On December 19th, metamaterials startup Echodyne announced a $15 million Series A investment round led by Bill Gates and Madrona Venture Group. As the fourth spinout from Intellectual Ventures, Echodyne is seeking to commercialize novel metamaterials-based radar technology, whose origins trace back to research at Duke University and UC San Diego. The investment demonstrates the building of commercial momentum for the technology and is good impetus to define and discuss metamaterials, identifying why they will be important, especially in the context of advanced manufacturing:
Metamaterials: “Beyond Nature”
The exact definition for metamaterials can vary depending on the source, but it is important to realize that the term “metamaterial” does not refer to one specific material, but rather to a design concept for materials. By arranging very small structures with particular geometries into specific patterns, the resulting system can achieve electromagnetic or acoustic properties which are not inherent to the materials from which the system is constructed. For example, small aluminum “U”s can be embedded in plastic causing the resulting system to respond to radio waves in a way unlike either aluminum or plastic, and furthermore, in a way that it is not simply a mixture of aluminum and plastic responses (as would be the case for a traditional composite).
For “normal” materials, electromagnetic properties are dependent on the identity, placement and bonding of atoms and molecules. In metamaterials, man-made structures (if constructed and tuned correctly) act in the place of atoms or molecules. The placement, shape and interactions of these structures define the electromagnetic behavior. The name is cleverly descriptive: “meta” = beyond. Through intelligent construction of man-made composite materials using the metamaterial design philosophy, one can design electromagnetic properties that, in some cases, surpass the limitations of traditional materials.
The details of what passing those limits really means gets very technical, but the ramifications are explained when discussing applications.
Determining how to construct structures and understanding interactions is incredibly difficult. Without the use of sophisticated computer models and simulations, developing these materials would be nearly impossible. Thus, the field has gained momentum in recent years as computational models and power have become readily available. This trend will continue to accelerate, fueling future possibilities and increasing the number of researchers working on the technology. John Pendry at Imperial College London established the theoretical framework for the field in the late 1990s and early 2000s, while David Smith and his colleagues at UC San Diego were able to experimentally demonstrate the first “double-negative” metamaterial in 2000. So while the field is still relatively young, these early results triggered huge interest in the research community and led to significant development in the last 10 years.
An example of a metamaterial structure is shown above. Metal ring resonators and wires are carefully arranged on circuit boards, in a similar fashion to the groundbreaking work by David Smith and his colleagues at UC San Diego.
How small is small?
The question of “how small is small?” is particularly important with regards to metamaterial structures and explains why fabricating them is non-trivial.
The scale of the structures is measured in relation to the wavelength at which it is designed to operate. For a composite material to be considered a metamaterial, generally the size of the structures should be, at largest, one-tenth the size of the incident wave. That means for radio wave tuned metamaterials, structures may need to be as small as 100 microns, and for light wave tuned metamaterials, the feature size needs to be less than 100 nm (and in many cases, much smaller).
(Interesting materials behavior can also be generated by structures larger than one-tenth the size of the incident wavelength, for example photonic crystals, which utilize wave diffraction. However, these effects are fundamentally different and do not offer the flexibility and possibilities that the metamaterial approach does.)
Fabricating such small structures can be a huge challenge, especially when dealing with structures that have a response to visible light. Furthermore, manufacturing “large” areas or volumes of metamaterials – inches, feet, meters and beyond – can be very difficult, time-consuming and incredibly costly. Currently, techniques for fabricating metamaterials are far from perfect and rarely prioritize, or even consider, structural demands. However, as their commercial use becomes more common and the use of advanced manufacturing techniques more prevalent, these challenges can be overcome and metamaterials could offer tailored electromagnetic/acoustic performance combined with optimized structural properties.
The ability to push electromagnetic and acoustic properties into new domains opens the door for a number of new, and sometimes fantastic, applications. A few notable examples:
- Antennas – certainly not a novel application, but metamaterials can enhance their performance in a number of significant ways, including making them smaller, more directional, with improved conformal performance and dynamic bandwidth / operational frequency.
- Cloaking – the most hyped of applications, but also one that has a long way to go to achieve the cloaking depicted in sci-fi films. By utilizing their unique ability to channel, focus and reform waves perfectly, a shell made of metamaterials could be used to completely hide an object (or space) from specific wavelengths of electromagnetic or acoustic waves. Although such cloaking has been demonstrated in labs (at very specific frequencies), the state of the art still suffers from limited dimensionality and bandwidth, as well as a shell size many times the size of the volume of cloaked space. Simply put, we are a long way away from seeing a cloaked human, but less fantastic applications for metamaterial cloaks are much closer on the horizon, including isolation or shielding of electrical or acoustic components.
- (Super) Lensing – the resolution of a traditional lens system is limited. By using a special class of metamaterials, this (diffraction) limit can be overcome and lenses can be constructed that allow resolution of features much smaller than otherwise detectable. Applications include semiconductor lithography and medical ultrasound.
- Absorbers – by controlling parameters associated with loss, metamaterials can be constructed to absorb or pass precise wavelengths of light (or other electromagnetic radiation) or sound, potentially useful for applications such as noise shielding or solar cells.
Evolving in Stages
Metamaterial technology is still in a relatively nascent stage and complete understanding of all principles (and their ramifications) is still being actively pursued. Although there are a few companies starting to use the design technique, most activity is concentrated in university research labs. The majority of mature metamaterials work is found at operating frequencies in the microwave regime, mainly due to the ease of fabricating the relatively large structures (many in the millimeter range or larger). Pushing to higher frequencies (infrared, visible, etc) adds additional challenges, most notably the aforementioned difficulties with fabricating very small structures, and is less mature. Translation of metamaterials techniques into acoustics is even newer; very few research groups have made significant inroads, but interest is growing.
The technology will first manifest itself in passive structures, whereby the metamaterial has been designed to operate at a specific frequency with very little bandwidth. Over time, metamaterials’ bandwidths will widen and active control will be achieved, allowing the metamaterial to change operational frequencies. Finally, metamaterials will become imbedded and integrated in structural materials, such as carbon fiber or fiber glass.
Outlook for the Future – Materials by Design
There are a number of incredibly interesting things that can be done with metamaterials, but in my opinion, the most important aspect of this technology is the design control that can be achieved over electromagnetic and acoustic properties. By employing metamaterials, engineers will be free to use materials that maximize function, rather than pick from imperfect options provided by nature. Designs that in the past would have needed to be compromised in order to conform to materials-limitations, can be realized and fully optimized by the inclusion of metamaterials.
As mentioned earlier, fabricating metamaterials at scale is difficult; imbedding and integrating them into traditional structural composites is even more challenging. However, these challenges come with great reward, as successful integration of metamaterials will compress an additional dimension of functionality in structural materials. (With the rise of the internet of things and the associated need for all these new devices to communicate with one another, it is very convenient for materials needed for structure and communications to converge.)
Although it remains in its very early stages (and I do not see a glut of startups likely to occupy the space anytime soon), metamaterials could very well fundamentally change the way that we design and manufacture in the future. Metamaterials’ near term commercial success is very likely strongly coupled to effectively leveraging new manufacturing techniques, such as additive manufacturing, to unlock its potential at scale. The future is bright for metamaterials, but those working towards its success will need to be mindful of the challenges associated with manufacturing, if they truly wish to make an impact in the startup scene.