Cardiopulmonary Implant (Localized Tissue)

    Over the last century, technology has allowed mankind to push the bounds of good health and old age further then ever. However, most of the care and technology to do this is centralized in large hospitals, or simply isn't available. What can be done for the unfortunate individuals who are injured when all alone, or those desperately needing an organ transplant? Sadly, it's very little. Someday remote health monitoring devices and organ cloning may be commonplace, but until then, the most promising technology lies in cybernetic implants. While not the simplest type of implant to build, the most useful for today could be a device that provides cardiopulmonary support to localized tissue. A closed loop, artificial cardiopulmonary system could, in theory, provide oxygen to poorly circulated portions of the body, severed limbs, and even keep the brain alive in heart attack victims. The basic idea of the cardiopulmonary system is simple: blood is circulated by a pumping mechanism (usually the heart) and is occasionally exposed to bond with oxygen (in the lungs). However, even in biological systems this is an extremely difficult and complex task. Is it even possible to build a small, implantable device that could provide some of the same functions?

    There are, of course many challenges to building any implantable devices. Generally, the most important component is a system to provide an adequate power supply to the implant. Researchers working on pacemakers and other devices have developed some novel power solutions, but the most commonly used are simple sealed batteries. If equipped properly, these batteries can be recharged inside the body through induction using an external charger or radio waves. Besides the simplicity offered by utilizing current battery technology, they are so widely used that batteries with special outputs can be manufactured without much trouble.

    A more elegant design, though it would take considerable development, is the option of using a bioelectric power source. Bioelectric energy is the energy produced by muscles and nerve cells. A simple bio-battery might be little more then a capsule of involuntary Cardiac muscle cells. "Not only is it involuntary, but also when excited, it generates a much longer electric impulse than does skeletal muscle, lasting about 300 ms" (Malmivuo 2.3). With many cells firing alternately, and connected properly, bioelectric power could drive a medical implant, or recharge another battery. Bioelectric power is appealing because the energy is continually produced inside the body.

    Another solution could use nuclear energy. In 2002, researchers at Cornell University developed a new type of battery that "may be capable of supplying power to remote sensors or implantable medical devices for decades." And because a prototype device can "convert energy from the radioactive material directly into mechanical motion..." it could produce "...rotary motion using a cam or ratcheted wheel" ("Atomic Battery"). Of course, one of the best perks about using nuclear energy is the battery life. The proposed atomic battery runs on nickel-63, a beta ray emitting radioisotope with a half-life of more than a hundred years.

    While a power source may be the most important part of any implant, the most difficult part of building a cardiopulmonary implant is avoiding complications arising from blood coagulation. The first Cardiopulmonary Bypass Machine (CBM), built by John Gibbon in 1937, killed most of the experimental animals through "haemolysis (clotting of blood), air embolism introduced into the body, and blood-foreign surface interaction. The problem of haemolysis was alleviated somewhat in 1939 when the anticoagulant Heparin was discovered" ("Bypass Machine"). However, Heparin would not be a viable option for an implant. If the implant is dormant, blood still needs to pass through it to reach other parts of the body. It would be unreasonable and dangerous to rely on medication to prevent coagulation over long periods of time because of side effects, and possible death if a dose was missed.

    To combat coagulation, it's important to understand why it occurs. Coagulation is caused by blood vessel injury. When this happens, "platelets adhere to macromolecules in the subendothelial tissues and then aggregate to form the primary hemostatic plug" ("Blood Coagulation"). Coagulation also occurs when coagulation factors in blood plasma comes into contact with tissue factor (lipoproteins on cells not normally in contact with plasma) and abnormal environmental conditions. Basically, this means blood won't coagulate as long as it stays in blood vessels. The solution then, is to model the parts of the implant in contact with blood after blood vessels.

    Large blood vessels, like the aorta, are composed of many different layers of tissue and cells that would be very difficult to recreate in a lab. Most of this material though, is support mass that helps keep the vessel strong and rigid. In an implant, plastics could easily accomplish the same result. Since strength of the blood vessel isn't an issue, the simplest blood vessels, capillaries, can be used as a model. "Capillaries, intermediate between arteries and veins, are composed of only the endothelium and a sparse basal lamina" (Marieb 719). The endothelium is a layer of cells that line the inside of blood vessels. "The basal lamina is a thin sheet of proteoglycans and glycoproteins, especially laminin, secreted by cells as an extracellular matrix" ("Gene Ontology Browser"). This suggests that coagulation problems could be solved by coating the inside of the implant with endothelial cells. These cells would presumably form their own extracellular matrix by secreting proteins (and thereby creating the basal lamina). The extracellular matrix is an important piece of the coagulation issue. Various molecules are produced there that regulate the viscosity of the blood, as well as cellular adhesion molecules that bind cells together.

    Dr. William J. Federspiel of the McGowan Institute for Regenerative Medicine is conducting similar research to build an artificial lung. In his research, "The blood flow pathways currently being designed which will provide a natural surface that minimizes problems associated with blood clotting, are being developed through various tissue engineering techniques which will line the blood flow channels with endothelial cells." In both Dr. Federspiel's lung and the proposed cardiopulmonary implant, the blood flow channels will need to bond somehow with the endothelium. Currently, research into blood vessel grafts and scaffolds (to help regrow blood vessels) have yielded several materials with these properties. Ideally, they should bond with the cellular adhesion molecules in the extracellular matrix, creating a very strong molecular bond.

    With coagulation problems under control, the next important component of the implant is the pump. While the human heart must pump large quantities of blood, non-stop for long periods of time, the proposed implant will only have to move small quantities of blood for a few hours. Thus, a much less robust and simple pumping system can suffice. While almost any pump could move blood through a tube, many would damage blood cells in the process. However, peristaltic pumps are gentle enough to prevent damage to cells, and are often used in blood banks and CBMs. Peristaltic pumps work by squeezing a tube with rollers. These are attached to a simple electric motor by three or more arms extending from a central axis. However, if something similar to the prototype nuclear battery is used, an electric motor would be redundant, as the battery itself could provide the rotary motion.

    With a working pump, the only major component remaining is an air exchange system. Somehow, the implant needs to pull air inside the body and expose the oxygen to blood, while expelling carbon dioxide. If the implant was specially installed, air could be drawn with an attachment from the lungs or perhaps the trachea. Obviously, this could only work for an implant installed near the lungs or throat, so a more general solution is desired. For implants installed elsewhere, a small hole would have to be cut through the surface of the skin. However, not only would this be unsightly, but a permanent hole in the skin could pose a high risk of infection. Therefore, the best way to get air to the implant seems to be a pair of telescopic needles.

    While the implant is inactive, the needles would reside underneath the skin. Then, once the implant is activated, the needles would penetrate through the skin using a simple hydraulic system, or perhaps a gear and screw model. Of course, these needles will have to be large enough to allow the proper amount of airflow (roughly 750 cubic cm/min), so a fair estimate might be around the diameter of a drinking straw. To ensure body tissue won't clog the airway, the needles would not have a hollow tip, but instead have several vertical slits along the side. Body tissue, dust and other contaminants would be further blocked by a filter covering the slits from the inside. The filters could be replaced periodically to increase the lifespan of the implant.

    With an open airway into the body, a mechanism for pulling the air inside is still needed. Certainly a fan or pump could work, but additional hardware increases the size, complexity and power requirements of the whole implant. A better solution is to reuse the peristaltic pump already in place. The tubes the peristaltic pump must squeeze would be constructed of a material that can bond with the endothelium, and is also gas permeable. If an airtight shell was built around the pump, and the space between each arm of the pump was airtight, the revolving action would be enough to pull air inwards. One of the needles would simply have to pierce a section of the shell to supply air, while another would pierce the shell 90 or 120 degrees (depends on how many arms the pump has) counter the direction of rotation to expel the used air. The air would be trapped between the arms of the pump and the tubes, and because the tubes are gas permeable, oxygen would be free to bond with the blood.

    The shell containing the peristaltic pump and gas exchange area would also be convenient for housing the rest of the implant components. This would include some simple electronics and a radio transceiver for controlling pump speed, needle position, and a blood/oxygen sensor. If configured for emergency cardiopulmonary support, data from a blood/oxygen sensor would be very important. If too little oxygen is in the blood, the pump could be instructed to operate at a different speed. Many commercial devices can measure the amount of oxygen in blood, but they all work on pretty much the same principals. A sensor shines red and infrared light through a blood vessel and detects the fluctuating signals caused by blood flow. The ratio of the fluctuation of the red and infrared light signals is used to calculate the blood oxygen saturation of hemoglobin.

    Depending on the use of the implant, and where it was installed, simple modifications to this electronics package could greatly expand its functionality. A very useful addition might involve adding some sort of network support. While the electronics package allows each implant to compute sensor signals, interpret data, and make flow changes, network support would allow multiple implants to work together and exchange information. A single implant would be sufficient for a small area of the body, but support for the head or an entire limb would require several implants working together in unison.

    Without a doubt, considerable time, funding and testing would be required to bring a product like this to market. However, there are no major technical issues here that lack a solution. Power isn't a problem because standard batteries and nuclear energy have already been developed. Coagulation can be dealt with by using endothelial cells, and peristaltic pumps have been in use for decades. Undoubtedly, this implant could save lives, and the technology developed to produce it could be applied to more complex and specialized tasks. By taking this first step, cybernetic implants could soon be in use to assist or replace failing organs, augment the human body, and possibly extend the working lifespan by many years.

Malmivuo, Jaakko and Robert Plonsey. Bioelectromagnetism: Principles and Applications of Bioelectric and Biomagnetic Fields. Oxford: Oxford University Press, 1995

"Atomic Battery Could Power Medical Implants." Medical Device & Diagnostic Industry. December 2002

""Internal" Workings of the Cardiopulmonary Bypass Machine".http://www.cheresources.com/cardiopul.shtml 16 Feb. 2004.

"Blood Coagulation". http://tollefsen.wustl.edu/projects/coagulation/coagulation.html 18 Feb. 2004

Marieb, Elaine N. Human Anatomy & Physiology. 5th Edition. Boston: Benjamin Cummings, July 2000

Gene Ontology Browser. http://www.informatics.jax.org/searches/GO.cgi?id=GO:0005605 2 Mar. 2004

Extracorporeal Organ Support

Nutrient Source
Holding Tank
Monitoring Equipment; solution analysis to detect bacterial infection, temperature and pressure control.

Limb Growth and Reconstruction

Limb growth would be possible using a Cardiopulmonary Implant and the Extracorporeal Organ Support hardware described above.

Bioelectric Energy