A triboelectric nanogenerator (TENG) can convert mechanical energy into electricity/electrical signal based on the triboelectric effect and electrostatic induction [1,2,3,4,5,6]. In detail, after two different materials rub against each other, due to the difference in their ability to adsorb electrons, one material will carry a positive charge, while the other material will carry a negative charge. At the same time, induced charges will be generated on the back electrode of the two materials. When two materials are separated, positive and negative charges separate, and this separation of positive and negative charges creates a potential difference between the upper and lower electrodes of the material. As the distance between the two materials changes, the potential also undergoes periodic changes. Similar to piezoelectric nanogenerators, connecting the outer sides of two materials through external circuits or loads can generate alternating induced currents [7,8,9,10]. TENG harnesses electricity generation through multiple distinct working modes, each offering unique mechanisms for energy harvesting [11]. The contact–separation mode involves two dissimilar materials making contact and then separating vertically. During contact, electron transfer occurs, leading to charge accumulation and subsequent potential difference upon separation, resulting in electricity generation. The single-electrode mode is a variation of the contact–separation mode, where only one triboelectric material is used, and the other material is replaced with an electrode or a conductive surface. The single material is mechanically moved or flexed to induce contact and separation with the electrode, generating electricity through the triboelectric effect. The sliding mode relies on two triboelectric materials in direct contact, which are laterally slid against each other. TENGs operating in the freestanding mode comprise a charged moving object and two stationary triboelectric layers. Electrodes positioned near these triboelectric layers are externally connected to a load. The reciprocating movement of the charged object between the two friction layers induces a potential difference between the two electrodes, initiating electron flow back and forth within the external circuit loop. As a result, electricity is generated, harnessing the triboelectric effect and transforming mechanical energy into a usable electrical output. TENGs provide a sustainable and renewable energy-harvesting solution that directly converts mechanical energy into electricity without relying on fossil fuels. They offer a cleaner and greener alternative to traditional power generation methods, contributing to reduced greenhouse gas emissions and environmental impact [12,13,14].

Compared to solar power, TENGs can generate electricity from various mechanical sources, making them suitable for both indoor and outdoor applications. They are not dependent on sunlight, making them viable for use in low-light or nighttime scenarios. TENGs and piezoelectric generators share similarities as both convert mechanical energy into electricity. However, TENGs offer advantages like higher power density and the ability to harvest energy from a broader range of mechanical movements, including sliding and rotational motion. Compared to the batteries, TENGs can serve as self-powered devices, eliminating the need for frequent battery replacements and reducing electronic waste. They offer sustainable power generation without relying on external energy storage. TENG technology has been widely researched and applied, including wearable electronics, smart sensors, biomedical devices, self-powered systems, and other fields [7,15,16,17,18,19].

With the development of TENG technology, implantable TENG (iTENG) has also received more and more attention and research [20,21,22,23]. One of the advantages of this technology is that it can use the mechanical energy generated by the body’s own movement to generate electricity in vivo, so there is no need to use an external power source or battery [11,24,25]. iTENGs can be used to prepare implantable medical devices, such as implantable cardiac pacemakers, EEG monitors, drug pumps, etc., thereby reducing the frequency of battery replacement and the risk of surgery [26,27]. iTENGs can not only reduce the volume and weight of the implant but also improve the service life of the implant, reducing the frequency of battery replacement and surgery [15,28,29].

Materials and structures play a crucial role in the design and application of iTENGs [30,31]. The choice of materials and structures is tailored to the specific requirements and constraints of each application [32]. By carefully selecting materials and optimizing structural designs, iTENGs can be customized to meet the requirements of various applications, including biomedical sensing, power supply, and other specific uses [33]. This approach ensures the functionality, biocompatibility, and efficiency of the iTENGs in their respective applications, leading to advancements in the field of implantable medical devices [34].

Biocompatibility, mechanical stability, and flexibility are three essential requirements for iTENGs [35,36]. These characteristics are crucial to ensure the safe and reliable operation of the devices within the human body. First of all, biocompatibility means that the material will not cause immune reaction and rejection when it comes into contact with human tissue and will not cause damage to human tissue and organs [37]. Therefore, in the preparation process of iTENG, it is necessary to select materials with good biocompatibility and conduct strict biocompatibility tests to ensure that it is harmless to the human body after implantation. Second, mechanical stability means that the material will not fail or be damaged due to mechanical stress, wear, or other factors during the implantation process [38]. Therefore, in the preparation of iTENG, it is necessary to select materials with good mechanical stability and conduct strict mechanical performance tests to ensure that its performance and stability can be maintained for a long time after implantation [39]. Flexibility is also a key consideration for iTENG as they need to conform to the shape and movement of the human body. Flexible materials and structural designs allow the devices to adapt and integrate seamlessly with the surrounding tissues or organs. This flexibility ensures patient comfort, minimizes the risk of damage to surrounding tissues, and enables unhindered bodily movements [40].

The iTENGs can be traced back to the development of triboelectric nanogenerators in the early 2010s [41,42,43,44]. As the field progressed, researchers also explored the use of flexible conductive polymers and carbon-based materials, such as graphene and carbon nanotubes, to achieve better compatibility with the human body [30,32,45]. As the field advanced, researchers focused on developing advanced encapsulation strategies, such as thin film coatings or biodegradable materials, to minimize device size, improve biocompatibility, and enable localized drug release [46]. Throughout the history of iTENGs, there has been a continuous effort to refine the materials and structures used in these devices [47]. The evolution of materials and structures has paved the way for the development of more advanced and functional iTENGs with the potential to power a wide range of biomedical devices and enable innovative healthcare solutions [48,49,50].

iTENG, as an emerging technology for energy harvesting and conversion, has broad application prospects. Although there are still some challenges, such as durability and stability, with the continuous advancement and improvement in technology, it is believed that iTENGs will be used in the future medical.

2. Structures

2.1. Contact–Separation Mode

2.2. Single-Electrode Mode

2.3. Free-Standing Mode