Zhu, a chemist at the University of Montreal, hit upon the idea while developing a bile-acid-based gel for tissue repair. He found that combining modified bile acids with silica created a hard plastic—perfect for patching broken pearly whites. The bile plastic matches the durability of plastic composite fillings and is more resistant to cracks. And it doesn't leach mercury, as many commonly used metal fillings do. Bile occurs naturally in the body, so it wouldn't cause any harm if the fillings were to decompose over time.