Beyond Ordinary: What are Metamaterials?

The word “metamaterial” literally means “beyond material”. These are engineered materials with unique properties not found in nature. They get their unusual characteristics, like being incredibly lightweight or possessing exotic mechanical responses, from their intricate internal structures. For decades, scientists have made tremendous progress in designing these “lattice metamaterials” with complex internal geometries.

However, a big challenge remained: once these materials were made, their properties were fixed. The exciting leap in this new paper is the ability to actively and on-demand modulate these properties in realtime. Our recently published paper named ‘Active heterogeneous mode coupling in bi-level multi-physically architected metamaterials for temporal, on-demand and tunable programming’ proposes an “active lattice design”. The initial idea for this active architecture came serendipitously two years ago from experimenting with traditional hexagonal lattices and piezoelectric patches.

The Secret Sauce: Electro-Active Elements and Programmable Coupling

Our research focuses on achieving “heterogeneous mode coupling” in conventional symmetric lattice geometries in the first place by intuitively adding electro-active elements. Simply put, traditional materials often show separate mechanical responses to different forces, like stretching when you pull and twisting when you shear. But what if pulling also caused a controlled twist, and you could program that interaction? This is where “active heterogeneous mode coupling" comes in.

The key to this “material magic” lies in embedding “electro-active elements", specifically, piezoelectric patches, into the metamaterial's structure. Piezoelectric materials have a fascinating property: they deform when an electric voltage is applied to them. By strategically placing these elements within a hexagonal lattice, our “hybrid alternating tri-layered piezo-metallic strip architecture” allows macroscale elastic properties to depend on external electric voltages.

This means we've achieved “programmable voltage-dependent normal-shear constitutive mode coupling” and also active multi-modal stiffness modulation. In simpler terms, by just changing the voltage, you can control how much a material stretches or compresses, and how much it twists or shears at the same time. For example, imagine a “smart block” made of this metamaterial. If you stand on it, you'll feel a downward jump (normal deformation), but also a simultaneous horizontal translation (shear mode). The amazing part is that the amount of this horizontal translation or vertical jump can be controlled by adjusting the external voltages! Interestingly, with zero voltage, there's no sliding between surfaces. This active coupling phenomenon is rarely explored in mechanical metamaterials, especially not in regular symmetric 2D geometries. 

Sneak peek at possible exploitations: real-world relevance for shaping our future

The implications of this breakthrough are vast and exciting, touching various aspects of advanced mechanical applications to be explored by the future researchers.
Such as aircraft wings or wind turbine blades that can dynamically change their shape during operation to optimize performance. This technology could enable truly adaptive structures, allowing for on-demand span, chord, camber, and sweep morphing in aerofoils.

Soft robots are flexible and can navigate complex environments by changing their shape. Our proposed piezo-embedded hexagonal metamaterial can achieve and control such “deformation-based locomotion” in real-time using external voltages (refer to Video 1). If the voltage is zero, there's no locomotion!

“This development presented in this work allows for active and on-demand seamless transition between stiff and flexible characteristics. Unlike conventional materials where high stiffness often means high density and weight, our novel metamaterial can achieve higher stiffness (in both normal and shear modes) on demand through applied voltage, without adding much weight. This means aircraft wings could actively become stiffer under increased aerodynamic loads. Conversely, on-demand flexibility can improve energy absorption and avert resonance,” explains the authors of the research paper published in Communications Engineering [1].

Therefore, it is now possible to have a load-bearing stationary metamaterial working on a particular spot and then have locomotion to move it to a different spot for performing further load-bearing operations in the new location. Such simultaneous on-demand robotic motion and load-bearing performance are proposed for the first time in this work.

That said, there's more to unpack. The “invisibility hats” or “invisibility cloaks” like in the Harry Potter, a character in J.K. Rowling’s novels shows a frictional exemplification of making objects invisible to the human eyes. Such notion of “optical invisibility” is later realized with the advent of various electromagnetic metamaterials. What if instead of optical illusion, we extend the same notion (“mechanical invisibility”) using mechanical metamaterials? Let’s first define the mechanical mode cloaking here. This mode cloaking can be referred to a phenomenon where certain mechanical deformation modes (or responses), such as strain or deformation in a specific direction, are suppressed or made “invisible”, even when stress is applied. It’s like “hiding” the material's mechanical responses in a certain mode or direction. Our work proves the possibility of “partial mode cloaking,” where, for instance, only shear strain exists even with normal stresses. This enables real-time identification and suppression of unwanted stresses in intelligent mechanical, aerospace, and biomedical structures. This could lead to advanced “digital twins” that can actively cancel unwanted stresses through programmed voltages, ensuring uninterrupted mechanical performance and averting failure conditions.

References ( Don’t forget to check! ):

1. Mondal, S., Mukhopadhyay, T., & Naskar, S. (2025). Active heterogeneous mode coupling in bi-level multi-physically architected metamaterials for temporal, on-demand and tunable programming. Communications Engineering, Nature Publication, 4(1), 1-15.