In recent years, America's blood shortage has propelled the biotechnology of blood substitutes. Artificial blood does not contain the plasma, red and white cells, or platelets of human blood, but functions to transport and deliver oxygen to the body's tissues until the recipient's bone marrow has regenerated the missing red blood cells. Current blood substitutes are either hemoglobin-based oxygen carriers (HBOCs) or perfluorocarbons (PFCs). While HBOCs utilize hemoglobin, an actual component of red blood cells, PFCs rely solely on synthetic chemical processes. Like most technological advances, there are still a number of advantages and disadvantages to consider. In the short term, the prospective benefits of a blood substitute overshadow the shortcomings. In addition to carrying oxygen, such compounds can be sterilized against infectious diseases and used in patients whose religious beliefs prevent them from accepting blood transfusions.
The Crisis Emerges
The word "blood" evokes vivid images of pain and fear. Despite its associations with injury and suffering, blood is inextricably linked with human life. A baby is born with only a cup of blood, yet the average adult body contains ten pints, or twenty cups, of the life-preserving liquid.
Unfortunately, one out of every three individuals will at one point in life not have enough blood to sustain his or her life. For this reason, blood is needed every three seconds, but only five percent of the American population donates blood (American Blood Centers). These statistics evoke a chilling and frightening reality. What happens when the blood supply, the river of life, runs dry? After the attacks on September 11, 2001, over 500,000 Americans responded to the need for blood donations (American Blood Centers). Scientists and engineers are replying to the same need, developing a blood substitute to combat the threat of catastrophic blood shortage.
The Search Begins
In response to impending blood shortages, scientists and engineers have begun a quest to discover an ideal blood substitute. Within the human body, blood's two primary functions are the transportation of oxygen to various tissues and the removal of carbon dioxide from the body (Campbell 811). Therefore, a blood substitute need not be a direct, identical replacement for blood. It is instead designed to imitate the oxygen-carrying capacity of the red blood cells. Artificial blood will only serve as a temporary substitute until the recipient's own body has enough time to reproduce the necessary blood cells to compensate for lost blood (Yen). The two principle categories of blood substitutes are known as hemoglobin-based oxygen carriers (HBOCs) and perfluorocarbons (PFCs).
Before one can completely understand the underlying scientific and engineering principles of blood substitutes, he or she must have a basic knowledge of the interaction between oxygen and the blood. In all red blood cells, a molecule known as hemoglobin binds oxygen from the lungs and carries it to the tissues of the body. Structurally, hemoglobin consists of two alpha and two beta chains.
Hemoglobin is often referred to as a tetramer, since it is a molecule composed of four units (Chang, "Red Blood" 601). Each of the alpha and beta chains binds to a heme group, which contains iron. The heme group is responsible for attaching oxygen to the red blood cell (Squires 1004). A cofactor called 2,3-diphosphoglycerate (2,3-DPG) is located in all red blood cells, and without it, the hemoglobin could not readily release oxygen to the body's tissues (Chang, "Red Blood" 602). However, the outward simplicity of hemoglobin belies its true complexity, for the molecule plays a complicated role in maintaining human life.
Hemoglobin retains its capacity to bind to oxygen even when it is outside of a red blood cell (Squires 1002). Therefore, the tetrameric molecule has served as a focal point in the search for a blood substitute. Unfortunately, the answer is neither simple nor apparent, for hemoglobin must be modified before it can be introduced into the body's circulatory system. This is the consequence of several factors. Hemoglobin is extracted by removing the red blood cell membrane, forming stroma-free hemoglobin (Yen). However, the removed hemoglobin lacks 2,3-DPG, which limits the hemoglobin's ability to deliver oxygen to tissues (Squires 1004). In essence, the hemoglobin will bind to oxygen and transport it throughout the body, but without the help of 2,3-DPG, it holds onto the oxygen molecule. An additional problem develops when pure hemoglobin enters the blood stream. The tetramer of four units is rapidly degraded into dimers of two units. The smaller subunits contribute to renal (kidney) toxicity as they are rapidly excreted by the kidneys (Chang, "Red Blood" 602). While pure hemoglobin intuitively appears to be the ideal blood substitute because of its affinity for oxygen, it must be modified before that concept can become a medical and scientific reality.
The word "blood" evokes vivid images of pain and fear. Despite its associations with injury and suffering, blood is inextricably linked with human life. A baby is born with only a cup of blood, yet the average adult body contains ten pints, or twenty cups, of the life-preserving liquid.
Hemoglobin is often referred to as a tetramer, since it is a molecule composed of four units (Chang, "Red Blood" 601). Each of the alpha and beta chains binds to a heme group, which contains iron. The heme group is responsible for attaching oxygen to the red blood cell (Squires 1004). A 
