Jia Ju*,Feixue LiangXiaoxin ZhangRan SunXiaoguang Pan,Xiaoyun GuanGuanning CuiXuan HeMengyan Li

1 College of Chemistry,Chemical Engineering and Environmental Engineering,Liaoning Shihua University,Fushun 113001,China

2 School of Computer and Communication Engineering,Liaoning Shihua University,Fushun 113001,China

Keywords:Blood purification Organic inorganic hybrid/blend membrane Albumin-bound toxins Preparation and modification Ligand density

A B S T R A C T Blood purification refers to the extra corporeal therapies of removing potentially toxic substances,in which blood is circulated through an adsorption system loading separation materials.High-efficient inexpensive separation materials are critical to success.In this review,separation materials such as polymers and nanomaterials are summarized and compared.Combining the advantages of the adsorptive membranes and nanomaterials,organic-inorganic hybrid/blend membranes have been developed explosively.These hybrid/blend membranes have both the characteristics of high permeability,easy fabrication,good biocompatibility of adsorptive membranes,and characteristics of fast adsorption rate and high adsorption capacity of nanomaterials.The preparation and modification methodology of the separation materials is reviewed.For affinity separation materials,the relationship of ligand chemistry,ligand density and pores of the matrix is discussed.This paper also summarizes some interesting applications in separation materials for removal of bilirubin,endotoxin,toxic metal ions,cytokine,etc.?2019 The Chemical Industry and Engineering Society of China,and Chemical Industry Press Co.,Ltd.All rights reserved.

1.Introduction

Blood purification is a treatment that directly removes the pathogenic substance or toxic substance from the patient's blood by means of extracorporeal circulation equipment.Due to its rapid and effective detoxification,it is called“the third treatment”besides drug therapy and surgical treatment.Although blood purification is not an etiological treatment,it may provide patients with numerous opportunities for organ transplantation or other cases.For example,before and after liver transplantation,the body's toxic substance such as bilirubin and bile acids will increase extremely in short time,which threatens the patient's life seriously.Therefore, detoxification through blood purification therapy is required,which benefits the recovery of organs after surgery[1,2].It is also found that in recent decades the most efficient way of treating sepsis and septic shock is extracorporeal blood detoxification[3,4].

Blood purification therapy originates from hemodialysis,and derives a variety of purification technologies, mainly including hemodialysis,hemofiltration,hemoperfusion,molecular adsorbents recirculating system,blood replacement,immunoadsorption,and plasma adsorption.The above technical joint uses frequently in clinical operation.For example,albumin dialysis-combined molecular adsorbents recirculating system removed the body's water-soluble or protein-bound toxins[5].Hemodialysis can remove small molecules of ammonia,nitrogen and virus molecules of systemic lupus erythematosus using dialysis membranes.Hemofiltration can separate macromolecules such as the blood protein and blood cells using an ultrafiltration membrane,avoiding the loss of beneficial proteins in the purification process.The above two used in combination,namely hemodiafiltration[6],separate blood cells in the upstream process and remove downstream small molecules. Hemoperfusion is mainly used to remove fat-soluble molecules such as bilirubin,bile acid and endotoxin from the blood with an adsorption column,and is usually used in combination with the above two processes[7].

Based on the above blood purification therapy,the key components of extracorporeal circulation equipment can be divided into two types of dialysis membranes or adsorbents. Adsorbents play essential and ubiquitous roles in the treatments of hemoperfusion,molecular adsorbents recirculating system,plasma adsorption,and immunoadsorption.And increasing reports on the adsorbents for blood purification have been presented in recent years.Thus,the adsorbents are summarized in the application of blood purification in this paper.Activated carbon was the earliest adsorbent for blood purification therapy.At present,resins as adsorbents are mostly used in clinical blood purification treatment.However,these materials also have some disadvantages,such as prolonged treatment time,high treatment costs,and adsorption efficiency that can be further improved.Therefore,biomedical workers devoted to developing novel adsorbents with the characteristics of money-saving,speeding the separation process,high adsorption capacity,and renewability.

For extracorporeal blood purification, high-efficient inexpensive separation materials are critical to success.Adsorbents as separation materials are generally divided into polymers,nanomaterials,and hybrid or blended both.First,we summarize polymers such as resins,adsorptive membranes, and molecular imprinting material in blood purification use. Secondly, nanomaterials as adsorption materials catch much of our attention.Thirdly,organic-inorganic hybrid/blend materials, especially adsorptive membranes blended or mixed with nanomaterials have begun to be used in blood purification.The preparation and modification methodology of the separation materials is summarized. For affinity separation materials, the relationship of ligand chemistry, ligand density and pores of the matrix is discussed. This paper also summarizes some interesting works in separation materials for removal of bilirubin,endotoxin,toxic metal ions,cytokine,urea,lysozyme,etc.

2.Polymers

Polymer adsorbents have been used for many years in blood purification to remove toxins,because of its high adsorption efficiency,easy processing and molding, no leakage, and good blood compatibility.Polymers,from natural to synthetic,are frequently adopted as adsorbent matrix.They include natural polymers such as chitosan and chitin,modified natural polymers such as cellulose acetate,cellulose nitrate and regenerated cellulose,and synthetic polymers containing polyesters,aliphatic polyamides,polyolefin,polyvinyl alcohol,synthetic copolymer,and so on.These polymers,hardly be directly used,need to be activated,functionalized and grafted with functional ligands to become blood purification adsorbents with specific adsorption capacity.The performance of adsorbents depends on not only the properties of ligands,but also the morphology of polymer matrix.Polymer materials can be processed into different shapes,such as particle or bead resins,adsorptive membranes,hydrogels,monolithic column,and molecular imprinting polymers, etc. In the following sections, particle or bead resins, adsorptive membranes and molecular imprinting polymers are reviewed.

2.1.Resins

Resins,beads or particles,as adsorbents for blood purification,have attracted much attention.At present,the disposable blood purification separator is made of resin,which can be used for hemoperfusion and immunoadsorption to remove bilirubin,endotoxin,bile acid,tumor necrosis factor-alpha,interleukin-6,and so on.Resins generally include non-polar adsorptive resins,polar adsorptive resins and ion exchange resins.The common hemoperfusion separators for removal of bilirubin are BR-350 and BL-300 produced by Asahi Kasei Co.,Japan.Both of them are packed with styrene-divinylbenzene anion exchange resin,which has the advantages of high bilirubin adsorption capacity and fast adsorption rate[8].The HA330 hemoperfusion separator manufactured by Jafron Biomedical Co.,Ltd.in China accounts for a large proportion of the Chinese market.The adsorption capacity of neutral microporous resin in HA330 depends on the screening effect of its threedimensional network structure and the molecular interaction between the functional groups of the resin and the targeted molecule[9].However,the accumulation of resin particles packed in the column and the tortuosity of apertures into the resin particles result in low flow rate and long operation time of resin separation,which enhances the suffering of patients.

2.2.Adsorptive membranes

Functional adsorption membranes,known as affinity membranes,combine the advantages of resins and sieving membranes.Compared with resin,as shown in Table 1,it could not only adsorb blood toxins efficiently using specific ligands,but also separate blood macromolecules by sieving.Moreover,adsorptive membranes have the characteristics of high permeability flux,which can remove blood impurities at lower operating pressure and higher flow rate.The device of adsorptive membrane is simple and convenient to operate. It has broad application prospects in medical examination,extracorporeal circulation treatment and other fields.

Table 1Comparison between adsorptive membranes and resins

Because adsorptive membranes were elaborated in previous reviews by Klein[10]and Suen[11].This article focuses on the research progress of polymer membranes as adsorbent for blood purification after these reviews.Chen et al.[12]and Ju et al.[13]prepared a cellulose acetate/polyethyleneimine blend microfiltration membranes and applied in removal of copper ion and bilirubin.Zhang and Gu[14]developed a new adsorbent system for bilirubin removal that composed of HSA as the specific ligand and PTFE capillaries as the carrier matrix.These new adsorbents have both the advantage of membrane and micro-column,and it has a mass transfer of higher velocity, better adsorption capacity,less fouling, longer useful life and can be connected to recirculation flow system directly without any auxiliary equipment. Bayramoglu et al.'s synthesized acrylic hydrogel copolymer membrane was from 2-hydroxyethylmethacrylate(HEMA)and glycidyl methacrylate(GMA)monomers. The polyethylenimine and Cibacron Blue F3GA were immobilized subsequently onto poly(HEMA-co-GMA)membrane surface to obtain two types of bilirubin adsorption membranes[15].Shi et al.prepared a poly-L-lysine-modified nylon membranes and show high bilirubin adsorption capacity[16].

These adsorptive membranes were prepared by blending,radiation grafting,chemical modification and other methods to immobilize general ligands such as PEI and dyes and showed good bilirubin adsorption capacity.However, up to now,affinity membranes as adsorbents for blood purification to remove bilirubin and other toxins are rarely used in clinical applications.It can be made clear that the adsorption rates of the targeted molecules are slower in the membrane systems than in the column systems sometimes. Slow binding rates between the targeted species and the immobilized ligands will result in greatly reduced dynamic adsorption capacities,except for the use of low flow rates [11]. In addition, the design and preparation of the optimized membrane separator may be one reason why the adsorption membrane has not been commercialized.

2.3.Molecular imprinting polymers

Molecularly imprinted polymers (MIPs) are a kind of intelligent polymers,which form selective sites in polymer matrix with the memory of the template.MIPs are prepared by copolymerization of functional monomers and crosslinkers in the presence of template molecules.After removal of template molecules,selective site is formed in highly crosslinked polymer matrix,which is complementary to template molecules in shape,size and chemical functions.Template molecules(targeted)can be selectively adsorbed from adsorption systems that have similar structures to template molecules. Compared with other molecular recognition technology, the MIP technology has remarkable characteristics,such as low cost,easy synthesis,chemical physics high stability, and good recovery, etc. Therefore, MIPs have attracted attention in many fields,especially in solid-phase extraction or micro-extraction, artificial antibodies, biochemical sensors and blood purification,etc.[17].

MIP materials have been explored in blood purification for removal of bilirubin and other toxins,because of its high adsorption selectivity in complicated blood systems.The higher the adsorption selectivity of the materials,the less beneficial substances with similar structure will be removed. Syu et al. first prepared bilirubin imprinting materials and applied it to the separation and detection of bilirubin [18]. In these studies,methacrylic acid(MMA),ethylene glycol methacrylate(EGDMA), and azodiisobutyronitrile (AIBN)were used as functional monomer,crosslinking agent,and initiator to prepare poly(MMA-co-EGDMA)molecular imprinting materials for bilirubin removal.Continued studies respectively investigated the synthetic conditions such as the polymerization methods(heat-and ultraviolet radiation-graft polymerization),the mass ratio of crosslinker to monomer(EGDMA/MAA),categories of monomers and metal complexation[19].Competitive adsorption model was put forward to describe the separation selectivity of bilirubin to biliverdin[20].Although the maximum bilirubin adsorption capacity is not higher than 1 mg/g MIP, it can be utilized in the preparation of serum bilirubin detection sensor[21].

To enhance bilirubin adsorption capacity of MIPs,Baydemir and Denizli[22,23]prepared a molecular imprinted macroporous gel for the adsorption of plasma bilirubin.They inserted molecular imprinting particles into macroporous hydrogel,which not only improved adsorption selectivity,but also increased bilirubin adsorption capacity of 3.6 mg/g MIP to 10.3 mg/g MIP [24]. The maximum adsorption capacity was higher than that of imprinting materials prepared by Syu. Since the imprinted particles are evenly distributed in macroporous hydrogels,sufficient contact opportunities are given for bilirubin and selective sites on imprinted particles.The increase in the accessibility of selected sites with bilirubin offers a good way to increase the adsorption properties of MIP block materials.Ligand density and adsorption accessibility will be discussed in detail in Section 5.2.

3.Inorganic Nanomaterials

Activated carbon is the first inorganic material used for blood purification to remove toxins from the body.However,activated carbon is gradually replaced by polymer materials because of its low removal efficiency and poor blood compatibility.Recently,inorganic nanomaterials such as carbon nanotubes, titanium dioxide and silicon materials have caught much attention of biomedical researchers.Several papers on the removal of toxins have been reported by using nanomaterials.Compared with polymer materials,the separation mechanism of removing toxins from inorganic materials is complex and diverse. The separation mechanism includes adsorption with microporous and mesoporous structure,photocatalytic degradation,specific adsorption with grafted organic ligands,and in many cases.Several mechanisms are cooperative.Both the adsorption capacity and adsorption rate of nanomaterials such as carbon nanotubes are higher than those of polymer materials in different morphology.On the other hand,their hemocompatibility,easy leakage in adsorption system, and capability for surface modification may not be competitive.

Carbon-based materials such as activated carbon particles[25],hollow mesoporous carbon spheres [26], single-wall carbon nanotubes(SWCNTs)and multi-wall carbon nanotubes(MWCNTs)[27,28],bone char[29],and three-dimensionally porous graphene[30]are mainly adsorbed toxins with microporous and mesoporous structure.Titanium dioxide(TiO2),as a photocatalytic adsorbent,cannot only degrade bilirubin under ultraviolet radiation,but also adsorb bilirubin with porous structures[31].A kind of nano-TiO2for bilirubin removal was prepared by sol-gel method and heat treatment.The size and surface hydrophobicity of the TiO2particles were regulated by changing the temperature and heat treatment time,aiming to improve bilirubin adsorption[32].A detection method for bilirubin can be established by Yang and Zhang through the preparation of TiO2film combined with quartz microcrystal balance technology[33].

In order to prevent nanomaterial leakage,Ando et al.prepared thick laminar structure bilirubin adsorbent with SWCNTs as the support and MWCNTs as the filler,and thus increasing blood compatibility[28].Another strategy is to blend or compound magnetic materials so that the composite materials can be readily collected in a magnetic field.MWCNTs/iron oxides magnetic composites were developed to remove bilirubin from albumin solution in bound solute dialysis.The magnetic adsorbents synthesized from acid pre-treated MWCNTs were well dispersed in aqueous solution,and proved to be recycled by application of an external magnetic field[34].Ma et al.successfully synthesized a magnetic nitrogen-doped porous carbon (m-NpC) through a simple one pot route using chitosan and Fe(NO3)3?9H2O as precursors, and NaCl as template agent,respectively.The adsorbent demonstrated its superior adsorption capacity for BSA-bound bilirubin and good hemocompatibility due to its nitrogen-doped carbon full of high affinity sites for acidic molecules,which competed effectively with albumin for bilirubin binding.The adsorbent could also be easily separated from the medium just by the addition of an external magnet [35]. A bloodcompatible adsorbent based on sol-gel derived alumina was applied to remove low-density lipoprotein for blood purification[36].

4.Organic-Inorganic Hybrid or Blended Materials

As described in Section 3, inorganic materials, especially nanomaterials,are widely used as adsorption materials in blood purification due to its high adsorption efficiency.However,nanomaterials as blood adsorbents have their defects in clinical application,for example,poor blood compatibility and easy leakage.There is still a gap in clinical application of inorganic nanomaterials.Many researchers paid much attention to solve these problems.It is a research hotspot that fabricates a new type of composite materials for blood purification through organic modification of inorganic materials or organic-inorganic hybrid or blended materials[37-39].

4.1.Beads or particles

Kevlar nanofiber-based porous beads and its blending CNT beads were successfully prepared and found to be promising materials for bilirubin removal.These beads showed good blood compatibility,which was revealed in terms of the low hemolysis ratio,prolonged clotting time and suppressed platelet activation and complement activation[40].Poly(N-vinyl-2-pyrrolidone)-modified silica particles were prepared through sol-gel process for adsorption of bilirubin.The adsorption capacity of these beads for bilirubin is highly dependent on the quantity of incorporated PVP in the silica matrix.In this study,PVP as imprinted agent to form the mesoporous structure with larger pore size[41].Tang et al.prepared a series of modified SBA-15 adsorbents for the adsorption of bilirubin by grafting methyl,amine and methyl/amine based on silica with high specific surface area and narrow pore size distribution[42,43].

Cheng et al.reported a novel nano-CaCO3/polystyrene nanocomposite adsorbent for the efficient removal of human tumor necrosis factor[44]. The incorporation of nano-CaCO3into polystyrene matrices could enhance the mechanical strength of macroporous resins and also significantly improve their bilirubin adsorption capacity. Thus,nano-CaCO3/polystyrene nanocomposite adsorbents have a good potential to be employed as effective adsorbents for blood purification[45].In total,the success of organic-inorganic composite adsorbents might provide a choice for the improvement of the adsorbents used in hemoperfusion.

4.2.Membranes

Shi and Xue et al.grafted lysine and arginine by different methods on the surface of porous alumina membrane,and the modified alumina membrane has the bilirubin adsorption capacity comparative to that of polymeric membranes [46-48].Nie et al.synthesized successfully heparin-mimicking polymer brush using surface-initiated atom transfer polymerization-grafted CNT,and then constructed functionalized-CNT/polyethersulfone composite membranes by a liquid-liquid phase separation technique. The composite membranes combined good cytocompatibility of heparin-mimicking polymer with the high adsorption capacity of nanomaterials for uremic toxins[49].

These hybrid/blend membranes have both the characteristics of high permeability, easy fabrication, good biocompatibility of adsorptive membranes,and characteristics of fast adsorption rate and high adsorption capacity of nanomaterials.Thus,preparation and application of organic-inorganic hybrid membranes in blood purification will be the next research direction.

5.Ligand Structure and Ligand Density of Adsorption Materials

5.1.Selection of ligands

Ligands are substrates,products,inhibitors,coenzymes,allosteric effectors,or any other molecule that specifically and reversibly interact with a target protein or macromolecules.Affinity separation based on molecular recognition between the ligands and the target molecules that can be removed or separated easily. According to interaction mechanism, ligands can be divided into specific ligands and general ligands.In general,the binding constants of specific ligands are about 105L·mol-1,while those of general ligands are relatively low,about 103L·mol-1.The latter belongs to weak affinity ligands,but the accumulation of multiple general ligands can show high selectivity and fast adsorption kinetics.General ligands are frequently adopted in grafting rather than specific ones,because of its stability,no deactivation and easy application in industrial scale.For affinity adsorption materials,the following ligands are most commonly used: reactive dyes,cyclodextrin, and proteins and amino acids, and polyethyleneimine.Polyethyleneimine immobilized as a polycationic ligand can effectively remove bilirubin [12,13], endotoxin and other anionic toxins. The other ligands will be introduced as follows.

5.1.1.Reactive dyes

Dye ligands have been considered an important alternative for natural ligands,because they can bind to most proteins by electrostatic force,hydrophobic interaction and hydrogen bonding.Dye modification is an effective way to remove blood toxins by affinity adsorption.However,the selectivity is poor because they adsorb proteins at the same time.Dye-modified hollow fiber membranes were prepared by hydrolyzing parts of amide groups with hydrochloric acid and grafted with Cibacron blue F3GA to remove plasma bilirubin.The bilirubin adsorption capacity of the dye-modified membranes reached 48.9 mg·g-1at 37°C.Bilirubin was not only directly adsorbed on the membranes,but also indirectly adsorbed by albumin [50].Ma et al.prepared cellulose matrix membranes with electrospinning technology and regenerated them by heat treatment and alkali hydrolysis.The dye-modified regenerated cellulose affinity membranes were prepared by grafting Cibacron blue F3GA onto the surface. The adsorption capacities of BSA and bilirubin were 13 mg·g-1and 4 mg·g-1,respectively[51].

5.1.2.Cyclodextrin

Cyclodextrin(CyD)famous with its stereo-chiral hydrophobic cavity can form inclusion compounds with many guest molecules according to Van der Waals force, hydrophobic interaction and molecular size matching.Wang et al.synthesized a water-soluble adsorbent by grafting β-CyD to branched polyethyleneimine matrix for removing albuminbound toxins from the plasma of hyperbilirubinemia patient.The results indicated that the adsorbents could be an effective adsorbent for blood purification aiming at the removal of not only bilirubin but also bile acids and amino acids[52,53].Sakata et al.developed water-insoluble copolymer particles of γ-CyD-polyurethane copolymer to remove endotoxin(lipopolysaccharides,LPSs),and investigated the effect of γ-CD content within the adsorbent and the buffer conditions on the LPS adsorption capacity.It is found that the copolymer particles can selectively remove endotoxin from DNA solutions,as shown in Fig.1,superior to other reported materials,which remove both LPSs and DNA by ionic interactions[54].

Fig.1.Schematic diagram of selective adsorption behavior of LPSs for cross-linked γ-CyD adsorbent in DNA solution containing LPSs[54].

5.1.3.Protein and amino acids

The protein can tightly interact with many blood toxins and thus is often used as a ligand for removing these toxins. Researchers immobilized human serum albumin(HSA)[55]and bovine serum albumin(BSA)[56]on activated carbon,macroporous resins or membranes by direct coating and surface modification,and the removal efficiency of toxins and the biocompatibility of the process were improved.HSA was chosen as ligand for competing with HSA-bound bilirubin, and immobilized onto the poly (2-hydroxyethylmethacrylate) (HEMA)-based matrix following Cyanogen bromide(CNBr)activation.The adsorbent had a higher adsorption capacity with high specificity for bilirubin removal[57].With the study of the interaction between toxins and albumin,it was found that the binding sites of toxins on albumin are composed of one or more amino acid fragments.These amino acids include lysine, histidine, tyrosine and phenylalanine [58]. At present,there are many studies on the preparation of affinity adsorbent with amino acid as ligand to remove endotoxin[59]and bilirubin[60]and to purify protein.Zhang et al.and Huang et al.grafted l-serine ligand onto PVDF and PSf hollow fiber membrane for extracorporeal elimination of endotoxin from blood[61,62].

5.2.Ligand density and accessibility

Ligand density,the content of ligand on the unit of mass or surface area,is a principal characteristic parameter of the adsorbent,which affects not only the adsorption capacity of adsorbents,but also the elution of ligates and regeneration of the adsorbents.

The adsorbent with different ligand density can be prepared by adjusting the activation conditions of the matrix and the reaction conditions of ligand immobilization.Carboxyl terminal ligand was grafted on the surface of the activated organosilicon by adjusting reaction time,reaction temperature and the concentration of acid anhydride.The ligand density can be regulated from 1%to 100%[63]. L-Lysine immobilization yield on regenerated cellulose membranes was adjusted as a function of the reaction conditions[64].Improvement in grafting method can also increase ligand density,thus increasing protein adsorption capacity of adsorbents.Gebauer et al.constructed a three-dimensional adsorption layer by crosslinking to enhance the ligand density of the membrane,of which the static adsorption capacity was 100 times higher than that of the direct crosslinking case[65].

Ligand density is an intrinsic property of adsorbent,which has a significant effect on the separation behavior of albumin or antibody on affinity chromatography[66],ion exchange chromatography [67],and hydrophobic binding chromatography[68].The effects of ligand density on the hydrophobic charge-induced chromatographic separation protein was studied by the molecular dynamics simulation [69]. It was found that the adsorption capacity and reversibility of the adsorbent increased with the increase of the ligand density,but due to the high binding resistance between ligands and proteins,the optimized adsorption rate appears in the case of slightly lower than the maximum ligand density. A novel type of hydrophobic charge-induced chromatography(HCIC)resin with different pore size and ligand density was prepared to investigate the adsorption equilibrium and kinetics of bovine serum immunoglobulin G[70].The maximum adsorption capacity and the effective pore diffusion coefficient increase with ligand density and pore size.For a given protein,the protein adsorption can be improved by optimizing the ligand density and pore size of the HCIC resin.

The separation performance of ion exchange resins or affinity adsorption resins is related to ligand chemistry,ligand density,and ligand accessibility.Franke and co-workers indicated that ligand density directly influences the porosity,pore diffusion coefficient and adsorption capacity of the materials [71]. The accessible surface area of the adsorbent declined with the increase of ligand density.Pore size directly affects mass transfer in the resin.In details,macropore is beneficial to the mass transfer of protein and increase the ligand accessibility on the adsorbent.The use of spacer arm increases the accessibility of the ligand[72].

Ligand chemistry,density and accessibility also have an effect on the adsorption of toxins such as bilirubin into the separation materials,but the relevant studies are less.Yu et al.grafted such as ethylenediamine and 1,6-hexane on the surface of cross-linked chitosan microspheres,and found that bilirubin adsorption capacity increased by about 5%as the number of methyl groups increased from 2 to 6,which indicated that increasing ligand accessibility was conducive to the adsorption of bilirubin. It was further showed that bilirubin adsorption increased with the increase in the number of amino groups, i.e. ligand density[73]. Jia et al. demonstrated that the adsorption rate of bilirubin decreased with the increasing density of HSA,and the equilibrium time for bilirubin adsorption was prolonged[74].This may be explained by the large volume of human albumin, resulting in a larger steric hindrance of bilirubin interaction with it.The conflict results between the above two studies might attribute to the difference in ligand size.

In our study,the bilirubin adsorption capacity of the four modified membranes was 100%higher than that of the original,and the adsorption selectivity of bilirubin to albumin was enhanced,due to the introduction of a spacer arm and the immobilization of specific ligands.Ligands containing amine and hydrophobic groups could enhance bilirubin adsorption capacity. Bilirubin adsorption capacity of modified membrane could not be enhanced via ligand density with large steric hindrance such as benzene ring. For long carbon chain ligands with small steric hindrance, the adsorption capacity of bilirubin increases with the increase of its ligand density[75].Salehi and Madaeni found that the addition of poly(ethylene glycol) as porogen in membrane preparation process can increase adsorption capacity of copper ion of chitosan/poly(vinyl alcohol) membrane. This can be explained that the increase in size and density of macrovoids can enhance the accessibility of the ions to the active sites[76].

6.Preparation and Modification of Methodology

One step or one-pot step is an extremely ideal preparation strategy.Indeed,the yield of ligands or active sites decreases gradually with the increase in preparation steps.However,studies in Section 5.2 showed that separation performance and albumin activity are not directly proportional to ligand density. Moreover, an ideal separation material must assemble the characteristics of uniform porosity,stability,optimized ligand density,high adsorption capacity and good blood compatibility.However,the matrix of separation materials could not be found to have these characteristics at the same time. Accordingly, in most cases,a multi-step process that couples various methods is found to prepare separation materials.

Organic-inorganic hybrid materials are a typical multi-step preparation process.Shi et al.,from Xiamen University,successfully prepared an organic and inorganic hybrid affinity membrane,and its bilirubin adsorption capacity reaches the level of organic materials.They first generated active layers on the surface of inorganic alumina membrane with few active sites by means of gas-phase deposition polymerization[46],silane crosslinking[47],sol-gel[48],etc.Then they immobilized ligands of lysine or arginine through chemical modification-used bifunctional-group agents of 1,4-butanediol diglycidyl ether and glutaraldehyde to prepare a bilirubin adsorptive membrane.In this study affinity membrane was successfully prepared by combined conventional preparation methods in inorganic materials with general chemical modification methods in polymers.Wu et al.developed a kind of efficient,safe and blood compatible adsorbent for bilirubin removal from human blood.The adsorbents combined the high adsorption of CNTs and inherent biocompatibility of chitin(Ch)and lysine(Lys).The Ch/CNT/Lys hybrid nanofibrous microspheres have been fabricated successfully from the chitin solution dissolved in NaOH/urea aqueous system at low temperature by blending with modified CNTs via thermalinduced method and further immobilized with lysine[77].

In order to purify α1-antitrypsin (AAT) from pretreated human plasma fraction IV in one-step membrane chromatography,Fan et al.offered a new approach to design the ligands on membrane adsorbent for specific application[78].First,polyvinylidene fluoride(PVDF)membranes were coated with polydopamine(PDA),then grafting cationic polyelectrolyte ligands of polyethylenimine and polyallylamine.The attached polyelectrolytes were quaternized to further improve hydrophilicity. Four different ligands and their quaternization forms were obtained on the PDA-coated PVDF membranes,the preparation process was illustrated in Fig.2.The outcome of this study would not only offer a facile method to design and screen ligands for membrane adsorbents,but also provide further insights in rapid purification of biomolecules from complex streams using ion-exchange membrane adsorbent with polyelectrolytes as ligands.

7.Other Interesting Separation Materials for Blood Purification

Recently, interesting separation materials for blood purification were applied to adsorb bilirubin, metal ions, cytokines through hemoperfusion and hemoadsorption and remove urea,lysozyme by hemodialysis and hemodiafiltration,as shown in Table 2.The preparation methods included non-solvent-induced phase separation, in situ crosslinking,electrospun,etc.In situ crosslinking[80]or polymerization[85]is an easy and effective method for the modification of polymeric membranes.The method can simplify preparation procedure and thus reduce membrane fabrication costs.A thin-film nanofibrous composite membrane, consisting of an ultrathin hydrophilic separation layer of polyvinyl alcohol (PVA), and an electrospun polyacrylonitrile(PAN) nanofibrous supporting layer exhibited high permeability(～290.5 L·m-2·h-1at 0.1 MPa)and excellent selectivity which should be attributed to its unique structure with ultrathin separation layer and highly porous supporting layer[84].

Fig.2.Schematic diagram of preparation of mussel-inspired membrane adsorbers with polyelectrolyte ligands[78].

Recent researches paid considerable attention to blood compatibility of separation materials.The blood compatibility of the separation materials was examined by means of plasma protein adsorption,plasma coagulation, and platelet adhesion. A novel non-ionic macroporous polystyrene adsorbent (SZ-9) with rich mesopores and high surface area was established by suspension polymerization,and it performed better adsorption capacity for bilirubin and better blood compatibility than BL-300[82].

8.Summary and Perspective

Blood purification begins with activated carbon, but is gradually replaced by polymer materials.With the development of nanotechnology,inorganic nanomaterials have been paid attention again.Each substitution is the result of improvement in separation efficiency or blood compatibility.In this review,separation materials for blood purification therapy such as polymers and nanomaterials are summarized and compared.Adsorptive membranes as polymer adsorbents showed high permeability flux,low pressure drops,good bilirubin adsorption capacity and good blood compatibility.However,adsorption rates of the targeted molecules are slower in the membrane systems,which have a negative effect on clinical applications,while nanomaterials showed higher adsorption capacity and adsorption rate than those of polymer materials in different morphology.However,their hemocompatibility,easy leakage,and capability for surface modification may not be competitive.Combining the advantages of the adsorptive membranes and nanomaterials,organic-inorganic hybrid/blend membranes begun to be developed.These hybrid/blend membranes have both the characteristics of high permeability,easy fabrication,good biocompatibility of adsorptive membranes,and characteristics of fast adsorption rate and high adsorption capacity of nanomaterials.The new material may have some shortcomings of low adsorption selectivity.It will be a good choice to embed inorganic molecularly imprinted materials into adsorptive membranes in order to enhance the adsorption selectivity of the hybrid/blend membranes.

Table 2Recent development in separation materials for blood purification

In the preparation of separation materials,some studies have focused on the ligand density and accessibility.Improving the accessible density of ligands is the key to improve the adsorption efficiency of adsorbents.This is closely related to the ligand structure and the microstructures of adsorbents. Thus, researchers will grasp the functional relationship between separation performance and ligand density and accessibility, thus realizing the controllable adjustment of material properties.In order to obtain excellent separation materials,preparation process tends to be complicated,which might be not conducive to large-scale production.Thus,simplifying preparation process represents one of the future research hotspots.