Synergies in Materials and Manufacturing: A Review of Composites and 3D Printing for Triboelectric Energy Harvesting

Abstract

Sophisticated energy-harvesting technologies have swiftly progressed, expanding energy supply distribution and leveraging advancements in self-sustaining electronic devices. Despite substantial advancements in friction nanomotors within the last decade, a considerable technical obstacle remains for their flawless incorporation using printed electronics and autonomous devices. Integrating advanced triboelectric nanogenerator (TENG) technology with the rapidly evolving field of composite material 3D printing with has resulted in the advancement of three-dimensionally printed TENGs. Triboelectric nanogenerators are an important part of the next generation of portable energy harvesting and sensing devices that may be used for energy harvesting and artificial intelligence tasks. This paper systematically analyzes the continual development of 3D-printed TENGs and the integration of composite materials. The authors thoroughly review the latest material combinations of composite materials and 3D printing techniques for TENGs. Furthermore, this paper showcases the latest applications, such as using a TENG device to generate energy for electrical devices and harvesting energy from human motions, tactile sensors, and self-sustaining sensing gloves. This paper discusses the obstacles in constructing composite-material-based 3D-printed TENGs and the concerns linked to research and methods for improving electrical output performance. The paper finishes with an assessment of the issues associated with the evolution of 3D-printed TENGs, along with innovations and potential future directions in the dynamic realm of composite-material-based 3D-printed TENGs.

1. Introduction

Researchers around the globe are concentrating on improving energy harvesting technologies, especially by making use of easily accessible environmental energy sources, in response to the growing demand for sustainable energy solutions. Solar, thermal, RF, and mechanical ambient energy sources are abundant. Nanogenerators, which employ piezoelectric, triboelectric, or pyroelectric energy to collect minuscule quantities of energy, are one of these sustainable energy alternatives. Triboelectric nanogenerators (TENGs) are one of the latest advancements in energy harvesting technology. The triboelectric effect allows them to transform mechanical energy into electrical energy [1]. The superior efficiency of triboelectric nanogenerators in producing electricity is a result of the complementary processes of electrostatic induction and triboelectrification. When two surfaces come into contact, triboelectrification occurs, resulting in the generation of charges. An external load is used in electrostatic induction to transmit electrons between electrodes [2]. A triboelectric nanogenerator (TENG) functions in four modes: the freestanding triboelectric layer mode, the vertical contact–separation mode, the lateral-sliding mode, and the single-electrode mode. A TENG is capable of detecting vibrations, cyclic rotations, and intermittent impacts. The periodic alternation between two contact surfaces and their separation underpins the power-generating mechanism. A typical TENG mode has distinct advantages and structural attributes, and the vertical contact–separation mode was the mode that was developed first. Since TENGs’ inception in 2012, numerous studies have been undertaken to improve them and broaden their application scope, with a continual focus on enhancing output performance [3]. The selection of materials for the acquisition or loss of electrons is essential for enhancing the output efficacy of TENGs. Thus, to enhance manufacturing performance, the vast range of materials for TENG manufacture needs more study. Materials exhibiting the triboelectric effect are crucial to the internal and external structures of TENGs [4]. Recent studies show how certain composite formulations can improve TENG performance, highlighting composites’ potential in sustainable energy solutions [5]. Composites assist in boosting the performance of triboelectric nanogenerators (TENGs) by varying the materials to optimize the triboelectric properties. The addition of carbon nanotubes (CNTs) serves as a conductive filler, improving the electrical conductivity of the composite material, which is crucial for TENG performance [6]. Composites take advantage of unique structural features, such as dynamic molecular pulleys, which promote thorough mixing to maximize the energy interactions and lower the resistance within highly dispersed composites, hence improving power density and overall performance in sustainable energy applications. This is especially important for AI because TENGs can be used as a power source for AI-based wearable and sensor network devices without reliance on traditional batteries [7,8,9]. In addition, researchers have utilized techniques such as electrospinning [10], spin-coating [11], and solvent casting [12] to enhance the production of triboelectric nanogenerators. Due to the potential for a rapid turnaround, cost-effective modeling, and user-friendly capabilities in producing complex and configurable designs with high material efficiency, the 3D printing of TENGs represents the most promising production method for TENG development [13]. Three-dimensional printing enables the customization of length and size according to desired specifications, facilitating the production of more complex and high-resolution TENG structures. Moreover, extrusion-based 3D printing for TENGs may fabricate electrical components directly without requiring a preliminary template, decreasing production time and costs relative to conventional TENG manufacturing methods. Extrusion-based 3D printing for TENGs, with its rapid fabrication time and controlled material deposition into specified configurations, is the most reliable method for creating TENGs among other 3D printing techniques [14]. In extrusion-based 3D printing, the filament is an essential component for achieving potent geometries and the surface architecture of triboelectric layers. The most significant frequently utilized auxiliary materials include acrylonitrile butadiene styrene (ABS) [15], polyethylene (PE) [16], and polylactic acid (PLA) [17]. On the other hand, polytetrafluoroethylene (PTFE), polyamide (PA), and polyethylene glycol (PEG) comprise the functional layers.

Chen et al. [18] first elucidated the 3D printing of TENGs using the well-known hydrogel as an electrode and composite resin as a triboelectric film. Since that time, other researchers have developed nanogenerator topologies using 3D printing technologies. Tong et al. [19] developed 3D-printed triboelectric nanogenerators for elastic membranes, meshes, and hollow structures created on flat, rotating, and uneven anatomical substrates using elastic metal-core silicon–copper fibers via fused deposition modeling. They also developed wearable mechano-sensors that monitor human and organ activity, perfused organs, and voice recognition without auditory input from the speaker. Qiao et al. [17] created a TENG using fused deposition modelling (FDM) and friction layers made of polylactic acid (PLA), nylon (PA), a mix of polypropylene and polyethylene (PP/PE), and poly (ethylene terephthalate 1,4-cyclohexylenedimethylene terephthalate) (PETG). This made the digital optimization of TENG device designs easier for effective noise energy harvesting. Mallineni et al. [20] delineated innovative high-performance TENGs using an additively manufactured gPLA nanocomposite. These TENGs may transform mechanical energy into electrical power and transfer the generated energy wirelessly, without requiring supplementary electronics or other power sources. They integrated a gPLA nanocomposite, created through 3D printing on a polyimide (Kapton) film, with a complementary polytetrafluoroethylene (PTFE or Teflon) sheet to create a gPLA-based triboelectric nanogenerator (TENG). On the other hand, triboelectric nanogenerators (TENGs) employ composite materials to boost energy harvesting efficiency via triboelectric effects. Recent research demonstrates that specific composite formulations can markedly enhance TENG performance, underscoring their promise in sustainable energy solutions. Zhang et al. [21] explores enhancing the current density of triboelectric nanogenerators (TENGs) by introducing dipole–dipole interactions between a nylon filter membrane (NFM) and graphene oxide (GO) through hydrogen bonds. The chemical interactions, specifically hydrogen bonds, between materials in composites have not been extensively studied. The improvement in charge transport of triboelectrification is observed through the hydrogen bonding between materials by using nylon film and GO-coated nylon filter membranes as triboelectric layers. The results showed that an ultrahigh current density of 1757 mA·m⁻² was achieved with the novel nylon/GO-NFM TENG. The study also demonstrated the feasibility of using the TENG as a motion sensor to detect finger movements.

Given the rapid advancements in TENG technologies, it is crucial to conduct frequent assessments to assess the technology’s readiness and promptly address any urgent concerns. Recently, several articles have come out that are useful for researchers in related fields because they show how 3D-printed TENGs, which are integrated into composite materials, and 3D printing have improved in areas like energy harvesting, active sensing, and theoretical modeling [22,23,24,25,26,27]. Recent reports of substantial progress in our fundamental understanding and practical use of TENGs have provided us with more thorough insights into them from both scientific and practical viewpoints. This review aims to provide a framework for future modifications, rather than just enumerating accomplishments. It also examines the principles of TENGs, encompassing their theoretical foundations, the mechanism of triboelectrification, performance-optimizing elements, criteria, and energy-regulation methodologies. It then examines recent notable breakthroughs in TENG applications, choosing broadly applicable solutions or innovative initiatives in a highly influential field. It also proposes a strategic roadmap for TENG research to determine the principal paths and provide a schedule for TENG development. This review presents an overview of composite materials in TENG, the integration of 3D printing with composite materials, advancements in energy harvesting, and the use of 3D-printed TENGs in sensors and energy-harvesting applications. Figure 1 demonstrates the potential applications of energy harvesting and sensors via composite-based 3D-printed triboelectric nanogenerators (TENGs).

2. Fundamentals of Triboelectric Nanogenerators (TENGs)

2.1. TENGs’ Functioning Modes

Since the inaugural report on TENGs in 2012, and based on their electrode configurations and orientations or polarization alternations, they have been divided into four distinct modes, as illustrated in Figure 2: vertical contact–separation (CS) mode, lateral-sliding (LS) mode, single-electrode (SE) mode, and freestanding triboelectric layer (FT) mode. A critical element of the TENG design is the effective induction of triboelectrification. Establishing an appropriate operational mode ensures the efficacious use of ambient external mechanical energy and provides triboelectrification suitable for a specific TENG configuration. The three typical operational modes are the sliding mode, the contact–separation mode, and the freestanding mode. Every mode is accessible for electrode arrangements: single and double electrodes. The double-electrode design primarily facilitates the above three operating modes [36].

2.1.1. Contact–Separation Mode TENG

The contact–separation mode is the used in most imperative and direct kind of triboelectric generator. Figure 2a illustrates this configuration, where electrodes attach to two distinct dielectrics that are oriented towards each other. Upon contact between the dielectrics—one operating as an anode and the other operating as a cathode—their surfaces acquire charge. A potential difference arises between the two substances upon separation due to the retention of charge on the surfaces of the dielectrics. Consequently, an electrostatic field transfers electric charges between the electrodes. The electric field dissipates with the reconnection of the materials, allowing the electrons to return. This continuous process generates alternating current electricity [37].

2.1.2. Lateral-Sliding Mode TENG

Sliding triboelectric nanogenerators work in a way that resembles the function of the vertical contact–separation mode. The only distinction is that, as can be seen in Figure 2c, the “friction” between the two materials has transitioned from vertical contact–separation to sliding motion. Lateral-sliding mode triboelectric nanogenerators function through the friction between two triboelectric layers. Unlike in the vertical contact–separation mode, the sliding mode generates a rather consistent output, fulfilling the need for a continuous power supply over a broader range of applications [38].

2.1.3. Single-Electrode (SE) Mode TENG

In SE mode, the freely flowing triboelectric layer lacks contact wires or electrodes. The system links only one electrode, or it operates independently as a triboelectric layer. Figure 2b demonstrates the operation of a single-electrode triboelectric nanogenerator (TENG). The surface charge transfer occurs when the triboelectric layer is in complete contact with the electrode. This causes opposing charges to form on both sides without any electrons moving. When the two electrodes separate, they facilitate charge exchange with the ground and equilibrate the electric potential, reducing the induced charges on the single electrode’s surface. To achieve equilibrium and ensure complete contact between both surfaces, electrons will transfer from the ground to the electrode upon approaching contact [39].

2.1.4. Freestanding Triboelectric Layer TENG

In the freestanding mode, a dielectric layer is mobile over the surfaces of two stationary metal electrodes, as shown in Figure 2d. The dielectric layer’s displacement between the two electrodes establishes an asymmetrical charge distribution, resulting in electron movement inside the circuit. This can achieve a somewhat higher energy conversion efficiency than other TENG modes. This capability enables the use of the freestanding mode in efficient energy-harvesting applications, including airflow measurements, autonomous sensors, and human dynamics [40].

3. Triboelectric Materials

3.1. Material Choice for Triboelectric Nanogenerators

The hunt for clean and efficient energy sources has resulted in extensive research into novel energy harvesting technology. Among these, triboelectric nanogenerators (TENGs) have emerged as a promising approach for converting mechanical energy into electrical energy [41]. Triboelectricity, or contact electrification, has been observed since ancient times. As early as 600 BC, the Greeks noted that rubbing amber with fur could attract light objects, a manifestation of static electricity [42]. When two dissimilar materials come into contact, a transfer of electrons occurs between their surfaces. Once the two materials are separated, they carry opposite charges. If one or both of those charged materials are connected to an electrode, the changing electric field due to their relative motion induces a flow of electrons in the external circuit [43]. Unlike in TENG, a pair of triboelectric materials with different charge affinities is always employed to generate charges. Theoretically, the selection of triboelectric materials depends heavily on the operating environment, required output voltage and current, mechanical stresses involved, cost, and scalability of fabrication. To generate more charge or obtain high output, a significant difference in charge affinities of two materials is preferred [44]. However, material density is the only material figure of merit (FOM). The performance FOM (FOM_P) includes structural FOM (FOM_S) and material FOM (FOM_M). FOM_S is based on device design, and FOM_M is based on the material’s surface charge density. The different TENG working models follow the FOM_S trend of contact: FT (CFT) > C-S > sliding FT (SFT) > LS > SE contact mode (SEC) [45]. The FOM is essential for the commercialization of TENG-based sensors and systems. In 2019, Wang’s group at Georgia Institute of Technology developed a new method that can quantify the triboelectric charge density (TECD) of different materials based on their triboelectrification with mercury operating with the contact–separation mode. However, contact electrification is generally referred to as triboelectrification in conventional terms [46]. The key parameters for contact electrification, surface charge density, polarity, and strength of charges are strongly dependent on the materials [47]. In 2020, Zou et al. [48] measured nearly 30 kinds of common inorganic non-metallic materials, and their triboelectric series is presented in Figure 3.

3.2. Composite Materials in TENGs

Composite materials are designed to take advantage of individual components such as polymers, ceramics, and even conductive fillers to create new materials with better dielectric characteristics, mechanical strengths, and affinities for charges. These characteristics are important for TENGs as they directly affect a device’s electric energy generation and storage capabilities [49]. The application of composites to TENGs has seen widespread use because of their ability to circumvent the drawbacks posed by single-material systems. For example, polymer-based composites tend to have high flexibility alongside tunable permittivity, while ceramic-based composites have better dielectric properties and thermal stability. The innovation of these materials into composites has enhanced the performance output of TENGs concerning voltage, current, and power density [50]. The most encountered materials that are utilized in the construction of TENGs are polymer-based composites due to their formability and customizability for various functions. Polydimethylsiloxane (PDMS), polyvinylidene fluoride (PVDF), and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) are polymers that are frequently employed with dielectric fillers, such as barium titanate (BaTiO₃) or graphene quantum dots (GQDs), to improve their triboelectric characteristics [51]. Ceramic-based composites have high permittivity, thermal stability, and greater operational performance in conditions that require sustained energy output, which makes them valuable for certain applications. Barium titanate (BaTiO₃) and lead zirconate titanate (PZT) are good examples because they are used with polymers or conductive fillers to achieve appropriate flexibility and dielectric properties [52]. Hybrid composites exploit the advantages of both polymers and ceramics, enabling flexibility and mechanical endurance. Due to the soft and flexible nature of these composites, they provide high permittivity while maintaining low weight and structural rigidity. In addition, these composites often contain conductive fillers for improved charge transfer and storage, such as carbon nanotubes (CNTs) and graphene [53]. The composite materials used in TENG devices largely affect their performance, owing to their dielectric traits. To attain the required permittivity and dielectric levels, various researchers have been developing various composites. One of the approaches is adding MWCNTs into chitosan matrices, which improves the dielectric constant and thus improves the output of TENGs [54]. The generation and retention of surface charges are very important in determining the efficacy of TENGs.

Table 1. Overview of composite TENGs: fabrication techniques and performance.

Sl. No	Composite Materials	Fabrication Technique	Application	Output Parameters	Reference
1	ZnO/PDMS	Thermal oxidation and composite formation	Surface charge density detection	-	[61]
2	PET/TPU/Fabric/PDMS	Layering and screen printing	Energy harvesting, sensor application	-	[62]
3	PMMA/PTFE/Al/C	DC Magnetron sputtering, spin-coating	Energy harvesting for IoT devices	-	[63]
4	Sn/PTFE/Al	Cold spray deposition	High-performance energy generation	-	[64]
5	Multiwall carbon nanotubes, PEDOT:PSS	Vacuum filtration	Wearable electronics	7.9 V	[65]
6	Aluminum, PDMS, AuNPs	Simple layering and spraying	Self-powered systems, wearable devices	169 mV (voltage), 120.4 µA (current), 6.006 µW (power)	[66]
7	FEP, Al	Modified stamp holder	Energy harvesting, sensing, data prediction	310 V, 165 μA, 14.8 W/m2	[67]
8	PDMS/ZnSnO3/MWCNT	Aqueous solution synthesis	Portable electronics, sensors	475 V, 36 mA, 4.2 mW	[68]
9	PVDF/tourmaline	Electrospinning	Powers LEDs, electronic watches	107 mW/m2, 267 V open-circuit voltage	[69]
10	Conductive and staple fibers, triboelectric materials	Core-spun yarn coating	Smart wearables, motion detection	117 V, 213 mW/m2	[70]

gives an overview of the composite materials that are used to obtain output voltages using different types of fabrication techniques. For example, dual nanocomposite layers such as PVDF/MXene and PDMS/NaNbO₃ can improve charge generation and retention and therefore increase a TENG’s output voltage to 150 V and current to 4.3 μA [55]. The TENG is mechanically robust so that it serves its purpose for a long period and needs to be constructed from materials that do not weaken when mechanical stress is repeatedly applied. As an example, soft composites like the ethyl cellulose/thermoplastic polyurethane (EC/TPU) nanofiber membranes with barium titanate nanoparticles retain their structure and electrical output even after extreme cycling [56]. The natural and synthetic composites for triboelectric nanogenerators (TENGs) reveal significant differences in sustainability, performance, and application potential. Natural composites, derived from renewable resources, offer ecofriendly alternatives with promising triboelectric properties. The natural rubber composites, particularly those enhanced with silver nanoparticles (AGNPs), have significantly improved dielectric constant, which is crucial for enhancing charge separation. The NR-Ag composite improves electrical output and exhibits antibacterial properties, making it suitable for applications like shoe insoles that convert human motion into electricity while preventing foot odor [57]. On the other hand, combining chitosan and silk fibroin into a composite film demonstrates excellent electron-donating ability, enabling efficient energy harvesting from human movements. The CSA-TENGs can be integrated into devices like hand clappers and trampolines, effectively harvesting biomechanical energy while monitoring human motion [58]. In contrast, synthetic composites for triboelectric nanogenerators (TENGs) are engineered materials that enhance energy harvesting capabilities by combining various polymers and conductive materials. Commonly used polymers include poly(ethylene-co-vinyl acetate) (EVA) and ethylcellulose (EC), which provide flexibility and structural integrity. At the same time, materials like polyaniline (PANI) and silver nanoparticles (AgNPs) are incorporated to improve electrical conductivity and enhance triboelectric performance [59]. Synthetic composites show great promise in enhancing TENG performance; challenges remain in scaling production and ensuring long-term stability under varying environmental conditions. The evolution of composite materials has pioneered the development of TENGs, enabling increased energy output, greater durability, and more diverse uses. It is expected that further studies will be directed towards the synthesis of new composite materials with tailor-made attributes such as higher permittivity, efficient charge retention, and a reduction in stress sensitivity. With these advancements, higher opportunities will arise regarding the use of TENGs in wearable electronics, self-powered biomedical devices, and smart systems [60].

3.3. Role of Composite Materials in Enhancing TENG Performance

The development of composites is equally vital in advancing the technology of TENGs. TENGs have undergone significant changes owing to the integration of composites in these devices, which has broadened the scope of research towards enhancing performance efficiency and durability and improving the efficiency and lifetime of TENGs. The numerous studies conducted on TENGs focus on their integration with composites, suggesting that the impact of composites in these devices has already been widely acclaimed. Thus, it can be noted that the use of composite materials enhances the mechanical, physical, and electrical properties of TENGs, thereby improving performance efficacy [71]. Table 2 explicitly delineates the principal characteristics acquired by the composite materials in TENGs.

As depicted in Figure 4A Wang et al. [72] developed (B₄C) particles to improve the area-wise performance of poly(vinylidene fluoride) (PVDF)-based triboelectric nanogenerators (TENGs) by increasing their wear resistance and thermal conductivity for prolonged operation under difficult circumstances. There is an increasing need for sustainable power technologies to energize wearable electronics and sensing nodes; here, TENGs are emerging as a viable solution for mechanical energy–electricity conversion. Nonetheless, TENGs encounter obstacles like heat buildup, surface wear, and abrasion that can negatively impact their lifespan and efficiency. To overcome these problems, researchers have introduced B4C particles into PVDF due to B4C’s high hardness, thermal conductivity, and corrosion resistance. The composite film showed 600% higher thermal conductivity relative to pure PVDF film, alongside a reduction of 39.7% in average frictional mass loss. B4C/PVDF TENG showcased an increase in electrical output performance along with improved anti-corrosion attributes, achieving an open-circuit voltage of 155.4 V, a short-circuit current of 7.9 µA, and maximum output power density of 0.33 W/m², surpassing the performance of pure PVDF devices.

As demonstrated in Figure 4B, Cao et al. [73] implemented changes in the construction materials of TENGs to enhance their functionality. TENGs, or triboelectric nanogenerators, are devices that convert mechanical energy into electrical energy. They are constructed of several tiers, each with a silencing layer, and the conversion of energy depends on the triboelectric properties of the materials used. Cao et al. aimed to improve the qualities of 2D nanomaterial fillers using chemical modification. For this purpose, they specifically used Ti₃C₂ and MXene, which can serve to improve polymer performance in TENGs. This effort involves the chemical modification of Ti₃C₂–MXene to alter their electrical polarity. The MXene was aminated to positive (NH2-Ti₃C₂) and negative (N-Ti₃C₂) nitrogen terminals to make it triboelectrically positive and triboelectrically negative, respectively. By incorporating these modified MXene constituents into specific polymers, NH₂-Ti₃C₂ with Nylon 11 for the positive extenuating layer and N-Ti₃C₂ with PVDF-TrFE for the negative layer, the composite friction layer can be synthesized. Each layer’s surface potential was modified by functionalized Ti₃C₂ with varying polarities, including positive and negative friction layers. The composite formation increased the surface potential in the desired (negative or positive) direction, which resulted in outperforming composites. Furthermore, the surface potential was enhanced by electrically polarizing each friction layer in the required direction. The PVDF-TrFE/N-Ti₃C₂ and Nylon 11/NH2-Ti₃C₂ friction pair produced TENGs with a peak output voltage of 250 V, a short-circuit current density of 280 µA/cm², a charge density of 210 µC/m², and a power density of 13 mW/cm².

On the flip side, Johnson et al. [74] proposed a new environmentally friendly biodegradable and recyclable triboelectric material from a composite of polyethylene oxide (PEO) and mica. The tribo-positive polarity of PEO was improved by adding muscovite mica micro-platelets to the polymer. The PEO/mica film shows good triboelectric properties. The composite film resulted in a voltage of 296 V PP with a current density of 24.2 mA while used in combination with PTFE. The addition of mica and the dielectric permittivity of the composite enhanced the overall triboelectric properties. The material also demonstrated good long-term stability and flexibility, which makes it ideal for self-powering small electronic systems. Among the most notable features of this PEO/mica composite is its ecofriendly nature. The ability of PEO to readily dissolve in water enables the recovery of mica, which can subsequently be used to make new PEO/mica films without deteriorating the triboelectric performance. This is shown in Figure 5.

Focusing on novel applications for energy harvesting, Kaur et al. [75] assess the feasibility of using eggshell membranes in composite zinc oxide (ZnO) triboelectric nanogenerators. Their work illustrates a method of powering nanoscale systems with the nanogenerators developed from the composite membranes, noting their ability to power green LEDs, charge capacitors, and power digital watches. The behavior of the materials made of the membranes that Kaur et al. created was also examined. They concluded that the composite material had a positive tribo-charging behavior, confirming the specific charging behavior of the nanostructures. The incorporation of eggshell membrane waste from the food industry significantly complements the biocompatible materials in the field of triboelectric devices, as depicted in Figure 6.

In 2023, Luo et al. [76] researched the implementation of a self-powered smart agriculture sensor based on a living plant leaf triboelectric nanogenerator (LPL-TENG). The LPL-TENG must be capable of monitoring eco-physiological stresses impacting a plant’s health and productivity, as this function is essential for food production in smart agriculture. A solution-casting technique was used to form stable and humidity-sensitive composite films with multilayer graphene oxide (GO) and spherical polyvinylsilsesquioxane (PVSQ) and polyvinyl alcohol (PVA). These composite films, along with the living plant leaves, are used by LPL-TENG to convert mechanical energy into electrical energy. This energy conversion allows the LPL-TENG to power small electronic devices and sense plant leaf humidity. The sensor can respond linearly to variations in plant leaf humidity and can also detect high wind speed. This is shown in Figure 7.

On the other hand, Pandey et al. [77] focus on the design of a high-performance triboelectric nanogenerator (TENG) powered by Nafion-functionalized BaTiO₃ NPs embedded in a PVDF composite nanofiber mat. Their primary objective aims to develop a self-sufficient human–machine interface intended for smart control systems. The incorporation of inorganic compounds such as BaTiO₃ into polymer matrices like PVDF offers the possibility of improving the mechanical properties and triboelectric responses of composite materials. Nevertheless, one major problem remains in the attainment of homogenous distribution and the stability of the composite materials over time. Nafion is used to guarantee the effective dispersion of BaTiO₃ NPs into the PVDF matrix because of its hydrophilic and hydrophobic nature. This modifies the distribution of BaTiO₃ NPs in the PVDF solution, where they withstand precipitation and agglomeration for up to six months. The composite nanofibers gained improved negative surface potential, which is expected to increase the stress transfer at the interface of BaTiO₃ and PVDF. The output performance of NBP-TENG in Figure 8 is impressive. It attains an output voltage of 307 V with a current density of 1.8 µA/cm² and a power density of 1.12 mW/cm², which shows a 6.3-, 7.1-, and 3.7-fold increase in output voltage, current density, and power density, respectively, compared to pristine PVDF nanofibers. NBP-TENG’s multifunctional capabilities were further validated by being able to power multiple commercial LEDs in series through manual tapping. It also maintained stable output performance over 10,000 cycles of repeated contact–separation, indicating its enduring durability and reliability.

In 2021, Choi et al. [78] describe the attempt to improve the performance of a triboelectric nanogenerator (TENG) in the context of a polymer composite of PDMS and conductive carbon black particles. The goal was to charge the storage effect of the composite to improve the long-term output performance of the TENG. A PDMS-based carbon black composite (PCC) was used for the TENG’s contact layer. The charge storage ability of this composite significantly decreased the TENG’s long-term output. They also increased the TENG’s long-term output performance by more than two times by increasing the carbon black content in the composites. The scotch yoke mechanism was used for the effective operation of the TENG with a PCC layer. This configuration provided the benefit of harvesting almost anywhere, using rotational motion; it produced an output current of 320 µA, which is enough to power hundreds of LEDs or commercial electronic devices, as described in Figure 9.

Mai et al. [79] explored the enhancement of droplet triboelectric nanogenerators (droplet-TENGs) using composite polymer films and electrowetting-assisted charge injection (EWCI), as shown in Figure 10A. Droplet-TENGs are identified as a promising renewable energy resource. The authors mainly focused on enhancing the dielectric surface properties, which are crucial for increasing the output performance of TENG devices. The fabrication of the droplet-TENG on an iodium tin oxide glass substrate was performed by using three different dielectric films: a pure amorphous fluoropolymer (AF) film, a composite AF film doped with silk powder (AF/Silk), and a composite AF film doped with polytetrafluoroethylene (PTFE) particles (AF/PTFEp). The energy-harvesting performance of the three devices was tested with and without the application of EWCI. The results showed that, for the AF and AF/silk devices, the output current increased by approximately 12 times following the application of EWCI. The composite AF/silk device also demonstrated improved current stability. The AF/PTFEp TENG exhibited a five-fold increase in output current compared to the TENG with a pure dielectric layer, and this improvement increased to around 25 times with the application of EWCI. From all three devices, reducing the dielectric layer thickness increases the output current.

In 2023, Lazar et al. [80] proposed an inventive method of using La0.8Sr0.2CoO3 (LSCO) ceramic particles with high permittivity for the charge transfer enhancement in triboelectric nanogenerators (TENGs) by adding them to a PDMS polymer matrix. A TENG based on PDMS films is fabricated to improve the dielectric properties of the triboelectric material. They observed that the film with 20 wt% LSCO had better electrical properties than pure PDMS film. The LP-TENG device (Figure 10B) of size 4 × 4 cm², when tapped, was able to illuminate 25 light-emitting diodes. Moreover, the vertical stack of PDMS/LSCO was able to harness the energy of mechanical vibrations and produce a current of 43 µA and an output voltage of 90 V. Strontium-doped Lanthanum Cobalt Oxide (LSCO) ceramics showed that their combination with polymers yields composites of remarkable electric and dielectric properties. LSCO exhibits a high dielectric constant around 600, which is beneficial. While LSCO has great properties, its triboelectric properties have yet to be reported. LSCO’s properties make it a unique candidate for charge transfer improvement in PDMS/LSCO composites with copper electrodes and increase the performance of TENGs made from PDMS/LSCO composites.

Zhao et al. [81] developed a TENG with a particular focus on a wind-energy-harvesting application that features PVC/MoS₂ composite membranes and PA membranes as triboelectric materials with Al sheet electrodes that have micro–nanostructures, as depicted in Figure 11. The incorporation of a small proportion of MoS₂ enhances the surface charge density and abrasion resistance of the composite films, which increases the TENG’s performance while also prolonging its lifespan. The optimized TENG is capable of producing an output voltage of 398 V, a current of 40 µA, and a maximum power of 1.23 mW, thus powering several commercial LEDs and a water thermometer. Molybdenum disulfide (MoS2) is a type of layered crystal that has weak Van der Waals bonds between the layers, which results in a high specific surface area, good mechanical properties, and large elastic strain. Under vacuum and high-temperature conditions, it can maintain a low friction coefficient while exhibiting excellent lubricating properties. The friction coefficient of the PVC/MoS₂ composite membrane is 0.29, which is 19.4% lower than that of the pure PVC membrane, thus significantly prolonging the lifespan of the TENG. The device demonstrates remarkable stability with steady output current for roughly 15 h at a wind speed of 17.7 m/s.

In 2022, Wu et al. [82] published a work in which the tribo-positivity of cellulose paper was modified by incorporating branched polyethylene imine (PEI), as shown in Figure 12. By varying the amount of PEI, the authors synthesized PEI–paper composites with different structures and mechanical properties. It was found that, with greater loading weight of PEI, these composites showed a marked increase in triboelectric output performance due to a steep increase in relative permittivity. For instance, when PEI was loaded to 7.5 mg/cm², the composite formed a network structure, while at 22.5 mg/cm² loading, it transformed to a hydrogel-like structure. The output triboelectric performance of the PEI–paper-composite-based TENG increased by almost 4 and 6 times, respectively, and approximately 7.5 and 16 times the increase in power densities was also noted. The ability of the PEI–paper-composite-based TENG to operate as a self-powered pressure sensor enables it to respond to multiple external disturbances in different operational modes, thus paving the way for its application in self-powered electric skin. In addition, the PEI endows the PEI–paper composites with water resistance, antibacterial properties, and flame retardation, while still allowing the paper to be easily discarded.

Kang et al. [83] made a biodegradable TENG device using an x-carrageenan–Agar (xC–Agar) composite, which has been labeled as a high-performance triboelectric friction material, as represented in Figure 13. The advancement of biodegradable TENGs is necessary for the development of bio-transient electronics that are meant to be absorbed or dissolved within the body after a set duration; thus, surgically removing the device is not necessary, as illustrated in Figure 10. The xC–Agar composite is a natural polymer that consists of polysaccharides from red seaweed. The composite possesses a 3D aggregated porous network structure with an abundant number of charge-trapping sites, which are required for improving the triboelectric properties of the material. These charge-trapping sites, made of Ca2+ cations and sulphate ester groups, enhance the chances of an electron being withdrawn, i.e., they are unoccupied states, leading to a greater ability to donate electrons. They found that the xC–Agar composite demonstrated enhanced electron-donating properties, with the surface potential increasing by up to 57.5% compared to bare materials when the composite had an optimized xC concentration of 80 wt%. The fabricated, fully biodegradable TENG using the optimized xC–Agar composite, which is very thin (0.3 mm) and flexible, achieved a high-output root mean square (RMS) current density of 0.45 mA·m⁻² and an output RMS power of 0.15 mW·m⁻² at optimized impedance. The TENG successfully powered 10 commercial blue light-emitting diodes (LEDs) and charged 1 µF and 5 µF capacitors without requiring an external power source. Overall, they demonstrate the potential of the nature-derived xC–Agar composite as a high-performing triboelectric material for biodegradable TENGs, offering a promising avenue for powering bio-transient electronics.

Table 2. Materials, features, and performance of composite-material-based TENGs.

Sl. No	Composite Materials	Key Features	TENG Performance Metrics	Reference
1	PDMS/BTO/GQD nanocomposite	High permittivity, conductive media	VOC: ~310 V; power density: ~1.6 W/m2	[84]
2	P(VDF-HFP)/NiFe2O4 nanofiber composite	High β-phase content, ferrimagnetic properties	VOC: 584 V; current: 25 μA	[85]
3	ZIF-72/PDMS nanocomposite	Enhanced dielectric constant, surface adhesion	VOC: 578 V; power density: ~5 W/m2	[86]
4	PVDF-GnP fibrous composite	High mechanical stability, flexibility	VOC: 134.4 V; current: 12.9 μA	[87]
5	EC/TPU/BTO nanofiber composite	High roughness, piezoelectric enhancement	VOC: 125.8 V; power density: 1.68 W/m2	[56]

Table 2. Materials, features, and performance of composite-material-based TENGs.

Sl. No	Composite Materials	Key Features	TENG Performance Metrics	Reference
1	PDMS/BTO/GQD nanocomposite	High permittivity, conductive media	VOC: ~310 V; power density: ~1.6 W/m2	[84]
2	P(VDF-HFP)/NiFe2O4 nanofiber composite	High β-phase content, ferrimagnetic properties	VOC: 584 V; current: 25 μA	[85]
3	ZIF-72/PDMS nanocomposite	Enhanced dielectric constant, surface adhesion	VOC: 578 V; power density: ~5 W/m2	[86]
4	PVDF-GnP fibrous composite	High mechanical stability, flexibility	VOC: 134.4 V; current: 12.9 μA	[87]
5	EC/TPU/BTO nanofiber composite	High roughness, piezoelectric enhancement	VOC: 125.8 V; power density: 1.68 W/m2	[56]

4. Three-Dimensional Printing for Triboelectric Nanogenerators (TENGs)

4.1. Advantages of 3D Printing in TENG Fabrication

Autonomous sensing systems and next-generation distributed energy harvesters greatly benefit from three-dimensional printed triboelectric nanogenerators (3D-printed TENGs) for their integration, reliability, adequate performance, and independence from external power sources. Based on their operational modes and 3D printing procedures, the classification of the structural characteristics of 3D-printed triboelectric nanogenerators (TENGs) is divided into two primary groups: stiff frameworks and adaptable frameworks. The conventional structure has many advantages: it is simple, straightforward to assemble, produces outstanding results, is very practical, and serves several purposes. Of these, FDM technology is widely used to design and document stiff structural 3D-printed TENGs. Structured, layer-to-layer 3D-printed flexible parts can be used as triboelectric and flexible substrate layers in real life, which is good for worn or implanted electronics. SLS and other 3D printing methods can be used to produce composite-structured 3D-printed TENGs. Several limitations, including printing resolution, aspect ratio, and processing temperature, hinder the utility of 3D printing for different applications, despite its many benefits and use in the production of TENG devices. The broader tolerances inherent in 3D printing may compromise the consistency of multiple prints of the same component. Such deviations often fail to meet quality control standards when produced in larger volumes [88]. Figure 14 demonstrates the milestones in the development of 3D-printed TENGs and composite-material-based 3D-printed TENGs. Consequently, TENG production has historically depended exclusively on regulatory oversight, focusing on certain craft methods, tools, and materials. Figure 15 demonstrates the 3D printing part-fabrication analysis and data analytics for 3D-printed components, which fall into two primary categories based on their structural strength: rigid and flexible constructions. It can be seen in Figure 15a that, of the 3D-printed components, 72.5% were stiff and 27.5% were flexible. It is evident that various rigid 3D-printed components offer similar benefits, and a significant proportion of these components are in actual use for the fabrication of 3D-printed TENGs. This is mainly because 3D printing technologies and materials facilitate the construction of stiff-structured TENGs more efficiently than flexible-structured TENGs. Figure 15b shows the real-world implementations of 3D printing fabrication functionalities, which include 52.5% of the triboelectric layer and 42.5% of the supporting frame being utilized for fabrication of 3D-printing-based TENGs [89].

4.2. Three-Dimensional Printing Techniques for TENGS

One can categorize 3D printing based on the printing materials and forming principles used. The categories include e-jet printing [90], screen printing [91], digital light processing (DLP) [92], stereolithography (SLA) [93], fused deposition modeling [20], direct ink writing [23], and selective laser sintering (SLS) [89]. Research suggests that extrusion-based 3D printing and direct ink writing (DIW) have more distinguished characteristics relative to other process types. The predominant 3D printing method for fabricating nanogenerators is fused deposition modeling, owing to its cost-effectiveness and user-friendliness. FDM extrudes a thermoplastic filament onto the substrate by heating it at the nozzle beyond the glass transition temperature. Upon extrusion, the printed component solidifies as the temperature decreases, forming the 3D shape. The FDM approach is applicable for fabricating many triboelectric layer configurations, such as thin films, zigzag patterns, and pyramid arrays. Another known extrusion-based 3D printing technique for TENG fabrication is direct ink writing (DIW) [94]. This approach extrudes the printing material via a nozzle under a particular pressure. Materials stack according to a digitally defined trajectory to form triboelectric layers in DIW. Liquid or paste ink can be placed into a syringe. The syringe nozzle moves along all three axes under the regulation of a computer program. The DIW printer simultaneously extrudes the ink. Shear thinning maintains the integrity of liquid ink, followed by one of the curing methods, including solvent evaporation, UV curing, or thermal curing. In contrast to FDM, DIW can be used to print a varied assortment of materials, including composites, polymers, and ceramics, and it enables the use of a broader range of ink types [95].

The fabrication of 3D-printed TENGs has utilized a diverse array of materials, including distinct surface morphologies, ink types, and surface topologies. The majority of researchers have attempted to optimize their results by using several materials of differing thicknesses. Triboelectric materials exhibit considerable diversity and need appropriate categorization and standardization. One can fabricate TENGs using any material that exhibits surface charge affinity. Conventional fabrication refers to a variety of materials with distinct nanogenerator combinations and thicknesses. Since 1757, contemporary smart self-powered gadgets have extensively utilized triboelectrification, a process that harnesses and transforms the conversion of environmental mechanical power into electrical energy. Material selection is crucial for optimum charge generation and efficient functioning in 3D TENG printing. We often use the triboelectric series to identify appropriate triboelectric materials. Triboelectrification and electrostatic induction are the two interrelated phenomena that facilitate the operation of TENGs, which gives the transfer of electrical charge between two substances with differing triboelectric properties to produce electric potential [96]. Consequently, the first stage in enhancing the triboelectric effect and output is to choose the optimal combination of materials exhibiting the greatest disparity in triboelectric affinity. Recent research has predominantly supported certain polymers because of their robust mechanical and triboelectric properties. The research focuses on modifying the polymer material after selecting the most promising materials. We diminish surface functionalization, elevate electrical permittivity, enlarge the contact area, refine the device architecture, and use enhanced manufacturing techniques. Nonetheless, choosing a suitable material is crucial for enhancing the triboelectric effect and producing a TENG with significant output [97]. Making high-performance 3D-printed triboelectric nanogenerators (TENGs) is a difficult task that requires a careful balance of the characteristics of the materials, the design of the structure, and the printing settings. Old-fashioned trial-and-error methods of experimentation take a lot of time and money. Digital optimization technologies like artificial intelligence (AI) and multiphysics simulations have the power to make big changes here [98]. These methods make the design and manufacturing process more efficient, smart, and predictive, which results in TENGs that operate better, last longer, and can be customized more easily. The main idea of the function of TENGs is as follows: when mechanical deformation happens (such as contact–separation, sliding, or bending), it creates charge (triboelectrification), which then leads to electrostatic induction, which makes electrical output. Accordingly, one can simulate the distribution of mechanical stress and strain in the 3D-printed structure under different loading circumstances, as well as the electrostatic fields and charge transfer at the triboelectric interfaces at the same time. This helps us estimate the output voltage, current, and power. AI and ML methods add to multiphysics simulations by making the TENG design and optimization process more automated and based on data. One may use ML algorithms to look at databases of material parameters (such as triboelectric potential, dielectric constant, and mechanical strength) to guess how novel, untested material combinations will work for TENGs. AI can create new material compositions or microstructures with the right qualities, speeding up the search for high-performance triboelectric and conductive composites for 3D printing. ML models, including neural networks and support vector machines, can learn how different 3D printing settings (like temperature, speed, infill, and orientation) affect TENG production, mechanical durability, and surface quality [99].

Table 3. Overview of 3D printing techniques and their applications and material combinations.

Sl. No	Fabrication Method	Power Output	Teng Dimensions	Material	Application	Reference
1	Electrospun nanofibrous tribo-surfaces	Voltage—880 V; current—3.75 μA	PDMS layer thicknesses of 40, 80, and 800 µm	Al sheet and a poly(vinylidene fluoride–trifluoroethylene) (PVDF-TrFE) nanofibrous structure	Human interactive triboelectric system	[103]
2	Direct ink writing (DIW)	Voltage—3.25 V	6 × 6 mesh structure	Graphene powder, PDMS prepolymer, and PTFE particles	Self-powered wearable tactile sensing	[8]
3	Fused deposition modelling (FDM)	Voltage—5.75 V; current—0.38 μA	Length—30 mm; width—30 mm	Silicon (SI 595CL) and polylactic acid	Wearable triboelectric electronic devices	[19]
4	Fused deposition modelling (FDM)	Voltage—241 V; current—1.52 mA	Length—120 mm; size—30 mm × 30 mm	Positive—PA and PLA; negative—PP/PE and PETG	Environmental energy harvesting	[12]
5	Digital light processing (DLP)	Voltage (RMS)—1.7 V and 2.3 V in vertical and rotational directions	-	PTFE and ABS	Self-powered dust-filtration systems	[102]
6	Screen-printing technology	Voltage—11.45 V; current—4.46 μA	Thin-film—30 mm × 30 mm × 1.4 mm	PTFE	Wearable power source	[104]
7	SLM, FDM, and SLA	RMS voltage—231 V; RMS current—18.9 μA	-	ABS, PLA, NYLON, and MMA	Sensors, electronics, and energy storage modules	[105]
8	Fused deposition modeling (FDM)	Voltage—308 V; current—61.6 μA	-	PDMS, PA, and PE	Energy harvesting	[106]

delineates the material combinations and their corresponding output performances. Chen et al. [100] introduced a straightforward, scalable, and adaptable method for fabricating bio-based, biodegradable, integrated, and elastic 3D-printed triboelectric nanogenerators (TENGs). They utilized a single-electrode mode structure, solely composed of two materials, i.e., poly(glycerol sebacate) (PGS) and carbon nanotubes (CNTs), and employed DIW and salt particulates. They fabricated a 3D-printed TENG, which measured 3 cm in length, 3 cm in width, and 0.5 cm in height, using 15% carbon nanotubes (CNTs). They then systematically subjected the 3D-printed TENG to a 40% compressive strain at a frequency of 3 Hz during electrical testing. The voltage reached a maximum of 45 V in an open circuit, while the volume current density peaked at 190 mA m⁻³ in a short circuit. Moreover, the 3D-printed TENG achieved a peak instantaneous power density of 1.11 W m⁻³ with an external load resistance of 50 MΩ. Likewise, Liu et al. [101] proposed a comprehensive manufacturing procedure for TENGs using 3D printing technology. They first incorporated PTFE microparticles and nickel powders in varying amounts into a Dragon Slow Skin 10 (DSS10) substrate to create various functional printing inks. Then, they 3D-printed the grid structure using 70 weight percent nickel/DSS10 and 40 weight percent PTFE/DSS10. This allowed them to identify the expansion of the 3D structure and illustrate how the VOC significantly increased with the rising pressure of the 3D-PTENG. A maximum power of 72.6 μW was attained with an external resistance of 12.3 MΩ, and the highest VOC of 17 V was recorded at pressures of 0.85 kPa. The author asserts that they employed an innovative, economical, ecofriendly, and non-toxic production technique in making their TENG, using programmable 3D printing technology to produce functional inks. Yoon et al. [102] also used DLP-type 3D printing to make a biomimetic–villus TENG (BV-TENG) using PTFE powder and ABS. They created the external-circuit electrode by applying a thin layer of Ag on the outside of the columns. This involved the application of a positive charge on ABS and a negative charge on PTFE. With an increase in frequency, the output charge density rose by 0.75 μC/m², while the power density escalated to 13.9 μW/m². The purpose of this BV-TENG was to manufacture a dust filter with the maximum possible surface area. The constructs developed in 3D-printed TENGs can energize numerous low-power wearable gadgets and transform a significant amount of mechanical, vibrational, and biomechanical energy. Previous sections have discussed the materials, structural fabrication techniques, and performance outputs used and achieved in 3D-printed TENGs. The following section will delineate the contemporary applications of 3D-printed TENGs across several domains, including sensing apparatuses, storage systems, and electronic device charging applications.

5. Applications of 3D-Printed Pure- and Composite-Material-Based TENGs

5.1. Three-Dimensional Printed TENGs for Energy Harvesting

A bidirectional rotary hybrid nanogenerator (BR-HNG) structure was found to have a peak voltage of 290 V and an average current of 9.5 µA. Figure 16 illustrates the working structure of this BR-HNG [107]. This was made possible by combining six HNGs with a porous NaNbO₂/PDMS sheet that covered an area of 2 × 2 cm². To transform the rotational energy generated by pedaling into electrical power, bicycles commonly used for daily transportation were connected to the BR-HNG.

Table 4. Overview of 3D-printed triboelectric nanogenerators (TENGs) (energy harvesting).

Sl. No	3D Printing Technique	Power Output	Teng Dimensions	Material	Application	Reference
1	Fused deposition modelling (FDM)	Voltage—37.5 V	-	BR-HND-PLA+ and porous sodium niobate and polydimethylsiloxane	Energy harvesting	[107]
2	Fused deposition modeling (FDM)	Voltage—5.68 V; current—20 nA	Volume—108.2 cm2; thickness 2 mm ABS	ABS, PI tape, aluminum	Water energy harvester	[108]
3	Direct ink writing (DIW)	Voltage—124 V	Length—30 mm; width—30 mm	Silicone elastomer	Wearable sensors and display	[109]
4	Digital light processing (DLP)	Peak voltage—47.7 V; current—2.5 μA	2 × 2 cm2 and 1 mm thickness	Acrylate polydimethylsiloxane	Energy harvesting from human movements	[110]

concisely outlines the 3D printing techniques and voltages obtained from the TENG and its composite materials. Furthermore, the BR-HNG may effectively produce extensive power while simultaneously energizing tiny electronic devices. Kim et al. [108] created a freestanding mode called DR-TENG by coating an ABS surface with acetone and placing electrodes on both sides of the surface between two rollers. Figure 17 depicts the operational framework of the DR-TENG. They then tested how well this mode could be used for wave energy harvesting. A 3D printer was used to construct the DR-TENG frame with an acrylonitrile butadiene styrene (ABS, ABS-A100) filament. The assembled apparatus, with a volume of 108.2 cm³, was encased in a 2 mm thick acrylic sheet and affixed using silicone hot-melt glue. We affixed the electrode layers on both sides using rectangular Al tape measuring 40 mm by 2 mm. The medium-curvature small and big roller tracks were separated by 6.75 mm and 11 mm, respectively, and the four diameter pairs were used to print the small and large rollers in 3D. The curvature parameter was set to 0.0333 mm, and multiple rollers were used to maximize electrical output from the 3D-printed DR-TENG. The use of high frequency and a substantial movement angle among the experimental parameters led to enhanced electrical outputs. Electrodes with two side covers worked better electrically than electrodes with only one cover. Through the model, the researchers achieved a VOC of 27.6 V, an ISC of 102.5 nA, and a power density of 69.34 Wm⁻² at 200 MΩ. The DR-TENG functions as an ecofriendly water energy harvester due to its enhanced contact area and space-efficient design. Li et al. [109] also created an experimental system for DIW energy harvesting. The assembly comprised a 3D printer motion control platform, an appropriate syringe, a progressive cavity pump using printed viscoelastic ink, and a nozzle tip. The silicone elastomer (Dow Corning 737 neutral cure sealant, Dow Corning) was chosen as the viscoelastic ink due to its exceptional properties, including air curing, maintaining flexibility between −65 and 177 °C, rapid curing (3 to 6 min) to form a skin-over surface, non-tacky curing for 14 min, and complete curing within 24 h. Accordingly, Hui et al. created a flexible TENG which can harness mechanical energy from ambient vibrations. Figure 18 exhibits the functional architecture of a DIW TENG. The directional layer that links to the expanded surface area produces the viscoelastic inks. This TENG model attained a maximum power density of 608.5 mW/m², producing an output voltage of 124 V at a frequency of 6 Hz and a force of 20 N. Chiappone et al. [110] developed a 3D-printed contact–separation mode TENG using digital light processing technology for energy harvesting. The triboelectric series included TEGORAD, a silicone acrylate resin, as the most tribo-negative material, and EB4740, a polyurethane acrylate, as the tribo-positive material. Figure 19 illustrates the operational framework of their DLP printer and the TENG structures that the authors produced. The TENG structure was fabricated using 3D printing and had dimensions of 22 mm × 22 mm per layer. They concluded that the DLP method can be used to make complex triboelectric nanogenerators that can obtain energy from different kinds of mechanical motions. In addition, the authors presented a triboelectric series of the most common photocurable resins, providing a solid base for future research into triboelectric nanogenerator devices made with this very flexible printing method.

5.2. Three-Dimensional Printed TENGs for Sensing

Sensing apparatuses can utilize 3D-printed TENGs to enhance their sensor efficacy through the integration of human–machine interfaces and intelligent sensing capabilities. Hazarika et al. [111] developed a multifunctional wearable device that amalgamates TENGs to monitor human movement with radiative cooling and a moisture-wicking technique. They utilized a rapid and economical 3D printing technique for the fabrication of hydrophobic–hydrophilic composites which feature PDMS printed unilaterally and h-PVDF printed in the other direction. The WKF-TENG presented by the authors exhibited reliable and enduring performance through precise and consistent identification of a diverse array of bodily motions, such as walking, leaping, knee elevation, and hand–wrist motions. Figure 20A displays the practical structure of the radiative cooling textile-based composite fabrication process. Furthermore, they proposed a potential form of multifunctional smart wearables for the future that might include moisture management and thermal comfort. Robotic arms and other dynamic elements of precision equipment may use mounted TENGs to monitor movements and vibrations that could compromise the accuracy of the machining process. Wearable exoskeletons designed to assist manufacturing workers in transporting heavy items may include triboelectric nanogenerators (TENGs). By monitoring the worker’s motions and exertions, the TENGs could be used to ensure that an exoskeleton provided enough support. A worker’s motions may fuel the sensors and control the systems inside the exoskeleton.

Wen et al. [112] created a PS-TENG. Silk fibroin served as the material in this 3D-printed triboelectric nanogenerator, which collects biomechanical energy, measures humidity, and functions as an autonomous active movement sensor. It comprises three fundamental layers: initially, they used PDMS to fabricate a microstructured polymeric thin film as the substrate, which gave the device remarkable flexibility. They then coated the PDMS sheet with screen-printed graphite interdigital electrodes. These electrodes functioned as both output electrodes for biomechanical energy harvesting and sensor electrodes for detecting water molecules. They created a silk fibroin layer to encapsulate the graphite interdigital electrodes, and this served as the medium for sensing the current states of water molecules. Figure 20B illustrates the practical aspects of silk-fibroin-based TENGs. The researchers documented many wrist movements, including twisting and bending at angles of 15°, 30°, 45°, and 60°. The differing friction durations and intensities of these movements resulted in the connected device generating distinct signals. In the same way, Zhang et al. [113] made a self-sustaining smart glove using very flexible fabric electrodes from a toroidal triboelectric sensor and pyramidal-structured MXene/Ecoflex nanocomposites. Three-dimensional printed templates were used to fabricate the MXene/Ecoflex nanocomposites. The built STTS operated in contact–separation mode, using a single electrode with finger skin as the positive layer and MXene/Ecoflex as the negative layer. The contraction and expansion of the finger muscles underpin the design of the STTS. Figure 21 depicts the practical elements of the flexible wearable glove. The peak-to-peak voltage output is 19.91 V, the sensitivity is high at 0.088 V/kPa⁻¹, and the pressure detection range is extensive, spanning from 0 to 120 kPa. This self-sustaining smart glove has promising applications for interaction and human–machine interaction technology in the future.

Zhu et al. [114] developed a breath-driven triboelectric nanogenerator (TENG) using 3D printing, with the goal of contributing to advancements in human–machine interfaces and sensing. The whole mechanism of the breath-driven TENG entails generating a triboelectric charge when the CNT-printed paper oscillates against the top wall of the resin channel. This phenomenon arises from the varying electrons that are attracted to carbon nanotubes (CNTs) and resin, generating positive and negative charges between their surfaces. This TENG was successfully used to detect diverse patterns of human respiration characterized by differing intensities, durations, and frequencies by generating reactive electrical signals that correspond to respiratory airflows. A group of researchers led by Bartłomiej Nowacki [115] made a triboelectric nanogenerator (TENG) that can directly sense ultrasonic vibrations in water. They used fused deposition modeling (FDM) and stereolithography (SLA) to print their TENG. Figure 22 illustrates the ability of the TENG assembly to detect ultrasonic waves. The TENG comprises a movable polymer pellet positioned between flat electrodes and an enclosing device shell. The researchers used stereolithography (SLA) to 3D-print the device casing and utilized fused deposition modeling (FDM) for the pellets; they used this device as a self-sustaining sensor to detect ultrasonic waves in water. The presented TENG functions as a contact-mode-independent mechanism. The polymer pellets were inserted into the cylindrical opening of the cubic plate; the capping layers were affixed to the cubic plate to construct a cell that retained the pellets between the aluminum electrodes. Stereolithography (SLA) was used to generate a planar surface for the printed result, making it appropriate for cellular fabrication. The objective of this technique was to reduce the energy dissipation resulting from the friction of the pellets against the cell walls. The cylindrical opening of the cell measured 5 mm in depth and 9 mm in diameter. When exposed to ultrasonic radiation, the pellets inside the cell were electrified and often alternated between the electrodes; the pellets were found to descend to the base of the device cell when the TENG was exposed to ultrasonic pulses exhibiting negative pressure; then, the collision of the pellets with the aluminum electrode generated triboelectric charges. The research involved aligning theoretical dependencies with actual data and analyzing voltage responses to ultrasonic stimulation using the FFT technique. The results of these two different tactics were similar. The fundamental frequency of the ultrasonic reactor, 40 kHz, was the sinusoidal signal that comprised the majority of the voltage waveforms. The small peaks in the FFT spectrum at frequencies above 400 kHz were caused by the interactions between pellets that were oscillating in the TENG or which were oscillating with the cavitation noise. The authors showed that their technique significantly outperformed a conventional ultrasonic sensor which is only capable of evaluating sound production. This sensor has significant potential for applications in sonochemistry and sonocatalysis. Table 5 explicitly highlights the performance and operational parameters of 3D-printing-based TENG sensors.

Li et al. [116] engineered a self-sustaining sensor that exhibits crosstalk among adjacent sensing units in TENGs; this sensor was is intended for use in wearable TENGs and flexible electronics. The proposed TENG device is a cohesive, flexible TENG sensor array, measuring 7.5 cm × 7.5 cm and including 100 sensing components. It is also proposed for use in a direct, cost-effective, and scalable method facilitated by 3D printing. The TENG sensing system comprises a AgNWs electrode, a PDMS sealing layer, and a soft substrate. Similarly, Haque et al. [117] developed a vertical contact–separation mode TENG using 3D-printed functional layers and multijet modeling (MJM). The fabricated TENG employs polydimethylsiloxane (PDMS), acrylonitrile butadiene styrene (ABS), polyamide (PA), and tango black (TB) as functional layers for use as a touch sensor. Moreover, the 3D-printed materials used to construct the TENG sensor enable it to detect applied forces and operating frequencies, subsequently providing outputs based on these observations.

Table 5. Performance and operational parameters of 3D-printing-based TENGs (sensors).

Sl. No	Fabrication Method	Power Output	Teng Dimensions	Material	Application	Reference
1	Stereolithography (SLA) and fused deposition modelling (FDM)	-	Diameter—1.5 mm; height—0.2 mm	ABS and aluminum foil	Ultrasonic sensing	[115]
2	Liquid additive manufacturing	Voc—405 V; Isc—38 µA	(40 mm × 25 mm × 1 mm)	PDMS and barium titanate	Tactile sensor for onboard wear detection system	[118]
3	Fused deposition modelling (FDM)	Output power density—56 mW/m2	4 cm × 4 cm	Silver nitrate (AgNO3) and poly vinylpyrrolidone (PVP)	Energy harvesting and self-sustaining angle sensors for human–robot cooperation in the detection and operation of the robot	[29]
4	Fused deposition modeling (FDM)	Power output 4.33 mW	-	Poly lactic acid (PLA) and FEP	Intelligent speech recognition	[33]
5	Direct ink writing (DIW)	Voltage—10 V	3.2 cm × 3.2 cm (length × width)	Cellulose nanofiber (CNF) aerogel, PET, and PDMS	Multifunctional electronic applications and sensitive humidity sensors	[119]

Table 5. Performance and operational parameters of 3D-printing-based TENGs (sensors).

Sl. No	Fabrication Method	Power Output	Teng Dimensions	Material	Application	Reference
1	Stereolithography (SLA) and fused deposition modelling (FDM)	-	Diameter—1.5 mm; height—0.2 mm	ABS and aluminum foil	Ultrasonic sensing	[115]
2	Liquid additive manufacturing	Voc—405 V; Isc—38 µA	(40 mm × 25 mm × 1 mm)	PDMS and barium titanate	Tactile sensor for onboard wear detection system	[118]
3	Fused deposition modelling (FDM)	Output power density—56 mW/m2	4 cm × 4 cm	Silver nitrate (AgNO3) and poly vinylpyrrolidone (PVP)	Energy harvesting and self-sustaining angle sensors for human–robot cooperation in the detection and operation of the robot	[29]
4	Fused deposition modeling (FDM)	Power output 4.33 mW	-	Poly lactic acid (PLA) and FEP	Intelligent speech recognition	[33]
5	Direct ink writing (DIW)	Voltage—10 V	3.2 cm × 3.2 cm (length × width)	Cellulose nanofiber (CNF) aerogel, PET, and PDMS	Multifunctional electronic applications and sensitive humidity sensors	[119]

5.3. Advancements in Composite-Based 3D-Printed TENG

The combination of 3D printing technology with TENGs has provided new space for the focused design of high-performance energy harvesting, detection, and wearable devices. Composite materials are of great importance in improving the performance and sustainability of TENGs for many applications. From energy collectibles to self-powered sensors and wearable electronics, 3D-printed TENGs showed their ability to dramatically change the way we interact with and capture the energy of the world around us. Table 6 precisely indicates the material composition/technique and performance of 3D-printed composite TENGs.

5.3.1. Direct Energy Harvesting

As depicted in Figure 23, He et al. [120] emphasize eliminating the weaknesses of conventional 3D printing to create micro/nanostructures and thermosetting materials for triboelectric nanogenerators (TENGs). TENG performance greatly relies on the material structure and properties used. To overcome these weaknesses, scientists have utilized polymer tubes as carrier boats to directly print using polydimethylsiloxane (PDMS) and polyethylene glycol (PEG). This technique enables the attainment of high-output performance with the inclusion of a more electronegative material and the formation of a sponge-like structure due to PEG removal after printing. This novel technique made it possible to produce micro/nanostructures in the triboelectric film and widen printable materials without requiring threadlike filaments. This innovative approach keeps all the benefits of traditional 3D printing, like being budget-friendly, quick to produce, and capable of easily creating intricate designs without needing templates. The study’s results showed that the TENG device made with this technique produced an impressive output voltage of 306 V, an instantaneous current of 6.14 mA, an energy conversion efficiency of 74.4%, and a power density of 236.67 W/m³. Plus, it could even directly light up a processed LED bulb (85 V, 3W). These findings push TENG technology forward, presenting a simple and cost-effective way to harvest energy that holds great promise for real-world applications.

5.3.2. Hybrid Energy-Harvesting Devices

Meena et al. [118] introduce an exciting new approach for boosting the performance of triboelectric nanogenerators (TENGs) through a hybrid design and 3D printing. They point out that traditional TENGs often struggle with low-output power, which makes them less practical for powering electronic devices, as shown in Figure 24. To tackle this issue, a hybrid piezoelectric–triboelectric nanogenerator (HTPENG) was created, which merges the triboelectric effect with the piezoelectric effect. The standout feature of their method is the fabrication process, which cleverly combines 3D printing with transfer printing. They start by using 3D printing to craft a flexible polydimethylsiloxane (PDMS) film; then, they transfer-print a layer of barium titanate (BTO) onto this film. This bilayer design enhances charge separation and improves impedance matching, resulting in better output performance. The HTPENG device they developed showed a significantly higher power density compared to traditional non-hybrid PDMS-based TENGs. Additionally, the authors investigated the use of an HTPENG as a self-powered sensor for monitoring tire tread wear. Impressively, the sensor was able to detect tire wear with high precision, achieving an accuracy of less than 1 mm. This research offers a promising method for creating high-performance hybrid nanogenerators using 3D printing and transfer printing, with the HTPENG showing great potential for various applications, including energy harvesting and self-powered sensing, especially in smart tires and tire wear monitoring systems.

5.3.3. Stretchable and Flexible TENG

As illustrated in Figure 25, Montero et al. [121] created a fully printed triboelectric nanogenerator (TENG) by combining freeze-casting, freeze-drying, and printing techniques. This innovative TENG utilizes a porous aerogel made from polyvinylidene fluoride–trifluoroethylene (P(VDF-TrFE)). They explored how porosity and poling affect the stretchability and energy harvesting efficiency of P(VDF-TrFE). In their study, they compared the mechanical, ferroelectric, and triboelectric characteristics of the porous structure with those of solid P(VDF-TrFE) films. The findings revealed that altering the structure of P(VDF-TrFE) significantly boosts its stretchability, increasing it from 7.7% in solid films to an impressive 66.4% in porous films. Additionally, this modification led to a 66% increase in output voltage and a 48% rise in generated charges for non-poled P(VDF-TrFE) porous aerogel films compared to their non-poled solid counterparts. The fully printed TENG, made with stretchable materials, achieved a peak power of 62.8 mW·m⁻² and an average power of 9.9 mW·m⁻² over 100 tapping cycles at a frequency of 0.75 Hz. Remarkably, this TENG could light up LEDs by capturing mechanical energy from human movement.

5.3.4. Self-Powered Sensors and Devices

Tung et al. [122] introduced an exciting new polymer composite designed for high-performance triboelectric nanogenerators (TENGs), featuring polyhexamethylene guanidine hydrochloride (PHMG). This innovative composite, made up of PHMG, polyvinyl alcohol (PVA), and glutaraldehyde (GA) as a crosslinking agent, was carefully optimized for ingredient ratios and then 3D-printed directly onto conductive substrates to function as positive TENG electrodes. The fabrication of the TENG electrode involved a 3D printing technique where the PHMG-GA-PVA gel was printed directly onto copper adhesive tape, following specific printing parameters, as shown in Figure 26. The method used a dual-pass approach with orthogonal infill patterns, resulting in a cross-hatched internal structure. The TENG device itself consisted of 3D-printed components, with printed PHMG-GA-PVA electrodes acting as positive friction electrodes, while an FEP membrane served as the negative friction electrode, separated by rubber buttons. This research successfully harnessed PHMG, known for its strong antibacterial properties, in a TENG device as a positive electrode. The amount of PHMG in the positive electrode plays a crucial role in determining the TENG device’s output, and the combination of amine functional groups in PHMG with GA and PVA leads to remarkable triboelectric performance. This device shows great promise for a variety of practical applications, such as self-powered devices, sensors, antibacterial face masks, and smart shoes.

5.3.5. Wearable Energy Harvesting and Health Monitoring

Gunasekhar et al. [123] presented an innovative wearable triboelectric nanogenerator (TENG) aimed to harvest energy and monitor human health. This TENG features a unique design, utilizing an electrospun polyvinylidene fluoride (PVDF) blend nanoweb as the tribo-negative layer, paired with a melt-blown thermoplastic polyurethane (TPU) film serving as the tribo-positive layer. The PVDF is mixed with a fourth-generation aromatic hyperbranched polyester (Ar.HBP-G4). Their research delves into identifying how different amounts of Ar.HBP-G4 in the PVDF/Ar.HBP-G4 blend affect performance. Interestingly, they found that a blend with 10 wt.% of Ar.HBP-G4 produced a significantly higher triboelectric output voltage than both pure PVDF and other ratios of Ar.HBP-G4. The TENG also demonstrated its ability to power portable electronic devices, successfully lighting up 45 LEDs. Additionally, the team assessed the device’s output voltage and sensitivity across various parts of the human body, showcasing its potential for health monitoring. This study underscores the increasing significance of tracking human physiological signals for diagnosing diseases, guiding therapy, and assessing health. Unlike traditional wireless monitoring systems that depend on rechargeable batteries—which can be bulky and have limited life—TENGs present a promising solution by harnessing mechanical energy from the environment.

Table 6. Material composition/technique and performance of 3D-printed composite TENGs.

Sl. No	Technique	Key Features	Performance Metrics	Reference
1	Laser surface patterning	Increase in surface area and charge accumulation	Maximal VOC: 98.87 V; JSC: 0.10 µA/cm2	[124]
2	Light-cured 3D printing	Couples surface structures for enhanced body friction	Improved output performance and pressure sensitivity	[125]
3	Bi2WO6-PDMS composite	Enhances dielectric properties and electrical output	Voltage: 200 V; current: 4 μA; charge: 5 nC	[126]
4	FRGO-PI composite	Amino-functionalized graphene oxide for improved tribological performance	Voltage: 58 V; current: 12 μA; charge: 33 nC	[127]
5	PEGDA surface treatment	Passivates surface defects in perovskite-based TENGs	VOC: 276.86 V; JSC: 68.61 mA/m2; charge: 56.08 nC	[128]
6	Plasma treatment	Enhances charge transfer efficiency	Performance improvement of up to 80%	[129]
7	3D-printed PA6,6	Flexible triboelectric materials for wearable electronics	VOC: 63 V; current: 0.8 μA	[130]
8	3D-printed TENG arrays	Minimizes crosstalk for tactile sensing	Sensitivity: 0.11 V/kPa; pressure range: 10–65 kPa	[116]

Table 6. Material composition/technique and performance of 3D-printed composite TENGs.

Sl. No	Technique	Key Features	Performance Metrics	Reference
1	Laser surface patterning	Increase in surface area and charge accumulation	Maximal VOC: 98.87 V; JSC: 0.10 µA/cm2	[124]
2	Light-cured 3D printing	Couples surface structures for enhanced body friction	Improved output performance and pressure sensitivity	[125]
3	Bi2WO6-PDMS composite	Enhances dielectric properties and electrical output	Voltage: 200 V; current: 4 μA; charge: 5 nC	[126]
4	FRGO-PI composite	Amino-functionalized graphene oxide for improved tribological performance	Voltage: 58 V; current: 12 μA; charge: 33 nC	[127]
5	PEGDA surface treatment	Passivates surface defects in perovskite-based TENGs	VOC: 276.86 V; JSC: 68.61 mA/m2; charge: 56.08 nC	[128]
6	Plasma treatment	Enhances charge transfer efficiency	Performance improvement of up to 80%	[129]
7	3D-printed PA6,6	Flexible triboelectric materials for wearable electronics	VOC: 63 V; current: 0.8 μA	[130]
8	3D-printed TENG arrays	Minimizes crosstalk for tactile sensing	Sensitivity: 0.11 V/kPa; pressure range: 10–65 kPa	[116]

6. Challenges and Future Perspectives

6.1. Challenges in Materials Development and Processing

The journey of developing and processing composite materials in 3D printing comes with its fair share of hurdles that prevent us from fully tapping into their potential. These hurdles range from choosing the right materials to the techniques used in processing, not to mention the challenge of integrating cutting-edge materials like graphene. Picking the right materials for composite filaments can be quite tricky, as it is crucial to ensure compatibility between the matrix and the reinforcing particles or fibers. This becomes especially tough for composites filled with metal and ceramic particles, where achieving a uniform mix and avoiding clumping is vital. Creating composite filaments demands meticulous control over the mixing and extrusion processes to guarantee consistent quality and performance. This also involves innovating new print nozzles to avoid clogging and enhance printability [131]. While material extrusion is a common method for printing pure plastics, its use for composite materials, especially those reinforced with continuous fibers, is still relatively new. Adding discontinuous fibers or particles to plastics can boost their mechanical properties, but continuous fiber reinforcement takes this to another level, providing remarkable strength and stiffness. These continuous fibers facilitate efficient load transfer, which is key when designing lightweight composite parts. Plus, these reinforcements can enhance various properties, including mechanical, optical, electronic, thermal, and even biomedical aspects [132]. The enduring environmental sustainability of 3D-printed composite-material-based triboelectric nanogenerators (TENGs) depends on the designer’s capacity to recycle and repurpose components. In contrast to conventional subtractive manufacturing methods, 3D printing exhibits superior efficiency in waste management throughout production. Recyclability is described as the theoretical capacity of a material or product to be recycled based on its qualities and chemical composition, irrespective of the actual existence of a recycling infrastructure. Several thermoplastic polymers, including Nylon 6, PVDF, and PLA, are used in the structural and triboelectric layers of TENGs. Components composed of a certain kind of thermoplastic may be potentially recycled. Although thermoplastics are recyclable, their mechanical qualities, such as tensile strength and molecular weight, may deteriorate with each recycling, owing to temperature exposure and shear pressures during reprocessing. This imposes a restriction on the frequency of their effective reuse. TENGs produced using multi-material 3D printing, which involves the printing of triboelectric layers, electrodes, and structural components in several materials followed by assembly, are very hard to recycle using conventional methods due to the irreversible amalgamation of materials. Although the separate components are recyclable, their close intermingling makes effective separation and reprocessing unfeasible. Figure 27 depicts the challenges that are encountered in the material development and processing of 3D-printed TENGs.

6.2. Performance Enhancement Strategies for 3D-Printed Composite TENG

Triboelectric nanogenerators (TENGs) are gaining traction as exciting technologies for energy-harvesting and sensing applications. They work by harnessing the principles of contact electrification and electrostatic induction. The effectiveness of TENGs is heavily dependent on surface characteristics like charge density, roughness, and the materials used. To boost their efficiency and longevity, surface modification techniques are essential, especially when combined with 3D printing technologies. One popular method is surface patterning, which enhances TENGs’ performances by increasing their effective contact area and surface roughness. Laser surface patterning has been utilized to create accurate and uniform patterns on composite materials, resulting in better charge accumulation and diffusion [133]. In a similar vein, light-cured 3D printing has been employed to create TENGs with fully integrated surface structures, turning traditional surface friction into body friction and greatly enhancing output performance [134]. Another vital strategy for boosting TENG performance is the design of micro-/nano-morphology. Researchers have come up with methods for fabricating micro- and nanostructures on the surfaces of friction layers, which not only increase the contact area but also improve electrical output. These structures can be divided into surface and internal morphologies, with surface morphologies proving to be more effective in enhancing charge density [135]. Chemical functionalization is a fantastic way of boosting the performances of TENGs. For instance, poly(ethylenee glycol) diacrylate (PEGDA) has been utilized to enhance the surface of perovskite-based TENGs, effectively sealing surface defects and elevating device performance. Thanks to this treatment, there was a remarkable 54% increase in open-circuit voltage and a 64% rise in short-circuit current density [128]. Plasma treatment has been used to tweak the surface properties of TENGs. The treatment of Si substrates with plasma can really boost a device’s performance through enhancing its charge transfer efficiency. This method has been proven to increase electrical output by as much as 80%, all while keeping other material properties intact [136]. Hybrid methods that combine various surface modification techniques have been investigated to improve the performances of TENGs. For example, researchers have shown that integrating both piezoelectric and triboelectric effects within a single device can be achieved through a hybrid nanogenerator (HNG). This strategy takes advantage of the combined effects of different energy harvesting methods, resulting in a notable increase in electrical output [137]. Narong Amorntep and his team utilized a 3D laser etching process to craft intricate patterns on the surface of a composite-based triboelectric nanogenerator (C-TENG). This technique boosts the contact surface area, leading to better charge accumulation. They etched various designs—like circular, line, square, hexagonal, and X-shaped patterns—onto the C-TENG surface using a fiber laser marking machine. To further enhance the performance of the C-TENG, they applied a graphite coating to the laser-patterned surface. This coating not only increases the contact area but also improves charge transfer, thanks to graphite’s excellent electrical conductivity and its two-dimensional structure. Consequently, the optimized C-TENGs, especially those featuring line patterns and a graphite coating, achieved an impressive maximum open-circuit voltage of 98.87 V and a short-circuit current density of 0.10 µA/cm² when subjected to a 20 N external force. Numerous obstacles exist regarding the selection of printed materials, electrical output, device framework design, and connecting 3D-printed TENGs to data retention devices. The thickness of the triboelectric materials significantly influences the output efficacy of TENGs [138,139]. The main measure that controls the 3D printing process is ensuring that the correct thickness of triboelectric layers is maintained, allowing them to produce the optimum voltage while still being very delicate. The diameter of the filament dictates the minimum layer thickness in fused deposition modelling (FDM). The literature for FDM documents a minimum layer thickness of 0.2 mm; any attempt to reduce this thickness compromises the system’s durability and heightens its brittleness. The choice of printed triboelectric materials significantly influences the power density of TENGs; hence, it is essential to choose appropriate materials for the improvement of 3D-printed TENGs. Despite the existence of several triboelectric materials, only a limited subset can provide the requisite output. Filaments and inks are the principal materials used in 3D-printed TENGs. Prior to deciding on the use of any material for TENG manufacture, it is vital to assess it. Polymers are the predominant printable materials used in FDM TENG fabrication, resulting in rigid, structured TENGs. On the other hand, DIW is utilized to create structures that are both biocompatible and flexible. They frequently use polymers like ABS, PA, PC, and PLA to create printable TENG layers. Polymers can reinforce fibers and nanoparticles to improve their mechanical strength and durability. Efficient fabrication of extrusion-based 3D-printed TENGs requires precise and seamless printing. Specific issues emerge when using certain materials in 3D printing, particularly with the encapsulation of molten substances ejected from the nozzle. The material’s volume diminishes during solidification because of the varying cooling rates of various printed substances. We can circumvent this challenge by maintaining the bonding structures of materials at a low strength, which facilitates effortless melting and fracturing during the extrusion process. Further study and development are necessary for optimizing the viscosity impact, use of nanofillers, and post-printing treatment. Furthermore, the materials used for EB 3D-printed TENGs must be flexible and stretchable due to their frequent use worn under clothes with direct skin contact. It is necessary to examine the fracture properties of triboelectric materials to achieve the required level of stretchability. Eventually, the aim is to obtain materials that exhibit superior fracture resistance and flexibility. Employing various energy-dissipation strategies is essential for improving structural design. To achieve optimal stretchability, it is essential to attain elevated critical strain, superior conductivity, minimal hysteresis, high fracture strength, and skin conformance. The challenges of biocompatibility and self-healing substantially hinder the effectiveness of wearable and implantable 3D-printed TENGs. Although 3D-printed TENGs have superior performance in wearable and implantable devices, the biocompatibility of these devices needs improvement for future research and widespread commercialization. Enhancing the biocompatibility of TENGs necessitates substantial advancements in skin affinity, breathability, comfort, structural stability, and controlled degradability. The self-repair capability of TENG is crucial for its long-term stability, since it functions as a mechanical energy harvester that must withstand various environmental stresses and damage.

6.2.1. Electrical Performance Analysis

The main thing that is stopping the progress of 3D-printed TENGs involves the incorporation of an electrode layer in conjunction with a triboelectric functional layer. It is essential to place the electrode directly on the triboelectric layer, devoid of any intervening space. This is because even a little air gap leads to considerable loss in charge transmission. Moreover, several micro–nano electronic devices continue to need substantial amounts of inconsistent and much lower output electrical power, despite attempts to augment the output power of 3D-printed TENGs. This discrepancy hinders the viable commercialization of 3D-printed TENGs. We may improve the device’s surface area, architecture, and power management circuitry to address these challenges. The result of 3D-printed triboelectric nanogenerators is inconsistent due to the unpredictability and irregularity of the encompassing mechanical energy. Storage devices must combine with triboelectric nanogenerators (TENGs) to provide constant output power. It might be hard to connect TENGs to storage devices, so power management circuitry is needed to keep storage devices and 3D-printed TENGs from having impedance problems. Configuring the power management system is another technique for ensuring constant output and performance. The first strategy involves retaining the fluctuating energy in capacitors and batteries, along with other storage technologies. However, commercial use requires much more effort.

6.2.2. Challenges Associated with Application Selection

Three main categories apply to 3D-printed TENG applications: tiny power sources, self-sustaining sensors, and human–machine interfaces for activity monitoring. We must adjust the design and material selection according to the application’s requirements. To properly conform to the human body, 3D-printed TENGs, a tiny power source requiring attachment, must exhibit flexibility and shape adaptability. Therefore, if these nanogenerators are intended for installation under the shoe sole, they must possess a robust and durable design. To fabricate these miniature wind- and water-energy-harvesting generators, it is crucial to strengthen the supporting frameworks of the TENGs to withstand ecological limitations. This generator may function as an autonomous health monitoring sensor to assess and document human speech, breathing, heart rate, pulse rhythm, joint mobility, and more characteristics. Temperature and humidity, among other external elements, impact the signal amplitudes of equipment that monitors body motions and heart rate. Effective packing methods and material selections are essential in surmounting this challenge. As a self-powered sensor, 3D-printed TENGs must exhibit excellent electrical performance, sensitivity, detection range, and reaction time. Despite the high reaction time and slightly restricted detection range of pressure sensors under optimal environmental circumstances, TENGs operate very well and with precision. For instance, when TENG functions in a vacuum, its surface charge density and operational performance exhibit exponential enhancement.

6.2.3. Long-Term Stability of Composites

The long-term stability and durability of materials used dictate a TENG’s capacity to preserve its performance attributes (e.g., output voltage, current, power) and structural integrity throughout prolonged operational durations and diverse environmental circumstances. The intrinsic qualities of printed materials and the characteristics of the additive manufacturing process make the development of 3D-printed TENGs extremely hard. Mechanical fatigue in 3D-printed triboelectric nanogenerators (TENGs) dictates the deterioration and ultimate failure of the material or device, resulting from recurrent mechanical stress (e.g., bending, compression, or stretching). TENGs are intrinsically engineered for cyclic mechanical input, rendering fatigue a crucial factor for their durability. TENGs generally function by repeating cycles of contact, separation, or deformation. Each cycle exerts stress on the triboelectric layers and structural components. Microscopic fractures may form at stress-concentrated areas (e.g., printing errors, sharp edges, or material interfaces) under cyclic loading. Cyclic loading may induce changes in the material’s microstructure over time, such as polymer chain scission, filler–matrix debonding in composites, or wear of triboelectric surfaces. This diminishes a material’s mechanical qualities (e.g., stiffness, strength, and elasticity) and electrical performance. The inherent fatigue resistance of the selected polymer matrix and reinforcing fillers (such as carbon nanotubes, graphene, and metal nanoparticles) is crucial. Elastomeric composites often have superior fatigue resistance compared to rigid composites. Numerous polymers used in 3D printing (e.g., PLA, nylon, PETG, and certain resins in composites) are hygroscopic, indicating their capacity to absorb moisture from the atmosphere. The absorbed water may chemically interact with the polymer chains via hydrolysis, resulting in chain scission (the breakdown of lengthy polymer chains). This process compromises the material, rendering it brittle, diminishing its tensile strength, and altering its elastic characteristics. Water molecules function as plasticizers by intercalating between polymer chains, therefore augmenting their free volume. This renders the material more pliable, lowers its glass transition temperature (Tg), and may result in less stiffness and strength, hence affecting the mechanical dynamics of a TENG’s functionality.

Table 7. Summarized challenges corresponding to mitigation strategies for 3D-printed TENGs.

Sl. No	Technical Challenge	3D Printing Technique(s)/Material Type(s)	Impact on TENG Performance	Mitigation Strategies	Reference
1	Layer delamination	FDM and multi-material printing	Mechanical failure, reduced flexibility/durability, and inconsistent electrical output due to poor contact	Optimized printing temperature, controlled cooling rates, annealing, use of adhesion promoters, and interface engineering	[140]
2	Poor surface roughness control	FDM and extrusion-based methods	Suboptimal triboelectric contact (too smooth/rough) and increased wear	Post-processing intentional micro-patterning during design and optimization of layer height/nozzle diameter	[141]
3	Post-processing complexity	SLA/DLP and FDM (support removal)	Increased fabrication time and potential damage to parts	Design for minimal supports, water-soluble supports, automated post-processing, and development of self-curing materials	[100]
4	Long-term stability issues	Environmental exposure and mechanical fatigue	Performance degradation (output voltage/current drop) over time	Robust encapsulation, use of durable materials, self-healing materials, and optimized device architecture for stress distribution	[142]
5	Low throughput/scalability	Most lab-scale 3D printing techniques	Hinders industrial production and commercialization	Development of high-speed 3D printers, multi-nozzle/multi-material systems, process automation, and hybrid manufacturing approaches	[130]

briefly explains the challenges that are encountered in mitigating the long-term stability of 3D-printed TENGs.

Aging in materials refers to the irreversible alteration of a material’s properties over time, occurring even in the absence of movement, due to chemical and physical processes. The polymeric matrix constituting the majority of 3D-printed composite TENGs may be compromised via many mechanisms, including oxidation, photodegradation (UV radiation), heat degradation, and chemical degradation. As the triboelectric layers age, the chemistries of their surfaces may alter. Oxidation or hydrolysis may introduce new functional groups to the materials, modifying their electron affinity and repositioning it within the triboelectric series. This may result in the material producing a diminished or inconsistent charge. Environmental exposure may render triboelectric surfaces either rougher due to erosion or differential degeneration or smoother as a result of wear or flow. Both may alter the effective contact area and the mechanics of friction, hence affecting the output of a TENG.

6.3. Future Research Direction

From the point of view of advanced materials, composite-material-based 3D-printed triboelectric nanogenerators (TENGs) have a lot of promise. The combination of different fields, like materials science, 3D printing technologies, and sustainable energy solutions, has created a one-of-a-kind chance that is about to change the way TENGs work, how well they work, how they affect the environment, and how they can power themselves. There are many methods that can be used to reach these aims, but using 3D printing technology to make composite materials that are just right for the job is the most effective technique that might eventually improve TENG performance. To make new composite formulations better, the emphasis will be on adding other parts, including nanofillers or even bio-based polymers, that will considerably improve the materials’ mechanical strength and electrical conductivity. These materials are very important in improving the triboelectric characteristics, charge affinity, and mechanical strength of TENGs, which is necessary for better energy conversion efficiency. Adjusting the dielectric characteristics might be helpful. For example, using high-k dielectric fillers like bismuth tungstate can greatly enhance voltage, current, and charge output, which makes TENGs work better overall. Along with the material qualities, the study will also look at 3D printing settings, including raster angle, infill density, and filler content. This modification is needed to make composite TENGs stronger mechanically and thermally, which will make them more efficient and reliable when utilized in a broad variety of applications, from flexible biomedical devices to strong components that need to be structurally sound. More research on multi-material 3D printing will make it possible to construct advanced integrated TENGs with diverse functional layers that are built for maximal triboelectric performance and long-term durability. Adding smart materials to 3D-printed composites reflects a big step toward making TENGs that can respond and adapt to their environment. This makes them much more versatile, especially in complex smart IoT systems that need self-sufficient power, autonomous control, and advanced sensing capabilities. One of the most exciting areas of study is the development of hybrid nanogenerators that use both the triboelectric and piezoelectric effects smartly. These technologies work together to boost the total electrical output by a large amount. They also make it possible to use TENGs in wireless power transmission and more complex portable electronics. We will also look at the idea of electronic devices that can run on their own. As such, we will focus on fully integrated self-charging power systems that combine TENGs with static energy storage units like supercapacitors or micro-batteries that are included in the device during 3D printing. Other aspects of this progress require us to work on making sure that the materials are compatible and that nanofillers are evenly spread out in filament composites to improve manufacturing efficiency without sacrificing quality. These aspects need a lot of attention to make sure that flexible manufacturing frameworks run reliably and produce a lot of goods. For TENGs to be useful in a wide range of fields, it is important to address their long-term exposure resistance to environmental factors like humidity, temperature changes, mechanical stress, or fatigue, and to include autonomous healing capabilities. As the demand for sustainable energy sources grows, research is now focused on making TENGs work better while having as little influence on the environment as possible. This includes making TENGs out of materials that can be composted and recycled, which would reduce their impact on the environment during their whole life cycle. It will be vital to improve life cycle assessment (LCA) methods so that we can accurately measure the environmental impact of TENGs from the time they are made to the time they are thrown away. This will help us to make truly ecofriendly energy-harvesting devices.

7. Conclusions

Recent advances in composite-material-based 3D-printed TENGs for ubiquitous sensors and energy-harvesting systems are presented in this concise review. Emphasized are the main challenges in material selection and device architecture and sophisticated 3D printing techniques that target the use of composite materials and their useful applications. Self-sustaining electronic sensors have evolved from next-generation electronic technologies using effective energy-harvesting methods during the last ten years. Modern transducers and nano energy producers allow these sensors to run without batteries. Because of its fast, affordable, and efficient nature, 3D printing is a great way to create TENGs. This study presents a short overview of the techniques for choosing materials for composite-material-based 3D-printed TENGs and outlines the procedures for creating a composite-material-based 3D-printed TENG. With this evaluation, based on current experimental data, we evaluate the useful applications of composite material and 3D-printed TENGs. Correct material choice is critical in the never-ending search to maximize the high performance and efficiency of composite-material-based 3D-printed TENGs. Choosing triboelectric materials that will boost the stability, adaptability, and compatibility of a device depends on knowing the device’s running circumstances. Effective testing of a system depends on prototypes of the components that enable the implementation of the wireless transmission of self-sustaining sensors. Although TENGs have advanced for a variety of applications, including wearable devices, medical implants, and other self-sustaining sensors, there are still a few obstacles that need additional study before they can be used generally. When using 3D TENGs for daily use, effective designs with great electrical output should be given great thought. Making 3D-Printed triboelectric nanogenerators more robust, long-lasting, and durable for uses in energy harvesting and sensors requires the consideration of several environmental factors. Three-dimensional printing has greatly helped biomaterials for the production of unique, customized structures to meet specific patient demands. Still, there are issues like a shortage of funds and complex laws. To create unique components with higher strength-to-weight ratios that enable on-demand manufacture and hasten aircraft repairs, the aerospace sector has invested in 3D printing. Factors including the high cost and restricted availability of materials as well as the uneven quality of 3D-printed components have meant that the aerospace sector has been hesitant in adopting 3D printing. Still, 3D printing enables automation in the construction sector, therefore opening the path for labor-free lunar building. Although additive manufacturing has many benefits, it also has several drawbacks that need further research before it can be generally used in many various sectors. Reducing the contact between printed layers helps manufacturing gaps to arise easier. This may lead to higher porosity, which lowers mechanical performance, and vice versa. Typically, in 3D printing, isotropic behavior changes the mechanical properties of materials when they are squeezed or extended vertically, but has no influence when they are stretched or crushed horizontally. All that computer-aided design (CAD) offers is a basic form of design. The tessellation principle suggests that 3D-printed copies of CAD designs often include flaws and defects, especially in cases involving curved surfaces. Three-dimensional printing is revolutionary for tailored goods and specialist applications, but technology still has a long way to go before it can challenge accepted wisdom for mass-producing ordinary goods. Its slower processing speed and higher price tag help to explain this. Still, 3D-printed TENGs have advanced significantly within the last several years. Three-dimensional printed TENGs will replace conventional industrial techniques in no time, with increasing investment, research, and development occurring across the world.