Tissue engineering is an interdisciplinary field that applies the principles and methods of engineering and the life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function (Langer and Vacanti, 1993). Tissue engineering has also been defined as "understanding the principles of tissue growth, and applying this to produce functional replacement tissue for clinical use" (MacArthur and Oreffo, 2005). Taking these two definitions into account the overall goal of tissue engineering can be summarised into restoring function through the delivery of living elements which become integrated into the patient (Vacanti and Langer, 1999). This goal, which should lead to the fabrication of fresh, physiological, functioning tissue, must involve the combined efforts of cell biologists, engineers, material scientists, mathematicians, geneticists, and clinicians in order to be successful.
There are three major approaches that can be taken to restorative tissue engineering:
Guided tissue regeneration: This involves implanting an inert scaffold into the body, which encourages regeneration of the hosts cells residing at the site of implantation. This approach involves establishing primary cell-lines, placing the cells on or within the scaffold and then implanting the new system inside the body (Shinoka et al., 1995). Examples of this approach include repair of bone, muscle, tendon and cartilage, endothelial cell-lined vascular grafts and heart valve substitutes.
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Cell seeded scaffold: An inert scaffold is seeded with cells before culturing the engineered product in vitro in a bioreactor before implantation into the patient. The most common example is the skin graft, used for the treatment of burns (Eldad et al., 1987). Skin graft replacements have been grown in tissue culture and used clinically for over 10 years.
Cell therapy: The use of cells and/or growth factors (in vivo) being injected into a damaged organ (e.g. cartilage) to encourage regeneration of healthy tissue. This technique uses the human body as a bioreactor.
Tissue engineering consists of four essential components: cells, a matrix or scaffold, a bioreactor, and growth factors (Knight and Evans, 2004). Scaffold materials are three-dimensional tissue structures that guide the organisation, growth and differentiation of cells. Scaffolds must be biocompatible and designed to meet both nutritional and biological needs for the specific cell population (Vats et al., 2003). Growth factors are soluble peptides that are capable of binding cellular receptors and producing either a permissive or preventive cellular response toward differentiation and/or proliferation of tissue (Whitaker et al., 2001). Bioreactors must be capable of providing the optimal conditions for cell adhesion, growth, and differentiation within the construct by creating a system capable of controlling environmental factors such as pH, temperature, oxygen tension, and mechanical forces (Naughton, 2002).
A classic example of the tissue engineering process (shown in fig1) is when a scaffold/matrix becomes embedded with living cells and specific regulatory cytokines, and is placed into a bioreactor. Ideally, a suitable biochemical and biomechanical microenvironment is created and cell multiplication fills the scaffold with tissue and allows the cells to grow into the correct shape. Once implanted into the body, the seeded scaffold becomes integrated, supporting and directing cell proliferation. As the cells proliferate the scaffold slowly biodegrades, gradually allowing blood vessels and host cytokines to make contact with the cells. Via this process, the scaffold further biodegrades while the cells proliferate and differentiate into the desired tissue. Finally, the scaffold completely dissolves and the formed tissue starts functioning in its new surroundings.
Fig . The tissue engineering process, taken from Tissue engineering: the design and fabrication of living replacement devices, Langer R, Vacanti JP.Tissue engineering has now emerged as a potential alternative to tissue or organ transplantation. Using this technology, tissue loss or organ failure can be treated either by implantation of an engineered biological substitute or alternatively with ex vivo perfusion systems. The tissue-engineered products may be fully functional at the time of treatment (e.g., liver assist devices, encapsulated islets), or have potential to integrate and form the expected functional tissue upon implantation (e.g., chondrocytes embedded in a matrix carrier). In certain cases, biomaterials are modified to enhance migration and attachment of the specific cell populations, which repair or replace the damaged tissue (Chapekar, 2000).
Attempts have been made around the worldwide to engineer almost every human tissue. According to Vacanti, more than 30 body tissues have been investigated in his laboratories alone, which Robert Langer, from the Massachusetts Institute of Technology, thinks have ââ‚¬Å“the potential to treat everything from burns to paralysis.ââ‚¬Â
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Scientists are now engineering cardiovascular tissues such as heart valves (Fabiani et al., 1995) and blood vessels (Niklason et al., 1999) (Huynh et al., 1999). Encapsulated pancreatic islets have been implanted in the patients for the treatment of diabetes (Lanza and Chick, 1997) and liver assist systems containing encapsulated hepatocytes have been used clinically to provide extracorporeal support to the patients with liver failure (Chen et al., 1997b). A bioartificial bladder has been developed as a replacement engineered organ (Oberpenning et al., 1999).
The first engineered organ to be approved by the FDA was skin. Commercial products, including Dermagraft (Smith and Nephew) and Apligraf (Organogenesis), have been used in the clinical treatment of patients with burns and severe chronic pressure sores and ulcers secondary to diabetes or other chronic illnesses (Horch et al., 2005). However, the tissue engineered skin does not possess all of the biological characteristics of normal skin. It does not contain sweat pores for temperature and liquid balance, it does not possess any of the glands such as oil glands, and does not have hair follicles. Melanocytes are also lacking, as are other cellular features. In spite of these and other structural limitations, the engineered skin can allow one to reconstitute the integumentary covering of the body, solving the primary problem of possible infection.
Tissue engineering techniques can be used to potentially treat diabetes, by generating insulin-producing cells from stem cells and grafting them into the pancreas in diabetic patients (Tang et al., 2004). Clinical transplantation of islet cells for the treatment of type 1 diabetes has been introduced with enthusiasm (Ohgawara et al., 2004). The procedure is considered minimally invasive and potentially offers the possibility of being performed under donor-specific tolerant conditions. Recently, multipotent precursor cells from the adult mouse pancreas have been identified (Seaberg et al., 2004), and may be a promising candidate for cell based therapeutic strategies. Unlike isolated beta cells which release insulin in a monophasic, all-or-none manner without any modification for intermediate concentrations of blood glucose, the pancreatic precursor cells may be cultured to produce all the cells of the islet cluster in order to generate a population of cells that will be able to coordinate the release of the appropriate amount of insulin to the physiologically relevant concentrations of glucose in the blood (Bosco and Meda, 1997).
Despite much progress having been made in the field of tissue engineering, further work towards organ and tissue replacement is required. The optimal cell source, scaffold design, and in vitro bioreactors, the use and development of microfabrication technology to create vascularized tissues and organs are still being investigated. The search for and use of an appropriate multipotent or pluripotent stem cell in tissue engineering is an emerging concept. Many technical questions are yet to be answered and require close interdisciplinary collaborations of surgeons, engineers, chemists, and biologists, with the ultimate goal of functional tissue restoration.