Blood is a particularly tricky fluid to handle. There are primarily four reasons for this: coagulation, haemolysis, availability and behaviour. The challenges associated with each of these items are highlighted below.
Coagulation. When blood is outside the body, it will coagulate at every opportunity. Coagulation is the complex process whereby blood forms clots; this can be triggered by everything from exposure to a foreign solid or gaseous interface and shear forces to low flow rates and stagnation. Without the use of anticoagulants, such as heparin, or specialty coatings on the parts to prevent or reduce contact activation, blood clots can form in the device and prevent it from operating properly.
Haemolysis. This term describes the rupturing of the red blood cells, causing the contents to spill out into the surrounding fluid. Haemolysis can be caused by mechanical stresses, such as pressure changes, turbulence and shear forces on the blood cells. The product specification should clearly state the acceptable limits of haemolysis, especially if the blood is to be returned to the patient. Careful design of the fluid circuit—choosing the correct tubing bores and flow speeds, avoiding step changes in tubing diameters and carefully choosing the pumping techniques—can help reduce haemolysis.
Availability and handling. In the UK, human blood is available on application from the National Blood Service for the purposes of approved research and development. The blood donations are screened, but it is still possible that blood-borne viruses are present, so precautions should be taken to ensure safe handling. This resource is invaluable for testing systems during development, but again, caution needs to be taken in analysing the results of the tests. If the system has to work without anticoagulants, bear in mind that the bagged blood will have anticoagulants present for storage purposes. For example, blood parameter sensors integrated into the fluid circuit can behave differently depending on the anticoagulant used.
Behaviour. Blood is a multiphase, non-Newtonian, pseudo-plastic fluid. As such, its flow behaviour can be difficult to predict and model. In our experience of developing systems that handle various amounts of blood, this is why we always recommend rolling up your sleeves in the lab at the earliest opportunity.