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Polymer Degradation and Stability 95 (2010) 1356e1364

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Biodegradation-induced surface change of polymer microspheresand its influence on cell growth

Yu Zhang a,b, Lei Sun c, Jian Jiang c, Xiaolin Zhang b, Wenjun Ding b, Zhihua Gan a,*

aCAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, ChinabGraduate University of Chinese Academy of Sciences, Beijing 100039, ChinacBeijing Institute of Traumatology and Orthopaedics, Beijing 100035, China

a r t i c l e i n f o

Article history:Received 23 December 2009Received in revised form19 January 2010Accepted 19 January 2010Available online 28 January 2010

Keywords:Biodegradable polymersMicrocarriersEnzymatic degradationSurface changeCell culture

* Corresponding author. Tel./fax: þ86 10 62529194E-mail address: zhgan@iccas.ac.cn (Z. Gan).

0141-3910/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2010.01.025

a b s t r a c t

Biodegradable microspheres were fabricated by poly(3-caprolactone) (PCL) homopolymer and poly(3-caprolactone-b-ethylene oxide) (PCL-b-PEO) amphiphilic block copolymer. The regulation of microspheresurface morphology was successfully achieved by controlled enzymatic degradation. The morphologicalchanges induced by biodegradation and their influences on the growth of MG-63 human osteosarcomacells were studied. Results based on the evaluation of cytotoxicity and the morphological observation ofMG-63 cells cultivated on microspheres showed better growth of cells on the surface of degradedmicrospheres than on the surface of those undegraded microspheres no matter they were fabricated byhomopolymers or copolymers. The influences of morphological changes of microsphere surface beforeand after biodegradation on MG-63 cell growth were discussed. The results of this work indicated thatthe biodegradation-induced morphological changes of microspheres could be well controlled and werefavorable for MG-63 cell attachment and proliferation.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Tissue engineering applies engineering principles in life scienceto develop biological substitutes for restoring, maintaining, orimproving the functions of tissue or organ [1]. It has become aneffective method for restoring or reconstructing damaged tissuesand organs in the last several decades. In tissue engineering,biodegradable polymers such as poly(glycolide) (PGA), poly(3-cap-rolactone) (PCL) and poly(lactic acid) (PLA) in form of porous scaf-folds andmicrocarriers are the representative biomaterials [2e5]. Atraditional osteoconductive matrix was a prefabricated 3-dimen-sional scaffold composed of biodegradable polymers or ceramicssince they can provide physical and biochemical support for cellgrowth and tissue regeneration [6e11]. Microcarrier was anothersuitable candidate for tissue defect filling which showed manyadvantages such as injectable, filling exactly in irregular defects,offering a large surface area, performing a three-dimensional cellculturemode and providing perfect growth environment for in vitrocultivation [12e16]. Intensive research has been reported on poly-meric microcarriers for cell cultivation and defects filling [17e23].These microcarriers were mainly fabricated by biodegradable

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All rights reserved.

polymers such as PLGA [19,24e26], PCL [20,27], PLA [25], chitosan[28,29] and gelatin [21,30]. Due to their ability of injectable massconveyance, good biodegradability and no need for cell separation,biodegradable polymeric microcarriers are continuously attractingextensive interest.

It has been known that the growth of anchorage-dependent cellsstrongly relies on the surface properties of biomaterial substrates. Inorder to prepare effective carrier for cell growth, a lot of fabricationfactors should be considered. For polymeric biomaterials as thesubstrates for cell growth, the surface physicochemical propertiesincluding surface composition, surface charge [31], surface energy[32,33], surface oxidation [34], solidity [35], curvature [36] andmorphology [37] have been showngreat influences on cell adhesionand proliferation. A lot of methods such as surface coating [17,38],plasma treatment [39] and grafting [40,41] have been appliedfor surface modification. On the other hand, the biodegradabilityprovides advantages to microcarriers for direct injection withoutcell separation procedures. Meanwhile, it also leads to surface andbulk changes of microspheres which subsequently influence cellgrowth on surface. Though the biodegradation has been knownto display significant impact on cell culture in the previous research[42e45], much attention was focused on the change of polymerbulk properties such as weight loss, water uptake, degree of crys-tallization, thermal property and mechanical property which mayaffect cell behavior [8,46e48]. Few works were reported on the

Y. Zhang et al. / Polymer Degradation and Stability 95 (2010) 1356e1364 1357

effect of surface morphological change of microspheres induced bybiodegradation on cell adhesion and growth. Since the cells growdirectly on the surface of microspheres, the microsphere surfacemorphology and its change in the course of biodegradation havegreat influences on cell growth. Therefore, the aim of this work is toelucidate the effect of surface degradation of microspheres inducedby enzymatic degradation on cell attachment and growth in thecourse of cultivation.

In this work, we investigated the surface morphological changesof microspheres induced by the enzymatic degradation and theirinfluences on MG-63 cell growth. Microspheres were used asthe substrate to study cell growth behavior due to their charac-teristic core-shell structure. In addition, the structure and surfacemorphology of microspheres can be easily regulated by changingthe composition of polymer chains. Since the degree of biodegra-dation of polymer biomaterials plays a very important role in tissueengineering for cell attachment, spread and growth, we choseLipase Pseudomonas cepacia enzyme, which was proved to accel-erate degradation rate of PCL [49], to control the degradation ofmicrospheres via changing the time length of enzymatic degrada-tion. Here, biodegradable PCL homopolymer and PCL-b-PEOamphiphilic block copolymer were used to fabricate microspheresso as to study the effect of composition on surface degradation.MG-63 cells were seeded on microspheres with different surfacemorphologies induced by the controlled enzymatic degradation.The cytotoxicity, viability and proliferation of MG-63 cells wereevaluated in terms of the role of surface degradation of micro-spheres. The results were well discussed on the controlled surfacechange of microspheres by degradation and its influence on cellcultivation on microspheres.

2. Experimental methods

2.1. Materials and methods

Poly(3-caprolactone) (PCL) (Mn ¼ 42,500, Mw/Mn ¼ 1.5), andpoly(vinyl alcohol) (PVA) (Aldrich,87e89%hydrolyzed, Mw ¼85,000e146,000), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazo-lium bromide (MTT), dimethyl sulphoxide (DMSO), P. cepacia and0.25% steapsin were purchased from SigmaeAldrich and used asreceived without further purification. Dichloromethane (CH2Cl2)(Beijing Chemical Reagent Co.) was used as received. Amphiphilicblock copolymer PCL20k-b-PEO1k (the subscript “20 k” and “1 k”denote the number-average molecular weight of PCL and PEO blocks,respectively.Mw/Mn¼ 1.36. Hereinafter, we use a simple abbreviationPCL-b-PEO to represent PCL20k-b-PEO1k)was synthesized according toour previous work [20].

Dulbecco's Modified Eagle's Medium (DMEM) and Fetal BovineSerum (FBS) were purchased from Gibico. MG-63 human osteo-sarcoma cells were provided by Graduate School of ChineseAcademy of Sciences.

2.2. Preparation, degradation and characterization of microspheres

Microspheres were fabricated according to a modified water-in-oil-in-water (W1/O/W2) double emulsion solvent extraction/evaporation technique [50]. Briefly, PCL or PCL-b-PEO solution inCH2Cl2 (5%, w/v) and PVA in distilled deionized water (0.1%, w/v)were compounded as O and W1 phases, respectively. The W1solution was injected into the polymer solution and sonicatedfor 30s to form the W1/O emulsion. Then the W1/O emulsionwas dispersed into a 160 ml 0.25% (w/v) PVA aqueous solution(W2 phase) under mechanical stirring. The microspheres fabricatedafter the W1/O/W2 was stirred continuously at a speed of 400 rpmfor 2 h at room temperature for evaporating CH2Cl2 solvent.

The enzymatic degradation experiments of polymer micro-spheres were carried out at 37�1 �C in a shaker. Specifically, eachsample (about 20 mgmicrospheres) was placed in a vial containing1 ml phosphate buffer solution (0.05 M, pH ¼ 7) and 0.5 mg lipase.The vials were placed in a shaker at a rotating speed of 120 rpmat 37 �C. After predetermined periods of time, microsphere sampleswere taken out from the shaker, washed with distilled waterfor several times, and then lyophilized. The microspheres werecollected for the further characterization.

The surface chemistry properties of microspheres before andafter degradation were measured by X-ray photoelectron spectros-copy (XPS, ESCALab220I-XL, England). The banding energy spectrumwas recorded from 0 to 1000 eVwith pass energy of 30 eV under thefixed transmission mode.

The shape and surface morphology of microspheres wereinvestigated by scanning electron microscopy (SEM, JSM 6700F,JEOL, Japan) at an accelerating voltage of 5 kV. For SEM observa-tion, the freeze-dried microspheres were first fixed on metal stubby using double-sided adhesive tape, and then vacuum-coatedwith a thin layer of platinum in sputter coater (SCD 500, BAL-TEC)for 60 s.

2.3. In vitro cytotoxicity test

MG-63 human osteosarcoma cells were employed. DMEM with10% FBS was utilized as cell culture medium. Before experiment,cells were pre-cultured until confluence was reached in medium at37 �C in humidified environment of 5% CO2[51].

The microspheres were sterilized by immersing in 75% ethanolfor 3 w 4 h followed with UV irradiation for 1 h. After that, themicrospheres were immersed in cell culturemedium. 200 ml of MG-63 human osteosarcoma cells in medium with a concentrationof 2.0�104 cells/mlwere added intowells of a 96-well plate (Costar,IL, USA). After cultured in an incubator for 24 h and the cellconfluencewas reached, the culture mediumwas changed to 200 mlof medium containing microspheres. At given time intervals, thecultured cells were assayed for cell viability with the methyl-thiazolyldiphenyl-tetrazolium bromide (MTT)method. Briefly, 20 mlof MTT (5mg/ml, Sigma) was added into wells. After 4 h incubation,the culture medium was removed and the precipitate of each wellwas dissolved in 200 ml dimethyl sulphoxide (DMSO). The opticaldensity (OD) was measured by the microplate reader (MULTISKANMK3, Thermo Electron Corporation) with setting absorbancewavelength as 570 nm. The relative growth rate (RGR) of cells wascalculated by following equation:

RGR ¼ IntsIntcontrol

� 100%

where Ints is the intensity of the cells incubated with the micro-sphere suspension and Intcontrol is the intensity of the cells incu-bated with the culture medium only (positive control).

2.4. Cell cultivation on microspheres

The microspheres were put into 24-well plate and pre-cultured at 37 �C for 1 h. Then MG-63 cells in DMEM with theconcentration of 5.0 � 104 cells/ml were added to the wellscontaining microspheres. The cells were cultured on micro-spheres for 3 days. The samples were fixed by 2.5% glutaralde-hyde solution, then dehydrated in a gradient way by ethanol,freeze-dried and then vacuum-coated with gold. The morphologyof cells cultivated on microspheres was observed by phasecontrast microscopy (PCM, IX51, Olympus) and scanning electronmicroscopy (SEM).

Y. Zhang et al. / Polymer Degradation and Stability 95 (2010) 1356e13641358

2.5. Statistical analysis

For statistical analysis, each experiment was repeated at leastthree times with similar results. All data was analyzed by using SPSSsoftware and was shown as means � standard deviation. Thesignificance of the obtained data was performed using ANOVAanalysis of variance and P < 0.05 was considered to indicate a statis-tically significant difference. The error bars indicate the standarderror for the associated average value.

3. Results and discussion

3.1. Surface change of microspheres beforeand after enzymatic degradation

Fig. 1 shows the SEM images of microspheres fabricated by PCLhomopolymer and PCL-b-PEO amphiphilic block copolymer. Itwas found that the microspheres were spherical with narrow sizedistribution (Fig. 1a1 & b1). Microspheres fabricated by homopol-ymer or copolymer have similar average diameter varying from 100to 150 mm. However, their surfacemorphologies were very different.The surface of PCL microspheres was relatively smooth withoutapparent pores (Fig. 1a2), whereas the surface of PCL-b-PEO micro-spheres showed a coarse surface with small ridges and groove allover the outside (Fig.1b2). The distinctness of surfacemorphology ofmicrospheres could be explained by the introduction of hydrophilicPEO segments in the copolymer. Because of the W/O/W fabricationmethod and phase inverse process, the hydrophilic PEO blocks tendto be enriched on the surface layer of microspheres. Water pene-trates into PEO chains layer to form a swollen out-layer wrappingthe microspheres. After freeze-drying, the water-penetratedmorphology of PEO out-layer was fixed as spiny protuberancesstructures.

Fig. 1. SEM photographs of surface morphology of microspheres fabricated by PC

Fig. 2 shows the surfacemorphologies ofmicrospheres fabricatedby PCL homopolymer and PCL-b-PEO copolymer after enzymaticdegradation for different time scale. Compared to the images in Fig.1,the surface morphologies of microspheres after enzymatic degra-dation changed greatly. For microspheres fabricated by PCL homo-polymer, their surface became rough with some flowerlike cracksand few holes, as shown in Fig. 2a1 and a2. For microspheresfabricated by copolymer, the surface of microspheres showed moreroughness and scragglymorphology after enzymatic degradation for0.5 h (Fig. 2b1). In addition, an alveolate morphology with homo-geneously small holes in the size of about 1w2 mmwas observed forthe microspheres after enzymatic degradation for 1 h (Fig. 2b2).

Based on the different morphologies of microspheres before andafter enzymatic degradation as shown in Figs. 1 and 2, it could beconcluded that polymer composition had great influences on thesurface morphology of microspheres as-fabrication and after enzy-matic degradation. Furthermore, as shown in Fig. 2, it was found thatthe degree of biodegradation, i.e. the surface morphological change,could be well controlled by the degradation time. Therefore, control-lable biodegradation could provide suitable microsphere samples forour further investigation on the influence of surface morphologicalchanges induced by biodegradation on cell growth behaviors.

Fig. 3 shows the surface chemistry of the PCL and PCL-b-PEO-NH2

microspheres before and after enzymatic degradation. For PCLmicrospheres, there was no shift change of C 1s (atomic orbital 1sof carbon) and O 1s (atomic orbital 1s of oxygen) and no obviouschange of the carbon and oxygen element content in the surfaceafter enzymatic degradation. Similarly, no obvious changewas foundfor the copolymer microspheres after enzymatic degradation.However, comparedwith PCLmicrospheres, the N 1s (atomic orbital1s of nitrogen) signal was detected from the surface of copolymermicrospheres because of the amino group at the end of PEO chain ofPCL-b-PEO-NH2 block copolymer. Although theXPS results indicated

L homopolymer (a1, a2) and PCL-b-PEO amphiphilic block copolymer (b1, b2).

Fig. 2. SEM photographs of surface morphology of PCL microspheres after enzymatic degradation for 1 h (a1) and 3 h (a2) as well as PCL-b-PEO microspheres after enzymaticdegradation for 0.5 h (b1) and 1 h (b2).

Y. Zhang et al. / Polymer Degradation and Stability 95 (2010) 1356e1364 1359

only little difference of surface chemistry ofmicrospheres before andafter enzymatic degradation, the hydrolysis ofmicrospheres actuallyresulted in the change of chemical structure of microsphere surface.Taking PCL microspheres as examples, Fig. 4 shows the change ofsurface chemistry resulting from hydrolysis mechanism of PCLchains after enzymatic degradation. The ester bonds in PCL chainshydrolyzed into carboxyl groups and hydroxyl groups, which wereexposed on the surface of microspheres. Combined with the resultsin Figs. 2 and 4, the change in surface morphology and chemistry formicrospheres after enzymatic degradation should have great influ-ence on the growth of anchorage-dependent cells on the surface ofmicrospheres.

Fig. 3. The XPS wide scan spectra of PCL microspheres before (a) and after (b) enzy-matic degradation, PCL-b-PEO-NH2 copolymer microspheres before (c) and after(d) enzymatic degradation.

3.2. Cytotoxicity of MG-63 cells cultivated on microspheres

Fig. 5 shows the in vitro viability of MG-63 cells on four kinds ofmicrospheres after cultivated for 1, 2, and 3 days, respectively. Thefour kinds of microspheres are the PCL microspheres before andafter enzymatic degradation (denoted as PCL and PCL-d), the PCL-b-PEO copolymer microspheres before and after biodegradation

Fig. 4. Hydrolysis mechanism of PCL microspheres.

Fig. 5. Relative growth rate (RGR) of MG-63 cells on microspheres with differentsurface morphology in three days. The four kinds of microspheres are PCL homopol-ymer microspheres before and after enzymatic degradation (denoted as PCL and PCL-d,respectively), PCL-b-PEO block copolymer microspheres before and after enzymaticdegradation (denoted as copolymer and copolymer-d, respectively).

Y. Zhang et al. / Polymer Degradation and Stability 95 (2010) 1356e13641360

(denoted as copolymer and copolymer-d), which were used tocheck their cytotoxicity before studying the influences of surfacechanges induced by biodegradation on cell growth. It was foundthat the RGR values of MG-63 cells cultured on the microsphereswere all higher than 80% in three days, indicating the zero or firstlevel of cytotoxicity. In our experiment, microspheres were addedinto wells enough to overspread the bottom of the well. Thus theoptical density measured by MTT method was able to stand for thecytotoxicity of microspheres. As the degradation of microsphereswas performed prior to the cultivation of cells and the by-productsof the biodegradation were already washed off by distilled water

Fig. 6. Phase contrast microscopy (PCM) photographs of MG-63 cells cultivated on PC

completely, no further enzymatic degradation of microspheresprogressed during the cell cultivation. These results revealed thatthe microspheres after biodegradation did not cause cytotoxicity.Therefore in our further investigation, the different cell growthbehavior and morphology on microspheres before and afterbiodegradation should only be attributed to the different surfaceproperties of microspheres induced by the biodegradation. In otherwords, it is the changes of surface property induced by biodegra-dation that result in the different cell growth behavior.

3.3. Morphology of MG-63 cells on PCL microspheres

Fig. 6 shows thephase contrastmicroscopy (PCM) photographs ofMG-63 cells cultivated on PCL microspheres before and after enzy-matic degradation. In photographs, the black round shape regionwas the opaque microspheres while the bright green color regionwas the assembled cells. It was found that cells were not preferentialto cultivate on PCL microspheres before biodegradation, only a fewmicrospheres were attached by cells on surface (Fig. 6a1 & a2).However, as shown in Fig. 6b1 and b2, after the PCL microsphereswere degraded by Pseudomonas lipase, the color around mostmicrospheres become greener than the substrate, implying thatmore cells were likely to grow on the degraded surface of micro-spheres. Therefore, the results in Fig. 6 indicate that the surfacechanges induced by enzymatic degradation were favorable for MG-63 cell attachment.

Fig. 7 shows the scanning electron microscopy (SEM) images ofMG-63 cells cultivated on PCL microspheres before and afterdegradation. Fig. 7a1 and a2 further confirmed the results of Fig. 6a1and a2 that only a few cells cultivated on PCL microspheres beforebiodegradation. Although several microspheres aggregated togetherowing to the cell growth effect, the pictures indicated a feeblegrowth situation of cells in the culture surroundings (Fig. 7a1). Incontrast, for the PCL microspheres after enzymatic degradation,

L microspheres before (a1, a2) and after (b1, b2) enzymatic degradation for 3 days.

Fig. 7. SEM photographs of MG-63 cells cultivated on PCL microspheres before (a1, a2) and after (b1, b2) enzymatic degradation for 3 days.

Y. Zhang et al. / Polymer Degradation and Stability 95 (2010) 1356e1364 1361

the results of Fig. 7b1 and b2 showmuch better growth morphologyof MG-63 cells on degraded microsphere surface. It was found thatcells were attached to the surface andwell spreading in a large-scaleon the surface of microspheres. The magnified image of Fig. 7b2shows a fusiform shape which was the typical characteristicmorphology of mature MG-63 cells. It can be seen in Fig. 7b1 and b2that cells protruded their pseudopod to span the pores on thesurface or to another microsphere. These results indicated that PCLmicrospheres after enzymatic degradation results in a better surfacefor MG-63 cell adhesion and growth compared with PCL micro-spheres before enzymatic degradation.

3.4. Morphology of MG-63 cells on copolymer microspheres

Fig. 8 shows the phase contrast microscopy (PCM) photographsof MG-63 cells cultivated on the copolymer microspheres. Forthe copolymer microspheres before enzymatic degradation, it canbe seen in Fig. 8a1 that nearly all the cells were spread out intoa fusiform shape on the culture dish and had a tendency to gath-ering around the copolymer microspheres. Several microsphereswere also aggregated each other due to the cell growth effect(Fig. 8a2). While for the microspheres after enzymatic degradation,it can be seen that cells cultivated on the degraded copolymermicrospheres propagated very well and almost each microspherewas a good microcarrier as cells spreaded and congregated aroundeach microsphere clearly (Fig. 8b1 & b2). These results revealed thegood growth behavior of MG-63 cells cultivated on the surfaceof degraded microspheres. Compared to the PCL homopolymermicrospheres, the PCL-b-PEO copolymer microspheres were bettermicrocarriers for cell attachment and growth.

Fig. 9 shows the scanning electron microscopy (SEM) images ofMG-63 cells cultivated on the copolymer microspheres. As shown inFig. 9a1 and a2 for microspheres before enzymatic degradation,

MG-63 cells were found to cultivate well on microspheres.Compared to the situation of PCL microspheres, cells attached onthe surface of copolymer microspheres more tightly and spread ina larger area (Fig. 9a2), indicating that the moderate hydrophilicsurface of copolymer microspheres are favorable for cell cultivation.While for the degraded copolymer microspheres, cells also showedwell cultivation on the surface of degraded microspheres (Fig. 9b1).The magnified photograph of Fig. 9b1 clearly showed that MG-63cells attached to the surface tightly and grew layer by layer, whichwere very helpful for cell differentiation and regeneration. Further-more, even though there are many micropores on the surface layerof degraded copolymer microspheres, cells were favorable to formpseudopods with spreading along the bridge of two micropores(Fig. 9b2). Meanwhile cells were also able to span the micropores toform a large-scale of spreading on the surface of microspheres.

Based on the above morphological results, it could be concludedthat the MG-63 cells showed better attachment and growth on thesurface of copolymer microspheres than on that of PCL homopol-ymer microspheres no matter those microspheres before or afterenzymatic degradation. This could be explained by the moderatehydrophilic surface of copolymer microspheres and the consequentmorphological changes after enzymatic degradation.

Most importantly, no matter the microspheres fabricated byhomopolymer or copolymer, our results indicated that the MG-63cells cultured on surface of degraded microspheres grew betterthan cultured on the surface of the corresponding undegradedmicrospheres. One reason is due to the porous and rough surfacecaused by degradation of the microspheres, which favored thenutrient exchanges on and inside the microspheres. In addition,MG-63 human osteosarcoma cells used in this work were probablypreferred to adhere to a rough surface. Since the substratesincluding porous scaffolds and microspheres are mostly fabricatedby biodegradable polymers, we always wonder how the

Fig. 8. Phase contrast microscopy (PCM) photographs of MG-63 cells cultivated on PCL-b-PEO microspheres before (a1, a2) and after (b1, b2) enzymatic degradation for 3 days.

Fig. 9. SEM images of MG-63 cells cultivated on PCL-b-PEO microspheres before (a1, a2) and after (b1, b2) enzymatic degradation for 3 days.

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biodegradation affects the cell growth and new tissue regeneration.Here in this work, we fabricated microspheres by two kinds ofpolymers and controlled their degree of biodegradation for inves-tigating the influences of biodegradation on cell growth, it has beenwell concluded that the morphological changes of microspherescaused by enzymatic degradation were more favorable for MG-63cell attachment and spreading. Even though the microspheresfabricated by homopolymer and copolymer show different surfaceproperties and morphologies, the SEM and PCM results indicatedthat the surface morphologies of microspheres after enzymaticdegradation were stimulative for cell adhesion and proliferation.Therefore, it could be evidenced that the hydrophilic surfaceand biodegradation-induced surface changes of microspheres werefavorable for MG-63 cell attachment and growth.

4. Conclusions

In this work, the regulation of surface morphology of micro-spheres was successfully achieved by controlling the time scale ofenzymatic degradation. The surface change caused by the enzy-matic degradation showed great influences on MG-63 cell growthon microspheres. The results indicated that MG-63 cells showedbetter growth on the surface of copolymer microspheres than onthat of PCL homopolymer microspheres no matter those micro-spheres before or after enzymatic degradation. Most importantly, indespite of microspheres fabricated by homopolymer or copolymer,MG-63 cells cultured on surface of degradedmicrospheres attachedand grew better than cells cultured on the surface of thoseundegraded microspheres. This work indicates that morphologicalchanges induced by biodegradation are favorable for MG-63 celladhesion and proliferation on the surface of microspheres fabri-cated by biodegradable polymers.

Acknowledgment

This work was supported by the National Natural ScienceFoundation of China (Grant no. 50573085, 50521302).

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