Concepts of scaffold-based tissue engineering--the rationale to use solid free-form fabrication techniques

Maxillary sinus floor elevation using a tissue-engineered bone with calcium-magnesium phosphate cement and bone marrow stromal cells in rabbits

The objective of this study was to assess the effects of maxillary sinus floor elevation with a tissue-engineered bone constructed with bone marrow stromal cells (bMSCs) and calcium-magnesium phosphate cement (CMPC) material. The calcium (Ca), magnesium (Mg), and phosphorus (P) ions released from calcium phosphate cement (CPC), magnesium phosphate cement (MPC), and CMPC were detected by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the proliferation and osteogenic differentiation of bMSCs seeded on CPC, MPC, and CMPC or cultured in CPC, MPC, and CMPC extracts were measured by MTT analysis, alkaline phosphatase (ALP) activity assay, alizarin red mineralization assay, and real-time PCR analysis of the osteogenic genes ALP and osteocalcin (OCN). Finally, bMSCs were combined with CPC, MPC, and CMPC and used for maxillary sinus floor elevation in rabbits, while CPC, MPC, or CMPC without cells served as control groups. The new bone formation in each group was detected by histological finding and fluorochrome labeling at weeks 2 and 8 after surgical operation. It was observed that the Ca ion concentrations of the CMPC and CPC scaffolds was significantly higher than that of the MPC scaffold, while the Mg ions concentration of CMPC and MPC was significantly higher than that of CPC. The bMSCs seeded on CMPC and MPC or cultured in their extracts proliferated more quickly than the cells seeded on CPC or cultured in its extract, respectively. The osteogenic differentiation of bMSCs seeded on CMPC and CPC or cultured in the corresponding extracts was significantly enhanced compared to that of bMSCs seeded on MPC or cultured in its extract; however, there was no significant difference between CMPC and CPC. As for maxillary sinus floor elevation in vivo, CMPC could promote more new bone formation and mineralization compared to CPC and MPC, while the addition of bMSCs could further enhance its new bone formation ability significantly. Our data suggest that CMPC possesses moderate biodegradability and excellent osteoconductivity, which may be attributed to its Ca and Mg ion composition, and the tissue-engineered bone constructed of CMPC and bMSCs might be a potential alterative graft for maxillofacial bone regeneration.

Review paper: Critical Issues in Tissue Engineering: Biomaterials, Cell Sources, Angiogenesis, and Drug Delivery Systems

Tissue engineering is a newly emerging biomedical technology, which aids and increases the repair and regeneration of deficient and injured tissues. It employs the principles from the fields of materials science, cell biology, transplantation, and engineering in an effort to treat or replace damaged tissues. Tissue engineering and development of complex tissues or organs, such as heart, muscle, kidney, liver, and lung, are still a distant milestone in twenty-first century. Generally, there are four main challenges in tissue engineering which need optimization. These include biomaterials, cell sources, vascularization of engineered tissues, and design of drug delivery systems. Biomaterials and cell sources should be specific for the engineering of each tissue or organ. On the other hand, angiogenesis is required not only for the treatment of a variety of ischemic conditions, but it is also a critical component of virtually all tissue-engineering strategies. Therefore, controlling the dose, location, and duration of releasing angiogenic factors via polymeric delivery systems, in order to ultimately better mimic the stem cell niche through scaffolds, will dictate the utility of a variety of biomaterials in tissue regeneration. This review focuses on the use of polymeric vehicles that are made of synthetic and/or natural biomaterials as scaffolds for three-dimensional cell cultures and for locally delivering the inductive growth factors in various formats to provide a method of controlled, localized delivery for the desired time frame and for vascularized tissue-engineering therapies.

Effect of trehalose coating on basic fibroblast growth factor release from tailor-made bone implants

Artificial bone implants are often incorporated with osteoinductive factors to facilitate early bone regeneration. Calcium phosphate, the main component in artificial bone implants, strongly binds these factors, and in a few cases, the incorporated proteins are not released from the implant under conditions of physiological pH, thereby leading to reduction in their osteoinductivity. In this study, we coated tailor-made bone implants with trehalose to facilitate the release of basic fibroblast growth factor (bFGF). In an in vitro study, mouse osteoblastic cells were separately cultured for 48 hr in a medium with a untreated implant (T-), trehalose-coated implant (T+), bFGF-incorporated implant (FT-), and bFGF-incorporated implant with trehalose coating (FT+). In the FT+ group, cell viability was significantly higher than that in the other groups (P<0.05). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed that trehalose effectively covered the surface of the artificial bone implant without affecting the crystallinity or the mechanical strength of the artificial bone implant. These results suggest that coating artificial bone implants with trehalose could limit the binding of bFGF to calcium phosphate.

Vascularization is the key challenge in tissue engineering

The main limitation in engineering in vitro tissues is the lack of a sufficient blood vessel system - the vascularization. In vivo almost all tissues are supplied by these endothelial cell coated tubular networks. Current strategies to create vascularized tissues are discussed in this review. The first strategy is based on the endothelial cells and their ability to form new vessels known as neoangiogenesis. Herein prevascularization techniques are compared to approaches in which biomolecules, such as growth factors, cytokines, peptides and proteins as well as cells are applied to generate new vessels. The second strategy is focused on scaffold-based techniques. Naturally-derived scaffolds, which contain vessels, are distinguished from synthetically manufactured matrices. Advantages and pitfalls of the approaches to create vascularized tissues in vitro are outlined and feasible future strategies are discussed.

Maxillary sinus floor elevation using a tissue-engineered bone with rhBMP-2-loaded porous calcium phosphate cement scaffold and bone marrow stromal cells in rabbits

The aim of this study was to evaluate the effects of maxillary sinus floor elevation by a tissue-engineered bone complex with recombinant human bone morphogenetic protein-2 (rhBMP-2)-loaded porous calcium phosphate cement (CPC) scaffold and bone marrow stromal cells (bMSCs) in rabbits. bMSCs were cultured and osteogenically induced. The osteoblastic differentiation of expanded bMSCs was detected by alkaline phosphatase activity, and calcium deposits in vitro. Thirty-six rabbits were randomly allocated into week 2, 4 and 8 observation groups. At each time point, 24 maxillary sinus floor elevation surgeries in 12 rabbits were performed bilaterally and randomly implanted by (1) CPC materials alone (group A, n = 6), (2) rhBMP-2/CPC composite materials alone (group B, n = 6), (3) CPC/bMSCs complex (group C, n = 6) and (4) rhBMP-2/CPC/bMSCs complex (group D, n = 6). As for maxillary sinus floor elevation, rhBMP-2-loaded CPC could promote new bone formation as compared to CPC, while addition of bMSCs could further enhance its new bone formation and maturity significantly, as detected by histological findings, and fluorochrome labeling. Our data suggested that rhBMP-2/CPC possessed excellent osteoinductive ability, while combining with bMSCs could further promote new bone formation and maturation in maxillary sinus elevation.

Three-dimensional plotted scaffolds with controlled pore size gradients: Effect of scaffold geometry on mechanical performance and cell seeding efficiency

Scaffolds produced by rapid prototyping (RP) techniques have proved their value for tissue engineering applications, due to their ability to produce predetermined forms and structures featuring fully interconnected pore architectures. Nevertheless, low cell seeding efficiency and non-uniform distribution of cells remain major limitations when using such types of scaffold. This can be mainly attributed to the inadequate pore architecture of scaffolds produced by RP and the limited efficiency of cell seeding techniques normally adopted. In this study we aimed at producing scaffolds with pore size gradients to enhance cell seeding efficiency and control the spatial organization of cells within the scaffold. Scaffolds based on blends of starch with poly(ε-caprolactone) featuring both homogeneously spaced pores (based on pore sizes of 0.75 and 0.1 mm) and pore size gradients (based on pore sizes of 0.1-0.75-0.1 and 0.75-0.1-0.75 mm) were designed and produced by three-dimensional plotting. The mechanical performance of the scaffolds was characterized using dynamic mechanical analysis (DMA) and conventional compression testing under wet conditions and subsequently characterized using scanning electron microscopy and micro-computed tomography. Osteoblast-like cells were seeded onto such scaffolds to investigate cell seeding efficiency and the ability to control the zonal distribution of cells upon seeding. Scaffolds featuring continuous pore size gradients were originally produced. These scaffolds were shown to have intermediate mechanical and morphological properties compared with homogenous pore size scaffolds. The pore size gradient scaffolds improved seeding efficiency from ∼35% in homogeneous scaffolds to ∼70% under static culture conditions. Fluorescence images of cross-sections of the scaffolds revealed that scaffolds with pore size gradients induce a more homogeneous distribution of cells within the scaffold.

Biomimetic composite coating on rapid prototyped scaffolds for bone tissue engineering

The objective of this present study was to improve the functional performance of rapid prototyped scaffolds for bone tissue engineering through biomimetic composite coating. Rapid prototyped poly(ε-caprolactone)/tri-calcium phosphate (PCL/TCP) scaffolds were fabricated using the screw extrusion system (SES). The fabricated PCL/TCP scaffolds were coated with a carbonated hydroxyapatite (CHA)-gelatin composite via biomimetic co-precipitation. The structure of the prepared CHA-gelatin composite coating was studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Compressive mechanical testing revealed that the coating process did not have any detrimental effect on the mechanical properties of the scaffolds. The cell-scaffold interaction was studied by culturing porcine bone marrow stromal cells (BMSCs) on the scaffolds and assessing the proliferation and bone-related gene and protein expression capabilities of the cells. Confocal laser microscopy and SEM images of the cell-scaffold constructs showed a uniformly distributed cell sheet and accumulation of extracellular matrix in the interior of CHA-gelatin composite-coated PCL/TCP scaffolds. The proliferation rate of BMSCs on CHA-gelatin composite-coated PCL/TCP scaffolds was about 2.3 and 1.7 times higher than that on PCL/TCP scaffolds and CHA-coated PCL/TCP scaffolds, respectively, by day 10. Furthermore, reverse transcription polymerase chain reaction and Western blot analysis revealed that CHA-gelatin composite-coated PCL/TCP scaffolds stimulate osteogenic differentiation of BMSCs the most, compared with PCL/TCP scaffolds and CHA-coated PCL/TCP scaffolds. These results demonstrate that CHA-gelatin composite-coated rapid prototyped PCL/TCP scaffolds are promising for bone tissue engineering.

Custom-made composite scaffolds for segmental defect repair in long bones

Current approaches for segmental bone defect reconstruction are restricted to autografts and allografts which possess osteoconductive, osteoinductive and osteogenic properties, but face significant disadvantages. The objective of this study was to compare the regenerative potential of scaffolds with different material composition but similar mechanical properties to autologous bone graft from the iliac crest in an ovine segmental defect model. After 12 weeks, in vivo specimens were analysed by X-ray imaging, torsion testing, micro-computed tomography and histology to assess amount, strength and structure of the newly formed bone. The highest amounts of bone neoformation with highest torsional moment values were observed in the autograft group and the lowest in the medical grade polycaprolactone and tricalcium phosphate composite group. The study results suggest that scaffolds based on aliphatic polyesters and ceramics, which are considered biologically inactive materials, induce only limited new bone formation but could be an equivalent alternative to autologous bone when combined with a biologically active stimulus such as bone morphogenetic proteins.

Bioengineering strategies for regeneration of craniofacial bone: a review of emerging technologies

Although advances in surgical techniques and bone grafting have significantly improved the functional and cosmetic restoration of craniofacial structures lost because of trauma or disease, there are still significant limitations in our ability to regenerate these tissues. The regeneration of oral and craniofacial tissues presents a formidable challenge that requires synthesis of basic science, clinical science, and engineering technology. Tissue engineering is an interdisciplinary field of study that addresses this challenge by applying the principles of engineering to biology and medicine toward the development of biological substitutes that restore, maintain, and improve normal function. This review will explore the impact of biomaterials design, stem cell biology and gene therapy on craniofacial tissue engineering.

An optical method for evaluation of geometric fidelity for anatomically shaped tissue-engineered constructs

Quantification of shape fidelity of complex geometries for tissue-engineered constructs has not been thoroughly investigated. The objective of this study was to quantitatively describe geometric fidelities of various approaches to the fabrication of anatomically shaped meniscal constructs. Ovine menisci (n = 4) were imaged using magnetic resonance imaging (MRI) and microcomputed tomography (microCT). Acrylonitrile butadiene styrene plastic molds were designed from each imaging modality and three-dimensional printed on a Stratasys FDM 3000. Silastic impression molds were fabricated directly from ovine menisci. These molds were used to generate shaped constructs using 2% alginate with 2% CaSO(4). Solid freeform fabrication was conducted on a custom open-architecture three-dimensional printing platform. Printed samples were made using 2% alginate with 0.75% CaSO(4). Hydrogel constructs were scanned via laser triangulation distance sensor. The point cloud images were analyzed to acquire computational measurements for key points of interest (e.g., height, width, and volume). Silastic molds were within + or - 10% error with respect to the native tissue for seven key measurements, microCT molds for six of seven, microCT prints for four of seven, MRI molds for five of seven, and MRI prints for four of seven. This work shows the ability to generate and quantify anatomically shaped meniscal constructs of high geometric fidelity and lends insight into the relative geometric fidelities of several tissue engineering techniques.

Epidermal growth factor (EGF) transfection of human bone marrow stromal cells in bone tissue engineering

A novel therapeutic approach for the treatment of bone defects is gene therapy assisted bone tissue engineering using bone marrow stromal cells (hBMSC). The aim of this study was to investigate the influence of human epidermal growth factor (hEGF) on proliferation and alkaline phosphatase (AP) activity of primary hBMSC in vitro. hBMSC cultures were achieved by explantation culture of bone chips. Following exposure to 0-10 ng recombinant hEGF (rhEGF)/ml cell numbers were determined by automated cell counting and cell bound AP activity was measured spectrophotometrically. hBMSC were transfected with hEGF plasmids and the proliferative effect was studied by cocultivation of transfected and untreated cells using porous cell culture inserts. The persistence of hEGF expression even after cell transfer was studied by the generation of possibly osteogenic constructs introducing transfected hBMSC in fibrin glue and bovine cancellous bone. The maximum increase in proliferation (156 +/- 7%) and AP activity (220 +/- 34%) was detected after exposition to 10 ng rhEGF/ml. In the separation chamber assay transfected cells produced hEGF concentrations up to 3.6 ng/ml, which induced a mean proliferation increase of 93% which could be significantly inhibited by a neutralizing hEGF antibody. Further, EGFsecretion of transfected hBMSC in 3D-culture was verified. Recombinant and transgenic hEGF stimulate proliferation of primary hBMSC in vitro. Lipotransfection of hBMSC with hEGF plasmids allows the transient and site directed delivery of biologically active transgenic hEGF. The introduction of mitogenic, angiogenic and chemoattractive factors in gene therapy assisted bone tissue engineering is discussed by the example of EGF.

Periodontal tissue engineering and regeneration: current approaches and expanding opportunities

The management of periodontal tissue defects that result from periodontitis represents a medical and socioeconomic challenge. Concerted efforts have been and still are being made to accelerate and augment periodontal tissue and bone regeneration, including a range of regenerative surgical procedures, the development of a variety of grafting materials, and the use of recombinant growth factors. More recently, tissue-engineering strategies, including new cell- and/or matrix-based dimensions, are also being developed, analyzed, and employed for periodontal regenerative therapies. Tissue engineering in periodontology applies the principles of engineering and life sciences toward the development of biological techniques that can restore lost alveolar bone, periodontal ligament, and root cementum. It is based on an understanding of the role of periodontal formation and aims to grow new functional tissues rather than to build new replacements of periodontium. Although tissue engineering has merged to create more opportunities for predictable and optimal periodontal tissue regeneration, the technique and design for preclinical and clinical studies remain in their early stages. To date, the reconstruction of small- to moderate-sized periodontal bone defects using engineered cell-scaffold constructs is technically feasible, and some of the currently developed concepts may represent alternatives for certain ideal clinical scenarios. However, the predictable reconstruction of the normal structure and functionality of a tooth-supporting apparatus remains challenging. This review summarizes current regenerative procedures for periodontal healing and regeneration and explores their progress and difficulties in clinical practice, with particular emphasis placed upon current challenges and future possibilities associated with tissue-engineering strategies in periodontal regenerative medicine.

Bone repair by cell-seeded 3D-bioplotted composite scaffolds made of collagen treated tricalciumphosphate or tricalciumphosphate-chitosan-collagen hydrogel or PLGA in ovine critical-sized calvarial defects

The aim of this study was to investigate the osteogenic effect of three different cell-seeded 3D-bioplotted scaffolds in a ovine calvarial critical-size defect model. The choice of scaffold-materials was based on their applicability for 3D-bioplotting and respective possibility to produce tailor-made scaffolds for the use in cranio-facial surgery for the replacement of complex shaped boneparts. Scaffold raw-materials are known to be osteoinductive when being cell-seeded [poly(L-lactide-co-glycolide) (PLGA)] or having components with osteoinductive properties as tricalciumphosphate (TCP) or collagen (Col) or chitosan. The scaffold-materials PLGA, TCP/Col, and HYDR (TCP/Col/chitosan) were cell-seeded with osteoblast-like cells whether gained from bone (OLB) or from periost (OLP). In a prospective and randomized design nine sheep underwent osteotomy to create four critical-sized calvarial defects. Three animals each were assigned to the HYDR-, the TCP/Col-, or the PLGA-group. In each animal, one defect was treated with a cell-free, an OLB- or OLP-seeded group-specific scaffold, respectively. The fourth defect remained untreated as control (UD). Fourteen weeks later, animals were euthanized for histo-morphometrical analysis of the defect healing. OLB- and OLP-seeded HYDR and OLB-seeded TCP/Col scaffolds significantly increased the amount of newly formed bone (NFB) at the defect bottom and OLP-seeded HYDR also within the scaffold area, whereas PLGA-scaffolds showed lower rates. The relative density of NFB was markedly higher in the HYDR/OLB group compared to the corresponding PLGA group. TCP/Col had good stiffness to prepare complex structures by bioplotting but HYDR and PLGA were very soft. HYDR showed appropriate biodegradation, TCP/Col and PLGA seemed to be nearly undegraded after 14 weeks. 3D-bioplotted, cell-seeded HYDR and TCP/Col scaffolds increased the amount of NFB within ovine critical-size calvarial defects, but stiffness, respectively, biodegradation of materials is not appropriate for the application in cranio-facial surgery and have to be improved further by modifications of the manufacturing process or their material composition.

A delayed type of three-dimensional growth of human endothelial cells under simulated weightlessness

Endothelial cells (ECs) form three-dimensional (3D) aggregates without any scaffold when they are exposed to microgravity simulated by a random positioning machine (RPM) but not under static conditions at gravity. Here we describe a delayed type of formation of 3D structures of ECs that was initiated when ECs cultured on a desktop RPM remained adherent for the first 5 days but spread over neighboring adherent cells, forming little colonies. After 2 weeks, tube-like structures (TSs) became visible in these cultures. They included a lumen, and they elongated during another 2 weeks of culturing. The walls of these TSs consisted mainly of single-layered ECs, which had produced significantly more beta(1)-integrin, laminin, fibronectin, and alpha-tubulin than ECs simultaneously grown adhering to the culture dishes under microgravity or normal gravity. The amount of actin protein was similar in ECs incorporated in TSs and in ECs growing at gravity. The ratio of tissue inhibitor of metalloproteinases-1 to matrix metalloproteinase-2 found in the supernatants was lower at the seventh than at the 28th day of culturing. These results suggest that culturing ECs under conditions of modeled gravitational unloading represents a new technique for studying the formation of tubes that resemble vascular intimas.

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.

Is tissue engineering a new paradigm in medicine? Consequences for the ethical evaluation of tissue engineering research

Ex-vivo tissue engineering is a quickly developing medical technology aiming to regenerate tissue through the introduction of an ex-vivo created tissue construct instead of restoring the damaged tissue to some level of functionality. Tissue engineering is considered by some as a new medical paradigm. We analyse this claim and identify tissue engineering's fundamental characteristics, focusing on the aim of the intervention and on the complexity and continuity of the process. We inquire how these features have an impact not only on the scientific research itself but also on the ethical evaluation of this research. We suggest that viewing tissue engineering as a new medical paradigm allows us to develop a wider perspective for successful investigation instead of focusing on isolated steps of the tissue engineering process in an anecdotal way, which may lead to an inadequate ethical evaluation. We argue that the concept of tissue engineering as a paradigm may benefit the way we address the ethical challenges presented by tissue engineering.

Bone tissue engineering therapeutics: controlled drug delivery in three-dimensional scaffolds

This paper provides an extensive overview of published studies on the development and applications of three-dimensional bone tissue engineering (TE) scaffolds with potential capability for the controlled delivery of therapeutic drugs. Typical drugs considered include gentamicin and other antibiotics generally used to combat osteomyelitis, as well as anti-inflammatory drugs and bisphosphonates, but delivery of growth factors is not covered in this review. In each case reviewed, special attention has been given to the technology used for controlling the release of the loaded drugs. The possibility of designing multifunctional three-dimensional bone TE scaffolds for the emerging field of bone TE therapeutics is discussed. A detailed summary of drugs included in three-dimensional scaffolds and the several approaches developed to combine bioceramics with various polymeric biomaterials in composites for drug-delivery systems is included. The main results presented in the literature are discussed and the remaining challenges in the field are summarized with suggestions for future research directions.

Maxillofacial reconstruction using custom-made artificial bones fabricated by inkjet printing technology

Ideally, artificial bones should be dimensionally compatible with deformities, and be biodegradable and osteoconductive; however, there are no artificial bones developed to date that satisfy these requirements. We fabricated novel custom-made artificial bones from alpha-tricalcium phosphate powder using an inkjet printer and implanted them in ten patients with maxillofacial deformities. The artificial bones had dimensional compatibility in all the patients. The operation time was reduced due to minimal need for size adjustment and fixing manipulation. The postsurgical computed tomography analysis detected partial union between the artificial bones and host bone tissues. There were no serious adverse reactions. These findings provide support for further clinical studies of the inkjet-printed custom-made artificial bones.

Bilayered scaffolds for osteochondral tissue engineering

Osteoarthritis (OA) is a prevalent degenerative joint disease that places a significant burden on the socioeconomic efficacy of communities around the world. Tissue engineering repair of articular cartilage in synovial joints represents a potential OA treatment strategy superior to current surgical techniques. In particular, osteochondral tissue engineering, which promotes the simultaneous regeneration of articular cartilage and underlining subchondral bone, may be a clinically relevant approach toward impeding OA progression. The unique and complex functional demands of the two contrasting tissues that comprise osteochondral tissue require the use of bilayered scaffolds to promote individual growth of both on a single integrated implant. This paper reviews the three current bilayered scaffold strategies applied to solve this challenging problem, with a focus on the need for an innovative approach to design and fabrication of new optimized scaffold combinations to reinforce materials science as an important element of osteochondral tissue engineering.

Translating tissue engineering technology platforms into cancer research

Technology platforms originally developed for tissue engineering applications produce valuable models that mimic three-dimensional (3D) tissue organization and function to enhance the understanding of cell/tissue function under normal and pathological situations. These models show that when replicating physiological and pathological conditions as closely as possible investigators are allowed to probe the basic mechanisms of morphogenesis, differentiation and cancer. Significant efforts investigating angiogenetic processes and factors in tumorigenesis are currently undertaken to establish ways of targeting angiogenesis in tumours. Anti-angiogenic agents have been accepted for clinical application as attractive targeted therapeutics for the treatment of cancer. Combining the areas of tumour angiogenesis, combination therapies and drug delivery systems is therefore closely related to the understanding of the basic principles that are applied in tissue engineering models. Studies with 3D model systems have repeatedly identified complex interacting roles of matrix stiffness and composition, integrins, growth factor receptors and signalling in development and cancer. These insights suggest that plasticity, regulation and suppression of these processes can provide strategies and therapeutic targets for future cancer therapies. The historical perspective of the fields of tissue engineering and controlled release of therapeutics, including inhibitors of angiogenesis in tumours is becoming clearly evident as a major future advance in merging these fields. New delivery systems are expected to greatly enhance the ability to deliver drugs locally and in therapeutic concentrations to relevant sites in living organisms. Investigating the phenomena of angiogenesis and anti-angiogenesis in 3D in vivo models such as the Arterio-Venous (AV) loop mode in a separated and isolated chamber within a living organism adds another significant horizon to this perspective and opens new modalities for translational research in this field.

Localized delivery of growth factors for periodontal tissue regeneration: role, strategies, and perspectives

Difficulties associated with achieving predictable periodontal regeneration, means that novel techniques need to be developed in order to regenerate the extensive soft and hard tissue destruction that results from periodontitis. Localized delivery of growth factors to the periodontium is an emerging and versatile therapeutic approach, with the potential to become a powerful tool in future regenerative periodontal therapy. Optimized delivery regimes and well-defined release kinetics appear to be logical prerequisites for safe and efficacious clinical application of growth factors and to avoid unwanted side effects and toxicity. While adequate concentrations of growth factor(s) need to be appropriately localized, delivery vehicles are also expected to possess properties such as protein protection, precision in controlled release, biocompatibility and biodegradability, self-regulated therapeutic activity, potential for multiple delivery, and good cell/tissue penetration. Here, current knowledge, recent advances, and future possibilities of growth factor delivery strategies are outlined for periodontal regeneration. First, the role of those growth factors that have been implicated in the periodontal healing/regeneration process, general requirements for their delivery, and the different material types available are described. A detailed discussion follows of current strategies for the selection of devices for localized growth factor delivery, with particular emphasis placed upon their advantages and disadvantages and future prospects for ongoing studies in reconstructing the tooth supporting apparatus.

Image-guided tissue engineering

Replication of anatomic shape is a significant challenge in developing implants for regenerative medicine. This has lead to significant interest in using medical imaging techniques such as magnetic resonance imaging and computed tomography to design tissue engineered constructs. Implementation of medical imaging and computer aided design in combination with technologies for rapid prototyping of living implants enables the generation of highly reproducible constructs with spatial resolution up to 25 microm. In this paper, we review the medical imaging modalities available and a paradigm for choosing a particular imaging technique. We also present fabrication techniques and methodologies for producing cellular engineered constructs. Finally, we comment on future challenges involved with image guided tissue engineering and efforts to generate engineered constructs ready for implantation.

Computer-aided design and finite-element modelling of biomaterial scaffolds for bone tissue engineering

Scaffold biomaterials for tissue engineering can be produced in many different ways depending on the applications and the materials used. Most research into new biomaterials is based on an experimental trial-and-error approach that limits the possibility of making many variations to a single material and studying its interaction with its surroundings. Instead, computer simulation applied to tissue engineering can offer a more exhaustive approach to test and screen out biomaterials. In this paper, a review of the current approach in biomaterials designed through computer-aided design (CAD) and through finite-element modelling is given. First we review the approach used in tissue engineering in the development of scaffolds and the interactions existing between biomaterials, cells and mechanical stimuli. Then, scaffold fabrication through CAD is presented and characterization of existing scaffolds through computed images is reviewed. Several case studies of finite-element studies in tissue engineering show the usefulness of computer simulations in determining the mechanical environment of cells when seeded into a scaffold and the proper design of the geometry and stiffness of the scaffold. This creates a need for more advanced studies that include aspects of mechanobiology in tissue engineering in order to be able to predict over time the growth and differentiation of tissues within scaffolds. Finally, current perspectives indicate that more efforts need to be put into the development of such advanced studies, with the removal of technical limitations such as computer power and the inclusion of more accurate biological and genetic processes into the developed algorithms.

Current and future regenerative medicine - principles, concepts, and therapeutic use of stem cell therapy and tissue engineering in equine medicine

This paper provides a bird's-eye perspective of the general principles of stem-cell therapy and tissue engineering; it relates comparative knowledge in this area to the current and future status of equine regenerative medicine.The understanding of equine stem cell biology, biofactors, and scaffolds, and their potential therapeutic use in horses are rudimentary at present. Mesenchymal stem cell isolation has been proclaimed from several equine tissues in the past few years. Based on the criteria of the International Society for Cellular Therapy, most of these cells are more correctly referred to as multipotent mesenchymal stromal cells, unless there is proof that they exhibit the fundamental in vivo characteristics of pluripotency and the ability to self-renew. That said, these cells from various tissues hold great promise for therapeutic use in horses. The 3 components of tissue engineering - cells, biological factors, and biomaterials - are increasingly being applied in equine medicine, fuelled by better scaffolds and increased understanding of individual biofactors and cell sources.The effectiveness of stem cell-based therapies and most tissue engineering concepts has not been demonstrated sufficiently in controlled clinical trials in equine patients to be regarded as evidence-based medicine. In the meantime, the medical mantra "do no harm" should prevail, and the application of stem cell-based therapies in the horse should be done critically and cautiously, and treatment outcomes (good and bad) should be recorded and reported.Stem cell and tissue engineering research in the horse has exciting comparative and equine specific perspectives that most likely will benefit the health of horses and humans. Controlled, well-designed studies are needed to move this new equine research field forward.

Anabolic agents for improving muscle regeneration and function after injury

1. In the present review, we describe how muscles can be injured by external factors, internal factors or during the performance of some actions during sports. In addition, we describe the injury to a muscle that occurs when its blood supply is interrupted, an occurrence common in clinical settings. An overview of muscle regeneration is presented, as well as a discussion of some of the potential complications that can compromise successful muscle repair and lead to impaired function and permanent disability. 2. Improving muscle regeneration is important for hastening muscle repair and restoring muscle function and the present review describes ways in which this can be achieved. We describe recent advances in tissue engineering that offer considerable promise for treating muscle damage, but highlight the fact that these techniques require rigorous evaluation before they can become mainstream clinical treatments. 3. Growth-promoting agents are purported to increase the size of existing and newly regenerating muscle fibres and, therefore, could be used to improve muscle function if administered at appropriate times during the repair process. The present review provides an update on the efficacy of some growth-promoting agents, including anabolic steroids, insulin-like growth factor-I (IGF-I) and beta(2)-adrenoceptor agonists, to improve muscle function after injury. Although these approaches have clinical merit, a better understanding of the androgenic, IGF-I and b-adrenoceptor signalling pathways in skeletal muscle is important if we are to devise safe and effective therapies to enhance muscle regeneration and function after injury.

Combining electrospun scaffolds with electrosprayed hydrogels leads to three-dimensional cellularization of hybrid constructs

A common problem in the design of tissue engineered scaffolds using electrospun scaffolds is the poor cellular infiltration into the structure. To tackle this issue, three approaches to scaffold design using electrospinning were investigated: selective leaching of a water-soluble fiber phase (poly ethylene oxide (PEO) or gelatin), the use of micron-sized fibers as the scaffold, and a combination of micron-sized fibers with codeposition of a hyaluronic acid-derivative hydrogel, Heprasil. These designs were achieved by modifying a conventional electrospinning system with two charged capillaries and a rotating mandrel collector. Three types of scaffolds were fabricated: medical grade poly(epsilon-caprolactone)/collagen (mPCL/Col) cospun with PEO or gelatin, mPCL/Col meshes with micron-sized fibers, and mPCL/Col microfibers cosprayed with Heprasil. All three scaffold types supported attachment and proliferation of human fetal osteoblasts. However, selective leaching only marginally improved cellular infiltration when compared to meshes obtained by conventional electrospinning. Better cell penetration was seen in mPCL/Col microfibers, and this effect was more pronounced when Heprasil regions were present in the structure. Thus, such techniques could be further exploited for the design of cell permeable fibrous meshes for tissue engineering applications.

Upgrading Calcium Phosphate Scaffolds for Tissue Engineering Applications

The research on ceramic scaffolds for bone tissue engineering is, nowadays, one of the newest and most attractive topics in the field of materials for biomedical applications. These scaffolds are aimed to provide supporting or even enhance the reparative capacity of body. Biphasic calcium phosphates (BCPs) and silicon doped BCP are very interesting candidates to be used as materials for scaffolds fabrication in bone tissue engineering. BCPs and silicon doped BCP consist of a mixture of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) or HA and α-tricalcium phosphate (α-TCP), respectively. For the regenerative purposes BCPs show better performance than HA because of the higher solubility of β-TCP compound, which facilitate the subsequent bone ingrowth in the implant. On the other, silicon doped BCP involve silicon that substituted into the apaptite crystal lattice for phosphorous with the subsequent charge imbalance. HA/α-TCP based bioceramics exhibits an important improvement of the bioactive behaviour with respect to non-substituted apatites. This work reviews the procedures to synthesise and fabricate scaffolds based on HA/β-TCP and silicon stabilised HA/α-TCP. Special attraction has been paid in the different synthesis methods and to the shaping of final scaffolds. By knowing the scaffold features at the crystallinity and macrostuctural level, the biocompatibility and clinical performance can be better understood, which will be also considered in this review.

A review of rapid prototyping techniques for tissue engineering purposes

Rapid prototyping (RP) is a common name for several techniques, which read in data from computer-aided design (CAD) drawings and manufacture automatically three-dimensional objects layer-by-layer according to the virtual design. The utilization of RP in tissue engineering enables the production of three-dimensional scaffolds with complex geometries and very fine structures. Adding micro- and nanometer details into the scaffolds improves the mechanical properties of the scaffold and ensures better cell adhesion to the scaffold surface. Thus, tissue engineering constructs can be customized according to the data acquired from the medical scans to match the each patient's individual needs. In addition RP enables the control of the scaffold porosity making it possible to fabricate applications with desired structural integrity. Unfortunately, every RP process has its own unique disadvantages in building tissue engineering scaffolds. Hence, the future research should be focused on the development of RP machines designed specifically for fabrication of tissue engineering scaffolds, although RP methods already can serve as a link between tissue and engineering.