Mechanical Properties of Biological Tissues

Strength of Biological Materials The strength of biological materials is defined by the ability of the material to withstand stress without failure. The strength of a material is affected by: – Microstructure – Age – Fluid content – Type, direction and velocity of loading

Definitions: Stress, Strain, & Young’s Modulus Stress (σ or τ) is defined force per unit of area. Stress quantifies the internal forces acting on the object as a reaction to the external applied forces. The units for stress are N/m 2 or Pa. Strain (ε) is defined as the % change in deformation of the object. Young’s Modulus (ϒ) is the slope of the stress – strain curve of the material. It describes the stiffness of the material. The units for Young’s modulus are N/m 2 or Pa.

Effects of Compression Forces on Deformable Bodies Imagine applying a compression force on a soft deformable object. The object would deform, compress and the sides would bulge out, directly in proportion to the applied force. This effect also occurs in rigid objects like a steel rod. The internal forces acting on the object cause deformation.

Effects of Compression on a Rigid Object Unlike a soft object, when a compression force is applied to a rigid object the deformation may not be immediately observable. In the example above the 20 N force causes deformation which is not visible. An increase in the force to 40 N will eventually cause visible deformation as bending occurs.

Effects of Shear Forces on Deformable Bodies

Shear Force A shear force is applied sideways to the material and it results in parallel opposing forces. Examples of shear forces include: When a pair of scissors cuts a material. When a material is punched. When a weight is held perpendicular to the long axis of a bone.

Material Testing System Images courtesy of Instron

Effects of Muscle Weakness

Strain is a Normalized Variable

Young’s Modulus Young’s modulus is a measure of the material’s resistance to deformation. Young’s modulus quantifies how much stress is required to generate a give strain. It does not depend upon the size or shape of the object, but only the material the object is composed of. Copper has a modulus of 120 x 10 9 Pa. Steel has a modulus of 200 x 10 9 Pa. Thus, steel is more resistant to deformation then is copper.

Effects of Muscle Weakness

Viscoelasticity When an elastic material containing fluid is deformed the return of the material to it’s original shape is time delayed. Viscoelastic materials exhibit both an elastic response and viscous damping. Bones, tendons, ligaments, cartilage, muscle, and skin are all viscoelastic. Viscoelastic materials display both a time dependent and rate dependent response.

Stress – Strain for an Elastic Material The force required to deform an elastic spring is described by Hooke’s law, where x is deformation and k is the spring stiffness.

Properties of a Viscoelastic Material The mechanical response of a viscoelastic material are time and velocity dependent. Viscoelastic materials exhibit: – a hysteresis – a creep response – a force relaxation response – Are time dependent, if the force is held for a longer time they exhibit greater hysteresis.

Properties of a Viscoelastic Material Creep is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material

Stress – Strain for a Viscoelastic Material

Creep Creep is a time dependent response of viscoelastic tissues. The muscle-tendon complex is loaded with a weight. When the load is initially applied the muscle undergoes deformation. Following this initial deformation the muscle continues to deform at a much lower rate, this later deformation is creep.

Force Relaxation Force relaxation is a time dependent response of viscoelastic tissues. When the muscle is stretched the force (resistance to stretch) rises rapidly. Then when the stretch is held the force (resistance to stretch) slowly declines over time, this is force relaxation.