Lecture 8 Notes (BME 296)

Lecture Overview

  • Lecture 8 - BME 296

Mechanical Properties of Biomaterials

  • Understand the significance of mechanical properties in biomaterials.

Importance of Mechanical Properties

  • Cells sense physical cues, converting them into biochemical/electrical signals through mechanotransduction.

  • At tissue/organ levels, mechanical properties impact stress shielding, which can affect cell, tissue, and organ functionality.

    • Vascular dynamic bending (VDB) and vascular pulsation (VP) create alternating stress states in stents, impacting stress distribution, with VDB causes greater stress than VP.

Types of Mechanical Properties

  • Tensile/Compressive Properties: Resistance to stretching or compressing.

  • Shear/Torsion: Resistance to forces acting parallel or twisting.

  • Bending: Resistance to deformation under load.

  • Viscoelasticity: Material behavior is affected by time; combines elastic and viscous properties.

  • Hardness: Ability to resist localized deformation.

  • Deformation occurs at the molecular level and varies over time.

Mechanical Testing Equipment

  • Instron Mechanical Testing Machine: Used for conventional testing of biomaterials.

  • Specialized equipment required for small-scale biomaterials (e.g., cartilage) that can accommodate testing under wet conditions, temperature variations, and oxygen levels.

Tensile and Shear Testing

  • Tensile Testing: Measures material response to stretching.

    • Determines stress (force over area) and strain (change in length/original length).

  • Shear Testing: Applies force parallel to the surfaces of a test specimen, measuring the material's response to shear stress.

Stress-Strain Curves

  • Stress-strain curves illustrate the relationship between stress and strain.

    • Hooke's Law: Describes linear region where stress is proportional to strain. The slope indicates modulus of elasticity (stiffness).

  • Plastic Deformation: The region beyond the yield point where the material deforms permanently.

  • Ductility vs. Brittleness: Ductile materials show significant deformation before fracture; brittle materials fracture with minimal plastic deformation.

Poisson’s Ratio

  • Describes relationship between longitudinal strain and transverse strain.

  • Important for understanding deformation characteristics in isotropic materials (uniform properties in all directions).

Molecular Causes of Deformation

  • Elastic Deformation: Occurs when stress is applied, and material returns to shape after unloading. Relies on atomic bonds.

  • Plastic Deformation: Permanent change in shape under stress. In metals and ceramics, involves dislocation movements along slip planes.

    • Rubber-like materials exhibit different elastic behaviors based on directional stress.

Plastic Deformation in Polymers

  • Amorphous Polymers: Deform via viscous flow where atoms slide past one another, breaking and reforming bonds.

  • Semi-Crystalline Polymers: Exhibit unique deformation characteristics due to crystalline lamellae that allow for tie molecules to maintain structural integrity during loading.

    • Deformation stages include tie chain extension, lamellar sliding, and block orientation along loading axis.