Scientists at Sungkyunkwan University in South Korea developed a bone-healing 'gun' that 3D-prints biodegradable scaffolds directly onto fractures. The device extrudes a mix of polycaprolactone and hydroxyapatite, sometimes with antibiotics, to support bone growth. It melts special polymer "bullets" at 60°C, cool enough to protect tissue while forming custom-fit framework for new bone growth.