## 4 Application
### Multi-Parameter Selective Sensing  
Multi-parameter selective sensing with a single sensor is challenging. The sensor array discussed in Section 4.3 facilitates this. As shown in Figure 11, the array integrates diverse sensing units, split between those with Terfenol-D and those without. This distribution enhances spatial resolution and signal strength. Units with distinct structures serve different functions. Units without a magnetostrictive material output time-frequency diagrams. Based on heartbeat characteristics, an average stable interval in the curve is chosen as the \"stable frequency\". The \"stable frequency\" is unaffected by heartbeat stress; it enables the heart temperature to be determined at this point. According to the curve in Figure 7a, an accurate single-point temperature can be obtained. For instance, the stable frequency 1.88470 GHz in Figure 11 corresponds to 311.15K (38 \({}^{\circ}\)C). Heart rates are also derived from these units by averaging several heartbeat cycles in the time-frequency data. The measurement of the cardiac magnetic field requires both types of units due to overlapping between the cardiac magnetic field and heartbeat peaks; a single sensing unit cannot exclude the influence of heartbeat stress. Differentiating the two sets of time-frequency graphs yield the Terfenol-D contribution to the sensor output, which represents the cardiac magnetic field isolated from heartbeat stress. Converting frequencies to magnetic fields via the relationship in Figure 9b yields a point-specific cardiac magnetic field. By processing  
Figure 10: Sensing test of MCG signals. (**a**) Cardiomagnetic signal and recovery signal in arrhythmia patient; (**b**) signals from arrhythmia patient detected by MP-SAWS; (**c**) cardiomagnetic signal and recovery signal in healthy person; and (**d**) signals from healthy individuals detected by MP-SAWS. varied outputs, the array selectively measures heart temperature, heart rate, and cardiac magnetic field. According to specific application scenarios, the detailed design of the MP-SAWS array is shown in Figure 12a. The array layout adopts a regular periodic design. The sensing unit without Terfenol-D consists of four independent SAWS units, each in the shape of a square. Based on the design of 64 interdigital electrode pairs shown in Figure 3a, the SAWS units are squares with a side length of 0.2 mm. The sensing unit with Terfenol-D consists of two sets of the SAWS-Terfenol-D-SAWS structures depicted in Figure 8a. The Terfenol-D is designed as a rectangle with a length of 0.4 mm and a width of 0.2 mm to maintain consistency in the arrangement of the SAWS units. Two different types of sensing units are arranged in a cross pattern to form a larger periodic unit, as shown in Figure 12a. The SAWS units form a square lattice with equal spacing, ensuring uniform measurement. The average planar dimensions of the human heart plane are approximately 12 cm in length and 8 cm in width [73]. According to the size of the periodic unit in Figure 12a, an array of 64 by 48 periodic units was designed. Along the longer side, there are 256 SAWS units, and along the shorter side, there are 192 SAWS units. The total size of the MP-SAWS is approximately 15 cm by 11 cm, producing 256 by 192 SAWS units that can deliver two-dimensional cardiac data. Different parameters to be measured are extracted from the resonant frequency results of a single-channel output. When applying the MP-SAWS, the environment is kept free of external magnetic fields. During temperature measurement, each SAW unit provides a continuous output for a single point. By selecting a point and using appropriate algorithms, such as excluding outliers and taking the average, the average resonant frequency of the point in the stable state (stable frequency) of the heart is estimated to determine the heart temperature. To measure heart rate, the periodic information of the resonant frequency is extracted from the SAWS units.