Biomaterials for Tissue Engineering Applications

When approached to put together this compilation on recent advances on the material component of tissue engineering, we first asked ourselves whether there was a need for another book in this field. Tissue engineering books abound; however, a review of the existing literature soon made clear that no other work exists that focuses specifically on the material aspects of tissue engineering science and the approach that we and others use within our own research. Namely, we operate under the principle that rational material design, directed and informed by native tissue structure and function, will enhance the generation of engineered constructs with functional utility. Moreover, as advances are being made in this arena at an exceedingly rapid pace, it is important to routinely update the field. To address this important need, we asked several emerging experts, who we thought may contribute a fresh perspective on this topic, to contribute reviews on past and current work on a range of materials and the formation of engineered tissues from these materials. Further, we asked that each contributor provide their own opinion on the new directions that the field is taking, so as to identify the emerging consensus areas of focus for the future.

The successful engineering of synthetic hydrogels that exhibit key features of the natural extracellular matrices has led to significant advances in the field of tissue engineering. Various chemical and physical approaches have been developed for hydrogel synthesis. While the chemical methods rely on the presence of readily addressable functional groups for the formation of covalent bonds at the crosslinking points, the physical approaches utilize weak and reversible interactions for gelation purposes. In many cases, physical gels need to be covalently stabilized for their long term applications in tissue engineering. Over the past decade, hydrogels have evolved from passive scaffolding materials to bioactive and cell-responsive matrices that play a defining role in the regulation of cellular functions and tissue growth. Novel hydrogels with tunable microstructures, mechanical properties, and degradation rates have been engineered. Biological motifs or soluble factors have been successfully incorporated in the hydrogel matrices to allow for a higher level of cell-matrix communication. These synthesis methods have resulted in the production of a wide variety of functional hydrogels that support the growth of many different tissue types. The development of the next generation biomimetic hydrogels relies on parallel advancements in materials chemistry, cell biology and developmental biology.

Fibers are a continuous material structure that have an extremely high ratio of length to width, and are particularly suitable for fabrication into biomaterial scaffolds for tissue engineering since fibrous structures can morphologically resemble extracellular matrix components in tissues. In addition, fibers can be collected and processed into complex fibrous networks using conventional textile techniques, such as knitting, weaving, or braiding, to create three-dimensional (3D) structures with improved structural and mechanical properties. Recently, there is a growing interest in using nanofabrication techniques to fabricate nanometer-sized fibers for tissue engineering. Nano-sized fibers exhibit enhanced physical and biological properties that are favorable for effective biomaterial scaffolds, compared to micro-sized fibers. While great progress using fibrous scaffolds to grow various human tissues has been made, it is important for scaffold-based tissue engineering to develop the next generation “smart” scaffolds capable of promoting cell-matrix interactions through a bio-inspired surface, and inducing favorable biological activity via controlled release of incorporated biological molecules.

The rapid progress in cell and developmental biology has clearly revealed that substrate elasticity and mechanical stimulation significantly affect cell function and tissue development. Further, many engineered soft-tissue constructs such as vascular grafts, cardiac patches, and cartilage are implanted in a mechanically dynamic environment, thus successful implants must sustain and recover from various deformations without mechanical irritations to surrounding tissues. Ideal scaffolds for these tissue engineering applications would be made of biodegradable elastomers with properties that resemble those of the extracellular matrix, providing a biomimetic mechanical environment and mechanical stimulation to cells and tissues. However, traditional biodegradable scaffold materials such as polylactide, polyglycolide, and poly(lactide-co-glycolide) are stiff and are subjected to plastic deformation and failure under cyclic strain. Consequently, for the past decade, many novel bioelastomers have been developed and extensively investigated for applications in tissue engineering. Both thermoplastic elastomers such as polyurethane, poly(ε-caprolactone) copolyester, poly(ether ester) and thermoset elastomers such as crosslinked polyesters have been developed and evaluated to engineer various tissues such as heart muscle and valves, blood vessels, skin, and cartilage. This chapter will cover representative bioelastomers and their applications in tissue engineering to highlight recent advances in this area.

Microscale and high throughput technologies are powerful tools for addressing many of the challenges in the field of tissue engineering. In this chapter, we present an overview of these technologies and their applications in controlling the cellular microenviroment for tissue engineering applications. We focus on concepts and techniques that can be used to create two- and three-dimensional tissue engineering substrates and scaffolds. Common to these techniques is the ability to control one or more aspects of the cellular microenvironment, including chemical and mechanical cues, cell–cell, cell–matrix, and cell-soluble factor interactions. We also discuss recent developments in high throughput techniques that are used to explore the vast number of combinations of factors that comprise the cellular microenvironment.

Nanotechnology has enabled the creation of novel structures or devices with organization and structure at the molecular and atomic scale. Nano-dimensions impart distinctive properties compared to micron sized materials. In this chapter, we will review techniques for creation of micron and nanometer features on 2D substrates and 3D nanomaterials which have been used in tissue engineering applications. In the first half, we will review the latest advances in custom designed micro- and nano-substrates to enhance the phenotypic expressions of cells and mechanisms behind changes in cell behaviors. In the second half, we will focus on nanomaterials and discuss recent developments in both design and applications for tissue engineering and mechanisms behind cellular interactions with nanomaterials. In addition, an examination of the effects of nanomaterials on biocompatibility and cytotoxicity will be presented. Specific applications of nanomaterials in tissue engineering of connective, neural, muscular and boney tissue will also be examined.

This chapter is focused on the classification of bioceramics and their medical applications. Alumina, zirconia or alumina–zirconia-based composite bioinert ceramics are currently used as femoral heads, acetabular cups for hip replacement, and dental implants. Nano-structured bioinert ceramics with significantly improved toughness and stability are desirable for future clinical needs. Bioactive glass and calcium phosphates are being investigated as bone fillers, bone cements, coatings, and scaffolds for bone repair and regeneration. Cell-laden biodegradable bioceramic/biopolymer hybrid composites mimicking the bony hierarchical structure present the desired properties for bone substitution and tissue engineering and are creating a new generation of regeneration materials. Bioceramics for dental and cancer treatment are also introduced in this chapter. Further challenges in bioceramic scaffold fabrication for tissue engineering are also discussed.

Materials derived from natural sources are used extensively in tissue engineering. Consisting of proteins, polysaccharides, or ceramics, these materials may be harvested from a wide range of sources and possess an equally wide range of physical and biological properties. This chapter focuses upon seven of these materials, namely collagen, fibrin, elastin, hyaluronic acid, alginate, chitosan, and silk. These materials are first discussed with respect to their intrinsic features that are relevant to tissue engineering, such as structure, source, degradation, mechanics, immunogenicity, and recognition by cells. This is followed by a review of techniques for derivatizing natural materials, forming scaffolds, and tailoring these scaffolds, accompanied by select examples of how these natural materials have been used in tissue engineering applications. While natural materials possess many characteristics that render them attractive for use in tissue engineering, they are also accompanied by some unique challenges; both of these features are highlighted in this chapter.

Engineered polypeptides have emerged as attractive materials to construct artificial extracellular matrices (ECMs) in tissue engineering. These materials offer advantages over conventional synthetic materials in recapitulating essential characteristics of complex and dynamic native ECMs, which is one of the key requirements for successful tissue engineering, because proteins are major players in providing structural support, cell adhesion, and signal regulation in native ECMs. The structures and functions of these proteins and their domains, as well as those of de novo designed polypeptide domains having self-assembly and molecular recognition abilities, can be combined in engineered polypeptides in a modular manner to yield multifunctional, bioactive materials to mimic native ECMs and optimize tissue engineering outcomes. Engineered polypeptides can be synthesized both chemically and biosynthetically. In recent years, the biosynthetic methodology has received increasing attention because the advances in molecular biology and protein engineering have expanded its capacity. Biosynthetic preparation allows polypeptide materials to be genetically engineered in a modular manner and the resulting polymers have absolutely uniform sequence, composition, molecular weight, and consequently higher order structures and functions. These properties not only allow us to engineer novel multifunctional materials to elicit desired cell responses toward functional tissue regeneration, but also offer the opportunity to create well-controlled and tunable systems for systematic studies to enhance our understanding of the relationships among extracellular microenvironments, cell behavior and fate selection, and tissue assembly. Such understanding will provide valuable guidelines for design of future generations of artificial ECMs. In this chapter, engineered polypeptides that have been used or have the potential to be used in tissue engineering will be discussed with an emphasis placed on their molecular design as well as examples of their use in tissue engineering studies.

Articular cartilage provides the surface of articulating joints with frictionless movement while absorbing loading forces. The tissue’s extracellular matrix (ECM) is comprised mainly of type II collagen and proteoglycans which are maintained by chondrocytes, the resident cell population. Cartilage is a structurally complex tissue, with zones that exhibit different cell morphologies and extracellular matrix structure depending on distance from the articulating surface. The tissue is both alymphatic and avascular. All nutrient, oxygen, and waste exchange occurs through diffusion. This, along with low cell density and proliferation, contributes to the tissue’s limited ability to repair ECM damage. The high number of people suffering from arthritis has led to a plethora of cartilage engineering research. Recent efforts have focused on aiding the body in cartilage restoration through both cell-based and acellular biomaterials. A variety of synthetic and natural polymers have been created for this purpose, each with their benefits and drawbacks. To date, an ideal biomaterial has yet to be created that can optimally repair or regenerate damaged cartilage. Here we highlight current biomaterial trends in cartilage engineering and examine future directions within the field.

Ligaments and tendons play an important role in mediating normal movement and stability of joints in the musculoskeletal system, and their inability to undergo endogenous repair following injury leads to significant joint instability, injury of other tissues, and the development of degenerative joint disease. To restore their normal structure and function and address these clinical challenges, biomaterial scaffolds are being developed that incorporate cellular, morphogenetic, and mechanical cues into defined architectures that may be implanted as part of regenerative medicine therapies. This chapter explores the field of biomaterials for regeneration of tendons and ligaments with an emphasis on: (1) native tissue structure, function, mechanical properties, and interfaces with other orthopaedic tissues; (2) mechanisms of injury, healing responses, and limitations of current clinical approaches for repair; and (3) contemporary biomaterials-based approaches for tissue engineering of tendons and ligaments, including cell types used, design strategies, and results of their application in vitro and in vivo. Several challenges remain in achieving a successful biomaterial for tendon/ligament regeneration, yet significant design and engineering improvements have continued to enhance their functional sophistication and hold much promise for future tissue engineering strategies.

Bone is the principal component of the skeletal system. It is comprised of an extracellular matrix that is characterized by a hierarchical and heterogeneous structure with features that span from the nanoscale to the macroscale and interact to perform the various functions of the tissue. For large defects, traditional therapies for bone repair include tissue grafts, which are limited by supply (autografts) and the potential for disease transmission (allografts). Alternatively, commercially available products used for bone reconstruction do not necessarily approximate the hierarchical nanoscale structure of the natural tissue. This chapter will focus on recent advances in the development of select biomimetic, self-assembled and nanocomposite materials for use in the repair and regeneration of osseous tissues.

Fibrocartilage is a specialized tissue found in the intervertebral discs, the menisci of the knee and temporomandibular joint, and various symphyseal joints throughout the body. Its unique combination of tensile strength, compressive strength, and deformability makes it an ideal material for many structures, however a low intrinsic capacity for repair means that disease or damage can produce chronic debility. The fibrocartilages represent a significant challenge for tissue engineers. Biomaterials must be capable of withstanding significant mechanical stress while guiding formation of a complex microarchitecture. In this chapter, we will review the structure and biology of fibrocartilage and take a look at the biomaterial strategies that have been used. At present no material has satisfied all of the requirements for a successful tissue engineered therapy, however many promising developments have occurred.

The development of liver support systems has been in intensive investigation for over 40 years. The main driving force is the shortage of donor organs for orthotopic liver transplantation. Liver cell transplantation and extracorporeal bioartificial livers (BAL) may bridge patients with end-stage liver diseases to successful orthotopic liver transplantation, support patients with acute liver failure to recover, and provide a curing method to patients with certain liver metabolic diseases. Another frontier of current liver tissue engineering is to construct many functional liver units in vitro for drug toxicity and metabolism screening. Much progress has been made, with several artificial liver dialysis devices on the market, a few BAL systems in clinical trials, and other in vitro micro-liver models in development. On the other hand, many lessons have been learned as well. In this chapter, we will focus on the review of advancement, challenges and the critical issues that have to be solved in the development of BAL systems and hepatic cell transplantation as well as in vitro micro-liver models from a tissue engineering perspective.

This book chapter will explore the area of cardiac tissue engineering and biomaterials used for cardiac cell therapy. After describing the pathology of heart disease and presenting the motivation for pursuing tissue engineering for the heart, the chapter outlines some of the current clinical treatments used to correct the loss of function resulting from heart disease. Current collection of studies is then divided broadly into two sections: cell injection and engineered heart patches. Chemical modifications and biomolecule incorporation are examined for the cell injection studies. Types of scaffolds as well as cultivation techniques are discussed for the engineered heart patches. Finally, future directions in the field including finding a suitable cell source and increasing the vascularization of the patches are suggested.

Creating functional vasculatures remains one of the fundamental challenges that must be addressed before large, complex tissue-engineered constructs can be used in clinical applications. Our current understanding of stem cell biology and vascular morphogenesis has allowed tissue engineers to design biomaterials that mimic the properties of native tissue and promote vascularization. Biomaterials approaches in tissue engineering include differentiation of vascular cells, delivery of angiogenic factors, in vivo and in vitro prevascularization, as well as microfabrication of complex vascular networks. This chapter will discuss the processes involved in vascular network assembly; these processes inspire the design of biomaterials to fit tissue vascularization. Previous work in this field will be described to allow discussion of the current state of the art and to provide insights into its future directions.

Biomaterials have played a role in the nervous system as drug delivery vehicles and scaffolds. The nervous system, both the peripheral and central, are capable of repair and regeneration when the appropriate environment is presented and this suggests that biomaterials could fundamentally change treatment following injury and disease by building a permissive environment for repair. Yet, when engineers have used materials particularly as scaffolds, one of the most striking findings is how similar many of their results have been across types of materials and approaches. Clearly, there is much still to learn. Part of that learning process comes from looking at what has succeeded in the clinic and using that to design the next generation of translatable approaches to treatment.

Stem cell engineering has enormous potential to study developmental processes, disease progression, drug screening, and to provide new therapeutics. The interaction of stem cells with their microenvironment plays an important role in determining the stem cell fate. One of the most important environmental factors is the extracellular matrix, which defines the biochemical and biophysical niche from which the stem cells receive various regulatory signals. Artificial matrices based on natural and synthetic materials have been developed to direct stem cell fate such as proliferation and differentiation. Some of these approaches are being currently explored clinically and other more sophisticated multi-functional biomimetic materials with defined properties are currently under development. In this chapter we discuss some of the recent trends in the development of biomaterials and their applications in directing self-renewal and differentiation of stem cells.

The total product life cycle (TPLC) provides a framework by which medical device companies incorporate international regulatory guidelines and best practices to translate product concepts, including novel biomaterial therapeutics, into clinical and commercial reality. The TPLC emphasizes continuity between all phases of a medical product’s lifetime, including Concept/Feasibility, Design/Development, Validation, Commercialization/Production, Post-Market Surveillance/Support. This product development cadence evolves through definition of the unmet clinical need and market opportunity, application of design control methodologies, consideration of manufacturability and scalability, and execution of the evidence generation and regulatory strategies. Throughout each of these activities, design controls are leveraged to promote cross-functional engagement of sales and marketing, research and development, quality assurance, regulatory affairs, operations and finance to ensure the medical device will provide a safe and efficacious solution for patients and surgeons.