Tiny gold heat sinks fry bacteria on implants

In the fight against antibiotic resistance, new technology developed at Chalmers University of Technology in Sweden could make a big difference, for example in the surgical placement of hip and knee implants. By heating small gold nanorods with near-infrared (NIR) light, bacteria are killed and the surface of the implant becomes sterile. Now researchers are presenting a new study that expands our understanding of how gold rods are affected by light and how the temperature in them can be measured.

Infections can occur during surgical procedures, with the risk greatly increased when foreign materials such as knee replacements are implanted into the body. The presence of this material weakens the body’s immune system, so antibiotic treatment is usually used. If infected, high doses of antibiotics and long-term treatment are often required, sometimes lifelong. This carries the risk of increasing antibiotic resistance, which is considered by WHO as one of the biggest threats to human health.

Heat kills bacteria on the surface of the implant.

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

“The gold rods absorb light, the electrons in the gold move, and finally the nanorods emit heat. You could say that the gold nanorods act like little frying pans that fry the bacteria to death,” says Maia Uusitalo, a doctoral student. Chalmers student and lead author of the study, which was published in the journal Nano letters.

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

“The trick is to size the rods. If you make them a little smaller or a little larger, they will absorb the wrong wavelength of light. We want the absorbed light to penetrate well into the skin and tissue. Because once the implant is inside the body, light must reach the surface of the prosthesis,” says Martin Andersson, professor and director of research at Chalmers.

Accurate gold rod temperature measurements

To better understand how this technology works and how gold nanorods heated in the near-infrared affect both bacteria and human cells, the researchers needed to measure the temperature of the rods. Because of their tiny size, they cannot be measured with a regular thermometer; instead, the researchers used X-rays to study how gold atoms move. This method can accurately measure the temperature of gold rods and control the temperature using near-infrared light intensity.

“The temperature should not exceed 120 degrees Celsius, since at higher temperatures the nanorods lose their shape and turn into spheres. As a result, they lose their optical properties and can no longer effectively absorb near-infrared light, which prevents the rods from heating up. – says Maya Uusitalo.

She notes that the heating is very local, with low energy transfer to the environment. This is very important to avoid damaging surrounding tissue.

The researchers hope that this method can be used for implants made from a variety of materials, such as titanium or various plastics.

Gold rods become antibacterial when activated.

The gold nanorods themselves are completely passive on the surface before being heated by near-infrared light. Only then the rods are activated, heated and cause an antibacterial effect.

“We can control when a surface should be antibacterial and when it shouldn’t. When we turn off the light, the surface ceases to be antibacterial and returns to its original state. This is an advantage since many antibacterial surfaces tend to have a negative effect on healing. “says Martin Andersson.

The goal is to eventually bring this technology into healthcare.

“We primarily believe in using near-infrared radiation to provide heat shortly after implant placement and wound closure. By heating the gold nanorods, we can kill any bacteria that may have settled on the prosthesis during surgery.” says Martin Andersson.

All bacteria are killed by the heat of the gold nanorods, but even ordinary cells can be damaged during treatment.

“If several 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 of using gold nanorods heated in the near-infrared region has previously been studied in cancer research, but the research team at Chalmers is the first to use the technology to create an antibacterial surface on implants with high precision and control.

More details about the scientific article:

The article “Photothermal properties of solid-supported gold nanorods” was published in the journal “Photothermal properties of solid-supported gold nanorods.” Nano letters.

The researchers behind the study are based in the Applied Chemistry Unit of the Department of Chemistry and Chemical Engineering and the Chalmers Materials Analysis Laboratory at Chalmers University of Technology.

The research was funded by the Chalmers Area of ​​Advanced Materials and the Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program.

For more information contact:

Martin AnderssonProfessor of Applied Chemistry, Department of Chemistry and Chemical Engineering, Chalmers University of Technology.
+46 31 772 29 66
[email protected]

Maya UusitaloPhD student in the Department of Applied Chemistry, Department of Chemistry and Chemical Engineering, Chalmers University of Technology.
[email protected]

Contact persons speak English and Swedish. They are available for live and pre-recorded interviews. In Chalmers we have podcast studios and broadcast filming equipment and can help with TV, radio or podcast interview requests.

More about the study

In the experiments, glass was used as the base material, and gold nanorods were bonded through electrostatic interaction. This interaction is possible because the gold rods have a positive charge, and the glass surface has a negative charge.

The gold rods in the study were placed sparsely on the surface, occupying about 11 percent of the material’s area.

The gold nanorods measure 20 by 70 nanometers and absorb light in the near-infrared region around 800 nanometers.

When the gold rods absorb light, the electrons in the outer layers of the rods are set in motion and heat is generated. This phenomenon is called Surface plasmon resonance. The heat generated is very local and the bacteria are killed by the heat.

To preserve the shape of the rods and preserve the properties of the material, their temperature should not exceed 120 degrees Celsius. However, it is important to note that the heat generated is low, so the surrounding tissue will not reach the same temperature as the surface of the gold nanorods.

Illustration: Daniel Spacek, Neuron Collective, Neuroncollective.com

Signature: The illustration shows how gold nanorods heat up when illuminated with near-infrared light. At temperatures above 120 degrees Celsius, gold rods begin to change shape and their optical properties change.