At Tufts University in Medford, Mass., researchers loaded a tiny 3D model of the human brain into a plastic shell resting atop a spring-loaded platform. Inside this polymer skull, the donut-shaped ball of living brain tissue floated in a warm, salty bath, its neurons whispering to each other in the darkness. Then a piston struck the platform, whipping it back and forth, and sending the mini-brain sloshing. Days later, as the team assessed the damage, the results were stark. Some of the human bra...