Description:FIELD OF INVENTION
The field of invention is energy harvesting, wearable electronics, piezoelectric materials, biomedical sensors, self-powered devices, and health monitoring systems.
BACKGROUND OF INVENTION
With the rapid development of wearable electronics and health monitoring systems, there is a growing demand for self-powered devices that eliminate the need for frequent battery replacement or recharging. Piezoelectric energy harvesting has emerged as a promising solution by converting mechanical energy from body movements into electrical energy. Piezoelectric materials, such as lead zirconate titanate (PZT) and polyvinylidene fluoride (PVDF), generate electrical charge when subjected to mechanical stress, making them suitable for wearable applications.
Existing Methodology:
Conventional energy harvesting systems rely on bulky batteries or external power sources, limiting device longevity and wearability. Existing piezoelectric harvesters use cantilever-based or flexible thin-film designs to convert biomechanical energy into electrical power. However, these methods often suffer from low energy conversion efficiency, limited flexibility, and integration challenges with wearable fabrics. Recent advancements focus on nanostructured piezoelectric materials, hybrid energy harvesters, and flexible substrates to enhance efficiency, durability, and user comfort in wearable health monitoring devices.
The patent application number 202241001833 discloses a system and method for continuous energy harvesting from constant damping of resonant oscillations. A system utilizing resonant oscillations with controlled damping to achieve continuous energy harvesting for self-powered electronic and wearable applications.
The patent application number 202217044789 discloses a cantilever for a piezoelectric energy harvesting system. A system that continuously harvests energy by damping resonant oscillations, converting mechanical vibrations into electrical power for sustainable applications.
The patent application number 202211063783 discloses a novel bifacial perovskite photovoltaics for harvesting energy from artificial indoor led light sources. A bifacial perovskite solar cell designed to efficiently harvest energy from artificial indoor led light sources for sustainable power generation.
SUMMARY
The invention relates to a piezoelectric energy harvesting device designed for wearable electronics and health monitoring applications. This device efficiently converts biomechanical energy from human motion, such as walking, running, or even subtle body movements, into electrical energy. Utilizing advanced flexible piezoelectric materials, such as PVDF-based nanocomposites or lead-free ceramics, the device ensures high energy conversion efficiency, durability, and seamless integration into wearable textiles. It incorporates a multi-layered structure with optimized electrode design to maximize power output while maintaining flexibility and comfort for continuous wear.
Objective of the Invention
The primary objective is to develop a self-powered system for wearable health monitoring, reducing dependence on batteries while ensuring continuous operation of sensors measuring vital signs (e.g., heart rate, temperature, motion). Additional goals include enhancing energy conversion efficiency, improving mechanical flexibility, and ensuring compatibility with modern IoT-enabled health monitoring systems for real-time data transmission and analysis.

DETAILED DESCRIPTION OF INVENTION
The continuous evolution of modern society, driven by advancements in data science, IoT, artificial intelligence, and robotics, has facilitated seamless interaction between humans and smart devices. This progress has led to the integration of everyday life into an intelligent information network. In this interconnected landscape, electrical power serves as a crucial component, akin to the circulatory system, ensuring the efficient operation of all technological and societal elements.
Wearable Technology and Its Applications
Wearable devices have emerged as a new generation of intelligent electronics, designed for applications such as movement tracking and health monitoring. A key advantage of wearable technology lies in its ability to sense and communicate environmental data in real time. With continuous advancements in miniaturization and functionality, wearable devices are now widely utilized in various domains, including healthcare, education, disability assistance, and gaming. Initially developed for military use, these devices have evolved into essential tools for diverse applications.
Modern wearable electronics encompass attributes such as lightweight construction, compactness, flexibility, high sensitivity, adaptability, reliability, self-powering capabilities, and low power consumption. They manifest in different forms, including wristbands, smartwatches, smart clothing, smart glasses, and embedded sensors.
Challenges in Powering Wearable Devices
Most conventional wearable devices require power ranging from microwatts to watts and primarily rely on batteries as their energy source. However, battery-dependent power systems face several limitations, including:
• Limited lifespan, requiring frequent recharging or replacement.
• Environmental concerns due to battery disposal.
• Safety risks such as overheating, fire hazards, and explosions.
• Design constraints imposed by battery size and weight.
To address these challenges, extensive research is being conducted to explore alternative power sources capable of converting mechanical energy into electrical energy. These innovative energy-harvesting solutions aim to extend device longevity while eliminating the drawbacks associated with conventional battery-powered systems.
Human Motion as a Source of Energy
Vibrational energy, present across various environments and scales, serves as a viable mechanical energy source for power generation. The human body, in particular, produces substantial kinetic energy through muscular contractions, which can be leveraged for wearable energy harvesting. Movements such as foot impacts, joint articulations (knee, arm, ankle, elbow, hip), and shifts in the upper body’s center of gravity generate biomechanical energy. Among these, lower limb movements—specifically those involving the knee, hip, and ankle—produce the highest biomechanical energy output due to their greater torque.
Energy Harvesting Technologies for Wearables
Several transducers have been explored for converting human motion into electrical energy, including:
• Piezoelectric transducers – Convert mechanical strain into electrical energy.
• Triboelectric nanogenerators – Generate power through contact electrification.
• Electromagnetic transducers – Utilize magnetic fields to induce electrical current.
• Electret transducers – Employ charged materials to generate electricity.
The selection of a suitable transducer depends on factors such as power output requirements and device topology.

Piezoelectric Energy Harvesting for Wearables
Piezoelectric energy harvesters have gained significant attention due to their ability to directly convert mechanical energy into electrical energy. These devices offer several advantages, including:
• Lightweight and compact design.
• Cost-effectiveness and ease of fabrication.
• High energy conversion efficiency.
Piezoelectric energy harvesters can efficiently harness various types of biomechanical energy, such as walking, running, joint movements, and even subtle gestures like tapping or knocking. Figure 1 provides a schematic representation of a piezoelectric wearable energy harvesting system based on human motion. By integrating such technology, wearable electronics can achieve self-sustainability, eliminating dependence on traditional batteries while enhancing device performance and user convenience.

Figure 1: Schematic Diagram of Piezoelectric Energy Harvesting from Human Motion for Powering Wearable Devices.
Contribution of This Article
This article provides a comprehensive review and analysis of recent advancements in piezoelectric wearable energy harvesting technology. By examining the existing literature and offering insights into the design principles, materials, and applications of piezoelectric wearable energy harvesters, it serves as a valuable resource for researchers and practitioners. Additionally, the discussion on challenges, opportunities, and future recommendations enhances the understanding and practical implementation of piezoelectric wearable energy harvesting based on human motion.
The selection of piezoelectric materials plays a crucial role in determining the energy-harvesting performance, mechanical flexibility, and durability of wearable devices. The structural configuration of these materials, including their arrangement in devices such as cantilevers or stacked types, influences the efficiency and compatibility of energy harvesters in wearable applications. Different operational modes, such as d31, d33, and d15, offer distinct advantages in terms of voltage output and current generation, shaping the design and optimization strategies for wearable piezoelectric energy harvesters. This article explores these aspects, highlighting key considerations in material selection and design to advance the development and practical application of wearable energy harvesting technologies.
Principles of Piezoelectric Energy Harvesting
Briscoe and Dunn defined piezoelectricity as the accumulation of electric charge in materials with non-centrosymmetric crystal structures in response to applied mechanical stress. Erturk and Inman further characterized it as the interplay between mechanical and electrical behaviors in ceramics and certain classes of crystals.
The direct and converse piezoelectric effects represent two fundamental phenomena within this concept. In 1880, Pierre and Jacques Curie discovered the direct piezoelectric effect, which describes a material’s ability to generate an electric field when subjected to mechanical strain. A year later, Lippmann applied thermodynamic principles to deduce the converse piezoelectric effect, demonstrating that piezoelectric materials deform when exposed to an electric field.
Polarization changes induced by mechanical stress are another significant characteristic of piezoelectric materials. Several factors influence polarization strength and direction, including crystal symmetry, polarization orientation, and mechanical deformation. A piezoelectric energy harvester consists of two main components: the mechanical module, which converts mechanical motion into electrical energy, and the electrical module, which contains a circuit for rectifying and transforming the generated voltage.
Piezoelectric energy harvesting offers several advantages, including structural simplicity, high power and energy density, integration with hybrid materials for a wide range of voltage outputs, independence from external power sources, flexible transducer shapes, micro- and macro-scale fabrication, high Curie temperature, seamless integration into structures, and ease of implementation.
Historically, the use of human body movement for energy harvesting dates back to 1770, when Abraham-Louis Perrelet harnessed energy from arm motion to develop a self-sustaining automatic pedometer watch. Despite this early innovation, the potential of human-centric kinetic energy generators has remained largely untapped. Given that an average person takes approximately 10,000 steps daily, there is significant potential for energy harvesting from walking. Theoretical models suggest a power density of up to 343 mW/cm³ at a walking frequency of 1 Hz, with real-world applications reaching up to 19 mW/cm³.
The remarkable property of piezoelectricity is found in specific crystalline materials, including barium titanate, quartz, Rochelle salt, topaz, and tourmaline, as well as naturally occurring organic materials like cane sugar. This property enables these materials to generate electric charges in response to mechanical pressure. The direct piezoelectric effect is widely used in energy transduction and sensor technologies, while the converse effect is essential for actuation mechanisms.

Piezoelectric Operation Modes
Piezoelectric materials used in energy harvesting typically have a well-defined polar axis, and the orientation of applied stress relative to this axis significantly affects energy harvesting efficiency. Directions parallel to the polar axis are denoted as the “3” direction, while those perpendicular to it are referred to as the “1” direction. These orientations define the primary operational modes used in piezoelectric energy harvesting:
• 33-mode: The applied stress aligns with the generated voltage, producing higher voltage output.
• 31-mode: Stress is applied axially, resulting in voltage output perpendicular to the applied force, which enhances current generation.
The 33-mode is advantageous for applications requiring high voltage output, while the 31-mode is preferred for generating higher currents.

Figure 2: Illustration of 31-Mode and 33-Mode Piezoelectric Operation
Piezoelectricity in Materials and Operational Modes
Piezoelectricity in materials arises due to the presence of an asymmetric center within their molecular chains or crystalline structure. When subjected to an external force, this asymmetry leads to spontaneous polarization or induced polarization. The relationship between the electric field, displacement, stress, and strain is governed by fundamental equations, where the dielectric constant matrix is denoted by “ε,” the piezoelectric constant by “d,” and the compliance matrix by “c.” The subscripts 1, 2, and 3 represent distinct Cartesian coordinate directions, while additional notations describe shear motion around the 4, 5, and 6 axes, as well as rotational motion around the 1, 2, and 3 axes.
The 31-mode configuration is widely used due to its adaptability in creating samples of various shapes and sizes. For instance, one study demonstrated a 31-mode energy harvester comprising a piezoelectric patch bonded to a substrate, typically made of stainless steel or brass, using epoxy. This design was analyzed in terms of output voltage, charge generation, energy efficiency, structural optimization, and theoretical modeling.
In piezoelectric materials such as PZT, the d33 value is more than twice that of d31, making the 33-mode harvester potentially more efficient than its 31-mode counterpart. Various designs have been explored for 33-mode energy harvesters, including configurations utilizing MEMS cantilevers. One such design employs interdigital electrodes to establish the 33-mode operation, where spontaneous polarization aligns in-plane with the PZT layer after poling. Compared to the 31-mode, this configuration enhances voltage generation due to controlled capacitance and an increased piezoelectric coefficient, achieved by modifying the electrode gap.

Figure 3: Structural configuration of the (a) d31 mode, (b) d33 mode, and (c) d15 mode piezoelectric energy harvester.
The 15-mode energy harvester operates based on the shear mode. Its significance lies in the fact that, for PZT materials, the d15 coefficient is typically higher than both d33 and d31. Although fabricating 15-mode samples is more complex, they remain an attractive option for researchers. Zhao et al. integrated a pair of PZT elements in a 15-mode configuration within their energy harvester. To enhance the output voltage, these components were connected in a series arrangement. The selected material, PZT-51, possesses a d31 coefficient of __, a d33 coefficient of 460 __, and a d15 coefficient of __. The structural layout of this device is depicted in Fig. 3(c).
Different operational modes influence performance due to variations in mechanical deformation and motion patterns. Consequently, selecting the appropriate mode is crucial for optimizing the efficiency of a piezoelectric wearable energy harvester. The d31 mode, characterized by bending deformation, is suitable for applications where piezoelectric materials experience flexing or bending. The d33 mode, which undergoes compression or stretching, is ideal for applications involving axial strain. Meanwhile, the d15 mode, associated with shear deformation, is relevant for lateral motion or shear stress.
Piezoelectric materials exhibit distinct properties based on their composition. PZT ceramics offer high piezoelectric coefficients, ZnO is lightweight and can be integrated into textiles, and PVDF films are flexible, making them well-suited for wearable structures. The selection of an operational mode and material should align with the specific requirements of the wearable energy harvester.
Piezoelectric Materials
Piezoelectric materials are classified into natural and synthetic types [68]. Naturally occurring materials include Rochelle salt, quartz, tourmaline group, and topaz, while synthetic materials consist of lead titanate, barium titanate, lead zirconate titanate, and lithium niobate. These materials are further categorized into four groups: ceramics, single crystals, polymers, and nanocomposites, as illustrated in Fig. 4. The following section provides a detailed discussion of each category .

Figure 4: Classification of Piezoelectric Materials
Piezoelectric Ceramics
Piezoelectric ceramics are essential in modern technology due to their excellent electrical, piezoelectric, and dielectric properties. However, they are brittle and prone to fracturing under high strain. Lead Zirconate Titanate (PZT) is the most widely used piezo-ceramic, containing 60% lead, which poses environmental and health risks, affecting the brain, kidneys, and nervous system. The European Union classified PZT as hazardous around 2003, prompting research into lead-free alternatives such as potassium sodium niobate (KNN), barium titanate (BT), bismuth ferrite (BFO), and sodium bismuth titanate (BNT).
Piezoelectric Single Crystals
Piezoelectric single crystals are widely used in actuators and sensors due to their superior piezoelectric properties. Common examples include lithium niobate (LiNbO₃), lead zinc niobate-lead titanate (PZN-PT), and lead magnesium niobate-lead titanate (PMN-PT). These materials exhibit high piezoelectric coefficients, with the d₃₃ value of PMN-PT reaching up to 2000 pC/N, which is four to six times greater than that of PZT.

Piezoelectric Polymers
Piezoelectric polymers offer several advantages over ceramics, including lightweight construction, easy fabrication, and exceptional flexibility. These properties make them ideal for integration into flexible and wearable devices. Although they have a lower electromechanical coupling factor (k), their flexibility allows them to endure higher strain levels, making them suitable for applications involving significant bending or twisting. Examples of piezoelectric polymers include fluoropolymers, polypeptides, polyester, polyurea, polysaccharides, polyamides, and biopolymers such as collagen, silk, and cellulose. Polyvinylidene fluoride (PVDF) is the most commonly used piezoelectric polymer in sensors and actuators.
Piezoelectric Nanocomposites
Nanocomposites combine the advantages of ceramics and polymers, resulting in materials with enhanced properties suitable for diverse applications. By integrating piezoelectric ceramics like PZT with flexible polymers, researchers have developed materials that retain the exceptional piezoelectric performance of ceramics while incorporating the mechanical flexibility of polymers. This allows for the creation of robust and adaptable materials used in energy harvesting, sensing, and actuation applications. Compared to pure ceramic and polymer materials, nanocomposites exhibit improved mechanical strength, durability, and efficiency.
Selection of Piezoelectric Materials
The choice of piezoelectric material depends on various factors, including operational frequency, design flexibility, available volume, piezoelectric properties, and application requirements. Historically, lead-based materials such as PZT have been widely used, with variants like PZT-5H and PZT-5A dominating the field since their development in the 1950s. While PZT remains cost-effective and highly efficient, regulatory concerns over lead toxicity have driven the development of lead-free alternatives like barium titanate (BaTiO₃), which offers reduced transduction efficiency.
Researchers have extensively compared piezoelectric ceramics and polymers based on various parameters, highlighting their unique advantages and limitations. Studies also explore the properties of single crystals such as PMN-PZT and PMN-PT, as well as ceramics like PZT-5H and PZT-5A. The table below provides a comparative analysis of different piezoelectric materials, including their advantages, disadvantages, key properties, and applications in wearable technology.
Comparative Analysis of Piezoelectric Materials
Material Advantages Disadvantages Example Properties Applications in Wearable Devices
Ceramics Easy fabrication, good stability, cost-effective, strong piezoelectric properties Contains toxic lead PZT-5H d₃₃ = 593 pC/N, d₃₁ = -275 pC/N, d₁₅ = 741 pC/N, Curie Temperature = 193°C Used in wearable sensors, actuators, and vibration energy harvesting
Single Crystals High piezoelectric performance Expensive, complex fabrication, lower mechanical strength PMN-PT (33% PT) d₃₃ = 2200 pC/N, d₃₁ = -920 pC/N, Curie Temperature = 145°C Used in wearable devices for monitoring physiological signals such as pulse waves and respiration, as well as energy harvesting
Polymers Lightweight, chemically resistant, flexible Low piezoelectric coefficient, limited temperature range PVDF d₃₃ = 33 pC/N, d₃₁ = -23 pC/N, Curie Temperature = 100°C Used in flexible wearables, smart clothing, skin-attached health monitoring patches, wearable gloves, and gesture recognition bands
Nanocomposites Lightweight, enhanced mechanical strength, improved performance Processing challenges, compatibility with matrix materials Cellulose-BaTiO₃ d₃₃ = 149 pC/N, d₃₁ = -78 pC/N, Curie Temperature = 115°C Used in smart shoes, vibration sensors, and health monitoring wearables
Note: d = piezoelectric strain constant, g = voltage constant, k = electromechanical coupling factor, Q = mechanical quality factor.
Structural Configuration
Piezoelectric transducers are available in various shapes, including unimorph and bimorph cantilever beams, cymbal designs, circular diaphragms, and stacked configurations. The choice of a specific transducer depends on factors such as form factor, energy requirements, and the intended wearable application. Each design offers distinct advantages and limitations, along with unique fabrication processes and applications in wearable technology, as detailed in the following table.
Comparison of Piezoelectric Transducer Configurations
Configuration Description Advantages Disadvantages Fabrication Process Applications in Wearables
Unimorph/Bimorph Cantilever Beam Features one or two piezoelectric layers attached to a non-active substrate. The active layer generates electric charges when bent or flexed. Low fabrication cost, high mechanical quality factor, simple structure, low resonance frequency, increased power generation with larger proof mass Limited ability to withstand high-impact forces Layer deposition (sputtering, sol-gel, or chemical vapor deposition), electrode patterning, bonding, poling, integration Lightweight and suitable for integration into clothing and accessories such as gloves and shoe soles
Cymbal Type Composed of a piezoelectric disk sandwiched between two metal caps. The disk flexes due to mechanical vibrations, generating electric charges. Can endure strong impacts, high energy output Restricted to applications requiring high-magnitude vibrations Disk fabrication (ceramic processing such as tape casting and hot pressing), metal cap fabrication, assembly, electrode patterning, integration Durable and ideal for outdoor wearable applications
Circular Diaphragm Utilizes a circular piezoelectric diaphragm that flexes under external force, generating electric charges. Operates effectively in pressure mode, more robust than a cantilever of similar size High resonance frequency Disk fabrication (ceramic processing), electrode patterning, integration Used in flexible patches and smart textiles
Stacked Type Composed of multiple piezoelectric layers stacked together. Suitable for pressure mode operation, higher output in the d33 mode, capable of withstanding high mechanical loads High stiffness Layer deposition, electrode patterning on each layer, stacking, poling, integration Provides greater output power, making it ideal for wearables with higher energy demands

Energy Harvesting from Human Motion
The human body continuously expends energy to perform various activities such as walking, speaking, eating, breathing, and moving limbs. Every muscle movement involves both positive and negative work. Positive work occurs when muscles contract and shorten, with torque aligning with joint movement. Conversely, negative work involves lengthening contractions where torque opposes joint movement. Both forms of work are essential for daily activities.
Energy harvesting systems can assist muscles during negative work by slowing down motion, similar to regenerative braking in vehicles, while simultaneously capturing and converting mechanical energy into electrical power. This harvested energy can be used to power wearable devices such as smartwatches, fitness trackers, and health monitoring patches. Understanding the energy potential of various human movements is crucial, and researchers have analyzed the energy associated with different body motions under diverse conditions.

Figure 5: Human Body Energy Potential
Walking is one of the most energy-intensive human movements, with each step generating over 1 Joule of energy per joint, providing more than 1 Watt of available power for energy harvesters. Studies indicate that walking on flat ground can harvest approximately 6.6 J/step, while descending and ascending stairs yield 7.47 J/step and 3.12 J/step, respectively.
Table 3 presents the energy generated by different joints, with the ankle producing the highest power (66.8 W) and the knee experiencing the highest negative work (92%). Table 4 details various sensors powered by piezoelectric energy harvesters, including LEDs operated by devices placed in shoes, jackets, and other locations.
Human motion energy harvesting is classified into three categories:
1. External Excitation Harvesters: These harvest energy from macroscopic body movements (e.g., walking, running, joint motion). Examples include energy harvesters mounted on backpacks, knee braces, and waist-mounted devices.
2. Direct Deformation Harvesters: These employ flexible, stretchable materials that generate electricity when bent or deformed by body movements (e.g., elbow, wrist, and knee motion).
3. Gradual Pressure Harvesters: These capture energy from slow, minor displacements such as finger tapping, foot pressure, blood pressure changes, and breathing cycles.
Human motion energy harvesting presents a promising approach to generating sustainable power from everyday activities. Walking, in particular, is a highly energy-intensive movement, providing significant opportunities for energy extraction through various methods. Among these, external excitation harvesters, direct deformation harvesters, and gradual pressure harvesters each offer unique advantages in capturing biomechanical energy.
Research shows that the ankle joint generates the highest power output, while the knee experiences the most negative work, highlighting key areas for optimizing energy harvester placement. Advancements in piezoelectric and flexible nanogenerator technologies further enhance the efficiency and practicality of energy harvesting systems.
As wearable energy harvesting devices continue to evolve, improvements in efficiency, comfort, and integration with existing electronics will be crucial for widespread adoption. Future research should focus on optimizing energy conversion rates, minimizing power losses, and developing lightweight, user-friendly designs to maximize real-world applicability.

DETAILED DESCRIPTION OF DIAGRAM
Figure 1: Schematic Diagram of Piezoelectric Energy Harvesting from Human Motion for Powering Wearable Devices.
Figure 2: Illustration of 31-Mode and 33-Mode Piezoelectric Operation
Figure 3: Structural configuration of the (a) d31 mode, (b) d33 mode, and (c) d15 mode piezoelectric energy harvester.
Figure 4: Classification of Piezoelectric Materials
Figure 5: Human Body Energy Potential , Claims:1. Piezoelectric energy harvesting device for wearable electronics and health monitoring claims that a piezoelectric energy harvesting device designed for continuous power generation in wearable electronics and health monitoring applications.
2. Utilizes flexible piezoelectric materials to efficiently convert biomechanical energy from human motion into electrical power.
3. Capable of harvesting energy from multiple body movements, including walking, running, and joint flexion.
4. Integrates ultrathin, lightweight, and flexible structures for enhanced wearability and comfort during prolonged use.
5. Employs high-sensitivity piezoelectric materials with optimized electromechanical coupling for improved energy conversion efficiency.
6. Features an energy storage system that stabilizes and regulates the generated power for direct use in low-power medical and wearable devices.
7. Incorporates biocompatible and non-toxic materials to ensure safety and long-term usability for skin-contact applications.
8. Supports real-time health monitoring by powering sensors that track physiological parameters such as heart rate, temperature, and motion patterns.
9. Designed to be self-sustaining, reducing the dependence on conventional batteries and enabling long-term operation of wearable devices.
10. Compatible with wireless communication modules to enable seamless data transmission for remote health monitoring applications.