by Dr. Gavin Hazell
Medical devices are ubiquitous in modern medicine. Devices range from simple catheters to artificial cardiac devices and complex materials that can replace our own joints. Contemporary surgical procedures have revolutionised our approach to joint replacement with 160, 000 total hip and knee replacement procedures performed each year in England and Wales. Medical implants have seen a rapid expansion in use which has been facilitated by technological advances and reduced manufacturing costs. Today, these devices profoundly impact patient quality of life and disease outcome.
However, all of these devices suffer from a major weakness. They are susceptible to bacterial colonisation, which leads to a medical device associated infection. Once bacteria adhere to the surface of an implant they grow and proliferate until a dense bacterial film resides on the surface, known as a biofilm. The presence of such a bacterial layer leads to the failure of the medical device and puts the patient at risk of sepsis and death.
Biofilms on the surface of implantable materials (such as a titanium hip replacement) are difficult to treat as they are generally recalcitrant to conventional antibiotic therapy. It is therefore necessary for the surgeon to remove the device, thoroughly clean the infected area and implant a replacement. This comes at a significant financial cost to the NHS as well as being a very traumatic and invasive experience for the patient.
Another significant problem with these kinds of infections is the presence of antibiotic resistant bacteria. If the infection is composed of bacteria such as methicillin-resistant Staphylococcus aureus (MRSA, or other antibiotic resistant strains), this makes it even harder to treat as conventional antibiotics cannot be used.
What is required for the next generation of medical implants/devices are materials with surfaces that are lethal to adherent bacterial cells. If surfaces that kill bacteria upon contact could be used in medical devices, then the risk of infections associated with these materials would be significantly reduced. This would negate the need for revision surgery, lower the financial burden for the healthcare provider and significantly improve patient experience.
Recently it has been shown that the surface of the wing of the cicada fly is composed of periodic arrays of nanopillars. These are tiny pillars that are around 200 nm in height and only visible with a very powerful microscope (human hair is around 100, 000 nm in width). When bacteria hit such surfaces, their cellular membrane stretches across these nanopillars and is placed under mechanical strain. If the membrane is soft enough, it ruptures and the bacteria die (see figure below).
In our laboratory, we seek such inspiration from nature to modify the surface of medical implants/devices and render them bactericidal. We generate surfaces composed of tiny nanospikes and/or nanocones that are able to mechanically kill bacteria. Killing bacteria through mechanical means ensures that they cannot evolve resistance and it is possible to kill bacteria that are already resistant to antibiotic strains.
We generate nanopatterns on a vast array of materials. We are currently focused on forming nanocones on polymers for use in catheters, blood storage bags and contact lenses. Black silicon is used as a new material for biosensor electrodes. Here we can pattern extremely sharp spikes that are able to puncture bacterial contaminants when the electrode is working. Finally, a large interest is in the patterning of titanium dioxide for use in prosthetic joint replacement surgery. It is possible to form nanospikes on these surfaces that can also puncture adherent bacteria. Below is an image gallery of all the materials we are working with along with some microscope images of dead, punctured bacteria on the surfaces. A note on intended clinical application is also included. (Please click on image to expand)
Dr Gavin Hazell is a research scientist working in the biomaterials engineering group at the University of Bristol. He is an expert in materials science and his research interests lie in finding new ways to improve healthcare through the generation of novel, smart materials.