Cartilage: Biomimetic Study of the Extracellular Matrix
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Cartilage: Biomimetic Study of the Extracellular Matrix Chinedu I. Anyaeji, Peter J. Basser, Ferenc Horkay Section on Tissue Biophysics and Biomimetics, Program in Pediatric Imaging and Tissue Sciences, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA ABSTRACT Cartilage is a complex biological tissue that exhibits gel-like behavior. Its primary biological function is providing compressive resistance to external loading and nearly frictionless lubrication of joints. In this study, we model cartilage extracellular matrix using a biomimetic system. We demonstrate that poly(vinyl) alcohol (PVA) hydrogels are robust biomaterials exhibiting mechanical and swelling properties similar to that of cartilage extracellular matrix. A comparison is made between the macroscopic behavior of PVA gels and literature data reported for cartilage. INTRODUCTION Cartilage is located at the end of bones. It has multiple biological roles such as load bearing, cushioning, lubrication. Cartilage is aneural and avascular. Since it contains a limited number of cells cartilage regenerates very slowly after damage [1-3]. Cartilage is synthesized by chondrocytes. These cells represent about five percent of the tissue volume. In addition, the matrix is comprised of collagen (10% to 20%), proteoglycans (5% to 15%), and water (70% to 80%) (7).
Figure 1: Illustration of the complexity of cartilage tissue. Within the cartilage matrix, aggrecan is the most prominent proteoglycan. It has a bottlebrush architecture consisting of a protein core and mainly chondroitin sulfate and keratan sulfate side chains. These molecules possess high net negative charges that repel
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each other in aqueous solution resulting in an extended molecular configuration. The hydrophilic chains attract water molecules and form a microgel, which assists the matrix to resist external loading. In the cartilage matrix aggrecan bottlebrushes self-assemble along linear hyaluronic acid chains [4-6]. Collagen provides the skeleton of the cartilage matrix; it is a triple helical polypeptide chain. Type II collagen is the most common type of collagen found in cartilage. Collagen serves two major functions: (i) its triple helical structure gives cartilage the ability to sustain tensile stress; (ii) it contains the aggrecan-hyaluronic acid complex, which is essential to maintain structural integrity [7-10]. The objective of our work is to develop a biomimetic model system that possesses cartilage-like mechanical and swelling properties. We characterize well-defined poly(vinyl alcohol) gels and compare these results with those obtained for cartilage tissue. BIOMIMETIC MODEL There are several requirements for an appropriate biomimetic model system. These include but are not restricted to (i) Tissue-like architecture, (ii) Easily controllable chemical composition, (iii)Controllable mechanical and swelling properties, (iv) High hydration capability and enhanced stability in the physiological milieu, (v) F
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