Magneto-responsive nanocomposites: Preparation and integration of magnetic nanoparticles into films, capsules, and gels

Advances in Colloid and Interface Science xxx (2013) xxx–xxx

CIS-01313; No of Pages 11

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Advances in Colloid and Interface Science

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Magneto-responsive nanocomposites: Preparation and integration of magneticnanoparticles into films, capsules, and gels

Francesca Ridi, Massimo Bonini, Piero Baglioni ⁎Department of Chemistry “Ugo Schiff” and CSGI, University of Florence, via della Lastruccia 3-Sesto Fiorentino, I-50019 Florence, Italy

⁎ Corresponding author. Tel.: +39 055 457 3033; fax: +E-mail address: [email protected] (P. Baglioni).URL: http://www.csgi.unifi.it (P. Baglioni).

0001-8686/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.cis.2013.09.006

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Keywords:Magnetic nanoparticleNanocompositeFilmCapsuleGel

This review reports on the latest developments in the field of magnetic nanocomposites, with a special focus onthe potentials introduced by the incorporation of magnetic nanoparticles into polymer and supramolecular ma-trices. The general notions and the state of the art of nanocomposite materials are summarized and the resultsreported in the literature over the last decade on magnetically responsive films, capsules and gels are reviewed.Themost promising concepts that have inspired the design of magneto-responsive nanocomposites are illustrat-ed through remarkable examples where the integration of magnetic nanoparticles into organic architectures hassuccessfully taken to the development of responsive multifunctional materials.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01.1. Nanotechnologies and nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01.2. Polymer nanocomposites: improving host performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01.3. Green polymers: matching the performances of synthetic plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01.4. Multifunctional nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01.5. Smart nanocomposites: introducing responsiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01.6. Magnetic nanocomposites: superparamagnetic nanoparticles heat them up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01.7. Magnetic nanocomposites: moving them around . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 01.8. Magnetic nanoparticles in supramolecular systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

2. Magnetic nanocomposite films, capsules and gels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.1. Magnetic tapes: platelet nanoparticles revamp them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.2. Magneto-responsive Layer-by-Layer assemblies: towards multi-functional platforms . . . . . . . . . . . . . . . . . . . . . . . 02.3. Magnetoliposomes: surface chemistry of MagNPs modulates drug release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.4. Magnetic hydrogels: a versatile scaffold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.5. Magneto-responsive PNIPAM gel particles: magnetically triggered release of drugs . . . . . . . . . . . . . . . . . . . . . . . . . . . 02.6. Magnetically triggered membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

1. Introduction

The aim of this review is to provide an overview of the latest re-search activity in the field of magnetic nanocomposites, especially

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eto-responsive nanocomposit2013), http://dx.doi.org/10.1

highlighting those concepts that have inspired the design ofmagnetical-ly responsive films, capsules and gels.

Many reviews have been published over the last few years aboutnanocomposite materials (i.e., multi-phasic materials composed by amatrix incorporating units with at least one dimension in the 100 nmsize range or smaller). A comprehensive overview of the latest efforts to-wards functional hybridmaterials can be found in a paper byKao et al. [1].The potential applications of polymer-based inorganic nanoparticle

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composites have been recently summarized [2], showing that thestability and processability of polymers and the peculiar electronicand magnetic properties of inorganic nanoparticles could be syner-gistically combined to generate improved materials [3].

Including magnetic nanoparticles in composite materials introducesresponsiveness to magnetic fields, opening up new perspectives interms of functionalities and applications. For instance, several caseswhere magnetic nanocomposites are effective in environmental re-mediation have been lately highlighted in the literature [4]. Smart com-posite materials have also been obtained through the combination ofmagnetic nanoparticles and thermally responsive polymers, where analternating magnetic field (AMF) is used to trigger localized heating,which in turn causes a change in the structure of the composite matrix[5]. The potentials introduced by peculiar properties of the matrix arewell illustrated by recent papers reviewing the progress in the fieldsof stimuli-responsive membranes [6], mesoporous structures [7,8],and liposomes [9].

In such a crowded scenario, this review is not intended to providethe reader with an overview of the preparationmethods and the poten-tial applications ofmagnetic nanocomposites. To this purpose, the read-er is referred to the reviews cited within the introduction of this paperand the references therein. The focus of this report is indeed onmagnet-ically responsive films, capsules and gels. Most importantly, rather thanon the availablematerials andmethods to obtain them, our review is es-pecially focused on those concepts that have recently inspired their de-sign, their preparation and their application.

Following a brief introduction to the general notions and the state ofthe art of nanocomposite materials, the progress over the last decade inthe field of magnetically responsive nanocomposites is shortly reviewed.The most promising concepts that have inspired the design of magneto-responsive films, capsules and gels are then critically discussed. To thisaim, this review is mainly focused on the illustration of selected casestudies where the integration of magnetic nanoparticles into organic ar-chitectures has successfully taken to the development of responsivemul-tifunctional materials. In particular, examples are used to highlight thepossible key aspects for the advances in the field, which are outlined inthe last section of this paper.

1.1. Nanotechnologies and nanocomposites

Nanotechnology aims at designing, building, and manipulating ma-terials characterized by at least one dimension below 100 nm. Whenthe dimensions of a material are reduced in the size range of nanome-ters, the physico-chemical behavior significantly departs from that ofthe bulk state. Nanostructuredmaterials display peculiar electrical, me-chanical, chemical, magnetic, and optical properties that are of interestto a broad range of applications. Tailoring their nanostructure producesthereforematerials that can be exploited to overcome the shortcomingsof traditional approaches. Among many others, sectors like electronics,medicine, and optics greatly extended their potential over the last de-cade by taking advantage of these new tools.

Nowadays nanometric building blocks are available through innova-tive synthetic routes and technologies making possible to arrange theminto functional structures and/or to include them into a supportingma-terial. It is worthwhile to recall that nanocomposites combine the prop-erties of both the supporting material (matrix) and the nanoparticles(often referred to as the filler or the guest) generating new functionalmaterials able to match specific needs.

Thematerials constituting thematrix of a nanocomposite (also indi-cated as the host) can be extremely diverse: metals, ceramics, and poly-mers have all been employed in the past, even though the use of metalsand ceramics asmatrices is confined to a limited number of applications.The primary role of the matrix is to provide a support for the filler,imparting stability and processability to thefinal product. Polymers rep-resent so far the most popular choice: in fact, inorganic nanostructuresare often easily dispersed and stabilized (or even directly prepared) in

Please cite this article as: Ridi F, et al, Magneto-responsive nanocomposicapsules, and gels, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1

polymer solutions. Furthermore, many technologies developed in thepast for plastics were straightforwardly transferred to the productionof polymer nanocomposites (PNCs).

1.2. Polymer nanocomposites: improving host performances

Very often the first step in the manufacture of PNCs is thechoice of the proper fabrication method to ensure a good disper-sion of the nanoparticulate material in the matrix [2,10,11]. Var-ious techniques have been developed for preparation of PNCs,and are divided into two categories: in situ synthesis and directcompounding.

In the in situmethods the nanoparticle precursors and/or themono-mers are first mixed and, successively, the formation of nanoparticlesand/or the polymerization are activated by means of a physico-chemicaltrigger or a chemical initiator. This approach commonly ensures a highhomogeneity of the final composite [12].

In the direct compounding methods, nanofillers and polymer areseparately prepared, and then they are mixed together, commonly bymeans of mechanical forces or fusion. These preparations are cheapand suitable for the production of large amounts of material. Directcompounding methods are therefore the most used at the industriallevel and they are often intended to improve and eventually extend theproperties of the host.

Polymers offer a wide range of individual properties that can beexploited in a nanocomposite material: mechanical, thermal, opticalproperties, as well as biodegradability, toxicity, hydrophobic/hydrophilicbalance can be introduced and adjusted into the final composite materialby choosing the most suitable polymer. An application where polymersare particularly useful is the preparation of films. Polymer thin films arenowadays essential to many technologies and play a ubiquitous role inimproving people's everyday life. Packaging, coatings, andmicroelectron-ics are just fewexamples of the areaswhere polymericfilms have becomemore and more important. Thanks to the development of nanotechnol-ogies, the last decade has seen a growing interest in providing addedvalue to polymer thin films. This is particularly evident in the improve-ment of the mechanical properties of plastic films through the introduc-tion of either organic or inorganic nanometric fillers. The incorporationof low amounts of nanometric clays in the pristine polymer increasesthe mechanical performances and enable producing films with lowercontent of plastics, which is a beneficial effect from an environmentalpoint of view [10,13].

1.3. Green polymers: matching the performances of synthetic plastics

Besides the reinforcement of the synthetic plastics, several “green”polymers derived from natural sources have been identified as possiblesubstitute of petroleum-derived plastics. The great benefit of these poly-mers produced from renewable material sources is related to their envi-ronmental friendly disposal,which occurs naturally anddoes not producepolluting gases [14]. Unfortunately, when compared to conventionalpolymers and plastics, the performances of green and bio-polymers arenot always matching the needs of existing technologies, generatingmajor drawbacks for their use. This issue has been tackled by includingnanofillers in the polymeric matrix to obtain bionanocomposites. Inparticular, the introduction of fillers such as natural fibers, anisotropicin shape, allows for the preparation of green biocomposites [15,16]with enhanced performances [17–19], especially in terms ofmechanicalproperties [20], while keeping the characteristic ductility properties ofthe polymer [21]. These bionanocomposites have been gaining a con-stantly increasing attention in recent years. Because of their intrinsic na-ture, thesematerials opened up newperspectives formany applicationswhere biocompatibility, stability in aqueousmedia, and biodegradationshould be addressed [21–23].

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1.4. Multifunctional nanocomposites

A step in the direction of designingmaterialswithmultiple functionsis represented by the use of a host material that, in addition to act as asupport, is also able to accomplish a specific function, especially in con-junction with the filler. In this framework, soft matter materials and inparticular polymers offer a wide range of properties that can be effec-tively exploited to address a number of different functions.

This approach is widely used by Nature to produce structures capa-ble of addressing themost various demands, with performances far sur-passing those of the synthetic analogs [24]. Many biological materialscombine inorganic and organic moieties, which are able to accomplishmultiple tasks thanks to their individual properties and to their arrange-ment in complex and hierarchical structures [25]. Bright examples arebones, composite architectures whose basic building blocks are hy-droxyapatite nanoparticles and collagen [26], assembled in such a wayto provide structural and protective support for cells, proteins, and ions.

Many different nanocomposites containing natural polymers (suchas chitin, chitosan, and collagen) and hydroxyapatite particles havebeen recently prepared to mimic the structure and the compositionof bone tissues [27]. Thesematerials combinemultiple aspects (biocom-patibility, biodegradability, mechanical properties, nanotopography,chemical affinity), highlighting the potential of nanocomposite mate-rials when their properties do not simply arise from the individualcomponents, but especially from the interplay dictated by the spatialarrangement and the mutual interactions.

1.5. Smart nanocomposites: introducing responsiveness

Smart materials display properties that can be tuned by externalstimuli, such as mechanical stress, temperature, pH, electric ormagnetic fields [28]. For instance, the functions accomplished by bonetissues are activated by a broad range of mechanical and bio-chemicalstimuli. The use of a host material that displays a tunable behavior,which could be controlled through an external stimulus, represents a vi-able approach towards smart nanocomposites.

Polymers and gels responsive to photonic, thermal, electric and sol-vent stimuli are already available [29], making possible the activationof the nanocomposite response through the direct interaction betweenthe stimulus and the host. The external stimulus could be alsomediatedby the filler. This is for instance the case of composites including noblemetal nanoparticles [30,31]. In such materials, the peculiar electronicand optical properties of metal nanoparticles [32] are exploited to ab-sorb a specific light frequency and generate heat [33]. The possibilityto remotely heat the nanocomposite allows for the controlled activationof a thermal event, such as a polymer swelling/de-swelling or a glasstransition.

1.6. Magnetic nanocomposites: superparamagnetic nanoparticles heat themup

Magnetic stimulation hasmany advantages over other types of stim-uli, especially in terms of penetration and invasiveness. Many materials(especially biological tissues) are much more transparent to magneticfields than to electric fields, making it possible to remotely activate anevent at a relevant distance from the magnet. To this aim, it is necessaryto load thematerial of interest with structures able to produce a responseto an externalmagnetic event.Magnetic nanoparticles (MagNPs) are verywell suited to this purpose, as their sizemakes themeasy to be embeddedwithin a variety of materials and they display peculiar properties whenexposed to static and/or alternating magnetic fields.

Several methods have been developed to prepare MagNPs in a largerange of compositions, shapes, dimensions, and surface properties [34].So far, thanks to their chemical stability, iron oxide nanoparticles haveattracted most of the attention [35]. The advantage of introducingmag-netic nanoparticles in a material is twofold, as magnetic field gradients

Please cite this article as: Ridi F, et al, Magneto-responsive nanocompositcapsules, and gels, Adv Colloid Interface Sci (2013), http://dx.doi.org/10.1

can be used to move the material, while alternating magnetic fieldscan be used to locally heat up the regions in the proximity of MagNPs.As the size ofmagnetic particles is reduced below a critical diameter (typ-ically in the order of tens of nm), particles behave as superparamagnets[36] i.e., the atomic moments of the nanoparticle are aligned into a giantmagnetic moment. In this oversimplified picture MagNPs can be classi-cally described as paramagnets, where nanoparticle moments replaceatomic moments corresponding to much larger values of the magneticsusceptibility. In single domain MagNPs, due to magnetic anisotropy,the magnetic moment displays two preferential orientations, one anti-parallel to the other, separated by an energy barrier that is typicallymuch smaller than room temperature thermal energy. As a result themagnetization direction in MagNPs at RT randomly flips, and the timebetween two consecutive flips is generally indicated as the Neél relaxa-tion time.

The magnetization of a system can be directed through an externalmagnetic field. When an alternating magnetic field is used, the magne-tization is cycled between two opposite directions. If the alternating fre-quency is much faster than the Neél relaxation time, the magnetizationcurve displays a hysteresis loop, corresponding to the generation ofheat. This is the case of superparamagnetic nanoparticles exposed tohigh frequency alternating magnetic fields (HF-AMFs, frequency typi-cally higher than 50 kHz), producing the so-called hyperthermic effect.Due to this property, MagNPs are useful in many biomedical applica-tions [35,37,38], where the heat generated in their proximity by AMFsis either used to thermally ablate pathological cells [39] or to activatethe release of drugs from composite materials [40].

Thermo-responsive polymers display a critical temperature atwhich atransition between coil and globule conformations takes place, resultingfrom the interplay between intra- and inter-molecular forces [29,33].When the system is in the coil state, polymer chains are swollen anddrugs and biomolecules are easily loaded and trapped within the matrix.As the system is thermally switched to the globule state, the matrix col-lapses and the loaded molecules are rapidly released to the surroundingenvironment. Systems displaying a coil-to-globule transition are inten-sively used in the design of triggerable drug carriers, as they can be easilyloaded at room temperature and the release is simply activatedbyheatingthe system above the lower critical solution temperature (LCST).

Among the polymers displaying a LCST, poly(N-isopropylacrylamide)(PNIPAM) is by far the most used, as it is biocompatible, its structure canbe easily adjusted by introducing co-monomers and/or copolymers, and,mostly important, the critical temperature can be tuned to the humanbody temperature.

During the last decade the introduction of superparamagneticnanoparticles within PNIPAM-based gels has been successfully investi-gated in order to exploit the heating properties of MagNPs. The applica-tion of a HF-AMF converts the nanoparticles into nano-heaters, able toinduce the coil-to-globule transition and the ensuing release of drugsand biomolecules embedded within the gel matrix. Compared to bulkheating, the control of the temperature and temperature rise withinthe region surrounding MagNPs offers several advantages, especiallyin terms of remotely controlling the process in vivo, as well as for the si-multaneous application of chemotherapeutics and hyperthermia [41].

1.7. Magnetic nanocomposites: moving them around

Superparamagnetic nanoparticles dispersed in a fluid randomlymove and rotate due to Brownian motions. When a homogeneousmagnetic field is applied, magnetic nanoparticles rotate to align theirmagnetization to the external magnetic field. As the magnetic fieldis switched off, MagNPs return to the equilibrium, both through Neéland Brownian relaxation. The application of a low frequency alternatingmagnetic field (LF-AMF, frequency typically in the order of few kHz orless) to a dispersion of MagNPs in a fluid leads mostly to the rotationof the particles. Thesemotions have been exploited tomechanically dis-turb the region surrounding the nanoparticles, especially when they




were embedded within a viscous environment such as of a soft-mattersystem (liquid crystal, protein, vesicle, etc.). In a non-uniformmagneticfield, MagNPs are first rotated to align their magnetization and thenmoved along the magnetic force lines. In an alternating magnetic field,this corresponds to cyclic rotations and motions of the nanoparticles,eventually taking to a loss of order in the region surrounding them. Amuch simpler situation takes place in the case of a static non-uniformmagnetic field: in fact, MagNPs, once aligned, are collected towards thehighest magnetic field.

This principle has been extensively used in magnetic separation. Forinstance, magnetic polymer nanocomposites represent matchless toolsfor environmental remediation [4,12]. Thanks to their adsorbent prop-erties, porous polymermatrices have proven as oneof themost effectivematerials in the treatment of pollutedwaters. In particular, their surfacechemistry can be adjusted to selectively target different contaminants,especially heavy metals [12]. The introduction of MagNPs in the polymermatrix offers themajor advantage of easily separating the adsorbent fromwater under a magnetic gradient. Furthermore, this approach allows notonly thedesign and the tailoring of composites able to sequestrate and re-move specific compounds from water but also to easily regenerate theadsorbent itself, as the interactions between the pollutant and the com-posite are typically reversible.

Magnetic composites have also been applied as separation tools inthe field of analytical biosciences [42]. To this aim MagNPs have beenembedded in several supports, such as polymers, biomaterials, and silica,typically to formmagnetic beads with size ranging from tens of nanome-ters to fewmicrons. The surface of these composites can be functionalizedwith specific receptors (such as antibodies) to selectively tag analytes insolution, including proteins, peptides, enzymes and even cells. The appli-cation of amagnetic gradient allows for the separation of the tagged com-posite, making isolation and purification of biostructuresmuch easier andfaster than with conventional methods.

1.8. Magnetic nanoparticles in supramolecular systems

A very promising approach to the production of composites takesadvantage of supramolecular materials. These materials are built uponnon-covalent interactions, such as electrostatic forces, hydrogen bonding,π–π interactions, van derWaals forces, and hydrophobic interactions. Na-ture provides the brightest examples of supramolecular nanocomposites[43]: the hierarchical architecture of bones, cellmembranes, and seashellsare just few exampleswhere supramolecular interactions are exploited attheir best.

The common feature among these weak forces is their reversibilitythat introduces the major advantages of supramolecular materials, i.e.,their ability to self-assemble into the energetically most favored systemand to self-heal it when altered by external factors.

The tendency of lipids and phospholipid-based surfactants to self-assemble into bilayers makes them ideal building blocks for compositematerials of biomedical interest [44]. Such bilayers can, either spontane-ously or under the application of an external stress, close around anaqueous core to form liposomes. The compatibility of lipids and lipid-like surfactantswith biological environments, togetherwith their abilityto solubilize hydrophilic molecules in the aqueous compartment andhydrophobic molecules within the bilayer, makes them the ideal vectorsfor therapeutics scopes. In the same way, MagNPs can be accommodatedwith a proper functionalization of their surface inside the aqueous coreor within the membrane. Several magneto-responsive nanocompositeshave been developed through the combination of liposomes andMagNPs,generally referred to as magnetoliposomes [9]. In particular, externalmagnetic fields (static or alternating and at different frequencies) can beused to move around liposomes, to induce the formation of pores intheir bilayers and eventually to fully disrupt them.

Supramolecular interactions are of outmost importance also in theself-assembly of polymers. Block copolymers (i.e., polymers made outfrom different polymerized monomers) spontaneously form ordered


arrays extending over macroscopic lengths, resulting from the short-range phase separation of the blocks. As MagNPs can be chemicallymodified to display different affinities for the monomers, nanoparticlesspontaneously segregate within the preferred phase, resulting in theformation of 2D and 3D magnetic nanostructures [45]. In addition,small molecules can be combined with block copolymers and MagNPsto finely tune their interaction, resulting in a well-controlled and mag-netically triggerable architecture of the final composite [46].

2. Magnetic nanocomposite films, capsules and gels

In this section we selected case studies highlighting the progress inspecific fields that have developed following to the introduction ofmag-netic nanoparticles in composite materials. This review does not intendto provide a comprehensive picture of all the applications and technol-ogies that have been advantaged by the responsivity of MagNPs. In-stead, the following examples were chosen to extract some generalconcepts and to emphasize their importance when designing magneto-responsive nanocomposites.

2.1. Magnetic tapes: platelet nanoparticles revamp them

When thinking aboutmagnetic films, audio and videomagnetic tapesare probably thefirst application that comes to everybody'smind (maybe“digital natives”would disagree on that). In the framework of this review,magnetic tapes are discussed to highlight the importance ofmutual inter-actions betweenmagnetic nanoparticleswhen they are embeddedwithina polymer matrix.

Together withmagnetic stripes, tapes are still the most popular appli-cationwhere polymerfilms andmagnetic nanoparticles are combined to-gether. Although their use as audio and video recording devices has beennow overcome by optical disks and hard disk drives (HDDs), magnetictapes are still the technology of choice for the storage of large amountsof data, as they still represent the cheapest option [47].

Historically, plastic-based magnetic tapes were first developed dur-ing the '30s, but the first system to become popular was introduced byIBM only during the '50s [48]. At that time tapes consisted of celluloseacetate films coated by iron oxide. Over the years cellulose acetatewas replaced first by polyvinyl chloride and then by polyethylene tere-phthalate (PET), which is still in use. Iron oxide particles, responsible forthe storage of data, have to fulfill very specific requirements: they mustdisplay a switchable magnetic state (which remains stable upon ther-mal fluctuations), high coercivity, and high remanence. Nowadays, themost effective materials are maghemite particles (γ-Fe2O3, a ferrimag-netic spinel ferrite), few hundreds of nanometers in size, doped with1–5% cobalt to have improved coercivity and storage capacity [49,50].In order to increase the amount of stored data, the density of magneticparticles in the organic film should be as high as possible, but avoidinginteractions between the magnetization of adjacent particles. To thisaim, a possible approach is the use of smaller particles. For this reason,the advent of nanotechnologies and, in particular, the development ofmany syntheticmethods for the preparation of nanosizedmagnetic par-ticles opened up promising perspectives to magnetic tape recording.However, as the size of particles is decreased, the homogeneity oftheir size, their precise order within the matrix and, most of all, themagnitude of themagnetization all become critical factors for the safetyof the recorded data.

During the late 80s, HDDs and optical disks overcame magnetictapes, and, for many years, magnetic tape recording was relegated to aniche market, preventing it from big investments and from furtherdevelopments.

The low-cost of magnetic tapes, together with the strongly increas-ing demand for data storage, recently boosted a renewed interest intheir innovation, with the aim to bring their storage capacity to a com-petitive level with HDDs. In recent years a joint research between IBMand Fujifilm, among the world leaders inmagnetic storage and polymer




thin films respectively, developed an ultra-high density magnetic tapethrough the incorporation of barium ferrite nanoparticles in PET filmsand the development of a new recording head [51].

The key features of these nanoparticles are their very high magneticcoercivity (deriving from the crystalline anisotropy) and their hexago-nal platelet shape, whichmakes their magnetization very easy to be ori-ented. When co-extruded or deposited on a polymer film, the particletends to lie flat on the film because of their shape. As a result, the easyaxis of magnetization of the particles protrudes perpendicularly to thesurface of the film. Compared to the conventional tapes, this setup allowsfor a large increase of the amount of storeddata and stronger nanoparticlemagnetic fields. Furthermore, the shape of the particles and their planararrangement allowed for a significant decrease of the required thicknessof the magnetic tape [51].

We believe that this case study highlights a central issue in the de-velopment of magnetic nanocomposites. A huge variety of nanostruc-tured materials are nowadays available, as well as new technologies tohandle and build upon them. In this case, the collaboration between re-searchers from two industries, each leading his field, took to the smartintroduction of a specific nanomaterial (platelet-shaped barium ferritenanoparticles), which could revamp a surpassed material such as PET-based magnetic tapes.

2.2. Magneto-responsive Layer-by-Layer assemblies: towards multi-functional platforms

The Layer-by-Layer (LbL)method has been extensively employed toprepare a variety of polymer supramolecular assemblies. The generalconcept behind this technique is very simple, as it consists in the prep-aration of a multi-layered film through the sequential deposition of al-ternating layers of molecules, driven by their affinities. The techniquewas originally developed by taking advantage of electrostatic forces be-tween oppositely charged polyelectrolytes [52]. Later, hydrogen bond-ing [53], hydrophobic interactions [54], host–guest interactions [55],and covalent bonding [56] have also been exploited. The first LbL as-semblies were built upon planar substrates, but the process was soonextended to spherical particles, which act as a template for the forma-tion of a polymeric corona. In this latter case, the core is used as a “sac-rificial template”, as it is typically dissolved once the desired number oflayers is attained, generating a hollow polymeric capsule [57,58].

Since their introduction, polyelectrolyte LbL assemblies have founduse in many applications and their success is still increasing, as theyare versatile and powerful systems that can act as protective containersand/or carriers of functional agents. Moreover, due to their chemical na-ture, LbL systems are intrinsically responsive to many external stimuli:in fact, their assembly is affected by pH, solvent, ionic strength, temper-ature, ultrasounds, etc. In addition, the flexibility of their preparationmakes it very simple to incorporate colloidal objectswithin their assem-blies. This has been used to introduce responsivity to additional stimuli,such as light [59] and magnetic fields [60].

LbL-basedmagnetic nanocomposites have been recently used in thedesign of electronic devices. In particular, through the modification oftheir surface properties, MagNPs can be easily integrated within filmsto make these nanocomposites responsive and to combine their individ-ual properties [61]. For instance, smart electronic devices can be obtainedincorporating MagNPs in conductive polymers, producing bi-functionalmaterials that simultaneously display electrical conductivity and magne-tization [62]. On the other hand, the incorporation of MagNPs in insulat-ing polyelectrolytes produces materials with capacitive behavior that isstill unexplored in the fabrication of electronic circuits [63].

The most stimulating property of LbL capsules is their ability to loadmolecules and/or colloidal structures within their pool, eventually re-leasing them under an external stimulus. LbL capsules have recentlybeen proposed as active components in smart coatings, where they actas reservoirs of corrosion inhibitors [64]. The corrosion process induces


a change in the pH,which triggers the destabilization of the LbL shell ac-tivating the release of the inhibiting species includedwithin the capsule.

Most of the LbL capsules' applications are in the biomedical field[65,66]. Many biocompatible polymers, as well as bioactive molecules,have been used to prepare capsules of biomedical interest: polysaccha-rides [67], polypeptides [67,68], enzymes [69], nucleic acids [70], andlipids have all been employed. These objects are well tolerated by livingorganisms and can be effectively engineered to accomplish selectedfunctions (for example, to modulate the adhesion on cells [71]). Theycan also serve as protective carriers of pharmaceutical actives to deliverthem at specific sites.

For their effective administration, drugs need to be released on de-mand and at tunable rates [72,73]. Magnetic responsivity is very wellsuited to the release of drugs and biomolecules in biological systems:in fact, the human body is almost transparent to static and alternatingmagnetic fields. Moreover, they are innocuous for living organisms, atleast for limited exposure times. The formulation of MagNPs in LbL cap-sules generally takes place through the surface functionalization of theparticles, in order to make them compatible with the polymer shell[60,67]. The application of a HF-AMF heats up the MagNPs, eventuallyinducing the release of drugs contained in the internal pool. Dependingon the composition of the system, the activation of different releasemechanisms has been reported. In some formulations, the local heatinginduced byMagNPs causes the formation of pores in the shell of themi-crocapsules, followed by its rupture and the sudden release of the drug[60]. Other compositions take advantage of a phase transition of thepolymeric shell induced by the local heating [74], suggesting the possi-bility of a pulsed release.

Recently, Ai [75] pointed out that LbL capsules are ideal candidatesfor theranostics, i.e. to associate therapeutic and diagnostic agents in asingle platform. Thanks to the modularity and flexibility of the proce-dure to prepare these capsules, several drugs, probes, and triggers canbe easily introduced, as illustrated in Fig. 1. Powerful multifunctionalsystem can be obtained through this approach. For instance, the pres-ence of MagNPs, in addition to their heating properties, allows for themagnetic targeting of the composites and for its use as a probe for mag-netic resonance imaging (MagNPs arewell known contrast agents [76]).

2.3. Magnetoliposomes: surface chemistry of MagNPs modulates drugrelease

Beside the polymer LbL capsules, magnetoliposomes are another classof colloidal systems where the advantages of the magnetic responsivitycan be fully exploited, especially for biomedical applications [77–79].These structures originate from the combination of MagNPs and lipo-somes, artificial self-assembled shells of lipids. The capacity of liposomesto host both hydrophilic and hydrophobic species, in their internalaqueous pool [80] or within the lipid bilayer [81], respectively, hasbeen demonstrated. They are fully biocompatible, nontoxic, and canbe easily modified on their outer membrane to promote specific molec-ular targeting mechanisms: all these characteristics make liposomesamong themost promising systems for innovative therapeutic solutions[82]. The incorporation of MagNPs produces the benefits already indi-cated in the previous paragraph in the case of LbL systems. These struc-tures can be used as contrast agents for magnetic resonance imaging,due to the presence ofMagNPs [83], but their potential is fully exploitedin drug delivery applications. As in the polymeric capsules, the releaseof therapeutics from magnetoliposomes can be modulated accordingto the requirements of each specific application by applying a controlledalternating magnetic field. The application of HF-AMF is used to gener-ate a local heating (due to the hyperthermic effect) that enhances thepermeability of the bilayer and promotes the outflow of the encapsulat-ed drug [84]. The magnetically actuated release is effectively obtainedalso by applying a low frequency alternating magnetic field (LF-AMF).As described in the Section 1.6, the LF-AMF produces the rotationof the particles rather than the inversion of the magnetization. This



Fig. 1. Step-by-step template-assisted LbL self-assembly of multifunctional polyelectrolyte capsules. Step I: LbL self-assembly of polyelectrolytes onto hybrid templates. Step II: templatedecomposition. Step III: purification of polymer matrix containing polyelectrolyte capsules. Step IV: loading of therapeutic agents into the capsules. Step V: addition of imaging, targeting,and protection moieties into capsule systems.Reproduced with permission from [75], copyright Elsevier 2011.


introduces a mechanical disturbance in the magnetoliposomes at themembrane level, which enhances the release process. In drug deliveryapplications, the possibility to take control of the mechanism of thedrug release, in terms of rates and kinetics, is of upmost importance.The literature describes that the modification of the MagNP surface al-lows themodulation of the release process. A relevant example to high-light the possibility of controlling the release process by modifying thesurface chemistry of the MagNPs was reported in a series of papers(whose content is schematically illustrated in Fig. 2) where the authorsstudied the effect of changing the polarity of CoFe2O4 nanoparticleson the release of carboxyfluorescein from L-phosphatidylcholineunilamellar magnetoliposomes. When subjected to LF-AMF cycles, themagnetoliposomes, containing in their aqueous pool uncoated CoFe2O4

nanoparticles synthesized by a Massart-modified method [85,86], showa drug release characterized by a so called anomalous transport (predict-ed by the Ritger and Peppas equation), which dominates on the diffusiveprocess typical of the non-magnetically activated systems. Despite thefact that the release obtained for this formulation was effective, the load-ing efficiency was quite low. In order to increase the MagNP loading in-side the aqueous core of magnetoliposomes and to evaluate the effectof a hydrophilic modification of MagNPs, the particles have been func-tionalizedwith a citrate shell [87]. Thismodification induced a significantincrease of the loaded amount, which is a beneficial effect in terms of thepossibility to control their position in the body through the application ofa static magnetic field (magnetophoresis). Furthermore, the study of thekinetics of the release process showed that the citrate coated particlesproduced a slower release in the first hours after themagnetic treatment,as compared to that obtained with the uncoated particles. The character-ization of themagnetic properties showed that this behavior is due to thepropensity of the uncoatedMagNPs to aggregate. Upon the application ofLF-AMF, themotion of these aggregates produces a higher disturbance onthe lipid bilayer than the one obtained by the non-aggregated citrate


coated MagNPs. This results in a faster release during the first hoursafter the magnetic treatment. The citrate shell, thus, allows a more grad-ual administration of the drug. Finally, whenMagNPs are coatedwith hy-drophobic moieties, such as oleic acid, they were loaded in the bilayerrather than in the aqueous pool [88]. In this case, a peculiar kinetic behav-ior was found, as the leakage of the drug from themagnetoliposomes oc-curs in two steps: during thefirst hours a slow release occurs, followed bya faster release a fewhours after the field treatment, till the completion ofthe process. The investigation revealed that this behavior is due to the ini-tial formation of local pores or defects at themembrane level, followed bystructural changes in the bilayer that produce an increased permeabilityof the membrane and a faster release.

2.4. Magnetic hydrogels: a versatile scaffold

Surface modification of MagNPs has been used to functionalize andembed them within polymeric gels. In particular, hydrogels (i.e., a gelnetwork formed by hydrophilic polymer chains in water) are of interestto many applications [29], such as drug delivery [89], sensors [90], andtissue engineering [91]. The network of a gel commonly results from ei-ther physical (physical gels) or chemical interactions (chemical gels), oreventually from a combination of both of them. Acrylamide-based gelsprovide a typical example, as the gel network results from the chemicalbonds betweenmonomers and cross-linkers, as well as from the physi-cal interactions between the polymer chains.

The chemical inclusion, i.e., the formation of a chemical bond be-tween the surface of NPs and the gel matrix, offers several advantages,especially when the particles are engineered to respond to externalstimuli, such as electric and magnetic fields. In particular, chemicallyconnecting MagNPs to the polymer matrix generally allows for higheramounts to be embedded and ensures that the particles do not phaseseparate when subjected to external magnetic fields.



Fig. 2. The release of drug from magnetoliposomes can be modulated by modifying the surface chemistry of MagNPs.


The co-precipitation of iron oxides from water is by far the mostcommon method to prepare MagNPs [92]. The surface of the resultingparticles typically consists of hydroxyl groups bonded to iron atoms,making them reactive towards several functionalization strategies [35].For instance, iron oxide nanoparticles can be effectively coated with a sil-ica shell through a simple condensation reaction with various silicates[93,94]. Much more interesting in view of their chemical interactionwith polymers are the reactions that are known to occur between iron ox-ides and functional groups such as carboxylates, phosphates, and sulfates[95,96].

Alginate (i.e., a polysaccharide bearing one carboxyl groups permonomer) is known to bindMagNPs [35]. Recently, magnetic hydrogelsfor on-demand delivery of drugs and cells have been fabricated throughthe inclusion of Fe3O4 nanoparticles in alginate-based gel networks [97].In order to physically confine cells within the gel pores, peptides con-taining the arginine–glycine–aspartic acid (RGD) sequence were coupledto the alginate backbone. Different pore sizes and connectivity betweenthem were obtained through the lyophilization–rehydration procedure,resulting in pores ranging from few to hundreds of micrometers thatcould be used to host various drugs, biomolecules and cells. Under the ap-plication of non-uniformexternalmagneticfields, themagnetic hydrogelsundergo a large deformation, with a change in volume up to about 70%.Correspondingly, the water flow generated through the gel poresby the change in volume was shown to efficiently activate the re-lease process both in vitro and in vivo. The deformation of the gelsis reversible, suggesting an on-demand and reversible process, whichcan be repeated over multiple cycles. Furthermore, the release of smalldrugs over large biomolecules and cells can be achieved by tuning pep-tide density, the amount of embedded MagNPs, and the magnetic fieldintensity.


The reactivity of iron oxide nanoparticle surface towards carboxylateshas also been used to prepare acrylamide-basedmagnetic hydrogels [98].To this aim, a polyethylene glycol (PEG)-derivative was prepared by es-terification of PEG with maleic anhydride (MA). The reaction produceda linear polymer bearing carboxylic moieties at both endings (seeFig. 3a), allowing for the surface functionalization of CoFe2O4

nanoparticles. The reaction of PEG with MA also takes to the formationof double bonds, which were then exploited to chemically embedMagNP-PEG adducts into the gel network during the polymerizationof acrylamide and N,N′-methylene bis-acrylamide.

The resulting nanocomposite material combines the properties ofacrylamide-based hydrogels, such as porosity, viscoelasticity, and wateradsorption and retention, together with the responsivity of magneticnanoparticles. Themagnetic gel behavior reminds of a sponge, as differ-ent water-based formulations can be spontaneously absorbed withinthe gel network and eventually squeezed out either mechanically ormagnetically.

Thanks to their peculiar properties, nanomagnetic sponges wereshown to hold great potential in thefield of conservation of cultural her-itage [99,100]. Among the water-based formulations that could beuploaded, direct (i.e., oil-in-water)microemulsions are of especial inter-est for their detergency properties. In particular, since their first applica-tion to the field [101], microemulsions have beenwidely applied duringthe last decade to the cleaning ofworks of art [102]. Oilmicelles are veryeffective in the swelling and removal of polymer coatings that were in-opportunely used in the past to protect paintings and frescoes and thathave to be removed. Nanomagnetic sponges can be used as active car-riers of these formulations as they can be loaded simply by dippingthe gel into the microemulsion (Fig. 3b). Magnetic gels can be guidedthrough the application of an external magnetic field, applied to specific



Fig. 3. a) Scheme of the preparation of the PEG-MA derivative used to embed MagNPs within the gel network; b) sketch of the upload process of the magnetic nanosponge with amicroemulsion; c) photographic sequence of themagnetic recovery after the cleaning treatment of themicroemulsion-loadedmagnetic gel from the surface of a piece ofmarble previouslycoated with a polymer.Readapted with permission from reference [99]. Copyright American Chemical Society 2007.


areas and finally recovered and eventually washed and recycled (seeFig. 3c).

2.5. Magneto-responsive PNIPAM gel particles: magnetically triggeredrelease of drugs

Functional nanocomposites displaying reversibly switchable proper-ties are of special interest to biomedical applications. In particular, thethermo-responsive properties of PNIPAM-based gel particles haveattracted the interest of many research groups in the field of drug de-livery. As introduced in Section 1.6, the superparamagnetic propertiesof MagNPs can be used to induce the coil-to-globule transition ofPNIPAM gels under the application of a HF-AMF, resulting in the reduc-tion of the volume occupied by the gel and in the simultaneous releaseof the molecules sequestrated within the swollen polymer matrix (seeFig. 4).

Nanocomposites made of thermoresponsive polymers and MagNPsallow for the design of novel multi-modal cancer therapies. Onceuploaded into the compositematrix, the drug can bemagnetically guid-ed to the region of interest, followed by the application of a HF-AMFable to induce both the drug release and the hyperthermic treatment.This approach has been recently testedwith doxorubicin-loaded compos-ites prepared by polymerization of n-isopropylacrylamide monomer inthe presence of γ-Fe2O3 MagNPs, showing promising performancesboth in vitro and in vivo [103,104].

Recently a MagNP-PNIPAM-based hydrogel that is also injectableand biodegradable has been reported in the literature [105]. The hydro-gel is obtained in situ through the co-extrusion of two reactive water-based solutions through a double-barreled syringe [106]. One solutiontypically consists of a dispersion of PNIPAM-based oligomers functional-ized with hydrazine, while the other contains aldehyde functionalizedcarbohydrate polymers, such as hyaluronic acid, carboxymethyl cellulose,dextran, and methylcellulose [107]. In particular, the hydrazide-

Fig. 4. Sketch of themagnetic and thermal actuation of the drug release from a PNIPAMparticleand the consequent release of the drugs.


functionalized PNIPAM is able to spontaneously physisorbs onto thesurface of iron oxideMagNPs, easily embedding themwithin the hydro-gel matrix. Once the hydrazide- and aldehyde-functionalized systemsare mixed, the solutions turn into a hydrogel in few seconds thanks tothe formation of hydrazone cross-links. The process is summarized inFig. 5.

Through the variation of polymers and precursors, and the ratios be-tween them, hydrogels with different physical and chemical propertiesare obtained, making it finally possible to tune their drug loading andreleasing characteristics.

The resulting composites are of outmost interest in the biomedicalfield. The coupling of MagNPs and PNIPAM makes them triggerable, asdemonstrated through pulsatile, “on-demand” drug release activationupon the application of an AMF. The in situ preparation makes thesehydrogels injectable and well suited for subcutaneous injections. Final-ly, the materials used for the synthesis make the hydrogels inert andbiodegradable: in fact, the hydrazone cross-links hydrolytically degradeat physiological pH and temperature.

2.6. Magnetically triggered membranes

The combination of superparamagnetic nanoparticles andthermoresponsive polymers has also been exploited to design compositedevices, where on demand functions are activated through structuralchanges induced by an external magnetic field. In this framework, re-sponsive membranes are of particular interest as they are key compo-nents in a large number of applications where complex multi-functionalsystems are required, such as separation processes, sensors, and drug de-livery devices [6].

Due to its pH and temperature responsive properties, PNIPAM hasalso found use in composite membranes for separation and controlleddelivery [108]. PNIPAM particles are exploited within composite mem-branes according to two general approaches: they are used to generate

embeddingMagNPs. The coil-to-globule phase transition takes to a large change in volume



Fig. 5. Sketch of the preparation of an injectable formulation consisting of a magnetic thermo-responsive gel formed through hydrazone cross-links between aldehyde-functionalized car-bohydrates and hydrazine-functionalized MagNP-PNIPAM.Readapted with permissions from [105]. Copyright American Chemical Society.


pores during the preparation of the membrane and to fill them at theend of the synthesis or simply to fill the pores of an already existingmembrane. In both cases, the working principle of a PNIPAM-based re-sponsive membrane is the same as above outlined: upon an externalstimulus PNIPAM particles shrink, so that the pores are opened up andan increase of the permeability across the membrane is obtained (seeFig. 6).

Thermo-responsive membranes based on Nylon-6 (N6) wereeasily prepared employing this design [109]. The pores of a N6 mem-brane were filled through the free radical polymerization of N-isopropylacrylamide monomers. N6 provides mechanical and dimen-sional stability, as well as a porous pattern whose separation propertiesare well known. The hydrogel is responsible for the filling of the poresand, as a consequence, for the diffusional permeability across themem-brane and its temperature dependence. The flow of small molecules,such as vitamin B12, across the membrane could be controlled throughthe extent of the filling: i.e., larger PNIPAM/N6 ratios take to lower per-meabilities. Furthermore, as the temperature is increased, the diffusionacross the membrane reversibly and reproducibly increases as well.

Similar to PNIPAM-based systems for drug delivery, the inclusion ofMagNPswithin the hydrogel network allows, in addition to the thermo-responsivity, the magnetic actuation of the device. Composites consistingof ethyl cellulose membranes, Fe3O4 MagNPs, and PNIPAM have been

Fig. 6. Sketch illustrating the increase of permeation of a MagNPs-PNIPAM-based membrane utemperature.


recently designed to achieve on-demanddrug transport upon the applica-tion of a HF-AMF [110,111].

The effectiveness of magnetically actuated membranes in modulat-ing the permeability towards sodium fluorescein (used as a fluorescentmodel of a drug)was successfully demonstrated. The on–off temperatureof the membrane, i.e., the temperature at which the membrane perme-ability displays an abrupt variation, can be adjusted by tuning the chem-ical composition of the hydrogel. Pulsed drug release can be achieved byswitching on and off the external magnetic field, the amount of drug re-leased being proportional to the duration of the magnetic pulse. The per-meation kinetics can be tuned through the thickness of the membraneand the hydrogel density in it. Finally, molecules of various dimensions,as well as differently charged, can be controllably permeated throughthese biocompatible membranes.

3. Conclusions

The latest advances in the field of magnetic nanocomposites havebeen highlighted in this review, with a special focus onmagnetically re-sponsive films, capsules and gels. During the last decade compositema-terials have greatly benefitted from the progress of nanotechnologies.The availability of a broad range of nanomaterials and the developmentof new methods to make them compatible with host matrices opened

pon application of a HF-AMF or bulk heating above the PNIPAM coil-to-globule transition




up newperspectives formany applicationswhere specific requirementsneed to be matched.

The progressive replacement of petroleum-derived plastics withpolymer nanocomposites is a good example of the potentials of hybridmaterials. Many green and bio-polymers have been indicated as poten-tial substitutes, especially where biodegradability is an issue, such as inthe case of plastic films for packaging. In order to match the perfor-mances of conventional materials, a variety of nanosized fillers havesuccessfully been included within the host polymer.

The combination of different materials into a composite paved theway towards the design of multi-functional materials. In addition toact as a support, hostmaterials are often required to accomplish specificfunctions, either spontaneously or in response to an external stimulus.Responsive polymers and supramolecular assemblies, for instance, areamong the most frequently used hosts for the design of responsivemulti-functional nanocomposites. External stimuli can also bemediatedand amplified by the nanostructures embedded within the hybrid. Forinstance, the introduction of plasmonic nanoparticles within thermo-responsive polymer hydrogels allows for the triggered activation of athermal event, such as a glass transition or a swelling-shrinking process.

Among the nanostructures used as triggers, magnetic nanoparticlesoffer several advantages, especially in the biotechnology field. Nano-composites including magnetic nanoparticles allow for their magneticguidance and their remote actuation by using external static and/or alter-nating magnetic fields. In particular, magnetic gradients can be exploitedto guide magnetic nanocomposites to specific targets and to induce me-chanical changes. When exposed to alternating magnetic fields, thanksto their superparamagnetic properties, MagNPs can be used as nano-heaters, eventually able to hyperthermically ablate biological materialwithin their proximity or to trigger thermal events.

The responsive behavior of magnetic nanocomposites has beenexploited inmanydifferentmaterials. In this reviewwe selected fewex-amples to highlight the most general concepts that have recently in-spired the design of multi-functional magnetic nanocomposite films,capsules and gels.

i. Nanocomposite's architecture determines its properties. Embeddingplatelet-shapedmagnetic nanoparticleswithin polymer filmsmakesit possible to align magnetic dipoles perpendicularly to the film,allowing for a competitive amount of recorded data per unit areaand opening up renewed perspectives to magnetic tape recording.More generally, this case study highlights how the final propertiesof the composite result from its architecture, which could beprecise-ly induced by the combination of the shapes of both guest and hostmaterials.

ii. Magnetic nanoparticles modulate release from capsules. LbL cap-sules and liposomes are both effective platforms for the design ofmulti-functional carriers, especially for the delivery of drugs andbiomolecules. Depending on their surface functionalization, mag-netic nanoparticles can be included inside the aqueous pool, withinthe lipid bilayer or they can replace one of the LbL layers. Upon theapplication of alternatingmagnetic fields,MagNPs are able to inducea controlled increase in the permeability of the capsules throughtheir destabilization. By adjusting the frequency of the AMF andthe localization of theMagNPswithin the composite, the destabiliza-tion can be carefully tuned. Its extent ranges from the local weaken-ing of the intermolecular forces (taking to the dynamic formation ofpores at the membrane level) to the complete destruction of thecapsule.

iii. Magnetic gradients guide and mechanically deformmagnetic nano-composites. Once a composite is loadedwith a significant amount ofmagnetic nanoparticles, the hybrid materials can be guided by amagnetic gradient. This has a huge relevance in bio-applicationswhere therapeutic agents have to be targeted to specific regions ofthe body, as well as in magnetic separation processes. For instance,magnetically responsive hybrids can be designed to selectively


adsorb heavy metal ions from waste water, from which they aremagnetically separated and conveniently recycled. Magnetic nano-composites can also be engineered to mechanically deform underthe application of amagnetic gradient. To this aim, hydrogel embed-ding MagNPs are typically used. These materials, acting as sponges,are able to load significant amounts of water solutions and to squeezethem out when shrunk by the magnetic stimulus.

iv. Thermo-responsive polymers and magnetic nanoparticles combineinto multi-responsive nanocomposites. Hydrogels made of thermo-responsive polymers and MagNPs allow for the remote triggeringof the thermally-actuated transition of the polymer phase throughthe magnetic activation of the nanoparticles. In the typical case ofPNIPAM-based hydrogel, the application of a HF-AMF takes to themagnetic heating of the particles, which in their turn cause thephase transition of the network from a coil to a globule conforma-tion. This principle has been extensively employed in the designof magnetically-triggered drug delivery carriers, especially be-cause of the proximity between the PNIPAM phase transitiontemperature and the human body temperature.

v. Thermo-responsive magnetic nanocomposites modulate the per-meability of porous supports. The coil-to-globule (or the globule-to-coil) transition in thermo-responsive polymers is accompa-nied by a large decrease (increase) in volume. In addition to theload and release of water solutions, this process has also beenexploited for controlling the permeability of membranes. In particu-lar, the porous network of conventional membranes has been filledwith magnetic nanocomposites made of PNIPAM and MagNPs. Theapplication of a HF-AMF takes to the shrinkage of the hydrogel,with the subsequent opening of the pores and the increase of themembrane permeability.

A variety ofmagnetic nanocomposites have beendescribed through-out this review, and the concepts that inspired their design have beenhighlighted. Most of these materials were designed to answer specif-ic needs in existing applications. Nevertheless, their potential finallysurpassed the needs that drove their preparation. Thanks to theirresponsivity andmulti-functionality, magnetic nanocomposites are of in-terest to many sectors, ranging from bio-medical sciences to membranefiltration and magnetic separation. The inherent multidisciplinarity re-quires therefore a continuous dialog between all the academic and indus-trial partners to fully exploit the potentials of magnetic nanocompositematerials. In this framework,we believe that this reviewprovides a usefulsummary about themost recent advances in thefield, aswell as a valuableinput to its future development.

Acknowledgments

CSGI (Consorzio per lo Sviluppo dei Sistemi a Grande Interfase) is ac-knowledged for financial support.

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