A microgripper is a specialized microscopic tool designed to precisely manipulate objects on the micrometer scale, comparable to the width of a human hair or even smaller. These devices perform actions such as gripping, holding, transporting, and releasing microscopic components or specimens. Often less than 100 micrometers in size, the microgripper acts as the end-effector of a micromanipulation system, allowing controlled interaction where traditional machinery is ineffective. They are a foundational technology in Micro-Electro-Mechanical Systems (MEMS), addressing the need for handling components too small for conventional robotics.
The Engineering of Grasping
Designing a mechanism to reliably grasp objects at the microscale requires overcoming the dominant physics of this environment. At this size, inertial forces and gravity are negligible, while surface forces, such as van der Waals forces and capillary adhesion, exert a strong influence. These adhesive forces can be so powerful that they prevent the microgripper from releasing the object, a phenomenon known as stiction.
To achieve precision and movement without the friction and wear of traditional joints, microgrippers utilize compliant mechanisms. These are monolithic structures that gain motion from the elastic deformation of flexible beams, rather than separate, hinged parts. The structures include flexible hinges, which are thin sections of material that bend to allow movement, replacing conventional pin joints. This joint-less design allows for high-precision motion and leverage, amplifying a small input displacement into a larger jaw movement while maintaining smooth operation.
Powering the Microgripper
The movement of a microgripper is achieved through various actuation methods that convert energy into mechanical motion. Thermal actuation is a common method relying on thermal expansion. In an electrothermal microgripper, an electric current heats a material, causing it to expand, often using a “chevron” or U-shaped design to maximize displacement. Although this method generates large displacements and forces at low voltages, the resulting heat can be a drawback, especially when manipulating delicate biological samples.
Piezoelectric actuation utilizes materials that change shape when an electric voltage is applied. These microgrippers are valued for their fast response time, high force output, and sub-nanometer resolution, making them suitable for high-precision tasks. Drawbacks include limited displacement and hysteresis, where the material’s motion depends on its previous state.
Magnetic actuation uses external magnetic fields to remotely control the gripper. This offers the advantage of wireless, untethered operation, which is beneficial for procedures in enclosed environments like the human body. Magnetic grippers provide high control precision and are harmless to tissues, though they require larger external equipment to generate the necessary field.
Building the Miniature Tool
The creation of microgrippers requires specialized fabrication techniques adapted from the semiconductor industry to create micrometer-scale features. This manufacturing domain is known as Micro-Electro-Mechanical Systems (MEMS) fabrication. The process begins with photolithography, which transfers a geometric pattern onto a silicon or polymer wafer. A light-sensitive polymer, called photoresist, is coated onto the wafer and exposed to ultraviolet light through a patterned mask.
The exposed or unexposed photoresist is then chemically removed, leaving a stencil-like pattern on the wafer surface. This pattern acts as a protective layer for etching, the process of selectively removing material from the wafer. Deep Reactive Ion Etching (DRIE) is a common dry etching technique used to create deep, high-aspect-ratio structures. The combination of photolithography and precise etching allows for the batch fabrication of numerous, identical microgrippers on a single wafer.
Essential Uses in Science and Industry
Microgrippers enable the precise manipulation of small objects across various fields. In biomedical applications, they are used for cellular manipulation, allowing researchers to gently grasp and position individual cells for tasks like single-cell injection or tissue sample extraction. The ability to apply minute, controlled forces is useful for handling delicate biological structures, such as ova or sperm during artificial fertilization procedures.
In industrial settings, microgrippers are fundamental to micro-assembly processes used to build miniature machines and sensors. These devices precisely pick, orient, and place micro-components, such as small gears or components for hybrid electronic circuits, with high positional accuracy. The telecommunications and electronics industries also rely on microgrippers for the precise positioning of fiber optics and micro-lenses during the production of advanced communication devices.