Magnetic Resonance Imaging for Monitoring of MagneticPolyelectrolyte Capsule In Vivo Delivery

Qiangying Yi & Danyang Li & Bingbing Lin &

Anton M. Pavlov & Dong Luo & Qiyong Gong &

Bin Song & Hua Ai & Gleb B. Sukhorukov

Published online: 10 December 2013# Springer Science+Business Media New York 2013

Abstract Layer-by-layer (LbL) assembled polyelectrolytecapsules have been widely studied as promising deliverysystems due to their well-controlled architectures. Althoughtheir potential applications in vitro have been widely investi-gated, at present, it is still a challenging task to track their real-time delivery in vivo, where and how they would be locatedfollowing their administration. In this work, the noninvasivemagnetic resonance imaging (MRI) technique was applied tomonitor the delivery of polyelectrolyte capsules in vivo, in-corporating magnetite nanoparticles as imaging components.First, MRI scan was performed over 6 h after sample admin-istration at the magnetic field of 3.0 T; magnetic capsules, bothpoly(allylamine hydrochloride)/poly(styrenesulfonate sodiumsalt)-based and poly-L-arginine hydrochloride/dextran sulfate(Parg/DS)-based, were detected mostly in the liver region,where the transverse relaxation time (T2) was shortened andhypointense images were visualized, demonstrating acontrast-enhanced MRI effect between liver and adjacenttissue. A continuous MRI scan found that the contrast-enhanced MRI effect can last up to 30 h; in the mean time,

the Parg/DS-based capsules with smaller diameter were foundto have a pronounced clearance effect, which resulted in aweakened MRI effect in the liver. No obvious toxicity wasfound in animal studies, and all mice survived after MRIscans. Histology study provided evidences to support theMRI results, and also revealed the destination of these mag-netic capsules over 30 h after administration.

Keywords Magnetite . Capsule . MRI . Liver . Spleen

1 Introduction

Layer-by-layer (LbL) self-assembled polyelectrolyte capsuleshave been widely studied over the past few decades and havebeen developed as potential delivery systems for variousapplications [1, 2]. Driven by electrostatic interactions ofdesired building blocks, LbL capsules with precise controlledmultilayer architecture and properties can be easily obtained[3, 4]. Promisingly, these fabricated capsules could engineernumerous solutions to meet the diverse requirements in thefield of medical and pharmaceutical applications. These shell-like formations serve as steady and efficient carriers for load-ing of cargo substances with varied molecular weights,shapes, and types [5]. Moreover, the stepwise polymer depo-sit ion procedure facili tates the modification andfunctionalization of the capsule formations, which furtherallow the release of the encapsulated substances at targetedsites in a controlled manner [6, 7].

Prior to practical applications in vivo, such as drug deliv-ery, tracking, and diagnosis, basic considerations of fabricatedcapsules with biosafety and efficacy in organ or tissue need tobe well investigated. In vitro biological evaluations helpedresearchers narrow the choices of biocompatible subjects forfurther in vivo use. Recent contributions of the in vitro

The authors Q. Yi and D. Li contributed equally to this work.

Q. Yi :A. M. Pavlov :G. B. Sukhorukov (*)School of Engineering and Materials Science, Queen MaryUniversity of London, Mile End Road, London E1 4NS, UKe-mail: g.sukhorukov@qmul.ac.uk

D. Li :B. Lin :D. Luo :H. Ai (*)National Engineering Research Centre for Biomaterials,Sichuan University, 29 Wangjiang Road, Chengdu 610064,People’s Republic of Chinae-mail: huaai@scu.edu.cn

Q. Gong : B. Song :H. AiDepartment of Radiology, West China Hospital, Sichuan University,37 Guoxuexiang Road, Chengdu 610041,People’s Republic of China

BioNanoSci. (2014) 4:59–70DOI 10.1007/s12668-013-0117-2

research on polyelectrolyte capsules for delivery uses, com-prising of important features such as biostability, biocompat-ibility, intracellular fate, etc., have been well studied andsummarized [3, 8, 9]. However, the most essential work hereshould be emphasized on the investigation of the delivery,how and where these capsules would be transported in realphysiological and pathological condition in vivo.Unfortunately, there are very few research works concerningthe real-time detection of these capsules in vivo, due to thedifficulty to realize continuous monitoring of them in organsor tissues. As a powerful and reliable tool developed in(bio-)medical imaging use, magnetic resonance imaging(MRI) allows the real-time visualization of living organismsand related interactions at molecular or cellular level [10, 11].Specially, MRI offers the available technique here to monitorthe capsule delivery in vivo, with the help of possible imagingcomponents. Considering the well-controlled and uniquestructures of the capsule shells, iron oxide-based magnetic(e.g., magnetite) nanoparticles should be one series of optimalimaging components. Moreover, the magnetite nanoparticlesare easy to be introduced into capsule shells or cavities ascharged components after a simple modification process [12].These magnetite nanoparticles are able to dramatically shortenthe transverse relaxation time (T2) in a mononuclear phago-cyte system (e.g., liver, spleen) and thus provide decreasedsignal intensity in a T2-wieghted image [13]. For example,superparamagnetic iron oxide (SPIO) nanoparticle possessingextremely high T2 relaxivity has made essential contributionto liver MRI imaging and drug delivery tracking [14], benefit-ing from its selective uptake by Kupffer cells in liver, spleen,and bone marrow [15]. Furthermore, these magnetite nano-particles behave quite biosafe for in vivo application, becauseof the low administration amount as well as their degradationproducts would participate in the iron metabolism of thehuman body [16, 17].

Design of delivery vesicles for potential use in vivo re-quires considerations of many parameters, such as size andstability. Besides, the featured surface with functional siteswould also affect the in vivo delivery as well as potentialfunctionalities of fabricated delivery vesicles. For example,biocompatible building blocks could be used to eliminate orreduce possible biotoxicity to minimum level [8]; hydrophilicsegment, for instance the polyethylene glycol (PEG), could beused to repel protein absorption and to ensure a longer circu-lation time in bloodstream [18]; and unique functional groups/components could be used to achieve potential detection,controlled delivery, as well as site-specific manipulation[19]. Consequently, these parameters should be addressed asimportant considerations for the fabrication and delivery ofLbL polyelectrolyte capsule systems.

In this work, we cope with the challenges and report on themonitoring of the magnetic polyelectrolyte capsules in vivo.Generally, incorporating the magnetite nanoparticles as

imaging probes, typical organs or tissues with deposited cap-sules could be visualized as hypointense signal under a 3-Tclinical magnetic field. Comparing the continuousMRI scans,in vivo capsule delivery path and preservation duration couldbe estimated qualitatively. Practically, related informationcould provide useful data to estimate working time windowof the capsule systems for potential applications. Generally,five types of capsule systems with different shell structureswere studied as typical examples here. Besides the specificMRI signal intensity that was influenced by possible capsulearchitecture as well as accumulation, the distribution of thesemagnetic capsules over a certain period after administrationwas verified by histology studies.

2 Materials and Methods

2.1 Materials

Poly(L-lysine)20 kDa-graft -poly(ethylene glycol)2 kDa (PLL-PEG) was purchased from SuSoS AG. Poly(allylamine hy-drochloride) (PAH, 70 kDa), poly(styrenesulfonate) (PSS)sodium salt (70 kDa), poly-L-arginine (Parg) hydrochloride(15–17 kDa), dextran sulfate (DS) sodium salt (∼100 kDa),iron(II) chloride (FeCl2), iron(III) chloride (FeCl3), ammoni-um hydroxide solution (NH4OH, 28 wt%), citric acid, ethyl-enediaminetetraacetic acid (EDTA), and other chemicals werepurchased from Sigma-Aldrich.

2.2 Methods

2.2.1 Magnetite Nanoparticle Preparation

Superparamagnetic (Fe3O4) nanoparticles were synthesizedaccording to the previous well-knownMassart coprecipitationmethod and stabilized by modification of the particle surfacewith citric acid, as described by Minko and coworkers [12].After dialysis against water, magnetic nanoparticles withadsorbed citric acid layers were stabilized in water. The fresh-ly prepared nanoparticles were characterized by using a trans-mission electron microscope (TEM, JEOL 2010) operating at200 kV and dynamic light scattering (DLS) technique withMalvern Zetasizer Nano ZS (Malvern Instruments Ltd.).

2.2.2 Magnetic Capsule Fabrication

For capsule preparation, polyelectrolytes were alternativelydeposited on prepared CaCO3 microparticles (∼3 μm, thesame batch) by using LbL assembly technique [20].Particularly, formed superparamagnetic nanoparticles wereintroduced as the negatively charged layers for fabrication ofmagnetic capsules [21]. Adsorption of the magnetite nanopar-ticles occurred by immersion of the prepared CaCO3

60 BioNanoSci. (2014) 4:59–70

microparticles in diluted water suspension of the nanoparticlesin a ratio of 1:50. In the meantime, capsules without magneticnanoparticles were also prepared as controls. Basically, twodifferent multilayer systems, PAH/PSS-based and Parg/DS-based, were adsorbed on the templates. After core removalwith treatment of 0.2M EDTA, hollow capsules with differentshell structures were obtained. Prior to introducing capsules toin vivo tests, the capsules were autoclaved at 121 °C for30 min to sterilize and shrink them [22]. Diluted microcap-sules were coated with gold and observed under a scanningelectron microscope (SEM, FEI Inspect-F) with an accelerat-ing voltage of 10 kVand spot size of 3.5 at a working distanceof approximately 10 mm. Capsule size distribution wasexpressed as mean ± SD of at least 50 capsules per sampleof randommeasurement of SEM images by using the softwareImage-Pro Plus v 6.0.

2.2.3 In Vivo MRI Studies

Before an MRI effect study, the capsule samples were con-centrated, resulting in an Fe concentration of 1 mg/ml of allthe magnetite-containing capsule suspensions, which was de-termined by atomic absorption spectroscopy (AAS) (AA800,PerkinElmer, USA).

All the tests involving animals were carried out in theNational Engineering Research Centre for Biomaterials andWest China Hospital (Sichuan University, China). All animalworks were performed under guidelines determined by theinstitutional committees for animal welfare and use of humansubjects. In vivo MRI effect of all the capsule samples wasstudied by using a 3-T MRI imaging system (Philips MedicalSystem) incorporating a mouse coil (Philips) for transmissionand reception of the signal. Commercial Philips clinical se-quences T2-weighted spin echo (T2W SE) was used to collectcorresponding information with the following parameters:repetition time (TR)=762 ms, TS=100 ms, FOV=40 mm×40 mm, and slice thickness=1 mm.

BALB/c mice (20∼25 g) were anesthetized with 150 μl of1 % pentobarbital sodium through intraperitoneal injection.Through intravenous injection in the tail, 50 μl of capsulesuspensions with/without magnetic nanoparticles was adminis-trated. Continuous MRI scans were performed 6 and 30 h aftersample injection. In the meantime, a healthy BALB/c mousewithout sample injection was studied as the negative control.

2.2.4 Histology Analysis

After the second MRI scan, mice were sacrificed, and liverand spleen tissue were removed and fixed with 4 % parafor-maldehyde before histology analysis. Then, the fixed tissueswere embedded in paraffin and cut into slides of 5 μm.Adjacent slides were prepared for histological analysis usingPrussian blue staining (iron staining).

3 Results and Discussion

3.1 Magnetic Capsules

Coprecipitating ferric (Fe3+) and ferrous (Fe2+) ions in aque-ous solution at high pH allows the simplest approach togenerate superparamagnetic nanoparticles [12]. Figure 1shows the characterization of the prepared magnetite nano-particles. These nanoparticles were polydispersed as present-ed in the TEM image; most of them were found to be lessthan 20 nm in diameter (Fig. 1a) [23]. The DLS data illus-trated that these particles has an average size of 48.5 nm inwater, with distribution ranging from 32.7 to 91.3 nm(Fig. 1b). This difference obtained from two measurementscould be explained as the aggregates of magnetite nanoparti-cles in water. However, comparing with the microscaledcapsule formations, such magnetic particle aggregates wouldhave negligible influence on capsule fabrication. On the otherhand, such aggregates of magnetite naonoparticles have beenfound to have abilities to greatly improve T2 relaxivityover single ones [24]. These magnetite nanoparticles werenegatively charged due to the existence of citric acidcoating on the surfaces. Their zeta potential was measuredto be −25.6 mV in water (Fig. 1c), which made them goodbuilding blocks as the negatively charged layers for LbLcapsule fabrication.

After LbL assembly and template removal process, hollowcapsules with different shell structures were obtained, asshown in Table 1. Samples #1 to #5 referred to these capsuleswith magnetite nanoparticles in their shells, while the samplesC1 and C2 represented the PAH/PSS and Parg/DS capsuleswithout magnetite, respectively. Mainly, these capsules couldbe classified into two types: PAH/PSS-based and Parg/DS-based capsules. The former is the most commonly studiedsynthetic capsules in many research works [25, 26]; the latterhas been investigated as a biocompatible system in manycases [27–29]. The capsule samples size was about 3 μm indiameter after fabrication, as shown in Table 1 and Fig. 2. Forall the magnetic capsules, much rougher surfaces were ob-served due to the existence of magnetite nanoparticles in theirshells.

Size control of the candidate delivery systems/vehicles isnecessary for their in vivo studies; a longer circulation time inbloodstream usually requires smaller vehicle size [30]. Typicalexamples have been demonstrated in related research works,where nanoscaled (<200 nm) particles were needed for spleenclearance and macroscaled (20 μm) particles were preferredfor pulmonary drug delivery [30, 31]. In this study, effortswere devoted to monitor possible in vivo delivery of thesepolyelectrolyte capsules by using MRI; thus, magnetic cap-sules with relatively smaller size, for instance 1 μm or less,were favored. In order to make such small capsules withoutobvious aggregations, autoclaving (121 °C, 30 min) was

BioNanoSci. (2014) 4:59–70 61

therefore applied to shrink them due to possible heat-inducedrearrangement of polyelectrolytes [32, 33]. Such heat treat-ment was found to have a great influence on the capsulemorphologies, which was demonstrated in the observed de-crease in size and in the changes in surface roughness. Inparticular, a heat-shrinking effect was found more obvious inthe Parg/DS-based capsules (#4 and #5) than that of the PAH/PSS-based ones (#1, #2, and #3) (Table 1). It was obvious thatthe former two were shrunk approximately to be 1 μm indiameter and observed as separated dense particle-like forma-tions. The latter three still kept their size to about 2 μm, andsome of these PAH/PSS-based capsules were found relativelyflat and slightly aggregated under SEM observation (Fig. 3).For the capsules without any magnetite nanoparticles in theshells, autoclaving had a significant influence on their struc-ture, as most of themwere shrunk to less than 1 μmwithmuchsmoother surfaces. It is worth mentioning that a heat-inducedfusion effect was observed in the pure polyelectrolyte capsulesuspension. Heating at 121 °C led to formations of muchpolydisperse (PAH/PSS)3 capsules. As shown in Fig. 3 (sam-ple C1), besides the capsules that completely shrunk to denseparticles, big particles with irregular shapes (e.g., dumbbell-like) and with size of up to 5 μm, which was considerablylarger and denser than the initial capsules, were observed. Thisphenomenon could be explained as heat-induced shell defor-mation and following rearrangement of two or more contactedcapsules [34].

3.2 In Vivo MRI

Synthesized magnetite (Fe3O4) nanoparticles, especially thesuperparamagnetic ones, have been reported as promisingMRI contrast agents for in vivo imaging [35]. Theoretically,these magnetite nanoparticles can substantially shorten the T2relaxation time and hence enhance image contrast betweeninterested tissue and adjacent area, illustrating as hypointensesignals (darkness) at targeted locations [36]. Consequently,these magnetite nanoparticles have been studied widely asuseful candidates for various imaging applications in humandiseases and animal models [37]. By controlling over theaggregation state of these magnetite nanoparticles into theircarriers, the corresponding T2 relativity could be improved.With the help of magnetite nanoparticles, many researchworks have been done to realize the visualization of deliveryvesicles in their form of micelles [38], liposome [39], andpolymeric coatings [40, 41] under MRI observation.

Here, the authors proposed the application ofMRI to detectthe in vivo delivery of polyelectrolyte capsules. In order toavoid possible aggregation of such magnetic capsules undermagnetic field and to get a better circulation in whole blood-stream, the first MRI scan was performed over several hoursafter sample administration. Typically, an example was dem-onstrated here, of which the capsule samples were injectedinto mice 6 h before observation. The signal obtained fromT2W SE sequence scan presented a series of MR images

Fig. 1 Characterization of synthesizedmagnetite nanoparticles: TEM image (a), DLS data of particle size distribution (b), and zeta potential (c) in water

Table 1 Description of fabricat-ed microcapsule samples Samples Shell structures Size change due to autoclaving (μm)

Before After

#1 PAH/PSS/PAH/Fe3O4/PSS/PAH/PSS 3.09±0.680 2.22±0.373

#2 PAH/PSS/PAH/Fe3O4/PSS/PAH/DS 2.99±0.668 2.01±0.392

#3 PAH/PSS/PAH/Fe3O4/PSS/PAH/PLL-PEG 3.02±0.579 2.14±0.361

#4 Parg/DS/Parg/Fe3O4/DS/Parg/DS 3.04±0.482 1.06±0.183

#5 Parg/DS/Parg/Fe3O4/DS/Parg/PLL-PEG 2.98±0.624 1.07±0.190

C1 (PAH/PSS)3 2.94±0.473 1.44±0.811

C2 (Parg/DS)3 2.93±0.516 0.70±0.163

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Fig. 2 SEM images of fabricatedcapsules. Images a to g presentedthe capsules from #1 to C2,respectively

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Fig. 3 SEM images of fabricatedcapsules after autoclaving.Images a to g presented thecapsules #1 to C2, respectively

64 BioNanoSci. (2014) 4:59–70

captured in certain duration (or TR). Under the same scanningcondition, acquired images could be comparable to judge theMRI effect of corresponding capsules. As shown in Figs. 4and 5, the MR images detected at the same echo time (TE=30ms) were discussed as clear examples. During all the scans,a water tube was placed near the tested mice as a signalintensity reference, which could be seen as the big bright dotson the right up corner. Accumulation of the efficient imagingprobes, for example the magnetic capsules discussed here,generates hypointense signals (darkness) at local regions.

Liver plays a major role in metabolism and has a number offunctions in the body. The mouse liver consisting of four lobesis the prominent upper abdominal organ [42]. MRI scan of themouse liver gave us brief information of the capsule deliveryin this area. As shown in Fig. 4, it was clear that the magneticcapsules containing magnetite nanoparticles in their shells(samples #1∼#5) were detected by MRI, as they exhibited astrong effect on liver contrast enhancement over 6 h afteradministration, demonstrating as dark image in liver tissue,and clear signal difference when compared with the brightgallbladder containing bile. This result revealed the fact that

the capsules, at least some of them, have been delivered toliver over 6 h after administration. In comparison, withoutmagnetic nanoparticle, either for the two pure polymeric sam-ples (C1 and C2) or the sample without injection (B), noobvious MRI signal intensity decrease in liver region couldbe observed either before or after injection.

A second scan was performed over 30 h after sampleinjection, in order to achieve a continuous study on the mon-itoring of the capsule delivery in vivo, as shown in Fig. 5. Forall the magnetic capsule samples, it was quite clear that therewas a decrease in MRI signal intensity, which could be visu-alized as slightly brighter liver tissue directly. Possible reasonshould be attributed to iron metabolism in the mouse’s body,transporting the magnetic capsules to other organs throughreticuloendothelial (RES) system. However, comparing withthe other laboratory-studied MRI contrast agents, e.g., man-ganese ferrite nanoparticle-loaded micelles [38], these mag-netic capsules in our work possessed longer time windows foreffective MR imaging, especially for these PAH/PSS-basedmagnetic capsules. For the two Parg/DS-based capsule sam-ples (#4 and #5), the signal intensity decrease of the liver

Fig. 4 BALB/c mice liverT2-weightedmap images over 6 hafter administration of differentsamples. The right bottom imageshowed the cross section ofmouse anatomy and possiblecapsule distribution schematic

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region was not that significant; one possible reason could bethat the capsules are more biodegradable than the PAH/PSSformulations, and this led to magnetite particle dissociationand resulted in lower T2 effects.

3.3 Histology Analysis

By staining the tissue slides with Prussian blue, the capsulesamples containing magnetite nanoparticles in liver andspleen tissues could be visualized directly. As shown inFigs. 6 and 7, the histology images of adjacent liver and spleentissue slides stained with Prussian blue were demonstrated. Asignificant difference between the samples with and withoutmagnetite nanoparticles can be easily noticed. That is, for themice injected with magnetic capsules, blue dots can be foundin the liver and spleen tissue slides, illustrating the existence ofmagnetite nanoparticles in tissue. However, not a single bluedot can be found in the other three slides, to which no mag-netite was introduced.

Generally, the administered magnetic capsules were foundto be randomly distributed in liver. Specially, it was clear fromFig. 6, for the Parg/DS-based capsules (#4 and #5), that a quite

small amount of blue dots can be found in liver (as pointed outby the arrows), which was in accord with the results from theMRI scan performed over 30 h after sample injection. Thisdemonstrated a fast removal effect of these capsule samples inliver and loweredMRI contrast effect, which was possibly dueto dissociated magnetites that can be easily taken up bysurrounding cells for iron reutilization. While these PAH/PSS-based magnetic capsules (#1∼#3) showedmore blue dotsin liver, these blue dots were observed as small aggregates ofthe magnetic capsules at high magnification (×630). Most ofthese aggregates should be formed in the process of autoclav-ing, as shown in Figs. 2 and 3. After the heat treatment, thePAH/PSS-based capsules exhibited a limited decrease in sizedecrease and tended to form small aggregates, whereas theParg/DS-based capsules decreased their size significantly todense particles. Besides biodegradability, larger MRI agents(or aggregates of such agents) sometimes might have diffi-cultly to be removed from liver cells, although they wouldprovide strong MRI signals and long time window forobservation.

Different from the smaller amount of Parg/DS-based mag-netic capsules found in liver tissue, the blue dots representing

Fig. 5 BALB/c mice liverT2-weighted map images over30 h after administration ofdifferent samples. The rightbottom image showed the crosssection of mouse anatomy andpossible capsule distributionchange schematic

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magnetic capsules were predominately located in spleen withfeatured distribution over 30 h after sample injection, whichwas even observed at low magnification. As shown in Fig. 7,for all the five samples, most of the magnetic capsules werefrequently found in red pulp (RP, R2) and only a smallamount were found in the white pulp (WP, R1). The mainreason could be attributed to the uptake and clearance effectof the splenic macrophages in red pulp and phagocytes inmarginal zone [30].

Surface characteristic is another key factor which couldgreatly influence the clearance and tissue distribution of theadministered contrast agents [30]. For they circulate for a longtime in the body, the desired delivery vesicles should beresistant to protein absorption and should thus avoid clearanceby the RES system. To achieve the goal, PEGylation on thecarrier surfaces is commonly used as a utility strategy [43, 44],by providing a dynamic hydrophilic layer at the carrier surfaceto increase plasma half-life [45]. A modified capsule surfacewith a noncharged hydrophilic outmost layer was hence pre-pared by introducing the PLL-PEG into the capsule multilayer

shells. Comparably, the other two polyelectrolytes DS andPSS were also used for capsule fabrication as controls. Thepositively charged PLL segment would provide charges forpolymer deposition, and the biocompatible small molecularPEG segment (below 2.5 kDa) would benefit a prolongedcirculation in bloodstream without adsorption of plasma pro-teins [46]. However, no obvious difference of the capsuledistribution in liver or spleen, which might be attributed tothe difference of the outermost layer, could be found amongthese five types of magnetic capsules. Besides, a possiblesignificant variation of the MRI effect caused by the capsulematrix was also found negative, as no obvious differencecould be found from the MRI images or tissue slides betweenthe two PAH/PSS-based and Parg/DS-based capsule sys-tems(#3 vs. # 1 and #2, #5 vs. # 4). Possible explanationmight be attributed to the influence of the relatively largecapsule size, which would weaken the potential effect ofsurface coating. Emphasis of our further work will be focusedon the investigation of smaller (e.g., nanoscaled) polyelectro-lyte capsules with varied surface layers.

Fig. 6 Histology images of liver tissue slides stained with Prussian blue. The images were demonstrated as overview (×200, left) and magnified view ofinterested region R (×630, right)

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Fig. 7 Histology images ofspleen tissue slides stained withPrussian blue. The images weredemonstrated as overview(×200, left) and magnified viewof interested regions R1(×630, middle) and R2(×630, right)

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4 Conclusions

LbL polyelectrolyte capsules containing superparamagneticnanoparticles were fabricated, and their delivery in vivo wasmonitored by MRI. Over 6 h after administration, these mag-netic capsules were detected in liver, where they exhibited astrong contrast-enhanced MRI effect. Continuous study onMRI effect up to 30 h demonstrated a long preservationduration in liver of these magnetic capsules, especially thePAH/PSS-based ones. Histology analysis revealed a relativelyfaster clearance of Parg/DS magnetic capsules than the PAH/PSS ones in liver, and a mass of magnetic capsules werepresented in spleen over 30 h after sample administration. Inour work, these magnetic capsules did not present obviousacute toxicity in vivo.

As a preliminary study, the LbL polyelectrolyte magneticcapsules described here demonstrated a good MRI effect inliver, offering a simple but efficient method to engineer con-trast agents for animal studies, where detection, diagnosis,and, likely, therapy of liver lesions and diseases could beapplied. Furthermore, quantification of the mass of magnetitenanoparticles deposited in liver and spleen will be investigatedin our future work. Besides, the distribution of the magneticcapsules in other organs will also be studied.

Acknowledgments The authors thank the Radiology Department ofWest China Hospital (Sichuan University, China) for the support on theMRI measurement. The authors also acknowledge the National NaturalScience Foundation of China (NSFC 51173117) and National Key BasicResearch Program of China (2013CB933903) for the financial support.This researchwas supported by an EPSRC “Global Engagement” grant toestablish research links between Queen Mary University of London, andSichuan University.

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