european polymer journal - msu · olga philippovaa,⇑, anna barabanovab, vyacheslav molchanova,...

European Polymer Journal 47 (2011) 542–559

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Feature Article

Magnetic polymer beads: Recent trends and developments in syntheticdesign and applications

Olga Philippova a,⇑, Anna Barabanova b, Vyacheslav Molchanov a, Alexei Khokhlov a

a Physics Department, Moscow State University, 119991 Moscow, Russiab Institute of Organoelement Compounds, 119991 Moscow, Russia

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 August 2010Received in revised form 7 November 2010Accepted 10 November 2010Available online 8 January 2011

The paper is dedicated to Professor NikosHadjichristidis on occasion of his birthday.

Keywords:Magnetic polymer beadsSwellingOil industryGelation

0014-3057 � 2010 Elsevier Ltd.doi:10.1016/j.eurpolymj.2010.11.006

⇑ Corresponding author. Tel.: +7 495 939 14 64; fE-mail address: [email protected] (O. Phili

Open access under CC BY

The paper describes the synthesis, properties and applications of magnetic polymer beads.State-of-the-art, future challenges, and promising trends in this field are analyzed. Newapplications in oil recovery are described.

� 2010 Elsevier Ltd. Open access under CC BY-NC-ND license.

1. Introduction

Smart polymer beads are designed to respond to theexternal stimuli [1,2], e.g. temperature, pH, solvent compo-sition, magnetic field etc. Among these stimuli, magneticfield has the additional advantages of instant action andcontactless control. The sensitivity to magnetic field is pro-vided by magnetic nano- or microparticles incorporatedinto the beads.

Synthesis and properties of magnetic polymer beadsconstitute a new topic of research rapidly developing last10 years. The magnetoresponsive polymeric beads benefitfrom the combination of features inherent to both theircomponents: magnetic particles and polymer. The particlesimpart the magnetic properties to the beads. These proper-ties allow for the rapid and easy separation of beads by theapplication of an external magnetic field. Also, due to theseproperties the magnetic particles can be exploited as heat

ax: +7 495 939 29 88ppova).

-NC-ND license.

generators, when they transform magnetic energy to heatdue to relaxation processes and hysteresis losses [3]. Thisphenomenon is of primary importance for cancer therapyby hyperthermia [4–9]. As to the polymer component, itstabilizes the magnetic particles and offers swelling abilityand elasticity to the beads. Also, polymer endows the par-ticles with functional groups necessary for the desiredapplications. For example, functionalization of the surfaceof beads by specific ligands makes possible their usage inimmunoassay methodologies [10], in the isolation of nu-cleic acid sequences [11], cells [12] and microorganisms[13] etc.

This review paper is focused on the description of thesynthesis, the properties and the applications of magneticpolymer beads with the special emphasis on the newtrends and developments.

2. Magnetic particles

As was mentioned above, the magnetic properties ofpolymer beads are determined by embedded magnetic

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Table 1Saturation magnetization and magnetization susceptibility of some mag-netic metal oxides [14].

Oxides Magnetizationsaturation (emu/g)

Magnetizationsusceptibility

c-Fe2O3 74 �5 � 10�6

Fe3O4 84 +18 � 10�6

Fe2O3–Fe3O4 �80 +7 � 10�6

CoO�Fe2O3 65 �110 � 10�6

Table 2Estimated single-domain sizes for spherical parti-cles with no shape anisotropy [17].

Material Dc (nm)

Co 70Fe 14Ni 55Fe3O4 128c-Fe2O3 166

O. Philippova et al. / European Polymer Journal 47 (2011) 542–559 543

particles. Among these particles, metal oxides are oftenpreferred over pure metals (Fe, Co, and Ni) [3] because theyare more stable to oxidation. The magnetic properties ofmetal oxides depend on their chemical composition, theirsize and shape, their crystal structure, the degree of crys-tallinity etc. Typical magnetization curve is schematicallyrepresented in Fig. 1. It reflects the ability of magneticmaterial to respond to an external magnetic field H. Asindicated on Fig. 1, the curve provides major characteristicparameters of magnetic materials: the saturation magneti-zation MS, the coercivity, and the remanence. The slope ofthe magnetization curve at H ? 0 gives the magnitude ofthe magnetic susceptibility. The values of the saturationmagnetization and of the magnetic susceptibility of somemetal oxides are presented in Table 1. It is seen that thesaturation magnetization of iron oxides is rather high,which makes them suitable for producing various mag-netoresponsive polymeric materials.

It should be emphasized that the size of iron oxide par-ticles plays an essential role in their behavior in magneticfield [15]. Small particles possess single domain structure.Single domains include groups of spins all pointing in thesame direction and acting cooperatively. By contrast, largerparticles possess multidomain structure consisting ofmany single domains separated by domain walls, whichproduce magnetic flux closures rendering the bulk mate-rial macroscopically non-magnetic [16]. The formation ofdomain walls becomes energetically favorable at somecritical particle diameter, Dc. Table 2 shows the estimatesof the single-domain diameter for some common materialsin the shape of spherical particles. Note that particles withsignificant shape anisotropy can remain single domain tomuch larger dimensions than their spherical counterparts.

The size of particles affects essentially the coercivity[18,19] as illustrated in Fig. 2. Let us first consider the sin-gle domain particles. It is seen that the coercivity is equalto zero, only when the single domain particles are quite

Fig. 1. Typical magnetization curve for ferromagnetic particles.

small. These particles are superparamagnetic. By contrast,larger single domain particles possess the coercivity, whichresults from the coherent rotation of spins required for thechanges in the magnetization [19]. As to multidomain par-ticles, they always demonstrate coercivity (Fig. 2). In thiscase, the magnetization reversal occurs through the nucle-ation and motion of the domain walls [19].

So, nanoparticles of iron oxides (typically in the sizerange of 5–15 nm) are superparamagnetic. At the sametime, microparticles (with the size in micrometer range)are ferromagnetic. Such large particles contain hundredsof single domains. Liquids with dispersed superparamag-netic single domain nanoparticles are called magnetic flu-ids, or ferrofluids [20–23]. The suspensions offerromagnetic micron-sized particles are called magneto-rheological fluids.

Depending on the magnetic properties and the size ofthe particles, the applications of the particles encapsulatedin polymer will change as well. For some applications

Fig. 2. Qualitative illustration of the behavior of the coercivity in ultrafineparticle systems as the particle size changes (Reprinted with permissionfrom Ref. [19]. � 1996, American Chemical Society).

Fig. 4. Preparation of uniaxially ordered polymer composite underuniform magnetic field [27].

544 O. Philippova et al. / European Polymer Journal 47 (2011) 542–559

including the separation technique and the drug deliveryparticularly interesting are small magnetic monodomainnanoparticles, because they do not possess remanence,when magnetic field is removed [19]. The additionaladvantages of using small nanoparticles, besides the possi-bility to easily switch off the magnetic state and particleinteractions by removing the external magnetic induction,are the minimum disturbance such a particle has on reac-tions of the molecules attached to its surface and the largesurface-to-volume ratio, which is of high interest for chem-ical binding. A disadvantage of such particles may be thatthe magnetic force is small due to the small volume, so thatviscous forces dominate and magnetic separations can takea long time (tens of minutes). This explains the interest ofusing larger magnetic particles (typically 0.2–5 lm indiameter) for some applications. Such microparticles dem-onstrate strong response to an external magnetic field.Also, they possess remanent magnetization and coercivity,which are important for their application as magneticmemory devices [24] and as blocking medium in oil recov-ery [25,26].

3. Synthesis of magnetic polymer beads

Magnetic polymer beads are composed of magneticnano- or microparticles embedded in a polymer matrix.The size of beads can vary from hundred nanometers tofew millimeters. The synthesis of magnetic polymer beadscan be performed by three general ways. In the first onethe magnetic particles are synthesized inside polymer ma-trix. In the second one polymer is synthesized in the pres-ence of magnetic particles. In the third one the beads areprepared from pre-formed polymer and magnetic particles.

The beads can have different structures (Fig. 3). In onekind of beads the magnetic particles are homogeneouslydistributed in the volume of polymer matrix (Fig. 3c).Other kinds of beads are characterized by core–shell struc-ture (polymer core–magnetic shell (Fig. 3a) or magneticcore–polymer shell (Fig. 3b)). Also, mixed systems are pre-pared, where the core–shell particles are homogeneouslydispersed in polymer matrix. To get ordering of the mag-netic filler the beads can be obtained under the action ofmagnetic field [27,28] (Figs. 4 and 5).

Below some of the methods of preparation of magneticpolymer beads are outlined.

Fig. 3. Schematic representation of different structures of magnetic polymer beamagnetic particles homogeneously distributed in the polymer bead.

3.1. In-situ formation of magnetic particles in polymer matrix

Nanoparticles of metal oxides, in particular, magnetiteFe3O4 and maghemite c-Fe2O3 are often synthesized bythe alkaline coprecipitation of ferric and ferrous salts, e.g.[29]:

FeCl2 � 4H2Oþ 2FeCl3 � 6H2Oþ 8NH4OH

¼ Fe3O4 þ 8NH4Clþ 20H2O

Magnetic polymer beads can be prepared by coprecipi-tation of iron salts directly in polymer matrix. In this case,polymeric matrix limits the growth of magnetic particles,as a result their size becomes smaller than in the absenceof polymer [30]. By in-situ coprecipitation of the iron saltsthe magnetic beads based on many polymers were pre-pared, including polystyrene (PS) [31,32], polyacrylamide[33], triblock polymer polyisopropene-block-poly(2-cinna-moylethyl methacrylate)-block-poly(tert-butyl acrylate)[34], dextran [30,35–39], alginate [40–44], poly(vinyl alco-hol) (PVA) [30,45], copolymers of acetoacetoxyethyl meth-acrylate and N-vinylcaprolactam [46], copolymers of N-isopropyl acrylamide (NIPA) and glycidyl methacrylate[47] etc.

The magnetic beads thus prepared can have all possiblestructures depicted in Fig. 3. When porous polymer beads(e.g. PS beads) are used as matrices for the formation of mag-netic particles, the structures of the type c (Fig. 3c) are pro-duced. When the iron salts do not penetrate inside thebeads and coprecipitate on their surface the polymer

ds: (a) polymer core–magnetic shell, (b) magnetic core–polymer shell, (c)

Fig. 5. Arrangement of carbonyl iron particles under the action of magnetic field seen by light microscope: suspension of particles in the absence of externalfield (a) and in the presence of 1 kOe uniform magnetic field (b). The direction of forming chains is parallel to the field direction, as shown by the arrow(Reprinted with permission from Ref. [28]. � 2005, Elsevier).


core–magnetic shell structure is formed (Fig. 3a). Whenmagnetic particles are synthesized in polymer solution, theybecome coated with polymer just after synthesis, whichgives magnetic core–polymer shell type structures (Fig. 3b).

To fix the magnetic particles in/on the polymer matrixthe functional groups interacting with particles can beintroduced in polymer chains [48–52]. For example, Kuma-gai et al. [52] report the formation of magnetic core–poly-mer shell type beads with tightly bound polymer chains byhydrolysis of FeCl3�6H2O in water and the subsequenttreatment with poly(ethylene glycol)–poly(aspartic acid)(PEG–PAsp) block copolymer. The polymer-coated nano-particles revealed excellent solubility and stability in aque-ous solution as well as in physiological saline. The FTIRexperimental results evidenced that PEG–PAsp moleculesare multivalently bound on the surface of the iron oxidenanoparticles via the coordination between the carboxylicacid groups in the PAsp segment of the block copolymerand Fe on the surface of the iron oxide nanoparticles.

Recent advances in the synthesis of magnetic nanopar-ticles in the presence of polymers are based on the use ofpolymer gels [53–55]. The advantages of using polymergels are multiple, but the most important one is that thenucleation and growth of iron oxide can be controlled bythe constrained architectures of the polymer network[54,55], i.e. the gel serves as a nanoreactor where ironoxide nanoparticles are formed in-situ. For example, Breul-mann et al. [54] investigated the preparation of magnetiteinside the pores of an elastic PS-polyacrylate copolymer geltemplate. The synthetic parameters of the polymerizationallow for the pore size and carboxylate functionality tobe tailored. The authors report that the iron oxide contentof the gels is 3.5–8% Fe3O4 with one reaction cycle and thatthe loading can increase up to 20% iron with successiveswelling/reaction cycles. The particles are 16 nm in diame-ter and are bound to carboxylate functional groups of thepolyacrylate component of the gel pore.

3.2. In-situ polymerization in the presence of magneticparticles

Another method to prepare magnetic polymer beadsconsists in the incorporation of the magnetic particles in-side polymer matrix in the process of polymerization.

3.2.1. Polymer core–magnetic shell beadsPolymer core–magnetic shell beads can be prepared by

polymerizing monomer droplets stabilized by magneticparticles in the Pickering emulsions [56]. The solid parti-cles first self-assemble at the liquid–liquid interface andact as effective stabilizers during polymerization processwithout the need for any conventional stabilizers. Afterthe polymerization, the particles armour the surface ofthe resultant polymer beads. Such solid-stabilized hetero-geneous polymerizations are very attractive in preparationof hybrid beads. Polymerizations based on Pickering emul-sion include Pickering miniemulsion polymerization, Pick-ering suspension polymerization, Pickering dispersionpolymerization and Pickering emulsion interface-initiatedatom transfer radical polymerization (ATRP). Recently, inaddition to ordinary polymer core–magnetic shell beadsmore sophisticated multihollow polymer beads with mag-netic shell were prepared by polymerizing droplets of mul-tiple W/O/W Pickering emulsion [56].

However, in such polymer core–magnetic shell struc-tures the exposure of magnetic particles directly to the sur-rounding medium limits the applications of the beads [24].These beads are developed, for example, for magnetorhe-ological fluid materials [57].

3.2.2. Magnetic core–polymer shell beadsMagnetic core–polymer shell beads are usually pre-

pared by three-dimensional polymerization of monomertogether with cross-linker on a surface of magnetic parti-cles [58–60] or by encapsulation of magnetic particles intoblock-copolymer micelles [61]. Representative TEM micro-graphs of such beads are shown in Fig. 6.

A new pathway for the preparation of magnetic core–polymer shell beads is a surface-initiated polymerization[62–67]. The technique is based on the growth of polymermolecules in-situ from surface-grafted initiators and re-sults in a covalently bound polymeric shell on the mag-netic core. By this method, various magnetic polymerbrush particles were obtained including those coated bypoly(e-caprolactone) by ring-opening polymerization [66]and coated by poly(2-methoxyethyl methacrylate) [67]and PS [68] by ATRP. A direct correlation between the mo-lar mass of the polymer arms as detected by GPC (afteracidolysis of the core) and the hydrodynamic radius of

Fig. 6. TEM images of some magnetic core - polymer shell beads: (a) Fe3O4/SiO2 core–PNIPAM shell beads (Reprinted with permission from Ref. [60]. �2005, American Chemical Society), (b) c-Fe2O3 nanoparticles in the shell of cross-linked amphiphilic block-copolymer poly(styrene-block-acrylic acid)PS250-b-PAA13 micelles (Reprinted with permission from Ref. [61]. � 2010, American Chemical Society).


the core–shell particles in DLS experiments was found[68,69]. The results obtained indicate that at rather highgrafting density the anchored polymeric chains are com-pletely stretched.

3.2.3. Magnetic beads with homogeneously dispersedmagnetic particles

Several approaches were used to produce magneticpolymer beads with homogeneously dispersed magneticparticles by in-situ polymerization. They include emulsionpolymerization, suspension polymerization, dispersionpolymerization, microemulsion polymerization, miniemul-sion polymerization [70–100].

One of the most widespread methods to produce mag-netic beads is emulsion polymerization. The monomer ismixed with the beads and then emulsified as a dispersephase. During polymerization the droplets of the dispersephase transform into beads. Both hydrophilic and hydro-phobic vinyl monomers can be used in this process viaW/O and O/W emulsions, respectively. Successful encapsu-lation of ion oxide particles in polymer beads requiresthese particles to be well dispersed in the monomer beforeemulsification to accommodate them within the droplets.It creates no problems in the case of hydrophilic mono-mers, which are miscible with hydrophilic iron oxides. Bycontrast, in the case of hydrophobic monomers, it is diffi-cult to disperse hydrophilic iron oxide particles in mono-mer without aggregation and phase separation. Toimprove dispersion, a surface treatment of the iron oxide,e.g. by oleic acid or sodium dodecyl sulfate, is performedin order to increase the hydrophobicity [95].

The synthesis of hydrophilic magnetic polymer beadsvia W/O emulsion is described in several papers[75,76,91]. Müller-Schulte et al. prepared hydrophilic mag-netic polymer beads from NIPA, acrylamide and N,N0-methylenebisacrylamide (BAA) [75,76]. The particle sizewas varied in the range of 10–200 lm by adjusting the stir-ring speed. Deng et al. [91] obtained hydrophilic submi-cron magnetic polymeric particles (75–285 nm indiameter) of acrylamide and BAA using W/O microemul-sion polymerization process.

The procedures of preparation of hydrophobic magneticpolymer beads via O/W emulsions were developed in pa-pers [77–80]. Several different hydrophobic magneticbeads were synthesized including poly(methyl methacry-

late-divinylbenzene), poly(styrene-divinylbenzene-glyc-idyl methacrylate), poly(styrene-methacrylic acid-acrylamide), poly(styrene-butyl acrylate-methacrylic acid)and poly(styrene-acetoacetoxyethylmethacrylate) (PS-AAEM) beads. In particular, Pich et al. [77] prepared PS-AAEM beads containing c-Fe2O3 nanoparticles by surfac-tant-free O/W emulsion polymerization. It was found thatmodifying the iron oxide nanoparticle surface with sodiumoleate significantly improves their encapsulation duringthe polymerization process. Variation of the AAEM concen-tration in the reaction mixture at constant content of ironoxide particles gives the possibility to control the particlesize of the hybrid beads.

Okassa et al. [81] synthesized biodegradable and bio-compatible submicrometer poly(lactide-co-glycolide)beads loaded with magnetite nanoparticles by a modifieddouble emulsion method (W/O/W) or an emulsion evapo-ration process (O/W). In the later the volatile organic sol-vent is evaporated from the emulsion droplets, when thepolymerization is finished.

In order to get magnetic PS latex, the miniemulsionpolymerization method was employed [98–100]. The sizeof miniemulsion droplets and consequently the size ofthe resulting beads can be controlled by the stabilizersystem and the shear applied to form the droplets. In pa-per [99] a water-soluble surfactant (sodium lauryl sul-phate, SLS) and a monomer-soluble long-chain alcohol,the so-called costabilizer (stearyl alcohol, SA), were usedfor stabilizing the styrene/water miniemulsion againstboth coalescence (with SLS) and diffusional degradationor Ostwald ripening (with SA), respectively. The sur-face-treated magnetite nanoparticles in toluene wereintroduced into the monomer under stirring to obtainthe stable magnetite/styrene dispersion. Preparation ofthe miniemulsion was carried out by ultrasound homog-enization. An oil-soluble initiator N,N0-azobis(isobutyro-nitrile) was used to initiate the polymerization reaction[99]. The encapsulation of the magnetic iron oxide inthe polymer was found to be complete; neither freemagnetite particles nor free PS latex particles (withoutmagnetite) were observed. Although the magnetite parti-cles were distributed uniformly in the starting styrenesystem, they aggregated and accumulated at one sideof the spheres during the microemulsion preparationand the subsequent polymerization.


In-situ polymerization gives the possibility to prepareporous magnetic polymer beads. Such beads of PNIPA withan average diameter of 2 mm and narrow size distributionwere synthesized by Zrinyi et al. [27,101] by the dropmethod. The chemical procedure developed by Park andChoi [102] was employed. First, an interpenetrating net-work (IPN) was prepared by simultaneous ionotropic gela-tion of alginate by Ca ions, with a concomitant free radicalpolymerization of NIPA and cross-linker within the beads.Then, the beads were put in ethylenediaminetetraacetate(EDTA) solution in order to extract calcium ions cross-link-ing alginate chains, which results further in the remove ofalginate itself from the IPN beads. The removal of one net-work from the other led to the formation of multiple chan-nels in PNIPA beads.

By in-situ polymerization several types of nanoparticlescan be simultaneously incorporated into the beads. In re-cent paper by Hawker and co-workers [103] MnFe2O4

and Au nanoparticles grafted with short PS ligands weredispersed in divinylbenzene (DVB) monomer followed byemulsification in aqueous media with cetyltrimethylam-monium chloride as surfactant. Subsequent free radicalpolymerization with 2,20-azobis(2-amidinopropane) dihy-drochloride yielded the cross-linked PDVB latex particlesembedded with the inorganic Fe and Au particles.

In-situ polymerization provides the easy means to tailorthe chemical and interfacial properties of polymer beadsby the incorporation of monomers with desired functionalgroups. For example, the above-mentioned PDVB beads[103] were grafted with short thiol-terminated PEG chainsvia thiol-ene coupling to provide the solubility of the beadsin organic medium. Magnetic polymer beads functional-ized by specific ligands were used in immunoassay meth-odologies [10], the isolation of nucleic acid sequences[11], the cell selection from complex matrices such aswhole blood [12], the isolation of microorganisms fromsamples in the food industry [13] etc.

One of the novel directions in the in-situ polymeriza-tion technique concerns the production of molecularly im-printed beads, which exhibit recognition properties[104,105]. The technique involves polymerization of func-tional monomers and a cross-linker around a template orprint molecule. Extraction of the template leaves behindrecognition sites of functional and shape complementarityto the template. For example, Ansell and Mosbach pre-pared [104] by a suspension polymerization methacrylicacid–1,1,1-trimethylolpropane trimethacrylate copolymermagnetic beads molecularly imprinted with the b-blocker(S)-propranolol. The resulting imprinted beads were capa-ble of binding the drug [3H]-(S)-propranolol more stronglythan a non-imprinted, otherwise identical, polymer. Therecognition properties of the imprinted particles were notaffected by the inclusion of iron oxide. Such (S)-proprano-lol imprinted magnetic polymer beads can be used for thedirect assay of (S)-propranolol in such multicomponentmedia like blood and urine [104].

Recent trends in the preparation of magnetic polymerbeads concern the use of membrane and microfluidicsemulsification techniques [24,106]. Compared with thetraditional emulsification methods, these techniques givesize-controlled mono- or narrowly dispersed droplets. In

membrane emulsification techniques a disperse phase isforced to pass through the micropores from one side ofthe membrane and forms individual droplets in a continu-ous flow liquid phase at the other side of the membrane.The continuous phase contains emulsifiers which stabilizethe droplets instantly while they are forming [24]. Alongwith the pore size and size distribution, the polarity ofmembrane surface plays an important role in the controlof droplet formation. It is required that the membrane sur-face, especially, the outlet surface of disperse droplets, tobe affinitive to the continuous phase, and less affinitiveto the disperse phase. These can avoid the spreading ofthe disperse phase on the membrane surface, so that thesizing of the droplets can be controlled by the microporesizes of the membrane [24]. So, hydrophilic membranes fa-vor the formation of O/W emulsions, while hydrophobicones are beneficial to the making of W/O emulsions. Forwell-formulated emulsion systems, the diameter of drop-lets produced is typically 2–8 times of that of membranepores [106].

Another route for controlled synthesis of polymericbeads provides a modern method of microfluidics [24].The beads can be prepared in a single-phase flow and amultiphase flow [107]. Single-phase microfluidic genera-tion of polymer beads utilizes UV illumination of the con-tinuous stream of a polymerizable liquid through apatterned mask [108]. Multiphase, droplet-based microflu-idic generation of polymer beads relies on (a) the emulsifi-cation of liquid monomers and (b) the solidification ofdroplets by chemical or physical means [108].

Although by membrane and microfluidic emulsificationfollowed by polymerization various hydrophilic andhydrophobic polymer beads were prepared, the elabora-tion of the procedure of the synthesis of polymer beadswith embedded magnetic particles by these methods isstill at the very beginning [109]. In the future, this seemsto be a very prospective route for the preparation of mag-netic polymer beads with narrow size distribution.

3.3. Mixing of pre-formed polymer and magnetic particles

3.3.1. Polymer core–magnetic shell beadsThe magnetic particles can be deposited on the surface

of polymer beads either by adsorption [110] or by layer-by-layer (LbL) coating [111,112]. For example, the latermethod was used [112] to deposit negatively chargedmaghemite nanoparticles on PS beads covered with a pos-itively changed polyelectrolyte. The TEM and SEM micro-graphs of such polymer core–magnetic shell beads arepresented in Fig. 7. To protect the magnetic shell an addi-tional polyelectrolyte layer may be deposited on the sur-face of the bead. When desired, the PS core can beremoved e.g. by dissolution, which results in hollow mag-netic shells [112].

3.3.2. Magnetic core–polymer shell beadsMagnetic core–polymer shell beads are prepared by

coating of magnetic particles with polymer either byadsorption [113,114] or by LbL technique [115]. For exam-ple, Thünemann et al. [115] used LbL technique to get mag-netic beads with maghemite core and two layer polymer

Fig. 7. TEM (a) and SEM (b) micrographs of magnetic nanoparticles-coated polystyrene beads. The SEM sample was sputter-coated with gold for bettercontrast. The thickness of the magnetic coating was estimated to be 25 nm on the basis of the difference in the diameters of the coated and the uncoatedbeads (Reprinted with permission from Ref. [112]. � 2005, American Chemical Society).


shell. The first layer around the maghemite core wasformed by polyethylenimine (PEI), while the second one– by poly(ethylene oxide)-block-poly(glutamic acid)(PEO-PGA). The hydrodynamic diameter of the particles in-creases stepwise from Dh equal to 25 nm (parent particles)via 35 nm (PEI-particles) to 46 nm (PEO-PGA/PEI-parti-cles). The coating is accompanied by a switching of the zetapotentials from moderately positive to highly positive andfinally slightly negative.

3.3.3. Magnetic polymer beads with homogeneously dispersedmagnetic particles

Such beads are often prepared by emulsion techniques.In this method, magnetic particles are dispersed in a poly-mer solution and emulsified as a disperse phase. Then eachdroplet of the emulsion is transformed into bead either bysolvent evaporation or cross-linking.

Hydrophilic magnetic polymer particles were obtainedfrom polymer aqueous solutions via W/O emulsions. Bothnatural and synthetic polymers were used including j-car-rageenan [116], chitosan [117,118], gelatin [119] and PVA[120]. Hou [116] prepared magnetite j-carrageenan beadsas follows. The mixture of j-carrageenan and magnetite inhot water was dispersed in hot vegetable oil to form W/Oemulsion. The droplet size was controlled by stirring rate.The emulsion droplets were solidified by rapid coolingand further gelified in a KCl aqueous solution. The surfaceof magnetic polymer beads was finally hardened by react-ing with PEI or 1,6-diaminohexane. The resulting beadshad a wide size distribution 30–1000 lm.

Magnetite chitosan beads were prepared by cross-link-ing the linear chitosan chains in W/O emulsion [117,118].Chitosan was dissolved in an aqueous solution of aceticacid containing Fe3O4 grains (1–5 lm) [117] or ferrofluid[118]. The dispersion was then added dropwise into anoil phase containing Span 80 as emulsifier under agitation.Then chitosan chains within droplets were cross-linked byglutaraldehyde. The particles obtained from the ferrofluidhad a mean particle size of 5 lm, whereas those preparedfrom 1 to 5 lm magnetite were 100–250 lm in size.

The hydrophobic magnetic beads of PS, poly(caprolac-tone), poly(l-lactic acid) and poly(l-lactide-co-glycolide)

were prepared by solvent evaporation technique [121].Asmatulu et al. [122] synthesized hydrophobic magnetitephenoxy resin and poly(lactide) beads by an emulsifica-tion–diffusion method. Hydrophilic magnetite (8–10 nm)particles coated with oleic acid were dispersed in dichloro-methane together with the phenoxy resin or poly(lactide).This phase was dispersed into a continuous aqueous phasecontaining PVA. The emulsion obtained was homogenizedagain in an isopropanol/water solution, stirred for solventdiffusion and formed magnetite embedded particles inthe size range of 0.5–7 lm.

Another approach to get magnetic polymer beads isdrop-by-drop ionotropic gelation. Khokhlov et al. preparedmagnetic alginate beads by dropping down the suspensionof magnetic filler particles in alginate solution into a cal-cium or barium chloride solution [25,26]. A spontaneouscross-linking reaction of negatively charged alginatechains by bivalent cations occurred resulting in sphericalbeads with an average diameter of about 3 mm. In additionto this so-called ‘‘external’’ gelation [25,26,123], an ‘‘inter-nal’’ gelation may be used. In the ‘‘internal’’ method, an‘‘inactive’’ calcium in the form of complex with EDTA isfirst added to the suspension of magnetic filler particlesin alginate solution. Then, calcium is slowly released byacidification of the medium using hydrolysis of gluco-d-lactone [124]. An ‘‘internal’’ gelation always leads to morehomogeneous beads than the ‘‘external’’ gelation. Theinfluence of different parameters, such as an uronic com-position of alginate (proportion of mannuronic and gulu-ronic acids units), the degree of substitution of Na+ intoCa2+ in alginate gel (that is defined by the immersion timeof beads in calcium chloride solution and the calcium chlo-ride concentration) on the properties of magnetic alginatebeads was studied. It was shown that at the complete sub-stitution of Na+ into Ca2+ the release of encapsulated mag-netic filler particles from alginate microspheres is notobserved.

In addition to emulsification and ionotropic gelationsome other techniques are used to produce magnetic poly-mer beads, in particular, spray drying [125,126]. In thistechnique, magnetic particles are dispersed in polymersolution and then spray dried. The method is useful to cre-

0 5 10 15 200

10

20

30

m-m

0/m0

[Fe3 O4 ], wt.%

Fig. 8. Degree of swelling as a function of Fe3O4 content for magnetite-loaded Ca-alginate beads in 0.75 wt.% NaCl solutions. The diameter ofdried beads in 2 mm, the size of magnetite particles is 300 nm (m, m0 –mass of swollen and dry beads, respectively).


ate particles in the size range of up to 50 lm, wherein thepolymer has a sufficiently high glass transition tempera-ture to allow formation of discrete beads. The size andthe morphology of the beads depend on the nature of thepolymer and on the spray dryer operating parameters,including chamber volume, flow rate, and nozzle design[127]. A new trend consists in the use of supercritical fluidswith spray techniques (rapid expansion of supercriticalsolutions) [128]. It renders the spray process more control-lable due to the high solubility, low viscosity and rapidevaporation of supercritical fluids, but the cost of such pro-cesses is not attractive [24].

Recent advances in the preparation of homogeneousmagnetic polymer beads from pre-formed magnetic parti-cles and polymer concern the use of membrane and micro-fluidics emulsification [24]. Omi et al. [129] got magneticpoly(styrene-co-acrylic acid) or/and poly(styrene-co-butylacrylate) particles using glass membranes. The polymerswere dissolved in ferrofluid containing magnetite in tolu-ene and then pushed through the membranes. As a resultuniform O/W droplets with suspended magnetic particleswere produced. After evaporation of toluene the magneticpolymer beads with the size of 5–40 lm containing 30–40 wt.% of magnetite were obtained. It was demonstratedthat both the compatibility of the disperse phase compo-nents themselves and the compatibility of the dispersephase (especially the solvent) with the membrane surfacewere important in the production of uniformly dispersedparticles. Yang et al. [130] described a microfluidic assistedsynthesis of multi-functional polycaprolactone beads withmagnetite nanoparticles. Badescu et al. [131] demonstratethe production of magnetic alginate beads with varioussizes ranging from 45 to 875 lm by microfluidic technique.

3.4. Magnetic beads with inhomogeneously dispersedmagnetic particles prepared under the action of magnetic field

Magnetic beads with tailor-made anisotropy can besynthesized under an external field (Fig. 4). In particular,magnetochromatic beads were prepared [132] by emulsi-fying a liquid UV curable resin containing mainly PEG diac-rylate (PEGDA) oligomers, magnetic Fe3O4 particles coatedwith silica and a photoinitiator in mineral or silicone oil.Upon the application of an external magnetic field, themagnetic particles self-assemble into ordered structuresinside the emulsion droplets. An immediate 365-nm UVillumination quickly polymerizes the PEGDA oligomers totransform the emulsion droplets into solid polymer beadsand, at the same time, permanently fixes the periodicstructures of magnetite particles. Beads with different col-ors can be obtained by controlling the periodicity of theassembly of magnetic particles through the variation ofthe external magnetic field during the UV curing process.The excellent stability together with the capability of faston/off switching of the diffraction by magnetic fieldsmakes the magnetochromatic beads thus obtained suitablefor applications such as color display, rewritable signage,and sensors [132].

Thus, several techniques of magnetic polymer beadspreparation were developed. They allow the synthesis ofbeads in a wide range of dimensions from hundred nano-

meters to few millimeters. For intravenous injection, therequired magnetic polymer beads should be small (lessthan 200 nm) in order to ensure their free transportationin human body without agglomeration or blockage [24].In other area of applications larger particles can be em-ployed: for intracavity applications – from few microme-ters up to approximately 30 lm [133]; for separationapplications, the particle size may vary in the range fromsubmicrometer to about 100 lm [134,135]. In oil-fieldapplications, the optimum size of beads for blocking flowis about few millimeters [25,26].

4. Properties

4.1. Swelling

Magnetic polymer beads can swell in solvents. It partic-ularly concerns beads with the gel-like structure. Usuallythe degree of swelling of beads containing magnetic parti-cles is lower than for the corresponding unloaded beads[136,137] (Fig. 8), which can be explained by the formationof additional cross-links by nanoparticles interacting withpolymer chains [136]. The presence of such cross-links isconfirmed by the fact that added nanoparticles increasethe elasticity modulus of the gel beads [24].

In some non-covalently cross-linked beads non-monot-onous dependence of the degree of swelling on the contentof magnetic particles is observed (Fig. 9). At low content ofmagnetic particles their addition leads to the swelling ofthe beads, at higher content a routine decrease of theswelling takes place. An unexpected swelling, which is ob-served only at rather high concentration of salt (NaCl) inthe system, can be due to the disruption of some of theCa – cross-links in the network upon addition of magneticparticles [138].

Thus, usually the introduction of magnetic particles intopolymer beads decreases their degree of swelling andstrengthens the beads.

0 5 10 150

10

20

30

40m

-m0/m

0

[CoO.Fe2O3], wt. %

Fig. 9. Degree of swelling of the Ca-alginate beads as a function ofcontent of CoO.Fe2O3 particles in 1.5 wt.% NaCl solution in water. Thediameter of dried beads in 2 mm, the size of cobalt ferrite particles is500 nm (m, m0 – mass of swollen and dry beads, respectively).


4.2. Collapse

Polymer beads that combine sensitivity to magneticfield with temperature- and/or pH-responsive propertieswere elaborated [27,137,139–142]. It was observed thatmagnetic nanoparticles do not alter neither temperature-nor pH-sensitivity.

Several papers are devoted to the study of the effect ofmagnetic particles on the temperature-induced collapse ofpolymer beads. For this purpose different thermosensitivebeads were used including PNIPA [27,141], copolymer ofN-vinylcaprolactam and acetoacetoxyethyl methacrylate[46,137,140], copolymers of maleilated carboxymethylchitosan with NIPA [142]. It was observed that the incorpo-rated magnetite particles do not shift the temperature ofthe collapse transition. This means that, besides the inter-action of magnetite particles with polymer chains, thepolymer network still has enough freedom to swell andcollapse at different temperatures.

Moreover, collapse of thermosensitive beads can betriggered by the action of magnetic field, if it induces heat-ing [65,76]. This phenomenon can be a basis for the elabo-ration of a novel type of remote-controlled drug releasesystem, in which the delivery of drug is induced by appliedmagnetic field. In this system, a release of the encapsulateddrug results from collapse of thermosensitive beads trig-gered by heating of the magnetic particles in an alternatingmagnetic field.

4.3. Behavior in magnetic field

Most of the papers describing the magnetic propertiesof polymer beads deal with beads loaded with nanosizedmagnetic particles exhibiting superparamagnetic proper-ties. If the concentration of the nanoparticles is below thepercolation threshold, the bead contains a collection of sin-gle domain particles. From the magnetic viewpoint, thebead is a dilute ensemble of non-interacting magnetic mo-

ments. In the absence of an external magnetic field, themoments are randomly oriented, and thus the bead hasno net magnetization. As soon as an external field is ap-plied, the magnetic moments tend to align with the fieldand produce a bulk magnetization. The thermal agitationopposes this process. When the strength of the field be-comes high enough, all the particles eventually align theirmoments along the direction of the field, and as a result,the magnetization saturates [54]. When the field is turnedoff, the magnetic dipole moments begin to randomizereducing the bulk magnetization. Magnetic dipoles are ableto reorient by two mechanisms [3]: either by rotation ofthe whole particle (Brownian mechanism) or by collectiverotation of the atomic magnetic moments inside the parti-cles (Néel mechanism). In ferrofluids, both mechanisms areeffective. By contrast, in polymer beads, where the mag-netic particles are trapped by polymer chains, the Brown-ian rotation is restricted and the Néel mechanismdominates.

Cooling induces the transition from superparamagneticto ferromagnetic state, which proceeds at the blockingtemperature TB [54]. The value of TB is proportional tothe volume of the magnetic particles, which means thateven a modest increase in particle size can result in a sig-nificant increase of TB.

For superparamagnetic material the magnetic behaviorcan be described by the Langevin function. Assuming themagnetization of individual particles in the gel bead tobe equal to the saturation magnetization of the pure ferro-magnetic material, the magnetization, M, of the bead in thepresence of an applied field can be expressed as [54]:

M ¼ /mMSLðnÞ ¼ /mMS coth n� 1n

� �

where /m is the volume fraction of the magnetic particlesin the bead, and n is the parameter of Langevin functionL(n) defined as n ¼ mH=kT, where m is the giant magneticmoment of nanosized magnetic particles, H is the externalmagnetic field, k is the Boltzmann constant, T is thetemperature.

According to this equation, there is no hysteresis in thefield dependence of the magnetization (M is a single valuedfunction of H) and the magnetization of a bead is directlyproportional to the concentration of magnetic particlesand their saturation magnetization.

Fig. 10 shows a typical magnetization curve for mag-netic polymer beads. It is seen that the beads show asuperparamagnetic behavior at room temperature. Noremanence is observed when the magnetic field isremoved.

As to the magnetization values, in real systems they areoften lower than can be predicted from the saturationmagnetization of the pure ferromagnetic material[126,143,144]. In particular, Huang et al. [143] found thatthe saturation magnetizations of magnetic fluid composedof 15-nm magnetite nanoparticles (without polymer) andof magnetic poly(styrene-co-acrylamide) beads are equalto 17.2 and 1.45 emu/g, respectively. At the same time,the saturation magnetization of bulk magnetite is84 emu/g. Lower saturation magnetization of the magnetic

Fig. 10. Magnetization curve for submicron magnetic polystyrene beadscontaining magnetite particles (Reprinted with permission from Ref. [95].� 2007, Elsevier).


fluid in comparison with the bulk magnetite may be due tospin disorder arising from the larger particle surface area[145,146]. In its turn, lower saturation magnetization ofpoly(styrene-co-acrylamide) beads loaded with 15-nmmagnetite particles in comparison with that of magneticfluid with the same concentration of 15-nm magnetite par-ticles was assigned [143] to the oxidation processes duringsonication and polymerization, which can lead to the for-mation of some non-magnetic particles, and to the pres-ence of polymer on the surface of particles.

An important effect is the heating of magnetic particles[3,65] in an external AC high-frequency magnetic field (inthe kHz range). In such field, the particles are forced toconduct fast remagnetization accompanied by thermallosses. The magnetic heatability is under intensive investi-gation for tumor therapy by magnetic fluid hyperthermia[4–9].

When the concentration of magnetic particles in thebeads becomes rather high, the particles start to interactwith each other. In the absence of a magnetic field, parti-cles that are big enough to exhibit significant magneticinteractions form open-loop structures with no spatial ori-entation [3]. When exposed to an external magnetic field,magnetic particles coalesce under the influence of themagnetic dipole interaction into chainlike ‘‘columnar’’structures along the field direction (Fig. 11). The exact

Fig. 11. TEM images of ferromagnetic polystyrene-coated cobalt nanoparticles al[146]. � 2006, American Chemical Society).

shape of these structures depends on parameters such asthe particle concentration and applied magnetic field. Evenafter removal of the external magnetic field, such particlestructures can stay agglomerated, hindering separationand recovery of the magnetic beads.

The mutual particle magnetic interactions leading tochainlike structures affect the mechanical properties ofthe beads, in particular, they are responsible for the in-crease of the elastic modulus of the beads induced by anexternal magnetic field [27,28,147] (Fig. 12). This phenom-enon is called temporary reinforcement. In order to en-hance the effect, anisotropic samples are prepared byvarying the spatial distribution of the magnetic particlesin the elastic matrix. It was shown that uniaxial field struc-tured composites exhibit much larger increase in modulusthan random particle dispersions [148,149]. The temporaryreinforcement effect was most significant if the appliedfield, the particle alignment, and the mechanical stressare all parallel to each other [28,148,149] (Fig. 12). The in-crease of elastic modulus in an external homogeneousmagnetic field was also reported for magnetic elastomers[148–152]. Thus, magnetic field produces a significant im-pact on the mechanical properties of beads.

5. Applications

Magnetic nano- and microparticles are of great interestfor many technological applications: magnetic storagemedia [153], biosensing applications [154], medical appli-cations, such as targeted drug delivery [122,155–163], con-trast agents in magnetic resonance imaging (MRI) [164–176], bioseparation [11–13,177–183], and destruction ofthe tumor tissue under the action of high-frequency mag-netic fields (‘‘intercellular hyperthermia’’) [4–9], etc. Belowsome of these applications are briefly described.

5.1. Drug delivery

Targeted drug delivery by magnetic field is an innova-tive new approach for drug release. According to this ap-proach, the magnetic beads are intravenouslyadministered and their accumulation takes place onlywithin the area to which the magnetic field is applied[157]. Shortly after accumulation in targeted region, drugmolecules are gradually released, thus improving the ther-

igned under a magnetic field (1 kOe) (Reprinted with permission from Ref.

Fig. 12. Dependence of the elastic modulus of poly(dimethylsiloxane) beads on the magnetic field. The arrangements of the particles in the polymernetworks are parallel to the applied mechanical stress while the applied uniform magnetic field is parallel (a) or perpendicular (b) to the columnarstructure. The concentration of the carbonyl iron particles in the polymer matrix is indicated on the figure (Reprinted with permission from Ref. [28]. �2006, Elsevier).


apeutic efficiency of the drugs by lowering the collateraltoxic side effects on the healthy cells or tissues [158,159].

The three main mechanisms for releasing drug mole-cules from the polymeric magnetic beads into a blood ves-sel or tissue are diffusion, degradation, and swellingfollowed by diffusion [160,161]. Diffusion occurs whendrug molecules dissolve in body fluids around or withinthe beads and migrate away from the beads. Degradationtakes place when the polymer chains hydrolyze into lowermolecular weight species, effectively releasing drug mole-cules that were trapped by the chains. As to the swelling-controlled release systems, they are initially dry. Whenthey are placed in the body, they swell enabling the drugmolecules to diffuse from the swollen network.

Internalization of magnetic beads strongly dependsupon their size. After administration, larger beads with adiameter higher than 200 nm are easily sequestered bythe spleen and eventually removed by the cells of thephagocyte system, resulting in decreased blood circulationtimes. Small beads with diameters less than 10 nm are rap-idly removed through extravasations and renal clearance.Beads with a diameter ranging from 10 to 100 nm are opti-mal for intravenous injection and have the most prolongedblood circulation times. These beads are small enough toescape the reticulo-endothelial system of the body as wellas to penetrate small capillaries of the tissues and offer themost effective distribution in targeted tissues.

Magnetic drug targeting employing nanobeads as carri-ers is a promising cancer treatment avoiding the side ef-fects of conventional chemotherapy [122] due to precisetargeted delivery. Alexiou et al. demonstrated that a strongmagnetic field gradient at the tumor location induces accu-mulation of the nanobeads covered by starch derivativeswith phosphate groups, which bound mitoxantrone[162]. Gallo et al. [163] showed that, after administrationof magnetic polymer beads loaded with oxantrazole, thebrain contained 100–400 times higher oxantrazole levelsthan those obtained after the solution dosage form, indi-cating the successfulness of drug delivery via magneticparticles.

5.2. Hyperthermia

Hyperthermia is a promising approach [4–9] to cancertherapy based on the selective heating of the target tissueto temperature between 42� and 46� that generally reducesthe viability of cancer cells and increases their sensitivityto chemotherapy and radiation. The fatal technical prob-lem in hyperthermia is the difficulty of the uniform heatingof only the tumor region up to the required temperaturewithout damaging normal tissue. This problem can betackled by using magnetic particles, which can be accumu-lated only in the tumor tissue and then heated by externalAC magnetic field [3,65].

Also, there exists a combination therapy which includeshyperthermia treatment followed by chemotherapy. The ap-proach involves use of magnetic carriers containing a drug tocause hyperthermia using the standard procedure, followedby the release of encapsulated drug that will act on the in-jured cells. It is anticipated that the combined treatmentmight be very efficient in treating solid tumor [184–186].

5.3. Magnetic resonance imaging

MRI is considered to be one of the most powerful tech-niques in diagnostics, clinical medicine and biomedical re-search. In MRI, image contrast is a result of the differentsignal intensity each tissue produces in response to a par-ticular sequence of the applied radiofrequency pulses. Thisresponse is governed by the proton density and magneticrelaxation times so as MRI contrast depends on the chem-ical and molecular structure of the tissue [171]. In early1980s it was recognized that target-specific magnetic par-ticles can serve as contrasting agents, and they rapidly be-came an important and indispensable tool for the non-invasive study of biologic processes with MRI. As contrast-ing agents superparamagnetic iron oxide or paramagneticmacromolecular compounds may be used [172]. Paramag-netic metal ions reduce the T1 relaxation of water protonsand enhance the signal intensity, as a result the images be-come brighter. The most commonly used superparamag-netic material is Fe3O4 with different polymer coatings

0 10 20 30 40 50

0

150

300

450

600flo

w, m

l/min

time, min

Fig. 14. Kinetics of the blocking flow of 1.5 wt.% aqueous solution of NaClin a tube with a diameter of 16 mm by magnetic alginate beadscontaining 3 wt.% of cobalt ferrite. Magnetic field strength used for theformation of plug is 3.3 kOe.


[173–175] such as dextran [173] or silicone [175]. Super-paramagnetic iron oxide causes marked shortening of T2relaxation and hence the reduction of signal intensity inMR images. So far it was mainly used as a liver-specificcontrast agent for intravenous application. Also, it is verypromising for detection of metastases in non-enlargedlymph nodes. When contrast agent is interstitially applied,none of it is accumulated in the lesion since metastases donot have an intact phagocytosing system. Thus, a contrastagent induces a signal effect in normal tissues, but not inmetastases, thus enhancing the contrast [176].

5.4. Bioseparation

Another important application of magnetic nano- andmicroparticles is the in vitro separation of cells or biomol-ecules [11–13,177–183]. The cells or biomolecules whichare non-magnetic in nature can be attached to magneticresponsive beads and thus manipulated using an externalmagnetic field. The separation of cells or compounds maybe done by direct and indirect methods. In the direct meth-od, ligands are immobilized on magnetic beads, and incu-bated with the medium (cells or compounds) for sometime. The target cells bind with these ligands and the com-plex formed can be separated by a magnetic field. In theindirect mode, the target cell initially interacts with the li-gand (primary antibody). The secondary antibody is thenimmobilized on magnetic particles and added to the med-ium containing the cells. When antibodies have poor affin-ity or antigens are less accessible, indirect methods mightperform better [178]. The magnetic separation of cells orbiomolecules is more effective, when the superparamag-netic particles are used, because they exhibit magneticproperties only in the presence of the magnetic field. Bynow, with the aid of magnetic absorbents the isolation ofvarious macromolecules such as enzymes, enzyme inhibi-tors, DNA, RNA, antibodies, antigens etc. from differentsources including nutrient media, fermentation broth andbody fluids, was performed [11–13,177–183].

5.5. Oil industry

As was recently demonstrated by Khokhlov et al.[25,26], the magnetosensitive polymer beads are of special

Fig. 13. Schematic representation of the method of blockin

interest for oil industry, in particular, for blocking waterflow in a productive well [187], for zonal isolation andfor targeted delivery of chemicals to the desired place ofthe well.

For blocking water flow under the action of the mag-netic field the dried gel beads should be accumulated nearthe magnet and swell there forming a gel plug (Fig. 13). Forthis aim 3-mm alginate beads cross-linked by calcium orbarium ions with embedded magnetic microparticles(magnetite Fe3O4, maghemite c-Fe2O3 or cobalt ferriteCoO�Fe2O3) were prepared. The synthesis was performedby dropping the suspension of magnetic particles in algi-nate solution into a large volume of aqueous solution ofcross-linker (CaCl2, BaCl2). The resulting beads representmicrogel peaces with homogeneously dispersed magneticmicroparticles.

To study the formation of the gel plug the dried beadswere put in aqueous NaCl solution circulating in a tube, asmall part of which (2.5 cm long) was placed betweenthe poles of permanent magnet. Due to magnetic filler

g water flow in a tube by magnetic polymer beads.

Fig. 15. Plug formed by swollen magnetic alginate beads.


the beads accumulate inside the tube near the magnet,where the strength of field is maximal. In the course oftime the flow through the region of accumulated beads de-creases gradually with swelling of the beads as seen inFig. 14. Within 35 min the flow of solvent through beadsbecomes completely blocked (Fig. 14). Then the magnetcan be removed, and the gel plug remains in its place(Fig. 15). Thereafter, the pressure in the tube is graduallyincreased in order to determine the maximum value ofpressure, which can be kept by the plug.

It should be noted that the beads prepared for the plugformation swell significantly in water medium acquiringup to 40 g of aqueous solution per 1 g of the dry polymer.The swelling ability of the beads plays a crucial role in theformation of the gel plug blocking the water flow. As to themagnetic filler, the best results were obtained with cobalt

+ B

HO

HO

OH

Boric acid

H C

CH OH

OH

H C

CH OH

OH

+ Na2B4O7.10H2O

Borax

Fig. 16. Chemical reactions involved in the

ferrite, which has the higher residual magnetizationamong the studied samples. This observation may indicatethat magnetic interactions can also contribute in the stabil-ity of the plug. Note that when temperature increases, theplug is formed faster and it is stronger. This may be due tomore pronounced disruption of calcium or barium cross-links on the surface of the beads, which allows more effec-tive swelling and interaction between the beads.

Analysis of the results obtained shows that the blockingefficiency of the plug depends mainly on three factors. Thefirst factor is the magnetic properties of the beads deter-mining their ability to be accumulated under the actionof magnetic field and their magnetic interactions stabiliz-ing the plug. The second factor is the ability of the driedbeads for strong and rapid swelling. Due to the swellingthe beads produce high pressure on the walls of the tubefixing the plug on its place after the removal of magnet.The third factor is the formation of some cross-links be-tween swollen beads, making the plug stronger.

As a result of the studies, the optimum composition ofmagnetic alginate beads was found [25,26]. It was shownthat such plug can keep the pressure of up to 6 atm in atube with a diameter of 35 mm.

Another direction of the use of magnetosensitive poly-mer beads in oil industry is connected with targeted deliv-ery and controlled release of encapsulated chemicals in thedesired place of the well. In this case, the beads shouldcontain not only magnetic particles, but also the encapsu-lated compound. Such beads can be guided by magneticfield to a proper place and swell there releasing the encap-sulated substance.

The released chemicals can be necessary, in particular,to initiate various physicochemical reactions, e.g., gela-tion of the solution, in which the beads are immersed.In this case, the magnetic beads should be loaded witha cross-linker. Under the action of magnetic field thebeads release the cross-linker in the desired place, whereit interacts with polymer leading to the gel formation[25,26].

Such controlled gelation system was elaborated on anexample of PVA or guar aqueous solution cross-linked by

Monodiol complex(solution)

C OH

C O

OH

H

B OH

C OH

C OH

B

O

O

C

C

H

H

Didiol complex (gel)

cross-linking of PVA by borate [188].

Fig. 17. Illustration of the gelation induced by targeted delivery of cross-linker guided by external magnetic field. (a) The beads are accumulatednear the magnet. (b) The beads are disrupted by magnetic field, thereleased cross-linker (borate) induced the gelation of the externalsolution containing 5 wt.% of PVA. Magnetic field intensity is 4.1 kOe.


borate (Fig. 16). Plug formation was studied in the tubeswith the diameter of 35 mm placed in the magnetic fieldof 2 kOe. It was observed that the plug formed can keepthe pressure of about 8 atm.

To accelerate the release, the beads can be destroyedunder the action of magnetic field (Fig. 17). Beaddestruction is possible if the elastic modulus of the beadis less that 40 kPa. Most probably, the disruption pro-ceeds as a result of the alignment of magnetic micropar-ticles along the lines of force of the applied field (Figs. 4and 5) inducing strong stresses in polymer chains con-nected to the beads. A drastic effect of the displacementof microparticles on the integrity of beads indicates thatthe polymer chains are strongly attached to the particles,only in this case one can expect that the alignment ofthe particles will destroy the whole polymer matrix, inwhich they are embedded. Most probably, the attach-ment proceeds via binding of carboxylic groups of so-dium alginate to iron on the surface of magnetite as itwas observed in the case of other carboxyl-containingcompounds like oleic acid, PEG-PAsp [52] and PS-polyac-rylate [54] copolymers.

One more striking property of the proposed system con-sists in its ability to self-healing, when the plug is crackedunder pressure. Such damaged plug can recover its integ-rity with time, which may be due to the redistribution ofcross-linker in the system. The self-healed plug can keepthe same pressure as the initial one before cracking.

Thus, magnetic polymer beads offer great potential foradvancements in various fields.

6. Conclusion

Magnetic polymer beads attract considerable attentionof researchers due to their unique properties. Since thebeads have high magnetic susceptibility to an externalmagnetic field, they can be easily separated from othercomponents of the mixture with the help of magnets. Thisis the basis of various separation applications. Attachmentof specific functional groups or ligands to the shell of beadscan make the separation highly selective. When need, the

beads can be accumulated exactly in a given site by apply-ing an external field, which is the basis for the target deliv-ery of various chemicals including drugs. Also, the beadscan be heated by high-frequency magnetic field, whichdetermines their application in hyperthermia therapy orin heat-triggered drug release.

Last years new applications of magnetic polymer beadsappear. They concern, in particular, oil industry, where thebeads can be used for different purposes including block-ing water flow, zonal isolation and targeted delivery ofchemicals encapsulated in the beads.

Acknowledgements

The financial support of Russian Ministry of Educationand Science in the framework of the program ‘‘Scientificand educational staff of innovative Russia’’ in 2009–2013is gratefully acknowledged. V.M. expresses his gratitudeto the award No. KUK-F1-034-13 made by King AbdullahUniversity of Science and Technology (KAUST).

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Olga Philippova is graduated from the Mos-cow State University in 1981. She received aPh.D. degree in polymer chemistry from theMoscow State University in 1985 and D.Sc.degree in polymer physics from the Instituteof Chemical Physics in Moscow in 1999. Cur-rently, she is full professor at PhysicsDepartment of the Moscow State University.Her research is focused on polymer gels,associating polymers and viscoelastic surfac-tants.

Anna Barabanova was born in Moscow,Russia, in 1964. She is graduated from M.V.Lomonosov Institute of Fine Chemical Tech-nology in 1988. She received her PhD inL.Ya.Karpov Institute of Physical Chemistry in1996. Currently, she is working as seniorresearch associate in A.N.Nesmeyanov Insti-tute of Organoelement Compounds of theRussian Academy of Sciences. Her primaryresearch interests include polymer nanocom-posites.

Vyacheslav Molchanov (born 1984) is grad-uated from the Moscow State University in2006. He received his PhD in polymer physicsin 2008 for his research on highly responsiveself-assembled networks based on polymerand wormlike micelles of surfactant. In 2009he was awarded with the King Abdullah Uni-versity of Science and Technology ResearchFellows Program Award (Saudi Arabia). Pres-ently he is research associate at PhysicsDepartment of the Moscow State University.His research is focused on viscoelastic solu-

tions of wormlike micelles of surfactant with nano- or colloid inorganicparticles.

Alexei Khokhlov is graduated from the Mos-cow State University in 1977. He received aPh.D. degree in polymer physics in 1979 andD.Sc. degree in polymer physics in 1983 fromthe Moscow State University. Currently, he isvice-rector of the Moscow State University,member of the Russian Academy of Sciences,full professor and head of the chair at PhysicsDepartment of the Moscow State University.His research is focused on different aspects ofpolymer science including statistical physicsof macromolecules, computer modeling of

polymer systems, physical chemistry of polyelectrolytes and ionomersetc.

european polymer journal - msu · olga philippovaa,⇑, anna barabanovab, vyacheslav molchanova,...

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