Ying Zhong, Assistant Professor of Mechanical Engineering at the University of South Florida, and her team have invented a new way to print wearable sensors without polymer binders. They call their product e-skin.

Tech Briefs: What gave you the idea to use electrostatic force for printing flexible sensors?

Professor Ying Zhong: We had some collaborators who were manufacturing flexible devices — printing them with screen printing. That means you have to mix functional powders with binders, heat them, and wait for them to dry, which takes a long time. So, we were looking for ways to print more rapidly, without the heating and drying.

Prof. Ying Zhong

I had been working on another interesting project: using corona discharge to treat the surface of polymer films. We realized that the corona could generate a unique type of strong electric field that we could use to lift small objects. That got us to wonder if it could be possible to place dry powders underneath the film to attract the powders directly to the substrate — we tried it and it worked.

We discovered a problem and we realized that a technology we were already working on could be the solution.

Tech Briefs: I think that's the way progress often happens in technology. My next question is that once the particles are attracted to the substrate, how are they held there?

Zhong: As we described in our published paper, we are using eight-micrometer-thick medical tape, which has an adhesive coating to hold the particles. It is so thin that when you adhere it to your skin, you won’t feel it.

Tech Briefs: How would you incorporate different kinds of sensors when you're manufacturing this?

Zhong: This printing technology is “universal” — it can print different types of functional materials. So, for instance, we can use graphene or carbon nanotubes (CNTs), which change resistivity to reflect the change in strain. The working mechanism is that strain changes the distance between the particles, causing a change in resistance that reflects the magnitude of strain in the sensor.

We have also reported a temperature sensor based on thermo-chromic powders in our paper. As I said, this is a universal printing technology — you can print different types of functional materials. Now we are demonstrating that we can print temperature sensitive tattoos that change color when the temperature changes. We are also trying to print materials that can be sensitive to humidity and different types of gases such as ammonia, carbon monoxide, and others.

Corona-Enabled Electrostatic Printing prints electronic skin, or “e-skin,” by using corona discharge to create a strong electric field between binder-free functional powders, such as graphene, and flexible, non-conductive surfaces, such as medical tape. The electrostatic force enables a multitude of e-skin sensors to be printed within sub-seconds, compared to the 20 minutes it takes with polymer binders, and doesn't require heat. (Credit: University of South Florida)

Tech Briefs: Have you thought about how this could be implemented commercially. It seems to me you would have to build a whole different kind of 3D printer.

Zhong: Yes, we are currently collaborating with Jabil, inc. to build a roll-to-roll system, which can allow you to manufacture devices at high speed. Since we don't use binders, we don't have to apply heat and then wait for the material to dry. That that means by using this roll-to-roll system to print — to generate the electrostatic force — we can manufacture flexible devices and even 3D structures at a fast rate and at low cost.

Tech Briefs: Is it difficult to generate and handle the high voltage necessary to produce corona?

Zhong: Although we need 20 to 30 kilovolts to generate the corona, the current is very low, lower than 0.1 mA — our lab equipment uses less than 2 watts of power— it’s very energy-efficient.

Tech Briefs: Is it a problem to ensure that the high voltage can’t cause harm?

Zhong: Well, of course you must manage the system and cover up all the surrounding conductive areas. Based on our observations, as long as you cover up the holder for the electrode with insulating layers, it should work very well — we didn't run into too much trouble with that.

Tech Briefs: What kind of material do you use for the cover?

Zhong: For now, in our lab we are simply using insulating tapes. In the future we are planning to create a better controlled surface coating or insulating holders, which can be easily done. We can use polymers to realize the coating, and 3D printers to manufacture the prototype. We have multiple polymer-related projects in my lab, so that should be easy.

Tech Briefs: Do you have other applications in mind in addition to your “e-skin” strain sensor?

Zhong: Yes, for example, we have been talking with a company about making smart strain-sensing carpets, which becomes practical because ours is a very low-cost and ultra-fast manufacturing technology that can be used to print large-area products. The idea is to produce carpet that could identify how many people are standing on it. You could then, for example, adjust the energy consumption of the HVAC system based on the number of occupants in a room.

We are also collaborating with a colleague on an application for physical rehabilitation. Our colleague works with people who have difficulty controlling the behavior of their legs. Because our sensor is ultra-thin, it's very comfortable and you can make it large enough to wear. That could be used to help them identify how they can better control the behavior of their legs.

There are many possible future applications. For example, you could attach it to a robot’s hand. If you want it to pick something up, it could sense how much pressure is being applied and how much area it is in contact with.

Tech Briefs: Do you have a rough guess as to when this could be ready for commercial use?

Zhong: Right now, we are working with Jabil to set up a small roll-to-roll system in our lab. If it works well, we will set it up in Jabil and help them to manufacture sensors in large numbers or large sizes. That might take three to five years, but especially these days, it’s hard to predict.

An edited version of this interview appeared in the January 2022 issue of Tech Briefs.