REVIEW www.rsc.org/softmatter | Soft Matter

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View Article Online / Journal Homepage / Table of Contents for this issue

Surface engineering of macroporous polypropylene membranes

Ling-Shu Wan,a Zhen-Mei Liua and Zhi-Kang Xu*ab

Received 7th August 2008, Accepted 26th January 2009

First published as an Advance Article on the web 25th February 2009

DOI: 10.1039/b813600a

The surface properties of polymer membranes are crucial to their application. This Review provides

concise comments on surface engineering strategies for macroporous polypropylene membranes. The

applications of surface engineering concepts in membrane-based bioreactors, bioseparation,

biosensing, biosynthesis, environmental analysis, water purification, energy technology, medical

devices (artificial lung and liver), and some novel separation processes (intelligent membrane

separation) are summarized. The prospect of surface engineered biomimetic polypropylene membrane

is also looked at.

1. Introduction

As a kind of soft matter, separation membranes have been used

in many fields such as water treatment, artificial organs, fuel cells,

and industrial filtration. For each application, the process

conditions are important, but the performances are clearly

determined by the membrane itself.1,2 The in-service perfor-

mances of a polymer membrane mainly depend on the chemistry

of the membrane material, the structures and properties of the

porous layer, and the microstructures and surface properties of

the top layer. Although a large number of novel polymers have

been developed at a lab scale, so far less than 20 have been used

Ling-Shu Wan

Ling-Shu Wan received his PhD

degree in Polymer Chemistry

and Physics in 2007 from Zhe-

jiang University, China (super-

visor: Professor Zhi-Kang Xu).

Currently, he holds a post-

doctoral position and is

a Lecturer at the same institute.

His research interests include

controlled polymer synthesis,

self-organized ordered polymer

membranes, and surface

functionalization of polymer

membranes.

aInstitute of Polymer Science, Department of Polymer Science andEngineering, Key Laboratory of Macromolecular Synthesis andFunctionalization (Ministry of Education), Zhejiang University,Hangzhou, 310027, China. E-mail: xuzk@zju.edu.cn; Fax: +86 57187951773bState Key Laboratory of Chemical Engineering, Zhejiang University,Hangzhou, 310027, China

This journal is ª The Royal Society of Chemistry 2009

as materials for industrially established membranes. They

include cellulose and its derivatives, poly(vinylidene flouride),

polysulfone, polypropylene, polyethylene, polyacrylonitrile,

polyamide, and polyimide, etc. These polymers have different

properties which make the corresponding membranes suitable

for certain separation processes. For example, cellulose is

hydrophilic while polypropylene is hydrophobic; hence the

former is suitable for the ultrafiltration of aqueous solutions and

the latter can be used in membrane distillation. As we know, the

development of membrane technology requires sophisticated

membranes that have controllable separation performance, long-

term stability, and even desired functions. Because it is very hard

to bring completely new and specially designed polymers into the

membrane processes, surface engineering of polymer membranes

has emerged in recent years.

Surface engineering of polymer membranes embraces those

processes that modify membrane surfaces to improve their

in-service performance. It means modifying the surface of

a membrane to confer surface structures and properties which

Zhen-Mei Liu

Dr Zhen-Mei Liu is a Lecturer

in the Department of Polymer

Science and Engineering,

Zhejiang University. She

received her PhD degree in

Polymer Chemistry and Physics

in 2004 on the subject of surface

modification on microporous

polypropylene membranes under

the supervisor of Professor

Zhi-Kang Xu. After that she

held post-doctoral positions in

the Institute of European

Membrane, Montpellier, France

from September 2004 to August

2005, and Martin-Luther-University Halle-Wittenberg, Halle,

Germany from February 2006 to February 2008, respectively.

Joining Zhejiang Unviersity in 2008, her research interests focus on

the development of polymeric membranes for biomedical materials

applications.

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are different from the bulk. The aim may be to minimize fouling,

modulate hydrophilicity/hydrophobicity, enhance biocompati-

bility, act as a diffusion barrier, provide bio- or chemical func-

tionalities, mimic a biomembrane, fabricate nanostructures or

simply improve the aesthetic appearance of the membrane

surface.

Surface engineering of polymer membranes involves those

strategies for surface modification. Firstly, the adopted methods

depend on the chemistry of the membrane materials. In fact,

a majority of polymers used as membrane materials are chemi-

cally inert, such as polypropylene and polyvinylideneflouride. It

is very important to establish versatile surface engineering

methods for the membranes. Among the membranes, poly-

propylene membrane can be considered as a typical one. Table 1

shows the surface engineering methods for macroporous poly-

propylene membrane. It should be pointed out that some special

strategies have been established for other polymer membranes.

For instance, polyacrylonitrile containing nitrile groups provides

the possibility of hydrolysis to generate reactive groups such as

carboxyl and amide, while sulfonation can be performed for

aromatic polymers. Secondly, the strategies are also decided by

the requirements of membrane applications. For most filtration

applications hydrophilization is required for polypropylene

membranes. For other applications, however, charged, hydro-

phobic, or biocompatible surfaces may be expected.

To date, there are excellent reviews on membrane science and

technology available, which focus on water purification,3 medical

applications,4 membranes and microfluidics,5 battery separa-

tors,6 catalytic membranes,7 nanopore membranes8 and other

membrane-related topics.9–12 This article provides an overview

of the state-of-the-art surface engineered polypropylene

membranes used in biotechnology, environmental protection,

energy and natural resources, biomedical devices, and some

other novel filtration processes.

Zhi-Kang Xu

Dr Zhi-Kang Xu is a full

professor in the Department of

Polymer Science and Engi-

neering, Zhejiang University.

He received his PhD degree in

Polymer Physics and Chemistry

at the Chemistry Department of

Zhejiang University in 1991. He

is now on the editorial board

of the Journal of Membrane

Science. He was financed by the

National Natural Science

Foundation of China for Young

Distinguished Scholars in 2006.

His research interests focus on

the surface engineering of polymer membranes, which includes

biofunctional membranes through enzyme immobilization,

membranes with biomimetic surface and function, membrane

surface with nanostructures by electrospinning, membrane surfaces

with anti-fouling and/or biocompatible properties, applications of

membranes in biosensors and other biomedical devices.

1776 | Soft Matter, 2009, 5, 1775–1785

2. Macroporous polypropylene membranes inbiotechnology

Although membrane technology has infiltrated into many fields,

in most cases pristine polypropylene membranes are not appli-

cable in biotechnology. This is mainly due to the specificity of

biofluids and biomolecules, which are highly environmentally

sensitive. Biomolecules (e.g. enzyme) tend to irreversibly adsorb

on the hydrophobic membrane surface. The adsorption not only

results in membrane fouling but also induces changes of the

structures and functions of the biomolecules. Therefore, surface

engineering of polypropylene membrane aims at creating

a friendly microenvironment for the biomolecules (especially for

enzymes), a surface with biosensing function, an anti-fouling

surface, and so on.

Membrane bioreactor

A bioreactor may refer to any device or system that supports

a biologically active environment. As for membrane bioreactor

(MBR), it means a biologically reactive separation process in

which the membrane offers the advantage of integrating catalytic

conversion, product separation and catalyst recovery into

a single separation process. The biological reaction is often

executed by cells, enzymes or other active biomolecules. Cells can

be encapsulated in the lumen of porous hollow fiber membranes,

which allow the transport of both cell secretory/catalytic

products out of and substrates/nutrients into the membranes.

Also, immobilized enzymes are often used for membrane biore-

actors. Various methods including physical adsorption, entrap-

ment, chemical binding, and site-specific recognition have been

developed for enzyme immobilization, which can affect the

enzyme activity and stability remarkably.23,65 It is to be noted

that, on the one hand, to chemically immobilize enzymes reactive

groups must be generated beforehand on the polypropylene

membrane; on the other hand, although a hydrophobic matrix

(e.g. polypropylene membrane) is generally a much better carrier

for the enzymatic reactions performed in organic solvents than

a hydrophilic membrane,66 the hydrophobicity can lead to the

denaturation of enzymes. Coating or radiation induced graft

polymerization of glycidylmethacrylate endowed a poly-

propylene membrane with both reactive groups and a hydro-

philic, biocompatible surface for enzyme immobilization.58,67

We proposed to construct a ‘‘biomimetic microenvironment’’

for immobilized enzymes to retain or increase their activity and

stability. This motivation originates from a biomembrane. It is

well known that a cell membrane contains amphiphilic lipids

(phospholipids, glycolipids, and steroids), carbohydrates

(glycoproteins and lipoprotein) and proteins. From the chemical

viewpoint, we may thereby construct a biomimetic surface

through surface engineering methods. Phospholipid analogous

polymers (PAPs) having hydrophobic octyloxy (8-PAP), dode-

cyloxy (12-PAP), and octadecyloxy (18-PAP) groups were

tethered onto polypropylene membranes by photo-induced graft

polymerization followed by a ring-opening reaction (Fig. 1).46,54

The other two kinds of moieties, a-allyl glucoside, g-ethyl-L-

glutamate and g-stearyl-L-glutamate were also introduced

onto the polypropylene membrane by plasma-induced graft

polymerization.36,68 Then lipase from Candida rugosa was

This journal is ª The Royal Society of Chemistry 2009

Table 1 Summary of surface engineering methods for macroporous polypropylene membranes

Methods Modifiers Applications Details

Physical adsorptionor coating

Charged surfactants Filtration of nanoparticles Create charged surface to filteroppositely chargednanoparticles13

Metals Biotechnology (e.g. isolation of viralgene)

Gold metal is coated ontopolypropylene hollow fibers usingchemical metal coating methods14

Biomacromolecules (e.g. heparin) Medical devices (e.g. oxygenator) Heparin coating can reduce plateletadhesion and activation15,16

Conjugated polymers Environmental monitoring Conjugated polymer-coatedmembranes are prepared asmicroex traction devices for on-site analysis of persistent organicpollutants17

Filling or support Catalyst (e.g. palladium) Removing dissolved oxygen After cleaning and etching by acidor base, palladium is chemicallydeposited onto a membrane toremove oxygen to very low levels(less than 1 ppb)18

Carriers (e.g. crown ethers) Environment analysis and resourcerecycling

Membrane is impregnated withcarriers in the pores to forma liquid membrane, whichfacilitates transport19,20

Plasma treatment Gases (e.g. air, N2, O2, CO2, NH3,hydrofluorocarbon)

Various membrane processes Various gases can be used toimprove the hydrophilicity,hydrophobicity or functionalityof membranes21–25

Plasma polymerization Various monomers (e.g.fluorosilicone)

Various membrane processes On membrane surfaces ultrathinlayers can be formed by plasmapolymerization26

Interfacial polymerization Reactive bicomponent monomers Reverse osmosis, hydrophilization Two monomers (e.g. diamine anddisulfonyl chloride/trisulfonylchloride) react and crosslink onthe interface of membranes toform a thin layer27

Grafting byplasma Vinyl monomers (generally, other

modifiers such asbiomacromolecules are chosenfor further modification)

The resulted membranes withspecial properties or functions,such as hydrophilic/hydrophobic/charged/temperature-sensitive/biomacromolecule-immobilizedmembranes, can be used in manyfields, including bioreactors,biosensing, medical devices,environmental monitoring, andfiltration

Radicals produced by plasma caninitiate graft polymerization, thushydrophilic/hydrophobic/functionalized membranes willbe obtained28–38

ultraviolet radiation Photoinitiators39–55 or complexinitiators56 initiate graftpolymerization under ultraviolet.Two methods exist, i.e. a one-stepmethod and a two-step method

g-ray radiation Membrane is pre-irradiated underg-rays to generate peroxides bywhich graft polymerization isinitiated57–59

ozone Membrane is modified by ozonetreatment to generate peroxides60

chemical reaction Membrane is treated by certainchemicals such as (NH4)2S2O8 toinitiate graft polymerization61

surface initiated controlledpolymerization

By introducing an ATRP initiatoror effective chain transfer agent,surface initiated ATRP or RAFTpolymerization results ina controlled surface39

Chemical treatment Acetone/KOH Separator for alkaline batteries Membrane is saturated with acetoneand then treated in aqueousKOH solution. Aldolcondensation of acetone results ina hydrophilic polypropylenemembrane62

Surface molecularlyimprinting

Template molecule Molecular recognition Imprinted polypropylenemembranes can be prepared bycasting in the pores of or bygrafting on a macroporouspolypropylene membrane40,63,64

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Fig. 1 Schematic representation of the preparation of phospholipid analogous polymer-modified polypropylene membranes. The zwitterionic moieties

can preserve essential water and the hydrophobic alkyl chains can stabilize the open state conformation of lipase.

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immobilized on the modified polypropylene membrane by

physical adsorption. It was confirmed that all these membranes

consisting of biomimetic surface layers showed excellent

biocompatibility, which is believed to minimize the denaturation

of proteins. Furthermore, these three kinds of components

possess different characteristics: poly(a-allyl glucoside) is

hydrophilic, the polypeptides are hydrophobic, while the phos-

pholipid analogous polymers are amphiphilic. Results indicated

that the lipase on the polypropylene membrane modified with

18-PAP showed the highest activity retention, which can be

attributed to both the stabilization of the ‘‘open state’’ confor-

mation of lipase by the hydrophobic alkyl groups and the

preservation of ‘‘essential water’’ for catalysis by the polar

zwitterionic moieties. These lipase-immobilized membranes

show potential for biphasic membrane bioreactors to hydrolyze

olive oil.38

Bioseparation

Bioseparation is a process for the purification/separation of

biological substances such as proteins and cells. Taking the case

of protein bioseparation, generally most protein products show

the following characteristic features: (1) they are present at very

low concentrations in their respective biological feed streams; (2)

they are thermolabile; (3) they are sensitive to operating condi-

tions such as pH; (4) they are sensitive to chemicals such as

surfactant; (5) they occur along with many impurities.

Membrane filtration can provide a separation at mild operating

conditions, thus it has been developed for bioseparation.

For some bioseparation processes, affinity between the

membrane and one or more of the solutes is expected. Function-

alized polypropylene membranes constructed by various surface

engineering methods can be used to collect or separate proteins,69

genes,14 cells,70 microorganisms,71 bio-chemicals such as choles-

terol,16 and so on.72 For example, a polypropylene hollow fiber

membrane was coated with gold metal using a chemical metal

coating method. The membrane filtration of virus was then

combined with electrophoresis for viral gene isolation. This

membrane process can be considered as a novel gene isolation

technique.14 An ‘‘intelligent membrane’’ that is temperature-

sensitive was developed for selective separation of cells.70 This type

of membrane is generally composed of two functional systems, i.e.

one is the intelligent part such as a poly(N-isopropylacrylamide)

(PNIPAM) graft layer and the other is the functional part that is

specific to the target cell (e.g. monoclonal antibody is specific to

the target cell). Based on the specific recognition of carbohydrates

1778 | Soft Matter, 2009, 5, 1775–1785

by proteins, a series of methods to fabricate glycosylated

membrane surfaces for protein separation have been developed in

our group.28,39,41,49,50 The typical strategies for surface glycosyl-

ation of polypropylene membrane are shown in Fig. 2.

Biosensing

A biosensor is a system or device that detects the presence of

biological and chemical species. It is composed of a sensing

element and a transducer. The sensing element selectively

recognizes the species of interest through a reaction (e.g.

enzyme), specific adsorption, or other physical or chemical

processes (such as antibody-antigen and carbohydrate-protein

interactions). The transducer converts the results of this recog-

nition into an electrical or optical signal to be measured. There

are three so-called ‘‘generations’’ of biosensors, and the main

difference is the way of causing a response.73 However, the

requirements for immobilization of biologically derived recog-

nition entities (e.g. enzyme) are almost the same and even stricter

for the third-generation biosensor that requires direct electrical

contact of the enzyme to the electrode. Most enzymes strongly

adsorb on the metal electrode surface leading to denaturation,

thus some matrices have been explored as an integral part of the

electrode transducer for enzyme immobilization. These matrices

include gold nanoparticles, carbon nanotubes, conducting

polymers, and biocompatible polymers. Porous membranes can

also be used because of their high porosity, large surface area-to-

volume ratio, and good mechanical properties.

The aforementioned surface engineering strategies for enzyme

immobilization in bioreactors are also universally valid in

biosensor preparation. For example, Hsiue et al.37 prepared

amperometric creatinine biosensors from a covered platinum/

silver electrode with a thin layer of sarcosine oxidase/creatinase/

creatininase co-immobilized multienzyme membrane. To chem-

ically immobilize the enzymes they used a polypropylene

membrane with carboxyl groups, which were introduced by

plasma-induced graft polymerization of acrylic acid. With the

development of low temperature plasma technology, this method

becomes convenient and effective.

Biosynthesis

Surface engineered macroporous polypropylene membranes can

be used for biosynthesis. b-galactosidase and amylosucrase-

immobilized polypropylene hollow fiber membranes were used

to produce glucose74 and 1,4-a-glucan,44 respectively. Membrane

This journal is ª The Royal Society of Chemistry 2009

Fig. 2 Schematic representation for the surface glycosylation of macroporous polypropylene membrane.

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with amino groups introduced by plasma treatment was applied

to the in situ synthesis of oligonucleotides,24 which is a latent

matrix for the preparation of DNA microarrays.

3. Environment and energy: changing waste intoresources

Environmental deterioration and resource depletion have

become a worldwide problem. It is not only a problem in

industrialized countries. With the acceleration of industrializa-

tion in developing countries, the problem will probably get

worse, which includes contamination of groundwater with heavy

metals or organic contaminants such as chlorophenols and their

derivatives, shortage of freshwater, and depletion of resources

This journal is ª The Royal Society of Chemistry 2009

and energy. The best solution may be to develop techniques

to change waste materials into useful items.

Removal and recovery of organic contaminants and heavy metals

Membrane techniques have been used in the monitoring,

removal and recovery of pollutants such as organic contaminants

and heavy metals. For example, membrane contactors, MBRs,

and supported liquid membranes are very useful. Macroporous

polypropylene membrane is a candidate for these membrane

processes because of its chemical stability, low cost, good

mechanical properties, and being readily available. However,

different processes require membranes with different surface

properties and hence surface engineered polypropylene

Soft Matter, 2009, 5, 1775–1785 | 1779

Fig. 3 Illustration of (a) a membrane contactor and (b) a membrane bioreactor. For the membrane contactor, a hydrophobic membrane can be directly

used because organic solvent that wets the membrane is used to remove the contaminants in aqueous solution. The membrane bioreactor generally deals

with aqueous solution and hence a hydrophilized membrane is often adopted.

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membranes are often required. On the one hand, hydrophobic

polypropylene membranes with excellent chemical stability can

be directly used in membrane contactors, in which the organic

phase wets the porous membrane and slightly excessive pressure

should be applied to the other phase to stabilize the interface

and to avoid the mixing of fluids. The organic contaminants and

metals that often exist in the aqueous phase are transported from

the feed to the organic phase through the membrane (Fig. 3a).

Although polypropylene membrane can be used directly, in some

cases a hydrophobic thin layer of fluorosilicone was deposited on

the membrane surface to further enhance the hydrophobicity.75,76

On the other hand, in some other processes hydrophilization of

the membrane is necessary (Fig. 3b). For example, biodegrada-

tion of organic contaminants is an environmentally-friendly and

cost-effective method. Pseudomonas putida is capable of using

phenol as the sole source of carbon and energy for cell growth

and metabolism. MBR based on Pseudomonas putida cells can

therefore be used to remove phenols in water. In such a system,

the simplest way is to pre-wet the polypropylene membrane with

ethanol but the effects cannot stand the test of time.77 Modifi-

cation by plasma treatment or grafting of hydrophilic monomers

was widely adopted.22,53,78,79

Except for the membranes grafted or coated with polymers

such as anionic/cationic exchangers, amphiphilic poly-

hydroxylated polyparaphenylene, and conjugated polymer,17,80,81

those functionalized by forming supported liquid membranes,

especially facilitated transport membranes, were also applied

in contaminants removal.19,82,83 Compared with bulk liquid

membranes, supported liquid membranes can provide large

surface areas and well-defined diffusion pathways, which ensure

fast and reproducible transport rates. Crown ether is the most

widely used carrier. It should be noted that, from the economic

and environmentally-friendly point of view, miniature, robust,

and automated devices are preferred.

Water purification

Recently, Shannon et al. highlighted the science and technology

for water purification in the coming decades.3 They pointed out

1780 | Soft Matter, 2009, 5, 1775–1785

that addressing the problem with water scarcity requires robust

new methods of purifying water at low cost and with less energy,

while at the same time minimizing the use of chemicals and the

impact on the environment. Efforts should be made to increase

water supplies through the safe re-use of wastewater and efficient

desalination of sea and brackish water. They insisted that

membrane techniques especially MBR and reverse osmosis (RO)

play an important role in water purification.

As mentioned above, some miniature devices such as a micro-

contactor are preferred for the monitoring, removal or recovery

of trace contaminants. However, municipal and industrial

wastewater contains a wide variety of contaminants and patho-

gens, and has a very high loading of organic matters, all of which

must be removed or transformed to harmless compounds.3 In

principle, MBR belongs to a microfiltration or ultrafiltration

membrane process. Therefore, in such cases, like other

membrane separation processes, membrane fouling is the most

serious problem affecting the system performance. The fouling

leads to a significant increase in hydraulic resistance, manifested

as permeate flux decline or transmembrane pressure increase.

Frequent membrane cleaning and replacing are therefore

required, increasing the operating costs significantly. Generally,

membrane fouling is induced by the interaction between the

membrane surface and the components of activated sludge

liquid. These components include biological flocs formed by

a large range of living or dead microorganisms along with soluble

and colloidal compounds. Le-Clech et al.84 reviewed the fouling

mechanisms, the fouling parameters, and the mitigation of

fouling in MBR.

Surface modification has been adopted to reduce the fouling of

membranes in MBR. We developed a series of convenient

methods for the surface engineering of polypropylene

membranes.22,53,78,79 Plasma treatment or photo-induced graft

polymerization of hydrophilic monomers can increase the

hydrophilicity of polypropylene membranes and thereby

decrease the fouling induced by hydrophobic interaction. Most

importantly, these approaches, especially plasma treatment, can

readily be scaled up and hence benefit the practical applications.

Surface modification by atmospheric plasma treatment has also

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been carried out in our lab, in which vacuum is not necessary. An

alternative in situ approach to membrane surface modification

was developed by using comb copolymers.85 Nevertheless, this

approach is generally based on the conventional immersion

precipitation method, for example, the preparation of PVDF and

polyacrylonitrile membranes. It is difficult to apply this method

to polypropylene membranes, which are largely produced by

melt extrusion and stretching or the thermally induced phase

separation method.

RO is a membrane process frequently used for desalination.

Generally, the combination of MBR and RO followed by

ultraviolet (UV) disinfection can be used to produce drinking

water. The widely used membrane materials are cellulose

acetate and aromatic polyamide. But thin film composite (TFC)

membranes based on macroporous polypropylene membranes

are also suitable candidates. TFC membranes can be prepared by

lamination, dip-coating, plasma polymerization, and interfacial

polymerization.27,86 For example, plasma treatment with

hydrophilic materials was first conducted on polypropylene

membranes and then interfacial polymerization was performed

to prepare TFC membranes. The first step was aimed at better

soaking with aqueous amine solution during the interfacial

polymerization. Besides, a kind of anchored gel membrane

prepared by a pore-filling technique has been commercialized for

water treatment, which will be discussed in the following section.

Energy technological applications

In liquid electrolyte Li-ion batteries, the essential function of

a separator is to prevent the physical contact of positive and

negative electrodes while permitting free ion flow. Considering

the requirements of the separator for liquid electrolyte Li-ion

batteries, such as chemical stability, thickness, porosity,

pore size, permeability, mechanical strength, wettability,

dimensional stability, thermal shrinkage, shutdown, and cost,6

polypropylene, polyethylene, and their blends or composite

macroporous membranes have been commercially available (e.g.

hydrophilic Celgard 3501). One of the problems is still the

inherent hydrophobicity of polypropylene membranes, because

the membrane being used as a separator should be wetted easily

by the electrolyte and should retain the electrolyte permanently.

Surface modification by coating and graft polymerization may

be an approach for improving the surface wettability of the

polypropylene membrane. However, the effects on the pore wall

are found to be insignificant in some cases. Ciszewski and Ryd-

zynska reported a novel chemical method which consists of two

steps, i.e. the polypropylene membrane was saturated with

acetone and then dipped in aqueous KOH solution.62 It was

found that the products of the aldol condensation accumulated

and adsorbed onto the walls of pores and thus made the

membrane hydrophilic. This modified polypropylene membrane

has a relatively low electrolytic resistance in aqueous concen-

trated KOH electrolyte.

Surface engineered polypropylene membranes can also be

applied to other energy technologies, such as fuel cells. Nowa-

days, perfluorocarbon type polymer electrolyte membranes are

commercially available, which include Nafion� (DuPont de

Nemours), Aciplex� (Asahi Chemical), Flemion� (Asahi

Glass), and Dow XUS� (Dow chemical). Although

This journal is ª The Royal Society of Chemistry 2009

perfluorocarbon polymer membranes show high proton

conductivity and good chemical stability, they have some

weaknesses including high cost, large membrane thickness, and

reduction of efficiencies due to methanol cross-over for direct

methanol fuel cell application.32 Surface engineering of other

polymeric membranes may be an option. For example, poly-

imide, poly(tetrafluoroethylene) and polyethylene membranes

with non-porous structures were prepared by pore-filling, which

was first proposed by Yamaguchi et al.87–89 Macroporous

polypropylene membrane was grafted with polystyrene by

plasma-induced polymerization to prepare a pore-filled

membrane because the grafting took place on the surface as well

as inside the pores of the membrane. The polystyrene was then

sulfonated to prepare a composite electrolyte membrane. Results

indicated that the polypropylene-based composite membrane

showed lower methanol permeability but equal proton conduc-

tivity when compared with Nafion� 117.32 By pre-treatment

with Freon-116 plasma gas, methanol impermeable thin poly-

propylene electrolyte membrane was also prepared by impreg-

nating commercial Nafion� solution.90

4. Macroporous polypropylene membranes towardsmedical applications

Polymer membrane has been widely used in medical fields, such

as drug delivery, artificial organs (artificial kidney, liver,

pancreas, lung, etc.), tissue regeneration, diagnostic devices and

other blood purification processes (e.g. plasma treatment and cell

separation). In some cases biodegradable materials are required,

thus polypropylene is not suitable. However, the excellent

mechanical, chemical and biological stability of polypropylene

makes its macroporous membrane preferred for some other

applications. For all medical applications, the requirement

of biocompatibility is necessary. Recently, Stamatialis et al.

reviewed the medical applications of membranes including drug

delivery, artificial organs, and tissue engineering.4 In this part we

only focus on the specific surface engineering requirements

for polypropylene membranes in medical applications, taking

oxygenator (artificial lung) and artificial liver as examples.

Membrane oxygenator

The principal function of natural lung is to transport oxygen

from the atmosphere into the bloodstream, and to release CO2

from the bloodstream into the atmosphere. This exchange of

gases is accomplished in the mosaic of specialized cells that form

millions of tiny, exceptionally thin-walled air sacs called alveoli.

The lung is a very efficient gas exchanger due to its large surface

area. The ideal artificial lung should be able to oxygenate,

remove a certain level of CO2, be gentle to blood and avoid

hemolysis and protein denaturation, and so on.4 Hollow fiber

membrane is generally used for membrane oxygenator, i.e.

membrane-based artificial lung in which there is no direct contact

between the blood and the oxygen minimizing the risk of air

embolism (Fig. 4). Hydrophobic porous polypropylene

membrane allows the gas exchange and prevents the plasma

leakage to some extent. Efforts have been made towards

improving both the biocompatibility and the gas permeability.

Considering the excellent gas permeability, biocompatibility, and

Soft Matter, 2009, 5, 1775–1785 | 1781

Fig. 5 Surface engineering of the hollow fiber membranes in a bio-

artificial liver bioreactor. After separation from the blood cells, plasma is

perfused into the lumen-side of hollow fiber membranes. Medium flows

through the shell-side at which hepatocytes are inoculated. Exchange of

molecules takes place across the membrane. The membrane functional-

ized with carbohydrates such as galactose can both enhance hepatocyte

function and promote mass transfer (not in scale).

Fig. 4 Schematic illustration of the principle of a membrane oxygenator

(artificial lung). Without direct contact between the blood and the

oxygen, oxygenation takes place across a hydrophobic membrane

preventing the leakage of blood. Surface engineered membranes may

improve the gas permeability and reduce the adsorption of plasma

proteins and cells. In this illustration the shell-side fluid is blood and the

lumen-side is gas.

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low plasma leakage, silicone is often coated onto polypropylene

membrane surfaces in commercial hollow fiber membrane

oxygenator.91 Besides, the preparation of silanized membrane is

a facile process. On the other hand, reducing complement

activation,92 protein adsorption, and platelet adhesion15,93,94 on

the membrane are important to the clinic usage. Coating of

silicone, heparin, or siloxane/caprolactone oligomer is found to

be helpful.

In fact, so far membrane oxygenator is mostly used in acute

respiratory cases, where it acts as a short-term extracorporeal

system. The long-term objective is to develop a system that can

be implanted into the body to replace injured biological lung. For

such systems, excellent biocompatibility is specifically required.

Besides, it should be admitted that natural organs have a wide

variety of sophisticated functions. For example, in addition to

the respiratory functions, natural lung also has non-respiratory

functions including converting angiotensin I to angiotensin II by

an angiotensin-converting enzyme, influencing the concentration

of biologically active substances and drugs used in medicine in

arterial blood, etc. Therefore, membranes with some biological

functions should receive adequate attention.

Artificial liver

Membrane-based artificial liver system with hepatocytes, namely

a bioartificial liver system, is a successful example of a membrane

system with biological functions. A bioartificial liver device is

essentially a bioreactor with inoculated hepatocytes (liver cells)

that perform the functions of a normal liver. In the bioreactor,

hepatocytes are located at the shell-side and the plasma separated

from blood cells is perfused through the lumen of hollow fibers

(Fig. 5). This allows the free exchange of molecules between the

hepatocytes and the plasma. It is to be noted that hepatocytes

are anchorage-dependent cells surrounded by extracellular

matrix (ECM). Usually the communication between cells as well

as between cells and ECM provides essential signals for their

1782 | Soft Matter, 2009, 5, 1775–1785

survival, growth or function. When enzymatically isolated from

the liver and cultured in vitro, they rapidly lose adult liver

morphology and differentiation functions.61 Therefore, Strain

and Neuberger proposed that the greatest challenge for the

bioartificial liver bioreactor is how best to maintain viable

functional hepatocytes outside of the body.95

Because of the relatively inert surface, polypropylene

membrane should be modified for hepatocyte culture. Collagen-

coated surface can provide a mimic of the ECM environment in

which hepatocytes show the highest reaction rate of ammonia

elimination.96 Besides coating, chemical immobilization of

collagen can also be achieved.61 For example, hydroperoxides

generated by ammonium peroxydisulfate treatment can initiate

free radical polymerization of acrylic acid in the presence of

ferrous(II) ions (Fe2+), and then collagen can be covalently

immobilized on the polypropylene membrane surface using

water-soluble carbodi-imide as the coupling agent. Aggregating

form and normal liver-specific functions were confirmed for the

hepatocytes cultured on the collagen-immobilized membrane.

Galactose residues are also one of the most reliable candidates

as immobilized ligands to interact with hepatocytes. A series of

glycosylated polypropylene membranes were prepared in our lab,

which may be applied in artificial liver systems.28,39,41,49,50 On the

one hand, galactose-carrying surfaces bind hepatocytes through

a receptor-mediated mechanism, resulting in enhanced hepato-

cyte functions. On the other hand, unlike membrane oxygenator

that requires a hydrophobic membrane to prevent plasma

leakage, bioartificial liver bioreactors deal with plasma and the

exchange of molecules occurs across the membrane, thus the

hydrophilic galactose-carrying membrane is preferred (Fig. 5).

5. Separation technology: inheriting and innovatingthrough surface engineering

Up to now, many new separation processes based on membranes

have been developed. However, the developments of absolutely

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new polymers towards membrane preparation are still limited, as

this is time consuming and very expensive. Generally, macro-

porous polypropylene membrane is only suitable for a certain

separation processes. For example, it cannot be directly used in

separation processes that require a dense separation layer, such

as pervaporation, or that requires a membrane with small size

pores such as nanofiltration. Membranes with new structures

and/or new surface properties may inherit such traditional

separation processes. Furthermore, surface engineered

membranes with special surface properties can be applied to and

even give rise to some new separation processes.

Separation membrane with filled selective micropores

Surface engineering can lead to changes in the structures of the

membrane pores as well as the outer surface of membranes by

introducing a dense surface layer or pore-filling. It is to be noted

that pore-filling may result in changes of membrane bulk prop-

erties. But in this review it is still included because (1) it also

inevitably changes the surface of the membrane, (2) in some cases

it requires pre-treatment of the membrane surface/pore surface,

and (3) it is often experimentally performed using surface

modification techniques such as plasma-induced graft polymer-

ization. As mentioned above, TFC polypropylene membrane

can be used in RO. In addition, polypropylene membranes with

selective layers such as hydrophilic poly(acrylic acid) can be

applied to pervaporation. This kind of selective layer can be

introduced by a number of methods including photo-induced

graft polymerization, plasma polymerization and interfacial

polymerization.27,86,97 Pore-filling is an interesting method for the

preparation of composite membranes (Fig. 6). Childs et al.

Fig. 6 Illustration of the preparation of a pore-filled membrane. (a)

Nascent macroporous polypropylene membrane; (b) direct pore-filling

within nascent polypropylene membrane results in a composite

membrane which may lead to defects at the pore interface; (c) pore-filling

within a pre-treated polypropylene membrane may avoid the defects.

This journal is ª The Royal Society of Chemistry 2009

prepared a series of pore-filled membranes by photo-induced

graft polymerization using a photoinitiator as initiator.98,99 The

polymerization took place mainly within the pores. Poly(4-

vinylpyridine)-filled membranes can act as a chemical valve,

which means a pH-dependence of the membrane permeability.

By using a photoinitiator with a strong ability to abstract

hydrogen (e.g. benzophenone), grafted polymer chains are

mostly immobilized at the pore walls. Other initiators such as

azobis(isobutyronitrile) (AIBN) can also be used for pore-filling.

We found that, when using AIBN as initiator, UV-induced pore-

filling was more effective than the thermally induced one.

Although the pore-filling initiated by AIBN is very effective, it is

important to avoid the defects between the matrix and the filled

polymer because the filled polymer chains in this case are mostly

separated from the matrix. Plasma pre-treatment of the

membrane or pre-graft polymerization may be a latent solution

(Fig. 6c).

Intelligent separation membrane

In fact, a membrane that has chemical valve property is a kind of

so-called intelligent separation membrane, which can be induced

by temperature, electric field, magnetic field, light, pressure

and pH. Among them, temperature-sensitive membranes have

received considerable attention. Liang et al. prepared PNIPAM-

grafted polypropylene membranes by either UV-induced51 or

plasma polymerization.33 The flux and rejection of this type of

membrane were temperature-dependant, i.e. they changed

sharply around the lower critical solution temperature of

PNIPAM. These changes are because the effective pore size of

the membrane can be shrunk or enlarged as the PNIPAM chains

swell or deswell with temperature. Multifunctional separation

membranes can be prepared by combining the temperature-

sensitive property of PNIPAM with other functional matters.

For example, the above-mentioned intelligent membrane that

consists of both PNIPAM and monoclonal antibody is useful in

cell separation.70 The combination of PNIPAM with b-cyclo-

dextrin also makes a so-called binary membrane, which is useful

in chiral resolution.100

Surface molecularly imprinted separation membrane

If the intensity or specificity of molecular recognition is not very

strong (e.g. recognition between b-cyclodextrin and oligopep-

tide), molecular imprinting techniques are a way to enhance the

recognition. A molecularly imprinted polymer provides specific

recognition sites complementary in shape, size and functional

groups to the target molecule. For bulk molecularly imprinted

materials (e.g. membrane and particles), the recognition sites are

often synthesized during polymerization or phase inversion. This

strategy is appropriate to the preparation of membranes from

polyacrylonitrile etc. Molecularly imprinted polypropylene

membranes are often prepared by surface graft polymerization.

Ulbricht et al. prepared surface molecularly imprinted poly-

propylene membranes through UV-induced graft polymerization

in the presence of a photoinitiator, a crosslinker, a functional

monomer that is capable of interacting with the target molecule

(template), and a template.40,101 They compared the surface

molecular imprinted membrane with the conventional molecular

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imprinted particles and found that both the specificity and the

selectivity were much higher for the surface molecular imprinted

membrane. They ascribed the specificity and the selectivity to the

accessibility of the molecularly imprinted layer on the membrane

surface.

Other surface engineered separation membranes

There are some other interesting separation processes achieved

by surface engineered membranes. For example, porous

polypropylene membranes can be used in the separation of

nanoparticles after simple modification.13 It is well known that

large particles can be filtered by the entrapment mechanism. As

the size of the particles decreases, however, the removal becomes

more difficult and thus the interaction between the particles

and the collectors must be increased to improve filtration effi-

ciency. Kang and Shah modified macroporous polypropylene

membranes with a cationic surfactant to create a charged surface.

Negatively charged nanoparticles were then adsorbed and

filtered, which was facilitated by electrostatic interaction.

Although the modification is a simple physical adsorption

process, they found that the filtration efficiency increased from

10% to 95%.

6. Conclusion and outlook

Surface engineered polymer membranes have demonstrated their

importance in various applications. This Review provides

a summary of surface engineering methods for polymer

membranes, taking macroporous polypropylene membrane as an

example. Surface engineering requirements for polypropylene

membranes used in biotechnology, environmental protection,

energy and resources, biomedical devices, and novel filtration

processes are also highlighted. It is clear that membranes with

superior performance and multiple functions are needed for most

membrane processes. To this end, biomimetic surfaces that are

fouling-resistant, biocompatible, or biofunctional have been

preliminarily constructed on polymer membranes. Furthermore,

it is expected that membranes with high performance can

be prepared by mimicking the advanced functions of a bio-

membrane, such as by introducing water channels and ion

channels. On the other hand, it is important to obtain

membranes with a precise selective surface layer. As we know,

membranes manufactured by phase inversion have significant

pore size variation, which means that a membrane rated 0.2 mm

may have some pores up to 1.0 mm. Besides the product purity,

sometimes it is dangerous because some microbe cells also

display some variation in size and show strong cell deform-

ability.71 Controlled surface engineering methods or other

techniques fabricating ordered surface structures may be prom-

ising top–bottom approaches.

Acknowledgements

Financial support from the National Natural Science Founda-

tion of China for Distinguished Young Scholars (50625309), the

Zhejiang Provincial Natural Science Foundation of China

(Z406260) and the National Basic Research Program of China

(2003CB15705, 2009CB623400) is gratefully acknowledged.

1784 | Soft Matter, 2009, 5, 1775–1785

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