Microscopic machines that swim through the bloodstream to deliver drugs or perform minor surgeries have been a dream of scientists for decades. In the past 15 years researchers have created micro-engine variants that rely on chemical reactions, magnetism or vibration for thrust—but they often motor around erratically. The main challenge is guiding them to where they are needed, says University of Hong Kong chemist Jinyao Tang. Tang and his team have made progress on that front with a micro swimmer that can be smoothly and precisely steered with the help of light.

As reported in the December 2016 Nature Nanotechnology, the researchers built bottlebrush-shaped microparticles with silicon stems and titanium dioxide “bristle” heads. Both materials absorb photons, so when light is shined on the microparticle, the stem generates negative hydroxide charges and the bristles produce positive hydrogen ions. As the ions move to balance the uneven distribution of charge, they pull fluid with them, causing the micro swimmer to move toward the light—stem-first, like a dart.

As a test, researchers placed a swimmer in liquid on a glass slide and guided it with ultraviolet light to spell out the word “nano.” The 11-micron-long motor could cover about a millimeter in two minutes—slow for medical applications—but Tang says they are now designing new geometries to speed up the swimmers. “This unique way of precisely controlling speed and direction is amazing,” says Sámuel Sánchez, a nanoroboticist at the Max Planck Institute for Intelligent Systems in Stuttgart, who was not involved in the research.

This work is an early glimpse at medical robots that doctors could navigate through a patient's body from the outside with a focused beam of light, Tang says. The devices currently run on ultraviolet light—but the researchers are now working on micro swimmers that respond to a near-infrared wavelength, which can penetrate a few centimeters of tissue. For applications deeper in the body, surgeons could control the bots with optical fibers.

Credit: Brown Bird Design; Source: “Programmable Artificial Phototactic Microswimmer,” by Baohu Dai et al., in Nature Nanotechnology, Vol. 11; December 2016