Tiny gold radiators fry bacteria on implants

In the fight against antibiotic resistance, a new technology developed at Chalmers University of Technology, in Sweden, can be of great importance when, for example, hip and knee implants are surgically inserted. By heating up small nanorods of gold with near-infrared light (NIR), the bacteria are killed, and the surface of the implant becomes sterile.

The researchers are now presenting a new study that increases the understanding of how the gold rods are affected by light and how the temperature in them can be measured.

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Infections can occur during surgical procedures, with the risk increasing significantly when foreign materials, such as knee prostheses, are implanted into the body. The presence of the material weakens the body’s immune system and antibiotic treatments are commonly used. If infected, high doses of antibiotics are often required with long treatment times, sometimes lifelong. This entails a risk of increased antibiotic resistance, which is seen by the WHO as one of the greatest threats to human health.

Heat kills the bacteria on the implant surface

The technology developed by the researchers at Chalmers is a method in which nanometre-sized rods of gold are attached to the implant surface. When near-infrared (NIR) light hits the surface of the implant, the rods heat up and act as tiny heating elements. Because the heating elements are so small, there is a very local heating, which kills any bacteria on the surface of the implant without heating the surrounding tissue.

“The gold rods absorb the light, the electrons in the gold are set in motion, and finally the nanorods emit heat. You could say that the gold nanorods work like small frying pans that fry the bacteria to death,” says Maja Uusitalo, doctoral student at Chalmers and lead author of the study, which has been published in the journal Nano Letters.

NIR light is invisible to the naked eye but has the ability to penetrate human tissue. This property allows the gold nanorods to be heated on the surface of the implant inside the body by illuminating the skin. The gold rods are sparsely distributed and cover only about ten percent of the implant’s surface. This means that the material’s beneficial properties, such as the ability to attach to bone, are largely retained.

“The trick is to tailor the size of the rods. If you make them a little smaller or a little bigger, they absorb light of the wrong wavelengths. We want the light that is absorbed to penetrate skin and tissue well. Because once the implant is inside the body, the light must be able to reach the surface of the prosthesis,” says Martin Andersson, Professor and research leader at Chalmers.

Precise measurements for gold rod temperature 

To increase the understanding of how the technology works, and how the NIR-heated gold nanorods affect both bacteria and human cells, the researchers needed to measure the temperature of the rods. Due to their tiny size, it is impossible to measure with a regular thermometer, instead the researchers used X-rays to study how the gold atoms move. The method enables precise measurement of the temperature of the gold rods and how the temperature can be regulated using the intensity of the NIR light.

“The temperature must not exceed 120 degrees Celsius, as higher temperatures cause the nanorods to lose their shape and transform into spheres. As a result, they lose their optical properties and can no longer absorb NIR light effectively, which prevents the rods from heating up” says Maja Uusitalo.

She points out that the heating is very local with low energy transfer to the surroundings. This is crucial to avoid causing any damage to the surrounding tissue.

The researchers hope that the method can be used on many different implant materials, such as titanium or different plastics. 

Gold rods become antibacterial when activated

The gold nanorods themselves are completely passive on the surface before the NIR light heats them. Only then are the rods activated, becoming hot and triggering the antibacterial effect.

“We can control when the surface should be antibacterial and when it should not. When we turn off the light, the surface is no longer antibacterial and reverts to its original state. This is an advantage because many antibacterial surfaces usually have negative effects on healing,” says Martin Andersson.

The goal is to eventually bring this technology into healthcare.

“We primarily believe in using NIR light for heating shortly after the implant is placed and the wound is sutured. By heating up the gold nanorods, we can eliminate any bacteria that may have settled on the prosthesis during surgery.” says Martin Andersson.

All bacteria die from the heat from the gold nanorods, but even ordinary cells can be damaged during treatment.

“If a few human cells are destroyed during the NIR heating process the body quickly regenerates new ones, so the impact on healing is minimal,” says Martin Andersson.

The technology with NIR-heated gold nanorods has previously been studied in cancer research, but the research group at Chalmers is the first to use the technology to create an antibacterial surface on implants with high precision and control.