‘Micro-swimmers’ may soon help with drug delivery

In the 1966 Hollywood movie Fantastic tripA group of scientists penetrate a colleague’s bloodstream to clear a blood clot from his brain, shrinking themselves and their submarine, Proteus, to the size of a cell. This element of science fiction is on its way to becoming a reality, as recent research focuses on putting microbots into the bloodstream to deliver drugs. Varun Sridhar of the Max Planck Institute for Intelligent Systems (MPI-IS), Stuttgart, Germany, says of this work: “Our work has shown that it is possible to use light as a fuel to move microbots in physical conditions with intelligent drug delivery. which is selectively sensitive to cancer cells.” The research is led by MPI-IS and Max Planck Institute for Solid State Research (MPI-FKF), Stuttgart, Germany.

Imagine trying to swim in a pool of honey. Any attempt to push backwards to generate forward motion would be hindered by the honey’s high viscosity. At the microscopic level, the viscosity of even water is overwhelming. “A Hollywood movie can take liberties; miniaturizing a submarine is all that is [needed]† However, in real life, moving microscopic swimmers is not so easy,” said Metin Sitti, director at MPI-IS, which is part of the partnership.

Made from the two-dimensional compound poly(heptazine imide) carbon nitride (also known as PHI carbon nitride), these microbots are nothing like the miniaturized humans. They range from 1-10 micrometers (a micrometer is one millionth of a meter) in size, and can propagate themselves when activated by shining light.

How they swim

The PHI carbon nitride microparticles are photocatalytic. “Just like in a solar cell, the incident light is converted into electrons and holes. These charges cause reactions in the surrounding fluid,” explains Dr. Sridhar out. The charges react with the liquid surrounding them. This reaction, combined with the particle’s electric field, causes the microbots (microswimmers) to swim.

“As long as there is light, electrons and holes are produced on the surface of the swimmers, which in turn react to form ions and an electric field around the swimmer. These ions move around the particle and cause liquid to flow around the particle. So this fluid flow makes the micro-swimmers move,” said Dr. Sridhar, “With light, we not only move the microbots, but we can also direct their movement toward a specific target.”

Just as the scent of frankincense floats from an area of ​​high concentration to a low concentration, the ions move from the clear surface of the microswimmer to the back. The diffusion of the swimming medium in one direction propels the microswimmer in the opposite direction. This is like a boat moving in the opposite direction of the oars.

The particles are nearly spherical, and the incident light illuminates one half of the sphere, while the other remains dark. Because photocatalysis is powered by light, it only takes place in the illuminated hemisphere. As the ions move from the bright side to the dark side, micro swimmers march toward the light source.

the bummer

Designing micro swimmers or having them move in a certain direction is not new. “The body fluids and blood contain dissolved salts. When salts are present, the salt ions stop the reaction ions from moving freely because they will simply bind or recombine with them and stop them. So all chemically powered swimmers cannot swim in solutions that contain salts.” says Filip Podjaski, an author of the article published in Science Robotics.

For example, when table salt (NaCl) is dissolved in water, it is broken down into sodium (Na ^†) and chloride (Cl ^†) ions. These ions neutralize the ions created by the photocatalytic reaction, hindering self-propulsion.

To overcome this challenge, the researchers examined various materials such as titanium dioxide and cobalt monoxide and finally zeroed in on polyheptazine imide (PHI) carbon nitride. While carbon nitride is an excellent photocatalyst, the two-dimensional PHI has a sponge-like structure full of pores and voids and charge storage properties.

The researchers found that the ions in the salt solution passed through the pores of PHI carbon nitride. So there was little or no resistance from the salt ions. Experiments were conducted in sample solutions as highly concentrated as Dead Sea water. “Salt ions present in the swimming medium have no influence on the propulsion. Our organic material allows the ions to pass freely through it,” says Bettina Lotsch, director at MPI-FKF and co-author of the article.

Drug Delivery

In addition to transporting salt ions from the liquid, the cavities and pores on the microparticles acted as cargo spaces and allowed them to absorb large amounts of drug. The researchers found that doxorubicin, a drug used to treat cancer, is easily absorbed. By changing the pH of the solution or by triggering it with light, the researchers showed that the drug release could be activated.

“The material also has an intelligent charge storage property to store electrons when light is present. The environment of cancer cells is characterized by a low oxygen content. The stored electrons are sensitive to it. We use that to deliver drugs that target the cancer cells,” explains Dr. Sridhar out.

(TV Venkateswaran is a scientist at Vigyan Prasar, Dept. of Science and technology, and a science communicator.)