A soft microrobot with zinc oxide nanorods uses ultrasound to generate localized reactive oxygen species, offering a precise method to kill bacteria without relying on systemic antibiotics.
(Nanowerk Spotlight) Bacterial infections continue to test the limits of medicine. Antibiotics transformed treatment by allowing doctors to stop dangerous microbes quickly and with precision, yet their success also encouraged heavy use. As these drugs spread, bacteria developed ways to survive them, and this resistance now threatens even basic care.
Researchers have searched for new approaches that do not depend on drugs alone. They have examined bacteriophages, the viruses that infect bacteria, though these must match the microbe they target and can be difficult to deploy. They have tested chemical coatings that kill on contact, but these often weaken inside the body when blood proteins and other molecules block their action.
Light-based methods offer another option, but light does not penetrate deeply into tissue. Some engineers have built tiny machines that travel through liquid and disturb bacterial colonies, yet these devices still struggle to reach the exact location of infection or maintain control inside complex biological spaces.
Underlying many of these attempts is a simple idea. Some materials can create reactive oxygen species, or ROS, when they receive energy. These short-lived molecules, such as hydroxyl radicals and superoxide anions, break essential structures inside bacteria. They could serve as powerful antibacterial agents if they could be created exactly where they are needed.
But this is the central challenge. Light cannot reach deep tissue. Chemical fuels may not exist at the infection site. Movement alone does not always create enough force to activate the material. Even if ROS form, they survive for such short times that they must be produced directly at the surface of the target. If the ROS generating material is buried inside a polymer or coating, it may not move enough to create the effect at all.
Ultrasound has emerged as a way to solve some of these problems. Sound waves move through tissue and fluid with less loss of energy than light. They can bend or compress small structures. Some materials respond to this bending by separating electrical charges within their crystal structure. This behavior is called the piezoelectric effect. Zinc oxide is known for this effect. Its long, narrow crystals generate tiny electric fields when they bend under sound waves, and these fields drive the chemical reactions that form ROS. Zinc oxide is also approved for medical uses. However, when used as free particles, it can drift away from target sites and may not bend strongly enough to create an efficient antibacterial response.
A recent study in Advanced Functional Materials (“Reconfigurable Magnetic Soft Microrobot for Acoustically Triggered Targeted Bacterial Sterilization”) describes a soft microrobot that attempts to overcome these limits. The robot moves under magnetic fields, changes shape with temperature and carries zinc oxide nanorods grown directly on its surface. By combining motion, contact and chemical activation, the device focuses ROS creation exactly where bacteria gather.
Magnetically actuated shape-morphing soft microrobot for localized antibacterial therapy. a) Reversible transformation between planar and helical configurations in response to environmental temperature. b) Zinc oxide (ZnO) nanorods grown in situ on the hydrogel surface and embedded iron oxide nanoparticles (IONPs) perpendicularly aligned for magnetic actuation. c) Ultrasound-mediated piezoelectric ROS generation for bacterial sterilization. d) Navigation of the helical microrobot toward the infected area via corkscrew motion under a rotating magnetic field. e) Temperature-triggered flattening enhances contact for localized treatment. (Image: Reproduced from DOI:10.1002/adfm.202518017, CC BY) (click on image to enlarge)
The robot’s core is a hydrogel made from NIPAM, a polymer that absorbs water at low temperature and releases it at higher temperature. When cool, the hydrogel swells. When warm, it contracts. This shift occurs around 36 °C. The researchers attached two thin SU 8 layers to guide how the hydrogel bends. One layer lies flat while the other contains angled support beams. When the hydrogel swells at about 20 °C, the angled beams cause the structure to twist into a helical shape. When the hydrogel shrinks at about 40 °C, the robot relaxes into a flat sheet. The transformation occurs within about a minute.
The team added magnetic control by embedding and aligning iron oxide nanoparticles inside the hydrogel. In its helical form, the robot rotates forward under a rotating magnetic field, much like a screw. This lets it travel through fluid and reach specific locations. When it encounters a surface, it shifts to a rolling movement that preserves direction and control.
The zinc oxide nanorods distinguish this device from previous microrobots. Instead of mixing the rods inside the hydrogel, the researchers grew them directly on its surface. They began by depositing seed crystals, then placed the structure into a heated solution that allowed the crystals to grow into upright rods.
Each rod is about one micrometer long and has a hexagonal cross section. They form a dense layer that covers the surface like a miniature forest. Imaging showed that the rods are firmly attached and remain stable after months of storage and after exposure to ultrasound.
The researchers tested magnetic motion in fluid and found that the robot moves with reliable speed and can navigate confined spaces. Its shift from helical motion to rolling helps it handle boundaries and irregular surfaces. The shape change to a flat form increases contact with the surrounding environment, which is important because ROS must be generated precisely at the bacterial interface.
The team then evaluated antibacterial activity using Escherichia coli ATCC 25922. Without ultrasound, the zinc oxide coated robots did not kill bacteria. This indicates that the coating remains stable and does not release harmful substances under static conditions.
Free zinc oxide nanoparticles killed a fraction of the bacteria because zinc ions can disrupt cell structures. But when the coated robots received ultrasound, they killed more than 90 percent of the bacteria. Most of the effect occurred within 20 minutes and reached about 97 percent by 40 minutes. Robots without zinc oxide produced only a small reduction.
To confirm the source of this effect, the team used electron paramagnetic resonance spectroscopy along with a trapping molecule called DMPO. The tests showed clear signatures of superoxide and hydroxyl radicals after ultrasound activation. These signals grew in strength with longer exposure.
Computer simulations helped explain why the nanorod arrangement matters. The models showed that bending forces produce stronger charge separation inside the rods than simple compression. Because the rods sit on a soft hydrogel surface, they bend readily under the pressure changes created by ultrasound. This enhances the chemical reactions that produce ROS.
This microrobot brings together controlled movement, shape change and ultrasound activated chemistry. Its soft structure lets it conform to surfaces while its zinc oxide coating ensures that ROS form at the precise location of bacterial contact. Ultrasound can reach areas that optical methods cannot, and magnetic steering helps place the robot where it is needed. Although the work remains at the laboratory stage, it outlines a path for targeted antibacterial treatment in places that receive little blood flow and respond poorly to systemic drugs.
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