Introducing Antibacterial Properties Into Additive Manufactured Parts

Challenge

We talk a lot about the benefits of Additive Manufacturing, in particular for complicated geometries, but in some cases, this can also lead to unexpected issues!

With a global need to reduce reliance on antibiotics many products, ranging from those in high contact areas such as door handles through to invasive medical devices, benefit from some form of antibacterial protection to prevent or reduce bacterial transmission from surfaces.  

However, the more complex these products are (and in particular those containing internal geometries) the more difficult standard procedures such as cleaning, sterilisation or anti-bacterial coating become.

Solution

Our approach to this problem was to investigate the possibility of introducing antibacterial properties into Additive Manufactured parts, in this case focusing on the polymer Laser Sintering process, in order to produce parts with inherent protection.  By including this protection throughout the parts rather than just at the surface, we could also provide a safeguard against scratches and other surface damage. 

For those who aren’t familiar with it, the process works by selectively fusing cross-sections of a polymer powder layer with a CO2 laser.  It’s also great at producing complex internal geometries due to the lack of need for support structures; ‘loose’ powder in each layer acts to support subsequent overhanging areas.

Our approach was to take a Nylon-12 powder, one of the most common and well-understood Laser Sintering polymers, and mix it with a silver-based additive (in this case a silver phosphate glass).  We’ve known of silver’s antibacterial properties for millennia and it’s already used in plenty of antibacterial applications, so this was a good starting point for our work.

Engineering properties of parts (a) Photograph of a selection of parts made from PA2200 (left) alongside the 1% B65003 composite material (right). (b) Raw stress-strain curves from tensile testing. (c) A comparison of Young’s modulus (E), ultimate tensile strength (σ_uts) and elongation at break (ε_max) for both materials.

On any research project, we tend to work through some specific questions to help steer us; in this case, the following are some of the key questions which defined our approach:

• Does the mixture of polymer and silver-based additive work within the Laser Sintering process?  Some materials work better than others within the process, so this is an essential starter question for us in materials-related research.
• If it does work, does it affect our mechanical properties?  The ideal situation is one in which we can obtain the same mechanical performance but with the increased functionality we’re looking for, but it doesn’t always happen that way!
• Do our parts harm human cells?  If we’re talking about surface contact, we need to be sure that the parts we make aren’t toxic to the people who might come into contact with them…
• Do we see any antibacterial effect?  This is the most important question of all – does our approach actually lead to the antibacterial protection we think it should??

Results

Thankfully, the answers to the first of our research questions were positive!  The materials we selected worked well within the Laser Sintering process and produced parts with extremely similar mechanical properties.  Various tests also showed that the silver-based additive was distributed well throughout the parts themselves, and was behaving in the way we expected.

False-colour image of bacteria on a Laser Sintered part (image by Tom Paterson)

Cytotoxicity tests showed no significant differences between parts produced with and without the additive, which just left us needing to know how the parts performed in terms of antibacterial protection.

This is where things became really interesting because we found our approach was more effective in different conditions.  In particular, we could see a significant effect from our choice of test medium; in conditions where the bacteria were supplied with plenty of nutrients, we didn’t see much of an effect, whereas in conditions with less access to nutrients we could see a much bigger effect.  This is a great result for us as researchers because it leaves us lots of room to explore these effects in more detail and to understand the underlying reasons why they occur.

"Additive Manufacturing provides so many possibilities both in the research and in the education aspects of my role.  This project is one of several research themes my team is working on, all with the ultimate aim of helping the processes reach their full potential.

The versatility of the processes (and the amount of things we still don’t know about them!) means there are plenty of opportunities for researchers and students to find an area they’re specifically interested in.  This, in turn, means that, while it still takes hard work and study time, it also stays fun!"  Candice Majewski, Senior Lecturer in Mechanical Engineering and Individual Project Coordinator.

You can find the full details of these initial stages of our work in this paper - https://www.nature.com/articles/s41598-020-57686-4

Next Steps

At the moment we’re at an exciting point with this research, and we have several areas we’re looking to do more work on.  In particular, we’re starting to think about exploring the effectiveness of our approach when considering different types of anti-bacterial additives, different base polymers and different test conditions to replicate different situations which might be encountered in different application areas.

We’ve also got some ongoing collaborative work with Dr Ifty Ahmed and his team at the University of Nottingham, involving tailoring the performance of antibacterial additives for different situations.  

And finally, we’re starting to look into specific applications for this approach, and how we might adapt our approach to fit different scenarios.  Watch this space for more information as we progress with this!

CREATE Education would like to thank Candice Majewski and collaborators Bob Turner, James Wingham, Joanna Shepherd, Thomas Paterson for taking the time to share their additive manufacturing research at The University of Sheffield.

To learn more about additive manufacturing solutions and how CREATE Education can help and support your projects visit www.createeeducation.com or speak to our specialists at enquries@createeducation.com