The human digestive tract has a lot of places for tumours to hide. Treating them is a difficult challenge that often requires invasive surgery. But a US research team may be about to change that.
They’ve developed laser-guided micro-robots that can deliver drugs to intestinal tumours, and that let clinicians monitor and control their activity. The microbots could also become the world’s tiniest surgeons, performing microsurgeries in hard-to-reach parts of the body.
The team from Caltech and the Washington University of St. Louis, led by Wei Gao and Lihong Wang, build their microbots around 20-μm magnesium microspheres. They coat each one in a 100-nm-thick layer of gold, a layer of drug-laced gel, and finally a layer of acid-proof polymer.
But they leave a tiny hole in each layer - so when intestinal acid hits the magnesium core, it fizzes, and the bot shoots forward like a bubble rocket.
Microbot-loaded gelatin in a paraffin capsule creates a swallowable bot army. Then the team uses photoacoustic tomography (PAT) to track the capsule through the gut.
PAT starts with an infra-red laser pulse, which diffuses harmlessly through the body and makes the haemoglobin molecules in red blood cells vibrate ultrasonically. Sensors pressed against the skin pick up those vibrations and create an image - like an ultra-detailed x-ray for soft tissue.
The researchers carefully chose the wavelength of the laser pulse, 750 nm, to distinguish between the photoacoustic response of the bots, the blood cells, and the paraffin capsule.
When they see the capsules have arrived at the tumour, they zap the area with a powerful infra-red laser, which heats the gold on the microbots until the paraffin capsule melts - and unleashes the army. Intestinal acid sends the bubble-powered bots shooting into the mucus of the intestinal wall.
The bots have no steering mechanism, so they fly off in all directions - but by sheer force of numbers, many end up at the tumour site, where they deliver the drugs.
These bots can stay in the digestive tract for a long time, which improves medicine delivery. And because they’re made of magnesium, they’re biocompatible and biodegradable, so they won’t harm the patient.
This technique worked fine when the researchers tried it on chicken breasts. It also worked perfectly on mice, who showed no ill effects or signs of distress.
But there’s a problem with humans. They’re a lot bigger. The lasers used in PAT only penetrate about 7 cm deep.
The team believe they can overcome this by switching to thermoacoustic tomography, a similar approach using longer-wavelength, microwave radiation.
If they pull that off, they’ll be able to start testing the efficacy of microbot-delivered drugs on humans - and exploring the possibility of using microbots in other parts of the body. That will mean finding new propulsion methods that don’t rely on fizzy gut acid.
“The microrobot concept is really cool because you can get micromachinery right to where you need it,” says Wang. “It could be drug delivery, or a predesigned microsurgery.”