Wearable Magnetic Field Sensor with Low Detection Limit and Wide Operation Range for Electronic Skin Applications

Abstract

Flexible electronic devices extended abilities of humans to perceive their environment conveniently and comfortably. Among them, flexible magnetic field sensors are crucial to detect changes in the external magnetic field. State-of-the-art flexible magnetoelectronics do not exhibit low detection limit and large working range simultaneously, which limits their application potential. Herein, a flexible magnetic field sensor possessing a low detection limit of 22 nT and wide sensing range from 22 nT up to 400 mT is reported. With the detection range of seven orders of magnitude in magnetic field sensor constitutes at least one order of magnitude improvement over current flexible magnetic field sensor technologies. The sensor is designed as a cantilever beam structure accommodating a flexible permanent magnetic composite and an amorphous magnetic wire enabling sensitivity to low magnetic fields. To detect high fields, the anisotropy of the giant magnetoimpedance effect of amorphous magnetic wires to the magnetic field direction is explored. Benefiting from mechanical flexibility of sensor and its broad detection range, its application potential for smart wearables targeting geomagnetic navigation, touchless interactivity, rehabilitation appliances, and safety interfaces providing warnings of exposure to high magnetic fields are explored.

Keywords: amorphous magnetic wires; magnetic field sensor; magnetosensitive smart skins; wearable electronics; wide detection range.

Figure 1

Flexible amorphous wire‐based magnetic field sensor for wide‐range magnetic field measurements and low detection limit. a) Schematic diagram of magnetic field detection. The sensor can be applied to or integrated in a decorative fingernail. The sensor is in the form of a cantilever beam and can detect magnetic field of different strength. b) Detection principle of the flexible magnetic field sensor. When the magnetic field is small, the cantilever beam retains its bent state. The magnetic field is measured based on the GMI effect of the amorphous wire itself. When the magnetic field became stronger, due to the interaction between the magnetic field and the magnetic patch, the cantilever beam and the amorphous wire are bent in the parallel direction to the magnetic field. Thus, the response curve of the amorphous wire to the magnetic field and the impedance of the sensor changes. c) The preparation process of the sensor: an elastic permanent magnet patch is formed by mixing NdFeB and PDMS. The patch is magnetized in a pulsed magnetic field and placed on a cantilever beam accommodating a GMI wire‐based sensor. Conductive lines are formed by brushing liquid metal at appropriate places for contacting the sensors. The entire device is encapsulated in PDMS. d) Optical image of the sensor and e) a close‐up of the amorphous wire and liquid metal contacts. f) The sensor is applied to a decorative nail. g,h) Bendability of the sensor devices.

Figure 2

Characterization of the flexible magnetic field sensor. a) Response of the flexible magnetic field sensor to the magnetic field. The sensor is driven at different frequencies with PI substrate. The driving current is 1 mA. b) Response of the flexible magnetic field sensor to the magnetic field when the sensor is fabricated on different substrate material, with 3 MHz driving frequency and 1 mA driving current. c) Magnetic field response of sensors with prepared on cantilever beams of different length made of polyimide material. d) The maximum field, which can be detected with our sensor, when using magnetic patches with different concentration of magnetic particles. The insert shows the magnetic remanence of the composites with different content of magnetic particles. As the PDMS content increases, the remanence magnetization (M_r) of the magnet patch decreases, while the maximum field, which can be detected by the sensor, increases. e) MI performance of the sensor dependent on the position of the magnet patch, which is located at different positions on the cantilever beam.

Figure 3

Response of the flexible magnetic field sensor to the magnetic field of different strength. a) MI response of the sensor in the range of 0 – 600 mT. Insert is an enlarged view of the sensor response at 0‐0.1 mT and 100–600 mT. b) MI performance of the sensor (right y‐axis) exposed to magnetic field, which increases 22 nT every 10 s (left y‐axis). c) MI performance of the sensor (right y‐axis) exposed to magnetic field, which increases 100 mT every 10 s (left y‐axis). d) Stability of the MI performance of the sensor exposed to 100 cycles of an applied magnetic field of 200 mT. The sensor is made of PI substrate, with a measurement frequency of 3 MHz, a current of 1 mA, a cantilever beam length of 3 cm, and a magnet at the top.

Figure 4

Application scenarios of wearable magnetic field sensors. a) Gaming demonstration where a wearable sensor applied to a finger nail is used to guide the direction of a car relying on the interaction with geomagnetic field (about 40 uT). The displacement of the car to the left or to the right is controlled by the swing of the hand (Video S1). The insert is an enlarged view of the sensor applied to the finger. b) The change of the impedance with the swing of the finger, which is used to control the direction of the car in the game as shown in panel (a). c‐d) The measurement of the pulse beat using the sensor. A small flexible magnet patch is applied to the arm. The sensor on a finger is brought in proximity to the magnet patch, which allows to measure the pulse caused by the change in the magnetic field due to the tiny displacement of the magnet. Typical magnetic field, which is detected by the sensor in this demonstrator is 10 mT. e‐f) Safety application demonstration. The same sensor as in previous examples can be used to detect strong magnetic fields to warn the wearer on the undesired exposure. The magnetic field threshold for this particular demonstrator is set to 200 mT (Video S2). The sensor is made of PI substrate, with a measurement frequency of 3 MHz, a current of 1 mA, a cantilever beam length of 3 cm, and a magnet at the top.