The miniaturization of robotics represents a pivotal frontier in engineering, enabling access to and manipulation within confined spaces that are unreachable by larger-scale machines. This capability holds transformative potential across diverse fields, most notably in biomedical engineering for applications such as targeted drug delivery, minimally invasive surgery, and cellular manipulation, as well as in environmental monitoring and disaster response. The development of small-scale robots, however, is profoundly challenged by scale effects, where dominant forces shift from inertia to surface and viscous forces, rendering conventional robotic design principles inadequate. Consequently, bio-inspiration has emerged as a vital and fruitful paradigm for innovation in this domain. In nature, numerous small animals, such as insects and other microorganisms, have evolved remarkable locomotion strategies to navigate complex environments at liquid-vapor, solid-liquid, and solid-gas interfaces. Their success is deeply rooted in interfacial phenomena, leveraging properties like superhydrophobicity, dynamic adhesion, and tunable anisotropic friction. This review explores these biological strategies to illuminate pathways for enhancing the mobility and functionality of small-scale robots. With a focused lens on magnetic actuation—a premier method for untethered, safe, and precise control at small scales—this article synthesizes recent progress in motion mechanisms, micro/nano-fabrication techniques, and control methodologies for magnetic bionic robots. Finally, we analyze the untapped potential of interfacial engineering for the next generation of small-scale bionic robots, envisioning systems with unprecedented adaptability and performance.
The natural world abounds with small organisms that are masters of interfacial navigation, exploiting surface phenomena to survive and thrive in challenging habitats. Their adaptations provide a rich source of inspiration for overcoming the locomotion challenges faced by small-scale bionic robots. The behavior of these organisms is governed by fundamental interfacial physics, which must be considered in robotic design.
At the liquid-vapor interface (e.g., water striders, fire ants), the primary challenge is to support weight without breaking the surface tension. The key lies in creating a superhydrophobic surface. The wettability of a solid surface by a liquid is classically described by Young’s model for an ideal, smooth surface, where the equilibrium contact angle θY is given by:
$$ \cos\theta_Y = \frac{\gamma_{sg} – \gamma_{sl}}{\gamma_{lg}} $$
where γsg, γsl, and γlg are the solid-gas, solid-liquid, and liquid-gas interfacial tensions, respectively. However, natural surfaces like the legs of water striders are rough and hierarchical. This roughness leads to a composite Cassie-Baxter state, where the apparent contact angle θC is significantly enhanced:
$$ \cos\theta_C = \phi(\cos\theta_Y + 1) – 1 $$
Here, φ is the area fraction of the solid in contact with the liquid. The micro/nano-hairs on a water strider’s leg trap air, creating a high apparent contact angle (>150°) and enabling it to stand and propel itself on water using surface tension forces.
Locomotion at the solid-liquid interface (e.g., sea slugs, limpets) requires mechanisms to overcome buoyancy and achieve reversible adhesion underwater. Adhesion (FA) and friction (Ff) govern motion:
$$ F_f = \mu N $$
$$ F_A = -\frac{3}{2}\pi R \Delta\gamma \quad \text{(based on JKR theory)} $$
where μ is the sliding friction coefficient, N is the normal force, R is the characteristic radius of the contact, and Δγ is the work of adhesion. Biological strategies here are diverse: chemical bonding via secreted mucus (e.g., limpets), mechanical interlocking with microstructures, suction, or the use of trapped air bubbles to form capillary bridges (e.g., certain leaf beetles). The ability to rapidly switch adhesion states is crucial for controlled movement in these environments.
Movement on solid-gas interfaces (e.g., snakes, geckos) involves managing dry adhesion and friction. Snakes exhibit anisotropic frictional properties due to the oriented, “hook-like” nanostructures on their ventral scales, reducing friction during forward motion while increasing it backward. Geckos achieve remarkable reversible adhesion primarily through van der Waals forces, facilitated by a hierarchical structure of setae and spatulae that maximize real contact area. These principles of directional friction and tunable dry adhesion are highly instructive for designing climbing and terrestrial crawling bionic robots.
| Interface | Biological Example | Key Physical Principle | Relevant Formula/Model |
|---|---|---|---|
| Liquid-Vapor | Water Strider | Superhydrophobicity, Surface Tension | Cassie-Baxter Model: $$ \cos\theta_C = \phi(\cos\theta_Y + 1) – 1 $$ |
| Solid-Liquid | Limpet, Leaf Beetle | Wet Adhesion, Capillary Forces | JKR Adhesion Model: $$ F_A = -\frac{3}{2}\pi R \Delta\gamma $$ |
| Solid-Gas | Snake, Gecko | Anisotropic Friction, Dry Adhesion (van der Waals) | Friction: $$ F_f = \mu N $$ |
Driven by these biological insights, the field of small-scale bionic robots has advanced rapidly. Magnetic actuation stands out due to its ability to provide wireless, penetrating, and biocompatible forces and torques. Researchers have developed a variety of magnetic bionic robots capable of navigating complex interfacial environments. For instance, soft milli-robots inspired by caterpillars or multi-legged insects can crawl on solid surfaces, swim at liquid interfaces, and even transition between different media, performing tasks like cargo transport. These bionic robots often mimic the gait or deformation patterns of their biological counterparts, leveraging magnetic fields to induce controlled body undulations, rotations, or crawling motions. The design of these systems involves careful consideration of the robot’s morphology, the distribution of magnetic material within its soft body, and the temporal programming of the applied magnetic field to generate the desired locomotion gait suitable for the target interface.
The locomotion of a magnetic bionic robot is governed by the interaction between its embedded magnetic moment (m) and an external magnetic field (B). The magnetic force (f) and torque (τ) acting on the robot are:
$$ f = (m \cdot \nabla)B $$
$$ \tau = m \times B $$
These fundamental equations give rise to three primary actuation mechanisms. First, magnetic gradient propulsion relies on a spatially non-uniform field (∇B ≠ 0) to exert a pulling/pushing force on the robot, useful for direct dragging or steering. Second, helical propulsion is inspired by bacterial flagella. A rotating uniform magnetic field applies a torque to a robot with a helical tail, causing it to spin and generate forward thrust in viscous fluids via a corkscrew motion. Third, swinging/undulatory propulsion involves using an oscillating or precessing magnetic field to induce rhythmic deformations—such as bending, beating, or traveling waves—in the body of a soft bionic robot. This mode mimics the swimming of fish or the crawling of worms and is effective across a range of environments, from wet surfaces to within fluids. The choice of mechanism depends heavily on the operational environment (e.g., low-Re fluid vs. solid interface) and the desired function of the bionic robot.
The realization of sophisticated magnetic bionic robots is inextricably linked to advancements in micro/nano-fabrication. Traditional techniques like photolithography and replica molding are used to create 2D or simple 3D structures. However, 3D printing, especially direct ink writing (DIW) and projection micro-stereolithography (PμSL), has become a transformative tool. It allows for the freeform fabrication of complex 3D geometries with spatially controlled composition. A critical step in manufacturing magnetic soft robots is “magnetic programming”—defining the orientation of the magnetic particles (and thus the local magnetic moment m) within the soft matrix. This can be achieved during printing by applying a directional magnetic field at the nozzle, or after printing using heat-assisted methods (heating above the Curie temperature of the particles and cooling under a field) or sequential magnetization of folded “origami” structures. This capability to program heterogeneous magnetization profiles enables the creation of bionic robots that can perform complex, shape-morphing motions under simple global magnetic fields.
| Fabrication Method | Key Materials | Magnetic Programming Approach | Advantage for Bionic Robots |
|---|---|---|---|
| Direct Ink Writing (DIW) | PDMS / Elastomer with NdFeB particles | In-situ alignment at nozzle with field; Post-print pulse magnetization. | Complex 3D structures, heterogeneous magnetization. |
| Magnetic Molding | PDMS with Iron particles | Particle alignment within mold using a field before curing. | Simple, good for replicating 2D designs. |
| UV Lithography | Photoresin with magnetic particles | Selective exposure under a uniform field to lock particle orientation. | High-resolution 2D/2.5D patterns. |
| Spray Coating | Polymer glue with Fe particles | Spraying onto objects under a field, magnetizing the coating. | Rapid fabrication, can “magnetize” arbitrary small objects. |
Effective control is essential for deploying bionic robots in practical scenarios. Control strategies can be broadly classified into model-based and model-free approaches. Model-based control requires an accurate mathematical representation of the robot’s dynamics, including its soft body mechanics (often using hyperelastic material models like Neo-Hookean or Mooney-Rivlin) and its interaction with the magnetic field and environment. Finite element analysis is frequently used for simulation and model building. While powerful, creating precise models for highly deformable soft robots in complex environments is challenging. Consequently, model-free, data-driven control methods are gaining prominence. These techniques, such as visual servoing, iterative learning control, and reinforcement learning, use sensor feedback (e.g., from microscopes) to train controllers without an explicit dynamic model. They are particularly promising for in vivo biomedical applications where the environment (e.g., blood vessels) is dynamic and poorly characterized.
The future trajectory of small-scale bionic robots will be significantly shaped by deeper integration of interfacial science and advanced manufacturing. Drawing directly from biological principles, several promising directions emerge. Surface engineering will be crucial: endowing robots with bio-inspired, stimuli-responsive skins that can change wettability, switch adhesion, or alter friction in real-time could dramatically enhance multi-terrain adaptability. Material innovation will focus on creating multi-functional composites that combine magnetic responsiveness with other properties like self-healing, biodegradability, or on-demand stiffness change. From a fabrication perspective, the convergence of multi-material 3D/4D printing with in-situ magnetic and functional patterning will enable the monolithic creation of highly sophisticated, multi-modal bionic robots. Finally, the development of intelligent, adaptive control algorithms that can leverage the robot’s embodied compliance and environmental interactions will be key for robust operation in unpredictable real-world settings, such as the human body. By harnessing the principles observed at multiphase interfaces in nature, the next generation of magnetic bionic robots will move beyond simple locomotion to achieve true environmental synergy, performing complex medical interventions and micro-scale tasks with unprecedented dexterity and autonomy.
In conclusion, the study of small animals at multiphase interfaces provides an indispensable blueprint for overcoming the fundamental challenges of small-scale robotics. Magnetic actuation has proven to be a versatile and powerful platform for implementing these bio-inspired strategies, leading to robots capable of swimming, crawling, and climbing in diverse environments. Continued progress hinges on interdisciplinary efforts that fuse insights from biology, interfacial science, materials engineering, and robotics. By focusing on the intelligent design of surfaces, morphology, and control in the spirit of bionic robots, researchers are paving the way for microscopic machines that can navigate the complex interfacial worlds within living organisms and other confined spaces, ultimately bringing the vision of targeted medical microrobotics and other advanced applications closer to reality.