Artificial, soft robots can mimic soft biological systems and, as a result, hold great promise for applications at the interface of machine and human. For example, they can be used to provide non-invasive access to human tissue in surgery or for drug delivery. Now, writing in Nature, Metin Sitti and colleagues report a magnetically driven, elastic, micrometre-sized robot that can navigate through tissue using different types of motion, as well as transport and deliver cargo to a specific location.

Robots at the scale of millimetres to micrometres, which are made of soft materials, such as polymers, have been developed with sophisticated features. Jumping, gripping and 3D mobility can be realized through a variety of actuation strategies, involving materials that respond to a range of stimuli. However, current robots are usually limited to one mode of locomotion and localized delivery of cargo remains challenging.

Credit: R.Tremlett\Macmillan Publishers Limited

The robot designed by Sitti and co-workers is made of a soft polymer with embedded magnetic microparticles. It is rectangular in shape, untethered and operated through external actuation by a magnetic field. The strength and direction of the magnetic field determines the body shape and locomotion type of the robot owing to a magnetic torque caused by the interaction between the field and the magnetization profile of the robot's body. For example, a weak, rotating magnetic field creates a travelling wave along the body, whereas a magnetic field with a high magnitude causes the deformation of the robot into a ‘C’ or ‘V’ shape, similar to the movement of caterpillars. If the direction of the field is changed, the robot starts rotating. These changes in shape enable the robot to walk, jump, roll, climb, swim and crawl through obstacles, simply by deforming its body. Moreover, the hydrophobic surface allows the robot to skim across the water surface.

These features make these tiny robots ideal candidates for operating in unstructured, hybrid liquid–solid environments, such as in human organs. On integration of the robot with an ultrasound-based imaging system, the researchers can observe its movement in a surgical human stomach phantom and in chicken muscle tissue. “Our soft robot can easily access and navigate through complex tissues using its seven different types of locomotion,” explains Sitti. Also, a cargo can be added to the robot using an extra strap. The magnetically induced bending of the robot then unlocks the strap and the cargo is released at the desired location and time.

Our soft robot can easily access and navigate through complex tissues using its seven different types of locomotion

“We are aiming for therapeutic applications inside the digestive and urinary tract, because the current robot size is small enough to reach these regions. We are also trying to downsize the robot to the micron-scale in order to access regions of the human body that are currently difficult or impossible to reach using existing medical devices,” says Sitti. “We will further test our robot for controlled drug delivery, wound healing and for the targeted killing of cancer cells through remote heating by external radio-frequency electromagnetic waves.”

The application of these magnetic robots in vivo is currently hampered by the incorporation of toxic microparticles and the need for the robot's post-operative removal. “However, if we were able to make similar robots from just biodegradable and biocompatible materials in the near future, then these problems may be overcome,” concludes Sitti.