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

Potency of fish collagen as a scaffold for regenerative medicine

Cells, growth factors, and scaffold are the crucial factors for tissue engineering. Recently, scaffolds consisting of natural polymers, such as collagen and gelatin, bioabsorbable synthetic polymers, such as polylactic acid and polyglycolic acid, and inorganic materials, such as hydroxyapatite, as well as composite materials have been rapidly developed. In particular, collagen is the most promising material for tissue engineering due to its biocompatibility and biodegradability. Collagen contains specific cell adhesion domains, including the arginine-glycine-aspartic acid (RGD) motif. After the integrin receptor on the cell surface binds to the RGD motif on the collagen molecule, cell adhesion is actively induced. This interaction contributes to the promotion of cell growth and differentiation and the regulation of various cell functions. However, it is difficult to use a pure collagen scaffold as a tissue engineering material due to its low mechanical strength. In order to make up for this disadvantage, collagen scaffolds are often modified using a cross-linker, such as gamma irradiation and carbodiimide. Taking into account the possibility of zoonosis, a variety of recent reports have been documented using fish collagen scaffolds. We herein review the potency of fish collagen scaffolds as well as associated problems to be addressed for use in regenerative medicine.

Adapted chondrogenic differentiation of human mesenchymal stem cells via controlled release of TGF-β1 from poly(ethylene oxide)-terephtalate/poly(butylene terepthalate) multiblock scaffolds

Controlled release of TGF-β1 from scaffolds is an attractive mechanism to modulate the chondrogenesis of human bone marrow mesenchymal stem cells (hBMSCs) that repopulate articular cartilage defects. Here, we evaluated the ability of porous scaffolds composed of poly(ethylene oxide)-terephtalate and poly(butylene terepthalate) (PEOT/PBT) to release bioactive TGF-β1 for chondrogenesis of hBMSCs in a pellet culture model. Chondroinduction was compared with that promoted by direct addition of the recombinant factor to the culture medium. The data show a controlled release of TGF-β1 from scaffolds for at least 21 days in vitro, with ∼10% of TGF-β1 released during this period. The delivered TGF-β1 was bioactive, as confirmed by successful chondrogenic differentiation of hBMSCs monitored by morphological, histological, immunohistochemical, biochemical, and real-time reverse transcription polymerase chain reaction analyses. Third, semiquantitative histological evaluations revealed a similar pattern of chondrogenesis compared with the positive controls. Importantly, TGF-β1-loaded scaffolds allowed for a ∼700-fold upregulation of type-II collagen mRNA compared to when pellets were maintained in the presence of the soluble TGF-β1, reflected also in the highest score of immunoreactivity to type-II collagen, not significantly different from the positive controls. Likewise, aggrecan mRNA was ∼200-fold upregulated. Interestingly, most (>94%) of the glycosaminoglycan produced remaining associated with the pellets. Analysis of hypertrophic events showed no significant difference in the average total hypertrophy score compared with the positive controls. These results demonstrate the suitability of controlled TGF-β1 release from biocompatible scaffolds to promote hBMSC chondrogenesis at a physical distance and in the absence of soluble TGF-β1.

25th anniversary article: scalable multiscale patterned structures inspired by nature: the role of hierarchy

Multiscale, hierarchically patterned surfaces, such as lotus leaves, butterfly wings, adhesion pads of gecko lizards are abundantly found in nature, where microstructures are usually used to strengthen the mechanical stability while nanostructures offer the main functionality, i.e., wettability, structural color, or dry adhesion. To emulate such hierarchical structures in nature, multiscale, multilevel patterning has been extensively utilized for the last few decades towards various applications ranging from wetting control, structural colors, to tissue scaffolds. In this review, we highlight recent advances in scalable multiscale patterning to bring about improved functions that can even surpass those found in nature, with particular focus on the analogy between natural and synthetic architectures in terms of the role of different length scales. This review is organized into four sections. First, the role and importance of multiscale, hierarchical structures is described with four representative examples. Second, recent achievements in multiscale patterning are introduced with their strengths and weaknesses. Third, four application areas of wetting control, dry adhesives, selectively filtrating membranes, and multiscale tissue scaffolds are overviewed by stressing out how and why multiscale structures need to be incorporated to carry out their performances. Finally, we present future directions and challenges for scalable, multiscale patterned surfaces.

Current trends in the design of scaffolds for computer-aided tissue engineering

Advances introduced by additive manufacturing have significantly improved the ability to tailor scaffold architecture, enhancing the control over microstructural features. This has led to a growing interest in the development of innovative scaffold designs, as testified by the increasing amount of research activities devoted to the understanding of the correlation between topological features of scaffolds and their resulting properties, in order to find architectures capable of optimal trade-off between often conflicting requirements (such as biological and mechanical ones). The main aim of this paper is to provide a review and propose a classification of existing methodologies for scaffold design and optimization in order to address key issues and help in deciphering the complex link between design criteria and resulting scaffold properties.

In silico prediction of the cell proliferation in porous scaffold using model of effective pore

The mathematical prediction of cell proliferation in porous scaffold still remains a challenge. The analysis of existing models and experimental data confirms a need for a new solution, which takes into account cells" development on the scaffold pore walls as well as some additional parameters such as the pore size, cell density in cellular layers, the thickness of the growing cell layer and others. The simulations, presented below, are based on three main approaches. The first approach takes into account multilayer cell growth on the pore walls of the scaffold. The second approach is a simulation of cell proliferation in a discrete process as a continuous one. The third one is the representation of scaffold structure as a system of cylindrical channels. Oxygen (nutrient) mass transfer is realized inside these channels. The model, described below, proposes the new solution to time dependent description of cell proliferation in porous scaffold and optimized trophical conditions for tissue development.

Recent developments of functional scaffolds for craniomaxillofacial bone tissue engineering applications

Autogenous bone grafting remains a gold standard for the reconstruction critical-sized bone defects in the craniomaxillofacial region. Nevertheless, this graft procedure has several disadvantages such as restricted availability, donor-site morbidity, and limitations in regard to fully restoring the complicated three-dimensional structures in the craniomaxillofacial bone. The ultimate goal of craniomaxillofacial bone reconstruction is the regeneration of the physiological bone that simultaneously fulfills both morphological and functional restorations. Developments of tissue engineering in the last two decades have brought such a goal closer to reality. In bone tissue engineering, the scaffolds are fundamental, elemental and mesenchymal stem cells/osteoprogenitor cells and bioactive factors. A variety of scaffolds have been developed and used as spacemakers, biodegradable bone substitutes for transplanting to the new bone, matrices of drug delivery system, or supporting structures enhancing adhesion, proliferation, and matrix production of seeded cells according to the circumstances of the bone defects. However, scaffolds to be clinically completely satisfied have not been developed yet. Development of more functional scaffolds is required to be applied widely to cranio-maxillofacial bone defects. This paper reviews recent trends of scaffolds for crania-maxillofacial bone tissue engineering, including our studies.

TMJ Bioengineering: A review

Regeneration using scaffolds, growth factors, and stem cells is being investigated worldwide. Pubmed search for scaffolds for condyle resulted in 102 articles, of which 24 analyzed Temporomandibular joint (TMJ) scaffolds and only 6 evaluated hydroxyapatite scaffolds. 17 articles report studies on TMJ disc regeneration. The ideal bone construct for repair should be able to replicate the lost structure, restore function, be harmless, reliable and biodegradable. Scaffolds act as carriers for mesenchymal stem cells and/or growth factors and are useful for cell adhesion, migration, proliferation, and differentiation. Gene therapy has also led to the accelerated effective bone regeneration. The major materials used as scaffolds are natural or synthetic polymers, ceramics, composite materials, and electrospun nanofibers. Mesenchymal stem cells are responsible for the formation of virtually all dental, oral, and craniofacial structures. Tissue-engineered bone can possess the customized shape and dimensions. It has the potential for the biological replacement of craniofacial bones.

Photocrosslinkable biodegradable elastomers based on cinnamate-functionalized polyesters

Synthetic biodegradable elastomers are an emerging class of materials that play a critical role in supporting innovations in bioabsorbable medical implants. This paper describes the synthesis and characterization of poly(glycerol-co-sebacate)-cinnamate (PGS-CinA), a biodegradable elastomer based on hyperbranched polyesters derivatized with pendant cinnamate groups. PGS-CinA can be prepared via photodimerization in the absence of photoinitiators using monomers that are found in common foods. The resulting network exhibits a Young's modulus of 50.5-152.1kPa and a projected in vitro degradation half-life time between 90 and 140days. PGS-CinA elastomers are intrinsically cell-adherent and support rapid proliferation of fibroblasts. Spreading and proliferation of fibroblasts are loosely governed by the substrate stiffness within the range of Young's moduli in PGS-CinA networks that were prepared. The thermo-mechanical properties, biodegradability and intrinsic support of cell attachment and proliferation suggest that PGS-CinA networks are broadly applicable for use in next generation bioabsorable materials including temporary medical devices and scaffolds for soft tissue engineering.

Evaluation of the Effective Diffusivity of a Freeform Fabricated Scaffold Using Computational Simulation

In scaffold-based tissue engineering, sufficient oxygen and nutrient supply into cells within a scaffold is essential to increase cell viability and the proliferation rate. Generally, oxygen and nutrients reach the cells through the media by diffusion in vitro or in vivo, assuming there is no convection flow through a scaffold with small-sized pores. The scaffold diffusion rate depends mainly on the scaffold pore architecture. Thus, understanding the effect of scaffold pore architecture on the diffusion mechanism is necessary to design an efficient scaffold model. This study proposes a computational method to estimate diffusivity using the finite element analysis (FEA). This method can be applied to evaluate and analyze the effective diffusivity of a freeform fabricated 3D scaffold. The diffusion application module of commercial FEA software was used to calculate the spatial oxygen concentration gradient in a scaffold model medium. The effective diffusivities of each scaffold could be calculated from the oxygen concentration data, which revealed that the scaffold pore architecture influences its effective diffusivity. The proposed method has been verified experimentally and can be applied to design pore architectures with efficient diffusion by increasing our understanding of how the diffusion rate within a scaffold is affected by its pore architecture.

Craniofacial bone tissue engineering

There are numerous conditions, such as trauma, cancer, congenital malformations, and progressive deforming skeletal diseases, that can compromise the function and architectonics of bones of craniofacial region. The need to develop new approaches for treatment of these disorders arises from the fact that conventional therapeutic strategies face many obstacles and limitations. The use of tissue engineering in regeneration of craniofacial bone structures is a very promising possibility and a great challenge for researchers and practitioners. Developments in stem cell biology and engineering have led to the discovery of different stem cell populations and biodegradable materials with suitable properties. This review summarizes the current achievements in tissue engineering of craniofacial bone, temporomandibular joint, and periodontal ligament.

The effect of scaffold-cell entrapment capacity and physico-chemical properties on cartilage regeneration

An important tenet in designing scaffolds for regenerative medicine consists in mimicking the dynamic mechanical properties of the tissues to be replaced to facilitate patient rehabilitation and restore daily activities. In addition, it is important to determine the contribution of the forming tissue to the mechanical properties of the scaffold during culture to optimize the pore network architecture. Depending on the biomaterial and scaffold fabrication technology, matching the scaffolds mechanical properties to articular cartilage can compromise the porosity, which hampers tissue formation. Here, we show that scaffolds with controlled and interconnected pore volume and matching articular cartilage dynamic mechanical properties, are indeed effective to support tissue regeneration by co-cultured primary and expanded chondrocyte (1:4). Cells were cultured on scaffolds in vitro for 4 weeks. A higher amount of cartilage specific matrix (ECM) was formed on mechanically matching (M) scaffolds after 28 days. A less protein adhesive composition supported chondrocytes rounded morphology, which contributed to cartilaginous differentiation. Interestingly, the dynamic stiffness of matching constructs remained approximately at the same value after culture, suggesting a comparable kinetics of tissue formation and scaffold degradation. Cartilage regeneration in matching scaffolds was confirmed subcutaneously in vivo. These results imply that mechanically matching scaffolds with appropriate physico-chemical properties support chondrocyte differentiation.

2-D coupled computational model of biological cell proliferation and nutrient delivery in a perfusion bioreactor

Tissue engineering aims to regenerate, repair or replace organs or tissues which have become defective due to trauma, disease or age related degeneration. This engineering may take place within the patient's body or tissue can be regenerated in a bioreactor for later implantation into the patient. Regeneration of soft tissue is one of the most demanding applications of tissue engineering. Producing proper nutrient supply, uniform cell distribution and high cell density are the important challenges. Many experimental models exist for tissue growth in a bioreactor. It is important to put experiments into a theoretical framework. Mathematical modelling in terms of physical and biochemical mechanisms is the best tool to understand experimental results. In this work a mathematical model of convective and diffusive transport of nutrients and cell evolution in a perfusion bioreactor is developed. A cell-seeded porous scaffold is placed in a perfusion bioreactor and fluid delivers the nutrients to the cells for their growth. The model describes the key features of the tissue engineering processes which includes the interaction between the cell growth, variation of material permeability due to cell proliferation, flow of fluid through the material and delivery of nutrients to the cells. The fluid flow through the porous scaffold is modelled by Darcy's law, and the delivery of nutrients to the cells is modelled by the advection-diffusion equation. A non-linear reaction diffusion system is used to model the cell growth. The growth of cells is modelled by logistic growth. COMSOL (a commercial finite element solver) is used to numerically solve the model. The results show that the distribution of cells and total cell number in the scaffold does not depend on the initial cell density but depend on the material permeability.

Electrospun nanofibrous cellulose scaffolds with controlled microarchitecture

Introducing porosity in electrospun scaffolds is critical to improve cell penetration and nutrient diffusion for tissue engineering. Nanofibrous cellulose scaffolds were prepared by electrospinning cellulose acetate (CA) followed by saponification to regenerate cellulose. Using a computer-assisted design approach, scaffolds underwent laser ablation resulting in pores with diameters between 50 and 300 μm without damaging or modifying the surrounding scaffold area. A new mineralization method was employed in conjunction with microablation using commercial phosphate buffered saline (PBS) to soak carboxymethylcellulose surface-modified electrospun scaffolds. The resulting crystals within the scaffold on the interior of the pore had a calcium to phosphate ratio of 1.56, similar to hydroxyapatite. It was observed that porosity of the cellulose scaffolds enhanced osteoblast cell attachment at the edge of the pores, while mineralization enhanced overall cell density.

Progress of key strategies in development of electrospun scaffolds: bone tissue

There has been unprecedented development in tissue engineering (TE) over the last few years owing to its potential applications, particularly in bone reconstruction or regeneration. In this article, we illustrate several advantages and disadvantages of different approaches to the design of electrospun TE scaffolds. We also review the major benefits of electrospun fibers for three-dimensional scaffolds in hard connective TE applications and identify the key strategies that can improve the mechanical properties of scaffolds for bone TE applications. A few interesting results of recent investigations have been explained for future trends in TE scaffold research.

Mass transfer trends occurring in engineered ex vivo tissue scaffolds

In vivo the vasculature provides an effective delivery system for cellular nutrients; however, artificial scaffolds have no such mechanism, and the ensuing limitations in mass transfer result in limited regeneration. In these investigations, the regional mass transfer properties that occur through a model scaffold derived from the human umbilical vein (HUV) were assessed. Our aim was to define the heterogeneous behavior associated with these regional variations, and to establish if different decellularization technologies can modulate transport conditions to improve microenvironmental conditions that enhance cell integration. The effect of three decellularization methods [Triton X-100 (TX100), sodium dodecyl sulfate (SDS), and acetone/ethanol (ACE/EtOH)] on mass transfer, cellular migration, proliferation, and metabolic activity were assessed. Results show that regional variation in tissue structure and composition significantly affects both mass transfer and cell function. ACE/EtOH decellularization was shown to increase albumin mass flux through the intima and proximate-medial region (0-250 μm) when compared with sections decellularized with TX100 or SDS; although, mass flux remained constant over all regions of the full tissue thickness when using TX100. Scaffolds decellularized with TX100 were shown to promote cell migration up to 146% further relative to SDS decellularized samples. These results show that depending on scaffold derivation and expectations for cellular integration, specificities of the decellularization chemistry affect the scaffold molecular architecture resulting in variable effects on mass transfer and cellular response.

Tissue-engineered constructs: the effect of scaffold architecture in osteochondral repair

Cartilage has a poor regenerative capacity. Tissue-engineering approaches using porous scaffolds seeded with chondrocytes may improve cartilage repair. The aim of this study was to examine the effect of pore size and pore interconnectivity on cartilage repair in osteochondral defects treated with different scaffolds seeded with allogenic chondrocytes. Scaffolds consisting of 55 wt% poly(ethylene oxide terephthalate) and 45 wt% poly(butylene terephthalate) (PEOT/PBT) with different pore sizes and interconnectivities were made, using a compression moulding (CM) and a three-dimensional fibre (3DF) deposition technique. In these scaffolds, allogenic chondrocytes were seeded, cultured for 3 weeks and implanted in osteochondral defects of skeletally mature rabbits. At 3 weeks no difference in cartilage repair between an empty osteochondral defect, CM or 3DF scaffolds was found. Three months post-implantation, cartilage repair was significantly improved after implantation of a 3DF scaffold compared to a CM scaffold. Although not significant, Mankin scores for osteoarthritis (OA) indicated less OA in the 3DF scaffold group compared to empty defects and CM-treated defects. It is concluded that scaffold pore size and pore interconnectivity influences osteochondral repair and a decreased pore interconnectivity seems to impair osteochondral repair.

Unit cell-based computer-aided manufacturing system for tissue engineering

Scaffolds play an important role in the regeneration of artificial tissues or organs. A scaffold is a porous structure with a micro-scale inner architecture in the range of several to several hundreds of micrometers. Therefore, computer-aided construction of scaffolds should provide sophisticated functionality for porous structure design and a tool path generation strategy that can achieve micro-scale architecture. In this study, a new unit cell-based computer-aided manufacturing (CAM) system was developed for the automated design and fabrication of a porous structure with micro-scale inner architecture that can be applied to composite tissue regeneration. The CAM system was developed by first defining a data structure for the computing process of a unit cell representing a single pore structure. Next, an algorithm and software were developed and applied to construct porous structures with a single or multiple pore design using solid freeform fabrication technology and a 3D tooth/spine computer-aided design model. We showed that this system is quite feasible for the design and fabrication of a scaffold for tissue engineering.

Effects of oxygen on zonal marker expression in human articular chondrocytes

Articular cartilage is organized in depth zones with phenotypically distinct subpopulations of chondrocytes that are exposed to different oxygen tensions. Despite growing evidence of the critical role for oxygen in chondrogenesis, little is known about its effect on chondrocytes from different zones. This study evaluates zonal marker expression of human articular chondrocytes from different zones under various oxygen tensions. Chondrocytes isolated from full-thickness, superficial, and middle/deep cartilage from knee replacement surgeries were expanded and redifferentiated under hypoxic (5% O(2)) or normoxic (20% O(2)) conditions. Differentiation under hypoxia increased expression of hypoxia-inducible factors 1alpha and 2alpha and accumulation of extracellular matrix, particularly in middle/deep chondrocytes, and favored re-expression of proteoglycan 4 by superficial chondrocytes compared with middle/deep cells. Zone-dependent expression of clusterin varied with culture duration. These results demonstrate that zonal chondrocytes retain important phenotypic differences during in vitro cultivation, and that these characteristics can be improved by altering the oxygen environment. However, transcript levels for pleiotrophin, cartilage intermediate layer protein, and collagen type X were similar between zones, challenging their reliability as zonal markers for tissue-engineered cartilage from osteoarthritis patients. Key factors including oxygen tension and cell source should be considered to prescribe zone-specific properties to tissue-engineered cartilage.

Biofabrication of osteochondral tissue equivalents by printing topologically defined, cell-laden hydrogel scaffolds

Osteochondral defects are prone to induce osteoarthritic degenerative changes. Many tissue-engineering approaches that aim to generate osteochondral implants suffer from poor tissue formation and compromised integration. This illustrates the need for further improvement of heterogeneous tissue constructs. Engineering of these structures is expected to profit from strategies addressing the complexity of tissue organization and the simultaneous use of multiple cell types. Moreover, this enables the investigation of the effects of three-dimensional (3D) organization and architecture on tissue function. In the present study, we characterize the use of a 3D fiber deposition (3DF) technique for the fabrication of cell-laden, heterogeneous hydrogel constructs for potential use as osteochondral grafts. Changing fiber spacing or angle of fiber deposition yielded scaffolds of varying porosity and elastic modulus. We encapsulated and printed fluorescently labeled human chondrocytes and osteogenic progenitors in alginate hydrogel yielding scaffolds of 1×2 cm with different parts for both cell types. Cell viability remained high throughout the printing process, and cells remained in their compartment of the printed scaffold for the whole culture period. Moreover, distinctive tissue formation was observed, both in vitro after 3 weeks and in vivo (6 weeks subcutaneously in immunodeficient mice), at different locations within one construct. These results demonstrate the possibility of manufacturing viable centimeter-scaled structured tissues by the 3DF technique, which could potentially be used for the repair of osteochondral defects.

Integration of hollow fiber membranes improves nutrient supply in three-dimensional tissue constructs

Sufficient nutrient and oxygen transport is a potent modulator of cell proliferation in in vitro tissue-engineered constructs. The lack of oxygen and culture medium can create a potentially lethal environment and limit cellular metabolic activity and growth. Diffusion through scaffold and multi-cellular tissue typically limits transport in vitro, leading to potential hypoxic regions and reduction in the viable tissue thickness. For the in vitro generation of clinically relevant tissue-engineered grafts, current nutrient diffusion limitations should be addressed. Major approaches to overcoming these include culture with bioreactors, scaffolds with artificial microvasculature, oxygen carriers and pre-vascularization of the engineered tissues. This study focuses on the development and utilization of a new perfusion culture system to provide adequate nutrient delivery to cells within large three-dimensional (3D) scaffolds. Perfusion of oxygenated culture medium through porous hollow fiber (HF) integrated within 3D free form fabricated (FFF) scaffolds is proposed. Mouse pre-myoblast (C2C12) cells cultured on scaffolds of poly(ethylene-oxide-terephthalate)-poly(butylene-terephthalate) block copolymer (300PEOT55PBT45) integrated with porous HF membranes of modified poly(ether-sulfone) (mPES, Gambro GmbH) is used as a model system. Various parameters such as fiber transport properties, fiber spacing within a scaffold and medium flow conditions are optimized. The results show that four HF membranes integrated with the scaffold significantly improve the cell density and cell distribution. This study provides a basis for the development of a new HF perfusion culture methodology to overcome the limitations of nutrient diffusion in the culture of large 3D tissue constructs.

Oxygen tension affects terminal differentiation of corneal limbal epithelial cells

Oxygen concentration has been shown to be crucial in the proliferation and differentiation of various types of cells, while the impact of oxygen tension on the lineage commitment of epithelial cells remains elusive. In this study, we investigated the effect of hypoxia on the differentiation of corneal limbal epithelium using an ex vivo squamous metaplasia model. Under normoxic conditions when exposed to air, the hyperproliferation and abnormal epidermal-like differentiation of human corneal limbal epithelium was induced, whereas when exposed to air under hypoxic conditions, although we observed augmented proliferation, the abnormal differentiation was inhibited. The Notch signaling pathway was activated in hypoxic cultures, whereas the p38 MAPK signaling pathway was downregulated. The addition of Notch inhibitor under hypoxic conditions restored the activation of p38 MAPK and resulted in the recidivation of limbal epithelial cells to epidermal-like differentiation. Moreover, the epidermal-like differentiation of rabbit limbal epithelial cells was also blocked under hypoxic conditions in corneal epithelial cell sheets engineered ex vivo. We concluded that hypoxia can prevent abnormal differentiation while enhancing the proliferation of corneal limbal epithelial cells. Hypoxia coupled with air exposure can be used in the tissue engineering of corneal limbal epithelium.

Mathematical model and numerical simulation of the cell growth in scaffolds

A scaffold is a three-dimensional matrix that provides a structural base to fill tissue lesion and provides cells with a suitable environment for proliferation and differentiation. Cell-seeded scaffolds can be implanted immediately or be cultured in vitro for a period of time before implantation. To obtain uniform cell growth throughout the entire volume of the scaffolds, an optimal strategy on cell seeding into scaffolds is important. We propose an efficient and accurate numerical scheme for a mathematical model to predict the growth and distribution of cells in scaffolds. The proposed numerical algorithm is a hybrid method which uses both finite difference approximations and analytic closed-form solutions. The effects of each parameter in the mathematical model are numerically investigated. Moreover, we propose an optimization algorithm which finds the best set of model parameters that minimize a discrete l(2) error between numerical and experimental data. Using the mathematical model and its efficient and accurate numerical simulations, we could interpret experimental results and identify dominating mechanisms.

Supply of nutrients to cells in engineered tissues

A proper supply of nutrients to cells in engineered tissues is paramount for an optimal development and survival of these tissues. However, especially in tissues with clinically relevant sizes, the mass transport of nutrients into the tissue is often insufficient to sustain all the cells within the tissue. This is not only the case during in vitro culture. After implantation of an engineered tissue, a vascular network is not directly established. Therefore, the mass transport of nutrients is also critical during the initial period after implantation. This review introduces the basics of mass transport, leading to the conclusion that three main concepts can be used to increase nutrient supply in tissue engineering. These are; increasing the overall diffusion coefficient, decreasing the diffusion distance, or increasing convective transport. Based on these concepts, the main strategies that have been developed to enhance the supply of nutrients to cells in engineered tissues will be discussed.

In vitro platforms for tissue engineering: implications for basic research and clinical translation

Clinical translation of engineered tissues into regenerative medicine applications, and the effort to reduce the use of animals for the screening of drugs and other compounds, result in an increasing demand for human tissues engineered in vitro for implantation, in vitro screening systems and basic research. Further development and optimization of in vitro models for quantitative studies of biophysical stimulation and mass transport in engineered tissues is seen as one of the high research priorities. A recent international symposium held in The Netherlands discussed the state of the art and key challenges in the development of bioreactor platforms and microscale technologies. Here we summarize the current status and future development of in vitro tissue models, based on the findings presented at this symposium.

Effect of pore architecture on oxygen diffusion in 3D scaffolds for tissue engineering

The aim of this study was to maximize oxygen diffusion within a three-dimensional scaffold in order to improve cell viability and proliferation. To evaluate the effect of pore architecture on oxygen diffusion, we designed a regular channel shape with uniform diameter, referred to as cylinder shaped, and a new channel shape with a channel diameter gradient, referred to as cone shaped. A numerical analysis predicted higher oxygen concentration in the cone-shaped channels than in the cylinder-shaped channels, throughout the scaffold. To confirm these numerical results, we examined cell proliferation and viability in 2D constructs and 3D scaffolds. Cell culture experiments revealed that cell proliferation and viability were superior in the constructs and scaffolds with cone-shaped channels.

Topology Optimization of Three Dimensional Tissue Engineering Scaffold Architectures for Prescribed Bulk Modulus and Diffusivity

Tissue engineering scaffolds play critical roles in skeletal tissue regeneration by supporting physiological loads as well as enhancing cell/tissue migration and formation. These roles can be fulfilled by the functional design of scaffold pore architectures such that the scaffold provides proper mechanical and mass transport environments for new tissue formation. These roles require simultaneous design of mechanical and mass transport properties. In this paper, a numerical homogenization based topology optimization scheme was applied to the design of three dimensional unit microstructures for tissue engineering scaffolds. As measures of mechanical and mass transport environments, target effective bulk modulus and isotropic diffusivity were achieved by optimal design of porous microstructure. Cross property bounds between bulk modulus and diffusivity were adapted to determine feasible design targets for a given porosity. Results demonstrate that designed microstructures could reach cross property bounds for porosity ranging from 30% to 60%.

Bioreactors for bone tissue engineering

Engineering bone tissue for use in orthopaedics poses multiple challenges. Providing the appropriate growth environment that will allow complex tissues such as bone to grow is one of these challenges. There are multiple design factors that must be considered in order to generate a functional tissue in vitro for replacement surgery in the clinic. Complex bioreactors have been designed that allow different stress regimes such as compressive, shear, and rotational forces to be applied to three-dimensional (3D) engineered constructs. This review addresses these considerations and outlines the types of bioreactor that have been developed and are currently in use.

Engineering craniofacial structures: facing the challenge

The human innate regenerative ability is known to be limited by the intensity of the insult together with the availability of progenitor cells, which may cause certain irreparable damage. It is only recently that the paradigm of tissue engineering found its way to the treatment of irreversibly affected body structures with the challenge of reconstructing the lost part. In the current review, we underline recent trials that target engineering of human craniofacial structures, mainly bone, cartilage, and teeth. We analyze the applied engineering strategies relative to the selection of cell types to lay down a specific targeted tissue, together with their association with an escorting scaffold for a particular engineered site, and discuss their necessity to be sustained by growth factors. Challenges and expectations for facial skeletal engineering are discussed in the context of future treatment.

Mathematical modelling of tissue-engineered angiogenesis

We present a mathematical model for the vascularisation of a porous scaffold following implantation in vivo. The model is given as a set of coupled non-linear ordinary differential equations (ODEs) which describe the evolution in time of the amounts of the different tissue constituents inside the scaffold. Bifurcation analyses reveal how the extent of scaffold vascularisation changes as a function of the parameter values. For example, it is shown how the loss of seeded cells arising from slow infiltration of vascular tissue can be overcome using a prevascularisation strategy consisting of seeding the scaffold with vascular cells. Using certain assumptions it is shown how the system can be simplified to one which is partially tractable and for which some analysis is given. Limited comparison is also given of the model solutions with experimental data from the chick chorioallantoic membrane (CAM) assay.

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.

Hypoxia and stem cell-based engineering of mesenchymal tissues

Stem cells have the ability for prolonged self-renewal and differentiation into mature cells of various lineages, which makes them important cell sources for tissue engineering applications. Their remarkable ability to replenish and differentiate in vivo is regulated by both intrinsic and extrinsic cellular mechanisms. The anatomical location where the stem cells reside, known as the "stem cell niche or microenvironment," provides signals conducive to the maintenance of definitive stem cell properties. Physiological condition including oxygen tension is an important component of the stem cell microenvironment and has been shown to play a role in regulating both embryonic and adult stem cells. This review focuses on oxygen as a signaling molecule and the way it regulates the stem cells' development into mesenchymal tissues in vitro. The physiological relevance of low oxygen tension as an environmental parameter that uniquely benefits stem cells' expansion and maintenance is described along with recent findings on the regulatory effects of oxygen on embryonic stem cells and adult mesenchymal stem cells. The relevance to tissue engineering is discussed in the context of the need to specifically regulate the oxygen content in the cellular microenvironment in order to optimize in vitro tissue development.