Stretchable Electronics Bend and Fold Like a Rubber Band

In the following conversation, John Rogers, Lee J. Flory-Founder chair in engineering at the University of Illinois at Urbana-Champaign, discusses the application of stretchable electronics technology to balloon catheters and envisions its role in future medical device applications.

MPMN: Describe your stretchable electronics concept. How is stretchability achieved, and what technological hurdles must be overcome to fabricate stretchable electronics?

Rogers: How to create flexible, stretchable electronics is an interesting question and a challenging problem because if you think about conventional electronics, they are all built on the rigid, planar surfaces of semiconductor wafers based on silicon or gallium arsenide. What we were interested in is building circuits with the kinds of performance capabilities associated with state-of-the-art silicon-type devices, but in formats that can be bent and stretched and folded and even manipulated like a rubber band or a balloon. Our feeling is that if you can accomplish that outcome, it affords new opportunities for integrating electronics directly with the human body for all kinds of purposes, ranging from physiological status monitoring to new kinds of surgical and therapeutic devices.

How to achieve stretchable electronics is a materials-level challenge that we have been thinking about for a while. It turns out that there are some relatively simple ideas in elementary mechanics that allow you to achieve stretchable silicon devices. One of them is that if you make anything thin enough, it becomes flexible just by virtue of the fact that bending stiffness decreases rapidly with thickness. A very, very thin sheet of silicon is flexible because of those mechanics, and a wafer is not--it's a rigid, nonbendable type of material. Thus, step one is to make the silicon very, very thin--maybe a factor of a thousand or ten thousand times thinner than a silicon wafer.

Making a silicon substrate very thin is a pretty easy way to get bendability. But stretchability is a little bit different, and it's really what you need if you want to accomplish biointegration. You can bend a flexible sheet of silicon or even a sheet of plastic. You can also wrap them around certain kinds of curvilinear shapes such as cylinders and cones, but you can't wrap them around a sphere. And you certainly can't wrap them around a body part. Their mechanics are not really matched to the soft, elastic nature of tissues in the body. Thus, you need to go beyond flexible.

You do that by configuring your circuit into an open-mesh design that allows individual devices to be interconnected using springy-type wires that can deform and move. When these wires are bonded to an underlying rubber sheet, the entire mesh can accommodate stretching in a way that isolates strain away from the device nodes in the mesh and allows the interconnects to move, providing a reversible linear elastic response to applied force.

That's what we've done in our latest project. We've exploited these two ideas--thinness and mesh geometry--to integrate a range of sensor and electronic functionalities onto the surface of an otherwise conventional balloon catheter. When a balloon catheter is inflated, it undergoes hundreds of percent of strain deformation, but we've designed the circuit so that it can move with the balloon without constraining its motion and without compromising the performance characteristics of the electronic devices.

MPMN: How do your stretchable electronics perform such functions as mapping regions of the heart and ablating abnormalities in conjunction with a balloon catheter? Why is this functionality important for doctors?

Rogers: We're materials scientists and engineers, but we work closely with clinical cardiologists to define the most useful levels of functionality that we can possibly provide in this format. Conventional balloon catheters are just dumb devices. They're completely nonfunctional; they merely effect a mechanical intervention. They do not perform sensing or actuation in the sense that we're talking about now. Thus, the idea was to take a balloon and add the most useful functionality as defined by our collaborators.

We designed the system to afford functionality for interventional procedures that are used to treat certain classes of arrhythmias. Such procedures currently involve two steps. The first involves mapping the electrophysiology associated with the beating heart to identify regions of the tissue that are behaving in an aberrant or abnormal fashion. The second is an ablation, or resection, step, which essentially eliminates aberrant tissue to address the arrhythmia. One way to eliminate the tissue is to use radio-frequency energy to burn it thermally and destroy it. Currently, these two procedures are performed using separate point-type catheters--one for mapping and one for zapping.

The mapping process itself occurs in a point-by-point fashion, whereby a surgeon moves the mapping catheter from one place to another along the interior surface of the heart, measuring single-point ECG traces at each location. In this point-by-point way, you can stitch together an overall map for identifying spatial regions that need to be dealt with. And once that's done, you come in with another point-type catheter that provides ablation functionality. You go back through in a point-by-point fashion and ablate away aberrant tissue.

This procedure works extremely well, but the problem is that it's time-consuming. It also requires a fair amount of skill on the part of the surgeon, and its resolution--the repeatability of being able to identify just the aberrant tissue and remove just that tissue--can be challenging. And in these procedures, morbidity is most closely tied to the duration of the procedure. Thus, our cardiology colleagues have told us that anything we can do to make it more precise, faster, and less dependent on the skill of the surgeon would be valuable.

So that's what we did. Our circuit provides multipoint mapping functionality and--using the same catheter--the ability to perform multipoint ablation. Our vision is that you have a soft balloon catheter that you insert into the interior of the heart. Then you inflate it to push the sensors and the ablation electrodes against the soft, beating inner surface of the heart. First you map, and then you zap using the same catheter in the same position, and then you're out. It's much different from what's done today. Conceptually, it's the same kind of process, but it's carried out in a more advanced engineering mode.

MPMN: In addition to mapping and zapping, can your stretchable and flexible electronics be customized to perform other functions?

Rogers: We have design rules for making this kind of flexible mesh array, and we can incorporate just about any kind of semiconductor device technology into this platform. Thus, we have incorporated sensing and ablation electrodes, but we've also developed temperature, flow, and tactile sensors to accommodate this platform. Temperature sensing is important because the ablation process is intrinsically based on temperature. The ability to sense temperature allows you to monitor in situ what you're doing to the tissue with the ablation electrodes. Flow sensing is important because the rate of cooling and the contact of the electronics to the tissue can be impeded by the flow of blood through that interface. Thus, being able to measure local surface-level blood flow is also important. And tactile sensing is important for measuring exactly how hard you're pressing the balloon against the tissue.

To show that we could do it, we even demonstrated devices that had tiny LEDs mounted on them. In this case, clinicians did not tell us that they needed LEDs; our goal was merely to demonstrate a capability--basically to put a stake in the ground and say, "Look, any kind of semiconductor device technology you're interested in, we can do it. And here's an example."

MPMN: What are your future plans for this flexible, stretchable electronic technology?

Rogers: In future cardiology applications, such as the treatment of arrhythmias, it's a straightforward path for us. We're going to populate the surface of a balloon catheter with many more sensors to get a better, high-definition view of what's going on and improve precision. I think that we can also scale our technology in a straightforward manner. These advances are kind of obvious. The other thing we're interested in is getting this technology into a form that can be useful for patients. Thus, we intend to move it from the research lab and animal demonstrations into something that can be useful for people that are suffering from this kind of heart disease.

To that end, we have begun a startup company called mc10 (Cambridge, MA). This company also has close ties with Massachusetts General Hospital (Boston) and the Sarver Heart Center at the University of Arizona (Tucson). This startup is pursuing commercialization. And in my lab, we're pursuing higher levels of integration.

But more generally, I think that while this kind of technology allows you to perform different kinds of procedures on the heart, it enables almost any kind of interface with the body to take on a new complexion. Thus, we're building devices that can map electrical activity in the brain, for example. The focus there is on diagnosing and treating epilepsy.

I think that stretchable electronics are suitable for a whole host of different application areas. We're pushing in a variety of directions and trying to do as much as we can in close collaboration with surgeons so that we can direct our technologies to the things that are most important. I'm not a doctor; I'm trying to feel out the situation. But we are engaged in robust collaborations in the areas in which we're working.