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1.
在多孔胶原支架上构建三维工程的心肌组织   总被引:1,自引:0,他引:1  
背景:目前已有转化的SV40细胞系、C2C12成肌细胞、心肌细胞、骨骼肌细胞以及胚胎干细胞用于修复哺乳动物心脏的细胞移植治疗,然而细胞移植治疗对严重缺失或损伤心肌结构的治疗的作用却不大。组织工程技术为此类心肌疾病的研究及治疗提供了新的可能性,目前已有可替代和改善骨、软骨、肾、肝、神经及皮肤等组织和器官的生物合成结构物。目的:探索多孔胶原作为构建三维工程心肌组织支架材料的可行性。设计:非随机、对照的实验研究。地点和对象:实验地点:军事医学科学院生物工程研究所。动物:清洁级1日龄Wistar大鼠400-500只,雌雄不限,体质量8~10g;成年Wistar大鼠五六只,雌雄不限,体质量250g,由军事医学科学院实验动物中心提供,饲养温度22~26℃,湿度40%~60%。干预:在无菌条件下,将经胰蛋白酶消化获得的新生鼠心肌细胞分别接种于三维多孔胶原支架和培养板上,比较心肌细胞在两种培养模式的代谢差异。主要观察指标:检测葡萄糖比消耗率、乳酸比产率、乳酸转化率、肌酸激酶及乳酸脱氢酶活力变化。结果:培养于多孔胶原支架上的心肌细胞的代谢特性近似于心肌细胞在培养板上培养的二维生长模式。结论:心肌细胞与多孔胶原支架形成了具有自律性同步收缩的三维工程心肌组织,多孔胶原作为心肌组织工程的支架材料有良好的应用前景。  相似文献   

2.
背景:制备具有细胞识别信号的细胞外基质替代材料及仿生支架是目前组织工程支架材料研究的重点和热点。目的:制备并筛选出能够满足构建骨-软骨复合组织要求的多孔三维支架,并评价其生物学性能。方法:制备胶原-壳聚糖、明胶-硫酸软骨素-透明质酸钠、胶原-陶瓷化骨、明胶-陶瓷化骨支架材料,以新鲜关节为对照组。结果与结论:胶原-壳聚糖支架孔径50-200μm,孔隙率(90.5±2.1)%;明胶-硫酸软骨素-透明质酸钠支架孔径100-150μm,孔隙率(78.0±1.1)%;胶原-陶瓷化骨支架孔径400-500μm,孔隙率(67.5±2.1)%;明胶-陶瓷化骨支架孔径300-400μm,孔隙率(65.9±1.2)%。明胶-硫酸软骨素-透明质酸钠与明胶-陶瓷化骨支架基本符合实验要求,其结构与生物化学成分近似于自然细胞外基质,能够模拟细胞外微环境。说明明胶-硫酸软骨素-透明质酸钠与明胶-陶瓷化骨支架可作为复合组织的支架。  相似文献   

3.
背景:聚碳酸亚丙酯(PPC)具有良好的力学性能和生物相容性,但是也存在合成高分子的共性不足即缺乏生物活性.壳聚糖纳米纤维具有优异的生物活性,但力学性能较差,很难保持稳定的高强度三维结构.目的:将壳聚糖纳米纤维与聚碳酸亚丙酯复合,制备具有优异生物活性和良好力学性能的三维多孔组织工程支架.方法:用溶液浇铸/粒子沥滤法制备PPC多孔支架,再用相分离法原位复合三维壳聚糖纳米纤维制备聚碳酸亚丙酯,壳聚糖纳米纤维复合三维多孔支架(PPC/CSNF).用扫描电子显微镜观察PPC及PPC,CSNF多孔支架微观形态,并测定其压缩模量、孔隙率.用扫描电子显微镜观察PPC/CSNF多孔支架在新西兰大白兔大腿皮下埋植1,2个月后的细胞生长情况.结果:PPC多孔支架孔径分布为200-500 μm且孔连通性好,PPC/CSNF多孔支架中的壳聚糖纳米纤维分布均匀其直径在50~500 nm之间;各种质量浓度的支架孔隙率均为90%以上;各种支架的压缩模量随着PPC浓度的增加而增加,最高可达约15 MPa;体内埋植的实验结果表明PPC/CSNF多孔支架具有良好的生物活性,可促进骨髓基质干细胞向软骨细胞分化.结果提示溶液浇铸/粒子沥滤法与低温相分离法相结合成功制备了力学性能与生物活性优异的PPC/CSNF多孔支架.该支架可促进新西兰大白兔的骨髓基质干细胞向软骨细胞分化.  相似文献   

4.
背景目前已有转化的SV40细胞系、C2C12成肌细胞、心肌细胞、骨骼肌细胞以及胚胎干细胞用于修复哺乳动物心脏的细胞移植治疗,然而细胞移植治疗对严重缺失或损伤心肌结构的治疗的作用却不大.组织工程技术为此类心肌疾病的研究及治疗提供了新的可能性,目前已有可替代和改善骨、软骨、肾、肝、神经及皮肤等组织和器官的生物合成结构物.目的探索多孔胶原作为构建三维工程心肌组织支架材料的可行性.设计非随机、对照的实验研究.地点和对象实验地点军事医学科学院生物工程研究所.动物清洁级1日龄Wistar大鼠400~500只,雌雄不限,体质量8~10 g;成年Wistar大鼠五六只,雌雄不限,体质量250 g,由军事医学科学院实验动物中心提供,饲养温度22~26℃,湿度40%~60%.干预在无菌条件下,将经胰蛋白酶消化获得的新生鼠心肌细胞分别接种于三维多孔胶原支架和培养板上,比较心肌细胞在两种培养模式的代谢差异.主要观察指标检测葡萄糖比消耗率、乳酸比产率、乳酸转化率、肌酸激酶及乳酸脱氢酶活力变化.结果培养于多孔胶原支架上的心肌细胞的代谢特性近似于心肌细胞在培养板上培养的二维生长模式.结论心肌细胞与多孔胶原支架形成了具有自律性同步收缩的三维工程心肌组织,多孔胶原作为心肌组织工程的支架材料有良好的应用前景.  相似文献   

5.
人骨髓基质干细胞体外诱导向软骨细胞分化   总被引:6,自引:0,他引:6  
目的:探讨应用壳聚糖支架三维立体培养人骨髓基质干细胞(mesenchvmal stem cells,MSCs),体外诱导其分化成为软骨细胞的可行性。方法:实验于2004—02/11在中山大学附属第三医院中心实验室完成。相分离法制备三维多孔壳聚糖支架,检测支架的孔隙率、孔径。抽取人骨髓,密度梯度离心法分离纯化,体外培养扩增。将第3代MSCs复合于支架中培养,应用无血清诱导液诱导细胞向软骨细胞分化,通过组织学染色及扫描电镜观察细胞的形态、增殖及功能的改变。结果:通过相分离法可制备出高孔隙率的三维壳聚糖支架,孔隙率达86.5%,孔隙分布均匀且相互连通,平均孔径182μm。诱导分化的MSCs在支架中贴附良好,呈现典型的软骨细胞形态,并有细胞外基质分泌:结论:人MSCs在三维立体培养环境下可诱导分化为软骨细胞,作为种子细胞在组织工程软骨的构建及软骨损伤的修复中有良好的应用价值。  相似文献   

6.
背景:采用壳聚糖对磁性氧化铁纳米颗粒表面进行改性,一方面可改善磁性纳米氧化铁颗粒的团聚性,增加其稳定性,另一方面将来可用于肿瘤热疗及基因治疗。
  目的:制备将来可用于肿瘤热疗及基因治疗的磁性壳聚糖微球,评价其生物相容性。
  方法:采用改良的化学沉淀法制备磁性氧化铁纳米颗粒。采用超声乳化法将壳聚糖加入到磁性氧化铁纳米颗粒中制备磁性壳聚糖微球,进行以下实验:①MTT实验检测磁性壳聚糖微球浸提液的细胞毒性:分别以1640培养液、聚丙烯酰胺单体溶液、100%,75%,50%,25%的磁性壳聚糖微球浸提液培养L-929细胞。②溶血实验:在兔抗凝血中分别加入磁性壳聚糖微球浸提液、生理盐水及蒸馏水。③微核实验:在昆明小鼠腹腔分别注射含氧化铁磁流体5,3.75,2.5,1.25 g/kg的磁性壳聚糖微球混悬液、环磷酰胺及生理盐水。
  结果与结论:磁性壳聚糖微球粒径200-300 nm,分散效果有所提高。不同浓度的磁性壳聚糖微球浸提液对L-929细胞毒性为1级,属对细胞无毒性范畴。磁性壳聚糖微球浸提液的溶血率为0.69%,小于5%,符合医用材料的溶血实验要求。磁性壳聚糖微球混悬液未导致细胞DNA断裂和非整倍体化,未导致微核产生的遗传毒理作用,材料无致畸或致突变作用,因此磁性壳聚糖微球具有良好的生物相容性。  相似文献   

7.
背景:单独使用天然材料如胶原、明胶及纤维蛋白制备的支架虽然解决了生物相容性等问题,但由于其降解太快,在作为细胞支架时往往提前塌陷而达不到诱生新组织的目的.目的:探讨应用丝素与壳聚糖复合制作生物可降解三维多孔的支架,并对其特性进行检测.设计、时间及地点:材料学观察实验,于2008-06/2009-06在浙江省医学科学院生物工程研究所完成.材料:春蚕茧由浙江省海宁市马桥镇黄墩庙村蚕农赠送,壳聚糖为上海伯奧生物科技有限公司产品.方法:通过脱胶、溶解、透析这3个主要步骤制成质量浓度为15 g/L的丝素溶液,将壳聚糖溶解在2%的醋酸溶液中制成25g/L.的壳聚糖-乙酸溶液,然后将两者混合,配制成丝素/壳聚糖质量比分别为10:0,5:5,4:6,3:7,2:8,0:10的6种溶液,分别吸入24孔板中,4℃排出气泡后,-20℃预冷冻12 h,再冷冻干燥30 h.取出后梯度乙醇水化,再用NaOH-乙醇溶液中和稳定1h,漂洗后再次冻干.主要观察指标:光镜和扫描电镜观察各种质量比配制的支架孔径、结构;采用改良的液体替代法测定各种支架的孔隙率;测定各种支架在体外4周的降解率.结果:丝素/壳聚糖质量比为10:0制成的支架孔隙粗大,非常蓬松,易碎,溶失率太高;相反仅以壳聚糖制成的支架冻干后较硬,缺乏足够的弹性;以5:5,4:6,3:7,2:8制成的复合支架冻干后,支架较松软,类似海绵,随着壳聚糖浓度增加,支架硬度增加,支架上有分布均匀且细密的小孔.光镜下各个孔形态不规则,每个孔紧密相连并联通,孔径大小均匀,孔径20~100 μm,随着壳聚糖浓度增加,孔径逐渐减小.支架孔隙率测定结果显示,丝素/壳聚糖质量比为4:6组>5:5组>3:7组>2:8组,与丝素/壳聚糖质量比为2:8组比较,5:5组和4:6组的支架孔隙率均明显增大(P<0.05).各种质量配比制成的丝素-壳聚糖复合支架吸水膨胀率无明显差异(P>0.05).第4周时丝素/壳聚糖质量比为2:8组降解最慢,5:5组降解最快.结论:在理化性能方面,将丝素与壳聚糖混合制作的复合支架较单纯用丝素或壳聚糖制作的支架均显示出明显优势,其中用丝素/壳聚糖质量比为5:5及4:6制成的复合支架最符合软骨组织工程的需要.  相似文献   

8.
背景:骨髓间充质干细胞具有向多种间质细胞谱系分化的能力,且支架材料的性能对骨缺损的修复有重要影响。目的:观察聚左旋乳酸/壳聚糖纳米纤维三维多孔支架复合骨髓间充质干细胞治疗骨缺损。方法:对骨缺损模型兔分别采用空白植入、髂后上棘自体松质骨移植、聚左旋乳酸/壳聚糖纳米纤维多孔支架移植和复合了骨髓间充质干细胞的聚左旋乳酸/壳聚糖纳米纤维多孔支架移植修复缺损部位。结果与结论:至移植12周,移植复合了骨髓间充质干细胞的聚左旋乳酸/壳聚糖纳米纤维多孔支架的实验兔的缺损处有骨组织生成,支架材料降解,已完成缺损修复,其修复情况接近松质骨组;髂后上棘自体松质骨移植的实验兔的缺损修复完好,新形成的骨组织较规则;只植入聚左旋乳酸/壳聚糖纳米纤维多孔支架的实验兔有少量骨组织形成,材料部分降解;空白植入的实验兔缺损处无新生骨组织生成,主要由纤维结缔组织填充。说明新型的生物支架材料聚左旋乳酸/壳聚糖纳米纤维三维多孔支架与来源于新西兰大白兔的骨髓间充质干细胞复合培养后,植入同种异体兔股骨髁缺损处,使骨缺损的修复速度加快,表现为较好的体内诱导成骨的作用。  相似文献   

9.
背景:目前软骨支架材料的种类比较多,但还没有一种材料能完全符合软骨修复的要求。目的:观察在混合材料胶原-壳聚糖支架中软骨细胞的生长情况。方法:采用冷冻干燥法将质量分数为2%胶原与3%壳聚糖混合制备胶原-壳聚糖多孔支架。将分离培养的第2代兔软骨细胞接种到胶原-壳聚糖支架上,对照组将软骨细胞接种到无支架的培养板中。观察支架的孔隙率、吸水性及内部形态结构,MTT法检测软骨细胞在支架上的增殖情况,组织切片苏木精-伊红染色,扫描电镜观察细胞在支架的生长、贴附情况,RT-PCR检测细胞支架复合物蛋白聚糖和Ⅱ型胶原mRNA表达情况。结果与结论:胶原-壳聚糖支架的吸水性为(80.0±0.55)%,孔隙率为(88.5±1.5)%,孔径为100~150μm,复合细胞培养2周后,细胞增殖活力高,软骨细胞分泌的蛋白聚糖和Ⅱ型胶原mRNA表达明显高于对照组。说明质量分数为2%胶原与3%壳聚糖的混合支架适合软骨细胞生长和快速增殖,是一种良好的修复和重建软骨载体。  相似文献   

10.
目的:寻求在心肌组织工程中,能为培养的种子细胞提供可靠的仿生型心肌细胞外基质材料.方法:应用计算机检索CNKI和PubMed数据库中1996-01/2007-08关于组织工程支架材料的文章.结果:具有良好的生物相容性和机械性能的天然生物支架材料是心肌组织工程支架快速成形技术的首选材料.天然生物材料如胶原、壳聚糖等机械性能较差,可塑性不强,但生物相容性好,可用于心肌支架的快速成形.三维打印、激光烧结、立体印刷技术、选择性激光烧结快速成形技术制备出的支架均具有高的孔隙率,高的表面积体积比,孔与孔之间完全通透,宏观形状可控,孔隙率和孔径独立控制等优点.结论:采用离散,堆积成型原理,利用计算机和纳米高分子技术等高科技,结合传统心肌组织工程支架生物材料,对生物材料应用表面修饰或改性技术,有望为心肌组织工程提供较理想的支架材料.  相似文献   

11.
背景:以明胶为基体制备的组织工程支架材料具有良好的生物相容性和生物降解性能,但存在力学性能低,降解速率难以控制的缺陷。目的:制备一种软骨组织工程支架材料多孔聚乙烯醇/明胶复合物,并检测其理化性能和生物相容性。方法:采用乳化发泡法制备聚乙烯醇/明胶多孔支架,并通过电镜分析、力学测试、皮下植入实验,检测材料孔径和孔隙率、IR光谱、力学性能和生物相容性。结果与结论:多孔材料内部呈三维网状多孔结构,孔径均匀,有相似的孔隙率61.8%,含水率44.6%,抗拉强度为(5.01±0.03)MPa,抗压强度为(1.47±0.36)MPa,有较好的力学性能,IR光谱分析表明材料内部结构均匀。皮下植入后,炎症反应逐渐减轻,囊壁逐渐变薄,并趋于稳定,提示多孔聚乙烯醇/明胶支架材料具有较好的生物相容性和力学性能。  相似文献   

12.
The current study aimed to fabricate three‐dimensional (3D) polycaprolactone (PCL), polycaprolactone and β‐tricalcium phosphate (PCL–TCP) scaffolds via a selective laser‐sintering technique (SLS). Collagen type I was further coated onto PCL–TCP scaffolds to form PCL–TCP–COL scaffolds. The physical characters of these three scaffolds were analysed. The osteogenic potential of porcine adipose‐derived stem cells (pASCs) was compared among these three scaffolds in order to find an optimal scaffold for bone tissue engineering. The experimental results showed no significant differences in pore size and porosity among the three scaffolds; the porosity was ca. 75–77% and the pore size was ca. 300–500 µm in all three. The compressive modulus was increased from 6.77 ± 0.19 to 13.66 ± 0.19 MPa by adding 30% β‐TCP into a 70% PCL scaffold. No significant increase of mechanical strength was found by surface‐coating with collagen type I. Hydrophilicity and swelling ratios showed statistical elevation (p < 0.05) after collagen type I was coated onto the PCL–TCP scaffolds. The in vitro study demonstrated that pASCs had the best osteogenic differentiation on PCL–TCP–COL group scaffolds, due to the highest ALP activity, osteocalcin mRNA expression and mineralization. A nude mice experiment showed better woven bone and vascular tissue formation in the PCL–TCP–COL group than in the PCL group. In conclusion, the study demonstrated the ability to fabricate 3D, porous PCL–TCP composite scaffolds (PCL:TCP = 70:30 by weight) via an in‐house‐built SLS technique. In addition, the osteogenic ability of pASCs was found to be enhanced by coating COL onto the PCL–TCP scaffolds, both in vitro and in vivo. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

13.
Customized scaffold plays an important role in bone tissue regeneration. Precise control of the mechanical properties and biological functions of scaffolds still remains a challenge. In this study, metal and ceramic biomaterials are composited by direct 3‐D printing. Hydroxyapatite (HA) powders with diameter of about 25 μm and Ti‐6Al‐4V powders with diameter of 15–53 μm were mixed and modulated for preparing 3‐D printing inks formulation. Three different proportions of 8, 10, and 25 wt.% HA specimens were printed with same porosity of 72.1%. The green bodies of the printed porous scaffolds were sintered at 1,150°C in the atmosphere of argon furnace and conventional muffle furnace. The porosities of the final 3‐D‐printed specimens were 64.3 ± 0.8% after linear shrinkage of 6.5 ± 0.8%. The maximum compressive strength of the 3‐D‐printed scaffolds can be flexibly customized in a wide range. The maximum compressive strength of these scaffolds in this study ranged from 3.07 to 60.4 MPa, depending on their different preparation process. The phase composition analysis and microstructure characterization indicated that the Ti‐6Al‐4V and HA were uniformly composited in the scaffolds. The cytocompatibility and osteogenic properties were evaluated in vitro with rabbit bone marrow stromal cells (rBMSCs). Differentiation and proliferation of rBMSCs indicated good biocompatibility of the 3‐D‐printed scaffolds. The proposed 3‐D printing of Ti‐6Al‐4V/HA composite porous scaffolds with tunable mechanical and biological properties in this study is a promising candidate for bone tissue engineering.  相似文献   

14.
背景:软骨组织工程的核心是利用少量的细胞经体外培养、扩增后,在一定环境下附着在三维多孔支架上,再将细胞/支架复合体移植到体内形成新的组织.目的:拟构建兔骨髓间充质干细胞/壳聚糖-胶原支架复合体,探讨该支架作为软骨组织工程支架的可行性.设计、时间及地点:细胞-支架学体外观察,于2008-03/2009-02在武警医学院生物化学教研室完成.材料:日本大耳白兔6只用于分离培养骨髓间充质干细胞.医用壳聚糖粉末为山东奥康生物科技有限公司产品.Ⅰ型胶原为Sigma公司产品.方法:取3%的壳聚糖和2%的胶原混合的醋酸溶液,倒入72孔板内,-20℃预冷冻10 h,冻干机冷冻抽干24 h制作成壳聚糖-胶原支架.取第3代兔骨髓间充质干细胞,以1×109L-1密度接种于支架内,构建细胞/支架复合体.主要观察指标:傅里叶红外光谱、扫描电镜、液体置换法测定支架的理化性质,观察三维培养后细胞在支架上的生长情况.结果:壳聚糖-胶原支架孔径为160~380 μm,平均为270 μm,孔相通性好,支架孔隙率为(86.00±5.12)%.傅里叶红外光谱仪测定数据表明复合支架组成物有典型的壳聚糖、胶原峰,未发现有聚乙二醇峰.细胞/支架共培养24,48,72 h后,骨髓间充质干细胞可渗入支架多孔结构内,并黏附在支架上成簇生长,部分细胞已与支架融合.结论:壳聚糖-胶原支架基本符合软骨组织工程支架要求,能够作为种子细胞的承载体.  相似文献   

15.
背景:关节软骨修复的关键是软骨和软骨下骨的整体修复,然而目前尚缺乏理想的一体化支架。目的:制备聚羟基丁酸-羟基辛酸-胶原一体化支架,并分析其基本生物学特性。方法:以聚羟基丁酸-羟基辛酸、Ⅰ型胶原为材料,通过溶剂浇铸-颗粒沥滤法制备聚羟基丁酸-羟基辛酸-胶原一体化支架,观察支架超微结构,支架孔径及孔与孔的连通情况;液体置换法测定支架孔隙率。将乳兔骨髓间充质干细胞接种于聚羟基丁酸-羟基辛酸-胶原一体化支架上,扫描电镜观察细胞在支架上的黏附状态,MTT法测定细胞在支架上的生长曲线。结果与结论:一体化支架呈疏松多孔结构,软骨层孔径80-100μm,骨层孔径200-220μm,孔隙率(80.0±2.3)%。骨髓间充质干细胞在支架上黏附状态良好,增殖迅速。说明聚羟基丁酸-羟基辛酸-胶原骨软骨一体化支架具备适宜的孔隙结构和良好的生物亲和性。  相似文献   

16.
In recent years, various composite scaffolds based on hydroxyapatite have been developed for bone tissue engineering. However, the poor cell survival micro-environment is still the major problem limiting their practical applications in bone repairing and regeneration. In this study, we fabricated a class of fluffy and porous three-dimensional composite fibrous scaffolds consisting of hydroxyapatite and polyacrylonitrile by employing an improved electrospinning technique combined with a bio-mineralization process. The fluffy structure of the hydroxyapatite/polyacrylonitrile composite scaffold ensured the cells would enter the interior of the scaffold and achieve a three-dimensional cell culture. Bone marrow mesenchymal stem cells were seeded into the scaffolds and cultured for 21 days in vitro to evaluate the response of cellular morphology and biochemical activities. The results indicated that the bone marrow mesenchymal stem cells showed higher degrees of growth, osteogenic differentiation and mineralization than those cultured on the two-dimensional hydroxyapatite/polyacrylonitrile composite membranes. The obtained results strongly supported the fact that the novel three-dimensional fluffy hydroxyapatite/polyacrylonitrile composite scaffold had potential application in the field of bone tissue engineering.

A fluffy and porous (3D) HA composite fibrous scaffold was fabricated by employing an improved electrospinning technique combined with a bio-mineralization process.  相似文献   

17.
Repair of bone defects remains a major challenge in orthopaedic surgery. Bone tissue engineering is an attractive approach for treating bone loss in various shapes and amounts. The aim of this study was to prepare and evaluate the feasibility of a porous scaffold, which was composed of oligomeric proanthocyanidin crosslinked gelatin mixed with β‐tricalcium phosphate (GTP) and was seeded with bone marrow stromal cells (BMSCs) as a bone substitute. GTP scaffolds were made porous using a salt‐leaching method. The physicochemical properties of the scaffold were evaluated to determine the optimal salt:composite weight ratio. The results indicated that the GTP scaffold had a favourable macroporous structure and higher porosity when the salt:composite weight ratio was 4:1. Cytotoxic tests demonstrated that extracts from the GTP scaffolds promoted the proliferation of BMSCs. Rat BMSCs were seeded on a GTP scaffold and cultured in a spinner flask. After 2 weeks of culture, scanning electron microscopy observation showed that the cells adhered well to the surfaces of the pores in the scaffold. Moreover, this study explored the biological response of rat calvarial bone to the scaffold to evaluate its potential in bone tissue engineering. Bone defects were filled with BMSC‐seeded GTP scaffold and acellular GTP scaffold. After 8 weeks, the scaffold induced new bone formation at a bone defect, as was confirmed by X‐ray microradiography and histology. The BMSC‐seeded scaffold induced more new bone formation than did an acellular scaffold. These observations suggest that the BMSCs‐seeded GTP scaffold can promote the regeneration of defective bone tissue. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

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