Indentation micromechanics of three-dimensional fibrin/collagen biomaterial scaffolds
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The underlying relationships between the microstructure and time-dependent mechanical properties of hydrated fibrin, collagen, and fibrin/collagen composite materials have been explored using an adaptation of the classical rigid, cylindrical, flat punch loaded normally to a planar specimen surface. A suite of quasi-static elastic and viscoelastic indentation experiments have been conducted with uniformly mixed fibrin, collagen, and fibrin/collagen composites, in addition to macrolayered collagen materials. Coupled with insights obtained from optical and confocal fluorescence microscopy, a simple micromechanics model has been developed for the effect of local microstructural variables on the macroscopic mechanical stiffness. These results demonstrate the efficacy of this technique to efficiently and reproducibly probe hydrated engineered tissue replacement materials for local variations in viscoelastic material behavior without the need for extensive specimen preparation or grips, as well as being suitable for performing directly comparable measurements with explants of human skin.
I. INTRODUCTION
When skin is damaged, the resultant wound healing process is highly orchestrated.1 The first stage consists of fibrin clot formation within a few minutes of injury. This fibrin clot must fulfill several crucial mechanical requirements.2â5 Most immediately, it must adhere to the surrounding tissues and entrap platelets and other cells responsible for rapid hemostasis. In addition, the fibrin must be strong enough to withstand the forces imposed upon it by blood pressure, yet flexible enough to conform to normal bodily movements without cracking or detaching from the wound site. Finally, the fibrin must transmit vital mechanical signals from the surrounding tissues and organs into the newly forming replacement skin to ensure its proper development and function. Fibrin clot formation occurs when fibrinogen, a soluble plasma protein found within the blood, is converted into nanometer-dimensioned, insoluble fibrin monomers via proteolytic cleavage by thrombin. Thrombin also originates within the blood, and its production is triggered by the rupturing of the blood vessel lining. The resultant fibrin monomers then aggregate into submicron diameter fibrils that, in turn, develop into a hydrated,
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0258 J. Mater. Res., Vol. 21, No. 8, Aug 2006
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three-dimensional network with open, interconnecting pores approximately 1â10 âŪm in diameter.2 Thus, the macrostructure of fibrin comprises an open-celled porous network of fibrin fibrils within an interpenetrating aqueous solution, i.e., a hydrated gel. The mechanical properties of fibrin and collagen are clearly of critical importance to the wound healing process, yet, despite years of work,6â8 the detailed relationships between the mechanical properties of fibrin and its macro- and microstructure are not yet completely understood. This kno
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