The dynamics of various biological systems, especially circulatory system and respiratory system, has been the object of long-term research. Dynamics of circulatory system mainly studies the flow of blood in the heart, arteries, capillaries and veins, as well as the mechanical problems of the heart and heart valves. Respiratory system dynamics mainly studies the flow of gas in airway, the flow of blood in pulmonary circulation and the gas exchange between qi and blood during breathing.
All these works, including the study of rheological properties and dynamics of biomaterials, not only help to understand the physiological and pathological processes of human body, but also provide scientific basis for the design and manufacture of artificial organs. Biomechanics also studies the transport of plant body fluids.
The influence of environment on physiology is also a research content of biomechanics. As we all know, oxygen has a great influence on the development of organisms, which develops slowly in anoxic environment and rapidly in oxygen-enriched environment. Even in the short term, the environmental impact is obvious. Experiments show that even if young rats live in 10% oxygen and one atmospheric pressure for only 24 hours, a large number of fibrous cells will appear under the wall of pulmonary arterioles with a diameter of 15 ~ 30 microns. If it lasts for 4 ~ 7 days, fibroblasts will transition into typical smooth muscle cells, which will undoubtedly affect the blood flow of pulmonary circulation. Another example is that when a person is in a state of high acceleration, the inertia of his blood will change obviously, and the suspended organs will deviate from the original position, thus changing the flow state of blood in the body.
When designing tools for sailing in water, it is often necessary to consider the best shape, the best propulsion mode and the best control mode. Because of natural selection, aquatic organisms with these advantages are more likely to survive. Therefore, we can gain some valuable knowledge by studying the movement of some aquatic organisms.
For example, the dolphin is an advanced animal, which has an efficient propulsion mechanism and a good appearance. In particular, its skin is divided into two layers, which are filled with elastic fibers and adipose tissue. It has special drag reduction characteristics and can maintain laminar boundary layer state when swimming at high speed. This is because its skin is very sensitive to the change of pressure gradient in the boundary layer, and appropriate elastic deformation can be carried out to reduce the reverse pressure gradient. Therefore, when swimming at high speed, the epidermis can produce wave-like motion to suppress the occurrence of end flow. Another example is that the movement of ciliates is realized by the special movement of cilia, and the movement mode of this lower organism is also maintained in the human respiratory tract, that is, some foreign bodies in the respiratory tract are excluded by cilia. In a word, the significance of studying the biological movement in nature is obvious.
The dynamics of human organs and systems, especially the heart-circulatory system and lung-respiratory system, the thermodynamic balance between biological system and environment, and the specific functions are also the focus of current research. Biomechanics research involves not only medicine and sports, but also traffic safety, aerospace and military science.
Biosolid mechanics is to study the related mechanical problems in biological tissues and organs by using the basic theories and methods of material mechanics, elastic-plastic theory and fracture mechanics.
In approximate analysis, the standard formula of material mechanics can be applied to the strength theory and state parameters of human and animal bones in compression, tension and fracture. However, bones are anisotropic in morphology and mechanical properties. Since 1970s, there have been many theoretical and practical studies on the mechanical properties of bone, such as the combined bar hypothesis and the two-phase hypothesis. Finite element method, fracture mechanics, stress sleeve method and predicted elasticity method have all been applied to the study of bone mechanics.
Bone is a composite material, and its strength is not only related to the structure of bone, but also related to the material itself. Bone is a combination of collagen fibers and inorganic crystals. The bone plate consists of longitudinal fibers and circumferential fibers. Inorganic crystals in bone greatly improve the strength of bone, reflecting the functional adaptability of bone to bear the maximum external force with the least structural materials.
Wood and insect skin are composite materials composed of fibers embedded in other materials, which are similar to the mechanical properties of FRP composed of fine glass fibers embedded in synthetic resin. Animals and plants are polymers composed of polysaccharides and protein lipids. The mechanical properties of protein and polysaccharide can be obtained by applying the polymer theory of rubber and plastic. Viscoelasticity, elastic deformation and elastic modulus can be used not only for protein of amino acid composition, but also for analyzing the mechanical properties of cells. Such as the force on microfilament during cell division, the working mode and principle of myofilament, and the mechanical properties of cell membrane.
Biofluid mechanics studies biological cardiovascular system, digestive and respiratory system, urinary system, endocrine system, swimming, flight and other mechanical problems related to fluid mechanics, aerodynamics, boundary layer theory and rheology. It generally divides biological materials into body fluids, hard tissues and soft tissues, and muscles belong to a special category.
In body fluids, blood is the focus of research, which mainly studies the viscosity of blood and the factors affecting viscosity (such as pipe diameter, visible components and red blood cells), as well as the specific volume distribution of red blood cells in pipeline branches, the mechanical properties of red blood cells themselves, the interaction between red blood cells and the interaction between red blood cells and pipeline walls. The flow of blood and the transportation of plant body fluids in human and animals are similar to those in fluid mechanics, such as laminar flow, turbulent flow, seepage flow and two-phase flow.
When analyzing the mechanical properties of blood, blood can be regarded as a uniform fluid when it flows through large blood vessels. Because the diameter of microvessels is equivalent to the diameter of red blood cells, blood can be regarded as two-phase fluid in microcirculation analysis. Of course, the thinner the blood vessels, the more remarkable the non-Newtonian characteristics of blood.
Most of the blood flow in the human body belongs to laminar flow, and turbulence is easy to occur in the parts where the blood flows faster or the blood vessels are thicker. In the aorta, blood moving at peak speed is hardly laminar, but it will become turbulent in many cases. The urine flow in urethra is often turbulent; The exchange of substances through the capillary wall is a kind of seepage. For inflow such as blood flow, the blood flow fluctuates due to the beating of the heart, and the flow boundary is not fixed due to the elasticity of blood vessels. Therefore, the flow state of blood in the body is more complicated.
For soft tissue, it is mainly to study its rheological characteristics and establish the constitutive relation, because the constitutive relation is not only the basis for further analysis of its mechanical problems, but also has clinical significance. For hard tissue, besides its rheological characteristics, the relationship between the growth and decline of bones and stress has also been studied a lot.
The knowledge of fluid mechanics is also used in the study of animal swimming. If this fish is streamlined and flexible, it can propel itself by making waves. The water tunnel experiment shows that the velocity gradient in the fluid boundary layer is very large when fish swim, so the force to overcome the viscous resistance of fluid is also great.
The swimming of small organisms and single cells is also an outflow problem. The fluctuation of flagella and the flapping of cilia push the fluid on the cell surface and make the cell move forward. Sperm swimming with flagella, the inertia of water can be ignored, and its hydrodynamic force is proportional to the relative swimming speed of sperm. The resistance of protozoa moving in liquid can be obtained according to the formula (Stokes law) for calculating the resistance of small particles in flow field.
In addition, the principles and methods of aerodynamics are often used to study the flight of animals. The flight power of aircraft and flying animals consists of zero lift and induced force. The former is used to overcome the air viscous resistance in the boundary layer; The latter is used to accelerate the downward flow of air to provide lift equivalent to the weight of aircraft or flying animals. Birds can adjust the gliding angle by flapping their wings back and forth in the air, which is the same as the flap adjustment of gliders. Wind tunnel has been used to study the flight characteristics of flying animals, such as vultures and bats, and their gliding performance is very similar to that of model gliders.
Sports biomechanics is a subject that studies human motion based on the basic principles of statics, kinematics and dynamics, combined with anatomy and physiology. Studying biology with the principles and methods of theoretical mechanics is an early and in-depth field.
Research characteristics of biomechanics
To study biomechanics, we must first understand the geometric characteristics of biomaterials, then determine the mechanical properties of tissues or materials, determine the constitutive equation, derive the main differential equations and integral equations, determine the boundary conditions and solve them. The solution of the above boundary problems needs to be verified by physiological experiments. If necessary, a separate mathematical model is needed to make the theory consistent with the experiment.
The most important difference between biomechanics and other branches of mechanics is that its research object is organisms. Therefore, when studying biomechanical problems, the environment of the experimental object is very important. As experimental objects, biomaterials can be divided into in vivo and in vitro. Biological materials in the body are generally in a stressed state (such as blood vessels and muscles), and once they are free, they are in an unnatural state (such as blood vessels and muscles, once they are free, they contract immediately and shorten obviously). The experimental results of the two materials are quite different.
In vivo experiments are divided into anesthesia and non-anesthesia. As for in vitro experiments, after the object is free, it can be tested according to the overall position, or further processed into specimens for experiments. Different experimental conditions and processing conditions have great influence on the experimental results. This is the characteristic of biomechanical research.