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Written by: Sydney M. Thayer
Written on: July 1st, 2011
Tags: biomedical engineering, health & medicine, material science
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About the Author
Sydney Thayer is a junior pursuing a major in Biomedical Engineering and minors in Theatre Arts and Natural Sciences at the University of Southern California. In the future, Sydney hopes to become a practicing pediatric physician while continuing her involvement in community theatre productions.
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Volume XIII Issue III > Biology’s Approach to Construction: The Development and Use of Scaffolds in Tissue Engineering
The field of tissue engineering has seen significant improvements in the past 10 years, much of which is due to the development of tissue scaffolds. These 3-dimensional, porous structures are perfectly suited for cellular attachment and growth due to their physical similarities to the native extracellular matrix. The ability of scaffolds to be strong yet flexible further increases their application versatility. When constructing scaffolds for implementation, two elements are necessary: a cell-growth base derived from a tissue culture and the biodegradable, polymer-based scaffold itself. When these two elements are combined and inserted into the patient, tissue repair occurs through cellular proliferation. Such scaffolds are currently used in the regeneration of cartilage, bone, and elements of the cardiovascular system, with some electrically-conductive scaffolds even tailored for neural repairs. While these polymeric scaffolds have proven applicable in the above-mentioned fields, recent research suggests improvements on the current technology. Rather than a composition of polymer blends, new “biological scaffolds” use collagen fibers derived from either animal or human donors that can assemble spontaneously into scaffold structures. These biological scaffolds have shown to decrease immune system rejection and increase regenerative success, making them the next great technological development in tissue engineering.

Introduction

In the mid 1980s, tissue engineering was established as the next major biological breakthrough. As relevant technology developed, it became plausible for engineered tissues to replace organs and other living cells that had been damaged or lost. Successful regeneration showed exceptional promise with the use of biocompatible materials that function as connectors across an injured area [1]. These “biological bridges” allow for cell proliferation and, thus, reattachment and organ growth. With this goal in mind, increased funds and research have been invested in developing instruments for cell growth with the most effective and useful medium being the scaffold (Fig 1).
HIA/Wikimedia Commons
Figure 1: The tissue engineering process.

The Intricacies of Tissue Scaffolds

As would be expected with a name referencing construction, most tissue scaffolds physically resemble building scaffolding. Tissue scaffolds appear as 3-dimensional cubes supported by cross-linking. The resulting porous structure allows cells to grow and attach to regular tissue, as pictured below.
Many different materials have been used in the development of this technology, including metals, calcium phosphate ceramics, glass, and silk proteins. While ceramic and glass scaffolds are still used in some bone and cartilage engineering, currently the most widely-used scaffold materials are polymers, or chains of molecules that interact chemically to form a substance. Certain polymer compositions are more suited for particular fields, but most polymers used have biodegradable abilities. Biodegradability is important for the administration of scaffolds in the body because the removal of an implanted device is considered to be one of the most dangerous surgical procedures[2]. With biodegradable scaffolds, the scaffold can be absorbed by the body, eliminating the necessity for removal procedures. While certain tissues (such as bone) may require permanent support, the majority of soft tissue transplants are actually negatively affected by long-term foreign bodies. Thus, the development of polymer scaffolds has made these implants even more versatile.
The most significant parts of the tissue scaffold are the pores. Because scaffolds are cell growth instruments, it is important to maximize the surface-area to volume ratio of the structure. However, the engineering task of minimizing the structure of the instrument while maximizing available space for cells to grow is challenging. This issue is often addressed through strict maintenance of pore size. While the number and orientation of the pores may vary, most scaffold pores fall between 50 and 500 micrometers in diameter [3].
The size within this range is determined by the type of tissue repair. With hard tissue engineering, which is generally confined to the regeneration of bone, scaffold pores are often large and highly interconnected, allowing cells to efficiently grow all throughout the scaffold while maintaining a rigid structure. [4] Scaffolds for soft tissues, such as those for nerve and cardiac fibers, also have highly interconnected pores, but these pores are often smaller. Scaffold with smaller pores favor flexibility and a greater surface area rather than rigidity, as extensive structural support is unnecessary for soft tissue. [4]
The characteristic properties of scaffolds, strong, flexible, and porous, are significant to their role as biological substitutes. This is because the scaffold must mimic the extracellular matrix (ECM) of the body. The ECM is a network of connective tissue that supports and anchors body cells, similar to the role of scaffolds. Like the ECM, scaffolds serve primarily as structural support, providing the shape and rigidity of the physical tissue. Additionally, both scaffolds and the ECM are important in promoting cell proliferation and differentiation, allowing tissue to regenerate and form additional blood vessel connections [5]. Because tissue scaffolds are constructed like existing ECMs and use biocompatible materials, they are easily integrated into the body. These characteristics suggest that scaffolds will serve important roles in the current and future practices of tissue engineering.