Zwitterionic materials for antifouling membrane surface construction

Acta Biomaterialia xxx (2016) xxx–xxx

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Zwitterionic materials for antifouling membrane surface construction q Mingrui He 1, Kang Gao 1, Linjie Zhou, Zhiwei Jiao, Mengyuan Wu, Jialin Cao, Xinda You, Ziyi Cai, Yanlei Su, Zhongyi Jiang ⇑ Key Laboratory for Green Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

a r t i c l e

i n f o

Article history: Received 3 December 2015 Received in revised form 2 March 2016 Accepted 25 March 2016 Available online xxxx Keywords: Antifouling membrane surfaces Zwitterionic materials Surface grafting Surface segregation Biomimetic adhesion

a b s t r a c t Membrane separation processes are often perplexed by severe and ubiquitous membrane fouling. Zwitterionic materials, keeping electric neutrality with equivalent positive and negative charged groups, are well known for their superior antifouling properties and have been broadly utilized to construct antifouling surfaces for medical devices, biosensors and marine coatings applications. In recent years, zwitterionic materials have been more and more frequently utilized for constructing antifouling membrane surfaces. In this review, the antifouling mechanisms of zwitterionic materials as well as their biomimetic prototypes in cell membranes will be discussed, followed by the survey of common approaches to incorporate zwitterionic materials onto membrane surfaces including surface grafting, surface segregation, biomimetic adhesion, surface coating and so on. The potential applications of these antifouling membranes are also embedded. Finally, we will present a brief perspective on the future development of zwitterionic materials modified antifouling membranes. Statement of Significance Membrane fouling is a severe problem hampering the application of membrane separation technology. The properties of membrane surfaces play a critical role in membrane fouling and antifouling behavior/performance. Antifouling membrane surface construction has evolved as a hot research issue for the development of membrane processes. Zwitterionic modification of membrane surfaces has been recognized as an effective strategy to resist membrane fouling. This review summarizes the antifouling mechanisms of zwitterionic materials inspired by cell membranes as well as the popular approaches to incorporate them onto membrane surfaces. It can help form a comprehensive knowledge about the principles and methods of modifying membrane surfaces with zwitterionic materials. Finally, we propose the possible future research directions of zwitterionic materials modified antifouling membranes. Ó 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

1. Introduction Due to increasingly serious water pollution and excessive use of water resources, the demand for clean water supply becomes more imperative [1,2]. Membrane separation technology, due to the advantages of low energy consumption and carbon footprint, high efficiency and easy operation, has been extensively used as a key

q Part of the Special Issue on Zwitterionic Materials, organized by Professors Shaoyi Jiang, Kazuhiko Ishihara, and Jian Ji. ⇑ Corresponding author at: Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin City, China. E-mail address: [email protected] (Z. Jiang). 1 These authors contributed equally to this work and should be considered co-first authors.

technology in water/wastewater treatment or desalination from brackish water and seawater to supply clean water, such as ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO). Besides, membrane separation processes are also widely used in biomedical devices, such as hemodialyzers [3] and artificial organs [4]. Over the past few decades, researchers have devoted great efforts in developing novel membrane materials to acquire enhanced performance [5]. However, membrane fouling is still a bottleneck restricting the sustainable development of membrane separation technology because it can lead to serious decline of permeability and selectivity, ultimately increases operating pressure and shortens membrane lifespan [1]. In general, membrane fouling can be classified into inorganic fouling, organic fouling and biological fouling. Proteins are representative foulants in organic fouling for their multiple interactions with membranes, and biological fouling usually

http://dx.doi.org/10.1016/j.actbio.2016.03.038 1742-7061/Ó 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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starts with protein adsorption. Hence, it is necessary to modify membrane surfaces for improving resistance to proteins [6]. It is well recognized that surface hydrophilization is an effective strategy to resist nonspecific protein adsorption [7,8]. Poly(ethylene glycol) (PEG), one of the most representative hydrophilic polymers, has been broadly employed to modify surfaces for elevating antifouling properties [9–11]. The antifouling behavior of PEGbased surfaces is interpreted with hydration shell formed by hydrogen bonds between PEG and water molecules in addition to steric repulsion effect due to flexible chains [12]. Nevertheless, several researches have demonstrated that PEG is susceptible to oxidative damage in the presence of oxygen and transition metal ions, which significantly worsens its antifouling properties [13,14]. Therefore, it is essential to find out alternative antifouling materials with better stability and antifouling properties. Zwitterionic materials, biologically inspired by zwitterionic phosphatidylcholine (PC) headgroups abundant in phospholipid bilayer of cell membranes, possess both anionic and cationic groups with overall charge neutrality [8,15]. Zwitterionic polymers have been recognized as the next-generation of promising antifouling materials because they can form hydration shell via electrostatic interactions, which are much stronger than hydrogen bonds, resulting in denser and tighter adsorbed water [8,16]. As shown in Fig. 1, with quaternary ammonium as common cationic group, phosphate, carboxylate or sulfonate as common anionic groups, zwitterionic polymers can be classified into polybetaines and polyampholytes according to whether the charged groups are located on the pendant side chains of same or different monomer units [17]. For instance, the typical polybetaines, such as poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC) [18], poly (sulfobetaine methacrylate) (PSBMA) [19] and poly(carboxybetaine methacrylate) (PCBMA) [20], can be incorporated onto surfaces through free radical polymerization due to the existence of carbon-carbon double bonds in their monomers. Uniformity of charge distribution and charge neutrality are two important factors affecting the antifouling properties of zwitterionic polymers [21]. In recent years, a number of researches pertaining to antifouling surfaces based on zwitterionic materials have been reported [16,22,23]. About ten years ago, our group introduced zwitterionic N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl) (DMMSA) groups onto polyacrylonitrile (PAN) UF membranes for the first time [24], extensive researches on incorporating zwitterionic materials onto membrane surfaces for antifouling properties have been carried out since then [25–27]. Till now, there have been several reviews about zwitterionic materials [8,15,17,21,28–31] and antifouling membranes [32–34], but these reviews did not focus on the construction of antifouling membrane surfaces with zwitte-

rionic materials. For this reason, this review mainly focuses on the popular approaches to incorporate zwitterionic materials onto membrane surfaces, in addition to the antifouling mechanisms and potential applications of these modified antifouling membranes in diverse membrane processes. Finally a brief perspective on the future development of zwitterionic materials modified antifouling membranes is presented.

2. Antifouling mechanisms of zwitterionic materials biologically inspired by cell membranes 2.1. Membrane fouling mechanism Different from the surface fouling of industrial facilities, ships, medical apparatus [35–37], in the process of membrane separation, foulants are more likely to be pushed onto the surfaces and pores of membranes by transmembrane flux [38]. Then a cake layer will form on membrane surfaces and it can shrink and block membrane pores, causing the decrease of permeation flux and separation properties. There are many kinds of membrane foulants, which can be roughly divided into three types consisting of organic foulants, inorganic foulants and biological foulants. Although the type and intensity of fouling behavior of different foulants is different, they can be explained by a common thermodynamic mechanism, i.e. the minimization of Gibbs free energy. Foulants, water and membranes are regarded as a thermodynamics system and the Gibbs free energy of system is prone to reduce. Proteins are representative organic foulants, for their distinct feature is amphiphilic, zwitterionic and often dissolvable in water. In aqueous solution, proteins keep certain conformations with water molecules inside and around via hydrogen bonding interactions and electrostatic interactions [39]. Once the proteins contact membrane surfaces, they deteriorate with the destruction of conformations and the loss of integrated water molecules, which brings the increase of the Gibbs free energy. However, when the adsorption and deterioration of proteins come to a certain degree, the effect of new interactions, including electrostatic interactions, hydrogen bonding interactions, van der Waals interacions and hydrophobic interacions, formed between the proteins and the membrane surfaces begin to play a significant role with the decrease of the Gibbs free energy [40,41]. As a whole, the Gibbs free energy of system first increases and then decreases (Fig. 2, the Gibbs free energy changed with hydrophilic and low surface energy surface will be discussed in the Sections 2.3 and 3.2 respectively). When the sum of the changed Gibbs free energy is negative, the adsorption of proteins on membrane surfaces is feasible in

Fig. 1. The representative structures of zwitterionic polymers. (a) Polybetaines whose charged groups are located on same monomer units: PMPC, PCBMA and PSBMA respectively. (b) Polyampholytes whose charged groups are located on different monomer units.

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a

Ordinary

Gibbs free energy

Hydrophilic Low surface energy

Before

When

After

The progress of protein adsorption and deterioration on different surface

b

Fig. 2. (a) The Gibbs free energy changes before, when and after the adsorption and deterioration of proteins on ordinary, hydrophilic and low surface energy membrane surfaces. The different changes of Gibbs free energy are showed by different curves. (b) Schematic illustration for the adsorption and the deterioration of proteins on membrane surfaces. Source: Ref. [40], Copyright 2015; reproduced with permission from the Angewandte Chemie International Edition Publishing Group.

a

N

b

O O

P O

Glycoprotein

O O

O

Zwitterionic hydrophilic head

O

Phospholipid bistructure

O

Aliphatic hydrophobic tail

Protein Fig. 3. (a) The section structure diagram of the cell membrane including protein, glycoprotein and phospholipid bilayer. (b) The structural formula of the phospholipid consisting of zwitterionic hydrophilic head and aliphatic hydrophobic tail.

thermodynamics, however, when the maximum of the Gibbs free energy lowers, the membrane fouling is more likely to happen in membrane separation processes. Most of inorganic fouling belongs to colloidal fouling, which is easy to occur when the transmembrane operation pressure goes into the range of NF or RO [42]. Inorganic colloidal foulants, such as silica, aluminum silicate minerals and ferric oxides/hydroxide, deposit on membranes and form a cake layer, adversely affecting the membrane flux [43,44]. The interactions between the inorganic colloidal foulants and membranes are mainly van der Waals forces and electrostatic interactions, whose mechanism in thermodynamics is similar to the organic fouling. The biological fouling is based on organic fouling. The biological fouling mechanism is divided into three steps: the reversible adsorption, irreversible adsorption and proliferation [45,46]. The first two steps are closely associated with organic fouling, with the reversible adsorption achieved by fibronectin via nonspecific interactions and the irreversible adsorption achieved by the extra-

cellular polymers via covalent interactions [1,47]. In the last step, the biological foulants grow and proliferate, and then generate intact biofilms, which will sharply reduce the permeability and selectivity of membranes and are hard to be removed. 2.2. Antifouling prototypes of cell membranes The environment of cells is composed of tissue fluid and blood, consisting of proteins, carbohydrates, inorganic salts and metabolic products, etc. In spite of this, the surfaces of cell membranes can still keep clean without the uncontrollable adsorption of exogenous proteins, polysaccharides, peptides or other foulants, which is attributed to the delicate composition and hierarchical structure of cell membranes [48]. The cell membranes are mainly composed of phospholipids, proteins and glycoproteins. The phospholipid bilayer constitutes the articulated skeleton of cell membranes, with proteins embedded and the long-chain glycoproteins located outside [49] (Fig. 3). The ability to resist fouling is mainly attribu-

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ted to the hydrophilic property of phospholipids and the steric hindrance of glycoproteins [50]. The phospholipid has a polar hydrophilic head outward and a nonpolar hydrophobic tail inward, the head bears a negative phosphate group and a positive quaternary ammonium group and the tail includes long-chain aliphatic acids. The hydrophilic property of phospholipid is attributed to its zwitterionic headgroup [16], whose antifouling mechanism will be explained in the Section 2.3.

2.3. Antifouling mechanisms of zwitterionic materials There are two major antifouling mechanisms for the zwitterionic materials located on a surface immersed in water. The first mechanism is the formation of hydration shell. The zwitterionic materials combine quite a number of water molecules to form a firm hydration shell via electrostatic interactions, as an effective barrier to prevent the surface from directly contacting with the foulants [16]. From thermodynamics viewpoint, it needs a large number of energy for the foulants to break through the hydration shell. In other words, the maximum of Gibbs free energy with hydrophilic surfaces is higher than unmodified surfaces,

which means the adsorption process is not prone to happen (Fig. 2). Compared with PEG and its derivative materials, the hydration shell formed by zwitterionic materials is denser and thicker, for PEG molecular chains are consist of repeat units ACH2CH2OA, and each unit includes an oxygen atom integrated with one water molecule via hydrogen bonding interactions, while zwitterionic molecule chains are consist of equally charged units, and each unit includes a positive charged group and a negative charged group integrated with at most eight water molecules via electrostatic interactions [51] (Fig. 4). For biological macromolecule foulants, the hydration shell formed by hydrophilic materials provides a water environment similar to free water, which is conducive to maintaining their internal and surface water molecules with hydrogen bonding network and ensuring their conformation undamaged, endowing the materials with low tendency of adsorption and good biocompatibility [21]. Moreover, compared with the directional array of water molecules in the hydration shell formed via hydrogen bonds, the dipole array of water molecules in the hydration shell formed via electrostatic interactions are closer to free water, which means that zwitterionic materials are better than PEG-based materials in resisting biological macromolecule foulants and are more biocompatible [52].

a O

HO

O

H

H

H

H

O

O

O

O

H

H

H

HO

OH

O

Water molecule

b

Foulant

Surface

H H O

O

O

H

H H

O

H O H O

H O O

OH

H

H

N O H

H

O H

H S O H

H O H

O

Fig. 4. Schematic illustration for the formation of hydration shell. (a) Each unit of the representative PEG materials is integrated with one water molecule. (b) Each unit of the zwitterionic materials is integrated with eight water molecules.

ΔG>0

Stable

Unstable Foulant

Surface

Fig. 5. Schematic illustration for the steric hindrance effect in antifouling. The compression of the polymer by foulants lead to the increase of Gibbs free energy and the system state changing from stable to unstable in thermodynamics. (‘G’ in the figure means the Gibbs free energy.)

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M. He et al. / Acta Biomaterialia xxx (2016) xxx–xxx

The second mechanism is steric hindrance effect. The steric hindrance of zwitterionic polymer chains is the same as other hydrophilic polymer chains, which are endowed with large excluded volume by their hydrophilicity and motility [21]. When the foulants come in contact with zwitterionic polymer chains, compressing the excluded volume of and lowering their motility, the system Gibbs free energy increases. So the polymer chains tend to recover to the swelling state and stop the foulants from getting in touch with the surface [53] (Fig. 5). The density and thickness of hydration shell as well as the intension of steric hindrance is closely related to the charge distribution and density of zwitterionic materials. Compared to PEG and its derivatives, charge distribution is the unique characteristic of zwitterionic materials, about which Jiang’s group has made a great contribution. In 2005, Jiang’s group proposed that the balanced charge and minimized dipole of zwitterions were the two key factors for antifouling. The zwitterionic materials with balanced charge and minimized dipole can fully bind water molecules via electrostatic interactions and repulse charged proteins via electrostatic repulsion [54]. This group also proposed that the antiparallel orientation of zwitterionic head, just like membrane lipids, rendered the minimized dipole, and this conclusion was proved by molecular simulation calculation. Then, they performed protein adsorption experiments with unequally charged zwitterionic polymers and further confirmed that the unequally charged zwitterionic polymers led to worse antifouling properties [55]. That means that when we use zwitterionic materials to resist fouling, we should make sure that the charge distribution is indeed well controlled, especially pseudo-zwitterionic materials (prepared from positively charged and negatively charged monomers), otherwise it may lead to a worse surface packing of the membranes and decrease the amount of water molecules trapped. Jiang’s work was conducted on model interface, but the proposed mechanism also suits for a porous and rough membrane surface. The density is another important factor, which includes the distribution and the length of zwitterionic materials on surfaces. Jiang’s group prepared a hierarchical zwitterionic platform with an ultralow fouling first layer and high loading second layer, which had a better antifouling ability than single layer structure [56]. Then, Jiang’s group confirmed that the increased length and density of zwitterionic materials could improve the ability of resisting proteins in an experiment about ionic-zwitterionic polymers [55]. In 2015, they used molecular simulations and modeling as a versatile tool to study the structure–property relationships of zwitterionic materials at the molecular level, concluding that strong hydration, moderated self-associations and few protein interactions contributed to a good antifouling property [29], providing us with clear principles to design new zwitterionic materials. Besides the inherent characteristic of zwitterionic materials, the ambient environment of zwitterionic materials is also an important factor. In this regard, salting-in effect often plays a crucial role in manipulating the ambient environment of zwitterionic materials [57]. In deionized water, with strong interactions between the positive and negative charges, zwitterionic polymer chains tend to aggregate and expose hydrophobic hydrocarbon groups outside, which seriously lower the hydration shell and steric hindrance. However, with the addition of a little amount of salt, the interactions between the positive and negative charges are screened, so the polymer chains become swollen with dense hydration shell and strong steric hindrance.

3. Approaches to incorporate zwitterionic materials onto membrane surfaces Surface modification is essential to the high-efficiency and longterm utilization of membranes. Facile incorporation of zwitterionic

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materials onto membrane surfaces is vital to the surface modification. Up to now, many kinds of approaches have been exploited, among them, surface grafting, surface segregation, biomimetic adhesion and surface coating are four most popular approaches. 3.1. Surface grafting Surface grafting refers to the incorporation of polymer chains onto a solid surface with good long-term stability via stable chemical bonds, the processes of which can be divided into ‘‘grafting to” and ‘‘grafting from” [58]. ‘‘Grafting to” refers to the direct graft of preformed end-functionalized polymer chains onto surfaces while ‘‘grafting from” means graft polymerization of monomers from surfaces. To enhance the antifouling properties of membranes, many efforts have been dedicated to anchor zwitterionic polymers onto membrane surfaces through surface grafting. Most of the reported works are the modification of membrane surfaces via ‘‘grafting from” including conventional radical polymerization and living radical polymerization [32], however, rare research has been reported via ‘‘grafting to” [59]. We consider there is a possible reason. As is well known, macromolecular chains can form thicker hydration shell and stronger steric hindrance than micromolecule in water environment, and when macromolecular chains are expected to be grafted on a membrane, the grafting density via ‘‘grafting to” will be lower than via ‘‘grafting from” because of the macromolecular chains’ steric hindrance. Besides it may be another reason that there is no commercial zwitterionic polymer chain like PEG derivative. 3.1.1. Grafting zwitterionic polymers from membrane surfaces by conventional radical polymerization The radical polymerization usually consists of two steps: initiation of free radicals on membrane surfaces via specific approaches and subsequent free radical polymerization of zwitterionic monomers from the active sites of initiator [60]. According to different creation principles of free radicals, conventional radical polymerization includes photo-initiated, ozone-initiated, plasma-initiated and physisorption radical graft polymerization. Photosensitive polymer materials such as polysulfone (PSF) and polyethersulfone (PES) can generate free radicals with ultraviolet (UV) radiation and benzophenone (BP) as light initiator, followed by the graft polymerization of zwitterionic monomers [61,62]. Yu et al. [61] grafted zwitterionic 3-(methacryloylamino) propyl-dimethyl-(3-sulfopropyl) ammonium hydroxide inner salt (MPDSAH) from PSF UF membrane surfaces with BP series photosensitizer under UV irradiation. The increment of hydrophilicity as well as antifouling properties was directly relevant to the increase of grafting density, which was governed by monomer concentration, photosensitizer concentration and UV irradiation time. The modified membranes maintained high water flux and good antifouling properties in a wide pH range from 4.5 to 10.0 and showed the best performance in UF processes with the grafting density of 374 lg/cm2, with great application potential in UF processes under different pH. Razi et al. [62] applied UV-initiated graft polymerization to graft PMPC from PES membranes. After modification no bacteria were detected in the immersion test, showing good effectiveness to suppress biofouling. Yang et al. [25] modified microporous polypropylene membranes by UV-initiated graft of PSBMA. With the increase of grafting density, the pure water flux increased and reached the maximum at the grafting density of 445 lg/cm2. And the grafted membranes also showed great antifouling performance at this grafting density with a flux recovery ratio higher than 95% in bovine serum albumin (BSA) filtration after one circle and almost complete inhibition of bacteria adhesion. However, the pure water flux decreased obviously with further increasing the grafting density, while the surface

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hydrophilicity and antifouling performance had little change. Brinker et al. fabricated UF membranes by photografting zwitterionic polysulfobetain or polycarbobetain, which showed markedly different pH- and ion-responsivity [63]. Compared with UV-initiated graft polymerization, ozoneinitiated graft polymerization possesses the similarly high selectivity under mild conditions and unique advantage of uniformly introducing peroxides on membrane surfaces. The introduced peroxides are extremely unstable and are prone to decompose into surface free radicals, initiating graft of zwitterionic polymers from membrane surfaces [64,65]. Yuan et al. [66] used ozone to produce active peroxide groups on poly(ether-urethane) membrane surfaces, followed by the graft polymerization of zwitterionic N,N’dimethyl (methacryloyloxyethyl) ammonium propanesulfonate (DMAPS). The grafted membranes possessed relatively high hydrophilic surfaces and good hemocompatibility with no platelet adhesion, considered as a promising blood-contacting biomaterial. Ozone-initiated graft technique may result in the degradation of polymers, but plasma-induced surface graft technique can avoid this problem. Plasma is a mixture of atomic, molecular, ionic and radical species, which can be prepared with gases being activated into energetic states. Plasma can provide an active chemical environment where plasma-surface reactions can occur with the formation of free radicals or functional groups on membrane surfaces [67]. Zhao et al. [7] grafted MPDSAH onto polypropylene nonwoven fabric (NWF) membranes via O2 plasma pretreatment and UV-irradiated polymerization technique. The grafting density was primarily affected by plasma pretreatment time as well as monomer concentration. The modified NWF membranes with the grafting density of 327.7 lg/cm2 exhibited the lowest BSA adsorption, which had 80% reduction compared with the neat NWF membrane, showing excellent antifouling stability in cyclic BSA adsorption. The modified NWF membranes also showed good antifouling properties in BSA filtration experiment, with flux recovery ratio of 90% after one circle and thus could be potentially applied in plasma purification and separation owing to their good biocompatibility, hemocompatibility and high antifouling properties. A limitation of the above techniques is expense, leading to the difficulty of scaling up [68]. Physisorbed free radical graft technique is a facile and effective approach, which can be operated without the limitation of complicated procedures and easily modify the interior walls of tubing or pipe, showing a tendency to be widely used in constructing antifouling surfaces [69]. Zhou et al. [70] grafted a PCBMA layer on poly(vinylidene fluoride) (PVDF) membrane surfaces via physisorbed free radical polymerization with azo-bis-isobutyrylnitrile (AIBN) as initiator. The pore density of membranes gradually became lower and surfaces became smoother with the increase of grafting density, together with the improvement of antifouling performance. The grafted membranes with grafting density of 1.02 mg/cm2 showed superior water absorption ability, improved antifouling and inorganic salt rejection properties in a wide pH range as well as excellent stability and reversible stimuli-responsive property, showing great application potential in water desalination and acid or alkali wastewater treatment. 3.1.2. Grafting zwitterionic polymers from membrane surfaces by living radical polymerization Although these above-mentioned studies successfully prepared zwitterionic grafted membranes with high antifouling properties, there are still several disadvantages for conventional radical polymerizations. Firstly, initiation of free radicals in conventional radical polymerizations has low efficiency, which may lead to low grafting density of zwitterionic polymers. Secondly, the free growth of polymer chains may cause uneven distribution of grafted polymers and uncontrollability of grafting layer thickness. In order

to overcome these shortcomings, researchers put forward a new living/controlled radical polymerization. A specially designed compound is introduced into the polymerization system, which can produce reversible chain termination or chain transfer reactions with free radicals to make their dormant species lose activity. These dormant species can decompose into free radicals under certain conditions and the fast dynamic equilibrium between dormant species and free radicals can control molecular weight and molecular weight distribution of polymers. The common living/controlled radical polymerization includes atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain-transfer polymerization (RAFT). ATRP usually employs alkyl halide as initiator with transition metal complex as catalyst such as monovalent copper complex, establishing reversible dynamic equilibrium between free radicals and dormant species via redox reaction [71]. The amount of initiators immobilized on membrane surfaces determines the graft ratio of zwitterionic polymers on membrane surfaces, while the degree of polymerization is dependent on the reaction time of ATRP [20,60,72]. Chiang et al. [71] grafted PSBMA from the PVDF UF membranes through ATRP, with the synthesis product of 2hydroxyethyl acrylate and 2-bromo-isobutyryl bromide (BIBB) as initiator, and the complex of cuprous bromide and bipyridine (CuBr/BPY) as catalyst. Surface hydrophilicity confirmed by water contact angle test, along with grafting density, increased monotonically with the increase of zwitterionic monomer concentration. The cyclic filtration test of 1 mg/ml BSA water solution showed that the modified PVDF UF membranes hardly adsorbed any BSA, the water flux recovery in the first cycle was 87% and reached nearly 100% in the second cycle, implying its superior antifouling characteristics and potential application in protein purification and wastewater treatment. Blood compatibility and cytocompatibility are key requirements of blood-contacting biomaterials. Liu et al. [26] grafted phosphorylcholine (PC) and sulfobetaine (SB) class zwitterions from cellulose membranes used for blood purification via surface-initiated ATRP, and obtained functional membranes which effectively suppressed nonspecific protein adsorption and platelet adhesion (Fig. 6). ATRP method has relative mild reaction condition and less operating steps, above all the most reagents are easily available [26]. ATRP has become one of the most powerful methods in the functional modification of membrane surfaces with zwitterionic polymers. However, the removal of copper ion is still a problem to be considered. Compared with ATRP, the polymerization rate of RAFT is slower, but RAFT is suitable for a wide variety of monomers and can control the polymerization without the need of metal catalysts [73– 75]. RAFT triggers radicals by traditional initiators, such as AIBN, and usually regulates reaction by chain transfer agent 4cyanopentanoic acid dithiocarbamate (CPADB) to realize the active polymerization [74]. Yuan et al. [60] prepared polysulfobetaine brush-modified cellulose membranes through the RAFT polymerization of 3-dimethyl (methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) on the membrane surfaces, dramatically increasing hemocompatibility and elevated resistance to both E. coli and HeLa cell adhesion, which could be possibly utilized in medical field to solve thrombosis and bacterial infection. Membrane modification via living/controlled radical polymerization shows controlled introduction of high-density graft chains and stable antifouling properties. However, the complicated modification processes prevents the large-scale application. In addition, the surface grafting methods mostly result in a decrease of permeation flux because of membrane pore plugging caused by grafted zwitterionic polymers [76]. The grafting density is an important parameter in ‘‘surface grafting” closely related with permeability and antifouling performance of membranes, so we make a brief summary here.

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Fig. 6. Schematic illustration for surface-initiated ATRP on the cellulose membranes. The surface-initiated ATRP was performed by two steps. The first step was the immobilization of BIBB through the esterification of hydroxyl groups on cellulose membranes with BIBB. The second step was the surface-initiated ATRP of zwitterionic monomers from cellulose membranes with CuBr/BPY as catalyst [26]. Source: Ref. [26], Copyright 2015; reproduced with permission from the Journal of Membrane Science Publishing Group.

Almost every reported work shows that antifouling properties of membranes are prone to enhance with increase of zwitterion grafting density because of denser hydration shell and stronger steric hindrance, and keep almost unchanged when the grafting density reached a certain level. Meanwhile, the permeate flux first increases and then decreases with increase of zwitterion grafting density, which can be explained by improvement of hydrophilic performance, blocking and shrink of membrane pores respectively [62,70,77]. When a small amount of zwitterion is grafted on a hydrophobic base membrane, its hydrophilicity is obviously improved, but with increase of grafting density, the improvement of hydrophilicity slows down, while the blocking and shrink of membrane pores become more and more obvious. So there is a trade-off between permeability and antifouling in surface grafting. The grafting density of zwitterionic materials can be as much as 1.67 mg/cm2 [62], however, it seems to be too high to construct an avail antifouling layer, which requires us to keep the grafting density suitable. And the proper grafting density might be around 0.4 mg/cm2, as showed above. Besides, the increase of grafting density leads to the decrease of surface roughness [78], which might be a positive contribution to antifouling. Indeed, the improvement of antifouling properties caused by the decrease of surface roughness or by the increase of hydrophilicity is hard to distinguish when the zwitterion grafting density is increased. 3.2. Surface segregation Surface segregation process is actually a self-assembly process of amphiphilic block copolymers: the amphiphilic block copolymers are blended in membrane casting solution, during nonsolvent induced phase separation process, the hydrophilic segments of amphiphilic copolymers spontaneously segregate to the surface while the hydrophobic segments are embedded in the membrane matrix [79]. Surface segregation can be divided into free surface segregation and forced surface segregation. The primary natural prototypes of the two kinds of surface segregation are antifouling cell membranes with zwitterionic and amphiphilic phospholipids [76] and lotus leaves with super-hydrophobicity as well as selfcleaning property respectively [80]. With significant biomimetic characteristics, surface segregation is expected to overcome the drawbacks of grafting for the modification of membranes [2]. Different from surface grafting, surface segregation is an in-situ modification approach, which is regulated by the kinetics and thermodynamics of phase separation process. So the membrane obtained by surface segregation has a more hydrophilic surface, a

similar surface morphology and same or larger surface pore size compared to the unmodified membrane. Besides, in addition to membrane surfaces, membrane pores can be modified simultaneously, which can enhance the resistance to foulants in membrane pores. In addition, its facile operation realizes the synchronization of membrane fabricating and the surface enrichment of zwitterionic polymers instead of additional complicated grafting steps [81]. Free surface segregation of zwitterionic copolymers was firstly used by our group to incorporate zwitterionic DMMSA functional groups onto PAN UF membranes, with remarkably reduced fouling and high flux recovery ratio up to 95% after simple water flushing during the UF of BSA solution [24], which could be used for protein separation due to the excellent flux recovery property and longer operation life. During the process of free surface segregation, the zwitterionic brushes will segregate onto membrane surfaces, forming a hydration shell that can inhibit the adhesion of foulants [79]. Due to the great hydrophilicity of zwitterionic copolymers, the copolymers on membrane surfaces face the problem of dissolving in water. Li et al. [82] used a novel amphiphilic zwitterionic copolymer PVDF-g-PSBMA prepared by ATRP as additive in the preparation of PVDF membranes with the improvement of stability via increasing interactions between the copolymers and the membrane surfaces. The BSA adsorption amount was nearly 1/3 of the control membrane due to the enrichment of hydrophilic PSBMA brushes on the membrane surfaces. Forced surface segregation is based on free surface segregation, with block copolymers including hydrophilic segmentes and low surface energy segments as addictives in membrane casting solution. During nonsolvent induced phase separation, the low surface energy segments are carried by the hydrophilic segments onto membrane surfaces at the same time [83]. Because low surface energy can weaken the interactions between the interfaces with the increase of final minimum Gibbs free energy (Fig. 2), the foulants are unstable on membrane surfaces and can be removed easily through hydraulic cleaning, endowing the membrane surfaces with dramatic self-cleaning abilities, greatly improving the antifouling properties. Zhao et al. [84] successfully fabricated ultralow fouling PVDF membranes with the incorporation of amphiphilic copolymers consisting of zwitterionic brushes of poly[3-(methacry loylamino)propyl]-dimethyl (3-sulfopropyl) ammonium (PSPP) and low surface energy brushes of poly(hexafluorobutyl methacrylate) (PHFBM). The PSPP content at the surface ranged from 20.1 mol% to 25.0 mol%, obviously higher than the PSPP content in the whole membrane. The membranes showed superior antifouling and selfcleaning abilities during the separation of oil/water emulsion, pro-

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tein aqueous solution and cell suspension with flux nearly unchanged (more than 99% of the initial value) and flux recovery was almost 100% after simple hydraulic flushing (Fig. 7). Due to its facile and generic features, forced surface segregation method can be extended to many more membrane separation processes. 3.3. Biomimetic adhesion Biomimetic adhesion is inspired by mussels, which can adhere to nearly all types of inorganic and organic surfaces [85] with adhesives including dopamine [86]. Dopamine and its derivatives are proved as versatile adhesives to anchor zwitterionic polymers onto membrane surfaces efficiently, conveniently and stably, endowing the membranes with significant antifouling properties [87]. Biomimetic adhesives are prone to cover the whole surface under a long deposition time with the blocking and shrink of membrane pores as well as the decrease of surface roughness, so biomimetic adhesion is usually used to fabricate or modify dense membranes. Karkhanechi et al. [88] used polydopamine (PDA) coating as a precursor layer to immobilize PMPC copolymer onto a commercial RO membrane, with the change of surface charge density from negative to neutral as well as the improvement of durability and antibiofouling potential. At the same time, to simplify the modification processes, Zhou et al. [89] used PDA and PSBMA via a simple onestep co-deposition process to fabricate antifouling microporous polypropylene membranes (MPPM), which showed significant antifouling potential along with improvement of permeation (Fig. 8). Moreover, the density of PSBMA equals to those modified by the UV-induced graft polymerization method, while the utilization efficiency of PSBMA is about 9.13 wt%, 10 times higher than that of UV-induced graft polymerization method. With biomimetic adhesives, in addition to adhering zwitterionic polymers directly, we can also adhere initiators onto membrane surfaces followed by the graft polymerization of zwitterionic monomers. Zhu et al. [90] reported that a modification of poly(lactic acid) membrane was completed with the use of PDA to immobilize ATRP initiators and subsequent ATRP of SBMA. The modified membranes were demonstrated with hydrophilicity, antifouling and hemocompatibility, with great potential for biomedical and blood-contacting

Fig. 8. Schematic illustration for the construction of antifouling MPPM surfaces by one-step co-deposition of PSBMA/PDA. The modification process was simple as immersing MPPM samples in dopamine alkaline solution containing zwitterionic PSBMA of different concentrations. Under alkaline conditions, dopamine could be oxidized and self-polymerize to form PDA film. Source: Ref. [89], Copyright 2015; reproduced with permission from the Journal of Membrane Science Publishing Group.

applications such as hemodialysis. In addition, some dopamine derivatives are zwitterionic materials themselves, for instance, redox functional amino acid 3-(3,4-dihydroxyphenyl)-l-alanine (L-DOPA) was used to modify the porous side of membranes for forward osmosis (FO) and RO membrane surfaces and exhibited good antifouling properties [91,92]. 3.4. Surface coating Surface coating is a traditional modification method, and different from surface grafting, the interactions between coating layers and virgin membranes generally belong to nonspecific interactions. Similar to biomimetic adhesion, dense coating layers are prone to cover the whole membrane surfaces. Therefore, the membranes fabricated or modified via surface coating usually possess dense smooth surfaces with increased selectivity and decreased flux. Sur-

Fig. 7. Schematic illustration for the membrane surface structure prepared by forced surface segregation and its antifouling and self-cleaning performance. The zwitterionic PSPP brushes generated hydration shell via electrostatic interactions, preventing foulants from contacting with PVDF membrane surfaces; low surface energy PHFBM brushes could weaken the interactions between foulants and surfaces, preventing coalescence, migration and spread of oil droplets and rendering the surfaces with desirable selfcleaning ability [84]. Source: Ref. [84], Copyright 2015; reproduced with permission from the Journal of Membrane Science Publishing Group.

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face coating can be divided into several methods, among which initiated chemical vapor deposition (iCVD), adsorption and selfassembled monolayers (SAMs) have been used to coat zwitterionic materials on membranes. The most commonly used method is adsorption. In Abdelhamid’s work, the zwitterionic homopolymer poly[2-(methacryloy loxy)ethyl-dimethyl-(3-sulfopropyl) ammonium hydroxide was coated onto the surface of commercial polyamide RO membranes via adsorption [93]. Compared to the unmodified membranes, the water permeability, salt rejection and antifouling properties of modified membranes showed obvious improvement. Ma et al. further used immersion coating and flow-through two methods to incorporate zwitterionic copolymers of 2methacryloyloxyethyl phosphorylcholine (MPC) and n-butyl methacrylate (BMA) onto membranes and concluded that the membranes coated by flow-through methods had better antifouling properties. Steel meshes cloud also be coated on, He et.al coated zwitterionic poly(2-methacryloyloxylethyl phosphorylcholine) (PMPC) brushes onto steel meshes to produce oil water separation membranes [94]. The modified steel meshes showed great resistance to oil contamination and self-cleaning performance not only in a water-wetted state, but also in an oil-wetted state, which showed potential for practical oil-spill remediation. Besides antifouling properties in water treatment, zwitterionic coating also has good biocompatibility. Ye et al. completed an interesting work, hollow fiber membranes are modified with functional zwitterionic macromolecules for improved thromboresistance in artificial lungs [95]. The modified membranes showed expected antifouling properties in experiment of platelet deposition and no degradation in experiment of gas transfer. Compare with adsorption, iCVD is more controllable, equable and reproducible. In Yang’s work, a zwitterionic antifouling coating on RO membrane was obtained by synthesis of Poly[2(dimethylamino)ethyl methacrylate-co-ethylene glycol dimethacry late] (PDE) thin films via iCVD followed by reaction with 1,3propane sultone, which was the first time that zwitterionic coating was applied on RO membranes [96]. The modified RO membranes showed unchanged salt rejection, reduced permeability and improved resistance to microbial contamination. Later, Shafi et al. completed a similar work with the zwitterionic polymer poly(4-vinylpyridine-co-ethylene glycol diacrylate) (p(4-VP-coEGDA)) and also achieved good performance [97]. SAMs can contribute to forming a delicate structure of membrane surfaces. Bengani et al. fabricated novel NF membranes via zwitterionic copolymer self-assembly, and the self-assembled channel-type clusters ranged from 0.6 nm to 2 nm in size [98]. The novel NF membranes exhibited size-based rejection between small molecules and strong resistance to biomacromolecular fouling. Furthermore, the flux decline was less than 4% during filtration of protein solutions and oil emulsions. 3.5. Other approaches Except for the four incorporation approaches above, there are also some other approaches to incorporate zwitterionic materials onto membrane surfaces for antifouling properties. There are two recent approaches worth mentioning. The first one is to use N-aminoethyl piperazine propane sulfonate (AEPPS) to replace piperazine in interfacial polymerization. In Weng’s work, zwitterionic polyamide thin film composite membranes (TFCMs) were fabricated via this approach [99]. Compared with other TFCMs, the zwitterionic modified membranes showed an improved antifouling property due to their high hydrophilicity, low surface electrical charges and smooth surface roughness. Meanwhile, Mi et al. also fabricated novel NF membranes in a similar way [100].

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The other approach is to load zwitterion on inorganic materials and then fabricate hybrid membranes via blending. Chan et al. loaded zwitterion onto carbon nanotubes (CNTs) and then aligned the modified CNTs within the polyamide separation layer via a high-vacuum filtration step, the nanocomposite membranes fabricated by which showed great potential in apply of water desalination [101]. Zhu et al. fabricated novel loose NF membranes by firstly loading PSBMA onto graphene oxide (GO) surface via surface grafting and then blending the zwitterionic GO into PES casting solution, which showed excellent antifouling performance, good mechanical strength and good selectivity in dye/salt fractionation [102]. Zhao et al. fabricated PVDF UF membranes by firstly loading cysteine onto silver nanowires and then blending the modified silver nanowires into PVDF casting solution, which showed great separation efficiency, mechanical strength and antifouling ability [103].

4. Summary and perspective Enhancing the antifouling properties of membranes is critical to ensure the efficient and wide applications of membrane separation technology, which has evolved an essential tool in water treatment, desalination and bioseparations. Modifying membrane surfaces with zwitterionic materials, inspired by biological cell membranes, has been demonstrated as a promising avenue to solve membrane fouling problems. Zwitterionic materials have been recognized as the next-generation of promising antifouling materials because they can generate hydration shell via electrostatic interactions, which is much stronger than hydrogen bonds, and possess the stimuli-responsive characteristics. In this review, we introduced the antifouling mechanisms of zwitterionic materials and summarized the approaches to construct antifouling membrane surfaces with zwitterionic materials, including surface grafting, surface segregation, biomimetic adhesion and surface coating. In order to push the zwitterionic materials modified membranes to practical applications, it is imperative to simplify the modification steps, reduce the preparation cost and realize largescale industrial use. Besides, the evaluation method of antifouling membrane should better meet the requirements for the actual applications. For instance, there are multiple foulants in actual applications, so we had better evaluate the antifouling properties of membranes with multiple foulants to imitate actual system, such as proteins, humic acids, algae and bacteria. Moreover, membranes are used for much more circles in actual applications than in lab experiments, so we had better investigate more circles to confirm if the nonfouling layer is stable in lab experiments. Majority of current research focused on modifying membranes with polybetaines whose positive and negative charges are located on the same monomers. As an alternative, polyampholytes with equal positive and negative charges located on different monomers can be another potent candidate. Besides, zwitterionic inorganic/ organic hybrid polymers with higher stability have been increasingly explored in materials science field. It can be expected that incorporating polyampholytes and zwitterionic inorganic/organic hybrid materials onto membrane surfaces can significantly expand the application scope of zwitterionic materials.

Acknowledgements The authors thank the financial support from the National Science Fund for Distinguished Young Scholars (21125627), Tianjin Natural Science Foundation (14JCZDJC37400, 13JCYBJC20500), and the Program of Introducing Talents of Discipline to Universities (no. B06006).

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