## 4 Design  
Researchers have taken various approaches to designing energy-harvesting systems using different materials and configurations. One common strategy involves arranging piezoelectric transducers in a series-parallel configuration to boost both voltage and current output. The resulting AC voltage is then converted to DC using a full-wave bridge rectifier with smoothing capacitors [5]. For instance, one design placed six piezoelectric cells between two wooden blocks, with springs allowing the upper block to return to its initial position after being stepped on. Tests with participants of different body weights showed how much voltage was generated over time [5]. By connecting the piezoelectric sensors in a series-parallel arrangement, the voltage and current outputs were maximized, enhancing energy efficiency [16]. Another approach mounted piezoelectric sensors in a series-parallel configuration on a wooden board to harness footstep energy, rectifying the AC to DC and storing it in a battery [93, 94, 95]. However, [PERSON] and [PERSON] [96] report that each step produces only minimal energy, limiting this design to very low-power uses. [PERSON] et al. [63] compared different circuit setups--series, parallel, and series-parallel --to see which yielded the most energy. Their system also employed Maximum Power Point Tracking (MPPT) to optimize solar energy collection. [PERSON] et al. [4] introduced a spatially varying (SV) model for the electric field in piezoelectric energy-harvesting gyroscopes. They compared this model with open-circuit (OC), constant electric field (CE), and constant electric field with coupling (CE-\(\upgamma\)) models, testing a range of load resistances using the Differential Quadrature Method (DQM). Additionally, [PERSON] et al. [4] embedded a piezoelectric patch into a micro-electromechanical system (MEMS) gyroscope, modeling both energy harvesting and angular velocity sensing. [PERSON] et al. [3] integrated fourteen piezoelectric diaphragms into a floor tile to capture energy from footsteps. These diaphragms were connected in parallel to a rectification circuit with Schottky diodes to convert AC to DC. A filter capacitor then smoothed the output, and a DC-DC buck converter stabilized it at 5 V for small-scale applications. In contrast, [PERSON] and [PERSON] [19] replaced compression-only setups with a disc spring washer, achieving up to 22% better performance under varying force and 125% better under changing velocity than a crinkle washer. Although less efficient overall, the crinkle washer showed promise for fine-tuning future designs by altering the number and distribution of its waves. Figure 5 illustrates a typical piezoelectric tile design: six piezoelectric cells sandwiched between wooden blocks with springs to handle repeated compression. The generated AC is rectified, smoothed, and stored in a battery for use in low-power applications. In terms of mat design, [PERSON] et al. [27] focused on portability by creating six square modules that each house multiple PENGs, TENG stacks, and EMGs arranged in a scattered layout. This design increases both energy output and ease of transportation, making the mat suitable for high-foot-traffic areas. According to the authors, the scattered layout-unlike the stacked configuration often found in hybrid footwear-enables a greater number of energy harvesters to run at once, significantly increasing overall power generation [27]. Meanwhile, [PERSON] et al. [97] took a different route, using triboelectric nanogenerator (TENG) technology and textile-TENG (t-TENG) arrays. Their mat, which features a core-shell yarn structure, can sense pressure and blend seamlessly into typical household environments. This \"smart carpet\" detects falls by tracking pressure changes across various TENG cells, and it uses an algorithm to tell ordinary footstep patterns apart from falls, sending an alert if a fall occurs [97].