Blood substitutes (also called artificial blood or blood surrogates or synthetic blood) are substances that fulfil some functions of biological blood, especially in humans. They aim to provide an alternative to blood transfusion.
Currently there are two types:
v Volume expanders: they are inert and merely increase blood volume.
v Oxygen carriers: they mimic human blood’s oxygen transport ability. Oxygen substitutes are in turn broken into two categories based on transport mechanism: per fluorocarbon based oxygen carriers (PBOC), and haemoglobin based oxygen carriers (HBOC).
Volume expanders are widely available and are used in both hospitals and first response situations by paramedics and emergency medical technicians.
Oxygen carriers are in clinical trials in the U.S. and Europe. Some are already in use.
For example, an HBOC called Hemopure is currently used in hospitals in South Africa, where the spread of HIV has threatened the blood supply. A Per fluorocarbon-based (PFC-based) oxygen carrier called Oxygent is in the late stages of human trials in Europe and North America.
Oxygen carriers work primarily through passive diffusion. Passive diffusion takes advantage of gasses’ tendency to move from areas of greater concentration to areas lesser concentration until it reaches a state of equilibrium. In the human body, oxygen moves from the lungs (high concentration) to the blood (low concentration). Then, once the blood reaches the capillaries, the oxygen moves from the blood (high concentration) to the tissues (low concentration).
PBOC vaguely resemble blood. They are very dark red or burgundy and are made from real, sterilized haemoglobin, which can come from a variety of sources:
v modified from real, expired human blood
v RBCs from cow blood
v Genetically bacteria that can produce haemoglobin
v Human placentas
HBOCs work much like ordinary RBCs.
The molecules of the HBOC float in the blood plasma, picking up oxygen from the lungs and dropping it off in the capillaries.
The molecules are much smaller than RBCs, so they can fit into spaces that RBCs cannot, such as into extremely swollen tissue or abnormal blood vessels around cancerous tumours.
Most HBOCs stay in a person’s blood for about a day – far less than the 100 days or so that ordinary RBCs circulate.
However, HBOCs also have a few side effects.
The modified haemoglobin molecules can fit into very small spaces between cells and bond to nitric oxide, which is important to maintaining blood pressure.
This can cause a patient’s blood pressure to rise to dangerous levels.
HBOCs can also cause abdominal discomfort and cramping that is most likely due to the release of free radicals, harmful molecules that can damage cells. Some HBOCs can cause a temporary, reddish discoloration of the eyes or flushed skin.
Unlike HBOCs, PFCs are usually white and are entirely synthetic.
PFCs are chemically inert, but they are extremely good at carrying dissolved gasses.
They can carry between 20 and 30 % more gas than water or blood plasma, and if more gas is present, they can carry more of it. For this reason, doctors primarily use PFCs in conjunction with supplemental oxygen.
However, extra oxygen can cause the release of free radicals in a person’s body.
Researchers are studying whether PFCs can work without the additional oxygen.
PFCs are oily and slippery, so they have to be emulsified, or suspended in a liquid, to be used in the blood. Usually, PFCs are mixed with other substances frequently used in intravenous drugs, such as lecithin or albumin.
These emulsifiers eventually break down as they circulate from the blood. The liver and kidneys remove them from the blood, and the lungs exhale the PFCs the way they would carbon dioxide. Sometimes people experience flu-like symptoms as their bodies digest and exhale the PFCs.
PFCs, like HBOCs, are extremely small and can fit into spaces that are inaccessible to RBCs. For this reason, some hospitals have studied whether PFCs can treat Traumatic Brain Injury (TBI) by delivering oxygen through swollen brain tissue.
Ø USES AND ADVANTAGES:
Pharmaceutical companies are testing PFCs and HBOCs for use in specific medical situations, but they have similar potential uses, including:
v Restoring oxygen delivery after loss of blood from trauma, especially in emergency and battlefield situations
v Preventing the need for blood transfusions during surgery
v Maintaining oxygen flow to cancerous tissue, which may make chemotherapy more effective
v Treating anaemia, which causes a reduction in red blood cells
v Allowing oxygen delivery to swollen tissues or areas of the body affected by sickle-cell anaemia
Artificial blood has a longer shelf life than human blood. Since the manufacturing process can include sterilization, it doesn’t carry the risk for disease transmission. Doctors can administer it to patients of any blood type.
In addition, many people who cannot accept blood transfusions for religious reasons can accept artificial blood, particularly PFCs, which are not derived from blood. The artificial blood is O-negative, which can be used on all patients, regardless of their blood type.
Artificial blood may be in widespread use within the next several years.
The next generations of blood substitutes will also probably become more sophisticated. In the future, HBOCs and PFCs may look a lot more like red blood cells, and they may carry some of the enzymes and antioxidants that real blood carries.
Why Artificial Blood?
There is an immense need for a substance that could replace human blood. The reasons for which blood substitute is in /need right now are:
v Donations are increasing annually throughout the world, but demand is climbing much more rapidly as an aging population requires more operations that often involve blood transfusion.
v The blood supply is not always very safe in many regions of the world. Blood transfusion is the second largest source of new HIV infections in Nigeria. In certain regions of South Africa as much as 40% of the population has HIV/ AIDS, and thorough testing is not financially feasible. A disease-free source of blood substitutes would be incredibly beneficial in these regions.
v In battlefield scenarios it is often impossible to administer rapid blood transfusions. Medical care in the armed services would benefit from a safe, easy way to manage blood supply.
v Great benefit could be derived from the rapid treatment of patients in trauma situations. Because these blood substitutes do not contain any of the antigens that determine blood type, they can be used across all types without immunologic reactions.
v There is no practical way to test for prior transmitted diseases in donated blood, such as Mad Cow and Cruetzfeld- Jacob disease, and other diseases could emerge as problems for the blood supply, including Smallpox and SARS. Blood substitutes could eventually improve on this.
v Transfused blood is currently more cost effective, but there are reasons to believe this may change. For example, the cost of blood substitutes may fall as manufacturing becomes refined.
v Blood substitutes can be stored for much longer than transfused blood, and can be kept at room temperature. Most haemoglobin-based oxygen carriers in trials today carry a shelf life of between one and three years, compared to 42 days for donated blood, which needs to be kept refrigerated.
v Blood substitutes allow for immediate full capacity oxygen transport, as opposed to transfused blood which can require about 24 hours to reach full oxygen transport capacity.