Tissue engineering is a set of techniques to produce tissue, primarily for repair or improvement of human body functions. There are also some additional spin-off applications, such as screening of pharmaceutics and artificial meat. Tissue-engineered constructs are different than medical implants, which are technical devices made from man-made materials, without living tissue parts.
Today there are several examples of tissue engineering in clinical use, e.g. skin, cartilage, bone, heart valves and bladder. There are many more examples, which still are at the research and animal experiment levels, like artificial liver, pancreas and blood vessels. For most examples, tissue is grown outside the body and then implanted into a patient. There are, however, also parallel attempts and ideas for in-vivo tissue engineering, where e.g. a cell culture is implanted and develops into a desired tissue inside the body.
Key components of a tissue engineering process are a tissue, or cell sample, that constitutes the seed and starting point of the process, and secondly a scaffold that holds and supports the seed and steers its growth into desired geometrical shape and location. A third important component is an environment of signal substances and nutrition etc. that steers the growing cell culture, or tissue seed, towards the desired biological properties, including both geometrical size and shape and bio-functional properties.
The tissue engineering process is often implemented in a bioreactor designed to steer both the geometrical shape and the bio-functional properties. In addition, the bioreactor also provides pressure and temperature control and specific motional programs like agitation and rotation. To build complex tissue structures it has become increasingly popular to use 2D or 3D-printing to fabricate the structures.
In terms of hurdles and challenges, there is a clear dividing line between target tissues that require vascularization, i.e. blood vessels in the grown tissue, which deliver nutrients and oxygen and remove waste products, and those which do not. For example, tissue engineered heart valves can function without vascularization because they are surrounded by blood, carrying nutrients and oxygen, while an artificial tissue engineered liver or pancreas must have internal vascularization.
The starting tissue sample or cell culture for a tissue engineering process can be autologous, i.e. from the same individual that is being treated, or allogeneic, which means that it is from the same species, normally a human, but from a different individual. It can also be xenogeneic, which means that it is from a different species than a human. Of course, the autologous cells or tissue offer several advantages in terms of compatibility with the patient’s tissues and physiology and reduces immune responses.
A special case, offering significant advantages, is to use stem cells. There are different types of stem cells, for example, adult stem cells and embryonic stem cells. Adult stem cells are stem cells for a specific tissue type. They exist in a human’s body and repair or renew the tissues in the body, e.g., bone, muscle, skin. Embryonic stem cells are undifferentiated cells of embryos. They are pluripotent, i.e., they have the potential to develop to any type of cells and tissue as they do naturally when they develop from the initial embryonic stage to a living being. Their pluripotent ability is mainly a blessing but also a curse in the context of tissue engineering; Since these cells can differentiate into any type of cells and tissue it might be difficult to control their differentiation and steer them into the desired ones and at the same time avoiding differentiation into unwanted cell types.
Embryonic stem cells are connected with special ethical issues and therefore adult stem cells may be more attractive. An important discovery that solves the ethical dilemma, and gives general advantages for tissue engineering was Yamanaka’s discovery (Nobel Prize 2012) that mature differentiated cells can be converted back to up-stream stem cells. When successful, it eliminates the need to harvest embryonic stem cells.
As mentioned above, tissue engineering is mainly aiming at developing repair functions for the human body in the form of tissues and organs. However, there are also interesting applications that can be seen as spin-offs of normal tissue engineering.
A mind-tickling application for tissue engineering is so-called artificial meat. The idea is simple – to use a cell culture to grow meat, i.e., essentially muscle cells or muscle tissue, in a bioreactor. The ‘meat’ can e.g. be grown from the different types of stem cells mentioned above or from myocytes, which is a kind of muscle stem cell. So far, there is no commercial products available, but fabrication plants are under consideration. Potential advantages of artificial meat are that it has the potential of reducing climate gases from meat production and reducing the amount of farming land areas occupied for cattle and cattle feed production.
Maybe the most interesting spin-off application, however, is engineered tissues for testing and screening in pharmaceutic R&D. If a representative tissue or organ can be engineered, it can then be used as a test site to screen both the efficiency, dosage and possible side effects of drugs under development for that particular tissue. Such testing with engineered tissues might reduce the amount of animal testing and maybe also time for clinical testing, with large potential of reducing costs for drug development.
Biomaterials are man-made materials developed for, and used in, products intended for medical treatments. They often integrate in various ways with their surrounding tissue. Tissue engineering, in contrast, is a set of methods to produce living functional tissue from cells or tissue seeds. The reason that tissue engineering initially was regarded as a subarea of biomaterials was that the scaffolds and templates, and the materials for tissue engineering, had strong focus on the materials for the scaffolds etc. Today, tissue engineering is regarded as a field in its own right, overlapping with biomaterials primarily regarding the used biocompatible materials.
Learn more about tissue engineering in our podcast, Science on surfaces - a bigger perspective on the small. In this episode, we also cover more about 3D-printing, and what the future of tissue engineering looks like.
Learn about the distinction between the two concepts biomaterials and tissue engineering and how they relate to each other.
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