For a new field of research to thrive, there must be ways to make money from a partial product. In this way development can be incremental: make an advance and use revenues from it to fund the next step ahead. So it is for the development of artificial and tissue engineered organs, where one possible stepping stone involves improvements in cost and efficiency of testing new therapies: "Our artificial organ systems are aimed at offering an alternative to animal experiments. Particularly as humans and animals have different metabolisms. 30 per cent of all side effects come to light in clinical trials. ... The special feature, in our liver model for example, is a functioning system of blood vessels. This creates a natural environment for cells. We don't build artificial blood vessels for this, but use existing ones - from a piece of pig's intestine. ... All of the pig cells are removed, but the blood vessels are preserved. Human cells are then seeded onto this structure - hepatocytes, which, as in the body, are responsible for transforming and breaking down drugs, and endothelial cells, which act as a barrier between blood and tissue cells. In order to simulate blood and circulation, the researchers put the model into a computer-controlled bioreactor using a flexible tube pump ... This enables the nutrient solution to be fed in and carried away in the same way as in veins and arteries in humans. ... The cells were active for up to three weeks. This time was sufficient to analyse and evaluate the functions. A longer period of activity is possible, however."