Most guns are tools for doing harm, but a team of American and Korean scientists has developed one that does the opposite, helping to patch up bone injuries. It comes a bit short of the Medigun from Team Fortress 2 or Ana’s Biotic Rifle, which featured in Overwatch. But it’s probably one of the first shots we have at making healing guns real. 3D printing on the fly In more complex bone problems like severe, irregular fractures or resections done as part of bone cancer treatment, the bone won’t heal on its own. The most common means of stabilizing the injured site and making recovery possible is metal-based grafts, implants usually made with titanium alloys. The problem with such implants is that they are difficult and expensive to manufacture, and it’s very hard to make them patient-specific. “3D printing has been highlighted as a novel approach to make such personalized implants, but this also requires substantial time and money,” said Jung Seung Lee, a biomedical engineering researcher at the Sungkyunkwan University in Korea. So his team wanted to find a way to make bone implants that would be faster and cheaper than a 3D printer. What they came up with was a modified glue gun. The idea was to make the implant right at the injured site during surgery. The surgeon would point the bone-healing gun at the fractured bone, pull the trigger, and create a stabilizing scaffold by extruding a filament that would solidify in the fracture and hold the bone together. “It was basically a tweaked commercially available hot glue gun. We modulated the temperature, and by adjusting the tip module, we could control the resolution of the extruded scaffold,” Lee said. Coming up with the gun design, though, was the easy part. The hard part was figuring out the ammo. Bone-healing bullets A hot glue gun is usually loaded with a solid stick of adhesive that is melted in the device and extruded through the tip. Making a stick that could heal bones was quite a challenge for several reasons. First, there was the temperature problem. Most adhesive sticks need to heat up to well over 100° Celsius before they start to melt. Extruding anything that hot right onto living tissue would cause a lot of damage. Second, once the material solidified, it needed to have mechanical properties comparable to a natural bone. Third, Lee’s team wanted their scaffolds to degrade over time and be replaced with regrowing bone tissues.