By changing the properties of conventional semiconductor wafers -- from stiff and brittle to bendable and stretchable -- engineers at the University of Illinois have opened the door to such devices as light-emitting diodes on a suture thread and even implantable biomedical tattoos.
John Rogers, an Illinois engineering professor, said the researchers sliced silicon and other materials thinner by a factor of 100,000 -- as if creating a sheet of paper from a two-by-four. Then, to achieve mechanics similar to those of a rubber band, they structured the material in a wavy, accordion-like shape.
I spoke with Rogers last week about what this advance means for biomedicine.
Progress has been made to allow semiconductor wafers to be used in biomedicine. How did we get there?
If you think about conventional semiconductor devices -- whether they're in electronics or in lighting in the form of LEDs -- all of those technologies rely on semiconductor wafers. They're very flat. They're stiff and they break easily. Those geometric and mechanical properties are not a design constraint for lots of applications -- your laptop computer, your cell phone. If you want to bring those technologies to bear on problems that relate to the function of the human body, for example, then there some challenges.
The human body is curvilinear and soft and elastic, which is fundamentally different than the properties of semiconductor wafers. We've been trying to figure out ways to bridge the gap in mechanics and geometry between the body and semiconductor technologies. If you can do that, you can make new devices that could address important problems in human health -- things ranging from advanced surgical tools to interfaces between the body and machines.
The details of how we do that rely on advanced concepts in mechanics. Our approach has been: stick with materials that are already known well [such as silicon] and are already in commercial products. But configure them into mechanical designs and geometries that allow the materials at the device level to offer these soft, stretchy properties. It really builds on existing technology know-how, rather than reinventing all of that materials technology.
How far along are you in this process?
We've been working this problem for five, six years now in my university group. Over the last year or so, we've reached a level where we can do just about everything we need to do to begin to address some of these problems. We have techniques that allow us to build advanced surgical tools that have clinically relevant modes of use.
Last year, we stood up a venture-backed start-up company to begin to move this to the next level. That is, outside a university lab and into commercial forms that people can start using for beneficial outcomes in human health [and] monitoring diagnostic systems for sporting goods. Our work in this area has culminated to where it begins to look to be technically mature enough to pursue commercialization possibilities. That's not to say that commercial products are on the shelves yet, but the trajectory is in that direction.
How could these semiconductor wafers be used within the human body? Some news stories on your work referenced "biomedical tattoos."
The paper we published had a demonstration of an implantable thin sheet that supported LEDs. It implants underneath the skin in the area where a tattoo is typically located. We did not have in mind an advanced form of body art. We're thinking about how you can embed LEDs and other kinds of semiconductor devices into the body for therapeutic benefit.
What are the possible uses of an LED that's implanted in the same position as a tattoo? We think there are three:
- One [use] is having a light source in the tissue that allows you to do a characterization by looking at the way the emitted light is scattering off that tissue. As an example, you've probably used finger-clip pulse rate monitors which use LEDs located outside the body to monitor blood flow. You can extract a pulse rate from that measurement. It turns out you can do a lot more sophisticated things. You can determine blood profusion, you can do a lot of diagnostics of the tissue, the structural components of the skin and bone and tendons. Having a light source in the body can be used for diagnostics of that type. This paper demonstrated our newly established [ability] to put LEDs anywhere we want, whether that's in the body, on a rubber band, on a suture thread, on a piece of paper.
- There are certain classes of drugs that can be delivered to the body in an inactive form and subsequently be activated by exposing them to light. You can introduce this drug and use implanted light sources to activate the drug only in the position where it's needed.
- The third possibility is a little less certain, but still could be interesting. There are reports in the literature -- and this is a subject of ongoing debate -- that indicate that if you expose tissue to certain wavelengths of light, certain colors of light at certain intensities, you can accelerate in some cases the wound healing process. If that is true, then having an ability to deliver light in a patterned way at a wound site, whether it's a burn or a cut, could have benefits in accelerated wound healing.
What about the use of semiconductor wafers in biomedical tools?
That represents an area of commercialization activity the start-up is pursuing. Another is with a large sporting good manufacturer. The details of that will be released soon. One is consumer sporting good monitoring equipment. The other is an advanced surgical device that is based on a balloon catheter.
An existing way to do certain kinds of surgical procedures involves the insertion of a catheter that has an integrated balloon on its surface. It inserts into the body through a small incision in its deflated state. It's then located at the position of an internal organ where one wants to carry out a procedure. At that position the balloon is inflated. That's used in the treatment of clogged arteries. You position the balloon in a location where there's clogging and the balloon expands and eliminates that blockage. The balloon is interesting because it is soft and able to conform to the curvilinear surface of the tissue.
If you could integrate high-performance semiconductor devices on the balloon [then you could deliver all kinds of advanced sensing and therapeutic functionality]. But LEDs, electrodes, flow monitors and other active functionality you can achieve today on a flat wafer is completely incompatible with a balloon. That relies on a lot of the same concepts that allow us to do LED tattoos and stretchable electronics -- bringing all that to bear on this balloon catheter platform to provide more insight into what's going on in the tissue and to provide active functionality for manipulating the tissue with these devices on the surface of the balloon.
Image, top: Microscale light-emitting diodes and photodetectors on the fingertip region of a surgical glove / University of Illinois
Image, bottom: John Rogers / University of Illinois