Q&A: Harvard's Siyi Xu Designs All-New Strain and Force Sensors

Working with teams from Harvard, Beth Israel Deaconess Medical Center, and Boston Children's Hospital, Siyi Xu developed a soft, non-toxic, wearable sensor that attaches to the hand and measures grasp force and the motion of the hand and fingers. The sensor is designed to identify neuromotor disabilities in prematurely born children.

Tech Briefs: What was the motivation for this project?

Siyi Xu: We wanted to study the behavior of prematurely born children as they grew. That's why two of the most important criteria were to make the sensors really safe and really small.

Tech Briefs: What is the structure of the sensor?

Xu: We developed different designs for strain sensors and force sensors. For the strain sensors, there is a silicone substrate, a conductive liquid-filled channel, and silicone-coated wires. There is also a thickness gradient from the two ends to the center to concentrate the deformation onto the micro-channel. For the force sensors, we used a multilayer architecture. We added microcylinders to support the channels under deformation, thereby improving the linearity and decreasing the hysteresis.

Tech Briefs: How do you measure motion and resistance?

Xu: The strain sensor is attached to the top side of a finger. As the finger bends, the sensor is stretched and its cross-sectional area decreases. This causes the resistance of the fluid-filled channel to increase. By sensing how much the resistance has increased or decreased, you can tell the position of the finger.

Resistance is a bit tricky, because with the biocompatible solutions we're using, if you apply direct current/voltage or exceed voltage thresholds, the fluid could fail. We create an oscillator circuit where the resistance of the sensor affects the period of the oscillator. We have configurations with many of these sensors operated in parallel, so we inject a current and then pick off different voltages and they are fed to the oscillator circuit. We measure the period of the oscillator and convert that to a voltage.

We measure force by knowing the resistance. If you press on it, the resistance will increase, and when you release it, it will fall back.

Tech Briefs: What applications are possible?

Xu: Health monitoring, fatigue monitoring, everyday use for factory workers subject to ergonomic challenges, virtual reality, computer interfaces — anything where you want to measure and utilize motion of the body. You can have a glove with the soft sensors embedded to remotely measure or control the pressure you apply in sports training. If you're working with a physical therapist but you're at home, how does the therapist know you're doing the right exercises, conditioning the right muscles, and not the wrong muscles? It can give remote feedback for those types of things.

Tech Briefs: You would have these sensors all over your body at key points?

Xu: Depending on what you want. If you sprained your ankle, then you'd have them in a sock; if you had a leg injury, they could be in a sleeve on your legs.