(Nanowerk Spotlight) The materials we interact with every day—whether they are steel, glass, or rubber—have properties like strength, flexibility, or brittleness that stem from their chemical composition. Steel is strong because of how its molecules are arranged; rubber stretches because of its natural elasticity; and glass is brittle due to its rigid, tightly bonded molecular structure.
But a shift is taking place. Materials can now be designed not just based on their composition, but on their internal structure. Known as mechanical metamaterials, these are engineered materials with carefully designed architectures that allow them to exhibit properties that go beyond what their base material would typically offer. Mechanical metamaterials enable customized responses to forces like compression, torsion, or stretching by manipulating their internal geometries.
However, until recently, even these advanced materials had fixed mechanical responses. Now, a study published in Advanced Materials (“Digital Mechanical Metamaterial with Programmable Functionality”) introduces a new class of materials that can change their behavior on demand – digital mechanical metamaterials (DMMs). These materials function like programmable systems, where mechanical properties such as stiffness or deformation can be digitally controlled, enabling real-time adaptation to external forces. By encoding different behaviors into the material, DMMs bring unprecedented flexibility to how materials respond to their environment.
Digital mechanical metamaterial (DMM) with multiple deformation modes. a) Schematic representation of traditional materials with single deformation mode, such as expansion upon under compression. b) DMM is capable of multiple deformation modes, including twist, shear, and compression when subjected to under compression. c) Design details of the DMM, which is assembled using 8 bistable flexible beams, 4 flexible tri-fold beams, and 2 rigid cross beams, all interconnected through two types of mortise and tenon joints. The bistable flexible beams can exist in two states: an initial state signifying {0} input and a compressed state representing a {1} input. d–f) Deformation snapshots of three distinct deformation modes programmed into the DMM: compression–twist coupling (CTC), compression–shear coupling (CSC), and pure compression (PC). These deformations are configurable through the digital encoding input determined by adjusting the steady state of the 8 bistable beams. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Unlike traditional materials, which have one built-in response to external forces, DMMs can be reprogrammed to behave differently depending on the need. By manipulating binary states—much like flipping a switch on or off—engineers can control whether a DMM compresses, twists, shears, or stiffens in response to a load. This leap from passive to programmable materials opens up new possibilities in industries where flexibility and real-time adaptation are critical.
DMMs build on two previous material innovations: smart materials and traditional mechanical metamaterials. Smart materials react to changes in their environment—such as temperature or magnetism—but are limited by their reliance on external triggers. Mechanical metamaterials, on the other hand, have internal structures specifically designed to control their mechanical behavior. Yet, even with their advanced architectures, mechanical metamaterials have lacked the ability to change their properties on demand. This is where DMMs break new ground. They combine the tailored geometry of metamaterials with the programmability of digital systems.
The key to DMM technology lies in bistability—a concept borrowed from digital circuits. Just like a switch can flip between “on” and “off,” DMMs shift between two stable mechanical states, represented as binary “0” or “1.” Each unit cell of a DMM is made up of rigid and flexible components that snap between these two configurations when compressed. By programming how these unit cells interact, engineers can adjust the material’s response to external forces. For example, a DMM can be programmed to remain soft and flexible under one set of conditions, but switch to a rigid, load-bearing state when compressed in a certain way.
What sets DMMs apart from other materials is their modularity. Each unit cell functions independently but can also be combined into arrays to create more complex, system-wide behaviors. For instance, a single unit might twist when compressed, but in combination with hundreds of other cells, an entire array of DMMs can be programmed to absorb energy, dampen vibrations, or adjust force transmission. This modularity makes DMMs adaptable to a wide range of applications, from protecting sensitive equipment to improving the performance of robotics.
One particularly novel application explored by the researchers is the use of DMMs for mechanical encryption. In a proof-of-concept experiment, they designed an array of DMM cells equipped with polarizing filters. By programming the cells to switch between different states, the researchers were able to control whether light could pass through the array. This forms a mechanical encryption system, where information is encoded into the material’s physical state. The concept opens up new possibilities for secure data transmission and storage, where physical manipulation of a material adds an extra layer of security beyond conventional digital encryption.
Beyond encryption, DMMs have the potential to revolutionize areas like energy absorption and vibration isolation. The researchers showed that DMMs can be programmed to exhibit varying levels of stiffness, allowing them to absorb energy at different rates when compressed. This is a critical feature for applications such as protective cushioning, where the material needs to dynamically respond to impact. For example, DMMs could be used in car crash protection systems or aerospace equipment, where it is essential to absorb shocks and protect delicate instruments.
Vibration isolation is another promising field for DMMs. Industries like aerospace and heavy machinery require materials that can prevent vibrations from damaging equipment. DMMs offer a tailored approach to vibration control. By switching between different mechanical states, engineers can program the material to dampen or isolate vibrations as needed. This could be particularly valuable in environments that are subject to constant mechanical stress or high-frequency vibrations, such as in spacecraft or industrial machinery.
DMMs also offer new possibilities in force transmission. In robotics and mechanical systems, controlling how force is transmitted through materials is key to optimizing performance. By programming the stiffness or flexibility of a DMM, engineers can fine-tune how forces are transmitted through robotic joints or mechanical linkages. A DMM-equipped robotic arm, for example, could adjust its grip strength depending on whether it is handling a delicate object or performing heavy lifting – without requiring changes to the material itself.
The modular nature of DMMs means they can be scaled to fit a wide range of applications. At a microscopic level, DMMs could be integrated into biomedical devices, adjusting their behavior to interact more effectively with human tissues. At a larger scale, they could be incorporated into aerospace structures, where their adaptability would offer advantages in controlling forces and protecting components. The ability to digitally program mechanical properties into a material adds a new dimension of control, allowing DMMs to be used in conjunction with sensors and other technologies for real-time adaptation.
Looking ahead, the integration of DMMs with electronic control systems could lead to materials that not only respond to their environment but actively sense and adapt in real-time. Imagine a material that stiffens when subjected to pressure, providing additional support during heavy loads, and then softens again once the load is reduced. This type of intelligent response would have a profound impact across industries, from construction to medicine, where materials must adapt to changing conditions.
DMMs also present opportunities for customization in manufacturing. Engineers could program materials to perform specific functions, reducing the need for multiple types of materials and minimizing waste. In construction, for example, DMMs could be customized to handle different structural loads depending on the building’s design, offering a more efficient approach to material use.
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