Fiorenzo Omenetto, Frank C. Doble Professor of Engineering at Tufts University School of Engineering, Dean of Research, and Director of the Tufts Silklab led a team that has produced an array of printable microwave resonators using silk-based metamaterials.
Tech Briefs: What got you started on this work?
Fiorenzo Omenetto: As you may know, I run a lab called Silklab where we've been working for a long time with biomaterials. But instead of working with biomaterials in traditional disciplines that are more geared towards their use for integration in tissue or for medical applications, we started to work with these materials for technological applications. I define technological applications as photonics and electronics, which is my background. That led to the use of silk, which I was “dragged” into by a conversation in the hallway with one of my colleagues, David Kaplan, who's a tissue engineering pioneer. That brought silk into the domain of diffractive optics, and photonic crystals, and waveguides, and gates for semiconductors, and hybrid devices that were doped with biological dopants that maintained their chemical functionality.
I was familiar with these expressions of technology from my days in ultrafast optics and microelectronics, but not with a biological function embedded. As an example, a diffraction grating made of silk and hemoglobin is unheard of. So, what you can do with it is still up for debate, but I think it's an incredibly powerful tool.
We want to redefine some applications in these domains and, broadly defined, the photonics and microelectronics worlds — the high technology domains. I have a feeling that biomaterials can really narrow the gap between the natural world and the technological world, things like the methods of planar fabrication, and of exquisite microelectronic techniques, and optical techniques. We have at our disposal this bottom-up fabrication that is typical of nature and that has dimensional control down to the nanoscale, has exquisite material control, and is effectively very interactive with the environment that surrounds it. It's also two different mindsets, in a way. Technology is defined by rigid design rules and metrics, whereas the natural world is very much adaptable, self-healing, and variable, and dependent on environmental conditions. Those things could scare a technologist. But I think that there are lots of advantages in those qualities. And there are a lot of advanced transduction techniques available from our technological backbone that, if you merge the two worlds, really start coming to life to provide something that probes the environment but is technically shaped to interface with the technological front ends that we're very familiar with and use every day.
It’s very powerful and very exciting to do — working with all these materials that are multifunctional. That’s what got me started in this. And the “why did I do it” is because I had a conversation in the hallway about silk. It was about a tissue engineering implant that was supposed to replace the cornea and it needed a couple of holes for the cells to grow in. I had femtosecond lasers in the lab to poke the holes in it, and when I tried to align the laser onto this transparent silk film, I didn't see any scattering and I lost the beam. This told me that the surface was of exquisite optical quality. And then we started looking at other optical applications, one thing led to the other and here we are with all sorts of silk-based technology.
Tech Briefs: As a parenthesis: One time I was assigned to do an article on how to make some engineering companies more creative and one of my key points was that enabling random conversations in hallways is a source of much creativity.
Omenetto: And I think that the other thing is to have people who are willing to talk — that's a very underestimated thing when we talk about interdisciplinarity. I'm not a big believer in putting different disciplines together in the same building and hoping for the best. I'm a big believer in people who are curious and are unselfish in sharing their expertise. I suppose serendipity is involved so it’s hard to come up with a recipe. David is an exceptional person in this regard. And so are many other people that that I work with. It's generally a lot of fun.
Tech Briefs: It's hard for me to imagine how biological materials interact with what you call technological materials such as standard electronics. I can't picture the mechanism.
Omenetto: Silk from the silkworm is basically a polymer — a biopolymer. So, I think that the direct analogy is with some of the polymers that are used in a general technological context. For example, they could be polymers used in planar fabrication approaches like photoresists. There are also analogies between biopolymer formats and some of the dielectrics and some of the crystalline matrices, whether they're silicon or silicon oxides. I think the picture becomes clearer if you think of how we make our materials. So, when I say that we work with silk, we really work with the precursor to the silk fibers, that is the liquid that the silkworm has in its glands to spin silk fibers. We work with that liquid: a suspension of the constituent protein of silk — a water suspension of fibroin molecules. That is the precursor material to generate a lot of end material formats by directed assembly of the silk fibroin proteins, if you appropriately dose the concentration of the molecules, the viscosity of the solution, and so forth. A simple example is that you can spin coat the silk solution with high control and generate layers that are atomically smooth. After hundreds of man-years of work on this, we can control them down to the atomic level — to the monolayer level. You can drop cast these materials like you would with elastomers, as we would with polydimethylsiloxane for example. You can do soft lithography and nanoimprint them like you would with thermoplastics, albeit at lower temperatures and pressures. When a silk solution is laid out and left to dry, beautiful transparent freestanding films are formed that are conformal at the nanoscale, et cetera, et cetera. Broadly speaking, the fact that this is a protein matrix provides, if you will, the missing link between technology and biology. You start with a water-based suspension of fibroin molecules, which has a neutral pH. So, as long as you have something that is dissolvable in water, you can add it to the mix and generate a large library of “biologically doped” material formats. So, it's sort of like bulk doping a technical material. And that's powerful because you can pick your inorganics or your organics, mix them with this silk solution, and reshape them into to all sorts of material formats of technological interest.
If you choose certain sets of organic materials, it turns out that when materials are formed, going from the liquid to the solid state of silk, whatever was mixed into the precursor solution is entrained in the solid format, but, importantly, preserves its biological activity in this format.
To go back to serendipity, when we started exploring soft lithography with silk and making all sorts of diffractive optics with it, we were very impressed by their optical quality. It was around Halloween and we were helping somebody do some flow cytometry experiments and we had some blood in the lab. We decided that, since it was Halloween after all, we would make a blood diffraction grating. So, we added some of the blood to the silk solution and cast it. We made a 600 line-per-millimeter diffraction grating that was bright red and we all were incredibly pleased with ourselves.
Then we forgot about it for a couple of months and when we looked at it again, the grating was still bright red. We expected the blood to be dark brown as it usually is when you leave it outside on a surface, but it wasn’t. So, we started a series of experiments where we saw that the heme molecule contained in the grating was still capable of binding and unbinding oxygen. We saw that the molecule was still active within this format — you had sort of a self-sensing grating. That led to a lot of things — that’s what I mean by having a technical format that bridges the two worlds.
Imagine now that you have a material that is compatible with UV lithography, with e-beam lithography, with spin coating, with etching, and with integration of devices. And this material is capable of containing/preserving a biological function and being bioreactive. The behavior of the device that results from this is then modulated by an enzyme that picks up something from the environment, or something that reacts to the presence or the absence of oxygen, a virus, or whatever else. You then could imagine integrated devices that are silicon-based, with an interface tied to a biological factor that can be directly manufactured with traditional techniques changes the behavior of the device.
This is where our material creativity starts because you can build an unusual transduction mechanism with unusual behaviors. And, additionally, you can fabricate a wide array of devices with inorganic dopants in very easy ways as well. So, for instance, we can bulk-dope a photonic crystal with quantum dots, which is something that you can't ordinarily do if you work with silicon or more conventional etching fabrication techniques. Our process borrows from colloidal science and from traditional top-down fabrication to create a powerful platform.
The unique enabler is that you can mix in the silk labile biological molecules and store them in technical formats with resolutions of a few nanometers, shaped, and interfaced with an electronic backbone that is well understood and with a photonic backbone that is also well understood. You have decades and decades and decades of technology that helps you build around these things. So, it's very exciting to to me.
Tech Briefs: How did you print your microwave resonators?
Omenetto: How did we get there from the from the world of silk? It happened because silk is bio-compatible, silk is implantable, it's food safe, and so on. So, we started building passive microwave devices: the more usual RF frequency resonators, passive resonators, and then, GHz resonators could then be affixed on biological tissue such as the surface of a tooth.
The principle in this case is that you can build a regular high-Q resonator, such as a split ring resonator, but now it is built onto a biochemically responsive substrate. So, the idea is that if you if you have a resonator that is built on a biologically active substrate that then reacts to the environment around it, the resonator’s resonance frequency is modulated by the interaction of the substrate with the outside world.
The substrate is technically very good because it allows you to build small resonators with micron-sized gaps, and thus metamaterials in the in the Terahertz range and even more. With these devices that were that were deposited on films of silk, you could monitor small changes of bound water in the film, you could monitor different states of crosslinking of the film, and so forth.
This is where we started; but the concept of having an active material to assemble a device on, or make devices with, that would act as an interface to the environment at the gap in resonators, then sort of stuck. So, the the things that happened as a result were: improving the production at scale (and at different scales) of these devices, flexible electronics, devices that could be integrated with the skin, that could be put on the surface of teeth, that could be eaten, or just could be printed in large volumes. What happened as a consequence of this was that we started using inkjet printing to print the high Q resonators and then the gaps were filled with PEDOT:PSS.
Adding this modulator then allowed us to actively tune what happens in the antenna array by adding an electrolyte. Just by putting salts in different parts of the gap, you can tune the response of the material depending on where you expose the sheet of resonators.
Fast forward to definitions: you have a big array of split ring resonators, which then leads you to have a metamaterial where the individual gaps are active and can be tuned by putting electrolytes on them wherever you’d like. You can tune the behavior of the metamaterial, it’s not static anymore — it's dynamic. So, it's printable, it's large-scale, it can be scaled up easily, and then it can be reconfigured as you want.
Tech Briefs: So, you tune the resonator to a fixed frequency in advance?
Omenetto: The typical metamaterials are built with an array of antennas that are generally static. The global electromagnetic response and the behavior of the metamaterial is fixed, depending on the designs of the antennas that you use. The one difference here is that you design an array — a 2D array of antennas. Then you can tune the resonant frequency by modulating the resonance in the individual gaps. So, there's a mutual interaction between the resonators and then there's the modulation from the conductive polymer you put in the gap that allows you to make the metamaterial dynamic.
Tech Briefs: How will dynamic metamaterial be used?
Omenetto: It’s up to anyone’s imagination. The most overused and abused example is invisibility cloaks. At frequencies that are much higher than the ones we use, we try to get light to bend in different ways so that it bends and propagates around an object — it cloaks the object as a consequence. But when the material is manufactured and “static”, then you're stuck with a with a particular behavior, albeit a spectacularly interesting one from a physical point of view.
In this case I think that one could start thinking about designing the material to have a certain response — a certain electromagnetic response — with specific resonance/absorbance. Then it is possible to tune the frequency of the response: if you add a little bit of electrolyte in a particular section of the metamaterial, you can move the “operating point” of the material, which makes it more absorbent at one frequency and then in a wet state at another frequency, or even turns off the absorption. You can start playing with these things and start modulating what the material does.
Now, what you'll do with it is a different story. We have this object that operates at a certain frequency range, so what is the use case scenario here? Moving beyond half a gigahertz, maybe we can go somewhere else. Maybe towards optical domains, maybe you can use transfer printing to support new applications in those regions.
There are certainly sensing applications for this technology. There are maybe other exotic applications that are predicated on continuous tuning of the electromagnetic response of the metamaterial itself. Or maybe you want to play a prank on somebody and get their remote not to work anymore.
The jury is still out as to where this is going to be applied. I think that the main concept here is that you have large scale, flexible materials — metamaterials — that can be printed, therefore scaled up. And you're not bound to a static geometry; the geometry can be tuned so you can modulate them in frequency, amplitude, and phase.
Tech Briefs: If you're going to use this as a sensor, say on the body, where does the microwave energy come from?
Omenetto: Well, it could be from a passive reader. This is generally for applications in impedance spectroscopy, where a transmitter senses the bounce-back signal once interrogated. If you were to put a drop of liquid inside your credit card, for example, the near field communication would be perturbed to a certain point. So, the question is whether you can extract useful information out of that — what that change in electromagnetic interaction means.
Tech Briefs: What's the next step in your work?
Omenetto: How much time do we have?
As far as this project is concerned, although our approach was using inkjet printing, I think transfer printing could be equally scalable and quite compelling. It would reduce the dimensions of the material, which would extend the frequency range of operation of these devices.
We already did transfer printing of some of these devices on the Terahertz scale with much smaller resonators, which we put on the surface of food and things like that. So, thinking about scaling this approach to different to regions of the electromagnetic spectrum is certainly one area we will look into.
Sensing is another—seeing if we can improve on impedance spectroscopy through the ability to have different resonators coexist on the same surface.
And this is only as far as the electronics is concerned. I'll keep my answer to that, but there are lots of other possibilities. Certainly, there are a lot of applications for sensing and large-scale sensing, redundant sensors, redundant systems, passive sensing devices, and more functional materials. We will be working on hard format integration in silicon of these types of biomaterials and integration of biomaterials in conventional fab processes of these materials, in order to have these “biologically-connected devices.”
An edited version of this interview appeared in the September 2021 issue of Tech Briefs.