The effect of PEGT/PBT scaffold architecture on the composition of tissue engineered cartilage

Farnesol-modified biodegradable polyurethanes for cartilage tissue engineering

A bifunctionalized 3,7,11-trimethyl-2,6,10-dodecatrien-1-diaminobutane amide (isoprenoid) was obtained from 3,7,11-trimethyl-2,6,10-dodecatrien-1-ol (farnesol) in a three-step synthesis. The bifunctionalized isoprenoid was characterized using infrared spectroscopy and (1)H and (13)C nuclear magnetic resonance spectroscopy and was covalently incorporated (0.12 mmol x g(-1)) into the biodegradable aliphatic polyurethane formed on the polycondensation reaction of poly(epsilon-caprolactone) diol, 1,4,3,6-dianhydro-D-sorbitol and 1,6-hexamethylene diisocyanate. Although the covalent incorporation of the isoprenoid molecule into the polyurethane chain modified the surface chemistry of the polymer, it did not affect the viability of attached chondrocytes. Porous 3D scaffolds were produced from the modified and unmodified biodegradable segmented polyurethanes by a salt leaching-phase-inverse technique. The scaffolds were seeded with bovine chondrocytes encapsulated in fibrin gel and cultured in vitro for 14 days. The incorporation of bifunctional isoprenoid into the polyurethane affected the morphology of the scaffolds produced, when compared with the morphology of the scaffolds produced using the same technique from the unmodified polyurethane. As a consequence, there was more uniform cell seeding and more homogeneous distribution of the synthesized extracellular matrix throughout the scaffold resulting in a reduced cell/tissue layer at the edges of the constructs. However, glycosaminoglycan (GAG), DNA content, and chondrocytes phenotype in the scaffolds produced from these two polyurethane formulations did not vary significantly. The findings suggest that the change of surface characteristics and the more open pore structure of the scaffolds produced from the isoprenoid-modified polyurethane are beneficial for the seeding efficiency and the homogeneity of the tissue engineered constructs.

Analysis of collagen and glucose modulated cell growth within tissue engineered scaffolds

The strategy of tissue engineering includes seeding cells onto porous scaffolds. The cellular construct is cultured in vitro for a period of time before transplantation for the patient. Because of the intrinsic complexity of biological systems, it is valuable to have models of simulation that can assess the culture conditions and optimize experiments. This work presents a mathematical model to account for the effects of glucose and type II collagen on chondrocyte growth under static culture conditions. Dependence of cell growth on collagen was assumed as a biphasic function of collagen quantity, whereby the cell growth rate increases and then decreases with increasing collagen content. Results from simulation were compared with experimental data in literature. The model was then applied to investigate the effects of cell seeding area, demonstrating the spatiotemporal evolution of cell distribution in scaffolds. Results show that the conventional uniform seeding method may not be a good way of gaining uniform and large cell number densities at the final stage of cultivation. A seeding mode that has cells reside initially in the middle area of scaffold was shown to be able to not only reduce the diffusion limitation of nutrients but also weaken the inhibiting impact of aggregated collagen on cell growth. Therefore the middle seeding mode may result in better cell amounts and uniformities for developing tissue engineered constructs.

Experimental-computational evaluation of human bone marrow stromal cell spreading on trabecular bone structures

The clinical application of macro-porous scaffolds for bone regeneration is significantly affected by the problem of insufficient cell colonization. Given the wide variety of different scaffold structures used for tissue engineering it is essential to derive relationships for cell colonization independent of scaffold architecture. To study cell population spreading on 3D structures decoupled from nutrient limitations, an in vitro culture system was developed consisting of thin slices of human trabecular bone seeded with Human Bone Marrow Stromal Cells, combined with dedicated microCT imaging and computational modeling of cell population spreading. Only the first phase of in vitro scaffold colonization was addressed, in which cells migrate and proliferate up to the stage when the surface of the bone is covered as a monolayer, a critical prerequisite for further tissue formation. The results confirm the model's ability to represent experimentally observed cell population spreading. The key advantage of the computational model was that by incorporating complex 3D structure, cell behavior can be characterized quantitatively in terms of intrinsic migration parameters, which could potentially be used for predictions on different macro-porous scaffolds subject to additional experimental validation. This type of modeling will prove useful in predicting cell colonization and improving strategies for skeletal tissue engineering.

Rapid prototyping of anatomically shaped, tissue-engineered implants for restoring congruent articulating surfaces in small joints

BACKGROUND

Preliminary studies investigated advanced scaffold design and tissue engineering approaches towards restoring congruent articulating surfaces in small joints.

MATERIALS AND METHODS

Anatomical femoral and tibial cartilage constructs, fabricated by three-dimensional fibre deposition (3DF) or compression moulding/particulate leaching (CM), were evaluated in vitro and in vivo in an autologous rabbit model. Effects of scaffold pore architecture on rabbit chondrocyte differentiation and mechanical properties were evaluated following in vitro culture and subcutaneous implantation in nude mice. After femoral and tibial osteotomy and autologous implantation of tissue-engineered constructs in rabbit knee joints, implant fixation and joint articulation were evaluated.

RESULTS

Rapid prototyping of 3DF architectures with 100% interconnecting pores promoted homogeneous distribution of viable cells, glycosaminoglycan (GAG) and collagen type II; significantly greater GAG content and differentiation capacity (GAG/DNA) in vitro compared to CM architectures; and higher mechanical equilibrium modulus and dynamic stiffness (at 0.1 Hz). Six weeks after implantation, femoral and tibial constructs had integrated with rabbit bone and knee flexion/extension and partial load bearing were regained. Histology demonstrated articulating surfaces between femoral and tibial constructs for CM and 3DF architectures; however, repair tissue appeared fibrocartilage-like and did not resemble implanted cartilage.

CONCLUSIONS

Anatomically shaped, tissue-engineered constructs with designed mechanical properties and internal pore architectures may offer alternatives for reconstruction or restoration of congruent articulating surfaces in small joints.

PEOT/PBT based scaffolds with low mechanical properties improve cartilage repair tissue formation in osteochondral defects

The aim of our study was to compare the healing response of biomechanically and biochemically different scaffolds in osteochondral defects created in rabbit medial femoral condyles. A block copolymer comprised of poly(ethylene oxide terephthalate) and poly(butylene terephthalate) was used to prepare porous scaffolds. The 70/30 scaffold (70 wt % poly(ethylene oxide terephthalate)) was compared to the stiffer 55/45 (55 wt % poly(ethylene oxide terephthalate)) scaffold. Nine 6-month-old rabbits were used. Osteochondral defects were filled with 55/45 scaffolds (n = 6); 70/30 scaffolds (n = 6); or left empty (n = 6). Defect sites were allowed to heal for 12 weeks. Condyles were macroscopically evaluated and analysed histologically using the O'Driscoll score for evaluating repair of osteochondral defects. Repair tissue in 70/30 scaffolds consisted of cartilage-like tissue on top of trabecular bone, whereas the tissue within the 55/45 scaffolds consisted predominantly of trabecular bone. O'Driscoll scores for 70/30 scaffolds were significantly better (p = 0.024) in comparison to untreated osteochondral defects and 55/45 scaffolds. This study reveals that the biomechanical and biochemical properties of the scaffold play an important role by themselves, and can affect the healing response of osteochondral defects. Scaffolds with low mechanical properties were superior in cartilage repair tissue formation.

Brain cortex regeneration affected by scaffold architectures

Object

The aim of this study was to compare designed scaffolds with a random-pored sponge scaffold to determine what role scaffold architecture plays in a cortical injury model.

Methods

Cylindrical scaffolds (3 × 3 mm) were made of a poly-(ε-caprolactone) polymer with 2 different molds from a 3D printer and had either: 1) unidirectional channels and microgrooves oriented longitudinally within the cylinder or 2) orthogonally intersecting channels and axial microgrooves within the cylinder. Additional randomized porosity was imparted using a salt-leaching method. A control scaffold without channels or microgrooves but containing random pores was also made. Scaffolds were implanted for 1, 4, and 8 weeks in a cylindrical defect created 3 mm posterior to the bregma in rat cortex. Control animals had tissue removed but received no implant. Brains were coronally cryosectioned and sections were stained. Antibodies for nestin, glial fibrillary acidic protein (GFAP), and TUJ1 were used to identify neural progenitors, activated astrocytes, and neuronal axons. Tissue ingrowth (H & E), astrocytic infiltration (GFAP), parenchymal inflammation (GFAP), and defect width (H & E) were quantified from images.

Results

Defect widths grew and parenchymal inflammation decreased over time with no statistical difference between groups. Total tissue ingrowth and astrocytic infiltration increased over time and was greatest in the orthogonal design group. Specific cell ingrowth, which was aligned with microgrooves interiorly in the orthogonal group and exteriorly in the longitudinal channel group, was qualitatively assessed from nestin and TUJ1 labeling.

Conclusions

Scaffold architecture can benefit brain tissue regeneration by integrating the following design principles: 1) large (100s of micrometers) pores or channels oriented toward the parenchyma for increased astrocytic infiltration; 2) microgrooves oriented in the desired direction of cellular migration and neuronal alignment; and 3) fully interconnecting channels for cellular migration and tissue integration.

Regenerating articular tissue by converging technologies

Scaffolds for osteochondral tissue engineering should provide mechanical stability, while offering specific signals for chondral and bone regeneration with a completely interconnected porous network for cell migration, attachment, and proliferation. Composites of polymers and ceramics are often considered to satisfy these requirements. As such methods largely rely on interfacial bonding between the ceramic and polymer phase, they may often compromise the use of the interface as an instrument to direct cell fate. Alternatively, here, we have designed hybrid 3D scaffolds using a novel concept based on biomaterial assembly, thereby omitting the drawbacks of interfacial bonding. Rapid prototyped ceramic particles were integrated into the pores of polymeric 3D fiber-deposited (3DF) matrices and infused with demineralized bone matrix (DBM) to obtain constructs that display the mechanical robustness of ceramics and the flexibility of polymers, mimicking bone tissue properties. Ostechondral scaffolds were then fabricated by directly depositing a 3DF structure optimized for cartilage regeneration adjacent to the bone scaffold. Stem cell seeded scaffolds regenerated both cartilage and bone in vivo.

Critical factors in the design of growth factor releasing scaffolds for cartilage tissue engineering

BACKGROUND

Trauma or degenerative diseases of the joints are common clinical problems resulting in high morbidity. Although various orthopedic treatments have been developed and evaluated, the low repair capacities of articular cartilage renders functional results unsatisfactory in the long term. Over the last decade, a different approach (tissue engineering) has emerged that aims not only to repair impaired cartilage, but also to fully regenerate it, by combining cells, biomaterials mimicking extracellular matrix (scaffolds) and regulatory signals. The latter is of high importance as growth factors have the potency to induce, support or enhance the growth and differentiation of various cell types towards the chondrogenic lineage. Therefore, the controlled release of different growth factors from scaffolds appears to have great potential to orchestrate tissue repair effectively.

OBJECTIVE

This review aims to highlight considerations and limitations of the design, materials and processing methods available to create scaffolds, in relation to the suitability to incorporate and release growth factors in a safe and defined manner. Furthermore, the current state of the art of signalling molecules release from scaffolds and the impact on cartilage regeneration in vitro and in vivo is reported and critically discussed.

METHODS

The strict aspects of biomaterials, scaffolds and growth factor release from scaffolds for cartilage tissue engineering applications are considered.

CONCLUSION

Engineering defined scaffolds that deliver growth factors in a controlled way is a task seldom attained. If growth factor delivery appears to be beneficial overall, the optimal delivery conditions for cartilage reconstruction should be more thoroughly investigated.

Integrating novel technologies to fabricate smart scaffolds

Tissue engineering aims at restoring or regenerating a damaged tissue by combining cells, derived from a patient biopsy, with a 3D porous matrix functioning as a scaffold. After isolation and eventual in vitro expansion, cells are seeded on the 3D scaffolds and implanted directly or at a later stage in the patient's body. 3D scaffolds need to satisfy a number of requirements: (i) biocompatibility, (ii) biodegradability and/or bioresorbability, (iii) suitable mechanical properties, (iv) adequate physicochemical properties to direct cell-material interactions matching the tissue to be replaced and (v) ease in regaining the original shape of the damaged tissue and the integration with the surrounding environment. Still, it appears to be a challenge to satisfy all the aforementioned requisites with the biomaterials and the scaffold fabrication technologies nowadays available. 3D scaffolds can be fabricated with various techniques, among which rapid prototyping and electrospinning seem to be the most promising. Rapid prototyping technologies allow manufacturing scaffolds with a controlled, completely accessible pore network--determinant for nutrient supply and diffusion--in a CAD/CAM fashion. Electrospinning (ESP) allows mimicking the extracellular matrix (ECM) environment of the cells and can provide fibrous scaffolds with instructive surface properties to direct cell faith into the proper lineage. Yet, these fabrication methods have some disadvantages if considered alone. This review aims at summarizing conventional and novel scaffold fabrication techniques and the biomaterials used for tissue engineering and drug-delivery applications. A new trend seems to emerge in the field of scaffold design where different scaffolds fabrication technologies and different biomaterials are combined to provide cells with mechanical, physicochemical and biological cues at the macro-, micro- and nano-scale. If merged together, these integrated technologies may lead to the generation of a new set of 3D scaffolds that satisfies all of the scaffolds' requirements for tissue-engineering applications and may contribute to their success in a long-term scenario.

Oxygen consumption of chondrocytes in agarose and collagen gels: a comparative analysis

The growth of engineered cartilage tissue in vitro is often impaired by the problem of insufficient oxygen and nutrient supply to cells seeded in 3D constructs. Despite its central role in controlling most cell functions, the scaffolding material has generally been thought of only as a transport barrier and its potential active role in controlling oxygen uptake has never been addressed. In this work the role of cell-material interaction on oxygen metabolism in 3D in vitro cultures was surveyed. To this aim bovine chondrocytes, at a cell density of 400,000 and 4,000,000 cells/mL, respectively, were seeded in collagen type I and in agarose, while keeping all other culture conditions constant. A unidirectional oxygen gradient was induced in the culture through the application of a "sandwich" model and the oxygen concentration at the pericellular level was measured by phosphorescence quenching microscopy. Results show that the oxygen consumption rate is two-fold higher in agarose than in collagen, which indicates that the nature of the material strongly influences cell metabolic behaviour. Moreover, since different oxygen consumption rates are linked to different cell biosynthetic activity, our findings will prove beyond any doubt the active role played by materials in tissue regeneration.

Anatomical 3D Fiber-Deposited Scaffolds for Tissue Engineering: Designing a Neotrachea

The advantage of using anatomically shaped scaffolds as compared to modeled designs was investigated and assessed in terms of cartilage formation in an artificial tracheal construct. Scaffolds were rapid prototyped with a technique named three-dimensional fiber deposition (3DF). Anatomical scaffolds were fabricated from a patient-derived computerized tomography dataset, and compared to cylindrical and toroidal tubular scaffolds. Lewis rat tracheal chondrocytes were seeded on 3DF scaffolds and cultured for 21 days. The 3-(4,5-dimethylthiazol-2yl)-2,5-dyphenyltetrazolium bromide (MTT) and sulfated glycosaminoglycan (GAG) assays were performed to measure the relative number of cells and the extracellular matrix (ECM) formed. After 3 weeks of culture, the anatomical scaffolds revealed a significant increase in ECM synthesis and a higher degree of differentiation as shown by the GAG/MTT ratio and by scanning electron microscopy analysis. Interestingly, a lower scaffold's pore volume and porosity resulted in more tissue formation and a better cell differentiation, as evidenced by GAG and GAG/MTT values. Scaffolds were compliant and did not show any signs of luminal obstruction in vitro. These results may promote anatomical scaffolds as functional matrices for tissue regeneration not only to help regain the original shape, but also for their improved capacity to support larger tissue formation.

The Roles of Hypoxia in theIn VitroEngineering of Tissues

Oxygen is a potent modulator of cell function and wound repair in vivo. The lack of oxygen (hypoxia) can create a potentially lethal environment and limit cellular respiration and growth or, alternatively, enhance the production of the specific extracellular matrix components and increase angiogenesis through the hypoxia-inducible factor-1 pathway. For the in vitro generation of clinically relevant tissue-engineered grafts, these divergent actions of hypoxia should be addressed. Diffusion through culture medium and tissue typically limits oxygen transport in vitro, leading to hypoxic regions and limiting the viable tissue thickness. Approaches to overcoming the transport limitations include culture with bioreactors, scaffolds with artificial microvasculature, oxygen carriers, and hyperbaric oxygen chambers. As an alternate approach, angiogenesis after implantation may be enhanced by incorporating endothelial cells, genetically modified cells, or specific factors (including vascular endothelial growth factor) into the scaffold or exposing the graft to a hypoxic environment just before implantation. Better understanding of the roles of hypoxia will help prevent common problems and exploit potential benefits of hypoxia in engineered tissues.

Finite Element Analysis of Meniscal Anatomical 3D Scaffolds: Implications for Tissue Engineering

Solid Free-Form Fabrication (SFF) technologies allow the fabrication of anatomical 3D scaffolds from computer tomography (CT) or magnetic resonance imaging (MRI) patients’ dataset. These structures can be designed and fabricated with a variable, interconnected and accessible porous network, resulting in modulable mechanical properties, permeability, and architecture that can be tailored to mimic a specific tissue to replace or regenerate. In this study, we evaluated whether anatomical meniscal 3D scaffolds with matching mechanical properties and architecture are beneficial for meniscus replacement as compared to meniscectomy. After acquiring CT and MRI of porcine menisci, 3D fiber-deposited (3DF) scaffolds were fabricated with different architectures by varying the deposition pattern of the fibers comprising the final structure. The mechanical behaviour of 3DF scaffolds with different architectures and of porcine menisci was measured by static and dynamic mechanical analysis and the effect of these tissue engineering templates on articular cartilage was assessed by finite element analysis (FEA) and compared to healthy conditions or to meniscectomy. Results show that 3DF anatomical menisci scaffolds can be fabricated with pore different architectures and with mechanical properties matching those of natural menisci. FEA predicted a beneficial effect of meniscus replacement with 3D scaffolds in different mechanical loading conditions as compared to meniscectomy. No influence of the internal scaffold architecture was found on articular cartilage damage. Although FEA predictions should be further confirmed by in vitro and in vivo experiments, this study highlights meniscus replacement by SFF anatomical scaffolds as a potential alternative to meniscectomy.

Effect of pore size on in vitro cartilage formation using chitosan-based hyaluronic acid hybrid polymer fibers

In this study, we successfully developed three-dimensional scaffolds fabricated from the chitosan-based hyaluronic acid hybrid polymer fibers, which can control the porous structure. To determine the adequate pore size for enhancing the chondrogenesis of cultured cells, we compared the behaviors of rabbit chondrocytes in scaffolds comprising different pore sizes (100, 200, and 400 microm pore size). Regarding the cell proliferation, there was no significant difference among the three groups. On the other hand, glycosaminoglycan contents in the 400 microm group significantly increased during the culture period, compared with those in the other groups. The ratio of type II to type I collagen mRNA level was also significantly higher in the 400 microm group than in the other groups. These results indicate that our scaffold with 400 microm pore size significantly enhances the extracellular matrix synthesis by chondrocytes. Additionally, the current scaffolds showed high mechanical properties, compared with liquid and gel materials. The data derived from this study suggest great promise for the future of a novel fabricated material with relatively large pore size as a scaffold for cartilage regeneration. The biological and mechanical advantages presented here will make it possible to apply our scaffold to relatively wide cartilaginous lesions.

Design of biphasic polymeric 3-dimensional fiber deposited scaffolds for cartilage tissue engineering applications

This report describes a novel system to create rapid prototyped 3-dimensional (3D) fibrous scaffolds with a shell-core fiber architecture in which the core polymer supplies the mechanical properties and the shell polymer acts as a coating providing the desired physicochemical surface properties. Poly[(ethylene oxide) terephthalate-co-poly(butylene) terephthalate] (PEOT/PBT) 3D fiber deposited (3DF) scaffolds were fabricated and examined for articular cartilage tissue regeneration. The shell polymer contained a higher molecular weight of the initial poly(ethylene glycol) (PEG) segments used in the copolymerization and a higher weight percentage of the PEOT domains compared with the core polymer. The 3DF scaffolds entirely produced with the shell or with the core polymers were also considered. After 3 weeks of culture, scaffolds were homogeneously filled with cartilage tissue, as assessed by scanning electron microscopy. Although comparable amounts of entrapped chondrocytes and of extracellular matrix formation were found for all analyzed scaffolds, chondrocytes maintained their rounded shape and aggregated during the culture period on shell-core 3DF scaffolds, suggesting a proper cell differentiation into articular cartilage. This finding was also observed in the 3DF scaffolds fabricated with the shell composition only. In contrast, cells spread and attached on scaffolds made simply with the core polymer, implying a lower degree of differentiation into articular cartilaginous tissue. Furthermore, the shell-core scaffolds displayed an improved dynamic stiffness as a result of a "prestress" action of the shell polymer on the core one. In addition, the dynamic stiffness of the constructs increased compared with the stiffness of the bare scaffolds before culture. These findings suggest that shell-core 3DF PEOT/PBT scaffolds with desired mechanical and surface properties are a promising solution for improved cartilage tissue engineering.

Interaction of cells and nanofiber scaffolds in tissue engineering

Nanofibers and nanomaterials are potentially recent additions to materials in relation to tissue engineering (TE). TE is the regeneration of biological tissues through the use of cells, with the aid of supporting structures and biomolecules. Mimicking architecture of extracellular matrix is one of the challenges for TE. Biodegradable biopolymer nanofibers with controlled surface and internal molecular structures can be electrospun into mats with specific fiber arrangement and structural integrity for drug delivery and TE applications. The polymeric materials are widely accepted because of their ease of processability and amenability to provide a large variety of cost-effective materials, which help to enhance the comfort and quality of life in modern biomedical and industrial society. Today, nanotechnology and nanoscience approaches to scaffold design and functionalization are beginning to expand the market for drug delivery and TE is forming the basis for highly profitable niche within the industry. This review describes recent advances for fabrication of nanofiber scaffolds and interaction of cells in TE.

Advanced Tools for Tissue Engineering: Scaffolds, Bioreactors, and Signaling

This article contains the collective views expressed at the second session of the workshop "Tissue Engineering--The Next Generation,'' which was devoted to the tools of tissue engineering: scaffolds, bioreactors, and molecular and physical signaling. Lisa E. Freed and Farshid Guilak discussed the integrated use of scaffolds and bioreactors as tools to accelerate and control tissue regeneration, in the context of engineering mechanically functional cartilage and cardiac muscle. Edward Guo focused on the opportunities that tissue engineering generates for studies of mechanobiology and on the need for tissue engineers to learn about mechanical forces during tissue and organ genesis. Martha L. Gray focused on the potential of biomedical imaging for noninvasive monitoring of engineered tissues and on the opportunities biomedical imaging can generate for the development of new markers. Robert Tranquillo reviewed the approach to tissue engineering of a spectrum of avascular habitually loaded tissues- blood vessels, heart valves, ligaments, tendons, cartilage, and skin. Jeffrey W. Holmes offered the perspective of a "reverse paradigm''--the use of tissue constructs in quantitative studies of cell-matrix interactions, cell mechanics, matrix mechanics, and mechanobiology. Milica Radisic discussed biomimetic design of tissue-engineering systems, on the example of synchronously contractile cardiac muscle. Michael V. Sefton proposed a new, simple approach to the vascularization of engineered tissues. This session stressed the need for advanced scaffolds, bioreactors, and imaging technologies and offered many enlightening examples on how these advanced tools can be utilized for functional tissue engineering and basic research in medicine and biology.

Evaluation of chondrogenesis within PEGT: PBT scaffolds with high PEG content

Porous poly(ethylene glycol) terephthalate:poly (butylene terephthalate) (PEGT:PBT) scaffolds with high PEG molecular weight (1000 g/mole) and PEGT content (60%) were fabricated using two different processes-paraffin templating and compression molding-for cartilage engineering applications. This polymer composition has previously been shown to enable chondrocyte adhesion and maintain differentiated phenotype in 2D monolayer culture. The influence of 3D polymer scaffold processing on the formation of cartilaginous tissue was studied by seeding primary immature bovine chondrocytes within cylindrical scaffolds in mixed flask reactors for 3 days, followed by cultivation in culture plates for a total of 10 or 24 days. Tissue-polymer constructs were evaluated morphologically by SEM and histology, and quantitatively for cellularity, total collagen, and glycosaminoglycan content, all of which remained statistically equivalent for each time point tested, irrespective of fabrication method. These data demonstrate that the polymers engineered for this study were able to support chondrogenesis independent of scaffold fabrication process, with the influence of pore architecture lessened by the highly hydrated scaffold microenvironments induced by high PEG content.

Fiber diameter and texture of electrospun PEOT/PBT scaffolds influence human mesenchymal stem cell proliferation and morphology, and the release of incorporated compounds

Electrospinning (ESP) has lately shown a great potential as a novel scaffold fabrication technique for tissue engineering. Scaffolds are produced by spinning a polymeric solution in fibers through a spinneret connected to a high-voltage electric field. The fibers are then collected on a support, where the scaffold is created. Scaffolds can be of different shapes, depending on the collector geometry, and have high porosity and high surface per volume ratio, since the deposited fibers vary from the microscale to the nanoscale range. Such fibers are quite effective in terms of tissue regeneration, as cells can bridge the scaffold pores and fibers, resulting in a fast and homogeneous tissue growth. Furthermore, fibers can display a nanoporous ultrastructure due to solvent evaporation. The aim of this study was to characterize electrospun scaffolds from poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) copolymers and to unravel the mechanism of pore formation on the fibers. The effect of different fiber diameters and of their surface nanotopology on cell seeding, attachment, and proliferation was studied. Smooth fibers with diameter of 10microm were found to support an optimal cell seeding and attachment within the micrometer range analyzed. Moreover, a nanoporous surface significantly enhanced cell proliferation and cells spreading on the fibers. The fabrication of ESP scaffolds with incorporated dyes with different molecular dimensions is also reported and their release measured. These findings contribute to the field of cell-material interaction and lead to the fabrication of "smart" scaffolds which can direct cells morphology and proliferation, and eventually release biological signals to properly conduct tissue formation.

Differential cell viability of chondrocytes and progenitor cells in tissue-engineered constructs following implantation into osteochondral defects

Animal studies in cartilage tissue engineering usually include the transfer of cultured cells into chondral or osteochondral defects. Immediately at implantation, the cells are exposed to a dramatically changed environment. The aim of this study was to determine the viability of two cell types currently considered for cellular therapies of cartilage defects-chondrocytes and progenitor cells-shortly after exposure to an osteochondral defect in rabbit knees. To that end, autogenic chondrocytes and periosteal cells were labeled with CM-DiI fluorochrome, seeded or cultured in PEGT/PBT scaffolds for periods up to 2 weeks, transferred into osteochondral defects, harvested 5 days postimplantation, and analyzed for cell viability. In order to further elucidate factors effecting cell viability within our model system, we investigated the effect of serum, 2) extracellular matrix surrounding implanted cells, 3) scaffold interconnectivity, and 4) hyaluronan, as a known cell protectant. Controls included scaffolds with devitalized cells and scaffolds analyzed at implantation. We found that the viability of periosteum cells (14%), but not of chondrocytes (65-95%), was significantly decreased after implantation. The addition of hyaluronan increased periostium cell viability to 44% (p < 0.05). Surprisingly, cell viability in less interconnected compression-molded scaffolds was higher compared to that of fully interconnected scaffolds produced by rapid prototyping. All other factors tested did not affect viability significantly. Our data suggest chondrocytes as a suitable cell source for cartilage repair in line with clinical data on several chondrocyte-based therapies. Although we did not test progenitor cells other the periosteum cells, tissue-engineering approaches using such cell types should take cell viability aspects into consideration.

Analysis of cell growth and diffusion in a scaffold for cartilage tissue engineering

Developments in tissue engineering over the past decade have offered promising future for the repair and reconstruction of damaged tissues. To regenerate three dimensional and weight-bearing implants, advances in biomaterials and manufacturing technologies prompted cell cultivations with natural or artificial scaffolds, in which cells are allowed to proliferate, migrate, and differentiate in vitro. In this article, we develop a mathematical model for cell growth in a porous scaffold. By treating the cell-scaffold construct as a porous medium, a continuum model is set up based on basic principles of mass conservation. In addition to cell growth kinetics, we incorporate cell diffusion in the model to describe the effects of cell random walks. Computational results are compared to experimental data found in the literature. With this model, we are able to investigate cell motility, heterogeneous cell distributions, and non-uniform seeding for tissue engineering applications. Results show that random walks tend to enhance uniform cell spreads in space, which in turn increases the probabilities for cells to acquire nutrients; therefore random walks are likely to be a positive contribution to the overall cell growth on scaffolds.

Cartilage tissue engineering by expanded goat articular chondrocytes

In this study we investigated whether expanded goat chondrocytes have the capacity to generate cartilaginous tissues with biochemical and biomechanical properties improving with time in culture. Goat chondrocytes were expanded in monolayer with or without combinations of FGF-2, TGF-beta1, and PDGFbb, and the postexpansion chondrogenic capacity assessed in pellet cultures. Expanded chondrocytes were also cultured for up to 6 weeks in HYAFF-M nonwoven meshes or Polyactive foams, and the resulting cartilaginous tissues were assessed histologically, biochemically, and biomechanically. Supplementation of the expansion medium with FGF-2 increased the proliferation rate of goat chondrocytes and enhanced their postexpansion chondrogenic capacity. FGF-2-expanded chondrocytes seeded in HYAFF-M or Polyactive scaffolds formed cartilaginous tissues with wet weight, glycosaminoglycan, and collagen content, increasing from 2 days to 6 weeks culture (up to respectively 2-, 8-, and 41-fold). Equilibrium and dynamic stiffness measured in HYAFF M-based constructs also increased with time, up to, respectively, 1.3- and 16-fold. This study demonstrates the feasibility to engineer goat cartilaginous tissues at different stages of development by varying culture time, and thus opens the possibility to test the effect of maturation stage of engineered cartilage on the outcome of cartilage repair in orthotopic goat models.

Biodegradable scaffolds – delivery systems for cell therapies

The use of cells as therapies for disease, repair and regeneration of tissues is one of the new challenges in modern therapeutics. To facilitate the ability to localise, condition and protect cells, biodegradable scaffolds are being developed that will improve the efficiency of these treatments. Thus, cell delivery systems, either scaffolds or capsules, play a pivotal role in the success of these techniques. This review discusses these novel approaches. The selection of scaffold materials is addressed alongside issues of biocompatibility. The current research developments in smart scaffolds, which focus on the formation of biomimetic scaffolds, new fabrication techniques capable of controlling architecture and microstructure of scaffolds, and the production of injectable and in situ crosslinked scaffolds, are outlined. Finally, the continuing challenges that will drive future research in the cell therapies are highlighted.

Cartilage tissue engineering: its potential and uses

PURPOSE OF REVIEW

The prevalent nature of osteoarthritis, a cartilage degenerative disease that results in the erosion of joint surfaces and loss of mobility, underscores the importance of developing functional articular cartilage replacement. Recent research efforts have focused on tissue engineering as a promising approach for cartilage regeneration and repair. Tissue engineering is a multidisciplinary research area that incorporates both biological and engineering principles for the purpose of generating new, living tissues to replace the diseased/damaged tissue and restore tissue/organ function. This review surveys and highlights the current concepts and recent progress in cartilage tissue engineering, and discusses the challenges and potential of this rapidly advancing field of biomedical research.

RECENT FINDINGS

Cartilage tissue engineering is critically dependent on selection of appropriate cells (differentiated or progenitor cells); fabrication and utilization of biocompatible and mechanically suitable scaffolds for cell delivery; stimulation with chondrogenically bioactive molecules introduced in the form of recombinant proteins or via gene transfer; and application of dynamic, mechanical loading regimens for conditioning of the engineered tissue constructs, including the design of specialized biomechanically active bioreactors.

SUMMARY

Cell selection, scaffold design and biological stimulation remain the challenges of function tissue engineering. Successful regeneration or replacement of damaged or diseased cartilage will depend on future advances in our understanding of the biology of cartilage and stem cells and technological development in engineering.

Dynamic mechanical properties of 3D fiber-deposited PEOT/PBT scaffolds: An experimental and numerical analysis

Mechanical properties of three-dimensional (3D) scaffolds can be appropriately modulated through novel fabrication techniques like 3D fiber deposition (3DF), by varying scaffold's pore size and shape. Dynamic stiffness, in particular, can be considered as an important property to optimize the scaffold structure for its ultimate in vivo application to regenerate a natural tissue. Experimental data from dynamic mechanical analysis (DMA) reveal a dependence of the dynamic stiffness of the scaffold on the intrinsic mechanical and physicochemical properties of the material used, and on the overall porosity and architecture of the construct. The aim of this study was to assess the relationship between the aforementioned parameters, through a mathematical model, which was derived from the experimental mechanical data. As an example of how mechanical properties can be tailored to match the natural tissue to be replaced, articular bovine cartilage and porcine knee meniscus cartilage dynamic stiffness were measured and related to the modeled 3DF scaffolds dynamic stiffness. The dynamic stiffness of 3DF scaffolds from poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) copolymers was measured with DMA. With increasing porosity, the dynamic stiffness was found to decrease in an exponential manner. The influence of the scaffold architecture (or pore shape) and of the molecular network properties of the copolymers was expressed as a scaffold characteristic coefficient alpha, which modulates the porosity effect. This model was validated through an FEA numerical simulation performed on the structures that were experimentally tested. The relative deviation between the experimental and the finite element model was less than 15% for all of the constructs with a dynamic stiffness higher than 1 MPa. Therefore, we conclude that the mathematical model introduced can be used to predict the dynamic stiffness of a porous PEOT/PBT scaffold, and to choose the biomechanically optimal structure for tissue engineering applications.

Heterogeneous proliferation within engineered cartilaginous tissue: the role of oxygen tension

This article investigates heterogeneous proliferation within a seeded three-dimensional scaffold structure with the purpose of improving protocols for engineered tissue growth. A simple mathematical model is developed to examine the very strong interaction between evolving oxygen profiles and cell distributions within cartilaginous constructs. A comparison between predictions based on the model and experimental evidence is given for both spatial and temporal evolution of the oxygen tension and cell number density, showing that behaviour for the first 14 days can be explained well by the mathematical model. The dependency of the cellular proliferation rate on the oxygen tension is examined and shown to be similar in size to previous work but linear in form. The results show that cell-scaffold constructs that rely solely on diffusion for their supply of nutrients will inevitably produce proliferation-dominated regions near the outer edge of the scaffold in situations when the cell number density and oxygen consumption rate exceed a critical level. Possible strategies for reducing such non-uniform proliferation, including the conventional methods of enhancing oxygen transport, are outlined based on the model predictions.

3D fiber-deposited scaffolds for tissue engineering: influence of pores geometry and architecture on dynamic mechanical properties

One of the main issues in tissue engineering is the fabrication of scaffolds that closely mimic the biomechanical properties of the tissues to be regenerated. Conventional fabrication techniques are not sufficiently suitable to control scaffold structure to modulate mechanical properties. Within novel scaffold fabrication processes 3D fiber deposition (3DF) showed great potential for tissue engineering applications because of the precision in making reproducible 3D scaffolds, characterized by 100% interconnected pores with different shapes and sizes. Evidently, these features also affect mechanical properties. Therefore, in this study we considered the influence of different structures on dynamic mechanical properties of 3DF scaffolds. Pores were varied in size and shape, by changing fibre diameter, spacing and orientation, and layer thickness. With increasing porosity, dynamic mechanical analysis (DMA) revealed a decrease in elastic properties such as dynamic stiffness and equilibrium modulus, and an increase of the viscous parameters like damping factor and creep unrecovered strain. Furthermore, the Poisson's ratio was measured, and the shear modulus computed from it. Scaffolds showed an adaptable degree of compressibility between sponges and incompressible materials. As comparison, bovine cartilage was tested and its properties fell in the fabricated scaffolds range. This investigation showed that viscoelastic properties of 3DF scaffolds could be modulated to accomplish mechanical requirements for tailored tissue engineered applications.

[Determination of sterol content in the moss Climacium dendroides L]

The potential of porous poly(ether ester) scaffolds made from poly(ethylene glycol) terephthalate: poly(butylene terephthalate) (PEGT:PBT) block copolymers produced by various methods to enable cartilaginous tissue formation in vitro was studied. Scaffolds were fabricated by two different processes: paraffin templating (PT) and compression molding (CM). To determine whether PEGT:PBT scaffolds are able to support chondrogenesis, primary bovine chondrocytes were seeded within cylindrical scaffolds under dynamic seeding conditions. On day 3, constructs were transferred to six-well plates and evaluated for glycosaminoglycan (GAG) distribution (3, 10, and 24 days), type II collagen distribution, cellularity, and total collagen and GAG content (10 and 24 days). It was observed that better cell distribution during infiltration within PT scaffolds allowed greater chondrogenesis, and at later time points, than in CM scaffolds. The amount of GAG remained constant for all groups from 10 to 24 days, whereas collagen content increased significantly. These data suggest that PEGT:PBT scaffolds are suitable for cartilage tissue engineering, with the PT process enabling greater chondrogenesis than CM.

Porous scaffold design for tissue engineering

A paradigm shift is taking place in medicine from using synthetic implants and tissue grafts to a tissue engineering approach that uses degradable porous material scaffolds integrated with biological cells or molecules to regenerate tissues. This new paradigm requires scaffolds that balance temporary mechanical function with mass transport to aid biological delivery and tissue regeneration. Little is known quantitatively about this balance as early scaffolds were not fabricated with precise porous architecture. Recent advances in both computational topology design (CTD) and solid free-form fabrication (SFF) have made it possible to create scaffolds with controlled architecture. This paper reviews the integration of CTD with SFF to build designer tissue-engineering scaffolds. It also details the mechanical properties and tissue regeneration achieved using designer scaffolds. Finally, future directions are suggested for using designer scaffolds with in vivo experimentation to optimize tissue-engineering treatments, and coupling designer scaffolds with cell printing to create designer material/biofactor hybrids.

Effects of scaffold composition and architecture on human nasal chondrocyte redifferentiation and cartilaginous matrix deposition

We investigated whether the post-expansion redifferentiation and cartilage tissue formation capacity of adult human nasal chondrocytes can be regulated by controlled modifications of scaffold composition and architecture. As a model system, we used poly(ethylene glycol)-terephthalate-poly(butylene)-terephthalate block copolymer scaffolds from two compositions (low or high PEG content, resulting in different wettability) and two architectures (generated by compression molding or three-dimensional (3D) fiber deposition) with similar porosity and mechanical properties, but different interconnecting pore architectures. Scaffolds were seeded with expanded human chondrocytes and the resulting constructs assessed immunohistochemically, biochemically and at the mRNA expression level following up to 4 weeks of static culture. For a given 3D architecture, the more hydrophilic scaffold enhanced cell redifferentiation and cartilaginous tissue formation after 4 weeks culture, as assessed by higher mRNA expression of collagen type II, increased deposition of glycosaminoglycan (GAG) and predominance of type II over type I collagen immunostain. The fiber-deposited scaffolds, with a more accessible pore volume and larger interconnecting pores, supported increased GAG deposition, but only if a more hydrophilic composition was used. By applying controlled and selective modifications of chemico-physical scaffold parameters, we demonstrate that both scaffold composition and architecture are instructive for expanded human chondrocytes in the generation of 3D cartilaginous tissues. The observed effects of composition and architecture were likely to have been mediated, respectively, by differential serum protein adsorption and efficiency of nutrient/waste exchange.

The effect of alginate, hyaluronate and hyaluronate derivatives biomaterials on synthesis of non-articular chondrocyte extracellular matrix

Cartilage engineering consists of re-constructing functional cartilage by seeding chondrocytes in suitable biomaterials in vitro. The characteristics of neocartilage differ upon the type of biomaterial chosen. This study aims at determining the appropriate scaffold material for articular cartilage reconstruction using non articular chondrocytes harvested from rat sternum. For this purpose, the use of polysaccharide hydrogels such as alginate (AA) and hyaluronic acid (HA) was investigated. Several ratios of AA/HA were used as well as three derivatives obtained by chemical modification of HA (HA-C18, HA-C12(2.3), HA-C12(2.5)-TEG0.5). Sternal chondrocytes were successfully cultured in 3D alginate and alginate/HA scaffolds. HA retention in alginate beads was found to be higher in beads seeded with cells than in beads without cells. HA-C18 improved HA retention in beads but inhibited the chondrocyte synthesis process. Cell proliferation and metabolism were enhanced in all biomaterials when beads were mechanically agitated. Preliminary results have shown that the chondrocyte neo-synthesised matrix had acquired articular characteristics after 21 days culture.

Three-dimensional fiber-deposited PEOT/PBT copolymer scaffolds for tissue engineering: Influence of porosity, molecular network mesh size, and swelling in aqueous media on dynamic mechanical properties

Among novel scaffold fabrication techniques, 3D fiber deposition (3DF) has recently emerged as a means to fabricate well-defined and custom-made scaffolds for tissue regeneration, with 100% interconnected pores. The mechanical behavior of these constructs is dependent not only on different three-dimensional architectural and geometric features, but also on the intrinsic chemical properties of the material used. These affect the mechanics of the solid material and eventually of 3D porous constructs derived from them. For instance, poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) block copolymers are known to have mechanical properties, depending on the PEOT/PBT weight ratio in block form and on the molecular weight of the initial poly(ethylene glycol) (PEG) blocks. These differences are enhanced even more by their different swelling properties in aqueous media. Therefore, this article examines the influence of copolymer compositions in terms of their swelling on dynamic mechanical properties of solid material and porous 3DF scaffolds. The molecular weight of the starting PEG blocks used in the copolymer synthesis varied from 300 to 1000 g/mol. The PEOT/PBT weight ratio in the blocks used varied from 55/45 to 80/20. This corresponded to an increase of the swelling ratio Q from 1.06 to 2.46, and of the mesh size xi from approximately 9 Angstrom to approximately 47 Angstrom. With increased swelling, dynamic mechanical analysis (DMA) revealed a decrease in elastic response and an increase of viscoelasticity. Thus, by coupling structural and chemical characteristics, the viscoelastic properties of PEOT/PBT 3DF scaffolds may be fine tuned to achieve mechanical requirements for a variety of engineered tissues. Ultimately, the combination of 3DF and DMA may be useful to validate the hypothesis that mimicking the biomechanical behavior of a specific tissue for its optimal replacement is an important issue for at least some tissue-engineering applications.

The effect of PEGT/PBT scaffold architecture on oxygen gradients in tissue engineered cartilaginous constructs

Repair of articular cartilage defects using tissue engineered constructs composed of a scaffold and cultured autologous cells holds promise for future treatments. However, nutrient limitation (e.g. oxygen) has been suggested as a cause of the onset of chondrogenesis solely within the peripheral boundaries of larger constructs. In the present study, oxygen gradients were evaluated by microelectrode measurements in two porous polyethylene glycol terephthalate/polybutylene terephthalate (PEGT/PBT) scaffold architectures, a compression-molded and particle-leached sponge (CM) and a 3D-deposited fiber (3DF) scaffold. During the first 14 days in vitro, gradients intensified, after which a gradual decrease of the gradients was observed in vitro. In vivo, however, gradients changed instantly and became less pronounced. Although similar gradients were observed regardless of scaffold type, significantly more cells were present in the center of 3DF constructs after 2 weeks of in vivo culture. Our results stress the importance of a rationally designed scaffold for tissue-engineering applications. Organized structures, such as the 3DF PEGT/PBT polymer scaffolds, offer possibilities for regulation of nutrient supply and, therefore, hold promise for clinical approaches for cartilage repair.