Biomedical engineering

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Transport phenomena research in biological systems may be at the dawn of a new age, with, for example, new opportunities to understand and control transport phenomena at the cellular level. By doing so, novel targeted therapies, including therapies based upon advances in nanotechnology, might be developed. Advances in pharmacology including drug discovery and screening, pharmaceutical manufacturing and drug delivery are widespread, and have already had a profound effect on the quality of life globally. Other areas such as tissue engineering are replete with transport phenomena of several types. In these and other emerging areas as well as in traditional areas of bioheat and mass transfer, the thermal sciences will significantly contribute to improved health and environment. Heat and mass transfer in a single cell, tissue, and organ has been investigated. At the organ level, such as lung, liver, and kidney, their structure must allow rapid exchange of molecules between blood and tissues (Truskey et al., 2004). Although transport process occurs in all organs, the effect is very significant in some organs such as the cardiovascular and respiratory systems, the gastrointestinal tract, the liver and the kidneys. For example, the cardiovascular system consists of the heart, blood vessels and blood. The primary function of the cardiovascular system is to transport oxygen within red cells and remove carbon dioxide. In the respiratory system, oxygen is delivered to tissues when it is metabolized and carbon dioxide is removed. Digestion and absorption of nutrition are done in the gastrointestinal track. The liver performs metabolic functions such as carbohydrate storage and release, cholesterol metabolism, synthesis of plasma and transport proteins including removing toxic molecules from the blood. Kidneys are responsible for removal of urine and water products. In a number of diseases, such as atherosclerosis, cancer, and kidney diseases, alternation of transport processes are very important factors. In biomedical engineering, understanding of transport phenomena plays an essential role on design of replacement tissues and delivery of drugs. For example, while titanium is the most widely used metallic biomaterial for orthopedic implant, one major problem is the mismatch of the Young’s modulus between the bone (10-30 GPa) and the titanium implant (110 GPa); this mismatch causes the bone to be insufficiently loaded (referred to as stress-shielding) and retards bone remodeling and healing (Oh et al., 2003; Thieme et al., 2001). The mismatch can be overcome by introducing a graded porosity near the implant surface so that the Young’s modulus of the titanium implant can change continuously. The Drug Delivery Devices (DDDs) are polymer porous devices with pores for drug loading and release. These DDDs can be implanted or taken orally by the patient. The drug can be either released continuously or distributed in discrete alternating sections to achieve a pulse release. The pattern for drug release can be controlled by varying local porosity within the DDDs. Understanding of transport phenomena during the manufacturing processes is the key to achieving desired porosity and porous structure in the above examples. In addition, growth of new bone into the porous implant and release of drug from DDD are also dominated by the transport phenomena. A cell is a membrane-bound soup of water, proteins, lipids and nucleic acids that grows, reproduces and interacts with its environment. Cells are regarded as the basic organizing unit of life and cell biology plays essential roles in human disease, such as bacterial infections and cancer. Depending on the complexity of a cell, it can be either a prokaryote or eukaryote. While the former can be found in bacteria and only has a cell membrane and cytoplasm and no other organelles, the latter exists in plants, animals, fungi, and protista and has a number of different cell organelles (see Fig. 1.21; Peters, 2007). The cells are usually in thermal equilibrium and the transport phenomena encountered in the cells are mainly momentum and mass transfer. While most existing models treat a cell as a well-mixed homogeneous system, cell heterogeneity as indicated by Fig. 1.21 is essential to capture many important phenomena at the cellular level. Due to barriers between biology and thermal sciences, the role and importance of mass transfer in information processing is not realized by biologists. To fully understand the role of transport phenomena at the cellular level, it is essential to develop more sophisticated models incorporating non-Newtonian mechanics, heterogeneities, time-dependence and stochastic fluctuations. Lasers have been widely used in medical applications for more than three decades. The majority of those applications involve thermal effects. For example, in laser hyperthermia, the temperature of a pathological tissue is often elevated to 42 ~ 45°C so that the growth of malignant tumor can be retarded. In laser coagulation, laser beams can cause immediate irreversible damage to pathological cells by heating them to above 60oC. In laser surgery, the laser beam can vaporize and cut tissues like a scalpel when tissue is heated to a temperature of as high as 100°C. No matter which treatment a doctor performs, a thorough understanding of the damage distribution within both the pathological tissue and the surrounding healthy tissue is imperative. During laser processing of pathobiological materials, such as ophthalmic microsurgery, laser lithotripsy, and angioplasty, plasma can be formed and it will interact with the laser beam. In such applications, a phenomenon called optical breakdown will occur as an extremely high intensity laser beam is focused onto the tissues. During the optical breakdown, a very high free electron density, i.e., plasma, on the order of 1018-1021 electrons/cm3 is produced. Two mechanisms are responsible for the optical breakdown: multiphoton absorption and avalanche ionization (also called cascade or impact ionization). Both mechanisms require a minimum threshold intensity before optical breakdown can be initiated. Once breakdown occurs, it leads to a rapid heating of the material in the focal volume, followed by its explosive expansion and the emission of a shock wave. The expansion of the heated volume further results in the formation of a cavity if it occurs in solids or of a cavitation bubble if it takes place in liquids. Heat and mass transfer during laser-pathobiological material processing play a dominant role and must be investigated (Zhou et al., 2008). Cryopreservation that uses liquid nitrogen to deep-freeze, and thus preserve, biological materials is another example that heat and mass transfer is dominant. Among the biological materials that can be preserved through this process are oocytes, embryos, tissues, and even entire organs. Preservation of embryos is necessary, for example, if couples have medical reason to delay their reproductive choice. Such situations may occur when a woman has to undergo a cancer treatment that may risk her subsequent ability to give birth to a healthy child. The doctor can preserve her oocytes or embryos to help her conceive after she has recovered. Cryopreservation of tissues (bones, tendons, corneas, heart valves, etc.) permits storage of deep-frozen biomaterials in a storage bank until they are needed for transplant. Recently, Wang et al. (2002) reported that transplant of a whole organ after cryopreservation and thawing is feasible. Cryopreservation of biological materials presents special challenges because both freezing and thawing can cause severe damage to the cells. In terms of heat transfer, cryopreservation involves solidification of multicomponent substances similar to those in food freezing, but the temperature is much lower (-196 C). The cells in the biological materials must function properly after freezing and thawing. For this reason, it is very important to control the cooling rate and prevent severe cell damage. The mechanisms of damage to cells in suspension and in tissues are different (Asymptote Ltd., 2002). For cells in suspension, cell death can occur if the cooling rate is too low; in such situations, cells are exposed to a hypertonic condition for a long period and they become pickled. However, if the cooling rate is too high, intracellular ice formation can occur. The mechanism of cell damage in tissue is different from that in suspension and is still not fully understood. The water in tissues can exist in two different forms: (1) a continuous liquid phase in a small extracellular compartment, and (2) non-continuous phases within the individual cells. The manner by which ice forms in the extracellular compartment of tissues, and the process of cellular dehydration, play significant roles in cell damage during freezing (Asymptote Ltd., 2002). Thawing is the opposite process of freezing biological materials. As with the freezing process, it is necessary to find the optimum heating rate to minimize the cell damage during thawing.