UNIVERSITY PARK, Pa. — A research team, including Huanyu “Larry” Cheng, James L. Henderson Jr. Memorial Associate Professor of Engineering Science and Mechanics at Penn State, is using pressure sensors — tiny devices, roughly the size of a paperclip, that can measure the force applied over an area — to design a highly sensitive electronic “skin” to use alongside robots and prosthetic limbs.
Cheng is a corresponding author on a paper, recently published in Nano-Micro Letters, that introduces the improved pressure sensor design. The team’s sensors can be assembled into an interconnected array, offering researchers and clinicians a wireless approach to recognizing spatial pressure distribution, hand gestures and even different types of food based off their weight and texture.
In the following Q&A, Cheng discussed pressure sensing technology and how his team’s work could help robots accurately “feel” the sensation of touch.
Q: Why do current pressure-sensing technologies struggle to balance sensitivity and accuracy? How did you address these issues with your new design?
Cheng: It is still difficult for flexible pressure sensors to simultaneously achieve high precision and responsiveness to subtle pressures, despite extensive research and development. Conventional designs often provide abundant conductive networks, but their irregular arrangement weakens compressive strength, which limits detection range and long-term stability.
In this work, we designed a flexible pressure-sensing platform based on a material known as reduced graphene oxide aerogel (rGOA) — an incredibly lightweight, oxygen-rich material. Using freeze casting, a manufacturing technique that solidifies mixtures of liquids and solids into one material, we can form our sensors to have an anisotropic microstructure, meaning they have different mechanical strengths depending on the direction we apply stress.
With these adjustments, our sensors can simultaneously achieve ultrahigh sensitivity, a broad pressure detection range and long-term stability. Although a single sensor is only about eight millimeters in size, they can each support about three ounces of force and reliably load and unload weight over 20,000 times. By assembling individual sensors into an interconnected array, we can effectively create an artificial “skin” capable of precisely measuring extremely subtle changes in pressure.
Q: How are the sensors built? How did you test their effectiveness?
Cheng: The pressure sensor was fabricated by sandwiching rGOA between a synthetic, plastic-like film stamped with interdigital electrodes — small measurement devices printed onto the material in silver ink — and a layer of thin, silicon-based polymer material. Sandwiching the materials together ensures stable electrical contact, mechanical robustness and flexibility for practical applications.
We tested our sensors by measuring the current response under a wide range of applied pressures, while also assessing frequency response and stability under a range of temperatures and humidities. Our sensors proved extremely sensitive, offering almost twice as much sensitivity as sensors manufactured with traditional structures. Additionally, the sensors exhibited incredibly fast response and recovery times, responding to pressure changes in just over 100 milliseconds, and recovering from responses in only 40 milliseconds — a process that other sensor options can take over 250 milliseconds to fully cycle through.