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March 2016: Exploring the Power of Optimization
Stretchable Electronics You Can Wear: How Abaqus Helped Make It Happen
Abaqus user Yonggang Huang is Walter P. Murphy Professor of Mechanical Engineering and Civil Environmental Engineering at Northwestern University. With a BS in Mechanics from Peking University in Beijing and a PhD and MS in Engineering Science from Harvard University, Professor Huang has worked closely with Professor John Rogers of the University of Illinois to develop a wide variety of stretchable electronics applications. Recently, their teams developed a wearable, stretchable electronic device for skincare company, L’Oreal that was showcased at the Consumer Electronics Show in January of 2016.
SCN: When did you realize that you wanted to become a mechanical engineer?
HUANG: I grew up in a mechanical engineering family. Both of my parents, my aunts, uncles, cousins, and siblings are in the fields of either mechanical or civil engineering. My father is a professor of mechanics and my mother is a retired civil engineer. I started using Abaqus as a PhD student at Harvard in 1987-1990 and we found it to be very powerful. I now use it a lot, not only with my group, but also to teach.
SCN: Tell us about this latest skin patch you developed for L’Oreal. What does it do?
HUANG: Our goal is to take existing, rigid devices and make them both flexible and stretchable. The L’Oreal patch is a device that can detect and monitor a person’s hydration level to indicate the amount of sun exposure. It’s mechanically almost invisible, which means people will be able to shower, sleep, and even play sports while wearing the device, which will remain fully functional for five days.
SCN: How did the L’Oreal project happen?
HUANG: L’Oreal initially contacted Professor Rogers and his technology company, MC10, Inc., after hearing him give a talk about stretchable electronics. I have been collaborating with Dr. Rogers for years, both at the University of Illinois and at Northwestern, where I’ve worked since 2007.
SCN: What are stretchable electronics and how do they differ from flexible electronics?
HUANG: Flexible electronics have been around for more than 15 years. But the idea for stretchable electronics was created by Professor Rogers and myself, as well as our teams at the Universities. A flexible object needs to be thin, but flexibility does not ensure stretchability.
For example, you can easily wrap a piece of paper around a cylindrical object, such as a soda can, because paper is flexible. However if you try to wrap the piece of paper around a complex shape, like a soccer ball, the paper will wrinkle. The same principle applies to electronics. If you try and wrap the electronic around a complex body part, it will wrinkle which will cause the device to fail. To mitigate that, Professor Rogers and I came up with the idea for stretchable electronics.
SCN: How do you get the electronics to stretch?
HUANG: This is a key part of where mechanics comes into play—and when you develop a stretchable device, Abaqus FEA becomes a necessity. To make the L’Oreal patch stretchable, we had to come up with a curvilinear shape that would allow the device to stretch with the skin and remain functional. We used Abaqus to test different geometric shapes and ensure stretchability.
SCN: What role did Northwestern play in the development of the patch?
HUANG: At Northwestern we focus more on the design and mechanics aspects of the process while Professor Rogers’ team at the University of Illinois is in charge of design, fabrication, and device performance. In our lab we use Abaqus software to simulate, evaluate and optimize this technology and then the other team further develops the project towards fabrication and mass production. We had specific goals regarding electronic performance and stability. The device needed to stretch 20% or higher to accommodate for all the motions on the skin surface. Abaqus helped us find all the different possibilities and decide the best route for fabrication.
SCN: You mentioned that the patch will stay on your skin for five days. How do you ensure that the device will stick for that long?
HUANG: Some applications may use a very thin adhesive, but in other instances, such devices don’t need an adhesive at all. The patch relies on the van der Waals forces instead. Think of geckos. Their feet have a natural adhesion that allows them to not only stick to a surface, but easily detach as well. We try to use the same idea for our devices.
SCN: What was the main challenge for you from the design point of view?
HUANG: The main challenge was finding a way to use the current 2D fabrication technology for rigid devices to make 3D curvilinear, stretchable electronics. We transfer-printed the device from their rigid, growth substrate to a different, naturally-stretchable polymer substrate. Our idea was to first pre-stretch the polymer substrate, then place the device on top. When we let go of the stretched substrate, the device buckled to form a 3D shape to achieve stretchability.
SCN: When the device is buckled it still has to function. How do you ensure that happens?
HUANG: This is also a critical point for where mechanics come into place. Normally, buckling is considered to be bad. But here, we designed it in such a way that when it buckles, the geometric shape changes, but the strength inside the buckled device remains. The mechanics design, electronics, and circuits all have to go hand in hand to make sure that the device has certain functionality and at the same time won’t break during the buckling process.
SCN: What are the benefits of using simulation for product design?
HUANG: Simulation not only greatly reduces costs, but it cuts the development cycle down a lot. You don’t need to use raw materials for trial and error testing. We use software to find out what the best design is and then we collaborate with Professor Rogers’ group. Simulation reduces our total product development time tremendously.
SCN: What other stretchable electronics are you and Professor Rogers working on?
HUANG: MC10, Inc. has launched a new product called Biostamp based on stretchable electronics. It measures all skin activity and it transfers the biosignals to your cell phone, from which you can monitor your own health. We have also created stretchable and flexible EKG (for heart) and EMG (for muscles), allowing people to monitor their heart and muscle activity without having to go to the hospital.
SCN: Where do you see the future of stretchable electronics heading?
HUANG: There is always development within the electronics industry to make devices smaller and faster. But in addition, if we can make them flexible and stretchable, the possibilities are limitless. The federal government has invested $70 billion in this research for the next five years. I feel that this technology will be increasingly important for biomedicine, especially for people who care about their health and longevity. Abaqus can play a very important role as these technologies develop.
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