They may look like Play-Doh, but the colorful, spongy rings of goo (left) are alive and may one day be able to learn. The rings are engineered to mimic the structure and function of the six layers of human cortical brain tissue. Scientists coaxed neurons (right) to grow around stiff, porous matrices made of silk proteins immersed in collagen gel. Then, they colored the layers with food dye and pieced them together like a jigsaw puzzle. By tweaking the size and orientation of matrix pores, researchers attempted to emulate variations of cellular structure and function in a real cortex. Unlike flat neuron cultures grown in petri dishes, the structure provides cells with something to cling to as they branch out and make connections, forming complex, 3D networks that more closely mimic real neural circuits, the authors say. The rings live longer than other models; researchers hope to keep the neuronal sponge alive for at least 6 months. Already, researchers are using them to study how neural networks respond to drugs and heal after various insults, such as disease or a traumatic injury, they report online today in the Proceedings of the National Academy of Sciences. Eventually, they hope to study whether these neural circuits alter their activity in response to experience, a basic form of learning.