# A Novel Magnetic Respiratory Sensor for Human Healthcare
## 2 Materials and Methods  
The MLCR sensor that utilizes the advantages of the giant magneto-impedance (GMI) effect of an ultrasoft magnetic Co-rich microwire [18], and an LC resonance circuit of an inductive coil made of the Co-rich microwire, has recently been developed by our group for real-time respiratory monitoring [19] and for spintronics applications [20]. Details of the design and fabrication of the MLCR sensor have been reported in Ref. [19]. In this study, we aimed to optimize the sensing capacity of the MLCR sensor. To do so, we performed a systematic investigation on the sensitivity of the MLCR sensor as a function of the operation frequency and the distance between the sensor and the targeted sensing object. To determine the optimized operation frequency and distance from the MMC sensor to a tiny permanent magnet, a vibrator with a non-magnetic arm was used. The waveform driving the vibrator was a sinusoidal wave with 0.1 Hz frequency and 3.3 V amplitude, which was 0.5 cm from top to bottom. Change in the external magnetic field, due to vibration of the permanent magnet, drives a change in the impedance of the MLCR sensor, corresponding to the highest and lowest peaks, which is defined as the sensitivity of the MLCR sensor (i.e., sensitivity, \(\Delta Z=Z_{\text{h}}-Z_{\text{l}}\), where \(Z_{\text{h}}\) and \(Z_{\text{l}}\) are the highest and lowest values of impedance, respectively). It should be noticed herein that the initial value of impedance, \(Z\), was automatically set by the impedance analyzer. Since we consider changes in \(Z\) a result of breathing (e.g., the breath amplitude) and breathing pattern variations (e.g., the respiratory rate, abnormal breathings), this initial impedance value does not affect test results reported in this paper. Figure 1a displays a typical vibrating (or breathing) pattern measured by the MLCR sensor, as it detects the magnetic signal of a vibrating permanent magnet that was located 5 cm away from the sensor. By studying the effect of variation of operation frequency from 60 to 140 MHz on the sensor's sensitivity, we found the optimal operation frequency of 80 MHz to achieve the highest sensitivity (see Figure 1b). Using this optimal operation frequency, we investigated the effect of changing the distance between the sensor and the permanent magnet on the sensor sensitivity and found that the 5 cm distance is optimal for magnetic respiratory monitoring using the MLCR sensor (see Figure 1c). Therefore, in the thesis research, we operated the MLCR sensor at \(f\) = 80 MHz and placed it at \(d\) = 5 cm from the permanent magnet for all breathing monitoring experiments.  
To examine the sensing capacity and consistency of the MLCR sensor in monitoring human respiratory motion in real time, we designed experiments with the sensor held on a non-magnetic arm at a distance of 5 cm away from a tiny permanent magnet (e.g., NdFeB) mounted to the skin of a test subject. As the test subject breathes in and out, the chest will rise and fall, which generates a corresponding magnetic field change of the permanent magnet, tracked by the MLCR. In other words, the motion of the permanent magnet is converted into an impedance change of the MLCR sensor. To perform human respiratory motion tests, we designed a comfortable test configuration, as illustrated in Figure 1d. In this sensor, the sensing element consists of an ultrasoft magnetic Co-rich microwire coil (the inset of Figure 1d) that enables the sensing of magnetic fields at magnitudes as small as a pT.  
Figure 1: **(a)** Real-time monitoring of a vibrator’s motion with a vibration frequency of 0.1 Hz and an optimal distance of 5 cm from a tiny permanent magnet. This simulates the normal breathing patterns of a patient; (b) the sensitivity of the magneto-LC resonance (MLCR) sensor versus frequency at a distance 5 cm away from a vibrating tiny permanent magnet; (c) the sensitivity of the MLCR sensor versus distance (\(d\)) for \(f\) = 80 MHz; (d) a patient lays down on a comfortable bed for real-time respiratory monitoring.