Jesse Hamilton, Department of Chemical Engineering

Introduction

Many people in today’s society depend on devices that contact their blood. Devices such as dialyzers and blood oxygenators are used every day in hemodialysis or coronary bypass. More permanent fixtures are also used such as artificial kidneys and hearts. These devices are wonderful and have contributed to longer and fuller lives for many people. However, man’s attempt to simulate the functions of the human body are simple at best, and there are serious problems that arise. These problems are tolerated by the medical profession at present because (1) they are not completely understood, and (2) the risks posed by the problems are less serious than the condition that is being corrected by the device. This project attempted to evaluate the effects of different materials on human blood.

Suvery of the Problem and Experiment

W hen blood contacts artificial materials, a process begins that changes the composition of the blood. As the blood contacts a material, such as happens as blood flows through a small diameter tube, particles in the blood begin to aggregate on the side of the tube. As the platelets aggregate on the side of the tube, more and more platelets collect on the initial deposit, and a small embolus forms. As the blood continues to flow past the embolus, the particle gets larger, and the platelets become more tightly packed and the particle breaks free from the side and flows in the blood. The particle can be 20-50 11m in diameter. The case is not just the development of one embolus, but the development of many emboli. This may seem small and insignificant, however, the capillaries are only 8 11m in diameter. Thus, the emboli can block the capillaries and this causes problems. The problems that result range from decreased oxygen transfer to the tissues to blood clots, strokes and death. At present, few have studied the effects of microemboli, and the only person known to this author to have a method of measuring microemboli concentrations is Dr. Ken Solen of the Chemical Engineering Department of Brigham Young University.

This project attempted to evaluate the tendency of three different biomaterials to produce microemboli when contacted with blood. The three materials were polyvinyl chloride (PVC), siloxane®, and polyurethane. The materials were tested using small diameter (1116 inch) tubes. The materials were tested by using the method developed by Dr. Solen. First, blood from a human donor was collected in the same type of blood bags used by the Red Cross. One and a half units heparin per milliliter of blood was injected into the blood bag before the blood was collected. This anticoagulant is one that is used throughout the medical community to prevent blood coagulation. Once the blood was collected in the bag, the bag was manually massaged to ensure that the heparin was well mixed within the blood. The blood bag was then placed in a sealed box that was submerged in a water bath maintained at 37″ C. A syringe pump was used to pump water into the box and around the blood bag. This forced the blood to be driven out of the blood bag and through a one meter length of small diameter tube of the material that was being tested. The blood was collected in a 5 ml syringe and then tested using a device that drives the blood through a filter at constant pressure. The 15 Jlm filters that were used are small enough the catch the larger microemboli that are in the blood. As the blood is forced through the filter, an initial amount of microemboli block the filter, but as the pressure drives the blood through. some of the “soft” initial microemboli are broken down and the pass through the filter. The flow rate of the blood through the filter was measured, and a resulting plot of the natural log of the flow rate versus time yielded a measure of the concentration of microemboli in the blood. Control samples of the blood from the bag before the experiment began and also after the experiment was completed, were taken to determine the affect of the blood sitting in the bag. The results were then evaluated.

Results and Discussion

Three experiments per material were planned. However. only five experiments were performed in total. Three experiments using PVC and two experiments using siloxane were the only experiments performed. The data resulting from each experiment were evaluated after each run. The results indicated that the amount of the microemboli resulting from contact of the blood with the tube was dominated by microemboli resulting from the bag. In other words. the bag produced microemboli in quantities that were large enough to negate the effects of the tubing. Even though the sealed box containing the blood bag was turned and moved often, the blood still collected on the sides and produced emboli in quantities large enough to make the data resulting from the tube statistically insignificant.

This discovery was disappointing because the expectations of this experiment were that one material would behave differently than others. It was hoped that this experiment would lead to the ability to evaluate the quality of different biomaterials so that the best material could be identified for future use in the medical industry. and that a decrease in problems related to the use of artificial materials would decrease and the quality of health care would increase.

Conclusion

This project has validity and will continue to be important. The major obstacle is the development of a model that does not require a reservoir to hold the blood before it is driven through the tube. A model is needed that will allow blood to be taken directly from the donor and driven through the tube without the need of collecting the blood in a bag or syringe first. This will allow for the elimination of the dominating effect of the blood bag. and it will also allow for a experiment that better simulates the actual situation that happens when blood is contacted during dialysis or in devices that process blood during surgeries or in artificial organs.

The most important certifications that come from the experiments performed are: (I) there really is something taking place in the blood when it contacts an artificial surface, and (2) there is a method that is able to measure the amount of microemboli that are in the blood. This work will continue and will make a significant contribution to the medical community in the coming years. This work will someday allow for artificial hearts and other organs to be used with greater confidence and allow the patients greater freedom and greater mobility in their day to day lives and activities.