Lecture 9-10.pdf - MCDB/CHEM 145/151 Enough of...

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MCDB/CHEM 145/151 Enough of structure (for now) Function/Properties of Biomaterials Mid-term quiz due back
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Literature - biomechanics Bell, E.C., and J.M. Gosline (1996). Mechanical design of mussel byssus: material yield enhances attachment strength . J. Exp. Biol. 199 :1005-1017. Denny, M.W. (1988). Biology and the Mechanics of the Wave swept Environment. Princeton Univ. Press, Chapter 12, p. 76-94. Gosline, J.M., Guerette, P.A., Ortlepp, C.S., & Savage, K.N. (1999). The mechanical design of spider silks: From fibroin sequence to mechanical function. J. Exp. Biol. 202 : 3292-3305. Miserez, A., Wasko, S.S., Carpenter, C.F., & Waite, J.H. (2009). Non- Entropic and reversible long range deformation of an encapsulating Bioelastomer. Nature Materials 8 : 910-916. Vogel, S. (2005). Comparative Biomechanics. Life’s Physical World. Princeton Univ. Press, Chapters 15-18.
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organism cell external milieu Locations of Load bearing proteins
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Scleroprotein formation Presclerotin Activator#1 Cross- Linking Enzyme Activator#1 Presclerotin Sclerotin monomer Aggregated polymer Cross-linked polymer Proenzyme Enzyme Physical XLs b -pleated sheet Fibroin Silk fibroin Dityrosine Random coil Preresilin Resilin S-S, isopeptides a -Coiled coils Prekeratin Keratin isopeptide (Q-K) a -Coiled coils Fibrinogen Fibrin Desmosine Random coil preElastin Elastin Aldimine, aldol PPII trimer Procollagen Tropocollagen Cross-links 2˚ structure Precursor Scleroprotein
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calipers diameter fibrous sample tethers mounts motor (pulling speed) ruler Linear deformation load gauge force applied Gear for tensile properties Video recording drive
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Assumptions for mechanical testing Uniform geometry No flaws Homogeneous composition Uniform molecular arrangement
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load deformation Perfect spring A= p r 2 s =stress= Load (N) A e =strain= L f -L o L o L o L f s e Perfect spring From Hook’s law: stress = E i x strain Modulus or stiffness E i = s/e units N/ m 2
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Critical Properties Stress Strain 1. Initial stiffness or Young’s modulus Engineer’s strain ( e ) = [L f -L i ] /L i m/m Materials with high E i are stiff and there are multiple strategies for achieving stiffness True strain ( e ) = ln [L f /L i ] m/m Assumption: area constant (E); area changes at break (T) Assumption: same volume all strains (E); volume can change (T)
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2 & 3. Ultimate or breaking stress/strain Stress, s Strain e s ult = strength at break e ult extensibility = stretchiness at break Area = breaking energy = toughness s yield
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Stress N m -2 Strain e.g. 1/2 (m/m) x (Nm -2 ) = 1/2 Nm x m -3 = 1/2 J m -3 Strain energy density at yield or breaking J m -3 / specific gravity = specific strain energy density Specific strain energy density 570,000 Swnt fiber ~2, 500 7900 20 M Ht steel ~133,000 1200 160 M spider silk Specific strain energy Jg - 3 Specific gravity kg m -3 Strain energy density Jm -3 Material 4. Strain energy density or toughness s d e = toughness “strain energy”
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5. Energy dissipation: hysteresis stress strain Dissipated energy due to molecular friction Cyclic Dissipated = Stored energy - recovered energy Recovered energy/ Total energy x 100 = resilience (%) Dissipated energy/Total energy X 100 = hysteresis (%) Elastin 10% Tendon 7% Dragline 65% Viscid 65%
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s e Perfect spring s e Biol. Tissue A s e Biol. Tissue B s e Newton Fluid slow fast Kelvin-Voigt Maxwell
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