Safety evaluation is a critical part of drug and cosmetic development, but experimental considerations and tighter regulations require alternatives to animal testing. Now scientists have partnered to create a liver-on-chip device mimicking human physiology, with liver organs less than a millimeter in diameter that survive for more than a month. By adding nano-based optoelectronic sensors, the group identified a new mechanism of acetaminophen (Tylenol) toxicity using this human-on-a-chip technology.