Many tissues, such as blood vessels and bone, come under mechanical stresses and strains that are thought to influence patterns of growth and development. Tissue engineers hope to replicate this environment-sensitive nature in the scaffolds on which they construct new tissues. David Mooney and colleagues at the University of Michigan have achieved this goal in part, building a biomatrix that releases a growth factor in a reversible fashion when compressed (Nature 408, 998–1000, 2000). The team used an alginate hydrogel loaded with vascular endothelial growth factor (VEGF) and placed this under repeated mechanical stress. The stressed biomatrix induced a greater density of blood vessel growth in two different animal model than the unstressed biomatrix, suggesting that cyclical VEGF release was more effective at stimulating neovascularization in vivo. The next challenge, says Mooney, will be to test the system in a more physiological set up—such as a biomaterial to heal bone fractures. In the longer term, tissue engineers will need to develop biomaterials that can release multiple growth factors in physiologically relevant patterns orchestrating normal growth.