Outline of today’s class: STRESS-SUPPORTING STRUCTURES1. Tissues and organs differ greatly in compliance (deformability). Are the basic rules of linear elastic mechanics obeyed by most tissues?Tissues and organs differ greatly in deformability due to: Three basic structural families of macromolecules: Collagens, elastins and polysaccharides (GAGs)2. Collagen fibers: Orientation pattern in various organs match mechanical function. 3. Collagen fibers: Periodic structure of crystalline fibers at different scales.3. Collagen fibers: Structure at different scales (cont.)4. Elastin fibers are amorphous and rubber-like 4. Elastin fibers (cont.)4. Elastin fibers (cont.)5. Glycosaminoglycans (GAGs) in tissues are polysaccharides. 5.Glycosaminoglycans(GAGs)(cont.). SummaryOutline of today’s class:STRESS-SUPPORTING STRUCTURES1. Tissues and organs differ greatly in deformability even though made up of few components.2. Collagen fibers: Orientation patterns in different organs match mechanical function.3. Collagen fibers: Periodic structure of crystalline fibers at different scales.4. Elastin fibers are amorphous and rubber-like.5. Glycosaminoglycans (GAGs) are polysaccharides that keep water inside tissues.1. Tissues and organs differ greatly in compliance (deformability).tendonneck ligamentskinStress-strain curves (constitutive relations) for various tissues and organs:stressstrainAre the basic rules of linear elastic mechanics obeyed by most tissues?• Matter is a continuum.• Linearity. Directly proportional relation between stress and strain. No terms with exponent higher than 1.• Elasticity. Time-independent mechanical behavior. Loading and deformation independent of time.• Isotropy. The elastic constants are independent of loading direction.• Conservation of mass.Tissues and organs differ greatly in deformability due to:•crystalline vs. amorphous fibers(collagen vs elastin fibers)•fiber reinforcement(tendon vs liver)•crosslinking (newborn vs aged)•molecular configuration(collagen vs GAG)•ceramic content •(bone vs cartilage)Three basic structural families of macromolecules: Collagens, elastins and polysaccharides (GAGs)•Diversity in mechanical behavior among tissues due primarily to variations in the organization of fibersof these macromolecular substances.•Model of supporting tissues as fiber-reinforced composite materials. A soft, aqueous GAG "matrix" gel is reinforced with fibers of collagen and elastin. [Bone is further reinforced with ceramic particles.] The model illuminates the critical importance of fiber orientation on stiffness and strength of tissues.2. Collagen fibers: Orientation pattern in various organs match mechanical function.• Tendons. Thick fibrous bundles that connect muscle to bone. Support axial forces. Collagen fibers are uniaxially oriented. • Ligaments are similar to tendon in structure but connect bone to bone. • Cornea. Membrane protects curved eye surface from tangential forces. Planar orientation of collagen fibers.• Dermis (skin). Collagen fibers nearly randomly oriented (some orientation in plane of epidermis).• Articular cartilage. Membrane between apposed bones in joints. It “lubricates” joints. “Cathedral”architecture of collagen fibers.Tendon connects muscle to boneand supports axial forcesImage removed due to copyright restrictions.Diagram of human arm bone, muscle and tendon structure, showing elbow flexion and extension.Quaternary structure of collagen. Fibers are probably single crystals, about 100 nm diameter, with a 64-nm periodicity (“banding”). Image removed due to copyright restrictions.Electron microscope photograph of collagen fibers.Tendon fibers200 μmWhat is the orientation of 100-nm thin collagen fibers in 200-μm thick tendon fibers? Do they form helix round axis of tendon fiber? Or is their axis parallel to tendon fiber axis?Image removed due to copyright restrictions.Electron microscope photograph.Effort to identify orientation of 100-nm collagen fibers by dissolution in aqueous acetic acid. However, collagen fiber orientation is not clearly shown in this experiment.100 μmImage removed due to copyright restrictions.Electron microscope photograph.Effort to identify orientation of collagen fibers by fracture. Wet tendon fibers fracture in “fibrillar” mode. 100-nm collagen fiber orientation not shown.40 μmImage removed due to copyright restrictions.Electron microscope photograph.Fracture surface of dry tendon20 μmTry a different approach. Dehydrate first, and then fracture tendon.Image removed due to copyright restrictions.Electron microscope photograph.Detail of fracture surface for dry tendon showing parallel arrangement of fibers along major fiber axis. Individual fibers,about 100 nm diameter, are single crystals of collagen.2 μmnormal tofracture surfaceResearch question: how are collagen crystals bound to each other?Image removed due to copyright restrictions.Electron microscope photograph.The eyeConjunctivaTHE HUMAN EYEAqueous HumourLensPupilOptic NerveRetinaFoveaCorneaIrisZonulaFigure by MIT OpenCourseWare.Tadpole corneaCORNEA. Membrane protects curved eye surface from tangential forces. Planar orientation of collagen fibers.Research question: how can the cornea, a two-phase material, be transparent?Image removed due to copyright restrictions.Fish corneaImage removed due to copyright restrictions.Detailedview of skinImage removed due to copyright restrictions.Medical illustration (by Frank Netter) of the structure of human skin.SKINFigure by MIT OpenCourseWare.A, arteriolesN, peripheral nerveAlmost all else is quasi-randomassembly ofcollagen fibersDermis100 μm______Histology photo removed due to copyright restrictions._____20 μmDERMIS (Skin). Collagen fibers nearly randomly oriented (there is some orientation in plane of epidermis). Supports forces along all three axes. Microscope photo removed due to copyright restrictions.Collagen fibers in human dermis: 4 magni-ficationsGibson and KenediBandingvisible onindividual fibrils(quaternary structure)Microscope photos removed due to copyright restrictions.Skin extensibility varies over right side of chest. Langer’s lines coincide with axis of minimum extensibility.right nippleDiagram removed due to copyright restrictions.normal dermisscarlight scattering pattern shows axial orientation for scarImage removed due to copyright restrictions.Regeneration of the dermis in guinea pig skin wounds. Studied by histology (up)