Implantable biosensors that continuously monitor the concentrations of biomarkers in the body could transform the way we diagnose and treat chronic diseases. However, many existing technologies are not suitable for long-term use as they are either rejected by the body or their signal fades with time.
Researchers from the Johannes Gutenberg University Mainz have developed a novel sensor that can detect analytes in the bloodstream for several months without signal or functionality loss. The technology, which combines colour-stable gold nanorods with a tissue-integrating hydrogel scaffold, could offer a universal platform for round-the-clock monitoring of numerous target analytes in vivo. They describe the new sensor in Nano Letters.
Going for gold
Ensuring a steady stream of information is a top priority in medical sensing. For optical sensors like the one created by Carsten Sönnichsen’s research group, this means choosing a sensing element with excellent photostability (resistance to fading). Thanks to a phenomenon called the plasmon effect, rod-shaped gold nanoparticles absorb and scatter near-infrared light with indefinite photostability.
“We are used to coloured objects bleaching over time. Gold nanoparticles, however, do not bleach but keep their colour permanently,” explains first author Katharina Kaefer.
Importantly, gold nanoparticles are compatible with several molecular recognition elements. In their study, the researchers coated their gold nanorods with a special type of DNA receptor called an aptamer. When the aptamer binds to the target analyte, the optical absorption spectrum of the gold shifts – in other words, the nanoparticles change colour. The extent of this colour change, which is captured using an infrared camera, depends on the concentration of the analyte. By changing the type of aptamer used, the technology is not restricted to one specific analyte and can be easily adapted to measure a range of biomarkers.
Sensing under the skin
Less than 1 mm thick, Sönnichsen likens the sensor to an invisible tattoo. The nanoparticles are embedded in a hydrogel scaffold that, when implanted under the skin, integrates with the surrounding tissue (in this case, the skin of hairless rats).
The team demonstrated the response of their sensor by injecting the rats with the antibiotic kanamycin. They found that the extent of gold nanoparticle colour change increased with increasing kanamycin dosage – a phenomenon that was not observed when the rats were injected with saline. What’s more, the sensor remained well-perfused and responded to kanamycin in the bloodstream for over two months. The researchers also checked the device for fibrous encapsulation – a tell-tale sign of implant failure – and observed minimal fibrous tissue formation.
The results highlight the sensor’s potential for long-term implantable biosensing. To develop the technology further, the team hopes to explore features that are useful in personalized medicine, such as sensor read-outs.
