Composite Electrospun Scaffolds for Engineering Tubular Bone Grafts

Polyhydroxybutyrate-based osteoinductive mineralized electrospun structures that mimic components and tissue interfaces of the osteon for bone tissue engineering

Scaffolds for bone tissue engineering should enable regeneration of bone tissues with its native hierarchically organized extracellular matrix (ECM) and multiple tissue interfaces. To achieve this, inspired by the structure and properties of bone osteon, we fabricated polyhydroxybutyrate (PHB)-based mineralized electrospun fibrous scaffolds. After studying multiple PHB-based fibers, we chose 7%PHB/1%Gelatin fibers (PG) to fabricate mineralized fibers that mimic mineralized collagen fibers in bone. The mineralized PG (mPG) surface had a rough, hydrophilic layer of low crystalline calcium phosphate which was biocompatible to bone marrow stromal cells (BMSCs), induced their proliferation and was osteoinductive. Subsequently, by modulating the electrospinning process, we fabricated mPG-based novel higher order fibrous scaffolds that mimic the macroscale geometries of osteons of bone ECM. Inspired by the aligned collagen fibers in bone lamellae, we fabricated mPG scaffolds with aligned fibers that could direct anisotropic elongation of mouse BMSC (mBMSCs). Further, we fabricated electrospun mPG-based osteoinductive tubular constructs which can mimic cylindrical bone components like osteons or lamellae or be used as long bone analogues based on their dimensions. Finally, to regenerate tissue interfaces in bone, we introduced a novel bi-layered scaffold-based approach. An electrospun bi-layered tubular construct that had PG in the outer layer and 7%PHB/0.5%Polypyrrole fibers (PPy) in the inner layer was fabricated. The bi-layered tubular construct underwent preferential surface mineralization only on its outer layer. This outer mineralized layer supported osteogenesis while the inner PPy layer could support neural cell growth. Thus, the bi-layered tubular construct may be used to regenerate haversian canal in the osteons which hosts nerve fibers. Overall, the study introduced novel techniques to fabricate biomimetic structures that can regenerate components of bone osteon and its multiple tissue interfaces. The study lays foundation for the fabrication of a modular scaffold that can regenerate bone with its hierarchical structure and complex tissue interfaces.

Poly(ε-caprolactone)/bioactive glass composite electrospun fibers for tissue engineering applications

In this work, composite electrospun fibers containing innovative bioactive glass nanoparticles were produced and characterized. Poly(ε-caprolactone), benign solvents, and sol-gel B- and Cu-doped bioactive glass powders were used to fabricate fibrous scaffolds. The retention of bioactive glass nanoparticles in the polymer matrix, the electrospinnability of this novel solution and the obtained electrospun composites were extensively characterized. As a result, composite electrospun fibers characterized by biocompatibility, bioactivity, and exhibiting overall properties adequate for both hard and soft tissue engineering applications, have been produced. The addition of these bioactive glass nanoparticles was, indeed, able to impart bioactive properties to the fibers. Cell culture studies show promising results, demonstrating proliferation and growth of cells on the composite fibers. Wettability, degradation rate, and mechanical performance were also tested and are in line with previous results.

3D printing of biomedically relevant polymer materials and biocompatibility

ABSTRACT

Research on polymer materials for additive manufacturing technology in biomedical applications is as promising as it is numerous, but biocompatibility of printable materials still remains a big challenge. Changes occurring during the 3D-printing processes itself may have adverse effects on the compatibility of the completed print. This prospective will put emphasis on the different additives and processes that can have a direct impact on biocompatibility during and after 3D printing of polymer materials.

In vitro behavior of tendon stem/progenitor cells on bioactive electrospun nanofiber membranes for tendon-bone tissue engineering applications

Purpose

In order to accelerate the tendon-bone healing processes and achieve the efficient osteointegration between the tendon graft and bone tunnel, we aim to design bioactive electrospun nanofiber membranes combined with tendon stem/progenitor cells (TSPCs) to promote osteogenic regeneration of the tendon and bone interface.

Methods

In this study, nanofiber membranes of polycaprolactone (PCL), PCL/collagen I (COL-1) hybrid nanofiber membranes, poly(dopamine) (PDA)-coated PCL nanofiber membranes and PDA-coated PCL/COL-1 hybrid nanofiber membranes were successfully fabricated by electrospinning. The biochemical characteristics and nanofibrous morphology of the membranes, as well as the characterization of rat TSPCs, were defined in vitro. After co-culture with different types of electrospun nanofiber membranes in vitro, cell proliferation, viability, adhesion and osteogenic differentiation of TSPCs were evaluated at different time points.

Results

Among all the membranes, the performance of the PCL/COL-1 (volume ratio: 2:1 v/v) group was superior in terms of its ability to support the adhesion, proliferation, and osteogenic differentiation of TSPCs. No benefit was found in this study to include PDA coating on cell adhesion, proliferation and osteogenic differentiation of TSPCs.

Conclusion

The PCL/COL-1 hybrid electrospun nanofiber membranes are biocompatible, biomimetic, easily fabricated, and are capable of supporting cell adhesion, proliferation, and osteogenic differentiation of TSPCs. These bioactive electrospun nanofiber membranes may act as a suitable functional biomimetic scaffold in tendon-bone tissue engineering applications to enhance tendon-bone healing abilities.

Inflammatory response and biomechanical properties of coaxial scaffolds for engineered skin in vitro and post-grafting

Engineered skin (ES) offers many advantages over split-thickness skin autografts for the treatment of burn wounds. However, ES, both in vitro and after grafting, is often significantly weaker, less elastic and more compliant than normal human skin. Biomechanical properties of ES can be tuned in vitro using electrospun co-axial (CoA) scaffolds. To explore the potential for coaxial scaffold-based ES use in vivo, two CoA scaffolds were fabricated with bioactive gelatin shells and biodegradable synthetic cores of polylactic acid (PLA) and polycaprolactone (PCL), and compared with gelatin monofilament scaffolds. Fibroblast and macrophage production of inflammatory cytokines interleukin 6 (IL-6) and transforming growth factor β-1 was significantly higher when cultured on PLA and PCL monofilament scaffolds compared to gelatin monofilament scaffolds. The core-shell fiber configuration significantly reduced production of pro-inflammatory cytokines to levels similar to those of gelatin monofilament scaffolds. In vitro, ES mechanical properties were significantly enhanced using CoA scaffolds; however, after grafting CoA- and gelatin-based ES to full-thickness excisional wounds on athymic mice, the in vitro mechanical advantage of CoA grafts was lost. A substantially increased inflammatory response to CoA-based ES was observed, with upregulation of IL-6 expression and a significant M2 macrophage presence. Additionally, expression of matrix metalloproteinase I was upregulated and collagen type I alpha 1 was downregulated in CoA ES two weeks after grafting. These results suggest that while coaxial scaffolds provide the ability to regulate biomechanics in vitro, further investigation of the inflammatory response to core materials is required to optimize this strategy for clinical use. STATEMENT OF SIGNIFICANCE: Engineered skin has been used to treat very large burn injuries. Despite its ability to heal these wounds, engineered skin exhibits reduced biomechanical properties making it challenging to manufacture and surgically apply. Coaxial fiber scaffolds have been utilized to tune the mechanical properties of engineered skin while maintaining optimal biological properties but it is not known how these perform on a patient especially with regards to their inflammatory response. The current study examines the biomechanical and inflammatory properties of coaxial scaffolds and uniaxial scaffolds in vitro and in vivo. The results show that the biological response to the scaffold materials is a critical determinant of tissue properties after grafting with reduced inflammation and rapid scaffold remodeling leading to stronger skin.

Nanofibrous scaffolds for biomedical applications

Tissue engineering is an emergent and very interesting research field, providing potential solutions for a myriad of challenges in healthcare. Fibrous scaffolds specifically have shown promise as an effective tissue engineering method, as their high length-to-width ratio mimics that of extracellular matrix components, which in turn guides tissue formation, promotes cellular adhesion and improves mechanical properties. In this review paper, we discuss in detail both the importance of fibrous scaffolds for the promotion of tissue growth and the different methods to produce fibrous biomaterials to possess favorable and unique characteristics. Here, we focus on the pressing need to develop biomimetic structures that promote an ideal environment to encourage tissue formation. In addition, we discuss different biomedical applications in which fibrous scaffolds can be useful, identifying their importance, relevant aspects, and remaining significant challenges. In conclusion, we provide comments on the future direction of fibrous scaffolds and the best way to produce them, proposed in light of recent technological advances and the newest and most promising fabrication techniques.

PCL and PCL-based materials in biomedical applications

Biodegradable polymers have met with an increasing demand in medical usage over the last decades. One of such polymers is poly(ε-caprolactone) (PCL), which is a polyester that has been widely used in tissue engineering field for its availability, relatively inexpensive price and suitability for modification. Its chemical and biological properties, physicochemical state, degradability and mechanical strength can be adjusted, and therefore, it can be used under harsh mechanical, physical and chemical conditions without significant loss of its properties. Degradation time of PCL is quite long, thus it is used mainly in the replacement of hard tissues in the body where healing also takes an extended period of time. It is also used at load-bearing tissues of the body by enhancing its stiffness. However, due to its tailorability, use of PCL is not restricted to one type of tissue and it can be extended to engineering of soft tissues by decreasing its molecular weight and degradation time. This review outlines the basic properties of PCL, its composites, blends and copolymers. We report on various techniques for the production of different forms, and provide examples of medical applications such as tissue engineering and drug delivery systems covering the studies performed in the last decades.

Sustained delivery of recombinant human bone morphogenetic protein-2 from perlecan domain I - functionalized electrospun poly (ε-caprolactone) scaffolds for bone regeneration

Background

Biomaterial scaffolds that deliver growth factors such as recombinant human bone morphogenetic proteins-2 (rhBMP-2) have improved clinical bone tissue engineering by enhancing bone tissue regeneration. This approach could be further improved if the controlled delivery of bioactive rhBMP-2 were sustained throughout the duration of osteogenesis from fibrous scaffolds that provide control over dose and bioactivity of rhBMP-2. In nature, heparan sulfate attached to core proteoglycans serves as the co-receptor that delivers growth factors to support tissue morphogenesis.

Methods

To mimic this behavior, we conjugated heparan sulfate decorated recombinant domain I of perlecan/HSPG2 onto an electrospun poly(ε-caprolactone) (PCL) scaffold, hypothesizing that the heparan sulfate chains will enhance rhBMP-2 loading onto the scaffold and preserve delivered rhBMP-2 bioactivity.

Results

In this study, we demonstrated that covalently conjugated perlecan domain I increased loading capacity of rhBMP-2 onto PCL scaffolds when compared to control unconjugated scaffolds. Additionally, rhBMP-2 released from the modified scaffolds enhanced alkaline phosphatase activity in W20–17 mouse bone marrow stromal cells, indicating the preservation of rhBMP-2 bioactivity indicative of osteogenesis.

Conclusions

We conclude that this platform provides a sophisticated and efficient approach to deliver bioactive rhBMP-2 for bone tissue regeneration applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s40634-016-0057-1) contains supplementary material, which is available to authorized users.

Double-layered cell transfer technology for bone regeneration

For cell-based medicine, to mimic in vivo cellular localization, various tissue engineering approaches have been studied to obtain a desirable arrangement of cells on scaffold materials. We have developed a novel method of cell manipulation called “cell transfer technology”, enabling the transfer of cultured cells onto scaffold materials, and controlling cell topology. Here we show that using this technique, two different cell types can be transferred onto a scaffold surface as stable double layers or in patterned arrangements. Various combinations of adherent cells were transferred to a scaffold, amniotic membrane, in overlapping bilayers (double-layered cell transfer), and transferred cells showed stability upon deformations of the material including folding and trimming. Transplantation of mesenchymal stem cells from periodontal ligaments (PDLSC) and osteoblasts, using double-layered cell transfer significantly enhanced bone formation, when compared to single cell type transplantation. Our findings suggest that this double-layer cell transfer is useful to produce a cell transplantation material that can bear two cell layers. Moreover, the transplantation of an amniotic membrane with PDLSCs/osteoblasts by cell transfer technology has therapeutic potential for bone defects. We conclude that cell transfer technology provides a novel and unique cell transplantation method for bone regeneration.

The influence of anisotropic nano- to micro-topography on in vitro and in vivo osteogenesis

AIM

Topographically modified substrates are increasingly used in tissue engineering to enhance biomimicry. The overarching hypothesis is that topographical cues will control cellular response at the cell-substrate interface.

MATERIALS & METHODS

The influence of anisotropically ordered poly(lactic-co-glycolic acid) substrates (constant groove width of ~1860 nm; constant line width of ~2220 nm; variable groove depth of ~35, 306 and 2046 nm) on in vitro and in vivo osteogenesis were assessed.

RESULTS & DISCUSSION

We demonstrate that substrates with groove depths of approximately 306 and 2046 nm promote osteoblast alignment parallel to underlined topography in vitro. However, none of the topographies assessed promoted directional osteogenesis in vivo.

CONCLUSION

2D imprinting technologies are useful tools for in vitro cell phenotype maintenance.

Efficacy of novel synthetic bone substitutes in the reconstruction of large segmental bone defects in sheep tibiae

The treatment of large bone defects, particularly those with segmental bone loss, remains a significant clinical challenge as current approaches involving surgery or bone grafting often do not yield satisfactory long-term outcomes. This study reports the evaluation of novel ceramic scaffolds applied as bone graft substitutes in a clinically relevant in vivo model. Baghdadite scaffolds, unmodified or modified with a polycaprolactone coating containing bioactive glass nanoparticles, were implanted into critical-sized segmental bone defects in sheep tibiae for 26 weeks. Radiographic, biomechanical, μ-CT and histological analyses showed that both unmodified and modified baghdadite scaffolds were able to withstand physiological loads at the defect site, and induced substantial bone formation in the absence of supplementation with cells or growth factors. Notably, all samples showed significant bridging of the critical-sized defect (average 80%) with evidence of bone infiltration and remodelling within the scaffold implant. The unmodified and modified baghdadite scaffolds achieved similar outcomes of defect repair, although the latter may have an initial mechanical advantage due to the nanocomposite coating. The baghdadite scaffolds evaluated in this study hold potential for use as purely synthetic bone graft substitutes in the treatment of large bone defects while circumventing the drawbacks of autografts and allografts.

Potential of Electrospun Nanofibers for Biomedical and Dental Applications

Electrospinning is a versatile technique that has gained popularity for various biomedical applications in recent years. Electrospinning is being used for fabricating nanofibers for various biomedical and dental applications such as tooth regeneration, wound healing and prevention of dental caries. Electrospun materials have the benefits of unique properties for instance, high surface area to volume ratio, enhanced cellular interactions, protein absorption to facilitate binding sites for cell receptors. Extensive research has been conducted to explore the potential of electrospun nanofibers for repair and regeneration of various dental and oral tissues including dental pulp, dentin, periodontal tissues, oral mucosa and skeletal tissues. However, there are a few limitations of electrospinning hindering the progress of these materials to practical or clinical applications. In terms of biomaterials aspects, the better understanding of controlled fabrication, properties and functioning of electrospun materials is required to overcome the limitations. More in vivo studies are definitely required to evaluate the biocompatibility of electrospun scaffolds. Furthermore, mechanical properties of such scaffolds should be enhanced so that they resist mechanical stresses during tissue regeneration applications. The objective of this article is to review the current progress of electrospun nanofibers for biomedical and dental applications. In addition, various aspects of electrospun materials in relation to potential dental applications have been discussed.

Composite scaffolds for osteochondral repair obtained by combination of additive manufacturing, leaching processes and hMSC-CM functionalization

Articular repair is a relevant and challenging area for the emerging fields of tissue engineering and biofabrication. The need of significant gradients of properties, for the promotion of osteochondral repair, has led to the development of several families of composite biomaterials and scaffolds, using different effective approaches, although a perfect solution has not yet been found. In this study we present the design, modeling, rapid manufacturing and in vitro testing of a composite scaffold aimed at osteochondral repair. The presented composite scaffold stands out for having a functional gradient of density and stiffness in the bony phase, obtained in titanium by means of computer-aided design combined with additive manufacture using selective laser sintering. The chondral phase is obtained by sugar leaching, using a PDMS matrix and sugar as porogen, and is joined to the bony phase during the polymerization of PDMS, therefore avoiding the use of supporting adhesives or additional intermediate layers. The mechanical performance of the construct is biomimetic and the stiffness values of the bony and chondral phases can be tuned to the desired applications, by means of controlled modifications of different parameters. A human mesenchymal stem cell (h-MSC) conditioned medium (CM) is used for improving scaffold response. Cell culture results provide relevant information regarding the viability of the composite scaffolds used.

Tubular Scaffold with Shape Recovery Effect for Cell Guide Applications

Tubular scaffolds with aligned polylactic acid (PLA) fibres were fabricated for cell guide applications by immersing rolled PLA fibre mats into a polyvinyl acetate (PVAc) solution to bind the mats. The PVAc solution was also mixed with up to 30 wt % β-tricalcium phosphate (β-TCP) content. Cross-sectional images of the scaffold materials obtained via scanning electron microscopy (SEM) revealed the aligned fibre morphology along with a significant number of voids in between the bundles of fibres. The addition of β-TCP into the scaffolds played an important role in increasing the void content from 17.1% to 25.3% for the 30 wt % β-TCP loading, which was measured via micro-CT (µCT) analysis. Furthermore, µCT analyses revealed the distribution of aggregated β-TCP particles in between the various PLA fibre layers of the scaffold. The compressive modulus properties of the scaffolds increased from 66 MPa to 83 MPa and the compressive strength properties decreased from 67 MPa to 41 MPa for the 30 wt % β-TCP content scaffold. The scaffolds produced were observed to change into a soft and flexible form which demonstrated shape recovery properties after immersion in phosphate buffered saline (PBS) media at 37 °C for 24 h. The cytocompatibility studies (using MG-63 human osteosarcoma cell line) revealed preferential cell proliferation along the longitudinal direction of the fibres as compared to the control tissue culture plastic. The manufacturing process highlighted above reveals a simple process for inducing controlled cell alignment and varying porosity features within tubular scaffolds for potential tissue engineering applications.

Electrospun PLGA fibers incorporated with functionalized biomolecules for cardiac tissue engineering

Structural similarity of electrospun fibers (ESFs) to the native extracellular matrix provides great potential for the application of biofunctional ESFs in tissue engineering. This study aimed to synthesize biofunctionalized poly (L-lactide-co-glycolide) (PLGA) ESFs for investigating the potential for cardiac tissue engineering application. We developed a simple but novel strategy to incorporate adhesive peptides in PLGA ESFs. Two adhesive peptides derived from laminin, YIGSR, and RGD, were covalently conjugated to poly-L-lysine, and then mingled with PLGA solution for electrospinning. Peptides were uniformly distributed on the surface and in the interior of ESFs. PLGA ESFs incorporated with YIGSR or RGD or adsorbed with laminin significantly enhanced the adhesion of cardiomyocytes isolated from neonatal rats. Furthermore, the cells were found to adhere better on ESFs compared with flat substrates after 7 days of culture. Immunofluorescent staining of F-actin, vinculin, a-actinin, and N-cadherin indicated that cardiomyocytes adhered and formed striated α-actinin better on the laminin-coated ESFs and the YIGSR-incorporated ESFs compared with the RGD-incorporated ESFs. The expression of α-myosin heavy chain and β-tubulin on the YIGSR-incorporated ESFs was significantly higher compared with the expression level on PLGA and RGD-incorporated samples. Furthermore, the contraction of cardiomyocytes was faster and lasted longer on the laminin-coated ESFs and YIGSR-incorporated ESFs. The results suggest that aligned YIGSR-incorporated PLGA ESFs is a better candidate for the formation of cardiac patches. This study demonstrated the potential of using peptide-incorporated ESFs as designable-scaffold platform for tissue engineering.

Strategies for directing the structure and function of three-dimensional collagen biomaterials across length scales

Collagen type I is a widely used natural biomaterial that has found utility in a variety of biological and medical applications. Its well-characterized structure and role as an extracellular matrix protein make it a highly relevant material for controlling cell function and mimicking tissue properties. Collagen type I is abundant in a number of tissues, and can be isolated as a purified protein. This review focuses on hydrogel biomaterials made by reconstituting collagen type I from a solubilized form, with an emphasis on in vitro studies in which collagen structure can be controlled. The hierarchical structure of collagen from the nanoscale to the macroscale is described, with an emphasis on how structure is related to function across scales. Methods of reconstituting collagen into hydrogel materials are presented, including molding of macroscopic constructs, creation of microscale modules and electrospinning of nanoscale fibers. The modification of collagen biomaterials to achieve the desired structures and functions is also addressed, with particular emphasis on mechanical control of collagen structure, creation of collagen composite materials and crosslinking of collagenous matrices. Biomaterials scientists have made remarkable progress in rationally designing collagen-based biomaterials and in applying them both to the study of biology and for therapeutic benefit. This broad review illustrates recent examples of techniques used to control collagen structure and thereby to direct its biological and mechanical functions.

Characterization of a bioactive fiber scaffold with entrapped HUVECs in coaxial electrospun core-shell fiber

Human umbilical vein endothelial cells (HUVECs) were successfully entrapped in polyethylene oxide (PEO) core /polycaprolactone (PCL) shell electrospun fibers thus creating a "bioactive fiber." The viability and release of biomolecules from the entrapped cells in the bioactive fibers were characterized. A key modification to the core solution was the inclusion of 50% fetal bovine serum (FBS), which improved cell viability substantially. The fluorescein diacetate (FDA) staining revealed that the entrapped cells were intact and viable immediately after the electrospinning process. A long-term cell viability assay using AlamarBlue® showed that cells were viable for over two weeks. Secreted Interleukin-8 (IL-8) was monitored as a candidate released protein, which can also act as an indicator of HUVEC stress. These results demonstrated that HUVECs could be entrapped within the electrospun scaffold with the potential of controllable cell deposition and the creation of a bioactive fibrous scaffold with extended functionality.

A fast process for imprinting micro and nano patterns on electrospun fiber meshes at physiological temperatures

Electrospun fiber meshes are patterned at length scales comparable to or lower than their fiber diameter. Simple nano- and microgrooves and closed geometric shapes are imprinted in different tones using a fast imprint process at physiological temperatures. Human mesenchymal stromal cells cultured on patterned scaffolds show differences in cellular morphology and cytoskeleton organization. Microgrooved electrospun fibers support upregulation of alkaline phosphatase and bone morphogenetic protein-2 gene expression when cells are cultured in osteogenic medium.

Bone Regeneration Based on Tissue Engineering Conceptions - A 21st Century Perspective

The role of Bone Tissue Engineering in the field of Regenerative Medicine has been the topic of substantial research over the past two decades. Technological advances have improved orthopaedic implants and surgical techniques for bone reconstruction. However, improvements in surgical techniques to reconstruct bone have been limited by the paucity of autologous materials available and donor site morbidity. Recent advances in the development of biomaterials have provided attractive alternatives to bone grafting expanding the surgical options for restoring the form and function of injured bone. Specifically, novel bioactive (second generation) biomaterials have been developed that are characterised by controlled action and reaction to the host tissue environment, whilst exhibiting controlled chemical breakdown and resorption with an ultimate replacement by regenerating tissue. Future generations of biomaterials (third generation) are designed to be not only osteoconductive but also osteoinductive, i.e. to stimulate regeneration of host tissues by combining tissue engineering and in situ tissue regeneration methods with a focus on novel applications. These techniques will lead to novel possibilities for tissue regeneration and repair. At present, tissue engineered constructs that may find future use as bone grafts for complex skeletal defects, whether from post-traumatic, degenerative, neoplastic or congenital/developmental "origin" require osseous reconstruction to ensure structural and functional integrity. Engineering functional bone using combinations of cells, scaffolds and bioactive factors is a promising strategy and a particular feature for future development in the area of hybrid materials which are able to exhibit suitable biomimetic and mechanical properties. This review will discuss the state of the art in this field and what we can expect from future generations of bone regeneration concepts.

Enhanced human bone marrow mesenchymal stem cell functions in novel 3D cartilage scaffolds with hydrogen treated multi-walled carbon nanotubes

Cartilage tissue is a nanostructured tissue which is notoriously hard to regenerate due to its extremely poor inherent regenerative capacity and complex stratified architecture. Current treatment methods are highly invasive and may have many complications. Thus, the goal of this work is to use nanomaterials and nano/microfabrication methods to create novel biologically inspired tissue engineered cartilage scaffolds to facilitate human bone marrow mesenchymal stem cell (MSC) chondrogenesis. To this end we utilized electrospinning to design and fabricate a series of novel 3D biomimetic nanostructured scaffolds based on hydrogen (H2) treated multi-walled carbon nanotubes (MWCNTs) and biocompatible poly(L-lactic acid) (PLLA) polymers. Specifically, a series of electrospun fibrous PLLA scaffolds with controlled fiber dimension were fabricated in this study. In vitro MSC studies showed that stem cells prefer to attach in the scaffolds with smaller fiber diameter. More importantly, the MWCNT embedded scaffolds showed a drastic increase in mechanical strength and a compressive Young's modulus matching to natural cartilage. Furthermore, our MSC differentiation results demonstrated that incorporation of the H2 treated carbon nanotubes and poly-L-lysine coating can induce more chondrogenic differentiations of MSCs than controls. After two weeks of culture, PLLA scaffolds with H2 treated MWCNTs and poly-L-lysine can achieve the highest glycosaminoglycan synthesis, making them promising for further exploration for cartilage regeneration.

Coaxial PCL/PVA electrospun nanofibers: osseointegration enhancer and controlled drug release device

The failure of prosthesis after total joint replacement is mainly due to dysfunctional osseointegration and implant infection. There is a critical need for orthopedic implants that promote rapid osseointegration and prevent bacterial colonization, particularly when placed in bone compromised by disease or physiology of the patients. The aim of this study was to fabricate a novel coaxial electrospun polycaprolactone (PCL)/polyvinyl alcohol (PVA) core-sheath nanofiber (NF) blended with both hydroxyapatite nanorods (HA) and type I collagen (Col) (PCL(Col)/PVA(HA)). Doxycycline (Doxy) and dexamethasone (Dex) were successfully incorporated into the PCL(Col)/PVA(HA) NFs for controlled release. The morphology, surface hydrophilicity and mechanical properties of the PCL/PVA NF mats were analyzed by scanning electron microscopy, water contact angle and atomic force microscopy. The PCL(Col)/PVA(HA) NFs are biocompatible and enhance the adhesion and proliferation of murine pre-osteoblastic MC3T3 cells. The release of Doxy and Dex from coaxial PCL(Col)/PVA(HA) NFs showed more controlled release compared with the blended NFs. Using an ex vivo porcine bone implantation model we found that the PCL(Col)/PVA(HA) NFs bind firmly on the titanium rod surface and the NFs coating remained intact on the surface of titanium rods after pullout. No disruption or delamination was observed after the pullout test. These findings indicate that PCL(Col)/PVA(HA) NFs encapsulating drugs have great potential in enhancing implant osseointegration and preventing implant infection.

Plug and play: combining materials and technologies to improve bone regenerative strategies

Despite recent advances in the development of biomaterials intended to replace natural bone grafts for the regeneration of large, clinically relevant defects, most synthetic solutions that are currently applied in the clinic are still inferior to natural bone grafts with regard to regenerative potential and are limited to non-weight-bearing applications. From a materials science perspective, we always face the conundrum of the preservation of bioactivity of calcium phosphate ceramics in spite of better mechanical and handling properties and processability of polymers. Composites have long been investigated as a method to marry these critical properties for the successful regeneration of bone and, indeed, have shown a significant improvement when used in combination with cells or growth factors. However, when looking at this approach from a clinical and regulatory perspective, the use of cells or biologicals prolongs the path of new treatments from the bench to the bedside. Applying 'smart' synthetic materials alone poses the fascinating challenge of instructing tissue regeneration in situ, thereby tremendously facilitating clinical translation. In the journey to make this possible, and with the aim of adding up the advantages of different biomaterials, combinations of fabrication technologies arise as a new strategy for generating instructive three-dimensional (3D) constructs for bone regeneration. Here we provide a review of recent technologies and approaches to create such constructs and give our perspective on how combinations of technologies and materials can help in obtaining more functional bone regeneration.

Engineering tubular bone using mesenchymal stem cell sheets and coral particles

The development of bone tissue engineering has provided new solutions for bone defects. However, the cell-scaffold-based approaches currently in use have several limitations, including low cell seeding rates and poor bone formation capacity. In the present study, we developed a novel strategy to engineer bone grafts using mesenchymal stem cell sheets and coral particles. Rabbit bone marrow mesenchymal stem cells were continuously cultured to form a cell sheet with osteogenic potential and coral particles were integrated into the sheet. The composite sheet was then wrapped around a cylindrical mandrel to fabricate a tubular construct. The resultant tubular construct was cultured in a spinner-flask bioreactor and subsequently implanted into a subcutaneous pocket in a nude mouse for assessment of its histological characteristics, radiological density and mechanical property. A similar construct assembled from a cell sheet alone acted as a control. In vitro observations demonstrated that the composite construct maintained its tubular shape, and exhibited higher radiological density, compressive strength and greater extracellular matrix deposition than did the control construct. In vivo experiments further revealed that new bone formed ectopically on the composite constructs, so that the 8-week explants of the composite sheets displayed radiological density similar to that of native bone. These results indicate that the strategy of using a combination of a cell sheet and coral particles has great potential for bone tissue engineering and repairing bone defects.

The future of carbon dioxide for polymer processing in tissue engineering

The use of CO2 for scaffold fabrication in tissue engineering was popularized in the mid-1990 s as a tool for producing polymeric foam scaffolds, but had fallen out of favor to some extent, in part due to challenges with pore interconnectivity. Pore interconnectivity issues have since been resolved by numerous dedicated studies that have collectively outlined how to control the appropriate parameters to achieve a pore structure desirable for tissue regeneration. In addition to CO2 foaming, several groups have leveraged CO2 as a swelling agent to impregnate scaffolds with drugs and other bioactive additives, and for encapsulation of plasmids within scaffolds for gene delivery. Moreover, in contrast to CO2 foaming, which typically relies on supercritical CO2 at very high pressures, CO2 at much lower pressures has also been used to sinter polymeric microspheres together in the presence of cells to create cell-seeded scaffolds in a single step. CO2 has a number of advantages for polymer processing in tissue engineering, including its ease of use, low cost, and the opportunity to circumvent the use of organic solvents. Building on these advantages, and especially now with the tremendous precedent that has paved the way in defining operating parameters, and making the technology accessible for new groups to adapt, we invite and encourage our colleagues in the field to leverage CO2 as a new tool to enhance their own respective unique capabilities.

Physically crosslinked nanocomposites from silicate-crosslinked PEO: mechanical properties and osteogenic differentiation of human mesenchymal stem cells

The mechanical and biological properties of silicate-crosslinked PEO nanocomposites are studied. A strong correlation is observed between silicate concentration and mechanical properties. In vitro cell culture studies reveal that an increase in silicate concentration enhances the attachment and proliferation of human mesenchymal stem cells significantly. An upregulation in the expression of osteocalcin on nanocomposites compared to the tissue culture polystyrene control is observed. Together, these results suggest that silicate-based nanocomposites are bioactive and have the potential to be used in a range of biotechnological and biomedical applications such as injectable matrices, biomedical coatings, drug delivery, and regenerative medicine.

Electrospun polyvinyl alcohol-collagen-hydroxyapatite nanofibers: a biomimetic extracellular matrix for osteoblastic cells

The failure of prosthesis after total joint replacement is due to the lack of early implant osseointegration. In this study polyvinyl alcohol-collagen-hydroxyapatite (PVA-Col-HA) electrospun nanofibrous meshes were fabricated as a biomimetic bone-like extracellular matrix for the modification of orthopedic prosthetic surfaces. In order to reinforce the PVA nanofibers, HA nanorods and Type I collagen were incorporated into the nanofibers. We investigated the morphology, biodegradability, mechanical properties and biocompatibility of the prepared nanofibers. Our results showed these inorganic-organic blended nanofibers to be degradable in vitro. The encapsulated nano-HA and collagen interacted with the PVA content, reinforcing the hydrolytic resistance and mechanical properties of nanofibers that provided longer lasting stability. The encapsulated nano-HA and collagen also enhanced the adhesion and proliferation of murine bone cells (MC3T3) in vitro. We propose the PVA-Col-HA nanofibers might be promising modifying materials on implant surfaces for orthopedic applications.

Layer-by-layer tissue microfabrication supports cell proliferation in vitro and in vivo

Layer-by-layer biofabrication represents a novel strategy to create three-dimensional living structures with a controlled internal architecture, using cell micromanipulation technologies. Laser assisted bioprinting (LAB) is an effective printing method for patterning cells, biomolecules, and biomaterials in two dimensions. "Biopapers," made of thin polymer scaffolds, may be appropriate to achieve three-dimensional constructs and to reinforce mechanical properties of printed materials. The aim of this work was to evaluate the effect of the tridimensional organization of cells and biomaterials on cell proliferation in vitro and in vivo. The experimental LAB setup was comprised of an infrared laser, focused onto a glass ribbon coated with an absorbing layer of gold. The cell bioink was made of MG63 cells (50 millions cells/mL in culture medium and 1% alginate), transduced with Luciferase gene for tracking and quantification. The printing substrate was a 100-μm-thick polycaprolacton (PCL) electrospun scaffold. The building sequence comprised sequential layers of cells and PCL scaffolds stacked using two different tridimensional arrangements, which were compared in this study (layer-by-layer vs. seeding on a single locus of the scaffolds). Then the cell-seeded materials were cultured in vitro or implanted in vivo in NOD-SCID mice. The qualitative follow-up involved scanning electron microscopy (SEM) observations, live-dead assays, and histology. The cell amount was quantified by photon imager during 21 days in vitro and 2 months in vivo. Live- dead assay and SEM revealed that the cells survived after printing and spread onto PCL membranes. Circle-shaped patterns were maintained in vitro during the first week but they were no longer observable after 2 weeks, due to cell proliferation. Luciferase tracking displayed that the cell amount was increased in vitro and in vivo when the materials and the cells where stacked layer by layer. Histological sections of the in vivo samples revealed a thicker fibrous tissue in the layer-by-layer samples. We have demonstrated in this study that PCL electrospun biopapers can act as a shock-absorbing mattress for cell printing and could further support cell proliferation. The layer-by-layer printing provided an appropriate 3D environment for cell survival and enhanced cell proliferation in vitro and in vivo.

Increasing the pore sizes of bone-mimetic electrospun scaffolds comprised of polycaprolactone, collagen I and hydroxyapatite to enhance cell infiltration

Bone-mimetic electrospun scaffolds consisting of polycaprolactone (PCL), collagen I and nanoparticulate hydroxyapatite (HA) have previously been shown to support the adhesion, integrin-related signaling and proliferation of mesenchymal stem cells (MSCs), suggesting these matrices serve as promising degradable substrates for osteoregeneration. However, the small pore sizes in electrospun scaffolds hinder cell infiltration in vitro and tissue-ingrowth into the scaffold in vivo, limiting their clinical potential. In this study, three separate techniques were evaluated for their capability to increase the pore size of the PCL/col I/nanoHA scaffolds: limited protease digestion, decreasing the fiber packing density during electrospinning, and inclusion of sacrificial fibers of the water-soluble polymer PEO. The PEO sacrificial fiber approach was found to be the most effective in increasing scaffold pore size. Furthermore, the use of sacrificial fibers promoted increased MSC infiltration into the scaffolds, as well as greater infiltration of endogenous cells within bone upon placement of scaffolds within calvarial organ cultures. These collective findings support the use of sacrificial PEO fibers as a means to increase the porosity of complex, bone-mimicking electrospun scaffolds, thereby enhancing tissue regenerative processes that depend upon cell infiltration, such as vascularization and replacement of the scaffold with native bone tissue.

The three-dimensional vascularization of growth factor-releasing hybrid scaffold of poly (epsilon-caprolactone)/collagen fibers and hyaluronic acid hydrogel

A significant stumbling block in the creation of functional three-dimensional (3D) engineered tissues is the proper vascularization of the constructs. Furthermore, in the context of electrospinning, the development of 3D constructs using this technique has been hindered by the limited infiltration of cells into their structure. In an attempt to address these issues, a hybrid mesh of poly (ɛ-caprolactone)-collagen blend (PCL/Col) and hyaluronic acid (HA) hydrogel, Heprasil™ was created via a dual electrodeposition system. Simultaneous deposition of HA and PCL/Col allowed the dual loading and controlled release of two potent angiogenic growth factors VEGF(165) and PDGF-BB over a period of five weeks in vitro. Furthermore, this manner of loading sustained the bioactivity of the two growth factors. Utilizing an in-house developed 3D co-culture assay model of human umbilical vein endothelial cells and lung fibroblasts, the growth factor-loaded hybrid meshes was shown to not only support cellular attachment, but also their infiltration and the recapitulation of primitive capillary network in the scaffold's architecture. Thus, the creation of a PCL/Col-Heprasil hybrid scaffold is a step forward toward the attainment of a 3D bio-functionalized, vascularized tissue engineering construct.

Electrospun scaffolds for bone tissue engineering

Tissue engineering aims to regenerate native tissues and will represent the alternative choice of standard surgery for different kind of tissue damages. The fundamental basis of tissue engineering is the appropriate selection of scaffolds and their morphological, mechanical, chemical, and biomimetic properties, closely related to cell lines that will be seeded therein. The aim of this review is to summarize and report the innovative scientific contributions published in the field of orthopedic tissue engineering, in particular about bone tissue engineering. We have focused our attention on the electrospinning technique, as a scaffold fabrication method. Electrospun materials are being evaluated as scaffolds for bone tissue engineering, and the results of all these studies clearly indicate that they represent suitable potential substrates for cell-based technologies.

In vitro assessment of the differentiation potential of bone marrow-derived mesenchymal stem cells on genipin-chitosan conjugation scaffold with surface hydroxyapatite nanostructure for bone tissue engineering

Increasing evidence has revealed that the surface characteristics of biomaterials, such as chemical composition, stiffness, and topography, especially nanotopography, significantly influence cell growth and differentiation. In this study, we examined the effect of surface biomimetic apatite nanostructure of a new hydroxyapatite-coated genipin-chitosan conjugation scaffold (HGCCS) on cell shape, cytoskeleton organization, and osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells in vitro. Cell shape and cytoskeleton organization showed significant differences between cells cultured on genipin-cross-linked chitosan framework and those cultured on HGCCS with surface apatite network-like nanostructure after 7 days of incubation in the osteogenic medium. The result of specific alkaline phosphatase activity as an indicator of osteogenic differentiation showed that the alkaline phosphatase activity of rat bone marrow-derived mesenchymal stem cells was higher on HGCCS. Based on quantitative real-time polymerase chain reaction, HGCCS induced highest mRNA expression of osteogenic differentiation makers, runt-related transcription factor 2 by 7 days, osteopontin by 7 days, and osteocalcin by 14 days, respectively. The enhanced ability of cells on HGCCS to produce mineralized extracellular matrix and nodules was also assessed on day 14 with Alizarin red staining. The results of this study suggest that the surface biomimetic apatite nanostructure of HGCCS is a critical signal cue to promoting osteogenic differentiation in vitro. These findings open a new research avenue to controlling stem cell lineage commitment and provide a promising scaffold for bone tissue engineering.

Colonization and osteogenic differentiation of different stem cell sources on electrospun nanofiber meshes

Numerous challenges remain in the successful clinical translation of cell-based therapies for musculoskeletal tissue repair, including the identification of an appropriate cell source and a viable cell delivery system. The aim of this study was to investigate the attachment, colonization, and osteogenic differentiation of two stem cell types, human mesenchymal stem cells (hMSCs) and human amniotic fluid stem (hAFS) cells, on electrospun nanofiber meshes. We demonstrate that nanofiber meshes are able to support these cell functions robustly, with both cell types demonstrating strong osteogenic potential. Differences in the kinetics of osteogenic differentiation were observed between hMSCs and hAFS cells, with the hAFS cells displaying a delayed alkaline phosphatase peak, but elevated mineral deposition, compared to hMSCs. We also compared the cell behavior on nanofiber meshes to that on tissue culture plastic, and observed that there is delayed initial attachment and proliferation on meshes, but enhanced mineralization at a later time point. Finally, cell-seeded nanofiber meshes were found to be effective in colonizing three-dimensional scaffolds in an in vitro system. This study provides support for the use of the nanofiber mesh as a model surface for cell culture in vitro, and a cell delivery vehicle for the repair of bone defects in vivo.

Microcavitary hydrogel-mediating phase transfer cell culture for cartilage tissue engineering

Hydrogels have been widely used as cell-laden vehicles for therapeutic transplantation in regenerative medicine. Although the advantages of biocompatibility and injectability for in situ grafting have made hydrogel a superior candidate in tissue engineering, there remain challenges in long-term efficacy of tissue development using hydrogel, especially when more sophisticated applications are demanded. The major bottleneck lies in environmental constraints for neo-tissue generation in the gel bulk such as proliferation of encapsulated cells (colonies) per se and also accommodation of their endogenously produced extracellular matrices. In this study, we endeavor to develop an innovative tissue engineering system to overcome these drawbacks through a novel microcavitary hydrogel (MCG)-based scaffolding technology and a novel phase transfer cell culture (PTCC) strategy to enable phenotypically bona fide neo-tissue formation in an injectable artificial graft. For this purpose, microspherical cavities are created in cell-encapsulating hydrogel bulk via a retarded dissolution of coencapsulated gelatin microspheres. Based on proliferation and affinity selection, the encapsulated cell colonies adjacent to the gel-cavity interface will spontaneously outgrow the hydrogel phase and sprout into cavities, enabling neo-tissue islets to fill up the voids and further expand throughout the whole system for full tissue regeneration. The design of MCG-PTCC strategy was elicited from an observation of a spontaneous dynamic outgrowth of chondrocytes from the edge of a cell-laden hydrogel construct over prolonged cultivation--a phenomenon named edge flourish. This MCG-PTCC strategy potentially introduce a new application to hydrogels in the field of regenerative medicine through elevation of its role as a cell vehicle to a three-dimensional transplantable growth-guiding platform for further development of newly generated tissues that better fulfill the demanding criteria of scaffolds in therapeutic tissue regeneration.