The team used a vinyl cutter to create a stretchable smart mesh, which can be used for sweat sensing applications. (Image Credit: Peisheng He/UC Berkeley))
UC Berkley engineers devised a new technique that allows medical researchers to test new wearable sensor prototype designs quickly and at an affordable cost. Rather than relying on photolithography, this approach uses a $200 vinyl cutter. It also significantly reduces small quantities of sensors’ production time by approximately 90% while decreasing costs by nearly 75%.
“Most researchers working on medical devices have no background in photolithography,” Renxiao Xu, who developed the technique, said. “Our method makes it easy and inexpensive for them to change their sensor design on a computer and then send the file to the vinyl cutter to make.”
Traditionally, wearable sensors are fabricated via photolithography, requiring a cleanroom and sophisticated equipment. Such sensors feature an island-bridge structure containing electronics and components like resistors, capacitors, and carbon nanotubes. Each bridge connects one island to another.
The new technique has a simplified, quicker, and more economical sensor development process. First, an adhesive sheet of polyethylene terephthalate (PET) attaches to a Mylar (biaxially oriented PET) substrate. These are then shaped with a vinyl cutter using the tunnel and through cut. The tunnel cut involves slicing through the PET layer while keeping the Mylar substrate intact. Meanwhile, the through cut carves through each layer.
Island-bridge sensors are fabricated in this manner. Tunnel cuts trace the interconnects path in the top adhesive PET layer. Afterward, the PET pieces are removed, providing a pattern of interconnects on the Mylar surface. Then, the plastic layer gets covered in gold, and the leftover PET layer is removed. This results in a Mylar surface featuring well-defined interconnects. The islands also contain exposed metal openings along with contact pads.
Sensor elements are mounted on the contact pads. Electronic components like resistors rely on a conductive paste and heat place to secure the bond. Also, carbon nanotubes can attach to the pads without heat. After this process, the through cut shapes the sensor’s profile, which includes zigzags, spirals, etc.
The team showcased their new technique by creating stretchable elements and sensors. One of these attaches under the nose to measure human breath through small changes occurring in temperatures between the sensor’s front and back region.
“For a breath sensor, you don’t want to something bulky,” Liwei Lin, professor of mechanical engineering and co-director of the Berkeley Sensor and Actuator Center, said. “You want something thin and flexible, almost like a tape beneath your nose, so you can fall asleep while it records a signal over a long period of time.”
However, sensor cutting cannot produce features less than 200 to 300 micrometers wide. In comparison, photolithography creates features that are only tens of micrometers wide. Even then, many wearable sensors do not need those fine features. According to the team, this technique could be used in labs that focus on studying wearable sensors or new diseases. Prototypes can be designed with CAD software or apps designed for vinyl printers.
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