Wearable technologies are exploding in popularity in both the consumer and research spaces, but most of the electronic sensors that detect and transmit data from wearables are made of hard, inflexible materials that can restrict both the wearer's natural movements and the accuracy of the data collected.
A sensitive, soft capacitive sensor made of silicone and fabric was developed that moves and flexes with the human body to unobtrusively and accurately detect movement. The sensor could be integrated with fabric to make “smart” robotic apparel. A batch-manufacturing process also was developed to create custom sensors, enabling fabrication for a given application.
The sensor consists of a thin sheet of silicone (a poorly conductive material) sandwiched between two layers of silver-plated, conductive fabric (a highly conductive material), forming a capacitive sensor that registers movement by measuring the change in capacitance, or the ability to hold electrical charge, of the electrical field between the two electrodes. When strain is applied by pulling on the sensor from the ends, the silicone layer gets thinner, and the conductive fabric layers get closer together, changing the capacitance of the sensor in a way that's proportional to the amount of strain applied. This measures how much the sensor changes shape.
In the manufacturing process, the fabric is attached to both sides of the silicone core with an additional layer of liquid silicone that is subsequently cured. This method allows the silicone to fill some of the air gaps in the fabric, mechanically locking it to the silicone, and increasing the surface area available for distributing strain and storing electrical charge. The silicone-textile hybrid improves sensitivity to movement by capitalizing on the qualities of both materials: the strong, interlocking fabric fibers help limit how much the silicone deforms while stretching, and the silicone helps the fabric return to its original shape after strain is removed. Thin, flexible wires are permanently attached to the conductive fabric with thermal seam tape, allowing electrical information from the sensor to be transmitted to a circuit without a hard, bulky interface.
The sensor was tested using strain experiments in which various measurements are taken as the sensor is stretched by an electromechanical tester. Generally, as an elastic material is pulled, its length increases while its thickness and width decrease, so the total area of the material stays constant; however, the conductive area of the new sensor increased as it was stretched, resulting in greater-than-expected capacitance.
The hybrid sensor detected increases in capacitance within 30 milliseconds of strain application and physical changes of less than half a millimeter, confirming that it is capable of capturing movement on the scale of the human body. A set of sensors was integrated into a glove to measure fine motor hand and finger movements in real time. The sensors were able to detect capacitance changes on individual fingers as they moved, indicating their relative positions over time.