Growth factor delivery for bone tissue engineering

Bone is a dynamic tissue that undergoes significant turnover during the life cycle of an individual. Despite having a significant regenerative capability, trauma and other pathological scenarios commonly require therapeutic intervention to facilitate the healing process. Bone tissue engineering, where cellular and biological processes at a site are deliberately manipulated for a therapeutic outcome, offers a viable option for the treatment of skeletal diseases. In this review paper, we aim to provide a brief synopsis of cellular and molecular basis of bone formation that are pertinent to current efforts of bone healing. Different approaches for engineering bone tissue were presented with special emphasis on the use of soluble (diffusible) therapeutic agents to accelerate bone healing. The latter agents have been used for both local bone repair (i.e. introduction of agents directly to a site of repair) as well as systemic bone regeneration (i.e. delivery for regeneration throughout the skeletal system). Critical drug delivery and targeting issues pertinent for each mode of bone regeneration are provided. In addition, future challenges and opportunities in bone tissue engineering are proposed from the authors' perspective.     

Growth factor delivery to re-engineer periodontal tissues

Repair of tooth-supporting structures destroyed by the chronic inflammatory disease periodontitis is a major goal of oral therapy. The field of tissue engineering combines materials science and biology to repair tissues and organs. Periodontal tissue engineering has been achieved with limited success by the utilization of guiding tissue (cell occlusive) membranes and bone grafting techniques. Over the past decade investigators have begun to utilize signaling molecules such as growth factors to restore lost tooth support due to periodontitis, the most common bone disease affecting humans. This review will provide information on the status of growth factor therapies being applied in periodontology to treat advanced alveolar bone loss.     

Spatial regulation of controlled bioactive factor delivery for bone tissue engineering

Limitations of current treatment options for critical size bone defects create a significant clinical need for tissue engineered bone strategies. This review describes how control over the spatiotemporal delivery of growth factors, nucleic acids, and drugs and small molecules may aid in recapitulating signals present in bone development and healing, regenerating interfaces of bone with other connective tissues, and enhancing vascularization of tissue engineered bone. State-of-the-art technologies used to create spatially controlled patterns of bioactive factors on the surfaces of materials, to build up 3D materials with patterns of signal presentation within their bulk, and to pattern bioactive factor delivery after scaffold fabrication are presented, highlighting their applications in bone tissue engineering. As these techniques improve in areas such as spatial resolution and speed of patterning, they will continue to grow in value as model systems for understanding cell responses to spatially regulated bioactive factor signal presentation in vitro, and as strategies to investigate the capacity of the defined spatial arrangement of these signals to drive bone regeneration in vivo.     

Growth factor delivery for bone tissue repair: an update

Growth factors (GFs) are endogenous proteins capable of acting on cell-surface receptors and directing cellular activities involved in the regeneration of new bone tissue. The specific actions and long-term effects of GFs on bone-forming cells have resulted in exploration of their potential for clinical bone repair. The concerted efforts have led to the recent approval of two GFs, bone morphogenetic protein-2 and osteogenic protein-1, for clinical bone repair, and human parathryroid hormone (1-34) for augmentation of systemic bone mass. This review provides a selective summary of recent (2001-2004) attempts for GF delivery in bone tissue regeneration. First, a summary of non-human primate studies involving local regeneration and repair is provided, with special emphasis on the range of biomaterials used for GF delivery. Next, efforts to administer GFs for systemic augmentation of bone tissue are summarised. Finally, an alternative means of GF delivery, namely the delivery of genes coding for osteogenic proteins, rather than the delivery of the proteins, is summarised from rodent models. To conclude, future avenues of research considered promising to enhance the clinical application of GFs are discussed.     

Matrices and scaffolds for drug delivery in dental, oral and craniofacial tissue engineering

Current treatments for diseases and trauma of dental, oral and craniofacial (DOC) structures rely on durable materials such as amalgam and synthetic materials, or autologous tissue grafts. A paradigm shift has taken place to utilize tissue engineering and drug delivery approaches towards the regeneration of these structures. Several prototypes of DOC structures have been regenerated such as temporomandibular joint (TMJ) condyle, cranial sutures, tooth structures and periodontium components. However, many challenges remain when taking in consideration the high demand for esthetics of DOC structures, the complex environment and yet minimal scar formation in the oral cavity, and the need for accommodating multiple tissue phenotypes. This review highlights recent advances in the regeneration of DOC structures, including the tooth, periodontium, TMJ, cranial sutures and implant dentistry, with specific emphasis on controlled release of signaling cues for stem cells, biomaterial matrices and scaffolds, and integrated tissue engineering approaches.     

Growth factor release from tissue engineering scaffolds.

Synthetic scaffold materials are used in tissue engineering for a variety of applications, including physical supports for the creation of functional tissues, protective gels to aid in wound healing and to encapsulate cells for localized hormone-delivery therapies. In order to encourage successful tissue growth, these scaffold materials must incorporate vital growth factors that are released to control their development. A major challenge lies in the requirement for these growth factor delivery mechanisms to mimic the in-vivo release profiles of factors produced during natural tissue morphogenesis or repair. This review highlights some of the major strategies for creating scaffold constructs reported thus far, along with the approaches taken to incorporate growth factors within the materials and the benefits of combining tissue engineering and drug delivery expertise.     

Natural polymers for gene delivery and tissue engineering

Although the field of gene delivery is dominated by viral vectors and synthetic polymeric or lipid gene carriers, natural polymers offer distinct advantages and may help advance the field of non-viral gene therapy. Natural polymers, such as chitosan, have been successful in oral and nasal delivery due to their mucoadhesive properties. Collagen has broad utility as gene activated matrices, capable of delivering large quantities of DNA in a direct, localized manner. Most natural polymers contain reactive sites amenable for ligand conjugation, cross-linking, and other modifications that can render the polymer tailored for a range of clinical applications. Natural polymers also often possess good cytocompatibility, making them popular choices for tissue engineering scaffolding applications. The marriage of gene therapy and tissue engineering exploits the power of genetic cell engineering to provide the biochemical signals to influence proliferation or differentiation of cells. Natural polymers with their ability to serve as gene carriers and tissue engineering scaffolds are poised to play an important role in the field of regenerative medicine. This review highlights the past and present research on various applications of natural polymers as particulate and matrix delivery vehicles for gene delivery.     

Chitosan nanoparticles as a dual growth factor delivery system for tissue engineering applications

The effect of stabilizer type (small molecule vs. polymeric) and the amount of micellar solubilized drug on Ostwald ripening of nanosuspensions was investigated. Indomethacin nanosuspensions were prepared with small molecule stabilizers (sodium lauryl sulfate (SLS) and Dowfax 2A1 (DF)) and a polymeric stabilizer (hydroxypropyl methyl cellulose (HPMC)). Two different drug:stabilizer ratios were used to evaluate the effect of micellar solubilized drug. The Ostwald ripening potential of nanosuspensions was evaluated by subjecting them to various stress conditions (temperature (15, 25, 35 and 45°C), thermal cycling, and mechanical shaking) for three months. The mean particle size increased in all SLS and DF formulations stored under different stress conditions. No effect of micellar solubilized drug on the Ostwald ripening rate was observed. In the case of HPMC formulations only those stored at higher temperatures (35 or 45°C) exhibited an increase in mean particle size. The increase in size in the HPMC formulation stored at 45°C was attributed to dehydration of the HPMC chains and subsequent loss of protection of the nanoparticles. The cube of the mean particle diameter versus time plot was determined to be non-linear for all formulations exhibiting Ostwald ripening. Therefore, according to the Lifshitz, Slyozov and Wagner theory the process was not diffusion controlled. The most probable mechanism for Ostwald ripening was surface nucleation controlled.     

Gene delivery strategies for cartilage tissue engineering

Tissue engineering is a multifaceted technology developed with a purpose of regenerating complex tissues and organs. Cartilage regeneration continues to challenge engineers and a new wave of efforts focus on developing strategies that provide sustained stimulation to cells by growth factors and other biological molecules to promote their differentiation into chondrocytes. Though significant research is dedicated to developing controlled release systems that deliver growth factors directly, a simpler approach to resolving this dilemma involves converting cells into protein producing factories. This is done through gene delivery. Gene Therapy studies published for articular diseases such as rheumatoid and osteoarthritis provide valuable information regarding different types of cells, gene delivery vectors and genes that can potentially be used to regenerate cartilage. Tissue engineering approaches provide the opportunity to combine two or more strategies used for Gene Therapy thus far and create a cohesive system that addresses both cartilage degeneration and synthesis simultaneously. Adopting gene transfer techniques for tissue engineering is a relatively novel approach, as non-viral gene delivery vectors are continually optimized for therapeutic purposes, and reservations about viral vectors have increasingly dampened their appeal. However, every element involved in gene transfection (i.e., the cell, vector and gene) is a variable which decides the physiological and biomechanical properties of the cartilage produced, and significant work still needs to be done in understanding the contribution of each of these factors to cartilage regeneration.     

Materials for protein delivery in tissue engineering

The ability of protein agents to modulate cellular behaviors, such as motility, proliferation, adhesion and function, is the subject of intense research; new therapies involving proteins will likely result. Unfortunately, many proteins have short half-lives and the potential for toxicity after systemic delivery, so traditional routes of administration are not appropriate. Alternate methods for sustained delivery of these agents to the desired cells and tissues in biologically active conformations and concentrations are necessary. Techniques similar to those long used in the controlled delivery of drugs have been used to administer certain growth factors to cells and tissues; although clinical success has been limited to date, studies in animal models suggest the potential for tremendous advances in the near future. This review outlines the basic technology of controlled protein delivery using polymeric materials, and discusses some of the techniques under investigation for the efficient administration of proteins in tissue engineering.     

Bisphosphonate-modified biomaterials for drug delivery and bone tissue engineering

Introduction: Bisphosphonates (BPs) were introduced 45 years ago as anti-osteoporotic drugs and during the last decade have been utilized as bone-targeting groups in systemic treatment of bone diseases. Very recently, strategies of chemical immobilization of BPs in hydrogels and nanocomposites for bone tissue engineering emerged. These strategies opened new applications of BPs in bone tissue engineering.

Areas covered: Conjugates of BPs to different drug molecules, imaging agents, proteins and polymers are discussed in terms of specific targeting to bone and therapeutic affect induced by the resulting prodrugs in comparison with the parent drugs. Conversion of these conjugates into hydrogel scaffolds is also presented along with the application of the resulting materials for bone tissue engineering.

Expert opinion: Calcium-binding properties of BPs can be successfully extended via different conjugation strategies not only for purposes of bone targeting, but also in supramolecular assembly affording either new nanocarriers or bulk nanocomposite scaffolds. Interaction between carrier-linked BPs and drug molecules should also be considered for the control of release of these molecules and their optimized delivery. Bone-targeting properties of BP-functionalized nanomaterials should correspond to bone adhesive properties of their bulk analogs.     

Collagen gel systems for sustained delivery and tissue engineering

Collagen gels are flowable, suggesting the possibility of an easily injectable, biocompatible drug delivery matrix. Sustained release of therapeutic molecules from collagen matrices, however, is beset with difficulties. Fibrillar collagen gels have an effective pore size of several tens of nanometers, too large to control release by hindered diffusion. To control release, it is necessary to rely on binding of the active agent to collagen, either by covalent or non-covalent bonds, or on sequestering in a secondary matrix. Such steps rapidly increase the complexity of the system. Non-fibrillar collagen has a lower effective pore size (4-6 nm), but it dissolves rapidly in vivo (approximately 24 h). For tissue engineering applications, collagen gels are more attractive, since they can act as a "cage" to retain cells or as gene delivery complexes, which are larger than drugs and therapeutic proteins. The gels have limitations in terms of strength, but reinforcement with solid components and alignment during gelation and culture can improve performance.     

Matrices and scaffolds for DNA delivery in tissue engineering

Regenerative medicine aims to create functional tissue replacements, typically through creating a controlled environment that promotes and directs the differentiation of stem or progenitor cells, either endogenous or transplanted. Scaffolds serve a central role in many strategies by providing the means to control the local environment. Gene delivery from the scaffold represents a versatile approach to manipulating the local environment for directing cell function. Research at the interface of biomaterials, gene therapy, and drug delivery has identified several design parameters for the vector and the biomaterial scaffold that must be satisfied. Progress has been made towards achieving gene delivery within a tissue engineering scaffold, though the design principles for the materials and vectors that produce efficient delivery require further development. Nevertheless, these advances in obtaining transgene expression with the scaffold have created opportunities to develop greater control of either delivery or expression and to identify the best practices for promoting tissue formation. Strategies to achieve controlled, localized expression within the tissue engineering scaffold will have broad application to the regeneration of many tissues, with great promise for clinical therapies.     

Matrices and scaffolds for protein delivery in tissue engineering

The tissue engineering of functional tissues depends on the development of suitable scaffolds to support three dimensional cell growth. To improve the properties of the scaffolds, many cell carriers serve dual purposes; in addition to providing cell support, cutting-edge scaffolds biologically interact with adhering and invading cells and effectively guide cellular growth and development by releasing bioactive proteins like growth factors and cytokines. To design controlled release systems for certain applications, it is important to understand the basic principles of protein delivery as well as the stability of each applied biomolecule. To illustrate the enormous progress that has been achieved in the important field of controlled release, some of the recently developed cell carriers with controlled release capacity, including both solid scaffolds and hydrogel-derived scaffolds, are described and possible solutions for unresolved issues are illustrated.     

Surface-functionalized electrospun nanofibers for tissue engineering and drug delivery

Electrospun nanofibers with a high surface area to volume ratio have received much attention because of their potential applications for biomedical devices, tissue engineering scaffolds, and drug delivery carriers. In order to develop electrospun nanofibers as useful nanobiomaterials, surfaces of electrospun nanofibers have been chemically functionalized for achieving sustained delivery through physical adsorption of diverse bioactive molecules. Surface modification of nanofibers includes plasma treatment, wet chemical method, surface graft polymerization, and co-electrospinning of surface active agents and polymers. A variety of bioactive molecules including anti-cancer drugs, enzymes, cytokines, and polysaccharides were entrapped within the interior or physically immobilized on the surface for controlled drug delivery. Surfaces of electrospun nanofibers were also chemically modified with immobilizing cell specific bioactive ligands to enhance cell adhesion, proliferation, and differentiation by mimicking morphology and biological functions of extracellular matrix. This review summarizes surface modification strategies of electrospun polymeric nanofibers for controlled drug delivery and tissue engineering.     

Polymer carriers for drug delivery in tissue engineering

Growing demand for tissues and organs for transplantation and the inability to meet this need using by autogeneic (from the host) or allogeneic (from the same species) sources has led to the rapid development of tissue engineering as an alternative. Tissue engineering aims to replace or facilitate the regrowth of damaged or diseased tissue by applying a combination of biomaterials, cells and bioactive molecules. This review focuses on synthetic polymers that have been used for tissue growth scaffold fabrication and their applications in both cell and extracellular matrix support and controlling the release of cell growth and differentiation supporting drugs.     

Viral vectors for gene delivery in tissue engineering

The goal of tissue engineering is the production of functional, biocompatible tissues by seeding cells within biological or synthetic scaffolds. One tissue engineering approach involves the genetic modification of cells that are seeded onto (or into) scaffolds prior to implantation. The genetic modification is achieved through gene delivery, with can utilize viral transduction or non-viral transfection systems. Although novel non-viral systems have continued to emerge as innovative vehicles for controlled gene delivery, viruses remain the most efficient means by which exogenous genes can be introduced into and expressed by mammalian cells. Retrovirus, adenovirus, adeno-associated virus and herpes virus are widely studied viral gene transfer systems and have attracted the most attention in the field of transduction. This review thoroughly discusses the genomic structures of each virus type, along with the advantages and disadvantages of their use in tissue engineering applications.     

壳聚糖-丝素蛋白-磷酸三钙复合体用于牙周组织工程支架初探

目的构建壳聚糖-丝素蛋白-磷酸三钙多孔复合体,观察其微观结构与体内外降解情况,探讨其作为牙周组织工程软骨支架的可行性。方法采用二次冻干技术制备壳聚糖-丝素蛋白-磷酸三钙复合体,冰冻切片和扫描电镜观察微观形态;将壳聚糖-丝素蛋白-磷酸三钙复合体支架置于人工降解液及植入大鼠背部肌肉,观察体内外降解情况及生物相容性。结果壳聚糖-丝素蛋白-磷酸三钙复合体呈海绵状,孔隙较均匀,孔形态不规则,孔隙之间彼此联通,孔隙率约80%,孔径200μm左右。4周体外降解率60%,在植入早期可见一过性炎性反应,随后逐渐降解,12周时基本降解吸收。结论壳聚糖-丝素蛋白-磷酸三钙多孔复合体具有良好的三维立体结构及可降解性,是一种可行的牙周组织工程支架。