 comprises a gas reservoir 20. The gas reservoir 20 is partially enclosed by a first surface 16 of the membrane 15. In the example of FIG. 1, the gas reservoir 20 is also partially enclosed by a tubing 18. In other examples, the gas reservoir 16 is fully enclosed by the membrane 15. In some examples, the membrane 15 forms a closed gas reservoir 20 in which gas circulates.

The membrane 15 is configured to enable gaseous exchange between the gas reservoir 20 and water in contact with a second surface 17 of the membrane 15. The membrane 15 is configured to provide gaseous exchange such that gas within the gas reservoir 20 is breathable by a user. The membrane 15 is configured to provide gaseous exchange such that gas within the gas reservoir 20 is continuously breathable by a user, meaning oxygen inhaled from the gas reservoir 20 by a user is at least partially replaced or replenished and carbon dioxide exhaled by the user is removed from the gas reservoir 20.

The artificial gill 10 also comprises a water flow device 30. The water flow device 30 is configured to direct a flow of water over and/or across the second surface 17 of the membrane 15. The second surface 17 is opposite the first surface 16 of the membrane 15. Although it is not shown in FIG. 1, the water flow device 30 can be attached to the air reservoir 20 or any other part of the artificial gill 10, for example such that the components of the artificial gill 10 are connected to form a single unit or structure that is wearable by a user.

The water flow device 30 ensures that water passes over the second surface 17 of the membrane 15. The water flow device 30 may increase the flow rate of water passing over the second surface 17 of the membrane 15. This ensures that the membrane 15 is supplied with fresh water (i.e., water which has not previously undergone exchange with the membrane 15) from which to absorb oxygen and in which to expel carbon dioxide. In some examples, the water flow device 30 is a pump.

In some examples, the water flow device 30 is a diver propulsion vehicle (DPV). In such examples, the DPV, which is part of the artificial gill 10, is configured to be worn by a user or attached to the user. The DPV simultaneously provides a means of propulsion, which assists a user with moving in an underwater environment 5, whilst also ensuring that water is flowing relative to the membrane 15 and directed across the second surface 17 of the membrane 15.

In some examples, the water flow device 30 is an underwater vehicle, such as a submarine, which the artificial gill 10 is connected to. In such examples, the artificial gill 10 can be used to supply oxygen to, and remove carbon dioxide from, the inside of the underwater vehicle. The artificial gill 10 may therefore provide breathable gas to one or more people inside the underwater vehicle.

Although it is not shown in FIG. 1, the artificial gill 10 comprises attachment means for attaching the artificial gill to a user. In some examples, the artificial gill 10 comprises apparatus (for example, handles, belts or straps) enabling the user the wear the artificial gill 10 (for example, like a rucksack bag using straps 45 as shown in FIG. 3), although that is not essential. In the example shown in FIG. 3, the membrane 15 and the gas reservoir 20 are located within a container 50. The water flow device 30 may be in fluid communication with an internal space of the container 50 to direct a flow of water over the second surface 17 of the membrane 15, for example via one or more pipes or tubes (not shown). A user may be able to breathe the gas in the gas reservoir 20, for example via one or more other pipes or tubes and a mouthpiece or respirator (not shown but described further below) in fluid communication with the gas reservoir 20. In other examples, any other suitable means may be used for attaching the artificial gill 10 to a user. For example, the artificial gill 10 may comprise a portion configured to be worn by the user (e.g., over at least a part of the torso of the user) on which the other components of the artificial gill 10 are mounted. In the example shown, the artificial gill 10 comprises a supplemental oxygen tank 70 (described further below), although that is not essential.

FIG. 4 shows another artificial gill 210. The artificial gill 210 is substantially similar to the artificial gill 10 of FIG. 1, with like reference numbers indicating corresponding features.

The membrane 215 of artificial gill 210 comprises a plurality of fibres 212. The membrane 215 may comprise any number of fibres 212. In the example shown, the fibres 212 are hollow fibres. Having a membrane 215 comprising a plurality of hollow fibres 212 may increases the surface area of the first and second sides of the membrane 215.

In the artificial gill 210, gas within the gas reservoir 220 flows through the channel defined by the centre of the hollow fibres 212 as it passes through the membrane 215. The increased surface area provided by the fibres increases the contact area between the membrane 215 and the gas within the gas reservoir 220, and between the water in the surrounding environment 205 and the membrane 215. Increasing the contact area increases the rate of gaseous exchange, thus enabling the gas within the gas reservoir 220 to uptake more oxygen and expel more carbon dioxide whilst reducing or minimising a size of the membrane 215 compared, for example, to a planar or sheet membrane.

For clarity, the second surface of the membrane 215 comprises, or is defined by, the outer surfaces of the hollow fibres 212. The water flow device 230 is configured to direct water over the outer surfaces of the hollow fibres 212. The first surface of the membrane 215 comprises, or is defined by, the inner surface of the hollow fibres 212 (i.e., the surface of the fibres 212 which defines the hollow passage through the fibres 212).

In the example shown, the fibres 212 are woven into a cross-wound mat, with the mat then rolled and potted into a container 250 comprising a fluid inlet 252a and outlet 252b, and a gas inlet 254a and outlet 254b, as shown in FIG. 5. The gas inlet 254a and outlet 254b are in fluid communication with the internal channels of the hollow fibres 212, whilst the fluid inlet 252a and outlet 252b are in fluid communication with the space enclosed by the container 250 and surrounding the fibres 212. In the example shown, the fibres 212 comprise a non-porous skin on the outer surface of the fibre 212, although that is not essential.

In the example shown in FIG. 5, the respective directions of gas flow and water flow are opposite to one another, known as counter-current flow. That may maximise a concentration gradient across the membrane to optimise gas transfer. However, that is not essential, and the directions of gas flow and water flow may be substantially the same as one another, or may be in different non-opposing directions.

In other examples, the fibres 212 may be knitted into mats, such as angled mats, and cross-knitted mats. Alternatively, the fibres 212 may be incorporated into the artificial gill 210 as loose individual fibres.

In some alternative examples, the fibres 212 may be arranged into bundles. The fibres 212 may simply be grouped adjacent one another in a bundle. Alternatively, the fibres 212 may be twisted, coiled, folded, or woven into bundles. The fibres 212 may be arranged to maximise the outer surface area of the fibres 212 (which maximises the second surface of the membrane 215) configured to be exposed to water. Maximising the outer surface area of the fibres 212 is one example of how to maximise the rate of gaseous exchange through the membrane 215 while minimising or reducing an overall size of the membrane 215. In other examples, the fibres may be arranged in other ways to maximise the rate of gaseous exchange through the membrane 215.

FIG. 6 shows another artificial gill 310. The artificial gill 310 is substantially similar to the artificial gill 210 of FIG. 4, with like reference numbers indicating corresponding features.

FIG. 6 shows an enlarged view of a hollow fibre 312 of the membrane 315. As shown in FIG. 6, the gas reservoir 320 is at last partially formed by or passes through the hollow centre of the hollow fibre 312. The gas reservoir 320 is at least partially formed by or passes through the hollow centre of each of the hollow fibres 312 in parallel. The water environment 305 is in contact with the outer surface of the hollow fibre 312.

In this example, the hollow fibre 312 comprises a wavy, serpentine or sinusoidal surface. Having a non-straight or textured surface increases the surface area of the hollow fibre 312 over a given length. The increased surface area increases the rate of gaseous exchange between the gas reservoir 320 and the water environment 305. In other examples, the hollow fibres 312 of the membrane 315 comprise a microstructured texture. In some examples, the microstructured texture is configured to increase surface area of the second surface of the membrane 315. In some examples, the microstructured texture of the fibres 312 is additionally or alternatively configured to increase turbulence in water passing over the second surface of the membrane 315 which may improve a rate of gas exchange. In some examples, fibres 312 are designed and manufactured with specific surface patterns/textures using 3D printing or other suitable methods.

FIG. 7 shows another artificial gill 410. The artificial gill 410 is substantially similar to the artificial gill 110 of FIG. 2, with like reference numbers indicating corresponding features. For clarity, the water flow device is not shown in FIG. 7.

The artificial gill 410 of FIG. 7 comprises a respirator 440. The respirator 440 is in fluid communication with the gas reservoir 420. The respirator 440 may be in direct fluid communication with any part of the gas reservoir 420.

The respirator 440 is configured to be worn by a user. The respirator 440 provides a means for a user to breathe the gas within the gas reservoir 420. Gas within the gas reservoir 420 undergoes gaseous exchange with water via the membrane 415 such that gas breathed in and exhaled by the user is regulated (i.e., the levels of CO₂ and O₂ are maintained at a safe breathable level). The artificial gill 410 enables a user to breathe when submerged in an underwater environment 405 for an extended period of time or continuously.

The respirator 440 is configured to fit over a user's nose and mouth. The respirator 440 comprises means for securing the respirator 440 to the user's face.

FIG. 8 shows an overview of an experimental set-up 700 used to investigate the gas permeability of a membrane and performance of an artificial gill in accordance with the present invention, for example as described above. The set-up 700 comprises a gas reservoir 720, a container 715 containing a membrane as described above (for example, similar to container 250 described above with regard to FIG. 5), a water source 735, and a water waste outlet 740.

The gas reservoir 720 is in controllable fluid communication with a gas source 715 via a one-way valve (for example a check valve). In the example shown, the gas source comprises a tank (e.g., a compressed gas canister) containing a custom gas substantially identical to a typical exhaled breath of a human. The custom gas supplied by the gas source comprises, by volume, approximately 4% CO₂, 16% O₂ and 80% N₂. The gas reservoir 720 itself is formed by a closed fluid circuit (tubing in the example shown) comprising the container 715, such that the first surface of the membrane at least partially encloses the air reservoir 720. A gas pump 745, and a plurality of sensors (CO₂ sensor and O₂ sensor in the example shown) are included in the circuit.

The gas pump 745 is configured to circulate air within the gas reservoir 720, such that air repeatedly passes through the container 715 and over a first surface of the membrane. The gas source 750 is configured to periodically, or controllably, inject a predetermined volume of gas into the gas reservoir 720. In the example shown, the injection of gas from the gas source is controlled via a flow controller FC and pressure indicator PI.

The plurality of sensors are configured to determine the percentage volume of O₂ and CO₂ present in the gas reservoir 720.

The water source 735 is a tank of water in the example shown. The water is oxygenated by a pump (for example an aquarium pump) and optionally continuously, or periodically, mixed, to ensure oxygen saturation before testing begins, although that is not essential. A water pump 730 is configured to convey water from the water source 735, over and across the membrane 715, and into the waste water outlet 740.

The gas reservoir 720 is designed to model and/or replicate a gas reservoir used in an artificial gill, such as artificial gills 10, 110, 210, 310, or 410 shown in FIGS. 1-5. The water source 735 is designed to replicate and/or model a source of water in which an artificial gill would be used.

In FIG. 8, three containers or modules 715 each comprising a membrane are connected in parallel, although that is not essential. Each module 715 comprises a plurality of potted hollow PMP fibres. Each of the hollow PMP fibres have walls which are substantially 90 μm thick. Gas circulating in the gas reservoir 720 passes through the hollow centre and across the inner surface of the PMP fibres. The water conveyed by water pump 730 passes over/around and across the outer surface of the PMP fibres within the container 715 715. This set-up 700 enables gaseous exchange to occur between gas in the gas reservoir 720 at least partially enclosed by the PMP fibres and the water flowing over the outer surface of the PMP fibres. The total internal surface area of the membrane (defined by the plurality of fibres) in each container or module 715 is approximately 0.06 m², although that is not essential. Each module 715 comprises a cylindrical housing having internal dimensions approximately 15 cm in length and approximately 15 mm in diameter, although that is not essential. It will be appreciated any suitable size or shape may be used for the module 715 to provide a sufficient or desired surface area of the membrane.

Experiments were performed using a similar set-up to the set-up 700 of FIG. 8. These experiments are explained and discussed below with reference to FIGS. 9 and 10.

FIG. 9 shows the percentage volume of O₂ and CO₂ versus time for an experimental set-up comprising a single container or module 715 (rather than three modules connected in parallel as in FIG. 8). Line 800 on the graph shows that within five minutes, O₂ levels increased from approximately 14.5% to approximately 18.5%. Line 810 on the graph shows that within approximately two minutes, CO₂ levels decreased from approximately 3.7% to below 0.05%. The experimental data shown in FIG. 9 was obtained with the water pump 730 providing a flow rate of substantially 0.5 L/min.

FIG. 10 shows experimental data similar to that shown in FIG. 9, for an experimental set-up comprising four containers or modules 715 connected in series. However, the experiment was repeated three times using different flow rates. Lines 920 and 950 show O₂ and CO₂ levels against time respectively with the water pump 730 providing a flow rate of substantially 1 L/min. Lines 910 and 940 show O₂ and CO₂ levels against time respectively for the water pump 730 providing a flow rate of substantially 0.5 L/min. Lines 900 and 930 show O₂ and CO₂ levels against time respectively with the water pump 730 providing a flow rate of substantially 0.25 L/min. The experimental data demonstrates that the rate of gaseous exchange increases with increasing flow rate. Increasing flow rate also resulted in an increase in water pressure of the water flowing across the second surface of the membrane within the containers 715.

FIG. 11 shows another experimental set-up 1000 similar to the set-up 700 shown in FIG. 8. The experimental set-up 1000 differs from the set-up 700 in that the gas reservoir 1020 does not from a closed loop. Instead, after air passes through the container 1015 comprising the membrane, the air passes through a one-way valve into a waste gas container 1055. Although three containers or modules 1015 connected in parallel are depicted schematically in FIG. 11, four modules 715 connected in series were used as described with respect to FIG. 10 (although that is not essential, and any suitable number of modules 715 may be connected in series and/or parallel).

Using the set-up 1000 of FIG. 11, an experiment was performed wherein substantially 10 cm³ of the custom gas mixture was injected into the gas reservoir 1020 substantially every four seconds, to replicate the exhalation cycle of one human at approximately 1/50^th of the normal volume. The injected gas was designed to replicate the composition of an exhaled human breath as described above. The water pump 1030 was set to provide a flow rate of 1.9 L/min.

FIG. 12 shows the recorded data from the experiment. Line 1120 shows the CO₂ concentration against time and line 1110 shows the O₂ concentration against time. The small fluctuations present in the lines 1120 and 1110 indicate the periodic injection of the custom gas approximately every 4 seconds. The experimental data illustrates that the membrane is capable of rapidly expelling exhaled sufficient CO₂ from the gas reservoir 1020 to a concentration of substantially 0.05% or below (for example, below typical atmospheric concentration), within approximately 30 s—see FIG. 13, which shows an enlarged view of the line 1120 between approximately 0.25 minutes and approximately 2 minutes).

In addition, the membrane is capable of absorbing sufficient O₂ from water flowing over the second surface of the membrane to enable the artificial gill to increase the oxygen concentration within the gas reservoir 1020 to substantially 17.75%. That O₂ concentration is sufficient for a human to breath continuously and is comparable to effective O₂ concentrations at moderate altitude. The US Occupational Safety and Health Administration advises an O₂ concentration of 19.5% by volume as the minimum safe concentration. Exhaled breath contains approximately 16% O₂ by volume and the advised minimum safe concentration is 19.5% O₂ by volume, giving a difference of 3.5%. The membrane is able to increase the O₂ concentration within the gas reservoir 1020 to substantially 17.75% by volume, 1.75% higher than the O₂ concentration of exhaled breath. 1.75% is half of 3.5%, so in the example shown the artificial gill is capable of providing at least 50% of the required O₂ to meet the advised minimum safe concentration. Of course, depending on the surface area of the membrane the artificial gill may be configured to provide between 50% and 100% of the required O₂ to meet the advised minimum safe concentration. Alternatively, the artificial gill may also comprise a supplemental oxygen tank to provide supplemental oxygen to the gas reservoir 1020 when necessary to maintain the advised minimum safe concentration of O₂ within the gas reservoir 1020. It will be appreciated a supplemental oxygen tank may be used with any of the artificial gills described herein.

To demonstrate the effectiveness of the water pump 1030 in improving performance of an artificial gill (in addition to the gas exchange membrane), the pump 1030 was turned on at time position 1101 and turned off at time position 1102. Lines 1110 and 1120 of FIG. 12 show there is a clear and significant increase in the rate of gaseous exchange when the water pump 1030 is turned on, compared with when it is turned off, demonstrating the improvement in performance of an artificial gill with a water flow device directing a flow of water over the second surface of the membrane.

The results obtained from the experimental set-ups 700, 1000 described above demonstrate the ability of an artificial gill in accordance with the invention to enable a user to breathe in an underwater environment. With the larger surface area of membrane that could be provided by a larger container or module 715, 1015 and the greater water flow rate and/or water pressure a larger module would be able to withstand, the apparatus and process are scalable to enable a person to breathe in an underwater environment.

The results obtained from the experimental set-up 1000 described above also enable a required surface area of the PMP membrane (to sufficiently reduce a CO₂ concentration and optionally increase an O₂ concentration within the gas reservoir) for a real-use case to be determined, at least in respect of the experimental parameters used (atmospheric pressure, water flow rate of 1.9 L/min).

The CO₂ sensor measures the amount of CO₂ in the gas reservoir 1020 as a percentage. As the experiment progresses (e.g., water flows), CO₂ levels are seen to decrease. At each data point, the CO₂% may be converted into a volume (cm³) based on the volume of gas injected into the gas reservoir 1020.

To calculate a gas exchange flux across the membrane, the change in CO₂ (cm³) may be divided by the change in time (min), internal surface area of the fibres (cm²) and internal gas pressure (bar), resulting in units of cm³/min/cm²/bar for flux. The internal surface area of the fibres in the membrane contained within the container 1015 was approximately

The average flux is taken for each data point from the start of the experiment (e.g., water flow) until approximately 0.04% CO₂ is reached within the gas reservoir 1020 (substantially atmospheric level of CO₂). That value may then be used to calculate the approximate required surface area of the membrane using the following equation. Exhaled breath contains approximately 4% CO₂ by volume, with each exhaled breath having a volume of substantially 500 cm³ and 16 breaths per minute taken. That requires approximately 320 cm³ of CO₂ to be removed from the gas reservoir 1020 per minute. Similarly, at least approximately 140 cm³ of O₂ may need to be added to the gas reservoir 1020 per minute (in order to provide at least 50% of the required O₂ to meet the advised minimum safe concentration, given each exhaled breath contains approximately 16% O₂ by volume, with each exhaled breath having a volume of substantially 500 cm³ and 16 breaths taken per minute).

From the experimental data collected for the experimental parameters described above, it can be determined a membrane surface area of approximately 43 m² may be required in order to expel sufficient CO₂ from the air reservoir 1020.

It will be appreciated the required surface area may vary with variation in water flow rates and pressure (for example, as delivered by the water flow device and/or due to river and ocean currents etc.), and the particular construction of the membrane. Tables 1, 2 and 3 below demonstrate estimated required surface areas with variation in each of water flow rate, porosity of the PMP membrane and wall thickness of the PMP membrane. The estimated surface areas were determined using linear relationships experimentally determined for those parameters, for example as shown in FIGS. 14A and 14B, which respectively show predicted required surface areas for CO₂ removal from and O₂ addition to the gas reservoir as a function of flow rate in experiments performed using the experimental set-up shown in FIG. 8. A substantially linear relationship can be seen between flow rate and predicted required surface area of the membrane. Similar relationships are expected for predicted required surface area as a function of porosity or as a function of membrane wall thickness, so the linear relationship observed between flow rate and membrane surface area was also used to model the relationship between those parameters and membrane surface area. The exact form of the linear relationship may depend on experimental set-up and parameters, for example the linear relationship obtained using the experimental set-up of FIG. 8 may be different to the linear relationship obtained using the experimental set-up of FIG. 11.

For each of Tables 1, 2 and 3, the variation in required surface area with variation in the relevant parameter was calculated holding the other two parameters at the experimental values used (e.g., flow rate of substantially 1.9 L/min, porosity of substantially 50%, and membrane wall thickness of substantially 90 μm). The linear relationship used to determine the variation in required surface area with variation in each respective parameter was obtained using the experimental set-up in FIG. 11.

TABLE 1
Table showing variation in required membrane surface area for
CO2 removal and O2 addition with variation in water flow rate

Required surface area (m2)

	Flow rate	Fluid velocity	For CO2	For O2
	(L/min)	(m/min)	removal	addition

	1.90	16.8	43.50	699.58
	0.10	0.9	826.48	13292.06
	0.50	4.4	165.30	2658.41
	1.00	8.8	82.65	1329.21
	1.50	13.3	55.10	886.14
	2.00	17.7	41.32	664.60
	3.00	26.5	27.55	443.07
	10.00	88.5	8.26	132.92
	20.00	177.0	4.13	66.46
	100.00	885.0	0.83	13.29
	0.01	0.1	6887.34	110767.20
	0.12	1.1	688.73	11076.72
	1.20	10.6	68.87	1107.67
	12.00	106.2	6.89	110.77
	120.00	1061.9	0.69	11.08

TABLE 2
Table showing variation in required membrane surface area for
CO2 removal and O2 addition with variation in membrane porosity

Required surface area (m2)

Porosity	For CO2	For O2
(%)	removal	addition

50.0	43.50	699.58
0.1	21749.49	349791.16
1.0	2174.95	34979.12
10.0	217.49	3497.91
20.0	108.75	1748.96
40.0	54.37	874.48
80.0	27.19	437.24

TABLE 3
Table showing variation in required membrane surface area for
CO2 removal and O2 addition with variation in membrane wall thickness

Required surface area (m2)

Wall thickness	For CO2	For O2
(um)	removal	addition

90.0	43.50	699.58
1.0	0.48	7.77
10.0	4.83	77.73
20.0	9.67	155.46
40.0	19.33	310.93
80.0	38.67	621.85

Depending on the water flow device used, a water flow rate may be between substantially 0.10 L/min and substantially 10 L/min, preferably between substantially 1 L/min and substantially 10 L/min and further preferably between substantially 1 L/min and substantially 5 L/min, although any suitable water flow rate may alternatively be used.

From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of artificial gills and breathable membranes, and which may be used instead of, or in addition to, features already described herein.

Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

For the sake of completeness, it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, and any reference signs in the claims shall not be construed as limiting the scope of the claims.