Mihail Barboiu
email: mihail-dumitru.barboiu@umontpellier.fr

ARTIFICIAL WATER CHANNELS -TOWARD BIOMIMETIC MEMBRANES FOR DESALINATION

Natural systems have evolved for millions of years to transport metabolites across the cell membranes. Among them, Aquaporins (AQPs) are biological channels known for fast water transport with ion exclusion. AQPs can be mimicked using simpler artificial water channels-AWCs to explore the naturally evolved water transport in artificial systems and to assess the basis for biomimetic desalination processes. Selective water translocation has been shown to depend on water-wires spanning the pore that induce dipolar orientation. Toward this objective, AWCs combining a particularly broad range of self-assembling features: H-bonding, water recognition, hydrophobic effects, etc. This might play an important role in the ability to finely control the hydrophilic or hydrophobic the constitutional domains of the channel responsible, for selective recognition and highwater translocation, respectively. Densely packed AWCs incorporated within TFC polyamide membranes will lead to a greater fundamental understanding on how selective pores can be optimized at the nanoscale to facilitate ultrafast and highly selective transport of water for the treatment of saline feed streams. Future routes to robust desalination membranes including AWCs will solve pressing challenges, such as the stability, scalability and fabrication of membrane for water desalination, ultrapure water production and the storage of clean energy applications

Universal access to safe drinking water is a fundamental need and human right. [START_REF] Eliasson | The rising pressure of global water shortages[END_REF] Competition over increasingly contaminated freshwater during the last decades has made the availability of this resource a serious global challenge for the 21st century. Water scarcity and the problems associated with the lack of the clean water are well known. Because oceans hold 97.5% of the total water in our planet, [START_REF] Schewe | Multimodel assessment of water scarcity under climate change[END_REF] desalination is considered as one of important solutions to problem of freshwater scarcity, along with environmental remediation technologies and conservation policies. Currently, about 100 million m 3 /day of desalinated water is being produced, with an annual increase of more than 10% [START_REF] Kocsis | Artificial water channels-deconvolution of natural Aquaporins through synthetic design[END_REF]. To address the increasing demand for water supply, several cutting-edge technologies have been explored during the last decades. [START_REF] Mekonnen | Four billion people facing severe water scarcity[END_REF] Current technologies are based on thermal processes (Multi Effect Desalination -MED or Multi Stage Flash-MSF) and membrane processes (Reverse Osmosis-RO, Electrodialysis-ED, Membrane Distillation-MD and Forward Osmosis-FO), with RO being the dominant process used for seawater or brackish water desalination. [START_REF] Imbrogno | Membrane Desalination: Where Are We, and What Can We Learn from Fundamentals?[END_REF] Most of the RO membranes relies on an anisotropic polymerized thin-film composite (TFC) polyamide-PA layer produced by interfacial polymerization on an underlying porous support layer, commercially available polysulfone, all integrated into spiral wound modules for fitting in pressure vessels [START_REF] Park | Maximizing the right stuff: The trade-off between membrane permeability and selectivity[END_REF]. Current Reverse Osmosis-RO desalination can produce water at < $ 1/m 3 , which is about 5 times the cost of treated surface water. Energy use has dropped but is still about twice the thermodynamic minimum required when operating at 60% recovery, is around 1.2 kWh/m3 and this value increase above 1.5 kWh/m3 at 80% recovery. [START_REF] Park | Maximizing the right stuff: The trade-off between membrane permeability and selectivity[END_REF] Closed-circuit RO will soon allow operating at these high recovery regimes but with higher minimum energy requirements. There is need for membrane innovation. The target of the future membrane is to increase both the water permeability while achieving high salt rejection. As pointed out recently, in many cases selectivity matters as much if not more than permeability. Efficiency may increase in the coming years by changing the process conditions, or by changing the membranes. [START_REF] Werber | Materials for next-generation desalination and water purification membranes[END_REF] The RO desalination, using thin-film composite-TFC membranes, has evolved over the years bringing down the costs significantly. [START_REF] Kumar | Water Desalination: History, Advances, and Challenges[END_REF] But, the desalination is still relatively expensive. More than half of a century has passed since the first functional RO membrane was designed. [START_REF] Cadotte | Interfacially synthesized reverse osmosis membrane[END_REF] New materials bearing promise for higher productivity in desalination have been developed in recent years, consequentially leading to the emerging research fields of thin-film nanocomposites-TFN [START_REF] Pendergast | A review of water treatment membrane nanotechnologies[END_REF] and biomimetic membranes [START_REF] Barboiu | From natural to bio-assisted and biomimetic artificial water channel systems[END_REF]. Recent research propose to find an easily scalable Artificial Water Channels AWC approach that could be immediately applied to desalination systems to increase their energy efficiency.

Natural water channels -toward bio-assisted Aquaporins-embedded membranes. Natural systems have evolved for millions of years to accept functional biostructures to transport metabolites across the cell membranes. Most of natural physiological processes depend on selective exchanges between a cell and its environment and water is playing an important role. The natural water channels, the Aquaporins (AQP) are known for their fast transport rates (~10 8 -10 9 water molecules/s/channel) and the perfect rejection of ions. [START_REF] Agre | Aquaporin Water Channels (Nobel Lecture)[END_REF] Due to its energy efficiency, membrane technologies start to adapt the AQP water purification technologies. [START_REF] Tang | Desalination by biomimetic aquaporin membranes: Review of status and prospects[END_REF] The high permeability / selectivity of AQPs inspired their incorporation into membranes for water-purification applications. This way, a new branch of membrane technologies for water purification called AQP bioassisted membranes was developed. As so it was an attractive option to combine the potential of the AQPs with the current existing technology in membrane-based water filtration. As soon as the functions of the AQPs became know, it seems logical that in order to make use of its high selectivity and water transport, the protein need to be used in conjunction with a membrane barrier. Their large-scale applications it is of interest, owing to the high costs of the AQPs production, low stability and practical synthetic constraints of membrane fabrication processes. In the simplest fashion these membranes consisted of three main components: i) AQP proteins ii) amphiphilic molecules as building blocks for the construction of protecting liposomes in which the AQPs are embedded and iii) a polymeric support for mechanical resistance. The choice for the amphiphilic molecules is argued for by the fact that the AQPs are transmembrane proteins and their native environment is partly the hydrophobic region of the lipids found in cellular membranes, meanwhile keeping the "mouths" of the protein exposed to the water from the cytosol. As such, a first reasonable strategy was to fabricate membranes having an active barrier a lipid bilayer containing AQP for the enhanced water permeation. It was shown showed that the supported lipid bilayer (SLBs) membranes are suitable impermeable platforms for incorporation of AQPs for water filtration. However, no reliable results have been obtained with such AQPs/ bilayers. In a compromise to obtain high stability and relatively good performance bio-assisted membranes a different synthetic strategy was applied by using a mix-matrix approach, when bilayer vesicles incorporating AQPs has been incorporated into traditional polyamide thin films [START_REF] Zhao | Synthesis of robust and high-performance aquaporin-based biomimetic membranes by interfacial polymerization-membrane preparation and RO performance characterization[END_REF]. This way the AQPs can maintain their efficiency having a basically native lipid membrane environment and at the same time, having minimal defect influences on the composite membrane in interaction with the polyamide matrix (Figure 3). The hybrid AQP-PA based membranes proved to be 40% permeable than their simple thin film composites (TFC) counterparts, meanwhile being stable for periods relevant for industrial applications at a lower energy consumption and operating cost. [START_REF] Qi | Aquaporin-based biomimetic reverse osmosis membranes: Stability long-term performance[END_REF] Most of the current designs for AQP membranes involve polymeric host materials, which can make it viable for upscaling and use in industry. Active high flux AQP membranes, became the commercially available products. Production of AQP proteins is a feasible task since biological protocols and reagents are used, but the purification setup still represents an expensive and time-consuming obstacle. The question 'how the natural proteins /artificial host matrix systems can be efficiently used together' has been thus partially answered. [START_REF] Hinds | Aligned Multiwalled Carbon Nanotube Membranes[END_REF][START_REF] Holt | Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes[END_REF][START_REF] Hu | Enabling Graphene Oxide Nanosheets as Water Separation Membranes[END_REF] However, no clear strategies converge to the easy synthetic development of robust AQP bio-assisted reverse osmosis (RO) or Forward osmosis (FO) membranes, working at high pressure and high salt concentrations, incompatibles with biological AQPs. Most importantly, today not readily usable AQP-based membrane exists for desalination. They are many challenges to use the AQP as selective transporters related to their: a) Long term stability when embedded in artificial membranes; b) Bio-stability and unconventional processing requirements: high pressure and salinity; c) Production -membrane proteins are challenging to mass produce; d) AQPs present an important flux, but not an acceptable permeability per active area, the selectivity is not reaching more than 97%, a critical problem for desalination. A possible way forward is to replace AQPs with synthetic channels. Recent advancements have made it possible to synthesize artificial water channels-AWCs that feature efficient, yet selective transport of water based on size exclusion and H-bond clustering at the molecular level. [START_REF] Barboiu | Artificial Water Channels -incipient innovative developments[END_REF] Another of the current disadvantages of biomimetic AQPs membranes is the low density of water channels per unit area of ~ 40 channels/µm 2 .

Why are Artificial Water Channels important? Biomimetics or biomimicry is the science behind mimicking the well-tuned natural processes. Nature should provide answers for efficient desalination. A possible way forward is to replace AQPs with synthetic AWCs. Ionrejecting, highly permeable artificial biomimetic channels may provide a route to producing highly efficient membranes for separations, particularly desalination. In practice, there is currently no economically viable RO membrane on the market! Therefore, there are incentives to further reduce cost and energy use for desalination as it becomes more widespread. Recent advancements have made it possible to synthesize AWCs that feature efficient transport of water. AWCs exhibit the following potential advantages: a) they are easily tunable and may be chemically optimized for desirable properties; b) they can be prepared on a large scale at low cost; c) Engineering processing without bio-related steps becomes simpler and more reproducible; d) they should also be robust with a longer lifetime into membranes. Within this context, the next impact may be related to increase the incipient potential, imagining that high water conduction activity obtained with natural AQPs can be bio-mimicked using simpler compounds that can display functions like the natural ones, the Artificial water channels-AWCs, using synthetic biomimetic approaches. AWCs have been recently proposed as a biomimetic AQPs. [START_REF] Duc | Imidazole-Quartet Water and Proton Dipolar Channels[END_REF][START_REF] Barboiu | Artificial water channels[END_REF] They are constructed from artificial molecules defining a water-pore superstructure, surrounded by a hydrophobic exterior accommodating the membrane environment. [START_REF] Duc | Imidazole-Quartet Water and Proton Dipolar Channels[END_REF][START_REF] Barboiu | Artificial water channels[END_REF][START_REF] Barboiu | An artificial primitive mimic of the Gramicidin-A channel[END_REF][20][START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF][START_REF] Kocsis | Oriented chiral water wires in artificial transmembrane channels[END_REF][START_REF] Murail | Water permeation across artificial I-quartet membrane channels: from structure to chaos[END_REF] Within this context, mimicking AQPs by using simple AWCs is an important endeavor from both a) the fundamental science perspective related to the deeper understanding of water structure and b) its transport function and applied science perspective as AWCs might have timely impact on increasing water filtration efficiency by using natural principles to change the desalination paradigms. Artificial ion-channels have been extensively developed for decades (thousands of papers in literature) with the hope to mimic the ionic conduction of natural proteins. Amazingly, there was no systematic and little progress in the area of Artificial water channels before 2011. [START_REF] Barboiu | Artificial Water Channels -incipient innovative developments[END_REF] Where are we now? Where are we going in this newborn field? The target of the future membrane is to increase both the water permeability by 3 to 5 folds of the currently best TFC membranes while achieving/maintaining 99,9% salt rejection. As pointed out recently in many cases selectivity matters as much if not more than permeability. Next research might provide answers to essential question related to: a) complicated interactional scenarios related to water transport through biomimetic pores; b) bio-inspired desalination with membranes that mimic mechanisms of natural desalinators. Simpler architectures encapsulating water may provide excellent and valuable models for understanding/ novel biomimetic water channel systems. Although there is a rich inventory when it comes to molecular encapsulation of water, in most cases the host systems are highly polar which prohibits them to be used in conditions similar to the one offered by the membrane to AQPs. As so, the development of the AWCs is highly important. The first synthetic water transporting systems have been reported in 2011 [START_REF] Duc | Imidazole-Quartet Water and Proton Dipolar Channels[END_REF] and the term of Artificial water channels has been coined practically one year later [START_REF] Barboiu | Artificial water channels[END_REF]. The common features of the AWCs are the presence of a central pore, which is a confining hydrophilic or hydrophobic region that allows the passage of water, and a hydrophobic shell that permits the insertion into the lipid membranes and subsequent analysis for water transport. There are main approaches towards designing these systems:  Single molecular channels, as one molecular entity that can span the length of the bilayer;  Self-assembled supramolecular channels that can self-assemble from multiple molecular components into architectures spanning the bilayer.  Nanochannels related to Carbon Nanotubes and GO materials and membranes Single Molecular Channels. The pillar [START_REF] Imbrogno | Membrane Desalination: Where Are We, and What Can We Learn from Fundamentals?[END_REF]arenes, PAP1 are the first reported single molecular channels [START_REF] Hu | Single-Molecular Artificial Transmembrane Water Channels[END_REF]. Intramolecular H-bonds between lateral hydrazide arms confer robustness, keeping the membrane spanning channels intact, when inserted into a lipid bilayer. PAP1 have a relatively low water transport rate: up to 40 molecules of water/s/channel. Like in AQPs, PAP1display water selectivity against H + and OH -ions. The explanation for this is the impeded proton hoping through a broken, discontinuous water wire, while the cation transport can be attributed to carrier transport mechanism. Following up on these results a second generation of more hydrophobic pillar [START_REF] Imbrogno | Membrane Desalination: Where Are We, and What Can We Learn from Fundamentals?[END_REF]arenes, PAP2 containing poly-Phe arms showed a osmotic water permeability of 3.5*10 8 water/s/channel [START_REF] Shen | Highly permeable artificial water channels that can self-assemble into two-dimensional arrays[END_REF], being in the range of AQPs and CTNs.

Self-assembled supramolecular channels. The second category of AWCs use a non-covalent selfassembly strategy, that often confer the supramolecular channels properties that are not predictable and differ completely from that of their building blocks. Based on this concept it has been reported that AWCs that can self-assembly in tubular imidazole I-quartet channels, mutually stabilized by water wires [START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF]. They show similar structural features as found in the AQP pores, with a central pore size of ~2.6 Å. The water molecules form a single molecular wire adopting a unique dipolar orientation inside the chiral I-quartets. The I-quartet channels are able to transport ~1.5 x 10 6 water/s/channel, which is within two orders of magnitude of AQPs' rates but most important, reject all ions except protons. [34] This ion-exclusion selectivity is important and unique for the AWCs. It is based on dimensional steric reasons, whereas hydrophobic and hydrodynamic effects appear to be less important. Recently, the research on water channels has been pushed a step further showing that a self-assembled "T-channel" is formed with morphological characteristics reminiscent to natural Gramicidin-A [START_REF] Barboiu | An artificial primitive mimic of the Gramicidin-A channel[END_REF]. The formation of an aqua-lubricated channel is generated via H-bonding between water molecules and carbonyl groups identical to the carbonyl strings in AQps.

Nanochannels. At the nanolevel, carbon nanotubes-CNTs offer an exciting source of inspiration to mimic natural channels. [START_REF] Hinds | Aligned Multiwalled Carbon Nanotube Membranes[END_REF][START_REF] Holt | Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes[END_REF] The frictionless water flow is 4-5 orders of magnitude increasing over what would be seen in other conventional nanoporous or AQP structures. Important studies show ionrejection at low salt concentrations which is promising for applications. In the past few years, membranes based on graphene and graphene oxide (GO) have gained considerable interest for their potential for desalination. [START_REF] Hu | Enabling Graphene Oxide Nanosheets as Water Separation Membranes[END_REF] The swollen membrane allows any selectivity as water molecules are transported with ions. However, the spacing in a GO membrane can be controlled by physically restraining the membrane from swelling. When interlayer spacing is reduced to 7.0 Å, the ion permeation rate was reduced by two orders of magnitude while the water permeation was only halved.

Outline beyond state of art:

All these examples studied must restore some balance to our understanding of the biomimicking of natural AQPs and remains an important exploring challenge. These findings show that natural channels can be bio-mimicked using simpler artificial compounds displaying constitutional functions like the natural ones (Table 1).The current research on AWCs is focused on their synthesis, molecular simulations and transport properties in lipid bilayer membranes. The osmotic water permeability of artificial systems ranges from several orders of magnitude lower than that of AQP1, with similar or even better values for Carbon Nanotubes CTNs (Table 1). The two main challenges for AWCs are i) to design tailored channels that reject salt but allow the easy passage of water molecules and ii) the transfer of the AWCs from the nanomolecular level to that of large-scale membrane materials. Although this has been partly achieved using AQPs, no studies have been reported yet on integrating the AWCs within active layers casted onto membrane supports. Designing stable polymer matrixes capable of keeping AWCs in their active state will prove crucial for future hybrid membranes. The use of lipid bilayers or hydrophobic polymers as host matrixes, which certainly can be affected by the presence of organic solutes or surfactants in water, is not a necessary option. They have been used until now to compare the AWC with AQP One strategy to obtain active layer is to incorporate the AWC networks while forming the Polyamide PA layer of RO membrane or to imagine new cross-liked polymers in order to obtain networks of AWC within the thin layers. If AWC are covalently attached to TFC, there is no degradation of layer, only temporary channel destabilization. This way the AWCs are dense and highly robust, would be encapsulated in the dense part of the membrane while the PA matrix should be responsible for the rejections of organics.

To compete with polymeric reverse osmosis membranes, the current gold standard for desalinating seawater, water flux and rejection rates for artificial water channels need to be very close to that of AQPs. While having even higher flux values will not have a substantial impact on membrane filtration performances, highly water specific membranes will improve the quality of water produced as well as remove unnecessary pretreatment processes. Given the importance of developing even more efficient membranes for desalination, this will be an exciting and challenging project, potentially redefining the paradigms of desalination (Figure 1).

Conclusions

The biomimetic water transport can solve/improve the performances and life-time of the bio-assisted membrane systems in which highly selective, but labile AQPs are used. In terms of economic gain and the economy of matter the artificial systems, might represent an interesting alternative. This project, scientifically challenging and of great fundamental interest, is leading to pioneering scientific or applicative discoveries, as literature can attest. Biomimetic water transport, fundamental in nature, should allow not only to develop new knowledge and a greater understanding on bottom-up design of multifunctional systems, but also to address different societal challenges. The thin-layer polyamide membranes, used for water desalination since 1972 has improved dramatically over the last 60 years [START_REF] Yang | The upper bound of thin-film composite (TFC) polyamide membranes for desalination[END_REF], however their low-resolution structure and the transport mechanism are still poorly understood. The ultimate performance of a membrane is usually determined by an increased flux (or permeance) due to the introduction of the defects in the membrane structure lowering the selectivity which is always decreasing. The first identification of Artificial water channels-AWCs, opened the door to new applicative desalination processes close to natural ones. We speculated at this time that biomimetic Artificial Water Channels -AWCs might have timely important influences to increase this driving force for the transport by using natural principles to change the water desalination paradigms.

Figure 1 .

 1 Figure 1. RO versus biomimetic artificial water channels for desalination strategies (image adapted with permissions from reference [3]).

Table 1 .

 1 Performance overview of artificial water channels (adapted with permission from[START_REF] Kocsis | Artificial water channels-deconvolution of natural Aquaporins through synthetic design[END_REF]).

	Water Channels	Net Permeability for water / Single	Selectivity
		channel permeability
	Aquaporins -AQPs.	167 µm/s/bar	High selectivity for water, perfect
		4 x 10 9 water molecules/s/channel	rejection of ions/H +
	Carbon Nanotubes-CTNs	300 µm/s/bar, 1.9 x 10 9 water	-
	Estimated diameter = 15 Å	molecules/s/channel
	Carbon Nanotubes-CTNs	520 µm/s/bar, 2.3 x 10 10 water	ion rejection in dilute solutions
	Estimated diameter = 8 Å	molecules/s/channel
	Hydrophilic I-quartets,	3-4 µm/s/bar	high selectivity for water, reject all
	channel (2.6 Å)	1.5 x 10 6 water molecules/s/channel	ions except protons
	Aquafoldamers, Hydrophilic	No permeability reported	high selectivity for water, reject all
	channel (2.8 Å)		ions except protons
	Hydropobic peptide tubular	51 μm/s/bar	no selectivity for water,
	macrocyclic pores (6.4 Å)	4.9 x 10 7 water molecules/s/channel	high conduction for K + and H +
	Hydrazide appended	8.6 × 10 -6 μm/s/bar	no selectivity for water conduction for
	Pillar[5]arene, PAP1	40 molecules/s/channel	alkali cations no conduction for H +
	unimolecular channel (6.5 Å)
	Peptide appended	swelling: 30 µm/s/bar or 3.5 10 8	no selectivity for water,
	Pillar[5]arenes, PAP2	shrinking: 1 µm/s/bar or 3.7x10 6	good conduction for alkali cations
	unimolecular channel (5 Å)	molecules/s/channel
	Double helical water T-channels	No permeability reported	Enhanced conduction states for cations
	(~2.5-4 Å)		and for H +
	RO membrane	2 µm/s/bar	99,8% rejection of salts versus water
	FO membrane	2,5 µm/s	-
	RO membrane	5,3 µm/s


Maria Di Vincenzo

Sophie Cerneaux

Istvan Kocsis

Mihail Barboiu
email: mihail-dumitru.barboiu@umontpellier.fr

Artificial Water Channels -toward next-generation Reverse Osmosis Membranes

Natural systems have evolved for millions of years to transport metabolites across the cell membranes. Among them, Aquaporins (AQPs) are biological channels known for fast water transport with ion exclusion. AQPs can be mimicked using simpler artificial water channels-AWCs to explore the naturally evolved water transport in artificial systems and to assess the basis for biomimetic desalination processes. Selective water translocation has been shown to depend on water-wires spanning the pore that induce dipolar orientation. Toward this objective, AWCs combining a particularly broad range of self-assembling features: Hbonding, water recognition, hydrophobic effects, etc. This might play an important role in the ability to finely control the hydrophilic or hydrophobic the constitutional domains of the channel responsible, for selective recognition and high-water translocation, respectively. Then, densely packed AWCs incorporated within TFC polyamide membranes will lead to a greater fundamental understanding on how selective pores can be optimized at the nanoscale to facilitate ultrafast and highly selective transport of water for the treatment of saline feed streams. Future routes to robust desalination membranes including AWCs will solve pressing challenges, such as the stability, scalability and fabrication of membrane for water desalination, ultrapure water production and the storage of clean energy applications

1.

Introduction

Universal access to safe drinking water is a fundamental need and human right. [START_REF] Eliasson | The rising pressure of global water shortages[END_REF] Competition over increasingly contaminated freshwater during the last decades has made the availability of this resource a serious global challenge for the 21st century. Water scarcity and the problems associated with the lack of the clean water are well known. Because oceans hold 97.5% of the total water in our planet, [START_REF] Schewe | Multimodel assessment of water scarcity under climate change[END_REF] desalination is considered as one of important solutions to problem of freshwater scarcity, along with environmental remediation technologies and conservation policies. Currently, about 100 million m 3 /day of desalinated water is being produced, with an annual increase of more than 10% [START_REF] Kocsis | Artificial water channelsdeconvolution of natural Aquaporins through synthetic design[END_REF].

Treated as a lack of essential resource; it is becoming increasingly urgent in the context of simultaneous growth of population and economic activities. To address the increasing demand for water supply, several cutting-edge technologies have been explored during the last decades. [START_REF] Mekonnen | Four billion people facing severe water scarcity[END_REF] Current technologies are based on thermal processes (Multi Effect Desalination -MED or Multi Stage Flash-MSF) and membrane processes (pressure driven Reverse Osmosis-RO, voltage driven Electrodialysis-ED, Membrane Distillation-MD and Forward Osmosis-FO), with RO being the dominant process used for seawater or brackish water desalination. [START_REF] Imbrogno | Membrane Desalination: Where Are We, and What Can We Learn from Fundamentals?[END_REF] Most of the reverse osmosis membranes relies on an anisotropic polymerized thin-film composite (TFC) polyamide-PA layer produced by interfacial polymerization on an underlying porous support layer, commercially available polysulfone, all integrated into spiral wound modules for fitting in pressure vessels [START_REF] Park | Maximizing the right stuff: The trade-off between membrane permeability and selectivity[END_REF]. Current Reverse Osmosis-RO desalination can produce water at < $ 1/m 3 , which is about 5 times the cost of treated surface water.

Energy use has dropped but is still about twice the thermodynamic minimum required when operating at 60% recovery, is around 1.2 kWh/m3 and this value increase above 1.5 kWh/m3 at 80% recovery. [START_REF] Park | Maximizing the right stuff: The trade-off between membrane permeability and selectivity[END_REF] Closed-circuit RO will soon allow operating at these high recovery regimes but with higher minimum energy requirements.

There is need for membrane innovation. The target of the future membrane is to increase both the water permeability while achieving high salt rejection. As pointed out recently, in many cases selectivity matters as much if not more than permeability. Efficiency may increase in the coming years by changing the process conditions, or by changing the membranes. [START_REF] Werber | Materials for next-generation desalination and water purification membranes[END_REF] The RO desalination, using thin-film composite-TFC membranes, has evolved over the years bringing down the costs significantly. [START_REF] Kumar | Water Desalination: History, Advances, and Challenges[END_REF] But, the desalination is still relatively expensive.

More than half of a century has passed since the first functional RO membrane was designed.

[9] Despite the fundamental knowledge, the material structure understanding is highly important for the up-scale from the laboratory films toward membranes, modules and further process designs. New materials bearing promise for higher productivity in desalination have been developed in recent years, consequentially leading to the emerging research fields of thin-film nanocomposites-TFN [START_REF] Pendergast | A review of water treatment membrane nanotechnologies[END_REF] and biomimetic membranes [START_REF] Barboiu | From natural to bio-assisted and biomimetic artificial water channel systems[END_REF]. The prime objective nowadays is to explore the naturally evolved desalination pathways and assess the possibilities of using them as the basis for 'engineered' desalination processes of enhanced performance. Recent research projects propose to find an easily scalable Artificial Water Channels AWC approach that could be immediately applied to desalination systems to increase their energy efficiency. This chapter will describe the incipient context and evolution of Artificial Water channels field.

Natural water channels -the Aquaporins

Natural systems have evolved for millions of years to accept functional biostructures to transport metabolites across the cell membranes. [START_REF] Ball | Water as an Active Constituent in Cell Biology[END_REF][START_REF] Ball | Water is an active matrix of life for cell and molecular biology[END_REF] Most of natural physiological processes depend on selective exchanges between a cell and its environment and water is playing an important role. [START_REF] Zhong | Biological water: A critique[END_REF] The natural water channels, the Aquaporins (AQP) are known for their fast transport rates (~10 8 -10 9 water molecules/s/channel) and the perfect rejection of ions. [START_REF] Agre | Aquaporin Water Channels (Nobel Lecture)[END_REF][START_REF] Tajkhorshid | Control of the Selectivity of the Aquaporin Water Channel Family by Global Orientational Tuning[END_REF][START_REF] Zeidel | Reconstitution of functional water channels in liposomes containing purified red cell CHIP28 protein[END_REF][START_REF] Kosinska Eriksson | Subangstrom Resolution X-Ray Structure Details Aquaporin-Water Interactions[END_REF] There are several features which are important for the AQPs efficient transport: A) Selectivity filter-SF: AQPs present an hourglass SF structure offering size restriction of ~3 Å and selectivity against cations, reinforced through electrostatic repulsion in the region known as the aromatic arginine (ar/R) constriction. Water in the SF is H-bonded to the protein and to other adjacent waters, allowing the passage of only a single file of molecules. The narrowest diameter of the pore itself is large enough for the water molecules to pass having themselves, but restrictive enough to block the passage of hydrated Na + or Mg 2+ cations, with hydration shells > 3 Å. B) Dipolar orientation of water wires: An interesting collective structuration of water is taking place in the SF, the interconnected water molecules adopting a dipolar orientation outward from the center of the pore. Although protons can pass from one water molecule to another through a Grotthuss mechanism, the inversion center breaks the water wire dipole and prohibits the proton translocation. More recent studies confirm the precise dynamic clustering of water molecules in SF is strongly favored via donor-acceptor H-bonding, excluding other small molecules, including the hydroxide anions, as they are not able to form a stable Hbonding within the SF. [START_REF] Kosinska Eriksson | Subangstrom Resolution X-Ray Structure Details Aquaporin-Water Interactions[END_REF]

Bio-assisted AQP-embedded membranes

Their high permeability/selectivity inspired the incorporation of AQPs into membranes for desalination. [START_REF] Shen | Biomimetic membranes: A review[END_REF] To tackle this problem an innovative approach was developed by using an amphiphilic artificial matrix hosting the AQPs. In terms of flexibility and stability the block copolymers offered a good solution. The first work to successfully incorporate AQPs into polymer-based vesicles, called polymersomes and prove the functionality of the protein. [START_REF] Kumar | Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z[END_REF] The vesicles were made of a block copolymer with poly-(2-methyloxazoline)-poly-dimethylsiloxane)-poly-(2-methyloxazoline) (PMOXA-PDMS-PMOXA) units and were used to host AQPZ, a bacterial type protein from the Aquaporin family. Using the stopped flow setup, it has been shown that the otherwise impermeable polymersomes undergo an increase of up to 800 times in water permeability when the AQPs are incorporated. (Figure 1) (image adapted with permission from reference [START_REF] Kumar | Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z[END_REF]).

The activation energy for water to pass through the polymersomes was comparable for that obtained in the case of native protein. The typical RO membrane shows a productivity of 1 µm/s/bar, while the modified AQPZ-ABA polymer not as a membrane is estimated for a productivity of 167 µm/s/bar. Indeed an improvement in permeability of ~2 orders of magnitude will not yield to a decrease in energy consumption by the same factor.

Due to its energy efficiency, membrane technologies start to adapt the AQP water purification technologies. [START_REF] Tang | Desalination by biomimetic aquaporin membranes: Review of status and prospects[END_REF][START_REF] Shannon | Science and technology for water purification in the coming decades[END_REF] The high permeability / selectivity of AQPs inspired their incorporation into membranes for water-purification applications. This way, a new branch of membrane technologies for water purification called AQP bioassisted membranes was developed. As so it was an attractive option to combine the potential of the AQPs with the current existing technology in membrane-based water filtration. As soon as the functions of the AQPs became know, it seems logical that in order to make use of its high selectivity and water transport, the protein need to be used in conjunction with a membrane barrier. Their large-scale applications it is of interest, owing to the high costs of the AQPs production, low stability and practical synthetic constraints of membrane fabrication processes. [START_REF] Tang | Desalination by biomimetic aquaporin membranes: Review of status and prospects[END_REF][START_REF] Shannon | Science and technology for water purification in the coming decades[END_REF][START_REF] Song | Design considerations for artificial water channel-based membranes[END_REF] Figure 2. Schematic presentation of the structure of the AQPZ-incorporated SLB a) on charged NF membrane supports [START_REF] Kaufman | Supported Lipid Bilayer Membranes for Water Purification by Reverse Osmosis[END_REF] and b) of a covalent amide bond 1,2-Dioleoyl-snglycero-3-phosphoethanolamine (DOPE) on top of a polydopamine (PDA) layer (orange), coated on a polyethersulfone (PSf) (gray) porous support. [START_REF] Ding | Fabrication of an aquaporin-based forward osmosis membrane through covalent bonding of a lipid bilayer to a microporous support[END_REF] (image adapted with permissions from references [START_REF] Kaufman | Supported Lipid Bilayer Membranes for Water Purification by Reverse Osmosis[END_REF][START_REF] Ding | Fabrication of an aquaporin-based forward osmosis membrane through covalent bonding of a lipid bilayer to a microporous support[END_REF]).

In the simplest fashion these membranes consisted of three main components: i) AQP proteins ii) amphiphilic molecules as building blocks for the construction of protecting liposomes in which the AQPs are embedded and iii) a polymeric support for mechanical resistance. The choice for the amphiphilic molecules is argued for by the fact that the AQPs are transmembrane proteins and their native environment is partly the hydrophobic region of the lipids found in cellular membranes, meanwhile keeping the "mouths" of the protein exposed to the water from the cytosol. As such, a first reasonable strategy was to fabricate membranes having an active barrier a lipid bilayer containing AQP for the enhanced water permeation. It was shown showed that the supported lipid bilayer (SLBs) membranes are suitable impermeable platforms for incorporation of AQPs for water filtration. However, no reliable results have been obtained with such AQPs/ bilayers (Figure 2a). [START_REF] Kaufman | Supported Lipid Bilayer Membranes for Water Purification by Reverse Osmosis[END_REF] It was also demonstrated that the use of covalently fixed Supported Lipid Bilayers on a Psf support can yield functional membranes with higher stability. [START_REF] Ding | Fabrication of an aquaporin-based forward osmosis membrane through covalent bonding of a lipid bilayer to a microporous support[END_REF] This was achieved through the covalent attachment of SLBs to a polydopamine coated polyehtersulfone. Using an amine functionalized lipid, the bottom half of the lipid bilayer is attached to the free carboxylic groups that can be found on the polydopamine. The covalent bonding between the bilayer and the support prevents the easy desorption of the lipid matrix, offering stability to the active layer of the membrane (Figure 2b). Nevertheless, the SLB is still prone to degradation when in contact with solutions containing detergents or high salinity, which can readily disrupt the lipid bilayer. The performances of these SLB membranes are remarkable in water filtration, although the reliability of these remains an issue for long term applications and upscaling.

In a compromise to obtain high stability and relatively good performance bio-assisted membranes a different synthetic strategy was applied by using a mix-matrix approach, when bilayer vesicles incorporating AQPs has been incorporated into traditional polyamide thin films [START_REF] Zhao | Synthesis of robust and high-performance aquaporin-based biomimetic membranes by interfacial polymerization-membrane preparation and RO performance characterization[END_REF]. This way the AQPs can maintain their efficiency having a basically native lipid membrane environment and at the same time, having minimal defect influences on the composite membrane in interaction with the polyamide matrix (Figure 3). The hybrid AQP-PA based membranes proved to be 40% permeable than their simple thin film composites (TFC) counterparts, meanwhile being stable for periods relevant for industrial applications at a lower energy consumption and operating cost. [START_REF] Qi | Aquaporinbased biomimetic reverse osmosis membranes: Stability and long-term performance[END_REF] Most of the current designs for AQP membranes involve polymeric host materials, which can make it viable for upscaling and use in industry.

Why are Artificial Water Channels important?

Biomimetics or biomimicry is the science behind mimicking the well-tuned natural processes.

Looking to nature as a model system paves the way for new ideas in exploring sustainable water solutions. Nature should provide answers for efficient desalination. Despite the tremendous imagination of synthetic chemists to produce sophisticated architectures confining water clusters, most strategies to selectively transport water have been related to the use of natural Aquaporin as the selectivity components. A possible way forward is to replace AQPs with synthetic AWCs. Ion-rejecting, highly permeable artificial biomimetic channels may provide a route to producing highly efficient membranes for separations, particularly desalination. Solute-rejecting thin film composite reverse osmosis is currently the best available technology for desalination, but it suffers from high energy consumption. In practice, there is currently no economically viable RO membrane on the market! Therefore, there are incentives to further reduce cost and energy use for desalination as it becomes more widespread. Recent advancements have made it possible to synthesize AWCs that feature efficient transport of water. AWCs exhibit the following potential advantages: a) they are easily tunable and may be chemically optimized for desirable properties; b) they can be prepared on a large scale at low cost; c) Engineering processing without bio-related steps becomes simpler and more reproducible, they should be easily immobilized in membrane in a scalable manner; d) they should also be robust with a longer lifetime after incorporation into membranes.

Within this context, the next impact may be related to increase the incipient potential, imagining that high water conduction activity obtained with natural AQPs can be biomimicked using simpler compounds that can display functions like the natural ones, the Artificial water channels-AWCs, using synthetic biomimetic approaches. AWCs have been recently proposed as a biomimetic AQPs. [START_REF] Duc | Imidazole-Quartet Water and Proton Dipolar Channels[END_REF][START_REF] Barboiu | Artificial water channels[END_REF] They are constructed from artificial molecules defining a water-pore superstructure, surrounded by a hydrophobic exterior accommodating the membrane environment. [START_REF] Barboiu | An artificial primitive mimic of the Gramicidin-A channel[END_REF][33][START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF][START_REF] Kocsis | Oriented chiral water wires in artificial transmembrane channels[END_REF][START_REF] Murail | Water permeation across artificial I-quartet membrane channels: from structure to chaos[END_REF] Despite the incipient efforts, only a few AWCs have been produced in attempts to attain the high-water permeability and significant ion rejection of AQPs. [START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF] Within this context, mimicking AQPs by using simple AWCs is an important endeavor from both a) the fundamental science perspective related to the deeper understanding of water structure and its transport function and applied science perspective as

AWCs might have timely impact on increasing water filtration efficiency by using natural principles to change the desalination paradigms.

Artificial ion-channels [START_REF] Gokel | Synthetic Models of Cation-Conducting Channels[END_REF][START_REF] Matile | En Route to Supramolecular Functional Plasticity: Synthetic β-Barrels, the Barrel-Stave Motif, and Related Approaches[END_REF][START_REF] Cazacu | Columnar Self-Assembled Ureidocrown-ethersan Example of Ion-channel Organization in Lipid Bilayers[END_REF][START_REF] Fyles | Synthetic Ion Channels in Bilayer Membranes[END_REF] have been extensively developed for decades (thousands of papers in literature) with the hope to mimic the ionic conduction of natural proteins. [START_REF] Hille | Ion Channels of Excitable Membranes[END_REF][START_REF] Hurtley | Crossing the Bilayer[END_REF] Amazingly, there was no systematic and little progress in the area of Artificial water channels before 2011. [START_REF] Barboiu | Artificial Water Channels -incipient innovative developments[END_REF] It was shown that structuration of water clusters within the pores, effects on their stability, dynamic behaviors or selective translocation, once inserted in the bilayer membranes. These studies provide detailed experimental data correlated with extensive theoretical simulations on water transport with efficiency similar to that of natural channels.

5.

Where are we now? Where are we going in this newborn field?

The target of the future membrane is to increase both the water permeability by 3 to 5 folds of the currently best TFC membranes while achieving/maintaining 99,9% salt rejection. As pointed out recently in many cases selectivity matters as much if not more than permeability.

Next research might provide answers to essential question related to: a) complicated interactional scenarios related to water transport through biomimetic pores b) bio-inspired desalination with membranes that mimic mechanisms of natural desalinators.

We know that the AQPs are the representative proteins for the water transport across cell membrane, but their structures are tremendously complicated. Simpler architectures encapsulating water may provide excellent and valuable models for understanding/designing novel biomimetic water channel systems. Although there is a rich inventory when it comes to molecular encapsulation of water, in most cases the host systems are highly polar which prohibits them to be used in conditions similar to the one offered by the membrane to AQPs.

As so, the development of the AWCs is highly important. The first synthetic water transporting systems have been reported in 2011 [START_REF] Duc | Imidazole-Quartet Water and Proton Dipolar Channels[END_REF] and the term of Artificial water channels has been coined practically one year later [START_REF] Barboiu | Artificial water channels[END_REF]. The common features of the AWCs are the presence of a central pore, which is a confining hydrophilic or hydrophobic region that allows the passage of water, and a hydrophobic shell that permits the insertion into the lipid membranes and subsequent analysis for water transport. There are main approaches towards designing these systems, when considering the structure of channels:

 Single molecular channels, as one molecular entity that can span the length of the bilayer;

 Self-assembled supramolecular channels that can self-assemble from multiple molecular components into architectures spanning the bilayer.

 Nanochannels related to Carbon Nanotubes and GO materials and membranes

Single Molecular Channels.

The pillar [START_REF] Imbrogno | Membrane Desalination: Where Are We, and What Can We Learn from Fundamentals?[END_REF]arenes, PAP1 are the first reported single molecular channels (Figure 4a) [START_REF] Hu | Single-Molecular Artificial Transmembrane Water Channels[END_REF]. Intramolecular H-bonds between lateral hydrazide arms confer robustness, keeping the membrane spanning channels intact, when inserted into a lipid bilayer. PAP1 have a relatively low water transport rate: up to 40 molecules of water/s/channel. Like in AQPs, PAP1display water selectivity against H + and OH -ions. The explanation for this is the impeded proton hoping through a broken, discontinuous water wire, while the cation transport can be attributed to carrier transport mechanism. Following up on these results a second generation of more hydrophobic pillar [START_REF] Imbrogno | Membrane Desalination: Where Are We, and What Can We Learn from Fundamentals?[END_REF]arenes, PAP2 containing poly-Phe arms showed a osmotic water permeability of 3.5*10 8 water/s/channel [START_REF] Shen | Highly permeable artificial water channels that can self-assemble into two-dimensional arrays[END_REF], being in the range of AQPs and CTNs. Nevertheless, PAP2 channels present a drawback when it comes to ionic selectivity. (image adapted with permissions from reference [START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF][START_REF] Hu | Single-Molecular Artificial Transmembrane Water Channels[END_REF][START_REF] Huo | Sticky'-Ends-Guided Creation of Functional Hollow Nanopores for Guest Encapsulation and Water Transport[END_REF][START_REF] Zhou | Self-assembling subnanometer pores with unusual mass-transport properties[END_REF])

Having an inner pore size of ~5 Å, they allow the passage of cations according to their hydration energy. More interestingly, PAP2 channels may be packed in a hexagonal arrangement in 2D membranes, thus the fabrication of active thin layers is possible. [START_REF] Hinds | Aligned Multiwalled Carbon Nanotube Membranes[END_REF][START_REF] Holt | Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes[END_REF][START_REF] Fornasiero | Ion exclusion by sub-2-nm carbon nanotube pores[END_REF] 5.2. Self-assembled supramolecular channels. The second category of AWCs use a noncovalent self-assembly strategy, that often confer the supramolecular channels properties that are not predictable and differ completely from that of their building blocks. Based on this concept it has been reported that AWCs based on alkylureido-imidazoles, can self-assembly in tubular imidazole I-quartet channels, mutually stabilized by water wires (Figure 4b) [START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF].

They show similar structural features as found in the AQP pores, with a central pore size of ~2.6 Å. The water molecules form a single molecular wire adopting a unique dipolar orientation inside the chiral I-quartets. The I-quartet channels are able to transport ~1.5 x 10 6

water/s/channel, which is within two orders of magnitude of AQPs' rates but most important, reject all ions except protons. [START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF] This ion-exclusion selectivity is important and unique for the AWCs. It is based on dimensional steric reasons, whereas hydrophobic and hydrodynamic effects appear to be less important. The I-quartets have been found to efficiently transport protons when an osmotic pressure is applied, in the absence of any pH gradient. This highlights the importance of oriented dipolar water-wires inside the channels, allowing polarization and synergetic antiport proton translocation through the bilayer membrane.

Small olygopyridine molecules that are able to self-assemble into helical Aquafoldamers with a pore of ~2.8 Å, close to the size of narrowest region in AQPs (Figure 4c). [START_REF] Huo | Sticky'-Ends-Guided Creation of Functional Hollow Nanopores for Guest Encapsulation and Water Transport[END_REF] The supramolecular nanopore contains a single file of water molecules. Interestingly, when water permeability tests were performed the channels showed no osmotic pressure induced water transport, but when under proton gradient was applied, they presented observable water transport. This underlines the dependence between the chain-like confinement of the water inside the pore and translocation capabilities of the channel, however a standardized value for water channel permeability needs to be determined. The channels showed no transport of Na + and K + cations, making them a viable option for desalination applications.

With a similar approach, giant macrocycle able of self-assembly into tubular channels via stacking and H-bonding interactions have been recently synthesized (Figure 4d). [START_REF] Zhou | Self-assembling subnanometer pores with unusual mass-transport properties[END_REF] Although the size of the pore of 4-5 Å would normally offer little selectivity, remarkably the channels can selectively transport K + against both Li + and Na + cations. The transport of water occurs in a similar fashion as the one observed in the case of CNTs, a hoping of water molecules between voids having no specific binding to the walls of the pore. The determined osmotic water permeability of these hydrophobic channels is ~4.9*10 7 water/s/channel.

Recently, the research on water channels has been pushed a step further showing that a selfassembled "T-channel" is formed with morphological characteristics reminiscent to natural Gramicidin-A [START_REF] Barboiu | An artificial primitive mimic of the Gramicidin-A channel[END_REF]. The formation of an aqua-lubricated channel is generated via H-bonding between water molecules and carbonyl groups identical to the carbonyl strings in AQps.

Vesicles incorporating "T-channels" show fifty times higher water permeability than control vesicles but this system does not show water/ion selectivity.

Nanochannels:

At the nanolevel, carbon nanotubes-CNTs offer an exciting source of inspiration to mimic natural channels. [START_REF] Hinds | Aligned Multiwalled Carbon Nanotube Membranes[END_REF][START_REF] Holt | Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes[END_REF][START_REF] Fornasiero | Ion exclusion by sub-2-nm carbon nanotube pores[END_REF] The CNTs can be considered as artificial systems where the selectivity can be modulated at the entrance, the net dipolar orientation of water molecules and the electroosmotic flow of actively pumped chemicals can be controlled through the inner CNT cores at important flow rates. The frictionless water flow is 4-5 orders of magnitude increasing over what would be seen in other conventional nanoporous or AQP structures. Important studies show ion-rejection at low salt concentrations which is promising for applications. In the past few years, membranes based on graphene and graphene oxide (GO) have gained considerable interest for their potential for desalination. [START_REF] Hu | Enabling Graphene Oxide Nanosheets as Water Separation Membranes[END_REF] The swollen membrane allows any selectivity as water molecules are transported with ions. However, the spacing in a GO laminar membrane can be controlled by physically restraining the membrane from swelling. When interlayer spacing is reduced to 7.0 Å, the ion permeation rate was reduced by two orders of magnitude while the water permeation rate was only halved.

Outline beyond state of art:

All these examples studied must restore some balance to our understanding of the biomimicking of natural AQPs and remains an important exploring challenge. These findings show that natural channels can be bio-mimicked using simpler artificial compounds displaying constitutional functions like the natural ones (Table 1). Biomimicking the complex superstructures of proteins at molecular level is an important exploring challenge. The artificial water channels with a simplified structure, allow the dynamic structural behaviours of confined superstructures of water to be elucidated. The current research on AWCs is focused on their synthesis, molecular simulations and transport properties in lipid bilayer membranes. The osmotic water permeability of artificial systems ranges from several orders of magnitude lower than that of AQP1, with similar or even better values for Carbon Nanotubes CTNs (Table 1).

The large cross-sectional area of AQPs (9.0 nm 2 ) is somewhat disproportionate in regards with the size of its active water pore. It is important to note that the effective cross-sectional area of the AWCs channels in 2D layers will take up significantly less space than proteins.

From an engineering point of view, this could potentially increase by orders of magnitude their permeability per surface area when compared with AQPs.

The two main challenges for AWCs are i) to design tailored channels that reject salt but allow the easy passage of water molecules and ii) the transfer of the AWCs from the nanomolecular level to that of large-scale membrane materials. Although this has been partly achieved using AQPs, no studies have been reported yet on integrating the AWCs within active layers casted onto polymeric membrane supports. Designing stable polymer matrixes capable of keeping AWCs in their active state will prove crucial for future hybrid membranes. The use of lipid bilayers or hydrophobic polymers as host matrixes, which certainly can be affected by the presence of organic solutes or surfactants in water, is not a necessary option. They have been used until now to compare the AWC with AQP One strategy to obtain active layer is to incorporate the AWC networks while forming the Polyamide PA layer of RO membrane or to imagine new cross-liked polymers in order to obtain networks of AWC within the thin layers. If AWC are covalently attached to TFC, there is no degradation of layer, only temporary channel destabilization. This way the AWCs are dense and highly robust, would be encapsulated in the dense part of the membrane while the PA matrix should be responsible for the rejections of organics.

To compete with polymeric reverse osmosis membranes, the current gold standard for desalinating seawater, water flux and rejection rates for artificial water channels need to be very close to that of AQPs. While having even higher flux values will not have a substantial impact on membrane filtration performances, highly water specific membranes will improve the quality of water produced as well as remove unnecessary pretreatment processes.

It was discovered that imidazole I-quartet i.e., stacks of four imidazoles and two water molecules, that can mutually stabilize oriented dipolar water-wires within 2.6-Å pores [START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF].

We know that a pore with a diameter of  3 Å very close to the narrowest constriction (2.8 Å) observed in AQP, is a critical prerequisite for ion-exclusion behaviours. Bigger pore diameters (> 3 Å) are at long last beginning to meet encouraging signs for high water permeability (10 6 -10 8 water molecules/second/channel), while most of these channels present ionic transport activity, thus not yet selectivity. The total ionic-exclusion by I -quartet channels suggests that they hold significant promise for the incipient development of the first effective artificial AQPs: high water permeability (10 6 water molecules/s/channel) and total ionic rejection. These key features must be combined in order to exploit both the selectivity of hydrophilic H-bonding channels like I-quartets 14 and the high permeability of hydrophobic carbon nanotubes. Shorter channels lengths, more dense channels per unit area of surface will play an important role in developing efficient water filtration systems based on AWC technology. Given the importance of developing even more efficient membranes for desalination, this will be an exciting and challenging project, potentially redefining the paradigms of desalination (Figure 5).

Conclusions

The biomimetic water transport can solve/improve the performances and life-time of the bioassisted membrane systems in which highly selective but labile AQPs are used. In terms of economic gain and the economy of matter the artificial systems, might represent an interesting alternative. This project, scientifically challenging and of great fundamental interest, is leading to pioneering scientific or applicative discoveries, as literature can attest.

Biomimetic water transport, fundamental in nature, should allow not only to develop new knowledge and a greater understanding on bottom-up design of multifunctional systems, but also to address different societal challenges. The thin-layer polyamide membranes, used for water desalination since 1972 has improved dramatically over the last 60 years [START_REF] Yang | The upper bound of thin-film composite (TFC) polyamide membranes for desalination[END_REF], however their low-resolution structure and the transport mechanism are still poorly understood. The ultimate performance of a membrane is usually determined by an increased flux (or permeance) due to the introduction of the defects in the membrane structure lowering the selectivity which is always decreasing. The first identification of Artificial water channels-AWCs, opened the door to new applicative desalination processes close to natural ones. We speculated at this time that biomimetic Artificial Water Channels -AWCs might have timely important influences to increase this driving force for the transport by using natural principles and to change the water desalination paradigms.

Figure 1 .

 1 Figure 1. Cryogenic transmission electron micrograph of a polymer vesicle b) electron micrograph of a cluster of vesicles c) Stopped-flow light-scattering experiments; increase in relative light scattering with and without reconstituted AQPZ into the polymer at 5.5°C(image adapted with permission from reference[START_REF] Kumar | Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z[END_REF]).

  [28] Active high flux AQP membranes, became the commercially available products. Production of AQP proteins is a feasible task since biological protocols and reagents are used, but the purification setup still represents an expensive and time-consuming obstacle. The question 'how the natural proteins /artificial host matrix systems can be efficiently used together' has been thus partially answered.[START_REF] Zhao | Synthesis of robust and high-performance aquaporin-based biomimetic membranes by interfacial polymerization-membrane preparation and RO performance characterization[END_REF][START_REF] Qi | Aquaporinbased biomimetic reverse osmosis membranes: Stability and long-term performance[END_REF][28]

Figure 3 .

 3 Figure 3. a) Scanning electron microscopy (SEM) image of the cross section of an AQPZ-thin film composite membrane b) Conceptual model of AQP proteoliposomes integrated into a polyamide matrix (image adapted with permissions from reference [26])However, no clear strategies converge to the easy synthetic development of robust AQP bio-

Figure 4 .

 4 Figure 4. a) Single molecular PAP1 water channels [43]; b) Single crystal structure of hexylureidoethylimidazole and its related urea H-bonding ribbons and of I-quartets that present d) dipolar orientation of water [34]; c) X-ray single crystal structure and d) representation of a single water file Aquafoldamers [48] d) Simulated structure of macrocyclic channels [49].(image adapted with permissions from reference[START_REF] Licsandru | Salt-excluding artificial water channels exhibiting enhanced dipolar water and proton translocation[END_REF][START_REF] Hu | Single-Molecular Artificial Transmembrane Water Channels[END_REF][START_REF] Huo | Sticky'-Ends-Guided Creation of Functional Hollow Nanopores for Guest Encapsulation and Water Transport[END_REF][START_REF] Zhou | Self-assembling subnanometer pores with unusual mass-transport properties[END_REF])

Figure 5 .

 5 Figure 5. RO versus biomimetic artificial water channels for desalination strategies (image adapted with permissions from reference [3]).

Table 1 .

 1 Performance overview of artificial water channels and pores (adapted with permission from reference[START_REF] Kocsis | Artificial water channelsdeconvolution of natural Aquaporins through synthetic design[END_REF]).

	Water Channels	Net Permeability for water /	Selectivity	Ref.
		Single channel permeability
	Aquaporins -AQPs.	167 µm/s/bar	High selectivity for	18,20
		4 x 10 9 water	water, perfect
		molecules/s/channel	rejection of ions/H +
	Carbon Nanotubes-CTNs	300 µm/s/bar, 1.9 x 10 9 water	-	45
	Estimated diameter = 15 Å	molecules/s/channel
	Carbon Nanotubes-CTNs	520 µm/s/bar, 2.3 x 10 10	ion rejection in dilute	45
	Estimated diameter = 8 Å	water molecules/s/channel	solutions
	Hydrophilic I-quartets,	3-4 µm/s/bar	high selectivity for	34
	channel (2.6 Å)	1.5 x 10 6 water	water, reject all ions
		molecules/s/channel	except protons
	Aquafoldamers,	No permeability reported	high selectivity for	48
	Hydrophilic channel (2.8		water, reject all ions
	Å)		except protons
	Hydropobic peptide	51 μm/s/bar	no selectivity for	49
	tubular macrocyclic pores	4.9 x 10 7 water	water,
	(6.4 Å)	molecules/s/channel	high conduction for
			K + and H +
	Hydrazide appended	8.6 × 10 -6 μm/s/bar	no selectivity for	43
	Pillar[5]arene, PAP1	40 molecules/s/channel	water conduction for
	unimolecular channel (6.5		alkali cations no
	Å)		conduction for H +
	Peptide appended	swelling: 30 µm/s/bar or 3.5	no selectivity for	44
	Pillar[5]arenes, PAP2	10 8	water,
	unimolecular channel (5	shrinking: 1 µm/s/bar or	good conduction for
	Å)	3.7x10 6 molecules/s/channel	alkali cations
	Double helical water T-	No permeability reported	Enhanced conduction	149
	channels (~2.5-4 Å)		states for cations and
			for H +
	RO membrane	2 µm/s/bar	99,8% rejection of	20
			salts versus water
	FO membrane	2,5 µm/s	-	5,6
	RO membrane	5,3 µm/s


H Miyake
email: hmiyake@tcu.ac.jp

V Griseri

U Chiba

K Enoki

Y Tanaka

Analysis of Charge Accumulation Phenomena by Physicochemical Analysis for fluorinated Polymer Films Irradiated by Proton

Keywords: Proton irradiation, Physico-Chemical analysis, Fluorinated films, chain scission, oxidization

In this paper, proton irradiated fluorinated polymer films are investigated by physicochemical analysis, such as PhotoLuminescence (PL), XPS and NMR. It has been observed that proton irradiated fluorinated films have a positive charge accumulation in the bulk under DC stress. The origin of this charge storage seems to be due to molecular chain scission. Therefore, this paper focused on physicochemical analysis results obtained on ETFE and FEP irradiated films. From the data, the scission, oxidization and crosslinking phenomena have been confirmed.

I. INTRODUCTION

Many dielectric materials are used for spacecraft as thermal control materials, such as MLI (Multi Layer Insulator), OSR (Optical Solar Refractor) and electrical insulation for wires harness. Those materials are charged up due to radioactive rays in space environment and discharges may occur. These discharging phenomena called ElectroStatic Discharges (ESD), are at the origin of some satellite fatal anomalies [START_REF] Koons | The Impact of the Space Environment on Space Systems[END_REF].

To understand the dielectric phenomena of thermal control materials on spacecraft irradiated by radio-active rays, such as electron, proton and -rays, an important experimental work has been performed in the last past thirty years [START_REF] Payan | Electrostatic behavior of dielectrics under GEO-like charging space environment simulated in laboratory[END_REF][START_REF] Perrin | Measurement of internal charge distribution in dielectrics using the pulsed electro-acoustic method in non contact mode[END_REF][START_REF] Nagasawa | Electric Charge Accumulation in Polar and Non-Polymers under Electron Beam Irradiation[END_REF][START_REF] West | Space-charge distributions in electron-beam charged Mylar and Kapton films[END_REF][START_REF] Kojima | Measurement of Space Charge Accumulation in Gamma-irradiated Polyethylene with DC Voltage[END_REF][START_REF] Miyake | Space Charge Distribution in Polymethyl Methacrylate and Quartz Glass Irradiated by Protons[END_REF][START_REF] Takada | Acoustic and optical methods for measuring electric charge distributions in dielectrics[END_REF].

In this paper, proton irradiated fluorinated polymer films are investigated. Until now, only space charge distribution on such materials has been studied. It was shown that pristine materials do not accumulate charges under DC voltage application. On the contrary, positive charges were observed in irradiated films. To go further in the understanding of such behavior it's necessary to establish the relationship between charge distribution and molecular structure. Therefore, in this study, we try to determine the molecular structure in irradiated fluorinated polymer using Photo-Luminescence (PL), FT-IR, XPS and NMR. We will then discuss the relationship between charge accumulation and molecular structure in proton irradiated fluorinated films.

This study was partly supported by the Inter-University Program for the Joint Use of QST (formerly JAEA and JAERI) Facilities and the Shared Use Program of QST Facilities. This work was also supported by JSPS/MEXT KAKENHI Grant Numbers JP18760242, JP23760292 and JP15KK0216.

II. EXPERIMENTAL PROCEDURE

A. Proton Irradiation Facility and Conditions

The proton beam irradiation was carried out in a vacuum atmosphere of approximately 10 -4 ~10 -5 Pa using the 3 MeV tandem type ion accelerator facility in Takasaki Advanced Radiation Research Institute of National Institute for Quantum and Radiation Science and Technology (former name is JAEA) of figure 1.

The acceleration energy and the irradiation current density of the proton beam were set to 1.5 or 2 MeV and 30 nA/cm 2 , respectively, while the irradiation duration was fixed to 30 minutes.

B. Samples

Commercially available ETFE (thickness : 100 m) and FEP (thickness : 100 m) films were selected for these measurements. Samples were irradiated by proton using the previously mentioned conditions.

III. SPACE CHARGE MESUREMENTS RESULTS

A. Space charge distribution in proton irradiated ETFE and FEP

The Space Charge (SC) distribution in ETFE and FEP irradiated by protons are shown in Figure 2 and3, respectively. These results were obtained by PEA method [START_REF] Takada | Acoustic and optical methods for measuring electric charge distributions in dielectrics[END_REF]. The measurements of proton beam sample were typically started one day after the end of the irradiation at atmospheric pressure. All the measurements were carried out at room temperature (approximately 25 ℃). The charge distribution measurements were carried out under DC voltage, which corresponded to an average electric field of 100 kV/mm, for 120 min to observe the charge accumulation process. In these figures, the red line shows the SC distribution after 5 s starting voltage application, and the blue line shows the SC distribution after 120 min under voltage application. The orange and green broken lines shows the initial charge distribution and 20 min after short-circuit, respectively. Furthermore, the perpendicular black broken line appearing in the figures corresponds to the calculated maximum penetration depth determine from the Stopping and Range of Ions in Matter (SRIM) tables [START_REF] Ziegler | The Stopping and Range of Ions in Matter[END_REF]. The energy of proton irradiation is 2.0 MeV. From the figure 2 and 3 (a), we cannot observe any charges in the bulk of pristine ETFE and FEP films. However, on the proton irradiated samples, we can see positive charge accumulation near the proton maximum penetration depth in both cases. It is interesting to note that those accumulated positive charges still remain after short-circuit. It is expected that positive charges appeared due to proton irradiation however it is not obvious, at first sight, that they should remain stable. These results are explained in details in the paper [START_REF] S? Yoishida | Space charge accumulation behavior in fluorinated Polymer Films Irradiated by Proton under DC stress[END_REF]. From those results, we considered the origin of positive charge accumulation in those bulks by physicochemical analysis which is described in following sections.

IV. PHYSICOCHEMICAL ANALYSIS AND DISCUSSION

A. PhotoLuminescence measurements

The PL measurements have been carried out in the chamber available at the LAPLACE [START_REF] Teyssedre | Temperature dependence of the photoluminescence in poly (ethylene terephthalate) films[END_REF]. The figure 4 shows the PL results of pristine and proton irradiated ETFE and FEP films. Although the excitation wavelength of this experiment was covering the range [200 -300 nm], only the result at 285 nm which shows the big differential between pristine and irradiated samples are shown in this paper. From the figure 4, big peaks are observed at 300 at about 550 and just above 600 nm in pristine FEP and ETFE. Concerning the irradiated FEP and ETFE, a broad peak centered on 400 and 450 nm is observed. Furthermore, the peak at 300 nm of pristine ETFE has disappeared completely, whereas the peak above 600 nm is hidden by the presence of the broad peak mentioned just before. It is considered that the new big peak is generated by the creation of new local state in the forbidden band due to the scission of molecular bonds. And judging from the observed wave form, the level of local states might be created with the several energies, continually in forbidden band.

B. FT-IR measurement

VERTEX 70 (Buker Optics) was used to obtain FT-IR spectra. FI-IR spectra with ATR-mode of pristine and proton (1.5 MeV) irradiated ETFE is shown in figure 5. These results are normalized on the value at 1040 cm -1 . The wavenumbers which are related to molecular vibration mode are shown in Table 1 [START_REF] Nasef | Electron beam irradiation effects on ethylene-tetrafluoroethylene copolymer films[END_REF][START_REF] Calleja | Conversion of poly(ethylene-alt-tetrafluoroethylene) copolymers into polytetrafluoroethylene by direct fluorination: A convenient approach to access new properties at the ETFE surface[END_REF]. After the ETFE irradiation new peaks were observed, between 2831 -2997 cm -1 and 1605 -1807 cm -1 . On the other hand, the peaks at 669 and 509 cm -1 are decreased, and the peaks between 972 -1425 cm -1 related to CF2, C-C and C-O molecular partly increased in amplitude and sprawled. From those results, it is considered that scissions of molecular chain were produced due to proton irradiation and unstable C molecular might be bonded with irradiated proton.

It is assumed that such unstable C made new bonding with C double bond, C=O and C=C. At least, these results show the possibility of oxidization and crosslinking. Concerning CF 2 mode, we could confirm both increase and decreasing phenomena.

Usually on PTFE films, formed by C and F atoms, scission type molecular species are observed after irradiation. On the contrary on PolyEthylene (PE) films, formed by C and H atoms, crosslinking type molecular species are identified after irradiaton [START_REF] Charlesby | Atomic Radiations and Polymers[END_REF][START_REF][END_REF]. As ETFE is elaborated by PE (CH 2 ) and PTFE (CF 2 ) parts, both scission and crosslinking phenomena might be produced due to proton irradiation, simultaneously.

Therefore, it is considered that 669 and 509 cm -1 was decreased due to scission. Furthermore, as ETFE is formed not only by CF 2 but also by CH 2 , it is assumed that the bonding state of CF2 was changed and shifted to dedicated states due to irradiation. This consideration may be related to the increasing and sprawled of peaks in the range 972 -1425 cm -1 .

C. XPS measurement SSX-100 (SURFACE SCIENCE INSTRUMENTS) was used

to obtain XPS spectra. The source of X-ray is Al K type anode (1487 eV) and the detecting energy range is 0 -1100 eV with 0.58 eV of FWHM. The results of XPS of FEP and ETFE are shown in Fig. 6 and7, respectively. In those figures, the horizontal axis shows binding energy [eV] and the perpendicular axis shows measurement signal intensity [a. u.]. From Fig. 6 and7, we can see the peak signal around 300 and 700 eV on the non -irradiated FEP and ETFE. Those peaks are related C1s and F1s signal, respectively. Concerning proton irradiated materials, we could obtain the differential signal between non -irradiated and irradiated materials. The intensity of F1s on FEP is decreased with irradiation energy progress. And O1s peak was newly created at the surround 500 eV. Furthermore, C1s signal was obtained precisely and shown in Fig. 6 irradiation proton energy progress and could not be observed on FEP irradiated by 2.0 MeV proton. CC or CH intensity increased with irradiation energy progress. Concerning the XPS spectra differential between irradiated and non -irradiated materials, although the intensities were different, we obtained same tendency on FEP and ETFE. On ETFE CF2 does not vanish completely. From the above results, we considered the bonding structure of irradiated sample. Concerning FEP, scission of CF2 bonding was produced due to irradiation and disconnected F particle was swept out form the bulk of sample. Therefore, it is considered that the peak of F1s and CF2 on C1s decreased. On the other hand, as the residual C chain create new binding with irradiated H+ or cross linking, it was thought that the peak signal of CC or CH increased. Furthermore, from new peak creation of O 1s , it seems that the irradiated samples were oxidized due to keep the sample under the air atmosphere after irradiation. And as the activated species were increased due to irradiation energy progress, we considered that the oxidation was proceeded.

Concerning of results of ETFE, the phenomena was little bit different from FEP results. CF molecule was also decreased. But the residual CF was still observed. And the oxidation was not proceeded to much compared with irradiated FEP. From the above results, we thought that same tendency of XPS results was observed at the above FT-IR results. At least, it is considered that scission of CF2 molecule might be produced from those results.

Next, when we compared between the SC accumulation phenomena and above XPS results, we should mention about the phenomena that the positive charges accumulated in the bulk near the cathode exceeding the proton penetration depth. The scission and oxidation in FEP may be reduced the injection barrier between anode and sample and produced easy charge injection condition. And those might also produce the high mobility condition for hole.

Furthermore, normally, materials become radioactive due to proton irradiation with the energy of more than 2.0 MeV. Therefore, as the scission and oxidation area will be expanded to non proton penetration area, it was considered that positive charge accumulation area was exceeded penetration depth.

D. NMR measurement

We tried to measure many spectra of irradiated sample. In this paper, we just introduce some big differential results between irradiated and non -irradiated sample.

Fig. 8 shows NMR spectra of ETFE focused on 1H. From the figure, we can see some defect of CH2 was observed at irradiated sample. It is considered that the irradiated H+ (proton) may play to give some defect to original structure. Fig. 9 shows NMR spectra on 13C of ETFE and on 19F of FEP. From the figure (a), although CH2 peak intensity was same between irradiated and non -irradiated sample, the peak became broad. And CF 2 intensity peak of irradiated material was about 2.5 larger than the intensity of non -irradiated sample. On the contrary, from the figure (b), many CF3 peaks in FEP were created. It is considered that CF 2 chain might be break due to irradiation and many terminations were produced as CF3. V. CONCLUSIONS Form the above results and discussion, concerning proton irradiated ETFE, we consider that morphology distortion, including scission and crosslinking, was produced due to irradiation. On the contrary, we also considered that the scission of molecule chain was dominant factor on FEP.

After irradiation, as unstable state still remained in the bulks, the oxidization could be continued during the sample storage in air atmosphere. These structural modifications are probably responsible for the positive charge accumulation observed in irradiated FEP and ETFE under high voltage application.

  (a) Space charge distribution in pristine ETFE (non-irradiated). (b) Space charge distribution in irradiated ETFE (2.0 MeV). (a) Space charge distribution in pristine FEP (non-irradiated). (b) Space charge distribution in irradiated FEP (2.0 MeV).

Fig. 2

 2 Fig. 2 Space charge distribution in pristine and proton irradiated ETFE under 100 kV/mm voltage application and subsequent short circuit.

Fig. 3

 3 Fig. 3 Space charge distribution in pristine and proton irradiated FEP under 100 kV/mm voltage application and subsequent short circuit.

  (b) -(d) and Fig. 7(b) -(c). From those figures, 284.5 and 276.5 eV show the origin of CF2 and CC or CH bonding, respectively. CF2 intensity decreased with

Fig. 4

 4 Fig. 4 PhotoLuminescence measurements results on 285 nm as an excitation light: (a) FEP and (b) ETFE.

Fig. 5 FT

 5 Fig. 5 FT-IR measurements results on pristine and irradiated ETFE.

Fig. 6 Fig. 8

 68 Fig. 6 XPS spectra of FEP irradiated by protons with the energy of 1.5 and 2.0 MeV

Fig. 9

 9 Fig. 9 XPS spectra of ETFE and FEP irradiated by protons with the energy of 2.0 MeV (a) Signal of 13C of ETFE (C-H) (b) Signal of 19F of FEP

Table 1

 1 Vibration mode of molecular on each wavenumber

	Wavenumber [cm -1 ]	Molecular	Mode
	2852	CH2	symmetric stretching vibration
	2921	CH2	asymmetric stretching vibration
	1740-1690	C=O	stretching vibration
	1654	C=C	stretching vibration
	1452	CH	deformation vibration
	1146	CF2	symmetric stretching vibration
	1199	CF2	symmetric stretching vibration
	732	CF2	symmetric stretching vibration