Nanorobots ferry drugs to kill bacteria deep in teeth

Researchers at the Indian Institute of Science in Bengaluru have developed magnetic nanorobots that can seek out and kill pathogenic bacteria in human teeth¹. Via a magnetic field, they generate heat that kills bacteria which resist conventional therapy.

The nanobots can help in surgical procedures and be engineered “to precisely deliver drugs to target tissues of teeth,” says lead researcher Ambarish Ghosh.

Of roughly 700 bacterial species that colonize the mouth, only a handful infect and cause dental decay. By evading the host defence system, some reach the dentine, a tissue inside teeth, and invade the pulp tissues which extend from the crown to the tip of the tooth’s root. It is difficult to remove deep-seated bacteria with root canal treatments.

The scientists embedded iron nanoparticles in silicon dioxide, giving magnetic properties to the helical structured robots. After being suspended in deionised water, these nanobots were injected into the cleared central canal of a tooth sample, and their movements, under a rotating magnetic field, were tracked with a near-infrared imaging technique.

The rotating magnetic field nudged the nanobots along a certain direction within the dentine, but the technique couldn’t spread the nanobots uniformly.

The team, which included Debayan Dasgupta and Shanmukh Srinivas Peddi, solved this by applying oscillating magnetic fields that induced a rocking motion in the nanobots, making them move along the length of the dentine. A simulation study on 40,000 particles showed that the nanobots interacted with the surface of dentinal tubules, a network of channels. This helped them reach the depths where pathogenic bacteria are found.

“These magnetically driven nanobots have directed motion that lead to higher passive diffusion rate beyond a certain depth, improving on the slower passive diffusion rate of therapeutic agents in conventional methods,” says Neetu Singh, an expert on nanomaterials at the Indian Institute of Technology in Delhi, who is not involved with this research.

To remove the nanobots from the dentinal tube, the researchers exposed them to rotating magnetic fields, retrieve them once their therapeutic action is completed, and reduce the chances of nanoparticle accumulation and toxicity, Singh explains.

In a separate experiment, the team cultured the nanobots with Enterococcus faecalis, one of the most resistant bacteria that invade human teeth. They used rotating magnetic field to drive the nanobots into a bacteria-infected tooth sample. A 15-minute exposure to oscillating magnetic fields generated heat that destroyed the bacteria. A staining agent revealed the dead cells of the bacteria as a red band in the middle section of the tooth.

“Multiple therapeutic agents such as siRNA, a type of tiny RNA molecules, and magnetic materials with antibacterial properties can be integrated with the nanobots that can release antibacterial agents over days or months, reducing pathogen load substantially,” Peddi says.

“The nanobots could be efficient for their ability to reach into confined spaces of a root canal in a tooth,” says Jayant Khandare, an expert on nanomedicine at the MIT World Peace University in Pune. Multi-centric studies with approved clinical protocol must be conducted to demonstrate the safety of the nanorobots in oral and systemic blood circulation, he adds.