(Nanowerk Spotlight) Microrobots, tiny machines typically measuring less than a millimeter in size, have the potential to revolutionize various fields, from medicine and environmental monitoring to manufacturing and space exploration. These miniature devices can be designed to perform a wide range of tasks, such as sensing, actuation, and manipulation, at scales that are difficult or impossible for larger machines to access.
In the biomedical domain, microrobots hold particular promise for applications such as targeted drug delivery, minimally invasive surgery, and diagnostic imaging. However, developing microrobots that can effectively navigate the complex environments found within living organisms has proven challenging. Biological fluids and tissues present unique obstacles, such as viscosity, surface adhesion, and immune responses, that can hinder the mobility and functionality of microrobots.
To overcome these challenges, researchers have explored a variety of materials, designs, and propulsion mechanisms for microrobots. Some have focused on developing biocompatible and biodegradable materials that can safely interact with living systems, while others have investigated novel actuation methods, such as magnetic fields, acoustic waves, and chemical reactions, to enable controlled motion and manipulation.
Despite significant progress, creating microrobots that can meet the demanding requirements for medical applications, such as high biocompatibility, stability, and precise control, has remained a difficult goal to achieve. Many designs have shown promise in laboratory settings but have struggled to translate to real-world biological environments.
Now, a team of researchers from China has developed a new type of micro- and nanorobot that combines the advantages of magnetic nanoparticles and hydrogels. In a paper published in the journal Advanced Intelligent Systems (“Design and Motion Controllability of Emerging Hydrogel Micro/Nanorobots”), they describe the creation of hydrogel micro/nanorobots (HMNRs) loaded with iron oxide (Fe3O4) particles. These HMNRs exhibit excellent biocompatibility, autonomous motility, and precise controllability, making them a promising platform for a wide range of biomedical applications.
Preparation method of of hydrogel micro/nanorobots. (Image: Reproduced from DOI:10.1002/aisy.202400339, CC BY)
The key innovation in this work lies in the use of hydrogels as a matrix to embed and stabilize the magnetic nanoparticles. Hydrogels are three-dimensional networks of polymer chains that can absorb large amounts of water, giving them a soft and elastic texture similar to biological tissues. By incorporating Fe3O4 particles into a hydrogel made from polyvinyl alcohol (PVA) and sodium tetraborate, the researchers were able to create microrobots with strong magnetic properties and improved mechanical stability compared to those made from magnetic particles alone.
To test the biocompatibility of the HMNRs, the researchers conducted experiments with human umbilical vein epidermal cells (HUVECs). They found that the HMNRs had no adverse effects on cell viability or growth, even at high concentrations. This is a crucial finding, as any material intended for use inside the body must be non-toxic and non-inflammatory to avoid causing harm to healthy tissues.
Next, the researchers investigated the motion capabilities of the HMNRs under the influence of different magnetic fields. When exposed to an oscillating magnetic field, the HMNRs exhibited a unique swinging motion, propelling themselves forward in a straight line. By adjusting the frequency and strength of the magnetic field, the researchers could control the speed and direction of the HMNRs with remarkable precision.
Notably, the HMNRs achieved faster propulsion speeds compared to microrobots made from Fe3O4 particles alone, likely due to the more efficient transfer of magnetic forces through the hydrogel matrix.
The researchers also explored the behavior of HMNR swarms, demonstrating that multiple microrobots could be magnetically assembled and controlled as a cohesive unit. These swarms exhibited coordinated motion and the ability to navigate through complex environments, such as branching channels and curved pipes. This swarming capability could be particularly useful for applications that require the delivery of larger payloads or the simultaneous targeting of multiple sites within the body.
To gain a deeper understanding of the motion mechanisms of the HMNRs, the researchers conducted theoretical simulations of the interaction between the microrobots and the surrounding fluid. They found that the movement of the HMNRs creates localized changes in fluid velocity and pressure, which in turn influence the motion of the microrobots. This complex interplay between the HMNRs and their environment highlights the importance of considering fluid dynamics when designing and optimizing microrobotic systems.
The development of HMNRs represents a significant step forward in the field of microrobotics for biomedical applications. By combining the strengths of magnetic nanoparticles and hydrogels, these microrobots offer a versatile and biocompatible platform for targeted drug delivery, minimally invasive surgery, and other therapeutic interventions. The precise control and autonomous motility demonstrated by the HMNRs open up new possibilities for navigating the intricate network of blood vessels and tissues within the human body.
However, while these initial results are promising, further studies are needed to evaluate the long-term stability, biodegradability, and immune compatibility of HMNRs in vivo. Advanced control systems and imaging techniques will also be required to guide the HMNRs to specific targets and monitor their activity in real-time.
Despite these hurdles, the potential benefits of HMNRs are immense. By enabling targeted and minimally invasive interventions, these microrobots could revolutionize the diagnosis and treatment of a wide range of diseases, from cancer to cardiovascular disorders. Moreover, the ability to control the motion of microrobots with external magnetic fields could pave the way for entirely new forms of therapy, such as the precise manipulation of individual cells or the stimulation of neural circuits.
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