Structure of Biological Materials

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1 ELEC ENG 3BA3: Structure of Biological Materials Notes for Lecture #8 Tuesday, October 3, 2006

2 3.3 Mechanics of biomaterials Mechanical issues for implanted biomaterials include: failure (e.g., fracture, plastic deformation or lack of rigidity) due to loading wear damage of biological tissues 2

3 The interaction of biomechanics and biomaterials is complicated by two factors: fatigue due to cyclic loading, and adaptation and healing of biological tissue. 3

4 Deformation 4

5 Multiple loading conditions must be considered: Most materials will undergo various loading conditions in different situations, e.g., torsional loading of a hip prosthesis when arising from a chair and a bending moment while standing. 5

6 Fatigue Fatigue is a reduction in the strength of a material due to cyclic loading. The fatigue behaviour of a material is characterized by a stress-life (S-N) diagram. This diagram plots the nominal stress amplitude S versus the number of cycles to failure N. 6

7 Fatigue (cont.): Typical S-N curves. 7

8 Fatigue (cont.): Some materials exhibit an endurance limit, i.e., an asymptotic failure stress (see curve A in the previous slide & the curves to the right). Such materials have an infinite lifetime if the applied stresses are always below the endurance limit. 8

9 Fatigue (cont.): Other materials do not exhibit an endurance limit and continue to fatigue with the number of cycles endured (see curve B on slide 7 & the curves to the right). The fatigue strength of such materials must be specified for a certain number of cycles. 9

10 Fracture Fatigue fracturing begins with an initial microscopic crack that propagates through the material with each loading cycle. Local stress concentrations can lead to fracturing of a material below the strength calculated for an isotropic material. Finite-element modeling of actual geometries can be used to determine local stress concentrations. 10

11 Fracture (cont.): An alternative is to empirically determine the effects of a certain geometry on the maximum local stress: 11

12 Wear Wear of the surface of a biomaterial can occur due to adhesion or abrasion between the material and a contact surface. Having smooth, hard surfaces on both materials will reduce both adhesion and abrasion wear. Maintaining a fluid film between the surfaces can reduce adhesion wear. 12

13 Wear (cont.): An important factor in the wear process is the contact stress: where L is the load and A c is the contact area. Higher contact stress implies higher local loads from decreased area, despite constant general loads need conforming surfaces. 13

14 Rigidity An appropriate rigidity of an implanted biomaterial will ensure consistent loading stresses and will allow natural healing and organic adaptation. In contrast, excessive movement of an implant may produce high local stresses and may prevent healing and adaptation of the biological tissue that it contacts. 14

15 3.4 Impact biomechanics The mechanics of a collision involving biological tissues or an entire person is important for understanding the effects of accidents in: motor vehicles, sports, and industrial or home environments. 15

16 Collision of objects In a collision between two objects, some or all of the kinetic energy of the objects may need to be dissipated. The fraction of the kinetic energy that is dissipated depends on the elasticity of the two objects. 16

17 Coefficient of restitution: 17

18 For two totally elastic objects, all of the kinetic energy is returned. For two partially elastic or totally inelastic objects, some of the kinetic energy must be dissipated according to: Energy being dissipated in human tissue in a collision is bad! 18

19 Viscoelastic behaviour Biological tissue may exhibit viscoelasticity, i.e., the elastic modulus increases as a function of loading rate. Consequently, tissues such as brain tissue and bone may act stiffer if a load is suddenly applied. 19

20 Viscoelastic behaviour (cont.): 20

21 Local stress concentration In a collision, it is desirable for the axial loadings on biological tissue to be uniformly distributed, so that local stress concentrations are minimized. This has important ramifications for safety gear and restraints. 21

22 Moments and shear stresses Likewise, it is important in a collision that no part of the body is subject to large moments or shear stresses because of being unrestrained relative to the rest of the body, e.g., the head and neck undergoing whiplash motions. 22

23 Moments and shear stresses (cont.): (from Siegmund, BC Medical Journal 2002) 23