As an interdisciplinary field, tissue engineering (TE) aims to regenerate tissues by combining the principles of cell biology, material science, and biomedical engineering. Nanotechnology creates new materials that might enable further tissue-engineering applications. In this context, the introduction of nanotechnology and nanomaterials promises a biomimetic approach by mimicking nature. This review summarizes the current scope of nanotechnology implementation possibilities in the field of tissue engineering of bone, muscle, and vascular grafts with forms on nanofibrous structures.
The replacement of damaged or injured parts of the body with new functional tissues is an age-old dream of mankind. Recently, the previously unreachable goal has moved within easy reach as the discipline of tissue engineering (TE) has opened a new area of science and medicine. The term “tissue engineering” was defined in 1987 as the “Application of the principles and methods of engineering and life sciences toward the fundamental understanding of structure-function relationships in normal and pathologic mammalian tissue and the development of biological substitutes to restore, maintain, or improve function” . Today, TE pursues three main strategies: (1) transplantation of cells through injection or transplantation into the body; (2) implantation of different types of matrices alone or in combination with growth factors and/or cells into the defect site in order to repair or regenerate tissue defects; and (3) seeding and culturing of three-dimensional biodegradable and/or biocompatible scaffolds with specific cell types in order to mimic the natural extracellular matrix (ECM). Nanotechnology is defined as the manipulation of matter over the scale of a nanometer, whereby, nanofibrous scaffolds can be produced containing nanofibers as scaffolds with two similar external dimensions in the nanoscale and the third dimension which is significantly larger. It is here that nanotechnology and nanomaterials come into play in order to fabricate optimal three-dimensional scaffolds for the attachment, proliferation, and ingrowth of cells aiming at new tissue formation. Here, the combination of the microporous structure and the high surface area to volume ratio of nanofibrous scaffolds favor cell adhesion, proliferation, migration and differentiation, thus, presenting all the desired properties for tissue engineering concepts . This review summarizes the current scope of nanofibrous implementation possibilities in the field of tissue engineering of muscle, bone, and vascular tissue.
Currently, nanofibers can be fabricated using a variety of techniques such as electrospinning, phase separation, or self-assembly. Molecular self-assembly peptide systems have been developed for the fabrication of nanofibrous scaffolds in order to mimic the human ECM [3, 4]. Peptide amphiphiles (PAs) consist of a hydrophobic tail group and a short peptide sequence. The drawing method continuously forms nanofibers by drawing and solidification of a viscous liquid polymer solution, which is pumped through a glass micropipette . The phase separation technique is a relatively simple procedure with minimal equipment requirements . It was first described by Ma and Zhang in 1999 and consists of five steps: polymer dissolution, phase separation process, gelation, extraction of solvent from the gel with water, freezing, and freeze-drying under vacuum . Here, scaffolds with arbitrary anatomical shapes can be fabricated. Macroporosity can be achieved by incorporation of porogens. Recently, Lee et al. used room temperature ionic liquids (RTILs) as a novel porogen, which were incorporated into a poly(lactic acid) scaffold with further selective dissolution . Using this technique, they were able to create an interconnected network with pore sizes of over 100 μm and a porosity of 86%–94%. The authors also studied these scaffolds for their interaction with mesenchymal stem cells (MSCs) derived from rat bone marrow. They were able to show good biocompatibility of the scaffolds in vitro as well as in vivo in a subcutaneous rat model as well as an increased osteogenic differentiation of MSCs seeded on the PLA scaffolds. Zhao et al. were able to create a porous biomimetic poly(propylene carbonate) (PPC) scaffold with a nanofibrous chitosan network using a dual solid-liquid phase separation technique . In the first phase separation, the PPC scaffold was built, and in the second phase separation, the nanofibrous chitosan structure was prepared in the macropores. The advantage of the phase separation technique is the relatively simple experimental setup. Furthermore, different architectures in nano- and macrostructure can be incorporated in the matrix design.
Out of these methods, electrospinning has emerged as a very efficient technique due to its relatively simple and inexpensive design . Electrospinning was first described by Formhals in 1934 and became popular in the 1990s. Electrospinning produces scaffolds with specific fiber orientation and high surface area. The electrospinning system basically consists of four major components:
a voltage power supply
a syringe pump, which determines the flow rate of the polymer solution
a needle shifts the polymer solution into the high electric field
a collector on which the nanofibers are collected .
Further adjustments can be made by varying the polymer solution properties, applied voltage, flow rate, or the distance between the needle and the collector. Depending on the desired tissue, different polymers can be used for electrospinning purposes. Typical polymers with relevance for biomedical applications can be divided into synthetic polymers such as polycaprolactone (PCL), poly(lactic acid) (PLA), poly(lactic acid-co-caprolactone) (PLCL), or poly(glycolic acid) (PGA) and natural polymers such as collagen, gelatin, or elastin. Synthetic polymers offer good mechanical stability but often lack biocompatibility, whereas natural polymers behave the other way. To overcome these shortcomings of natural and synthetic polymers, composites of different polymers have been created. Recently, Rim et al. reviewed the current approaches for the fabrication of electrospun nanofibers for tissue engineering . The author emphasizes, that the successful fabrication of the electrospun nanofibrous scaffolds starts with the selection of proper polymers, whereas the combination of multiple polymers as well as various postmodification techniques may provide desirable properties . Beachley and coworkers addressed the limited control over fiber organization within nanofiber structures when they are collected directly from an electrospinning jet . Furthermore, a novel assembly technique that utilizes parallel automated tracks to orient and collect nanofibers from the electrospinning jet has been established in order to allow a better control of the fiber organization, resulting in a continuous steady-state delivery of static stabilized nanofibers . Another study by Leverson and colleagues demonstrated the feasibility of fabrication of electrospun scaffolds containing two differently scaled fibers as well as scaffolds containing fibers of the two different materials fibrin and PCL using a dual extrusion electrospinning setup.
To date, there are only a few alternatives for functional restoration of damaged muscle tissue. The surgical techniques of muscle transplantation or transposition present a reasonable possibility with good results for restoration of function, especially for the paralyzed face or the upper extremity . Although the present clinically used flaps enable, in most cases, a functional satisfiable reconstruction, there are some procedural limitations: extended flaps cause significant morbidity at the donor site that can implicate long-term pain and functional limitations. As a new alternative, skeletal muscle tissue engineering may overcome problems associated with the autologous transfer of muscle tissue.
For muscle tissue engineering, fibrous matrices with a high degree of parallel orientation (Figure 1) are necessary to guarantee a high cell density and the application of longitudinal force after contraction [15, 16]. Several techniques have been described to obtain the aligned architecture, such as selective laser sintering, three-dimensional printing, or unidirectional freeze-drying [17–20]. Electrospinning as an alternative method offers the opportunity to produce nanofibers as mentioned below (Table 1). The technique allows the adjustment of matrix properties by varying multiple parameters such as the concentration and viscosity of the spinning solution, the voltage applied, or the flow rate of the spinning solution. A variety of different synthetic polymers and biopolymers can be used for electrospinning of matrices for muscle tissue engineering. Natural polymers, such as collagen [21–23], hyaluronic acid , or elastin , mimic the natural extracellular matrix perfectly and, therefore, offer the advantage of cell attachment and differentiation of cells. The main disadvantage of natural polymers is their low stability in vivo; therefore, these matrices are often combined with synthetic polymers. The most frequently used synthetic polymer in muscle tissue engineering is poly(ɛ-caprolactone) (PCL) [25, 26]. PCL is a synthetic polymer with a slow degradation rate, biocompatible but also hydrophobic and, therefore, shows poor cell attachment . To combine natural and synthetic polymers, different techniques can be used. The most frequently used methods are spinning polymer-blend solutions, core-shell spinning, or co-spinning of different polymer solutions. Using the core-shell spinning technique, matrices can also be used as drug delivery systems (DDS) in order to release incorporated drugs in a controlled manner. Yoon et al. used a core-shell laminated, structured, electrospun mat of hydrophobic PCL and hydrophilic poly(ethylene oxide) (PEO)/rhodamine-B fibers in a normal PCL matrix . Rhodamine was released from the scaffold by the drug delivery system, which was controlled by the thickness of the PCL layer. Matrices can also be loaded with growth factors such as angiogenic growth factor VEGF to reduce vascularization time  or growth factors to induce myogenic differentiation, e.g., insulin-like growth factor . Rende et al. were able to show an effect of the nerve growth factor (NGF) on the differentiation and proliferation of myogenic cells in vitro via the tyrosine kinase receptor TrKA . Furthermore, Lavasani et al. showed the effect of retroviral transduction of MDSCs with a CL-NGF vector and direct stimulation of MDSCs with an NGF protein on the muscle regeneration in dystrophic muscles . They were able to show that NGF acts as a modulatory device and improves muscle regeneration. Despite other tissue engineering applications, matrices with incorporated DDS have still not been applied for muscle tissue engineering.
|Author, year||Material||Animal model||Cells||Results|
|Yoon, 2011 ||PCL/PEO/rhodamine-B||In vitro||–||Initial burst in drug release was eliminated by thickness of PCL layer|
|Rende, 2000 ||–||In vitro||Myogenic cell line L6, primary cultures of adult human myoblasts, human rhabdomyosarcoma cell line TE-671||Effects of NGF are mediated by TrKA receptors, NGF is needed for increase fusion into myotubes|
|Baker, 2008 ||PCL/PEO||In vitro||MSC||>50% PEO fibers improved cell infiltration of scaffolds|
|Prabhakaran, 2011 ||PLLA/PANi||In vitro||Nerve stem cells||Electric stimulation of scaffolds resulted in extended neurite outgrowth|
|Guarino, 2013 ||PANi/PEGDA||In vitro||PC12/hMSC||PANi increased electric conductivity of matrices|
It has been noted that cell infiltration into dense electrospun scaffolds with reduced pore size and aligned orientation is limited . The co-spinning procedure holds the opportunity to control the pore size by using the so-called sacrificial fibers such as poly-(ethylene-oxide) (PEO), which dissolves in water . Baker et al. developed a dual-electrospinning method to produce scaffolds containing a PCL and a PEO component . They seeded the novel matrix and a pure PCL matrix with mesenchymal stem cells (MSCs) and were able to show an increased cell infiltration and distribution after 3 weeks in culture in the PCL/PEO group. In a recent study, Baker et al. removed 50% of the initial fibers resulting in a scaffold, which provides sufficient instruction to align cells and direct the formation of a highly organized ECM . After the electrospinning procedure, matrices have also been secondarily modified by coating or plasma treatment .
Using the electrospinning technique, electrically conductive matrices can be created by spinning conductive polymers such as polyaniline (PANi) . This can lead to an increased myogenic differentiation of seeded myoblasts  and neurite outgrowth of cultured neural stem cells . Prabhakaran et al. fabricated an electrospun scaffold containing poly-l-lactide (PLLA) that was blended with PANi to obtain PLLA/PANi nanofibers. In vitro electrical stimulation of nerve stem cells seeded on the PLLA/PANi matrices resulted in extended neurite outgrowth compared to the cells grown on nonstimulated scaffolds . In a recent study, Guarino et al. built a hybrid material that was fabricated by in situ precipitation of PANi in polyethyleneglycol diacrylate (PEGDA) solution, followed by crosslinking via UV irradiation resulting in a porous architecture by sodium chloride particle leaching . The hybrid matrices improved proton conductivity and the biological response of PC12 and human mesenchymal stem cells (hMSCs).
Tissue engineering of small-diameter vascular grafts is one of the most challenging and necessary fields of research as cardiovascular disease causes almost 1 million deaths in the US alone every year . For myocardial revascularization, autologous arterial or venous grafts still represent the gold standard. The availability of vessel grafts is limited because of poor quality, limited length, previous surgical treatment, or peripheral arterial occlusive disease. Therefore, many studies have focused on the generation of vascular grafts (Table 2). Although vascular grafts composed of expanded polytetrafluoroethylene (ePTFE) or polyethyleneterephthalate (Dacron) are already used in the replacement of large- or medium-diameter vessels, these materials are not suitable for small-diameter vessels because of thrombosis and intimal hyperplasia [43, 44]. According to Ercolani et al., the optimal small-diameter vascular graft should have mechanical strength, biocompatibility, an acceptable healing response, and ease of handling . Electrospinning has become an effective technique for the production of small-diameter vascular grafts because these grafts mimic the natural extracellular matrix with a high surface area, high porosity, and small pores. Different natural and synthetic materials have been used in the past for vascular tissue engineering. Poly(lactic-co-glycolic) acid (PLGA) is a synthetic polymer and has been used for small-diameter vascular grafts. Kim et al., for example, preseeded the grafts with endothelial cells and smooth muscle cells, which showed an increased patency rate compared to nonseeded grafts. One disadvantage of PLGA is its fast degradation rate, which makes this material unsuitable for vascular graft engineering [46, 47]. Diban et al. produced hollow fiber (HF) membranes via phase inversion using blends of PLGA with PCL . They investigated the influence of different ratios of PLGA and PCL regarding water transport and mechanical properties as well as cell attachment and proliferation of human adipose stem cell (hASC). They showed that PCL/PLGA85/15 HFs meet the mechanical requirements of small-caliber vessels. Furthermore, the blended PCL/PLGA85/15 HFs supported more hASC attachment and proliferation than PCL and PLGA HFs. Wang et al. fabricated pillared PLGA nanoscaffolds using the femtosecond laser ablation technique . They seeded bovine endothelial cells onto the scaffolds and were able to demonstrate cell adherence and growth around each branch of the pillared microvessel networks.
|Author, year||Material||Animal model||Explantation time point (max.)||Cells||Stenotic changes||Degradation||Results|
|Diban, 2013 ||PLGA/PCL||In vitro||–||hASC||–||–||PCL/PLGA85/15 blend best results|
|Wang, 2012 ||PLGA||In vitro||–||Bovine endothelial cells||–||–||Cells adhere and grow around the pillared microvessels|
|De Valence, 2012 ||PCL||Rat aorta||18 months||–||No thrombosis, limited intimal hyperplasia||Fiber fragmentation||Insufficient regeneration of vascular wall|
|Innocente, 2009 ||PCL/PTX||In vitro, rat aorta||6 months||–||No thrombosis||–||PTX loading delays endothelialization and cellular ingrowth|
|Kuwabara, 2012 ||PCL||Rat carotid||72 weeks||–||72.5% Patency||PCL degraded||Regeneration of media layer progressed over time|
|He, 2005 ||P(LLA-CL)||In vitro||–||HCAEC||–||–||Scaffolds enhance viability, spreading, and attachment of HCAEC|
|Dong, 2008 ||P(LLA-CL)||In vitro||–||PCASMC||–||Eight month degradation time||Generation of multilayered aligned oriented PCASMCs on the scaffolds|
|Bergmeister, 2012 ||PU||Rat aorta||6 months||–||95% Patency rate||–||Cell immigration and differentiation seemed to be promoted by the scaffold|
|Soldani, 2009 ||PEtU–PDMS||Sheep carotid||24 months||–||No thrombosis at 24 month||Complete degradation||Superior handling and patency rates of PEtU-PDMS grafts compared to ePTFEE|
|He, 2010 ||PU||Rat aorta||8 weeks||Pericytes||100% Patency rate in seeded grafts||Remodeling with collagen and elastin||Increased patency by seeded pericytes|
|Del Gaudio, 2012 ||PCL/PHBV||In vitro||–||Rat cerebral EC||–||–||PCL-based scaffolds promote cell attachment and migration|
|McClure, 2010 ||Silk fibroin, collagen, elastin, PCL||In vitro||4 weeks||–||–||Signs of degradation||COL-blended samples displayed better mechanical properties than SF-blended samples|
|Stroncek, 2012 ||ePTFE||In vitro, rat femoral artery||28 days||EPC/+AdTM||75%–88% Patency rate in cell seeded grafts||–||EPCs increase patency rate of ePTFE grafts|
PCL has been used in several studies for small-diameter vascular grafts. The advantages of PCL are its slow degradation rate, good mechanical properties, and fast endothelialization [50–52]. de Valence et al. recently used PCL vascular grafts in the rat abdominal aorta replacement model . They built vessels with an inner diameter of 2 mm by electrospinning of PCL. The grafts were implanted in the abdominal aorta replacement model in Sprague-Dawley rats for 1.5, 3, 6, 12, and 18 months and were tested regarding patency, compliance, and scaffold degradation. They reported graft thrombosis and no significant change in graft compliance over time. Also, a decrease in molecular weight and fiber fragmentation were observed at 18 months although this lead to no measurable limitations of its mechanical properties. Cellular infiltration and vascularization were detected in the early stages followed by a progressive regression over time.
Pektok et al. compared small-diameter vascular grafts made of PCL nanofibers with ePTFE grafts regarding in vivo degradation and patency . They implanted vascular grafts into the abdominal aorta of 15 Sprague-Dawley rats per group. After 3, 6, 12, 18, and 24 weeks, the infrarenal segment of the abdominal aorta, including the implanted graft, was explanted and processed to histological evaluation. At the explantation time point, all grafts were patent without significant stenosis detectable in the digital subtraction angiography. Histological analysis of the PCL grafts showed a rapid endothelialization of the luminal part and a neovascularization throughout the graft starting 3 weeks after implantation with further increase over time. In the ePTFE group, the grafts revealed a delayed endothelialization and a slow cellular infiltration. Also, no neovascularization was detected in the ePTFE grafts, and the increased neointima formation led to two stenotic lesions after 18 and 24 weeks.
To reduce intimal proliferation, Innocente et al. combined small-caliber vascular prostheses using electrospun PCL with slow-releasing paclitaxel (PTX), which should show an antiproliferating effect . PLX has been used to reduce in-stent restenosis, therefore, PLX-loaded PCL grafts were used in this study. The grafts were fabricated by electrospinning of PCL solutions (12% wt/vol) containing PTX (0.75 wt/wt). They were compared to plain PCL grafts in the abdominal aorta in vivo model in Sprague-Dawley rats. Grafts were explanted after 3, 12, and 24 weeks followed by histological evaluation. DSA showed no stenosis or aneurysm in both groups at any explantation time point. Histological evaluation revealed reduced intima formation in the PCL-PTX grafts compared to the plain PCL grafts. No differences were observed regarding cellular infiltration and neovascularization of vascular grafts.
Kuwabara et al. reported electrospun nano-scale PCL grafts with a diameter of 0.7 mm and a porosity of approximately 78% . They implanted the grafts in a rat carotid artery replacement model followed by explantation after 1, 2, 6, 12, 24, 48, and 72 weeks. Histological evaluation revealed a patency rate of 72.5% without any aneurysmal formation or calcifications. The endothelium was completed in the early phase without any further modifications, whereas the tunica media only partially regenerated, and degradation of the scaffolds was visible over time without completion.
“Poly(l-lactide-co-e-caprolactone) P(LLA-CL)” nanofibers show a slow degradation rate and were used in different studies for vascular tissue engineering over past years, also combined with smooth muscle cells [56, 57]. He et al. used electrospun collagen blended P(LLA-CL) nanofibers to fabricate vascular grafts . They were able to show improved cell attachment and viability compared to plain polymer nanofibers. Dong et al. used porcine coronary artery smooth muscle cells (PCASMCs) seeded and cultured on P(LLA-CL) scaffolds for up to 105 days . They were able to demonstrate that PCASMCs did not influence the degradation rate of the P(LLA-CL) scaffolds, although they did attach and proliferate on the scaffolds. They also created multilayers of aligned oriented PCASMCs and a significant ECM protein secretion.
Polyurethane (PU) has become a promising material for vascular tissue engineering because of its high tensile strength, good elasticity, and biocompatibility . Bergmeister et al. evaluated vascular grafts composed of PU with an inner diameter of 1.5 mm and a mean wall thickness of 70 mm . They used the rat abdominal aorta model to investigate their in vivo biocompatibility after 1 week, 1, 3, and 6 months. They were able to show a patency rate of 95% without any visible graft degradation. Within the first month, the luminal surface was covered with endothelial cells with no further hyperplasia except for one specimen. The grafts showed ongoing cell proliferation up to 6 months after implantation. Soldani et al. fabricated small-diameter vascular grafts with a poly(ether)urethane-polydimethylsiloxane (PEtU-PDMS) semi-interpenetrating polymeric network with a diameter of 5.0 mm and a wall thickness of approximately 500 mm . They implanted the grafts in the carotid artery of adult sheeps with ePTFE grafts as a control group. All PEtU-PDMS grafts were patent, while 50% of the control grafts were found to be occluded. Also PEtU-PDMS grafts showed signs of remodeling with partial degradation and no visible signs of calcification. He et al. produced bilayered elastomeric poly(ester-urethane)urea scaffolds, which were preseeded with pericytes . The inner scaffold was fabricated by the thermally induced phase separation (TIPS) method, whereas the outer surface was produced by electrospinning to achieve an inner porous layer and an external fibrous layer. Seeded and unseeded scaffolds were implanted in the rat abdominal aorta model, and patency was checked using angiography. After 8 weeks, the specimens were explanted and processed to histological evaluation. Angiograms showed a significantly increased patency in the pericyte seeded group compared to the unseeded specimens. In the TIPS layer, signs of remodeling were visible, whereas in the electrospun layer, no cell infiltration was detected.
Because synthetic materials often lack cell attachment properties, they are combined with natural polymers such as collagen or elastin. This can be obtained by either blending or co-electrospinning of the synthetic and natural polymer. To improve attachment of cells, mainly PCL was blended with multiple synthetic or natural polymers such as collagen, elastin, or gelatin with different blend ratios [63–66]. Recently, del Gaudio produced PCL scaffolds blended with hard poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a polymer of microbial origin, and evaluated these scaffolds regarding their mechanical properties and attachment and growth of rat cerebral endothelial cells . They were able to demonstrate that PCL and PCL/PHBV scaffolds promoted attachment and migration of rat cerebral endothelial cells.
The normal vessel architecture consists of three layers: the intima, media, and adventitia. To mimic this structure, layered vascular grafts have gained attention over the last years. Multilayered vascular grafts consist of usually two or three different natural or synthetic polymers such as PCL, collagen, elastin, or gelatin [67–69]. McClure et al. composed a tri-layered vascular graft of polycaprolactone, elastin, collagen, and silk to mimic the native vascular wall and its mechanical properties .
Also, cells such as endothelial cells or endothelial progenitor cells have been investigated to improve endothelialization . Stroncek et al. used small-diameter ePTFE vascular grafts seeded with endothelial progenitor cells (EPCs) or with EPCs transfected with an adenoviral vector containing the gene for human thrombomodulin (EPC+AdTM) . In vitro results showed an increase in activated protein C (APC) in the EPC+AdTM group compared to the EPC group. They also performed in vivo studies in athymic rats. They implanted EPC-seeded vascular grafts, EPC+AdTM grafts, and plain ePTFE grafts bilaterally into the femoral arteries for 7 and 28 days. Control grafts showed a patency rate after 7 days of only 25%, whereas EPC seeded and EPC+AdTM seeded specimens were found to have patency rates of around 90% after 7 days and 75%–88% after 28 days.
Scaffolds with a pore size between 100 and 350 μm and a porosity >90% have been identified as optimal for cell/tissue ingrowth and, thus, enhanced bone regeneration . Furthermore, the optimal nanofibrous scaffold for bone tissue engineering has to meet the physical requirements of natural bone, such as mechanical strength, porosity, pore size, rigidity, and above all, three-dimensional architecture [72–74]. A nanoscaled structure of the material surface has been found to have enhanced positive effects on osteoblastic cell response, increasing initial cell adhesion and subsequent proliferation, and finally, the osteogenic differentiation. This seems not to be surprising when we keep in mind that the natural environment of cells, the ECM, composed of organic and inorganic phase, is also organized in nanodimensions. Therefore, many of the novel bio-inspired scaffolds try to mimic this effect of ECM on cells by creating nanostructured surfaces (Table 3).
|Author, year||Material||Animal model||Explantation time point (max.)||Cells||Results|
|Peng, 2011 ||HA/PLLA||In vitro||–||Rat osteosarcoma ROS17/2.8 cells||HA/PLLA scaffolds showed higher cell viability and ALP activity than a pure PLLA scaffolds|
|Jegal, 2011 ||Gelatin-apatite-PLCL||In vitro, rat calvarium||6 weeks||–||More enhanced bone formation in composite scaffolds than in pure PLCL nanofiber|
|Liu, 2009 ||NF-gelatin/apatite||In vitro||–||Osteoblasts||Apatite incorporation showed increased osteogenic differentiation and mechanical strength of scaffolds|
|Woo, 2009 ||PLLA||Rat calvarium||8 weeks||–||Nanofibrous scaffolds showed more new bone tissue formation than solid-walled scaffolds|
|He, 2010 ||PLLA||In vitro||–||–||Possible modifications of matrix characteristics by varying processing parameters|
|Cheng, 2013 ||CNT/PLGA||In vitro||–||MC3T3-E1 osteoblasts||CNTs increased cell attachment and proliferation and osteogenic differentiation of scaffolds|
|Zhang, 2011 ||CNT/Ti6Al4V/nano HA||In vitro||–||–||CNT-reinforced HA coating increased coating hardness|
|Schofer, 2011 ||PLLA+BMP-2||Rat calvarium||12 weeks||–||PLLA/BMP-2 increased bone formation in defect model compared to PLLA or bovine spongiosa alone|
|Srouji, 2011 ||PEO/PCL/PEG+BMP-2||Rat calvarium/in vitro||8 weeks||MSC||Significantly increased bone generation in BMP-2 group|
|Seyedjafari, 2010 ||PLLA-COL||In vitro||–||USSC||Enhanced proliferation and osteogenic differentiation of USSC on PLLA-COL nanofibers|
|Lu, 2013 ||PHBV/HA||Rabbit radius/in vitro||12 weeks||MSC||Osteogenic differentiation and bone generation by introduction of HA|
|Binulal, 2012 ||nG/PCL||In vitro||–||hMSC||Faster degradation and increased cell attachment by gelatin nanoparticles|
|Wepener, 2012 ||HA/βTCP||In vitro||–||Osteoblasts/monocytes||Scaffolds are biocompatible|
|Li, 2012 ||CelluNF/HAp||In vitro||–||–||CelluNF/HAp scaffolds consist of micro-, meso-, and macropores|
|Guo, 2012 ||Wnf-CPC||Rabbit femur, in vitro||12 weeks||MG-63||Wnf-CPC showed increased bone formation and cell attachment compared to CPC alone|
|Wang 2012 ||Silicia-titania||In vitro||–||MSC||Osteogenic cell differentiation was enhanced on the Ti:Si=7:3 group|
|Ganesh, 2012 ||PCL/nSiO(2)||In vitro||–||hMSC||Increased osteogenic differentiation and scaffold strength by incorporation of nSiO(2)|
For bone tissue engineering, composite scaffolds with a mineral component such as hydroxyapatite (HA), calcium phosphate (CaP), or bioglass are commonly used. Wei et al. produced nano-hydroxyapatite (NHAP)/PLLA composite scaffolds with high porosity, well-controlled pore architectures, and improved mechanical properties using TIPS techniques and dioxane/water as a solvent . Peng et al. incorporated needle-shaped nano- or micro-sized HA particles into PLLA scaffolds using the electrospinning technique . The composite scaffolds revealed a higher viability of attached rat osteosarcoma ROS17/2.8 cells and higher ALP activity compared to plain PLLA specimens. Jegal et al. fabricated composite scaffolds using electrospun gelatin-apatite-poly(lactide-co-caprolactone) (PLCL) . By adding only a small concentration of gelatin-apatite into PLCL, the tensile strength was significantly increased compared to plain PLCL matrices. Also osteogenic differentiation of pre-osteoblast cells was increased in the composite specimens compared to the pure PLCL nanofibers. In vivo studies were carried out using a rat calvarial critical size model. Composite scaffolds showed an increased defect closure compared to pure PLCL nanofibers and the blank control group.
Liu and coworkers developed nanofibrous gelatin/apatite (NF-gelatin/apatite) composite scaffolds using the TIPS technique . They were able to show an increased surface area and mechanical strength of NF-gelatin scaffolds compared to commercially available gelatin foam. By incorporating apatite particles on the surface of NF-gelatin scaffolds, osteogenic differentiation of seeded cells was also improved.
Woo et al. tested the biological properties of nanofibrous PLLA scaffolds compared with solid PLLA scaffolds in the critical-size rat calvarial bone defect . After 4 and 8 weeks, specimens were explanted and analyzed using micro-CT and histological evaluation. They found out that 8 weeks after implantation, nanofibrous scaffolds lead to a significantly increased bone formation compared to solid-walled specimens. They also suggested, based on the results of the immunostainings, that nanofibrous matrices lead to an osteogenic differentiation of recruited cells. Jose and coworkers tested a combination of PLGA with collagen and nano-HA to produce bioactive multicomponent nanofibrous scaffold by electrospinning . They showed that nano-HA was beneficial in small amount regarding mechanical properties, whereas high concentrations were detected to be disadvantageous. The multicomponent scaffold revealed to be supportive for hMSC adhesion and proliferation.
He et al. developed a novel technique to accelerate calcium phosphate deposition on nanofibers . Using an electrodisposition technique, they were able to reduce mineralization time to under an hour.
Cheng et al. produced carbon nanotube (CNT)-PLGA composite scaffolds in order to enhance mechanical strength and surface roughness . CNT/PLGA scaffolds were fabricated by a dispersion of CNT in PLGA/dichloromethane solution followed by a solvent casting and particulate leaching step. The obtained results demonstrated an increased mechanical strength by incorporation of CNTs and an increased adhesion, growth, and osteogenic differentiation of MC3T3-E1 osteoblasts. Zhang and coworkers produced nanostructured HA coatings with nano-sized HA powder on titanium alloy Ti6Al4V by cathodic electrophoretic deposition (EPD) . Also, multiwalled CNTs were integrated into the HA coating, which lead to a markedly increased coating hardness compared to monolithic HA coating.
Although composite nanomaterials have been widely used for bone tissue engineering, pure synthetic polymers combined with growth factors have been tested to achieve osteoinductivity.
Schofer et al. incorporated bone morphogenetic protein 2 (BMP-2) into PLLA nanofibers . They implanted PLLA/BMP-2 scaffolds into a critical size rat calvarial defect model and compared these samples with pure PLLA specimens, bovine spongiosa, and blank defects. Explantation time points were 4, 8, and 12 weeks after scaffold implantation. Micro-CT and Histology revealed only small amounts of bone formation in the plain PLLA scaffolds, whereas PLLA/BMP-2 specimens showed an early onset of bone regeneration after 4 weeks and the fastest bone regeneration than any other tested group. Srouji et al. developed a core-shell fiber scaffold releasing BMP-2 for bone tissue engineering . The core of the matrix was made of an aqueous core solution of poly(ethylene oxide) with incorporated BMP-2, and the shell was made of polycaprolactone blended with poly(ethylene glycol). First, they demonstrated in vitro bioactivity of these scaffolds by measuring ALP levels using human mesenchymal stem cells. For in vivo testing, they implanted BMP-2-containing and pure core-shell scaffolds in a rat cranial defect model followed by explantation after 8 weeks. In vitro results demonstrated a significantly ninefold increased ALP activity in the BMP-2 group compared to the plain scaffolds and a slow-release kinetic of the BMP-2. Furthermore, the in vivo study showed an increased bone formation in rat calvarial defects of approximately 80% of the total area compared to 40% coverage in the plain group.
Seyedjafari et al. tested cord blood-derived unrestricted somatic stem cells (USSC) on collagen-grafted PLLA nanofibers . They found out that the nanofibrous structure of pure PLLA promoted USSCs to osteogenic differentiate and to proliferate, whereas PLLA-COL led to an increased expression of osteogenic markers.
Recently, electrospun poly-3-hydroxybutyrate-co-3- hydroxyvalerate (PHBV)/HA nanofibrous scaffolds were evaluated in a critical-sized rabbit radius defect model. The investigators demonstrated good capacity of the scaffolding system for repairing rabbit bone defects . PCL, as mentioned above, is a semisynthetic polymer with excellent biocompatibility and biodegradability, which make it a good candidate for bone tissue engineering applications [88, 89]. Herein, electrospun nanofibrous semisynthetic polymeric PCL scaffolds were modified with a maximum of 15 wt% of biopolymeric gelatin nanoparticles (nGs), resulting in a significant increase in the degradation rate of PCL. In addition, hMSCs attached and spread faster and better on PCL-nG scaffolds when compared with PCL-scaffolds alone . In another experimental study, either osteoblasts or osteoclast-like cells were cultured separately on electrospun nanobioceramic biphasic hydroxyapatite/β-tricalcium phosphate scaffolds, showing good biocompatibility of the scaffolds without significant negative effects on either osteoblasts or osteoclast-like . As another modification, electrospun cellulose nanofiber (CelluNF)/HA scaffolds were synthesized in simulated body fluid (SBF) in order to mimic the formation of collagen fiber/HA in bone tissue. As a modification, the phosphorylation of CelluNFs resulted in a highly effective guide of HA growth along the nanofibers . Guo and coworkers investigated a self-setting bioactive cement by incorporation of wollastonite nanofibers (WNFs) into calcium phosphate cement (CPC). There was no effect on the setting time or the compressive strength of wnf-CPC but the hydrophilicity and degradability of wnf-CPC were significantly improved by the addition of WNFs, which was attributed to the change of microstructure. Wnf-CPCs were then evaluated in a rabbit bone defect model and showed enhanced efficiency of new bone formation when compared with CPC alone . The excellent biocompatibility and bioactive properties predestine ceramic scaffolds for use as bone graft substitutes for bone regeneration applications. Furthermore, oxides such as silica as the major component of bioactive glasses and titania as the oxide complex of titanium have been found to enhance osteoblast differentiation . In this context, the osteogenic potential of electrospun silica-titania nanofibrous meshes was evaluated by seeding MSCs and resulted in enhanced cell differentiation toward osteoblasts, especially with a Ti:Si ratio of 7:3 . Despite its excellent biocompatibility, the relatively low bioactivity and inadequate mechanical properties are disadvantages of the native PCL for bone tissue engineering applications . As already mentioned, there is evidence in the literature that incorporating silica leads to an increase in scaffold osteogenicity, bioactivity, and mechanical properties [95, 97, 98]. Recently, Ganesh and coworkers investigated a modification of PCL nanofibrous scaffolds through adding nanoparticles of silica (nSiO2) in order to take advantage of the osteogenic potential of silica while at the same time reinforcing the PCL nanofibers and, thus, improving their mechanical properties for bone tissue engineering applications. A substantial increase in scaffold mechanical strength, protein adsorption, and osteogenic differentiation of MSCs could be demonstrated by embedding silica nanoparticles . The main advantage of natural polymers, such as collagen, elastin, gelatin, hyaluronic acid, and silk is the very similar structure to human macromolecular substances . Based on the idea that natural polymers can be recognized and interact with the biological environment, the main components of the extracellular matrix collagen and hyaluronic acid (HyA) have been used to fabricate electrospun nanosized scaffolds. Collagen/HyA scaffolds that are insoluble in aqueous solutions have been developed and promoted cellular attachment .
To achieve the clinical implantation of in vivo and in vitro tested scaffolds, Wang et al. developed anatomically shaped scaffolds using a 3D printing technique . Scaffolds were produced by phase separation to generate interconnected PLLA nanofibrous scaffolds showing proliferation and adhesion of osteoblasts in vitro as well as differentiation and bone formation.
The ECMs of various tissues in our body, like bone muscle and blood vessels, differ in the arrangement and composition of several proteins to maintain specific tissue formation, organ specific shape, and function. Therefore, the development of innovative scaffolds in regenerative medicine should vary according to the desired engineered tissue. The discussed nanotechnology approaches open the perspective to develop advanced nanomaterials that mimic the natural extracellular matrix of many tissues, and therefore, they hold great potential in tissue engineering applications. The availability of multiple natural and synthetic polymers with defined properties and the possibility of combining them during the electrospinning process represent an attractive opportunity to modulate the scaffold regarding the desired tissue. Nanofiber-based matrices show promising results to provide scaffolds with tissue-specific properties and versatility, which has been reviewed in this article for muscle, vascular, and bone tissue engineering.
We would like to thank the Xue-Hong and Hans Georg Geis, Deutsche Forschungsgemeinschaft DFG (KN 578/2-1) as well as the “Emerging Fields Initiative” of the University of Erlangen-Nuremberg and the AO Research Fund (AO Research Fund Grant S-10-36A).
 Skalak RFC. Tissue Engineering. Liss: New York, 1988. Search in Google Scholar
 Ma Z, Kotaki M, Inai R, Ramakrishna S. Potential of nanofiber matrix as tissue-engineering scaffolds. Tissue Eng. 2005, 11, 101–109. Search in Google Scholar
 Paramonov SE, Jun HW, Hartgerink JD. Self-assembly of peptide–amphiphile nanofibers: the roles of hydrogen bonding and amphiphilic packing. J. Am. Chem. Soc. 2006, 128, 7291–7298. Search in Google Scholar
 Matson JB, Stupp SI. Self-assembling peptide scaffolds for regenerative medicine. Chem. Commun. (Camb) 2012, 48, 26–33. Search in Google Scholar
 Jayaraman K, Kotaki M, Zhang Y, Mo X, Ramakrishna S. Recent advances in polymer nanofibers. J. Nanosci. Nanotechnol. 2004, 4, 52–65. Search in Google Scholar
 James R, Toti US, Laurencin CT, Kumbar SG. Electrospun nanofibrous scaffolds for engineering soft connective tissues. Methods Mol. Biol. 2011, 726, 243–258. Search in Google Scholar
 Ma PX, Zhang R. Synthetic nano-scale fibrous extracellular matrix. J. Biomed. Mater. Res. 1999, 46, 60–72. Search in Google Scholar
 Lee HY, Jin GZ, Shin US, Kim JH, Kim HW. Novel porous scaffolds of poly(lactic acid) produced by phase-separation using room temperature ionic liquid and the assessments of biocompatibility. J. Mater. Sci. Mater. Med. 2012, 23, 1271–1279. Search in Google Scholar
 Zhao J, Han W, Chen H, Tu M, Huan S, Miao G, Zeng R, Wu H, Cha Z, Zhou C. Fabrication and in vivo osteogenesis of biomimetic poly(propylene carbonate) scaffold with nanofibrous chitosan network in macropores for bone tissue engineering. J. Mater. Sci. Mater. Med. 2012, 23, 517–525. Search in Google Scholar
 Vasita R, Katti DS. Nanofibers and their applications in tissue engineering. Int. J. Nanomed. 2006, 1, 15–30. Search in Google Scholar
 Sill TJ, von Recum HA. Electrospinning: applications in drug delivery and tissue engineering. Biomaterials 2008, 29, 1989–2006. Search in Google Scholar
 Rim NG, Shin CS, Shin H. Current approaches to electrospun nanofibers for tissue engineering. Biomed. Mater. 2013, 8, 014102. Search in Google Scholar
 Beachley V, Katsanevakis E, Zhang N, Wen X. A novel method to precisely assemble loose nanofiber structures for regenerative medicine applications. Adv. Healthc. Mater. 2012, 2, 343–51. Search in Google Scholar
 Frey M, Michaelidou M, Tzou CH, Hold A, Pona I, Placheta E. Proven and innovative operative techniques for reanimation of the paralyzed face. Handchir. Mikrochir. Plast. Chir. 2010, 42, 81–89. Search in Google Scholar
 Klumpp D, Horch RE, Kneser U, Beier JP. Engineering skeletal muscle tissue – new perspectives in vitro and in vivo. J. Cell Mol. Med. 2010, 14, 2622–2629. Search in Google Scholar
 Huang NF, Patel S, Thakar RG, Wu J, Hsiao BS, Chu B, Lee RJ, Li S. Myotube assembly on nanofibrous and micropatterned polymers. Nano Lett. 2006, 6, 537–542. Search in Google Scholar
 Karande TS, Ong JL, Agrawal CM. Diffusion in musculoskeletal tissue engineering scaffolds: design issues related to porosity, permeability, architecture, and nutrient mixing. Ann. Biomed. Eng. 2004, 32, 1728–1743. Search in Google Scholar
 Yeong WY, Sudarmadji N, Yu HY, Chua CK, Leong KF, Venkatraman SS, Boey YC, Tan LP. Porous polycaprolactone scaffold for cardiac tissue engineering fabricated by selective laser sintering. Acta Biomater. 2010, 6, 2028–2034. Search in Google Scholar
 Madaghiele M, Sannino A, Yannas IV, Spector M. Collagen-based matrices with axially oriented pores. J. Biomed. Mater. Res. A 2008, 85, 757–767. Search in Google Scholar
 Mandal BB, Kundu SC. Cell proliferation and migration in silk fibroin 3D scaffolds. Biomaterials 2009, 30, 2956–2965. Search in Google Scholar
 Beier JP, Klumpp D, Rudisile M, Dersch R, Wendorff JH, Bleiziffer O, Arkudas A, Polykandriotis E, Horch RE, Kneser U. Collagen matrices from sponge to nano: new perspectives for tissue engineering of skeletal muscle. BMC Biotechnol. 2009, 9, 34. Search in Google Scholar
 Sell SA, McClure MJ, Garg K, Wolfe PS, Bowlin GL. Electrospinning of collagen/biopolymers for regenerative medicine and cardiovascular tissue engineering. Adv. Drug Deliv. Rev. 2009, 61, 1007–1019. Search in Google Scholar
 Fischer, RL, McCoy MG, Grant SA. Electrospinning collagen and hyaluronic acid nanofiber meshes. J. Mater. Sci. Mater. Med. 2012, 23, 1645–1654. Search in Google Scholar
 Buttafoco L, Kolkman NG, Engbers-Buijtenhuijs P, Poot AA, Dijkstra PJ, Vermes I, Feijen J. Electrospinning of collagen and elastin for tissue engineering applications. Biomaterials 2006, 27, 724–734. Search in Google Scholar
 BÖlgen N, Menceloğlu YZ, Acatay K, Vargel I, Pişkin E. In vitro and in vivo degradation of non-woven materials made of poly(epsilon-caprolactone) nanofibers prepared by electrospinning under different conditions. J. Biomater. Sci. Polym. Ed. 2005, 16, 1537–1555. Search in Google Scholar
 Kim GH. Electrospun PCL nanofibers with anisotropic mechanical properties as a biomedical scaffold. Biomed. Mater. 2008, 3, 025010. Search in Google Scholar
 Zhang H, Hollister S. Comparison of bone marrow stromal cell behaviors on poly(caprolactone) with or without surface modification: studies on cell adhesion, survival and proliferation. J. Biomater. Sci. Polym. Ed. 2009, 20, 1975–1993. Search in Google Scholar
 Yoon H, Kim G. A three-dimensional polycaprolactone scaffold combined with a drug delivery system consisting of electrospun nanofibers. J. Pharm. Sci. 2011, 100, 424–430. Search in Google Scholar
 Yang F, Cho SW, Son SM, Bogatyrev SR, Singh D, Green JJ, Mei Y, Park S, Bhang SH, Kim BS, Langer R, Anderson DG. Genetic engineering of human stem cells for enhanced angiogenesis using biodegradable polymeric nanoparticles. Proc. Natl. Acad. Sci. USA 2010, 107, 3317–3322. Search in Google Scholar
 Duan C, Ren H, Gao S. Insulin-like growth factors (IGFs), IGF receptors, and IGF-binding proteins: roles in skeletal muscle growth and differentiation. Gen. Comp. Endocrinol. 2010, 167, 344–351. Search in Google Scholar
 Rende M, Brizi E, Conner J, Treves S, Censier K, Provenzano C, Taglialatela G, Sanna PP, Donato R. Nerve growth factor (NGF) influences differentiation and proliferation of myogenic cells in vitro via TrKA. Int. J. Dev. Neurosci. 2000, 18, 869–885. Search in Google Scholar
 Lavasani M, Lu A, Peng H, Cummins J, Huard J. Nerve growth factor improves the muscle regeneration capacity of muscle stem cells in dystrophic muscle. Hum. Gene Ther. 2006, 17, 180–192. Search in Google Scholar
 Klumpp D, Rudisile M, Kühnle RI, Hess A, Bitto FF, Arkudas A, Bleiziffer O, Boos AM, Kneser U, Horch RE, Beier JP. Three-dimensional vascularization of electrospun PCL/collagen-blend nanofibrous scaffolds in vivo. J. Biomed. Mater. Res. A 2012, 100, 2302–2311. Search in Google Scholar
 Baker BM, Shah RP, Silverstein AM, Esterhai JL, Burdick JA, Mauck RL. Sacrificial nanofibrous composites provide instruction without impediment and enable functional tissue formation. Proc. Natl. Acad. Sci. USA 2012, 109, 14176–14181. Search in Google Scholar
 Baker BM, Gee AO, Metter RB, Nathan AS, Marklein RA, Burdick JA, Mauck RL. The potential to improve cell infiltration in composite fiber-aligned electrospun scaffolds by the selective removal of sacrificial fibers. Biomaterials 2008, 29, 2348–2358. Search in Google Scholar
 Martins A, Pinho ED, Faria S, Pashkuleva I, Marques AP, Reis RL, Neves NM. Surface modification of electrospun polycaprolactone nanofiber meshes by plasma treatment to enhance biological performance. Small 2009, 5, 1195–1206. Search in Google Scholar
 Chen D, Miao YE, Liu T. Electrically conductive polyaniline/polyimide nanofiber membranes prepared via a combination of electrospinning and subsequent in-situ polymerization growth. ACS Appl. Mater. Interfaces 2013, 5, 1206–1212. Search in Google Scholar
 Jun I, Jeong S, Shin H. The stimulation of myoblast differentiation by electrically conductive sub-micron fibers. Biomaterials 2009, 30, 2038–2047. Search in Google Scholar
 Ghasemi-Mobarakeh L, Prabhakaran MP, Morshed M, Nasr-Esfahani MH, Ramakrishna S. Electrical stimulation of nerve cells using conductive nanofibrous scaffolds for nerve tissue engineering. Tissue Eng. Part A 2009, 15, 3605–3619. Search in Google Scholar
 Prabhakaran MP, Ghasemi-Mobarakeh L, Jin G, Ramakrishna S. Electrospun conducting polymer nanofibers and electrical stimulation of nerve stem cells. J. Biosci. Bioeng. 2011, 112, 501–507. Search in Google Scholar
 Guarino V, Alvarez-Perez MA, Borriello A, Napolitano T, Ambrosio L. Conductive PANi/PEGDA macroporous hydrogels for nerve regeneration. Adv. Healthc. Mater. 2013, 2, 218–227. Search in Google Scholar
 Mensah GA, Brown DW. An overview of cardiovascular disease burden in the United States. Health Aff. (Millwood) 2007, 26, 38–48. Search in Google Scholar
 Ravi S, Qu Z, Chaikof EL. Polymeric materials for tissue engineering of arterial substitutes. Vascular 2009, 17(Suppl 1), S45–S54. Search in Google Scholar
 Tillman BW, Yazdani SK, Lee SJ, Geary RL, Atala A, Yoo JJ. The in vivo stability of electrospun polycaprolactone-collagen scaffolds in vascular reconstruction. Biomaterials 2009, 30, 583–588. Search in Google Scholar
 Ercolani E, Del Gaudio C, Bianco A. Vascular tissue engineering of small-diameter blood vessels: reviewing the electrospinning approach. J. Tissue Eng. Regen. Med. 2013. DOI:10.1002/term.1697. Search in Google Scholar
 Zeng J, Chen X, Liang Q, Xu X, Jing X. Enzymatic degradation of poly(L-lactide) and poly(epsilon-caprolactone) electrospun fibers. Macromol. Biosci. 2004, 4, 1118–1125. Search in Google Scholar
 Nottelet B, Pektok E, Mandracchia D, Tille JC, Walpoth B, Gurny R, Möller M. Factorial design optimization and in vivo feasibility of poly(epsilon-caprolactone)-micro- and nanofiber-based small diameter vascular grafts. J. Biomed. Mater. Res A 2009, 89, 865–875. Search in Google Scholar
 Diban N, Haimi S, Bolhuis-Versteeg L, Teixeira S, Miettinen S, Poot A, Grijpma D, Stamatialis D. Hollow fibers of poly(lactide-co-glycolide) and poly(epsilon-caprolactone) blends for vascular tissue engineering applications. Acta Biomater. 2013, 9, 6450–6458. Search in Google Scholar
 Wang HW, Cheng CW, Li CW, Chang HW, Wu PH, Wang GJ. Fabrication of pillared PLGA microvessel scaffold using femtosecond laser ablation. Int. J. Nanomed. 2012, 7, 1865–1873. Search in Google Scholar
 Pektok E, Nottelet B, Tille JC, Gurny R, Kalangos A, Moeller M, Walpoth BH. Degradation and healing characteristics of small-diameter poly(epsilon-caprolactone) vascular grafts in the rat systemic arterial circulation. Circulation 2008, 118, 2563–2570. Search in Google Scholar
 Drilling S, Gaumer J, Lannutti J. Fabrication of burst pressure competent vascular grafts via electrospinning: effects of microstructure. J. Biomed. Mater. Res. A 2009, 88, 923–934. Search in Google Scholar
 Wu H, Fan J, Chu CC, Wu J. Electrospinning of small diameter 3-D nanofibrous tubular scaffolds with controllable nanofiber orientations for vascular grafts. J. Mater. Sci. Mater. Med. 2010, 21, 3207–3215. Search in Google Scholar
 de Valence S, Tille JC, Mugnai D, Mrowczynski W, Gurny R, Möller M, Walpoth BH. Long term performance of polycaprolactone vascular grafts in a rat abdominal aorta replacement model. Biomaterials 2012, 33, 38–47. Search in Google Scholar
 Innocente F, Mandracchia D, Pektok E, Nottelet B, Tille JC, de Valence S, Faggian G, Mazzucco A, Kalangos A, Gurny R, Moeller M, Walpoth BH. Paclitaxel-eluting biodegradable synthetic vascular prostheses: a step towards reduction of neointima formation? Circulation 2009, 120(11 Suppl), S37–S45. Search in Google Scholar
 Kuwabara F, Narita Y, Yamawaki-Ogata A, Satake M, Kaneko H, Oshima H, Usui A, Ueda Y. Long-term results of tissue-engineered small-caliber vascular grafts in a rat carotid arterial replacement model. J. Artif. Organs 2012, 15, 399–405. Search in Google Scholar
 Dong Y, Yong T, Liao S, Chan CK, Stevens MM, Ramakrishna S. Distinctive degradation behaviors of electrospun polyglycolide, poly(DL-lactide-co-glycolide), and poly(L-lactide-co-epsilon-caprolactone) nanofibers cultured with/without porcine smooth muscle cells. Tissue Eng. Part A 2010, 16, 283–298. Search in Google Scholar
 Dong Y, Yong T, Liao S, Chan CK, Ramakrishna S. Long-term viability of coronary artery smooth muscle cells on poly (L-lactide-co-epsilon-caprolactone) nanofibrous scaffold indicates its potential for blood vessel tissue engineering. J. R. Soc. Interface 2008, 5, 1109–1118. Search in Google Scholar
 He W, Yong T, Teo WE, Ma Z, Ramakrishna S. Fabrication and endothelialization of collagen-blended biodegradable polymer nanofibers: potential vascular graft for blood vessel tissue engineering. Tissue Eng. 2005, 11, 1574–1588. Search in Google Scholar
 Grasl C, Bergmeister H, Stoiber M, Schima H, Weigel G. Electrospun polyurethane vascular grafts: in vitro mechanical behavior and endothelial adhesion molecule expression. J. Biomed. Mater. Res. A 2010, 93, 716–723. Search in Google Scholar
 Bergmeister H, Grasl C, Walter I, Plasenzotti R, Stoiber M, Schreiber C, Losert U, Weigel G, Schima H. Electrospun small-diameter polyurethane vascular grafts: ingrowth and differentiation of vascular-specific host cells. Artif. Organs 2012, 36, 54–61. Search in Google Scholar
 Soldani G, Losi P, Bernabei M, Burchielli S, Chiappino D, Kull S, Briganti E, Spiller D. Long term performance of small-diameter vascular grafts made of a poly(ether)urethane-polydimethylsiloxane semi-interpenetrating polymeric network. Biomaterials 2009, 31, 2592–2605. Search in Google Scholar
 He W, Nieponice A, Soletti L, Hong Y, Gharaibeh B, Crisan M, Usas A, Peault B, Huard J, Wagner WR, Vorp DA. Pericyte-based human tissue engineered vascular grafts. Biomaterials 2010, 31, 8235–8244. Search in Google Scholar
 Del Gaudio C, Fioravanzo L, Folin M, Marchi F, Ercolani E, Bianco A. Electrospun tubular scaffolds: on the effectiveness of blending poly(epsilon-caprolactone) with poly(3-hydroxybutyrate-co-3-hydroxyvalerate). J. Biomed. Mater. Res. B Appl. Biomater. 2012, 100B, 1883–1898. Search in Google Scholar
 Lee SJ, Liu J, Oh SH, Soker S, Atala A, Yoo JJ. Development of a composite vascular scaffolding system that withstands physiological vascular conditions. Biomaterials 2008, 29, 2891–2898. Search in Google Scholar
 Lee SJ, Yoo JJ, Lim GJ, Atala A, Stitzel J. In vitro evaluation of electrospun nanofiber scaffolds for vascular graft application. J. Biomed. Mater. Res. A. 2007, 83, 999–1008. Search in Google Scholar
 Heydarkhan-Hagvall S, Schenke-Layland K, Dhanasopon AP, Rofail F, Smith H, Wu BM, Shemin R, Beygui RE, MacLellan WR. Three-dimensional electrospun ECM-based hybrid scaffolds for cardiovascular tissue engineering. Biomaterials 2008, 29, 2907–2914. Search in Google Scholar
 Wise SG, Byrom MJ, Waterhouse A, Bannon PG, Weiss AS, Ng MK. A multilayered synthetic human elastin/polycaprolactone hybrid vascular graft with tailored mechanical properties. Acta. Biomater. 2011, 7, 295–303. Search in Google Scholar
 McClure MJ, Sell SA, Simpson DG, Walpoth BH, Bowlin GL. Tri-layered electrospinning to mimic native arterial architecture using polycaprolactone, elastin, and collagen: a preliminary study. J. Vis. Exp. 2011, 47. Search in Google Scholar
 McClure MJ, Sell SA, Ayres CE, Simpson DG, Bowlin GL. Electrospinning-aligned and random polydioxanone-polycaprolactone-silk fibroin-blended scaffolds: geometry for a vascular matrix. Biomed. Mater. 2009, 4, 055010. Search in Google Scholar
 McClure MJ, Sell SA, Simpson DG, Walpoth BH, Bowlin GL. A three-layered electrospun matrix to mimic native arterial architecture using polycaprolactone, elastin, and collagen: a preliminary study. Acta Biomater. 2010, 6, 2422–2433. Search in Google Scholar
 Stroncek JD, Ren LC, Klitzman B, Reichert WM. Patient-derived endothelial progenitor cells improve vascular graft patency in a rodent model. Acta Biomater. 2012, 8, 201–208. Search in Google Scholar
 Hutmacher DW. Scaffolds in tissue engineering bone and cartilage. Biomaterials 2000, 21, 2529–2543. Search in Google Scholar
 Strobel L, Rath S, Maier A, Beier J, Arkudas A, Greil P, Horch R, Kneser U. Induction of bone formation in biphasic calcium phosphate scaffolds by bone morphogenetic protein-2 and primary osteoblasts. J. Tissue Eng. Regen. Med. 2012. DOI:10.1002/term.1511. Search in Google Scholar
 Meng D, Rath SN, Mordan N, Salih V, Kneser U, Boccaccini AR. In vitro evaluation of 45S5 Bioglass(R)-derived glass-ceramic scaffolds coated with carbon nanotubes. J. Biomed. Mater. Res. A 2011, 99, 435–444. Search in Google Scholar
 Wei G, Ma PX. Structure and properties of nano-hydroxyapatite/ polymer composite scaffolds for bone tissue engineering. Biomaterials 2004, 25, 4749–4757. Search in Google Scholar
 Peng F, Yu X, Wei M. In vitro cell performance on hydroxyapatite particles/poly(L-lactic acid) nanofibrous scaffolds with an excellent particle along nanofiber orientation. Acta Biomater. 2011, 7, 2585–2592. Search in Google Scholar
 Jegal SH, Park JH, Kim JH, Kim TH, Shin US, Kim TI, Kim HW. Functional composite nanofibers of poly(lactide-co-caprolactone) containing gelatin-apatite bone mimetic precipitate for bone regeneration. Acta Biomater. 2011, 7, 1609–1617. Search in Google Scholar
 Liu X, Smith LA, Hu J, Ma PX. Biomimetic nanofibrous gelatin/apatite composite scaffolds for bone tissue engineering. Biomaterials 2009, 30, 2252–2258. Search in Google Scholar
 Woo KM, Chen VJ, Jung HM, Kim TI, Shin HI, Baek JH, Ryoo HM, Ma PX. Comparative evaluation of nanofibrous scaffolding for bone regeneration in critical-size calvarial defects. Tissue Eng. Part A 2009, 15, 2155–2162. Search in Google Scholar
 Jose MV, Thomas V, Xu Y, Bellis S, Nyairo E, Dean D. Aligned bioactive multi-component nanofibrous nanocomposite scaffolds for bone tissue engineering. Macromol. Biosci. 2010, 10, 433–44. Search in Google Scholar
 He C, Xiao G, Jin X, Sun C, Ma PX. Electrodeposition on nanofibrous polymer scaffolds: rapid mineralization, tunable calcium phosphate composition and topography. Adv. Funct. Mater. 2010, 20, 3568–3576. Search in Google Scholar
 Cheng Q, Rutledge K, Jabbarzadeh E. Carbon nanotube-poly(lactide-co-glycolide) composite scaffolds for bone tissue engineering applications. Ann. Biomed. Eng. 2013, 41, 904–916. Search in Google Scholar
 Zhang B, Kwok CT, Cheng FT, Man HC. Fabrication of nano-structured HA/CNT coatings on Ti6Al4V by electrophoretic deposition for biomedical applications. J. Nanosci. Nanotechnol. 2011, 11, 10740–10745. Search in Google Scholar
 Schofer MD, Roessler PP, Schaefer J, Theisen C, Schlimme S, Heverhagen JT, Voelker M, Dersch R, Agarwal S, Fuchs-Winkelmann S, Paletta JR. Electrospun PLLA nanofiber scaffolds and their use in combination with BMP-2 for reconstruction of bone defects. PLoS One 2011, 6, e25462. Search in Google Scholar
 Srouji S, Ben-David D, Lotan R, Livne E, Avrahami R, Zussman E. Slow-release human recombinant bone morphogenetic protein-2 embedded within electrospun scaffolds for regeneration of bone defect: in vitro and in vivo evaluation. Tissue Eng Part A 2011, 17, 269–277. Search in Google Scholar
 Seyedjafari E, Soleimani M, Ghaemi N, Sarbolouki MN. Enhanced osteogenic differentiation of cord blood-derived unrestricted somatic stem cells on electrospun nanofibers. J. Mater. Sci. Mater. Med. 2011, 22, 165–174. Search in Google Scholar
 Lü L-X, Zhang X-F, Wang Y-Y, Ortiz L, Mao X, Jiang Z-L, Xiao Z-D, Huang N-P. Effects of hydroxyapatite-containing composite nanofibers on osteogenesis of mesenchymal stem cells in vitro and bone regeneration in vivo. ACS Appl. Mater. Interfaces 2013, 5, 319–330. Search in Google Scholar
 Binulal NS, Deepthy M, Selvamurugan N, Shalumon KT, Suja S, Mony U, Jayakumar R, Nair SV. Role of nanofibrous poly(caprolactone) scaffolds in human mesenchymal stem cell attachment and spreading for in vitro bone tissue engineering – response to osteogenic regulators. Tissue Eng. Part A 2010, 16, 393–404. Search in Google Scholar
 Yoshimoto H, Shin YM, Terai H, Vacanti JP. A biodegradable nanofiber scaffold by electrospinning and its potential for bone tissue engineering. Biomaterials 2003, 24, 2077–2082. Search in Google Scholar
 Binulal NS, Natarajan A, Menon D, Bhaskaran VK, Mony U, Nair SV. Gelatin nanoparticles loaded poly(epsilon-caprolactone) nanofibrous semi-synthetic scaffolds for bone tissue engineering. Biomed. Mater. 2012, 7, 065001. Search in Google Scholar
 Wepener I, Richter W, van Papendorp D, Joubert AM. In vitro osteoclast-like and osteoblast cells’ response to electrospun calcium phosphate biphasic candidate scaffolds for bone tissue engineering. J. Mater. Sci. Mater. Med. 2012, 23, 3029–3040. Search in Google Scholar
 Li K, Wang J, Liu X, Xiong X, Liu H. Biomimetic growth of hydroxyapatite on phosphorylated electrospun cellulose nanofibers. Carbohydr. Polym. 2012, 90, 1573–1581. Search in Google Scholar
 Guo H, Wei J, Song W, Zhang S, Yan Y, Liu C, Xiao T. Wollastonite nanofiber-doped self-setting calcium phosphate bioactive cement for bone tissue regeneration. Int. J. Nanomed. 2012, 7, 3613–3624. Search in Google Scholar
 Kang YM, Kim KH, Seol YJ, Rhee SH. Evaluations of osteogenic and osteoconductive properties of a non-woven silica gel fabric made by the electrospinning method. Acta Biomater. 2009, 5, 462–469. Search in Google Scholar
 Wang X, Zhu J, Yin L, Liu S, Zhang X, Ao Y, Chen H. Fabrication of electrospun silica-titania nanofibers with different silica content and evaluation of the morphology and osteoinductive properties. J. Biomed. Mater. Res. A 2012, 100, 3511–3517. Search in Google Scholar
 Venugopal JR, Low S, Choon AT, Kumar AB, Ramakrishna S. Nanobioengineered electrospun composite nanofibers and osteoblasts for bone regeneration. Artif. Organs 2008, 32, 388–397. Search in Google Scholar
 Arcos D, Vallet-Regi M. Sol-gel silica-based biomaterials and bone tissue regeneration. Acta Biomater. 2010, 6, 2874–2888. Search in Google Scholar
 Chen QZ, Thompson ID, Boccaccini AR. 45S5 Bioglass-derived glass-ceramic scaffolds for bone tissue engineering. Biomaterials 2006, 27, 2414–2425. Search in Google Scholar
 Ganesh N, Jayakumar R, Koyakutty M, Mony U, Nair SV. Embedded silica nanoparticles in poly(caprolactone) nanofibrous scaffolds enhanced osteogenic potential for bone tissue engineering. Tissue Eng. Part A 2012, 18, 1867–1881. Search in Google Scholar
 Yannas IV, Tzeranis DS, Harley BA, So PT. Biologically active collagen-based scaffolds: advances in processing and characterization. Philos. Transact. A Math. Phys. Eng. Sci. 2010, 368, 2123–2139. Search in Google Scholar
 Kumbar SG, James R, Nukavarapu SP, Laurencin CT. Electrospun nanofiber scaffolds: engineering soft tissues. Biomed. Mater. 2008, 3, 034002. Search in Google Scholar
 Wang P, Hu J, Ma PX. The engineering of patient-specific, anatomically shaped, digits. Biomaterials 2009, 30, 2735–2740. Search in Google Scholar
©2013 by Walter de Gruyter Berlin Boston
This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.