Mechanical failure of soft tissues is characteristic of life-threatening diseases, including capillary stress failure, pulmonary emphysema, and vessel wall aneurysms. Failure occurs when mechanical forces are sufficiently high to rupture the enzymatically weakened extracellular matrix (ECM). Elastin, an important structural ECM protein, is known to stretch beyond 200% strain before failing. However, ECM constructs and native vessel walls composed primarily of elastin and proteoglycans (PGs) have been found to fail at much lower strains. In this study, we hypothesized that PGs significantly contribute to tissue failure. To test this, we developed a zipper network model (ZNM), in which springs representing elastin are organized into long wavy fibers in a zipper-like formation and placed within a network of springs mimicking PGs. Elastin and PG springs possessed distinct mechanical and failure properties. Simulations using the ZNM showed that the failure of PGs alone reduces the global failure strain of the ECM well below that of elastin, and hence, digestion of elastin does not influence the failure strain. Network analysis suggested that whereas PGs drive the failure process and define the failure strain, elastin determines the peak and failure stresses. Predictions of the ZNM were experimentally confirmed by measuring the failure properties of engineered elastin-rich ECM constructs before and after digestion with trypsin, which cleaves the core protein of PGs without affecting elastin. This study reveals a role for PGs in the failure properties of engineered and native ECM with implications for the design of engineered tissues.elastin ͉ strain ͉ stress ͉ trypsin T issue failure is characteristic of several life-threatening diseases, including capillary stress failure in the lung (1), pulmonary emphysema (2), vessel wall aneurysms (3, 4), and prosthetic heart valve failure (5). Whereas emergency surgery can be life saving for aneurysm patients (6), there is no cure for emphysema (7). These diseases are caused by various changes in the extracellular matrix (ECM) that lead to a weakening of ECM. A better understanding of the structure of the ECM, and how its components contribute to functional behavior (8), is necessary to provide insight into the development and progression of these diseases and will also be important if future tissue-engineered constructs are to be implanted in the body.Elastin, collagen, and proteoglycans (PGs) are the principle components that form the complex structural network of the ECM that is necessary for tissues to function properly (9). Elastin and collagen are fibrous proteins capable of carrying loads and resisting tension (10, 11). PGs make up the gel in which elastin and collagen fibers are embedded and are necessary for the stability of the ECM (12). Varying the structure and composition of the ECM has major effects on its mechanical (9, 13-15) and failure properties (3,(16)(17)(18)(19)(20)(21).Independently, 2 groups have obtained similar surprising results related to failure of t...

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Mechanical failure of soft tissues is characteristic of life threatening diseases, including emphysema and vessel wall aneurysms. Failure occurs when mechanical forces are sufficient to rupture the enzymatically weakened extracellular matrix (ECM). Elastin is an important structural protein of the ECM, and is known to stretch beyond 200% strain before failing. However, ECM constructs and native vessel walls composed primarily of elastin and proteoglycans (PGs) have been found to fail at much lower strains. In this study, we hypothesized that PGs significantly contribute to tissue failure. To test this, we developed a novel Zipper Network Model (ZNM), in which springs representing elastin are organized into long wavy fibers in a zipper-like formation and placed within a network of springs mimicking PGs. Elastin and PG springs possessed distinct mechanical and failure properties. The elastin does not percolate while the PGs can serve as bridges between elastin fibers as well as hinder folding of the fibers via bond-bending. During stretching, elastin fibers first become straight, then start stretching the PG bridges. Simulations using the ZNM showed that the failure of PGs alone reduces the global failure strain of the ECM well below that of elastin and hence digestion of elastin does not influence the failure strain. Network analysis also suggested that elastin determines the peak and failure stress while PGs transmit the load and define the failure strain of the network. Predictions of the ZNM were experimentally confirmed by measuring the failure properties of engineered ECM constructs before and after digestion with trypsin that cleaves the core protein of PGs without affecting elastin. This study reveals a novel role for PGs in the failure mechanics of engineered and native ECM with implications for the design of engineered tissues.