Biorheology

The study of the flow and deformation of biological materials. The behavior and fitness of living organisms depend partly on the mechanical properties of their structural materials. Thus, biologists are interested in biorheology from the point of view of evolution and adaptation to the environment. Physicians are interested in it in order to understand health and disease. Bioengineers devise methods to measure or to change the rheological properties of biological materials, develop mathematical descriptions of biorheology, and create new practical applications for biorheology in agriculture, industry, and medicine.

The rheological behavior of most biological materials is more complex than that of air, water, and most structural materials used in engineering. Air and water are viscous fluids; all fluids whose viscosity is similar to that of air and water are called newtonian fluids. Biological fluids such as protoplasm, blood, and synovial fluid behave differently, however, and they are called non-newtonian fluids. For example, blood behaves like a fluid when it flows, but when it stops flowing it behaves like a solid with a small but finite yield stress.

Most materials used in engineering construction, such as steel, aluminum, or rock, obey Hooke's law, according to which stresses are linearly proportional to strains. These materials deviate from Hooke's law only when approaching failure. A structure made of Hookean materials behaves linearly: load and deflection a relinearly proportional to each other in such a structure. Some biological materials, such as bone and wood, also obey Hooke's law in their normal state of function, but many others, such as skin, tendon, muscle, blood vessels, lung, and liver, do not. These materials, referred to as non-Hookean, become stiffer as stress increases. See Bone

In biorheology, so-called constitutive equations are used to describe the complex mechanical behavior of materials in terms of mathematics. At least three kinds of constitutive equations are needed: those describing stress-strain relationships of material in the normal state of life; those describing the transport of matter, such as water, gas, and other substances, in tissues; and those describing growth or resorption of tissues in response to long-term changes in the state of stress and strain. The third type is the most fascinating, but there is very little quantitative information available about it except for bone. The second type is very complex because living tissues are nonhomogeneous, and since mass transport in tissues is a molecular phenomenon, it is accentuated by nonhomogeneity at the cellular level. The best-known constitutive equations are therefore of the first kind. See Biomechanics

biorheology

Biorheology

The study of the flow and deformation of biological materials. The behavior and fitness of living organisms depend partly on the mechanical properties of their structural materials. Thus, biologists are interested in biorheology from the point of view of evolution and adaptation to the environment. Physicians are interested in it in order to understand health and disease. Bioengineers devise methods to measure or to change the rheological properties of biological materials, develop mathematical descriptions of biorheology, and create new practical applications for biorheology in agriculture, industry, and medicine.

The rheological behavior of most biological materials is more complex than that of air, water, and most structural materials used in engineering. Air and water are viscous fluids; all fluids whose viscosity is similar to that of air and water are called newtonian fluids. Biological fluids such as protoplasm, blood, and synovial fluid behave differently, however, and they are called non-newtonian fluids. For example, blood behaves like a fluid when it flows, but when it stops flowing it behaves like a solid with a small but finite yield stress.

Most materials used in engineering construction, such as steel, aluminum, or rock, obey Hooke's law, according to which stresses are linearly proportional to strains. These materials deviate from Hooke's law only when approaching failure. A structure made of Hookean materials behaves linearly: load and deflection a relinearly proportional to each other in such a structure. Some biological materials, such as bone and wood, also obey Hooke's law in their normal state of function, but many others, such as skin, tendon, muscle, blood vessels, lung, and liver, do not. These materials, referred to as non-Hookean, become stiffer as stress increases. See Elasticity, Stress and strain

In biorheology, so-called constitutive equations are used to describe the complex mechanical behavior of materials in terms of mathematics. At least three kinds of constitutive equations are needed: those describing stress-strain relationships of material in the normal state of life; those describing the transport of matter, such as water, gas, and other substances, in tissues; and those describing growth or resorption of tissues in response to long-term changes in the state of stress and strain. The third type is the most fascinating, but there is very little quantitative information available about it except for bone. The second type is very complex because living tissues are nonhomogeneous, and since mass transport in tissues is a molecular phenomenon, it is accentuated by nonhomogeneity at the cellular level. The best-known constitutive equations are therefore of the first kind. See Biomechanics