The bone marrow is a highly complex tissue, in which the blood- and the bone-forming system are closely intertwined. Hematopoietic stem cells (HSCs) are at the root of blood formation and give rise to all different types of blood cells including erythrocytes and immune cells. Nowadays, HSCs are the only stem cells routinely applied in the clinics to treat patients that suffer from hematological disorder such as leukemia. Therefore, being able to manipulate or multiply HSCs in vitro is highly desirable from a clinical point of view.
In vivo the action of HSCs is tightly controlled by their local microenvironment – the so-called stem cell niche in the bone marrow. This niche provides all the signals that HSCs need in order to maintain their stem cell properties. These signals include not only biological signals such as growth-factors, cell-cell contacts or cell-matrix interactions but also physical stimuli such as matrix stiffness, nanopatterning or the three-dimensional architecture of the environment. During the last years, we could show that each of these parameters influences HSC behavior. At the same time we found, however, that mimicking one parameter is not enough to gain control over HSC proliferation and differentiation. This finding is not surprising as the bone marrow composition and functions are complex and this complexity would have been lost during evolution if not necessary. Therefore, in our current studies we aim at increasing the complexity of bone marrow-mimicking biomaterials by integrating biological and physical stimuli into the systems. With the help of these multi-functional, biomimetic materials we are able to foster HSC expansion and, thereby, to do another step towards the in vitro multiplication of HSCs.