Most studies of human brain development rely on animals or on brain slices taken after death; both are useful but have limits. Brain models have been mostly two-dimensional or made with a three-dimensional gel, said Rosemarie Hunziker, program director of tissue engineering and biomaterial at the National Institute of Biomedical Imaging and Bioengineering, which funded Dr. Kaplan’s research. None of those systems replicate the brain’s gray or white matter, or how neurons communicate, Dr. Hunziker said. “Even if you get cells to live in there, they don’t do much,” she said. Dr. Kaplan’s team found that a spongy silk material coated with a positively charged polymer could culture rat neurons, a stand-in for gray matter. By itself, silk did not encourage neurons to produce axons, branches that transmit electrical pulses to other neurons. The researchers formed the silk material into a doughnut and added collagen gel to the center. Axons grew from the ring through the gel — the white matter substitute — and sent signals to neurons across the circle. They got “these neurons talking to each other,” Dr. Hunziker said. “No one’s really shown that before.” By adding nutrients and growth factors, scientists kept the brainlike tissue alive in an incubator for two months, at which point they experimented on it. Adding a neurotoxin essentially killed the neurons, as it would in a real brain. To simulate traumatic brain injury, they dropped weights from different heights.