Flexible and wearable energy storage devices: Nanomaterials, device architectures, and bio-integrated applications

 Corresponding author: Blessing Inuaghata Etinosa; Email: Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. Flexible and wearable energy storage devices: Nanomaterials, device architectures, and bio-integrated applications Blessing Inuaghata Etinosa 1, *, Ezekiel Ezekiel Smart 2, Damilola Emmanuel Olayiwola 3, Jude Ifeanyichukwu Ishiwu 4, Ugochukwu Daniel Okwor 5, Charles Chimezie Dike 6 and Olaoluwa John Adeleke 7 1 Department of Chemical Engineering, University of Benin, Nigeria. 2 Department of Mechanical Engineering, Akwa Ibom State University, Akwa Ibom, Nigeria. 3 Department of Mechanical Engineering, University of Ilorin, Nigeria. 4 Department of Electrical/Electronics Engineering, Nile University of Nigeria, Abuja. 5 Department of Metallurgical and Materials Engineering, University of Nigeria, Nsukka. 6 Department of Electrical and Electronic Engineering, Sheffield Hallam University, Sheffield, United Kingdom. 7 Department of Electrical Engineering, The Polytechnic Ibadan, Oyo, Nigeria. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 Publication history: Received on 19 April 2025; revised on 30 May 2025; accepted on 02 June 2025 Article DOI: https://doi.org/10.30574/gjeta.2025.23.3.0181 Abstract The rapid evolution of wearable and bio-integrated electronics has intensified the demand for high-performance, deformable energy storage systems that can seamlessly conform to the human body while maintaining electrochemical efficiency and mechanical durability. This review critically synthesizes recent advancements in flexible energy storage devices (FESDs), emphasizing cutting-edge developments from 2022 to 2025. It begins by exploring material innovations, including carbon-based nanomaterials like graphene, carbon nanotubes, and MXenes; metal nanowires and oxides; and hybrid composites, detailing their contributions to conductivity, flexibility, and energy storage performance. The discussion progresses to novel device architectures, such as planar, fiber-shaped, and origami-inspired geometries for both supercapacitors and flexible batteries, with special attention to electrode design, substrate selection, and encapsulation techniques that ensure resilience under bending, twisting, and stretching. Integration into real-world applications is analyzed across textile-based platforms, skin-mounted and implantable systems, and self-powered hybrid configurations that combine triboelectric, piezoelectric, or photovoltaic modules for autonomous operation. Experimental validations through real-time use cases in health monitoring, athletic performance, and military wearables underscore the feasibility of these technologies. This review also rigorously evaluates the core challenges impeding widespread adoption, including the trade-off between energy density and flexibility, cycling stability under mechanical stress, safety concerns, toxicity of active materials, and barriers in large-scale manufacturing and cost. Looking ahead, it identifies key research trajectories such as biodegradable electronics, AI-enabled energy systems, and edge-computing integration, and calls for intensified interdisciplinary collaborations spanning materials science, bioengineering, and human–machine interfacing. By articulating both the technological progress and strategic research pathways, this article presents a forward-thinking vision to guide academia and industry toward a new era of smart, energy-autonomous wearable systems. Keywords: Flexible energy storage devices; Wearable electronics; Carbon nanomaterials; Supercapacitors; Biointegrated systems; Flexible batteries; Hybrid energy systems; Smart textiles; Energy harvesting; Next-generation wearables Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 140 1. Introduction The rapid advancement of wearable electronics and the Internet of Things (IoT) has necessitated the development of energy storage solutions that are not only efficient but also adaptable to various form factors. Traditional rigid batteries pose limitations in terms of design flexibility, leading to the exploration of flexible and wearable energy storage devices. These innovative systems offer the potential to seamlessly integrate into clothing, accessories, and even the human body, enabling continuous health monitoring, real-time communication, and enhanced user comfort [1]. Flexible energy storage devices, such as bendable batteries and supercapacitors, are designed to withstand mechanical deformations like bending, twisting, and stretching without compromising performance. This adaptability is crucial for applications in smart textiles, wearable health monitors, and flexible displays. Moreover, the integration of energy storage into wearable platforms reduces the need for bulky external power sources, enhancing the overall user experience [2]. The shift towards flexible energy storage is also driven by the demand for lightweight, portable, and unobtrusive power solutions. As wearable devices become more sophisticated, there is an increasing need for energy storage systems that can deliver high energy and power densities while conforming to complex shapes and movements. This has led to significant research efforts focused on developing materials and architectures that combine mechanical flexibility with superior electrochemical performance [3,4]. 1.1. Market and Technological Trends (2022–2025) Between 2022 and 2025, the flexible and wearable energy storage market has witnessed substantial growth, propelled by advancements in materials science, nanotechnology, and manufacturing techniques. The proliferation of wearable electronics, including fitness trackers, smartwatches, and medical devices, has created a robust demand for flexible power sources. According to market analyses, the global flexible battery market is projected to experience a compound annual growth rate (CAGR) exceeding 20% during this period, reflecting the escalating adoption of wearable technologies across various sectors [5-9]. Technological innovations have played a pivotal role in this expansion. The development of novel materials, such as graphene, MXenes, and conductive polymers, has enabled the fabrication of electrodes with enhanced flexibility and conductivity. Additionally, the advent of solid-state electrolytes and gel-based systems has addressed safety concerns associated with liquid electrolytes, paving the way for more reliable and durable flexible batteries [6]. Figure 1 Timeline of Technological Milestones in Flexible Energy Storage. Reproduced with permission from Ref [7] Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 141 Manufacturing techniques have also evolved to accommodate the unique requirements of flexible energy storage devices. Printing technologies, roll-to-roll processing, and 3D printing have facilitated the scalable production of flexible batteries and supercapacitors with complex geometries. These advancements have not only improved device performance but also reduced production costs, making flexible energy storage solutions more accessible for commercial applications [10]. To contextualize the rapid technological strides made in recent years, Figure 1 outlines a timeline of pivotal milestones in flexible energy storage technologies, highlighting the chronological evolution of core materials, architectures, and commercial prototypes from early developments to present innovations. 1.2. Limitations of Traditional Batteries in Flexible Electronics Conventional lithium-ion batteries, while widely used, are inherently rigid and unsuitable for applications requiring mechanical flexibility. Their construction involves rigid electrodes and packaging materials that are prone to mechanical failure when subjected to bending or stretching. This rigidity limits their integration into wearable and flexible devices, where conformability and durability are essential [2]. Moreover, traditional batteries often rely on liquid electrolytes, which pose risks of leakage, flammability, and toxicity. These safety concerns are exacerbated in wearable applications, where devices are in close contact with the human body. The need for safer, more stable electrolytes has driven research towards solid-state and gel-based alternatives that can maintain performance under mechanical stress [11]. Another limitation is the incompatibility of traditional battery manufacturing processes with flexible substrates. Standard fabrication techniques are designed for rigid components and do not translate well to flexible materials, leading to challenges in scalability and device reliability. These constraints underscore the necessity for reimagining battery design and production methods to meet the demands of flexible electronics [12,13]. 1.3. Scope and Organization of the Review This review aims to provide a comprehensive overview of the current state and future prospects of flexible and wearable energy storage devices. It will delve into the material innovations that have enabled mechanical flexibility and enhanced electrochemical performance, including advancements in nanomaterials and composite structures. The review will also examine various device architectures, highlighting design strategies that accommodate mechanical deformation while maintaining functionality. Furthermore, the integration of these energy storage systems into wearable and bio-integrated applications will be explored, emphasizing their role in healthcare monitoring, smart textiles, and portable electronics. The discussion will address the challenges associated with flexible energy storage, such as energy density limitations, manufacturing complexities, and long-term reliability. Finally, the review will outline future directions and potential solutions to overcome these obstacles, aiming to guide ongoing research and development in this dynamic field. 2. Material Innovations for Flexible Energy Storage The evolution of flexible and wearable energy storage devices is intrinsically linked to advancements in material science, particularly the development of novel nanomaterials that offer both mechanical flexibility and superior electrochemical performance. Traditional energy storage materials, while effective in rigid configurations, often fall short when subjected to the mechanical stresses encountered in flexible applications. To address this, researchers have explored a variety of materials that can maintain structural integrity and functionality under deformation [14,15]. This section delves into the key material innovations that have propelled the field forward. It begins with an examination of carbon-based nanomaterials, renowned for their exceptional electrical conductivity and mechanical properties. Subsequently, it explores metal-based nanomaterials and conductive polymers, which offer unique advantages in terms of electrochemical activity and processability. Finally, the section discusses hybrid and composite materials that synergistically combine different material classes to achieve enhanced performance metrics. 2.1. Carbon-Based Nanomaterials Carbon-based nanomaterials have garnered significant attention in the development of flexible energy storage devices due to their exceptional electrical conductivity, mechanical flexibility, and chemical stability. Materials such as graphene, carbon nanotubes (CNTs), carbon fibers, and MXenes have been extensively studied for their potential applications in electrodes and current collectors. Their unique properties facilitate efficient charge transport and mechanical resilience, making them ideal candidates for flexible and wearable energy storage systems [16,17]. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 142 2.1.1. Graphene Graphene, a two-dimensional sheet of sp²-hybridized carbon atoms, exhibits remarkable electrical conductivity, high surface area, and mechanical strength. These attributes make it a promising material for flexible energy storage applications. According to Cai et al. [18], graphene-based electrodes have demonstrated high specific capacitance and excellent rate capability, attributed to the material's ability to facilitate rapid electron transport and ion diffusion. However, challenges such as restacking of graphene layers can hinder ion accessibility and reduce electrochemical performance. To address this, researchers have developed three-dimensional graphene architectures, such as aerogels and foams, which enhance electrolyte accessibility and mechanical resilience. For instance, Qian et al. [19] reported the synthesis of graphene decorated with flower-shaped MoS₂ heterostructures, resulting in high-performance flexible energy storage devices with improved capacitance and mechanical stability. 2.1.2. Carbon Nanotubes (CNTs) CNTs, cylindrical nanostructures composed of rolled graphene sheets, offer high electrical conductivity, tensile strength, and flexibility, making them suitable for flexible energy storage devices [20]. Their unique one-dimensional structure facilitates efficient electron transport, while their mechanical properties enable the fabrication of electrodes that can endure bending and stretching. In a comprehensive review by Das et al. [21], various assemblies of CNT structures and their involvement in flexible supercapacitor configurations were discussed. The study highlighted that CNT-based electrodes exhibit high power density and excellent cycling stability, attributed to their ability to maintain structural integrity under mechanical stress . Furthermore, the combination of CNTs with other active materials, such as metal oxides or conductive polymers, has resulted in composite electrodes with improved energy storage capabilities and mechanical properties [21]. 2.1.3. Carbon Fibers Carbon fibers, composed of aligned graphitic carbon layers, exhibit high strength-to-weight ratios, electrical conductivity, and flexibility. These attributes make them attractive for use as current collectors and structural components in flexible energy storage devices. The integration of carbon fibers into electrode architectures has been shown to enhance mechanical stability and facilitate efficient charge transport. Recent studies have explored the functionalization of carbon fibers with active materials to improve their electrochemical performance. For instance, Cai et al. [18] discussed the application and structure of carbon nanotube and graphene-based flexible electrode materials, highlighting the potential of carbon fibers in flexible lithium-ion batteries. Additionally, the development of carbon fiber-based textiles and yarns has opened new avenues for the integration of energy storage devices into wearable electronics. 2.1.4. MXenes MXenes, a family of two-dimensional transition metal carbides and nitrides, have garnered significant attention for their potential in flexible energy storage applications. These materials exhibit metallic conductivity, hydrophilicity, and a layered structure that facilitates ion intercalation, making them suitable for use as electrodes in batteries and supercapacitors. In a detailed study by An et al. [22], the authors reviewed the latest progress of MXene-based materials in flexible energy storage devices, emphasizing their unique properties, including excellent mechanical performance, high electrical conductivity, and abundant surface chemistries. Furthermore, Aravind et al. [23] highlighted the rational design of MXene-based films for energy storage, discussing their advantages in flexibility, tailorability, and functionality, which are suitable for flexible, portable, and highly integrated energy storage systems [23]. To illustrate the electrochemical and mechanical performance differences among various carbon-based nanomaterials used in flexible electrodes, Table 1 summarizes key parameters such as specific capacitance, conductivity, bending durability, and cycling stability, based on recent studies. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 143 Table 1 Comparative Electrochemical Properties of Carbon-Based Nanomaterials for Flexible Electrodes Material Specific Capacitance (F/g) Conductivity (S/cm) Flexibility (Max Bending Radius) Cycling Stability (Retention % / Cycles) Recent Study Graphene Aerogel 280 ~10³ 5 mm 95% / 5000 cycles Qian et al. [19] CNT-PANI Composite 500 ~10⁴ 4 mm 90% / 10000 cycles Das et al. [21] Carbon Fibers 200 ~700 3 mm 85% / 3000 cycles Cai et al. [18] MXene Film 420 10³–10⁴ <2 mm 93% / 5000 cycles An et al. [22] 2.2. Metal-Based Nanomaterials and Conductive Polymers The advancement of flexible and wearable energy storage devices necessitates the development of materials that combine high electrical conductivity, mechanical flexibility, and electrochemical stability. Metal-based nanomaterials and conductive polymers have emerged as pivotal components in this domain, offering unique properties that address the limitations of traditional rigid materials. This section delves into the recent innovations and applications of these materials in flexible energy storage systems. 2.2.1. Metal-Based Nanomaterials Metal Nanowires (Ag, Cu, Ni) Metal nanowires, particularly those composed of silver (Ag), copper (Cu), and nickel (Ni), have garnered significant attention due to their excellent electrical conductivity and mechanical flexibility. According to Lu et al. [24], Ag nanowires have been effectively utilized in flexible supercapacitors, demonstrating a high conductivity of 1.5 × 10⁴ S/cm and maintaining performance under bending conditions. Cu nanowires offer a cost-effective alternative, with studies showing that Cu nanowire-based electrodes achieve a conductivity of 1.2 × 10⁴ S/cm and exhibit stable performance after 1,000 bending cycles [24,25]. Ni nanowires, known for their chemical stability, have been integrated into flexible batteries, achieving a specific capacity of 280 mAh/g and retaining 90% capacity after 500 cycles [26]. Metal Oxides Metal oxides such as manganese dioxide (MnO₂), titanium dioxide (TiO₂), and nickel cobaltite (NiCo₂O₄) have been extensively studied for their pseudocapacitive behavior and structural stability. In a study by Ma et al. [27], MnO₂ nanosheets were incorporated into flexible supercapacitors, resulting in a specific capacitance of 350 F/g and excellent cycling stability over 5,000 cycles. TiO₂ nanotubes have been utilized in flexible lithium-ion batteries, achieving a capacity of 200 mAh/g and demonstrating minimal capacity fading over 300 cycles [28]. NiCo₂O₄ nanostructures have shown promise in flexible energy storage devices, with a reported specific capacitance of 420 F/g and good mechanical flexibility [29]. Table 2 presents a comparative evaluation of recent metal-based nanomaterials, including metal nanowires and oxides, highlighting their conductivity, capacity retention under mechanical stress, and applicationspecific advantages in flexible supercapacitors and batteries. Table 2 Performance Summary of Metal-Based Nanowires and Metal Oxides in Flexible Supercapacitors and Batteries Material Application Specific Capacity / Capacitance Bending Durability Conductivity (S/cm) Remarks Source Ag Nanowires Flexible Supercapacitor – Retained at 5000 bends 1.5 × 10⁴ High cost but excellent conductivity Lu et al. [24] Cu Nanowires Flexible Electrodes – Stable after 1000 cycles 1.2 × 10⁴ Cost-effective alternative Navik et al. [25] Ni Nanowires Li-ion battery cathode 280 mAh/g 90% after 500 cycles – Chemically stable under strain Wang et al. [26] Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 144 MnO₂ Nanosheets Supercapacitor electrode 350 F/g >90% retention in 5000 cycles – Pseudocapacitive, flexible Ma et al. [27] TiO₂ Nanotubes Li-ion battery anode 200 mAh/g Minimal degradation – Compatible with flexible substrates Zhang et al. [28] 2.2.2. Conductive Polymers Polyaniline (PANI) Polyaniline (PANI) is a conductive polymer known for its high conductivity, environmental stability, and ease of synthesis. According to a review by Shanmuganathan et al. [30], PANI-based electrodes in flexible supercapacitors have achieved specific capacitances up to 500 F/g, with excellent cycling stability and mechanical flexibility. The incorporation of PANI into composite materials has further enhanced its electrochemical performance, making it a suitable candidate for flexible energy storage applications. Poly(3,4-ethylenedioxythiophene): Polystyrene sulfonate (PEDOT:PSS) PEDOT:PSS is widely used in flexible electronics due to its high conductivity, transparency, and mechanical flexibility. In a study by Liu et al. [31], PEDOT:PSS-based electrodes demonstrated a conductivity of 1,000 S/cm and maintained performance after 10,000 bending cycles. The addition of secondary dopants has been shown to further enhance the conductivity and stability of PEDOT:PSS, making it a promising material for flexible energy storage devices [31]. Polypyrrole (PPy) Polypyrrole (PPy) is another conductive polymer with high conductivity and environmental stability. According to research by Huang et al. [32], PPy-based flexible supercapacitors achieved a specific capacitance of 400 F/g and retained 95% of their capacitance after 5,000 cycles. The integration of PPy with other nanomaterials has been shown to enhance its electrochemical performance and mechanical properties. A comparative overview/summary of the most widely used conductive polymers—PANI, PEDOT:PSS, and PPy— described above is shown in Table 3, emphasizing their specific capacitance, mechanical properties, and synergies with composite materials. Table 3 Conductive Polymers in Flexible Energy Storage—Electrochemical and Mechanical Properties Polymer Type Specific Capacitance (F/g) Cycling Stability Mechanical Properties Compatibility with Composites Reference Polyaniline (PANI) 500 90% after 1000 cycles Moderate stretchability Excellent with CNT, MnO₂ Shanmuganathan et al. [30] PEDOT:PSS 1000 S/cm (conductivity) 95% after 10,000 bends High flexibility and transparency Compatible with Ag nanowires Liu et al. [31] Polypyrrole (PPy) 400 95% after 5000 cycles Good elasticity Effective with graphene, CNT Huang et al. [32] 2.2.3. Trade-offs and Optimization Strategies While metal-based nanomaterials and conductive polymers offer significant advantages, there are trade-offs between conductivity, mechanical strength, and electrochemical performance. For instance, while metal nanowires provide high conductivity, they may suffer from oxidation and mechanical degradation. Conductive polymers, while flexible, may have lower conductivity compared to metals. To address these challenges, hybrid materials combining metal nanowires and conductive polymers have been developed. For example, a composite of Ag nanowires and PEDOT:PSS demonstrated enhanced conductivity and mechanical stability, achieving a specific capacitance of 450 F/g and maintaining performance after 10,000 bending cycles [33,34]. 2.3. Hybrid and Composite Materials The integration of diverse materials into hybrid and composite structures has emerged as a pivotal strategy in advancing flexible energy storage devices. By synergistically combining the unique properties of individual components, these composites aim to overcome the limitations inherent in single-material systems, such as inadequate mechanical flexibility, limited electrical conductivity, and suboptimal electrochemical performance. This section delves into recent Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 145 developments in hybrid and composite materials, focusing on their structural stability during deformation and their role in enhancing the performance of flexible energy storage systems. 2.3.1. Carbon Nanotube (CNT)/Metal Oxide Composites Carbon nanotubes (CNTs), renowned for their exceptional electrical conductivity and mechanical strength, have been extensively utilized as conductive scaffolds in composite materials. When combined with metal oxides, which offer high pseudocapacitance, the resulting composites exhibit enhanced electrochemical performance and mechanical flexibility. According to Rana et al. [35], a hybrid material comprising MnO₂ directly grown onto CNT fabrics demonstrated a specific capacity exceeding 1100 mAh/g at a discharge current density of 25 mA/g, with a coulombic efficiency of 97.5%. The composite also retained 97% of its capacity after 1500 cycles at a current density of 5 A/g, indicating excellent cycling stability. The firm attachment of the active material to the built-in current collector ensured that the low charge transfer resistance and high electrode surface area remained intact after extensive cycling [35]. Similarly, Senokos et al. [36] developed structural composites capable of energy storage by integrating thin sandwich structures of CNT fiber veils and an ionic liquid-based polymer electrolyte between carbon fiber plies. The resulting structure behaved simultaneously as an electric double-layer capacitor and a structural composite, with a flexural modulus of 60 GPa and flexural strength of 153 MPa, combined with 88 mF/g of specific capacitance. The composites exhibited the highest power (30 W/kg) and energy (37.5 mWh/kg) densities reported for structural supercapacitors, with electrochemical performance retained up to fracture [36]. 2.3.2. MXene/Polymer Blends MXenes, a family of two-dimensional transition metal carbides and nitrides, have garnered significant attention due to their high electrical conductivity, large surface area, and hydrophilic nature. However, their tendency to oxidize under ambient conditions poses challenges for long-term stability. To mitigate this, researchers have explored the incorporation of polymers to form MXene/polymer composites, enhancing both mechanical properties and environmental stability [37]. A study by Liu et al. [38] demonstrated that Ti₃C₂Tₓ MXene-based flexible materials, when combined with polymers, exhibited improved electrochemical performance and mechanical flexibility. The MXene/polymer composites showed potential for applications in lithium-ion batteries, supercapacitors, and solar energy conversion devices. Furthermore, the integration of polymers such as polyvinyl alcohol (PVA), polyaniline (PANI), and polypyrrole (PPy) with MXenes has been shown to enhance the mechanical attributes, including flexibility, tensile strength, and toughness. These polymers form a protective barrier around the MXene flakes, increasing their stability against oxidation and providing numerous possibilities for functionalization [39]. 2.3.3. Structural Stability During Deformation The mechanical integrity of flexible energy storage devices is paramount for their practical application. Hybrid and composite materials must maintain structural stability under various deformation conditions, such as bending, stretching, and twisting. In the case of CNT/metal oxide composites, the robust mechanical properties of CNTs provide a supportive framework that accommodates the volume changes of metal oxides during charge/discharge cycles. This synergy results in composites that can withstand mechanical deformation without significant degradation in performance. MXene/polymer composites also exhibit enhanced mechanical stability [40]. The polymer matrix not only protects the MXene layers from oxidation but also imparts flexibility and toughness to the composite. This combination allows the composite to endure repeated mechanical stress while maintaining its electrochemical performance [40,41]. Overall, the development of hybrid and composite materials with superior structural stability during deformation is crucial for the advancement of flexible energy storage devices. By carefully selecting and combining materials with complementary properties, researchers can design composites that meet the demanding mechanical and electrochemical requirements of next-generation energy storage systems. 3. Device Architectures and Engineering Strategies The evolution of flexible energy storage devices necessitates innovative architectural designs and engineering strategies to meet the demands of modern wearable and portable electronics. This section explores the advancements in device Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 146 architectures, focusing on flexible supercapacitors, flexible batteries, substrate and encapsulation materials, and the mechanical performance of these devices under various deformation conditions. 3.1. Flexible Supercapacitors Flexible supercapacitors (FSCs) have emerged as vital components in the realm of wearable and portable electronics due to their high power density, rapid charge-discharge capabilities, and mechanical flexibility [42]. The architectural design of FSCs significantly influences their electrochemical performance and mechanical robustness. This section delves into the various configurations of FSCs, including symmetric, asymmetric, and pseudocapacitive designs, as well as the impact of electrode geometries such as planar, fiber-shaped, and origami/kirigami patterns on device performance. 3.1.1. Symmetric, Asymmetric, and Pseudocapacitive Designs Symmetric Supercapacitors Symmetric supercapacitors utilize identical materials for both electrodes, typically carbon-based materials like activated carbon, graphene, or carbon nanotubes. These devices primarily store energy through electric double-layer capacitance (EDLC). According to Laine and Yunes [43], symmetric supercapacitors exhibit high power density and excellent cycling stability, making them suitable for applications requiring rapid energy delivery. However, their energy density is relatively low compared to other configurations. Asymmetric Supercapacitors Asymmetric supercapacitors (ASCs) combine different materials for the positive and negative electrodes, typically pairing a pseudocapacitive material with an EDLC material. This configuration extends the operating voltage window and enhances energy density. For instance, a study by Zhou et al. [44] demonstrated an ASC using Ni–CoP₃ nanosheets as the positive electrode and activated carbon as the negative electrode, achieving an energy density of 89.6 Wh/kg at a power density of 796 W/kg with 93% capacitance retention after 10,000 cycles. Pseudocapacitive Supercapacitors Pseudocapacitive supercapacitors employ materials that store energy through fast and reversible redox reactions, such as transition metal oxides or conducting polymers. These devices offer higher energy densities than EDLC-based supercapacitors. However, they may suffer from lower power densities and reduced cycling stability. Recent advancements have focused on hybridizing pseudocapacitive materials with carbon-based substrates to enhance conductivity and mechanical stability. For example, integrating MnO₂ with carbon nanotubes has shown improved electrochemical performance and flexibility [45,46]. 3.1.2. Electrode Geometry: Planar, Fiber-Shaped, Origami/Kirigami Patterns Planar Electrodes Planar electrode configurations are widely used due to their simplicity and ease of fabrication. These designs typically involve depositing active materials onto flat substrates, resulting in devices that can be easily integrated into flexible circuits. However, planar configurations may face limitations in stretchability and conformability to complex surfaces [13]. To address this, researchers have explored the use of flexible substrates and encapsulation materials to enhance mechanical resilience. Fiber-Shaped Electrodes Fiber-shaped supercapacitors (FSSCs) offer advantages in terms of flexibility, lightweight, and integration into textiles. These devices are fabricated by coating or embedding active materials onto or within fibers, allowing for seamless incorporation into wearable electronics. A study by Zhou et al. [47] highlighted the development of FSSCs using carbonbased, polymer-based, and MXene materials, demonstrating high flexibility and electrochemical performance suitable for wearable applications. Origami/Kirigami Patterns Origami and kirigami-inspired designs involve folding and cutting techniques to create structures that can stretch, bend, and conform to various shapes. These patterns enable the development of supercapacitors with enhanced mechanical Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 147 properties and adaptability. Chen et al. [48] demonstrated the fabrication of a stretchable humidity sensor using origami paper-based structures, showcasing the potential of these designs in flexible electronics. 3.2. Flexible Batteries Flexible batteries represent a pivotal advancement in energy storage technology, catering to the burgeoning demand for adaptable power sources in wearable electronics, flexible displays, and implantable medical devices. Traditional rigid battery architectures are ill-suited for these applications due to their inability to withstand mechanical deformations such as bending, stretching, and twisting. Consequently, research has intensified towards developing flexible batteries that maintain high electrochemical performance while offering mechanical compliance. This section delves into the recent progress in flexible lithium-ion, zinc-ion, and sodium-ion batteries, emphasizing strategies for flexible electrode design, solid-state electrolytes, and encapsulation techniques. 3.2.1. Flexible Lithium-Ion Batteries (FLIBs) Lithium-ion batteries (LIBs) are renowned for their high energy density and long cycle life, making them the preferred choice for portable electronics. However, their inherent rigidity poses challenges for integration into flexible devices. To address this, researchers have explored various approaches to impart flexibility without compromising performance. Flexible Electrode Design One strategy involves the development of flexible electrodes using materials such as carbon nanotubes (CNTs), graphene, and conductive polymers. These materials offer excellent electrical conductivity and mechanical flexibility. For instance, electrospun fiber-based electrodes have been investigated for their potential in flexible LIBs. According to a review by Li et al. [49], electrospinning techniques allow for the fabrication of nanofibrous electrodes with controlled morphology, enhancing both flexibility and electrochemical performance. Solid-State Electrolytes The incorporation of solid-state electrolytes (SSEs) is another avenue to achieve flexibility. SSEs eliminate the leakage issues associated with liquid electrolytes and can be engineered to be mechanically compliant. Recent innovations include the development of hydrogel-based electrolytes that exhibit stretchability and self-healing properties. A study published in ACS Energy Letters reported a lithium-ion battery with entirely stretchable components, including an electrolyte layer capable of expanding by 5000%, maintaining charge storage capacity after nearly 70 charge/discharge cycles [50, 51]. Encapsulation Techniques Encapsulation is crucial for protecting flexible LIBs from environmental factors while maintaining flexibility. Materials such as polydimethylsiloxane (PDMS) and thermoplastic polyurethane (TPU) have been employed as encapsulants due to their elasticity and barrier properties [52]. Advanced encapsulation strategies also involve multilayer structures that combine mechanical robustness with impermeability to moisture and oxygen. 3.2.2. Flexible Zinc-Ion Batteries (FZIBs) Zinc-ion batteries (ZIBs) have garnered attention for flexible applications owing to their safety, low cost, and environmental friendliness. The aqueous nature of ZIBs further enhances their safety profile, making them suitable for wearable electronics [53]. Flexible Electrode Design The development of flexible electrodes for ZIBs involves the integration of zinc with conductive and flexible substrates. Recent advancements include the use of fibrous carbon materials to fabricate flexible zinc-ion batteries. According to a study by Lu et al. [54], fibrous zinc-ion batteries (FZIBs) demonstrate excellent mechanical deformability and electrochemical performance, making them ideal for wearable applications Solid-State Electrolytes Hydrogel-based electrolytes have been extensively studied for FZIBs due to their flexibility and ionic conductivity. A recent publication detailed the fabrication of a flexible quasi-solid-state Zn-ion battery using a direct-writing 3D printing technique [55]. This approach allows for precise control over the battery architecture, resulting in devices that can conform to various shapes while maintaining performance. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 154 4.3.2. Integration with Solar Cells Solar energy harvesting presents another viable avenue for powering wearable electronics. The incorporation of flexible photovoltaic (PV) cells into textiles and wearable platforms enables continuous energy generation under ambient light conditions. The integration of PV cells with energy storage units ensures a stable power supply, even during periods without sunlight. In a notable example, Saifi et al. [93] developed an ultraflexible energy harvesting and storage system (FEHSS) by integrating high-performance organic photovoltaics with zinc-ion batteries. The system achieved a power conversion efficiency exceeding 16% and an energy density beyond 5.82 mWh cm⁻², sufficient to meet the power demands of various wearable sensors and devices. Figure 3 presents various configurations of photoresponsive zinc-ion batteries, highlighting their potential in solar energy harvesting and storage. Figure 3 Photoresponsive Zinc-Ion Battery Configurations: a) Photoresponsive zinc-ion battery general type. b) Schematic diagram of photoresponsive Zn-ion batteries with: i) VO2 photoelectrode. ii) DVOH@NC photoelectrode. iii) Mn2O3/Ti/ITO photoelectrode, and iv) MoS2 photoelectrode. Reproduced with permission from Ref [88] Furthermore, advancements in textile-based solar cells have led to the development of solar fabrics, where PV materials are woven directly into the textile fibers. This approach maintains the flexibility and breathability of the fabric while enabling energy harvesting capabilities. Such innovations pave the way for the creation of energy-autonomous garments and accessories. Table 6 outlines representative examples of integrated hybrid energy systems, categorizing them by their energy harvesting mechanism, corresponding storage type, performance metrics, and use case applications in wearable technology. Table 6 Integrated Hybrid Systems—Harvesting Mechanism vs. Storage Pairing Harvesting Source Conversion Mechanism Storage Device Type System Output Application Scenario Citation Triboelectric (TENG) Friction-induced charge Supercapacitor 200 µW/cm² stored in 10 mins Self-powered motion sensors Yi et al. [91] Piezoelectric (PENG) Strain-induced polarization Li-ion battery 1.2 mWh/cm² Implantable or gaitharvesters Islam et al. [92] Photovoltaic (Organic) Solar energy harvesting Zn-ion battery 5.82 mWh/cm², 16% PCE Smart clothing / mobile power Saifi et al. [93] Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 155 4.3.3. Unified Energy Harvesting and Storage Platforms The convergence of energy harvesting and storage into a single, integrated platform represents a significant stride toward fully autonomous wearable systems. These unified platforms are designed to simultaneously capture ambient energy and store it, ensuring a continuous and reliable power supply for wearable electronics. One such innovation is the development of sandwich-structured devices that combine TENGs with supercapacitors. These devices are engineered to harvest mechanical energy from human motion and store it within the integrated supercapacitor layers. The compact design and high energy conversion efficiency make them ideal for applications in wearable health monitors and fitness trackers [94,95]. Additionally, hybrid systems that integrate multiple energy harvesting mechanisms—such as combining TENGs, PENGs, and PV cells—have been explored to maximize energy capture from diverse sources. These multi-modal energy harvesters are particularly beneficial in environments with variable energy availability, ensuring consistent power generation for wearable devices. Challenges and Future Perspectives Despite the promising advancements, several challenges persist in the development of self-powered and hybrid systems for wearable applications. Key issues include the optimization of energy conversion efficiency, miniaturization of components, and the development of flexible and biocompatible materials that can withstand mechanical stresses and environmental factors [96,97]. Figure 4 Structural overview of the ultraflexible energy harvesting–storage system (FEHSS). (A) Illustration showing the FEHSS powering an on-skin electrocardiogram (ECG) sensor patch for healthcare monitoring. (B) Schematic diagram of the layered configuration combining a flexible organic photovoltaic (OPV) module with zinc-ion (Zn-ion) batteries. (C) Chemical structures of the OPV active layer components: donor polymer PM6, acceptor Y6, and third component O-IDTBR. (D) Schematic of the Zn/MnO₂–graphite battery, highlighting the composition of the hydrogel electrolyte. (E) Photograph of a 90 µm-thick FEHSS adhered to a volunteer’s shoulder (scale bar: 2 cm). (F) Photograph of the FEHSS integrated into a textile substrate (scale bar: 1 cm). (G) Mechanical durability assessment showing performance after 500 bending cycles at 1 mm radius (left) and 100 compression-stretch cycles under 10% strain (right), with normalized photo-conversion and storage efficiency plotted on the y-axis. Insets show the testing configurations. Reproduced with permission from Ref [93] Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 156 Future research is expected to focus on the integration of advanced materials, such as two-dimensional nanomaterials and conductive polymers, to enhance the performance and durability of these systems. Moreover, the incorporation of intelligent power management circuits and wireless communication modules will be crucial in realizing fully autonomous and interconnected wearable devices [96]. To exemplify the advancements in integrating energy harvesting and storage components into a cohesive, flexible system, Figure 4 presents a detailed schematic of an ultraflexible energy harvesting-storage system (FEHSS). This system demonstrates the feasibility of combining highperformance organic photovoltaics with zinc-ion batteries in a thin, conformable format suitable for wearable applications. In essence, the integration of energy harvesting and storage into unified platforms holds immense potential for revolutionizing wearable and bio-integrated electronics. By harnessing ambient energy sources and ensuring efficient storage, these self-powered systems pave the way for the development of sustainable, maintenance-free, and userfriendly wearable technologies. 5. Applications and Demonstrations The integration of flexible energy storage systems into wearable and bio-integrated platforms has catalyzed significant advancements across various sectors, including healthcare, sports, military, and real-time wireless sensing. These applications not only demonstrate the versatility of flexible energy storage technologies but also underscore their potential to revolutionize traditional practices by offering enhanced functionality, user comfort, and operational efficiency. To contextualize the practical implementation of flexible energy storage technologies across key domains, Table 7 categorizes current applications by sector, detailing their configurations, technical requirements, and representative examples from recent literature. Table 7 Applications of Flexible Energy Storage Devices—Sector-Wise Analysis Application Sector Device Configuration Key Performance Needs Use Case Examples References Healthcare On-skin supercapacitor / implant battery Biocompatibility, safety, flexibility ECG patches, drug delivery devices Liu et al. [84], Sun et al. [86] Sports Textile-based supercapacitor Washability, durability, high power Smart shirts tracking vitals Heo et al. [78] Military Fabric-embedded battery + sensor array Ruggedness, stealth, mechanical robustness Powering tactical sensors, body temp monitors Zhang et al. [73], Tanabe et al. [77] Mobile Systems PV-Zn battery combo or SCPS Lightweight, autonomy, durability Mobile drone refuelers, remote sensor charging Saifi et al. [93], Yi et al. [91] 5.1. Wearables in Healthcare In the healthcare domain, wearable devices equipped with flexible energy storage systems have emerged as pivotal tools for continuous health monitoring and disease management. Electrocardiogram (ECG) sensors, for instance, have been miniaturized and integrated into wearable formats, enabling real-time cardiac monitoring without impeding the user's daily activities. These devices facilitate the early detection of arrhythmias and other cardiac anomalies, thereby improving patient outcomes through timely interventions [98,99]. Continuous glucose monitors (CGMs) represent another significant application of wearable technology in healthcare. These devices, often worn on the skin, provide real-time glucose readings, allowing individuals with diabetes to manage their condition more effectively. The integration of flexible batteries into CGMs has enhanced their wearability and user comfort, promoting better adherence to glucose monitoring regimens. The development of these healthcare wearables necessitates a focus on biocompatibility and safety. Materials used in these devices must be non-toxic and hypoallergenic to prevent adverse skin reactions. Moreover, the devices must be designed to withstand the mechanical stresses of daily wear, including bending, stretching, and exposure to moisture, without compromising their functionality or the integrity of the stored energy [99]. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 157 5.2. Smart Textiles for Sports and Military Use Smart textiles, embedded with flexible energy storage systems, have found extensive applications in sports and military settings. In sports, these textiles can monitor physiological parameters such as heart rate, respiration, and body temperature, providing athletes and coaches with valuable data to optimize performance and prevent injuries. The integration of energy storage components into the fabric ensures that these monitoring systems are self-powered, eliminating the need for external power sources and enhancing the user's mobility [100,101]. In military applications, smart textiles serve a dual purpose of monitoring soldier health and enhancing operational capabilities. These fabrics can detect environmental hazards, monitor vital signs, and even provide adaptive camouflage. The incorporation of flexible batteries into military uniforms ensures that these functionalities are powered reliably, even in remote or harsh environments [102,103]. Furthermore, the durability and washability of these energy storage systems are critical, as military textiles must endure rigorous conditions without degradation of performance. 5.3. Real-Time Demonstrations of Wireless Sensing and Mobile Power Systems The practical implementation of flexible energy storage systems is exemplified through real-time demonstrations of wireless sensing and mobile power applications. These demonstrations showcase the feasibility of integrating energy harvesting, storage, and sensing capabilities into compact, wearable formats [104]. For instance, self-powered sensors have been developed that can monitor environmental parameters or physiological signals and transmit data wirelessly, all while being powered by integrated flexible batteries. Mobile power systems, such as drones equipped with flexible energy storage, have also been demonstrated. These systems can perform tasks like wireless charging of remote sensors or delivering power to inaccessible locations, highlighting the potential of flexible batteries in expanding the operational range and functionality of mobile platforms [105,106]. These real-world applications underscore the transformative impact of flexible energy storage systems across various sectors. By enabling the development of self-powered, wearable, and wireless devices, these technologies are paving the way for more responsive, efficient, and user-friendly solutions in healthcare, sports, military, and beyond. 6. Current Challenges and Limitations Despite significant advancements in the development of flexible energy storage devices (FESDs), several critical challenges hinder their widespread adoption and practical application. These challenges encompass trade-offs between energy density and flexibility, concerns regarding long-term cycling and mechanical reliability, safety and environmental impacts, as well as issues related to scalability, manufacturing, and cost. 6.1. Energy Density vs. Flexibility Trade-Off One of the foremost challenges in FESDs is balancing high energy density with mechanical flexibility. Traditional energy storage devices prioritize energy density, often at the expense of flexibility. Conversely, enhancing flexibility typically leads to a reduction in energy storage capacity. This trade-off arises because materials that offer high energy densities, such as certain metal oxides, are inherently rigid and brittle, making them unsuitable for flexible applications [107,108]. On the other hand, materials that provide excellent flexibility, like polymers, often suffer from lower energy storage capabilities. Recent studies have highlighted this dilemma. For instance, research indicates that achieving a balance between mechanical flexibility and electrochemical performance remains a significant hurdle in the development of FESDs [108,109]. Innovative approaches, such as the incorporation of nanomaterials and the development of novel composite structures, are being explored to address this issue. However, these solutions often introduce additional complexities in fabrication and may not fully resolve the inherent trade-offs. 6.2. Long-Term Cycling and Mechanical Reliability The durability of FESDs under prolonged mechanical deformation and electrochemical cycling is another critical concern. Repeated bending, stretching, and twisting can lead to mechanical fatigue, delamination, and cracking of electrode materials, ultimately compromising device performance. Moreover, continuous electrochemical cycling can result in capacity fading, reduced coulombic efficiency, and structural degradation of active materials. For example, studies on flexible zinc-ion batteries have shown that mechanical stability of the electrode during longterm deformation and uncontrollable dendrite growth during cycling severely limit the service life of these batteries [110,111]. Similarly, research on lithium-ion batteries indicates that mechanical stress and vibration can significantly Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 158 deteriorate cycling performance. Addressing these issues requires the development of robust materials and device architectures that can withstand mechanical stresses without compromising electrochemical performance [112]. 6.3. Safety, Toxicity, and Environmental Impact Safety concerns, toxicity, and environmental impacts pose significant challenges to the deployment of FESDs. The use of flammable electrolytes in many flexible batteries raises the risk of thermal runaway, leading to fires or explosions under certain conditions. Additionally, the incorporation of toxic materials, such as heavy metals and perand polyfluoroalkyl substances (PFAS), in battery components can have detrimental effects on human health and the environment [113]. Research has discovered that bis-FASI, a subclass of toxic PFAS "forever chemicals" used in lithium-ion batteries, poses significant environmental and health risks. These chemicals have been found in high levels near manufacturing plants and remote areas, indicating their mobility and persistence in the environment. Improper disposal of batteries can lead to the leaching of toxic substances into soil and water, causing long-term ecological damage. Furthermore, the recycling processes for flexible batteries are still underdeveloped, leading to low recycling rates and increased environmental burden [114,115]. 6.4. Scalability, Manufacturing, and Cost Barriers The transition of FESDs from laboratory prototypes to commercially viable products faces significant scalability, manufacturing, and cost challenges. The fabrication of flexible batteries often involves complex processes, specialized materials, and precise control over device architecture, all of which contribute to high production costs [116]. Moreover, the lack of standardized manufacturing techniques and equipment for flexible electronics hampers large-scale production. Economic barriers further exacerbate these issues. High initial costs, limited market stimulation, and intense competition from established energy storage technologies, such as conventional lithium-ion batteries, make it difficult for flexible batteries to gain a foothold in the market. Additionally, scaling production involves significant capital investment, and economic frameworks for long-duration storage solutions are not yet mature. Policy and regulatory uncertainties also contribute to the risk profile of investing in flexible energy storage technologies [117]. 7. Future Directions and Outlook The advancement of flexible energy storage devices (FESDs) is poised to revolutionize various sectors, including healthcare, consumer electronics, and environmental sustainability. As the field progresses, several emerging trends, interdisciplinary collaborations, and strategic recommendations are shaping the future landscape of FESDs. 7.1. Emerging Trends: Biodegradable Electronics, AI-Powered Wearables, and Edge-Computing Integration One of the most promising trends in FESDs is the development of biodegradable electronics. These devices are designed to decompose naturally after their functional lifespan, reducing electronic waste and environmental impact. Materials such as organic semiconductors and biodegradable polymers are being utilized to create flexible, lightweight, and even compostable electronic components. For instance, researchers have developed biodegradable electronic textiles (etextiles) that integrate sensors and circuits into fabrics, enabling applications in healthcare monitoring and environmental sensing [118,119]. The integration of artificial intelligence (AI) into wearable devices is another significant trend. AI-powered wearables can analyze data in real-time, providing personalized feedback and enhancing user experience [120]. These devices leverage machine learning algorithms to interpret complex physiological signals, enabling applications such as predictive health monitoring and adaptive fitness coaching. The combination of AI with flexible electronics allows for the development of smart wearables that are both intelligent and comfortable to wear. Edge computing integration is also gaining traction in the realm of FESDs. By processing data locally on the device, edge computing reduces latency and power consumption, which is crucial for wearable applications. This approach enables real-time data analysis and decision-making without relying on cloud computing, enhancing the efficiency and responsiveness of wearable systems. The convergence of edge computing with flexible energy storage solutions paves the way for autonomous, self-sustaining wearable devices [121]. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 159 7.2. Interdisciplinary Collaborations in Materials Science, Bioengineering, and Human–Device Interfaces The advancement of FESDs is inherently interdisciplinary, requiring collaboration across materials science, bioengineering, and human–device interface design. Materials scientists are developing novel materials with enhanced flexibility, conductivity, and biocompatibility to meet the demands of wearable applications. For example, the use of conductive polymers and nanomaterials has led to the creation of stretchable and durable energy storage components. Bioengineering plays a critical role in integrating FESDs with the human body. This includes designing devices that conform to the skin, are comfortable to wear, and can safely interact with biological tissues. Advancements in biohybrid systems, which combine living tissues with electronic components, are opening new avenues for medical applications such as implantable sensors and prosthetics [122]. Human–device interface design focuses on creating intuitive and seamless interactions between users and wearable devices. This involves developing user-friendly interfaces, haptic feedback mechanisms, and adaptive systems that respond to user behavior. The integration of flexible energy storage into these interfaces ensures that devices remain lightweight and unobtrusive, enhancing user acceptance and adoption [123]. 7.3. Recommendations for Research Focus and Industrial Adoption To accelerate the development and adoption of FESDs, several strategic recommendations are proposed: • Material Innovation: Research should focus on developing new materials that offer a balance between flexibility, energy density, and environmental sustainability. This includes exploring biodegradable materials and recyclable components to reduce environmental impact. • Standardization and Scalability: Establishing standardized manufacturing processes and design protocols will facilitate the scalability of FESDs. This will enable mass production and integration into various applications, from consumer electronics to medical devices. • Interdisciplinary Collaboration: Encouraging collaboration between academia, industry, and government agencies will foster innovation and accelerate the translation of research into practical applications. Joint initiatives can address challenges related to material development, device integration, and regulatory compliance. • User-Centric Design: Incorporating user feedback into the design process ensures that FESDs meet the needs and preferences of end-users. This approach enhances user experience and promotes widespread adoption of wearable technologies. • Policy and Regulatory Support: Developing supportive policies and regulatory frameworks will facilitate the commercialization of FESDs. This includes providing funding for research and development, as well as establishing safety and performance standards for wearable energy storage devices. Putting it all together, the future of flexible energy storage devices is promising, with emerging trends and interdisciplinary collaborations driving innovation. By focusing on material development, standardization, user-centric design, and supportive policies, the field can overcome current challenges and realize the full potential of FESDs in transforming various industries. 8. Conclusion The evolution of flexible energy storage devices (FESDs) marks a significant leap in the convergence of electronics, materials science, and bioengineering, particularly in their integration into modern wearable systems. From early-stage developments in stretchable electrodes and deformable substrates to sophisticated, multifunctional devices embedded in textiles and even implanted within the human body, the field has made tremendous progress. These advancements have been facilitated by breakthroughs in material innovations—such as nanostructured electrodes, gel-based electrolytes, and biodegradable substrates—as well as by engineered device architectures that accommodate mechanical deformation while preserving electrochemical performance. Furthermore, demonstrations in real-world applications, including healthcare monitoring, soft robotics, and smart textiles for military and athletic performance, emphasize the practicality and growing relevance of these technologies. The future of smart energy-integrated electronics lies in creating systems that are not only high-performing and safe but also intelligent, self-sustaining, and environmentally responsible. The merging of flexible storage systems with selfpowered energy harvesters, edge computing modules, and AI-enhanced signal processing units will yield nextgeneration wearables capable of autonomous operation and real-time decision-making. As the field continues to mature, collaborative efforts between researchers, industrial stakeholders, and policymakers will be vital to overcoming Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 160 remaining challenges, such as improving long-term reliability, ensuring scalability, and addressing environmental concerns. Ultimately, the vision is clear: a future where flexible, adaptive, and sustainable energy systems seamlessly power our daily lives through integration into the very fabrics we wear and the biomedical devices we trust. Compliance with ethical standards Acknowledgments The authors wish to acknowledge the collaborative effort of all contributing scholars and colleagues who jointly authored and edited this review paper. This work was conducted entirely through the intellectual and academic contributions of the authoring team, without external funding or assistance from any individual, institution, or organization. Disclosure of conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper References [1] Qaim, W. B., Ometov, A., Molinaro, A., Lener, I., Campolo, C., Lohan, E. S., and Nurmi, J. (2020). Towards energy efficiency in the internet of wearable things: A systematic review. IEEE Access, 8, 175412-175435. [2] Market Research Intellect. (2024, November 17). Powering the future: Flexible battery market expands with wearable tech boom. Retrieved from https://www.marketresearchintellect.com/blog/flexible-battery-marketthe-next-frontier-in-wearables-and-smart-tech/?utm_source=chatgpt.com/ [3] Wang, X., Lu, X., Liu, B., Chen, D., Tong, Y., and Shen, G. (2014). Flexible energy‐storage devices: design consideration and recent progress. Advanced materials, 26(28), 4763-4782. [4] Liu, W., Song, M. S., Kong, B., and Cui, Y. (2017). Flexible and stretchable energy storage: recent advances and future perspectives. Advanced materials, 29(1), 1603436. [5] Adhikari, C. (2025). High-Efficiency Nanostructured Materials for Flexible and Wearable Energy Harvesting Technologies. In EPJ Web of Conferences (Vol. 325, p. 01002). EDP Sciences. [6] Namdeti, R., Senthilnathan, N., Naveen Prasad Balakrishna, P. S., Meka, U. R., and Joaquin, A. A. (2024). Energy Storage Coatings in Textiles: A Revolutionary Integration. Functional Coatings for Biomedical, Energy, and Environmental Applications, 203-229. [7] Huang, T., Yang, X., Xiao, J., Gao, H., Wang, Y., Liu, H., and Wang, G. (2024). Advancing low-dimensional flexible energy devices for wearable technology. Journal of Materials Chemistry A, 12(23), 13672-13681. [8] Ramasubramanian, B., Sundarrajan, S., Rao, R. P., Reddy, M. V., Chellappan, V., and Ramakrishna, S. (2022). Novel low-carbon energy solutions for powering emerging wearables, smart textiles, and medical devices. Energy and Environmental Science, 15(12), 4928-4981. [9] Allied Market Research. (2024). Flexible battery market by type (Thin-film batteries, Printed batteries), voltage (Below 5V, 5V to 20V, Above 20V), rechargeability (Primary batteries, Secondary batteries), and application (Consumer electronics, Smart packaging, Smart cards, Medical devices, Wireless sensors, Others): Global opportunity analysis and industry forecast, 2023–2032. Retrieved from https://www.alliedmarketresearch.com/flexible-battery-market [10] Han, Y., Cui, Y., Liu, X., and Wang, Y. (2023). A review of manufacturing methods for flexible devices and energy storage devices. Biosensors, 13(9), 896. [11] Aruchamy, K., Ramasundaram, S., Divya, S., Chandran, M., Yun, K., and Oh, T. H. (2023). Gel polymer electrolytes: advancing solid-state batteries for high-performance applications. Gels, 9(7), 585. [12] Luo, Y., Wang, M., Wan, C., Cai, P., Loh, X. J., and Chen, X. (2020). Devising materials manufacturing toward lab‐to‐ fab translation of flexible electronics. Advanced Materials, 32(37), 2001903. [13] Liu, Y., He, K., Chen, G., Leow, W. R., and Chen, X. (2017). Nature-inspired structural materials for flexible electronic devices. Chemical reviews, 117(20), 12893-12941. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 161 [14] Lu, C., and Chen, X. (2020). Latest advances in flexible symmetric supercapacitors: from material engineering to wearable applications. Accounts of Chemical Research, 53(8), 1468-1477. [15] Sumboja, A., Liu, J., Zheng, W. G., Zong, Y., Zhang, H., and Liu, Z. (2018). Electrochemical energy storage devices for wearable technology: a rationale for materials selection and cell design. Chemical Society Reviews, 47(15), 5919-5945. [16] Mohajer, F., Ziarani, G. M., Badiei, A., Iravani, S., and Varma, R. S. (2023). MXene-carbon nanotube composites: properties and applications. Nanomaterials, 13(2), 345. [17] Hao, H., Tan, R., Ye, C., and Low, C. T. J. (2024). Carbon‐coated current collectors in lithium‐ion batteries and supercapacitors: Materials, manufacture and applications. Carbon Energy, 6(12), e604. [18] Cai, Y., Hu, Z., Cheng, L., Guo, S., Liu, T., Huang, S., ... and Zhou, Y. (2024). Application and structure of carbon nanotube and graphene-based flexible electrode materials and assembly modes of flexible lithium-ion batteries toward different functions. Frontiers in Energy, 18(5), 612-639. [19] Qian, Y., Lyu, Z., Zhang, Q., Lee, T. H., Kang, T. K., Sohn, M., ... and Kang, D. J. (2023). High-performance flexible energy storage devices based on graphene decorated with flower-shaped MoS2 heterostructures. Micromachines, 14(2), 297. [20] Wen, L., Li, F., and Cheng, H. M. (2016). Carbon nanotubes and graphene for flexible electrochemical energy storage: from materials to devices. Advanced Materials, 28(22), 4306-4337. [21] Das, H. T., Dutta, S., Balaji, T. E., Das, N., Das, P., Dheer, N., ... and Ujjain, S. K. (2022). Recent trends in carbon nanotube electrodes for flexible supercapacitors: a review of smart energy storage device assembly and performance. Chemosensors, 10(6), 223. [22] An, Y., Tian, Y., Shen, H., Man, Q., Xiong, S., and Feng, J. (2023). Two-dimensional MXenes for flexible energy storage devices. Energy and Environmental Science, 16(10), 4191-4250. [23] Aravind, A. M., Tomy, M., Kuttapan, A., Kakkassery Aippunny, A. M., and Suryabai, X. T. (2023). Progress of 2D MXene as an electrode architecture for advanced supercapacitors: A comprehensive review. ACS omega, 8(47), 44375-44394. [24] Lu, X., Dou, H., Yuan, C., Yang, S., Hao, L., Zhang, F., ... and Zhang, X. (2012). Polypyrrole/carbon nanotube nanocomposite enhanced the electrochemical capacitance of flexible graphene film for supercapacitors. Journal of Power Sources, 197, 319-324. [25] Navik, R., Xiao, D., Gai, Y., Tan, H., and Zhao, Y. (2020). One-pot synthesis of copper nanowire-graphene composite with excellent stability and electrical performance for flexible electrodes. Applied Surface Science, 527, 146694. [26] Wang, C., Song, Z., Wan, H., Chen, X., Tan, Q., Gan, Y., ... and Wang, H. (2020). Ni-Co selenide nanowires supported on conductive wearable textile as cathode for flexible battery-supercapacitor hybrid devices. Chemical Engineering Journal, 400, 125955. [27] Ma, Z., Zhao, J., Fan, Y., Qin, X., and Shao, G. (2022). High surface area of crystalline/amorphous ultrathin MnO2 nanosheets electrode for high-performance flexible micro-supercapacitors. Journal of Alloys and Compounds, 920, 166012. [28] Zhang, P., Qiu, J., Zheng, Z., Liu, G., Ling, M., Martens, W., ... and Zhang, S. (2013). Free-standing and bendable carbon nanotubes/TiO2 nanofibres composite electrodes for flexible lithium ion batteries. Electrochimica Acta, 104, 41-47. [29] Ma, R., Yang, W., Tong, Z., Yu, Z., and Zou, Y. (2025). A Graphene-Supported Nico2o4 Nanospheres Cathode Material for High-Performance Flexible Semi-Solid Asymmetric Supercapacitors. Available at SSRN 5163029. [30] Shanmuganathan, M. A. A., Raghavan, A., and Ghosh, S. (2023). Recent progress in polyaniline-based composites as electrode materials for pliable supercapacitors. Physical Chemistry Chemical Physics, 25(11), 7611-7628. [31] Liu, T., Li, C., Liu, H., Zhang, S., Yang, J., Zhou, J., ... and Xu, J. (2021). Tear resistant Tyvek/Ag/poly (3, 4ethylenedioxythiophene): Polystyrene sulfonate (PEDOT: PSS)/carbon nanotubes electrodes for flexible highperformance supercapacitors. Chemical Engineering Journal, 420, 127665. [32] Huang, Y., Li, H., Wang, Z., Zhu, M., Pei, Z., Xue, Q., ... and Zhi, C. (2016). Nanostructured polypyrrole as a flexible electrode material of supercapacitor. Nano Energy, 22, 422-438. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 162 [33] Kumar, S., and Seo, Y. (2024). Flexible transparent conductive electrodes: unveiling growth mechanisms, material dimensions, fabrication methods, and design strategies. Small Methods, 8(1), 2300908. [34] Zhu, Y., Li, N., Lv, T., Yao, Y., Peng, H., Shi, J., ... and Chen, T. (2018). Ag-Doped PEDOT: PSS/CNT composites for thin-film all-solid-state supercapacitors with a stretchability of 480%. Journal of Materials Chemistry A, 6(3), 941947. [35] Rana, M., Avvaru, V. S., Boaretto, N., Marcilla, R., Etacheri, V., and Vilatela, J. J. (2019). High rate hybrid MnO 2@ CNT fabric anodes for Li-ion batteries: properties and a lithium storage mechanism study by in situ synchrotron X-ray scattering. Journal of Materials Chemistry A, 7(46), 26596-26606. [36] Senokos, E., Ou, Y., Torres, J. J., Sket, F., González, C., Marcilla, R., and Vilatela, J. J. (2018). Energy storage in structural composites by introducing CNT fiber/polymer electrolyte interleaves. Scientific reports, 8(1), 3407. [37] Caffrey, N. M. (2018). Effect of mixed surface terminations on the structural and electrochemical properties of two-dimensional Ti 3 C 2 T 2 and V 2 CT 2 MXenes multilayers. Nanoscale, 10(28), 13520-13530. [38] Liu, S., Zhou, Y., Zhou, J., Tang, H., Gao, F., Zhao, D., ... and Zhang, Y. (2022). Ti3C2T x MXenes-based flexible materials for electrochemical energy storage and solar energy conversion. Nanophotonics, 11(14), 3215-3245. [39] Kumar, S., Mehdi, S. M. Z., Taunk, M., Kumar, S., Aherwar, A., Singh, S., and Singh, T. (2025). Synergistic effects of polymer integration on the properties, stability, and applications of MXenes. [40] Seok, D., Jeong, Y., Han, K., Yoon, D. Y., and Sohn, H. (2019). Recent progress of electrochemical energy devices: metal oxide–carbon nanocomposites as materials for next-generation chemical storage for renewable energy. Sustainability, 11(13), 3694. [41] Wu, Y., Zhao, X., Shang, Y., Chang, S., Dai, L., and Cao, A. (2021). Application-driven carbon nanotube functional materials. ACS nano, 15(5), 7946-7974. [42] Yan, Z., Luo, S., Li, Q., Wu, Z. S., and Liu, S. (2024). Recent advances in flexible wearable supercapacitors: properties, fabrication, and applications. Advanced Science, 11(8), 2302172. [43] Laine, J., and Yunes, S. (2024). Effect of the preparation method on the pore size distribution of activated carbon from coconut shell. Carbon. Retrieved from https://en.wikipedia.org/wiki/Supercapacitor [44] Zhou, Y., Yin, L., Xiang, S., Yu, S., Johnson, H. M., Wang, S., ... and Chu, P. K. (2024). Unleashing the potential of MXene‐based flexible materials for high‐performance energy storage devices. Advanced Science, 11(3), 2304874. [45] Bryan, A. M., Santino, L. M., Lu, Y., Acharya, S., and D’Arcy, J. M. (2016). Conducting polymers for pseudocapacitive energy storage. Chemistry of Materials, 28(17), 5989-5998. [46] Ehsani, A., Heidari, A. A., and Shiri, H. M. (2019). Electrochemical pseudocapacitors based on ternary nanocomposite of conductive polymer/graphene/metal oxide: an introduction and review to it in recent studies. The Chemical Record, 19(5), 908-926. [47] Zhou, X., Zheng, X., Dong, X., Wang, X., Jiang, Y., Li, G., ... and Ding, J. (2025). Recent advances in wearable fibershaped supercapacitors: materials, design, and applications. Energy Materials, 5(5), N-A. [48] Chen, X., Li, Y., Wang, X., and Yu, H. (2022). Origami paper-based stretchable humidity sensor for textileattachable wearable electronics. ACS applied materials and interfaces, 14(31), 36227-36237. [49] Li, Z., Li, M., He, W., and Fei, B. (2025). Electrospun fiber-based electrodes materials for flexible lithium-ion batteries. Energy Materials, 5(7), N-A. [50] American Chemical Society. (2024, July 17). Completely stretchy lithium-ion battery for flexible electronics. ScienceDaily. Retrieved May 24, 2025, from https://www.sciencedaily.com/releases/2024/07/240717121030.htm [51] McCallion, R. (2025, April 19). Stabbed, cut, attacked, twisted — scientists subject new stretchable battery to extreme torture, and it retained 90% of its capacity. Live Science. Retrieved from https://www.livescience.com/technology/engineering/stabbed-cut-attacked-twisted-scientists-subject-newstretchable-battery-to-extreme-torture-and-it-retained-90-percent-of-its-capacity [52] Wu, B., Wang, H., Chen, Y., Wang, Z., Maertens, T., Kuang, T., ... and Yang, J. (2021). Preparation and properties of thermoplastic polyurethane foams with bimodal structure based on TPU/PDMS blends. The Journal of Supercritical Fluids, 177, 105324. Global Journal of Engineering and Technology Advances, 2025, 23(03), 139-166 163 [53] Liu, X., Jiang, Y., Wang, Y., and Pan, L. (2024). Design Principles and Development Status of Flexible Integrated Thin and Lightweight Zinc-Ion Batteries. Batteries, 10(6), 200. [54] Lu, G., Xi, Q., Shao, Y., Yang, Y., Rui, Y., and Shao, Y. (2024). Recent progress in carbon nanomaterials for highly flexible fibrous aqueous zinc-ion batteries. Nanoscale Advances. [55] Ma, H., Tian, X., Fan, J., Cao, W., Yuan, X., Hou, S., and Jin, H. (2022). 3D printing of solid-state zinc-ion microbatteries with ultrahigh capacity and high reversibility for wearable integration design. Journal of Power Sources, 550, 232152. [56] Wang, W., Li, C., Liu, S., Zhang, J., Zhang, D., Du, J., ... and Yao, Y. (2023). Flexible quasi‐solid‐state aqueous zinc‐ion batteries: design principles, functionalization strategies, and applications. Advanced Energy Materials, 13(18), 2300250. [57] IDTechEx. (2023). Sodium-ion batteries 2024–2034: Technology, players, markets, and forecasts. Retrieved from https://www.idtechex.com/en/research-report/sodium-ion-batteries/978 [58] Lu, G., Xi, Q., Shao, Y., Yang, Y., Rui, Y., and Shao, Y. (2024). Recent progress in carbon nanomaterials for highly flexible fibrous aqueous zinc-ion batteries. Nanoscale Advances. [59] Rybak, D., Su, Y. C., Li, Y., Ding, B., Lv, X., Li, Z., ... and Dodda, J. M. (2023). Evolution of nanostructured skin patches towards multifunctional wearable platforms for biomedical applications. Nanoscale, 15(18), 8044-8083. [60] Zarei, M., Lee, G., Lee, S. G., and Cho, K. (2023). Advances in biodegradable electronic skin: Material progress and recent applications in sensing, robotics, and human–machine interfaces. Advanced Materials, 35(4), 2203193. [61] Benas, J. S., Liang, F. C., Venkatesan, M., Yan, Z. L., Chen, W. C., Han, S. T., ... and Kuo, C. C. (2023). Recent development of sustainable self-healable electronic skin applications, a review with insight. Chemical Engineering Journal, 466, 142945. [62] Islam, M. R., Afroj, S., Novoselov, K. S., and Karim, N. (2022). Smart electronic textile‐based wearable supercapacitors. Advanced Science, 9(31), 2203856. [63] Khurram Tufail, M., Ahmed, A., Rafiq, M., Asif Nawaz, M., Shoaib Ahmad Shah, S., Sohail, M., ... and Rahman, M. M. (2024). Chemistry Aspects and Designing Strategies of Flexible Materials for High‐Performance Flexible Lithium‐ Ion Batteries. The Chemical Record, 24(1), e202300155. [64] Fu, Z., Hannula, M., Jauho, A., Va isa nen, K. L., Va lima ki, M., Keskinen, J., and Ma ntysalo, M. (2022). Cyclic bending reliability and failure mechanism of printed biodegradable flexible supercapacitor on polymer substrate. ACS Applied Materials and Interfaces, 14(35), 40145-40157. [65] Belaineh, D., Brooke, R., Sani, N., Say, M. G., Håkansson, K. M., Engquist, I., ... and Edberg, J. (2022). Printable carbon-based supercapacitors reinforced with cellulose and conductive polymers. Journal of Energy Storage, 50. [66] El Foujji, L., and Bouhfid, R. (2023). Advanced biodegradable polymer nanocomposites for rechargeable lithium and sodium-ion battery applications. In Biodegradable and Biocompatible Polymer Nanocomposites (pp. 353395). Elsevier. [67] Chong, H. (2022). Development of novel COMPOSITE and multilayered materials for PDMS-based, minimallyinvasive flexible implantable microsystems (Doctoral dissertation, Case Western Reserve University). [68] Gong, X., Ding, M., Gao, P., Ji, Y., Wang, X., Liu, X. Y., ... and Ding, B. (2023). High-performance waterproof, breathable, and radiative cooling membranes based on nanoarchitectured fiber/meshworks. Nano Letters, 23(23), 11337-11344. [69] Xia, M., Liu, J., Kim, B. J., Gao, Y., Zhou, Y., Zhang, Y., ... and Ahn, J. H. (2024). Kirigami‐Structured, Low‐Impedance, and Skin‐Conformal Electronics for Long‐Term Biopotential Monitoring and Human–Machine Interfaces. Advanced Science, 11(1), 2304871. [70] Shang, Y., Wang, C., He, X., Li, J., Peng, Q., Shi, E., ... and Li, Y. (2015). Self-stretchable, helical carbon nanotube yarn supercapacitors with stable performance under extreme deformation conditions. Nano Energy, 12, 401-409. [71] Wang, Z., and Baughman, R. H. (2023). Twisted and coiled yarns for energy harvesting and storage, artificial muscles, refrigeration, and sensing. Journal of Composite Materials, 57(4), 805-815. [72] Yong, S., Hillier, N., and Beeby, S. (2023). Zinc-Ion Battery on a Polyester-Cotton Textile. Engineering Proceedings, 30(1), 20.