When talking about machines designed to fit inside the human body, space is obviously a valuable commodity. That means energy is usually outsourced to beyond the body, often delivered wirelessly throughelectromagnetic coils, embedded magnetic particles, or light. The micro-bots can then be triggered to release their payload by external stimulations like ultrasound, light, or infrared heat. But for the new research, the team wanted to design micro-motors that didn't require external control.
"We already know that micro-motors use different external driving forces – such as light, heat or magnetic field – to actively navigate to a specific location," says Kang Liang, corresponding author of the study. "In this research, we designed micro-motors that no longer rely on external manipulation to navigate to a specific location. Instead, they take advantage of variations in biological environments to automatically navigate themselves."
This is done using a system inspired by the way submarines sink and rise. The full-size machines fill ballasts with water to sink downwards or air to float upwards. The micro-subs, meanwhile, do much the same thing by either collecting or releasing gas bubbles. The subs are made of metal-organic frameworks (MOFs), scrunched-up materials that have extremely high surface areas and can hold huge amounts of a payload in a relatively small physical space.
The bubbles are created by a bioactive enzyme, before gathering inside the MOF's pores. The machine itself reacts to different pH levels in the body – in acidic environments, the MOFs hold onto the gas bubbles, helping the sub float upwards. When the pH level rises, these bubbles are released and the sub sinks again.
This mechanism allows for 3D movement, the team says, which could help steer the micro-submarines to the more acidic cancer cells to deliver drugs more precisely. And they're tiny too: one swallowable capsule could contain millions of the subs, and each sub would carry millions of drug molecules.
"Imagine you swallow a capsule to target a cancer in the gastrointestinal tract," says Liang. "Once in the gastrointestinal fluid, the micro-submarines carrying the medicine could be released. Within the fluid, they could travel to the upper or bottom region depending on the orientation of the patient. The drug-loaded particles can then be internalized by the cells at the site of the cancer. Once inside the cells, they will be degraded causing the release of the drugs to fight the cancer in a very targeted and efficient way."
Of course, there's still plenty of work to be done to get these up to scratch for human use. There's also the awkward requirement that a patient be either standing upright or lying down to direct the micro-submarines.