X-ray Visible and Uniform Alginate Microspheres Loaded with in Situ

Mar 2, 2015 - National Engineering Research Center for Nanomedicine, Huazhong. University of Science and ... also for real-time in situ tracking of em...
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X‑ray Visible and Uniform Alginate Microspheres Loaded with in Situ Synthesized BaSO4 Nanoparticles for in Vivo Transcatheter Arterial Embolization Qin Wang,⊥,† Kun Qian,⊥,§ Shanshan Liu,† Yajiang Yang,*,† Bin Liang,§ Chuansheng Zheng,§ Xiangliang Yang,‡ Huibi Xu,‡ and Amy Q. Shen*,∥ †

School of Chemistry and Chemical Engineering, and ‡National Engineering Research Center for Nanomedicine, Huazhong University of Science and Technology, Wuhan 430074, China § Department of Radiology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China ∥ Micro/Bio/Nanofluidics Unit, Okinawa Institute of Science and Technology Graduate University, Japan, Mechanical Engineering, University of Washington, Seattle 98195, United States S Supporting Information *

ABSTRACT: The lack of noninvasive tracking and mapping the fate of embolic agents has restricted the development and further applications of the transcatheter arterial embolization (TAE) therapy. In this work, inherent radiopaque embolic material, barium alginate (ALG) microspheres loaded with in situ synthesized BaSO4 (denoted as BaSO4/ALG microspheres), have been synthesized by a one-step droplet microfluidic technique. One of the advantages of our microfluidic approach is that radiopaque BaSO4 is in the form of nanoparticles and well dispersed inside ALG microspheres, thereby greatly enhancing the imaging quality. The crystal structure of in situ synthesized BaSO4 nanoparticles in ALG microspheres is confirmed by X-ray diffraction analysis. Results of in vitro and in vivo assays from digital subtraction angiography and computed tomography scans demonstrate that BaSO4/ALG microspheres possess excellent visibility under Xray. Histopathological analysis verifies that the embolic efficacy of BaSO4/ALG microspheres is similar to that of commercially available alginate microsphere embolic agents. Furthermore, the visibility of radiopaque BaSO4/ALG microspheres under X-ray promises the direct detection of the embolic efficiency and position of embolic microspheres after embolism, which offers great promises in direct real-time in vivo investigations for TAE.

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INTRODUCTION Interventional therapy has been widely employed in clinical practices as a complementary method with medicine and surgical treatments. Transcatheter arterial embolization (TAE) or transcatheter arterial chemoembolization (TACE) is a typical noninvasive vascular interventional therapy for treatments of unresectable tumors,1 intracranial aneurysms,2 and arteriovenous malformations.3 Notably, TAE has become the preferred choice for advanced liver cancer patients. In the treatment of TAE, the embolic materials are injected to the target site through a microcatheter guided by an accurate radiographical instrument (i.e., X-ray modalities), facilitating artery occlusion to prevent nutrient supplies to the tumor and necrosis of the tumor. Thereby, X-ray radiopacity in embolic materials is critical not only for controlled target delivery but also for real-time in situ tracking of embolic materials.4−6 Most embolic materials used for TAE lack inherent radiopacity.7 For instance, poly(vinyl alcohol) (PVA) microparticles, gelatin sponges, and alginate microspheres have been routinely used as embolic materials.7−10 Their radiopacity is © 2015 American Chemical Society

usually obtained through physically mixing with iodine-based contrast agents such as iohexol8 or heavy metal salts such as barium sulfate (BaSO4).2,11 The main disadvantage of this method is that the contrast agent can easily dissociate or leach from the host matrices, not only leading to fuzzy imaging and misdiagnosis but also causing systemic toxicity.12 Therefore, the lack of inherent radiopaque embolic agents has severely hindered the development of TAE. To synthesize inherent radiopaque embolic materials, a common method involves the copolymerization of acrylate and comonomers containing iodine.13−16 However, the preparation of such copolymers is rather complicated and labor intensive. For example, the copolymeric monomer containing iodine (2-(4′-iodobenzoyl)oxo-ethyl methacrylate (4IEMA)) needs to be initially prepared from 4-iodobenzoyl chloride and distilled hydroxyethyl methacrylate in an esterification reaction, followed by freeReceived: January 8, 2015 Revised: February 27, 2015 Published: March 2, 2015 1240

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Biomacromolecules radical polymerizations.13−16 Another method to achieve radiopacity involves encapsulation of heavy metal salts in polymeric microspheres.17−20 For instance, the heavy metal salt BaSO4, widely employed as an X-ray contrast agent in the gastrointestinal tract, has been encapsulated in the polymeric matrix and applied to implants,17 embolization,18−20 and immunoprotection.21 Although heavy metal salts themselves exhibit good imaging quality and are relatively safe,22,23 these inorganic powders are difficult to disperse homogeneously in the embolic material matrices due to the precipitation and aggregation of heavy metal salts, yielding inaccurate positioning.24 Moreover, these polymeric particles are usually nonuniform in size when prepared with the conventional emulsification polymerization methods. As indicated above, the radiopacity of the conventional embolic system is poor by simply mixing BaSO4 powder with the polymeric material. As we reported previously, X-ray visible and uniformly sized alginate microspheres loaded with in situ synthesized BaSO4 nanoparticles (denoted as BaSO4/ALG microspheres) can be prepared using a novel droplet-based microfluidics strategy.19 In this work, we attempt to use droplet microfluidics synthesized BaSO4/ALG microspheres as an embolic agent for TAE. Since radiopaque BaSO4 nanoparticles are homogeneously dispersed in alginate microspheres, excellent X-ray visibility and precise positioning could be achieved. In addition, droplet-based microfluidics can readily provide uniform microspheres, which may be beneficial for controlled embolization, controlled drug release, and enhanced embolization efficiency. Finally, alginate is a natural polymer with excellent biocompatibility and biodegradability in comparison to those of other existing embolic polymeric materials such as PVA. Herein, BaSO4/ALG microspheres containing varied amounts of BaSO4 were synthesized as embolic agents. Their radiopacity and embolic efficiency were evaluated by both in vitro and in vivo experiments. Commercially purchased pure calcium alginate microspheres mixed with commercial iodine-based contrast agent were also used as a reference.

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cross-linked by Ba2+ to form barium alginate microspheres. Simultaneously, BaSO4 nanoparticles were in situ synthesized by the reaction between SO42− premixed in the alginate solution and Ba2+ in the collection bath, yielding BaSO4 nanoparticles encapsulated in ALG microspheres. The resultant BaSO4/ALG microspheres were further centrifuged and washed with alcohol and DI water multiple times. The sample can be either dried in an oven for further characterization or be dispersed in 0.9 wt % NaCl solution for storage. Characterizations of BaSO4/ALG Microspheres. An aqueous dispersion of small amounts of BaSO4/ALG microspheres was deposited onto a clean silicon wafer and then dried in air. After coating with gold, the morphologies of the alginate microspheres were imaged on a scanning electron microscope (SEM, Quanta 200, FEI) at an accelerated voltage of 20 kV. The crystal structure of BaSO4 synthesized in situ in ALG microspheres was measured by a powder X-ray diffraction instrument (XRD, X’Pert PRO, PANalytical) with 2θ ranging from 20.0° to 70.0° in a step of 0.017° under Cu Kα radiation (λ1 = 1.540598 Å and λ2 = 1.544426 Å). The operation voltage was 40 kV, and the current was 40 mA. The X-ray visibility of resultant BaSO4/ALG microspheres was assessed by using a 16-slice spiral computed tomography (CT) instrument (SOMATOM Sensation, Siemens) under a scanning voltage of 80 kV and a current of 120 mA. Aqueous dispersion of the BaSO4/ALG microspheres was placed in glass vials for subsequent measurements. Commercial iodixanol solution with 150 mg I mL−1 was used as a reference. In Vivo Assessment on Embolic Efficacy and Visibility of BaSO4/ALG Microspheres. Adult New Zealand white rabbits with body weights of 2.5−3.0 kg were obtained from Experimental Animal Center in Tongji Medical College, Huazhong University of Science and Technology, China. All animal experiments were conducted according to “The Guide for the Care and Use of Laboratory Animals of Huazhong University of Science and Technology”, approved by the Animal Care Committee of Hubei Province, China. The rabbits were kept under standard feeding conditions, and no food feeding was allowed for 12 h prior to the following procedure. The commercial calcium alginate microspheres (KMG embolic microspheres with size of 100−300 μm) mixed with commercial iodine-based contrast agent−iodixanol solution were used as a reference. The experimental procedure was performed according to a method described elsewhere.26,27 Briefly, arteriography was performed by first injecting 2.0 mL of iodixanol solution, followed by injecting an aqueous suspension of BaSO4/ALG or KMG microspheres with or without iodixanol solution at 150 mg I mL−1 via microcatheter to the renal artery of rabbits, under the guidance of digital subtraction angiography (DSA, Siemens). The embolic efficacy of the kidneys of rabbits and the visibility of two types of alginatebased microspheres were further assessed via X-ray imaging by DSA and CT at designated time intervals. In addition, histological detection was performed via Elastica van Gieson staining at day 14 after embolization and investigated by using bright field microscopy (Olympus BX51, Japan).

EXPERIMENTAL SECTION

Materials. Sodium alginate (low viscosity) was purchased from Sigma-Aldrich. Na2SO4 (anhydrate, purity ≥99.0%), BaCl2 (dihydrate, purity ≥99.0%), Span 80 and liquid paraffin were purchased from Sinopharm Chemical Reagent Co. Ltd. Sylgard 184 silicone elastomer kits (polydimethysiloxane, PDMS, and curing agent) were purchased from Dow Corning Co. Calcium alginate microspheres (commercial name KMG, 100−300 μm) were purchased from Beijing Shengyiyao Technology & Development Co. Ltd. A nonionic iodine-based X-ray contrast agent, injectable iodixanol solution (commercial name Visipaque), was manufactured by GE Healthcare Company (Ireland Cork, Ireland). All chemicals were used as received. Preparation of BaSO4/ALG Microspheres. Barium alginate (denoted as ALG) microspheres loaded with varied amounts of BaSO4 were prepared via a droplet-based microfluidic device combined with off-chip external ion-cross-linking. The microfluidic device was fabricated according to a method described elsewhere.19,25 The mixture of sodium alginate aqueous solution (2 wt %) with varied amounts of Na2SO4 (0.05−0.4 mol L−1) was used as the dispersed phase. The continuous phase consisted of liquid paraffin containing 2 wt % of Span 80 as surfactant to stabilize the droplets formed in the microchannel. Through adjusting the flow rates and relative flow rate ratio of the two phases, monodispersed liquid alginate droplets containing Na2SO4 were formed and observed on an inverted microscope (IX71, Olympus). The resultant droplets were subsequently collected in a bath containing BaCl2 (0.6 mol L−1) and

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RESULTS AND DISCUSSION As shown in Scheme 1a, barium alginate microspheres encapsulated with BaSO4 nanoparticles (denoted as BaSO4/ ALG microspheres) were prepared by using a flow focusing droplet microfluidic device, combined with an external ionic cross-linking step. The aqueous mixture of sodium alginate aqueous solution (2 wt %) with varied amounts of Na2SO4 (0.05, 0.2, and 0.4 mol L−1) was used as the dispersed phase (marked with W, green), and liquid paraffin containing Span 80 (2 wt%) was used as the continuous phase (marked with O, blue). The monodispersed aqueous droplets containing sodium alginate and Na2SO4 were generated due to the competition between viscous and capillary forces. When these droplets dripped down into a collection bath containing BaCl2, the 1241

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Biomacromolecules Scheme 1. (a) Schematic Illustration of Radiopaque BaSO4/ ALG Microspheres Prepared via a Droplet Microfluidic Device, (b) Optical Micrograph of Hydrated BaSO4/ALG Microspheres 250 μm in Diameter,a and (c) Photo Image of a Stable Aqueous Suspension of BaSO4/ALG Microspheres Stored in a Vial under Room Temperature

Figure 1. XRD patterns of BaSO4/ALG microspheres measured by an XRD instrument with 2θ ranging from 20.0° to 70.0° in a step of 0.017° under a Cu Kα radiation. a The inset shows the SEM image of a single dried BaSO4/ALG microsphere with 60 μm in diameter.

were formed in situ during the cross-linking process between sodium alginate and barium chloride.19,28 The results of thermogravimetric analysis (TGA, see Figure S2 in Supporting Information) showed that BaSO4/ALG microspheres exhibited excellent thermal stability at temperatures below 220 °C in comparison with that of the ALG microspheres without BaSO4. This could be attributed to the existence of BaSO4 nanoparticles inside ALG microspheres. The final residues for BaSO4/ALG microspheres with initial SO42− concentrations of 0.05, 0.2, 0.4 mol L−1 in the dispersed phases are 33.6, 72.9, and 82.9 wt % in dried microspheres, respectively, which may exist in the form of BaSO4, BaO, or Ba(OH)2. To meet the visibility criteria, the minimum content of BaSO4 in microspheres is required to be approximately 5 wt % for CT and 10 wt % for fluoroscopy.29 Unlike physically mixing BaSO4 in microspheres, we introduced Na2SO4 in the dispersed phase and assumed that all SO42− ions could react with Ba2+ ions completely in the collection bath to form BaSO4. Thus, the weight percentage of BaSO4 in the BaSO4/ALG microspheres can be estimated by the initial Na2SO4 concentrations. For instance, when the initial concentration of Na2SO4 in the dispersed phase was 0.05, 0.2, or 0.4 mol L −1 , the corresponding content of BaSO4 was estimated to be around 1, 5, and 9 wt % in the hydrated BaSO4/ALG microspheres, and 37, 70, and 82 wt % in the dry BaSO4/ALG microspheres, respectively. These results are well in accordance with the residue data measured by TGA. In other words, within a certain range of initial concentrations of Na2SO4, in the dispersed phase, higher initial Na2SO4 concentrations in the dispersed phase yielded higher BaSO4 contents in ALG microspheres. Figure 2 shows CT images of BaSO4/ALG microspheres prepared by using varied initial concentrations of Na2SO4 in the dispersed phase. The brightness of BaSO4/ALG microspheres under X-ray increased with an increase of initial Na2SO4 concentration. Sample a shows a somewhat dim image due to the insufficient BaSO4 (1 wt %) content in the microsphere. Sample b contains approximately 5 wt % of BaSO4 and displays considerable visibility. The brightness of sample c containing approximately 9 wt % of BaSO4 was found to be similar to that of sample d, an aqueous iodixanol solution with approximately 30 wt % of iodixanol. To provide a more quantitative analysis, the CT value (in Hounsfield scale) reflecting the radiodensity of samples a−d shows 451.2 ± 13.5 HU, 924.3 ± 75.3 HU, 1491.33 ± 294.88 HU, and 1899.17 ± 67.33 HU, respectively. In our experiments, it was found that BaSO4/ALG micro-

cross-linking reaction between alginate and Ba2+ occurred. Simultaneously, the reaction between SO42− premixed in the aqueous droplets and Ba2+ present in the collection bath also took place, leading to the formation of BaSO4 nanoparticles. These two simultaneous reactions resulted in the formation of monodispersed BaSO4/ALG microspheres. We note that BaSO4/ALG microspheres with different dispersion densities of BaSO4 could be achieved by simply varying the initial concentration of Na2SO4 in the dispersed phase. The size of BaSO4/ALG microspheres can also be easily modulated by adjusting the flow rate ratio of the two phases. For example, BaSO4/ALG microspheres of 150 μm in diameter can be obtained by applying flow rates of the dispersed aqueous phase and the continuous oil phase at 30 μL h−1 and 500 μL h−1, respectively. Scheme 1b shows an optical microscopy image of the hydrated BaSO4/ALG microspheres with an average diameter of 250 μm. The microspheres are nearly monodispersed and spherical in shape, with the diameter ranging from 229 to 284 μm for the sample collected for several hours, while commercial pure calcium alginate microspheres (KMG, 100−300 μm) are polydispersed with the diameter ranging from 149 to 380 μm, as shown in Figure S1 in Supporting Information. The inserted SEM image in Scheme 1b shows the morphology of a dried BaSO4/ALG microsphere of 60 μm in diameter. As we reported previously, BaSO4 nanoparticles were dispersed within the BaSO4/ALG microspheres without aggregation due to the porous scaffold structure of the alginate.19 However, the distribution of BaSO4 nanoparticles is denser on the edge of the microspheres than those in the particle’s interior regions, which may be attributed to the gradual diffusion of Ba2+ in the collection bath from the surface of the inside of alginate droplet. As shown in Scheme 1c, aqueous suspension of hydrated BaSO4/ALG microspheres (>5 mL) with 250 μm in diameter was stored in 0.9 wt % aqueous NaCl solution for further use (stable for a minimum of 6 months). This stable BaSO4/ALG microsphere suspension is suitable for clinical applications as a radiopaque embolic agent. The crystal structure of BaSO4 nanoparticles in situ synthesized in ALG microspheres was identified by X-ray diffraction (XRD) (see Figure 1). All diffraction peaks and relative intensities in the XRD patterns match well with the reference of orthorhombic BaSO4 crystal structure (JCPDS card: 00-024-1035). This result implied that BaSO4 crystallites 1242

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also used as a reference. Before injecting BaSO4/ALG microspheres, contrast agent iodixanol solution was first injected to fill the blood vessel for superselective embolization. The DSA images of normal kidney of rabbits after iodixanol injection are shown in Figure 3a and c. The blood vessels can be clearly observed.

Figure 2. CT images of BaSO4/ALG microspheres prepared using 0.05 mol L−1 (sample a), 0.2 mol L−1 (sample b), and 0.4 mol L−1 (sample c) of Na2SO4 in the dispersed phase. As a reference, sample d is iodixanol solution containing approximately 30 wt % of iodixanol.

spheres were easily ruptured and led to irregular shaped particles when 9 wt % of BaSO4 was synthesized in the microspheres (sample c, see Figure S3 in Supporting Information), possibly caused by excessive BaSO4 formed on the surface of ALG microspheres during the external crosslinking process, which led to a relatively brittle surface and induced rupture of the microspheres. Since polydispersed irregular shaped particles would affect the embolic performance,30 all samples for the study of in vivo embolic efficacy and visibility were prepared by using 0.2 mol L−1 of Na2SO4 in the dispersed phase (i.e., 5 wt % of BaSO4 in ALG microspheres). The assessment of the cytotoxicity BaSO4/ALG microspheres is critical for their potential in vivo applications. Some assessment on the toxicity of barium compounds including BaCl2, BaSO4, and BaCO3 has been reported in the context of oral or inhalation studies.31,32 These reports showed that the barium toxicity usually originated from soluble salts such as BaCl2, while insoluble salts such as BaSO4 rendered promising biocompatibility via oral applications. However, we are not aware of any report on the toxicity of BaSO4 via intravascular diagnostic procedures, even though polymeric materials with BaSO 4 encapsulation have been reported as embolic agents.2,11,18−20 One possible scenario is that BaSO4 might migrate to important organs (such as the brain or heart) by ectopic embolism or random migration of BaSO4 upon degradation of embolic microspheres. However, ectopic embolism was not observed upon injection in our in vivo experiments. The X-ray visibility of our BaSO4/ALG microspheres can effectively prevent from the occurrence of ectopic embolism. In addition, cytotoxicity evaluation of BaSO4/ALG, pure ALG, and pure calcium alginate microspheres (purchased as KMG particles) was conducted by using liver hepatocellular cells (HepG2 cells) incubated with microspheres at various concentrations for 24 h by using the MTT assay and cell culture inserts method.33,34 The MTT assay is a colorimetric assay to assess cell viability by using the tetrazolium dye MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide, (Sigma-Aldrich, St Louis, MO, USA)) as a color indicator.33 Our results demonstrated that all three types of microspheres were relatively nontoxic even at a concentration level of 5 mg mL−1, as shown in Figure S4 (see more details in Supporting Information). The in vivo visibility of BaSO4/ALG microspheres in the renal artery of normal rabbits was evaluated by using DSA.5 Pure calcium alginate microspheres KMG (100−300 μm) were

Figure 3. DSA images of the right kidney of rabbits before and after embolization. (a and b) Normal right kidney before and after embolization with KMG microspheres. (c and d) Normal right kidney before and after embolization with BaSO4/ALG microspheres.

Subsequently, approximately 30 mg/mL BaSO4/ALG microspheres or KMG microspheres were injected into the right renal artery of the rabbit under the guidance of DSA. After 10 min, contrast agent iodixanol solution was again injected, and DSA images were captured immediately (Figure 3b and d). Renal arteries filled with contrast agent show dark color, and the terminal shadows almost disappeared, indicating that the blood vessels in the kidney have been successfully embolized by microspheres. The contour of the kidney embolized with BaSO4/ALG microspheres (Figure 3d) is more clear and complete with better contrast than that embolized with KMG microspheres (Figure 3b). Since KMG microspheres themselves are invisible under X-ray, the fuzzy DSA image in the kidney (Figure 3b) may be attributed to the residue of iodixanol. In the case of BaSO4/ALG microspheres, however, the blood vessels in the kidney are visible (as shown in Figure 3d) because evenly dispersed BaSO4 in the ALG microspheres act as an effective contrast agent. The results from Figure 3 verify that BaSO4/ALG microspheres possess excellent visibility under X-ray in vivo and can be used to embolize the blood vessel via the TAE procedure. Further embolization experiments using BaSO4/ALG microspheres as embolic agent were carried out in the left kidney (LK) of rabbits. The DSA images at various intervals are shown in Figure 4. Figure 4a is the DSA image of LK after immediate injection of iodixanol solution to display the blood vessel for embolization. Figure 4b illustrates the LK at 1.5 h after the embolization with BaSO4/ALG microspheres, showing that the renal artery is not visible under X-ray, suggesting that the injected iodixanol solution has dissipated completely. In contrast, the inserted microcatheter can be observed due to its inherent radiopaque property. In particular, the contour 1243

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BaSO4/ALG microspheres under X-ray is beneficial not only for evaluating the efficacy of embolization during embolic process but also for the follow-up assessment of embolic sites and efficacy. For further assessment of the embolic efficacy of BaSO4/ ALG microspheres, histological detection was performed via Elastica van Gieson staining at day 14 after embolization. In this procedure, elastic fibers were stained in blue, with the renal vascular system being surrounded by these blue fibers. Figure 6

Figure 4. DSA images of the left kidney of a rabbit: (a) immediately after injection of iodixanol solution; (b) at 1.5 h after the embolization with BaSO4/ALG microspheres; and (c) immediately after subsequent injection of iodixanol solution at the embolization with BaSO4/ALG microspheres for 1.5 h.

profile of LK is also visible under X-ray, which is derived from BaSO4 nanoparticles encapsulated in the embolic ALG microspheres. When iodixanol solution was injected again to test the embolic efficacy, the renal artery was visualized, and the contrast agent did not diffuse into the embolized kidney but refluxed to other unembolized blood vessels, as shown in Figure 4c. Figure 4b and c also demonstrated that the terminal embolization by BaSO4/ALG microspheres was accomplished successfully. These results demonstrate that BaSO4/ALG microspheres have excellent embolic efficacy with X-ray visibility in vivo. The visibility and embolic efficacy of BaSO4/ALG microspheres were also evaluated using computed tomography (CT). Figure 5 shows CT scan images of rabbits at designated time

Figure 6. Histopathological analysis of rabbit kidney tissues. (a) Control sample without embolization; (b) day 14 post-embolization with KMG microspheres; and (c) day 14 post-embolization with BaSO4/ALG microspheres. The magnification: ×200.

shows histological appearance of tissues of a rabbit kidney without embolization (Figure 6a), embolized by KMG microspheres (Figure 6b), and BaSO4/ALG microspheres (Figure 6c) at day 14 after embolization. As observed, there is nothing in the vessel of the kidney without embolization as shown in Figure 6a (see regions marked by yellow arrows), while interlobular arteries and arcuate arteries were almost filled completely with embolic microspheres, as shown in Figure 6b and c (see regions marked by black arrows). The result of pathology is similar between the two embolic microspheres, which indicates that the embolic efficacy of BaSO4/ALG microspheres is similar to that of commercial KMG microspheres.

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CONCLUSIONS In this work, the radiopaque alginate microspheres loaded with homogeneously dispersed in situ synthesized BaSO4 nanoparticles at various concentrations were prepared in a single step by using a droplet microfluidic device. The resultant BaSO4/ALG microspheres possessed excellent visibility under X-ray, evaluated by both in vitro and in vivo assays. In comparison with the commercially available alginate-based embolic agent (KMG), BaSO4/ALG microspheres possessed better and more durable X-ray visibility after embolization due to the existence of in situ synthesized BaSO4 nanoparticles as a contrast agent. Histopathological analysis also demonstrated that the embolic efficacy of BaSO4/ALG microspheres was similar to that of the commercial embolic agent. Our microfluidic synthesized BaSO4/ALG microspheres present exciting opportunities for potential clinical applications not only for embolization but also for noninvasive tracking and mapping of the fate of the embolic agents in vivo.

Figure 5. CT scan images of rabbits at designated time intervals after embolization using KMG microspheres mixed with contrast agent iodixanol (images a−d) and BaSO4/ALG microspheres (images e−h). RK means right kidney, and the white objects in the bottom center of each image represent the spine location of the rabbits.

intervals after embolization using KMG microspheres mixed with iodixanol solution (Figure 5a−d) and BaSO4/ALG microspheres (Figure 5e−h). At 20 min after the embolization, the right kidney (RK) contour (white zone) clearly appeared on the bottom left of images in Figure 5a and e because the iodixanol solution had not completely dissipated. As shown in Figure 5b, the white zone on the bottom left completely disappeared after 5 h, implying that iodixanol in KMG microspheres fully dissipated. By contrast, the white zone remained visible in the case of BaSO4/ALG microspheres due to the inherent visibility of BaSO4 (Figure 5f). After 7 days and 14 days, the outline of the kidney can no longer be identified for the KMG microspheres case (Figure 5c and d). In the case of BaSO4/ALG microspheres, a faded white ring (marked by arrows) consisting of small white spots surrounding the kidney can still be observed in Figure 5g and h, which may arise from the renal atrophy and potential fragmentation of the microspheres after embolizaton. The size and location of the white ring reflect those of the embolized right kidney. This observation further demonstrates that the good visibility of

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ASSOCIATED CONTENT

S Supporting Information *

Optical micrographs and size distributions of KMG microspheres and BaSO4/ALG microspheres, TGA measurement method and results, Na2SO4 concentration effect on the morphology of BaSO4/ALG microspheres, Digital Subtraction Angiography (DSA), and in vitro cytotoxicity of BaSO4/ALG 1244

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Biomacromolecules

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microspheres determination method and results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(Y.Y.) E-mail: [email protected]. *(A.Q.S.) E-mail: [email protected]. Author Contributions ⊥

Q.W. and K.Q. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51103051) and National Basic Research Program of China (2012CB932500). We thank the Analytical and Testing Center in Huazhong University of Science and Technology for the related measurements. A.Q.S. also thanks the OIST Graduate University for subsidy funding from the Cabinet Office, Government of Japan.

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DOI: 10.1021/acs.biomac.5b00027 Biomacromolecules 2015, 16, 1240−1246

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DOI: 10.1021/acs.biomac.5b00027 Biomacromolecules 2015, 16, 1240−1246