Material scientist Andreas Hermann , orthodontist Yijin Ren and their colleagues have made a 3D printing substrate which kills bacteria on contact. The first applications will be in dentistry, but other implants may follow. Their results were recently published in the journal Advanced Functional Materials.

'The director of Kolff, who is head of the Orthodontics Department, asked me if I could come up with an antimicrobial dental glue', Herrmann explains. Kids with braces have small metal blocks glued to their teeth, and these are an ideal breeding ground for the microbes that cause tooth decay. 'So when I saw all sorts of 3D printed objects for use in dentistry in her office, I said: why not incorporate the antimicrobials in 3D prints?' The results were published after some two-and-a-half years of work.

In dentistry, it is standard practice to work with materials that polymerize under UV light. Herrmann took monomers which are routinely used, and set out to add what are known as quaternary ammonium ions. These positively charged molecules interact with the negatively charged bacterial membrane and puncture a hole in it, killing the microbes.

The scientists used two approaches to make a printable antimicrobial material. In the first, they mixed two different monomers and an additional quaternary ammonium compound with a polymerizable unit and used UV light to polymerize the whole mixture. But some antimicrobials could still leach out of the polymer mesh.

In the second approach, they first polymerized the antimicrobial groups to form long chains. The resulting antimicrobial polymer was added to the 3D printing fluid, and became entangled with the other polymers during polymerization. Here only very little antimicrobial material diffused out.

'The trick in both approaches was to get the mixture right to enable 3D printing and minimize any leakage of the antimicrobials. You don't want them to enter the mouth and thus the intestines, where they could kill off gut microbes', Herrmann explains.

In the end, they succeeded. Herrmann: 'We have tested printed objects with saliva. All the components are already being used in humans, but more tests are needed before we can bring these 3D antimicrobials to the market.' The first applications will probably be in orthodontics, where 3D printed retainers and aligners are already in use. In the longer run, 3D printed crowns with antimicrobial properties could be an option.

The use of antimicrobials could solve a major problem in dentistry. 'Any artificial objects in the mouth can be colonized by bacteria', Herrmann explains. In the US alone, this causes billions of dollars in dental costs each year. But the process of 3D printed antimicrobial medical devices has even wider applications. 'All implants in medicine suffer from biofilm formation, so giving them antibacterial properties would be beneficial.'