Who could guess that scientists working on the holy grail of sensor implants would succeed with gold? Researchers at Johannes Gutenberg University Mainz (JGU) used gold nanoparticles as implanted sensors. The scientists took advantage of the particles’ reaction to changes in their environment. It turns out gold nanoparticles are antennas for light. The nanoparticles absorb photons (light particles) and scatter them which makes them appear to change color. Building on this color-changing feature, the JGU researchers developed implant sensors suitable for continuous long-term monitoring of drug concentrations.

As detailed in an report published in Nano Letters, JGU researchers embedded the gold nanoparticles in a porous hydrogel. The hydrogel solves a common implant problem because the body does not try to reject it. The hydrogel also keeps the tiny particles in place. Blood vessels and cells grow into the structure’s pores following implantation. According to Professor Carsten Sönnichsen, JGU’s head of the Nanobiotechnology Group, the sensor is “like an invisible tattoo.”

Photons of infrared light aren’t visible to humans, but the JGU scientists detect color changes through the skin of test animals with an infrared detection and measurement device. In the published report the JGU team implanted the hydrogel structure with gold nanoparticle beneath a hairless rat’s skin. The implant changed color in reaction to different levels of an administered antibiotic when drug molecules reached the sensors via the blood stream.

The JGU team states the potential permanency of gold nanoparticle-based sensor implants solves two of the problems seen with earlier sensor implants. Gold nanoparticles don’t bleach or change color over times, said study author Dr. Katharina Kaefer. Developers can coat the nanoparticles with different drug or biomarker receptors and external measurement devices will have a standard baseline for color change. Previous sensor implant designs lose their usefulness after days or week because their sensitivity degrades; this is not the case with the relatively inert gold nanoparticles. Changing particle sensitivity in conventional sensor implants makes color measurement unreliable as time passes. Also, the body may attempt to reject conventional sensor implants, but the hydrogel structure doesn’t provoke rejection.