12160 FACTA UNIVERSITATIS Series: Mechanical Engineering Vol. 21, No 4, 2023, pp. 751 - 772 https://doi.org/10.22190/FUME230925043H © 2023 by University of Niš, Serbia | Creative Commons License: CC BY-NC-ND Review article SENSING MECHANISM AND APPLICATION OF MECHANICAL STRAIN SENSOR: A MINI-REVIEW Heebo Ha1, Nadeem Qaiser2, Tae Gwang Yun3, Jun Young Cheong4, Sooman Lim5, Byungil Hwang6 1Department of Intelligent Semiconductor Engineering, Chung-Ang University, Republic of Korea 2Material Science and Engineering, Physical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Saudi Arabia 3Department of Molecular Science and Technology, Ajou University, Republic of Korea 4Bavarian Center for Battery Technology (BayBatt) and Department of Chemistry, University of Bayreuth, Germany 5Department of Flexible and Printable Electronics, LANL-JBNU Engineering Institute, Jeonbuk National University, Republic of Korea 6School of Integrative Engineering, Chung-Ang University, Republic of Korea Abstract. This study reviews the potential of flexible strain sensors based on nanomaterials such as carbon nanotubes (CNTs), graphene, and metal nanowires (NWs). These nanomaterials have excellent flexibility, conductivity, and mechanical properties, which enable them to be integrated into clothing or attached to the skin for the real-time monitoring of various activities. However, the main challenge is balancing high stretchability and sensitivity. This paper explains the basic concept of strain sensors that can convert mechanical deformation into electrical signals. Moreover, this paper focuses on simple, flexible, and stretchable resistive and capacitive sensors. It also discusses the important factors in choosing materials and fabrication methods, emphasizing the crucial role of suitable polymers in high-performance strain sensing. This study reviews the fabrication processes, mechanisms, performance, and applications of stretchable strain sensors in detail. It analyzes key aspects, such as sensitivity, stretchability, linearity, response time, and durability. This review provides useful insights into the current status and prospects of stretchable strain sensors in wearable technology and human–machine interfaces. Key words: Strain Sensor, Flexible, Nanomaterials, Mechanical Property, Deformation Received: September 25, 2023 / Accepted November 01, 2023 H. Ha, N. Qaiser, T. Yun, J. Cheong, S. Lim contributed equally to this work. Corresponding author: Byungil Hwang School of Integrative Engineering, Chung-Ang University, Seoul 06974, Republic of Korea E-mail: bihwang@cau.ac.kr 752 H. HA, N. QAISER, T. G. YUN, J. Y. CHEONG, S. LIM, B. HWANG 1. INTRODUCTION Flexible and wearable devices that adhere to the skin are garnering interest and are actively developed [1-5]. These devices offer valuable features, such as the ability to interact with and monitor the human body [6-11]. However, conventional strain sensors that rely on metals or semiconductor materials exhibit limited sensitivity and stretchability, making their integration into wearable devices challenging [12-15]. Notably, these sensors possess high stretchability to adhere to the skin or body joints effectively. Moreover, they must satisfy various requirements, including high elasticity, flexibility, durability, low power consumption, and biocompatibility [16, 17]. Fig. 1 State-of-the-art flexible, wearable strain sensors made of nanomaterials and polymer composites. Reprinted with permission from [11], copyright 2023 American Chemical Society. Reprinted with permission from [18], copyright 2022 Springer Nature. Reprinted with permission from [19], copyright 2021 Wiley. Reprinted with permission from [20] under the Creative Commons Attribution 4.0 International (CC BY 4.0) License. Reprinted with permission from [21], copyright 2020 American Chemical Society. Sensing Mechanism and Application of Mechanical Strain Sensor: a Mini-Review 753 Hence, carbon nanotubes (CNTs) [22-31], graphene [26, 32-36], metal thin films [5, 37], and nanomaterials, such as metal nanowires (NWs) [38-45] , have emerged as promising candidates for flexible strain sensors. These materials exhibit remarkable flexibility and conductivity, as well as exceptional mechanical, electrical, optical, and chemical properties. Using these nanomaterials, flexible strain sensors can be seamlessly integrated into clothing or adhered directly to the human body for the real-time monitoring of physical activities with impressive efficiency. These materials are often combined with polymers to achieve desired flexibility, durability, and biocompatibility. However, balancing stretchability and sensitivity remains a challenge, hindering the development of high-performance strain sensors [46]. A strain sensor converts mechanical deformations into electrical signals. Various types of strain sensors exist, including fiber Bragg gratings [47], triboelectric sensors [38], and piezoelectric sensors [48, 49]. However, using strain sensors for wearable applications poses challenges owing to limitations such as the need for sophisticated measurement equipment, low resolution, and dynamic performance issues [50]. Some highly sensitive strain sensors can detect small strains and offer excellent electrical responses. However, they tend to lose their conductivity when subjected to relatively large strains, which makes them incapable of detecting subtle changes. For instance [51], while graphene-based strain sensors have a high gauge factor (GF) of 103, indicating substantial changes in electrical resistance owing to deformation, their detectable range is limited to 6%. Conversely, highly elastic sensors are better equipped to handle substantial strains but exhibit poor sensitivity. Strain sensors based on materials such as finely crushed graphene, graphene nanopaper, and silver nanowires can detect a wide range of strains exceeding 70%. However, within the range of 20% or less strain, they exhibit a low GF value and insufficient sensitivity. Wearable sensors are broadly categorized into two types: resistive [52] and capacitive [53]. Both require relatively simple systems and offer high flexibility and stretchability. Resistive sensors typically consist of conductive sensing films integrated with flexible substrates. As the composite material expanded, the microstructure of the sensing film changed, altering its resistance in response to the applied strain. When the strain was removed, the composite returned to its original state, and the resistance returned to its initial value. However, capacitive sensors rely on electrodes and an insulator. The capacitance changes with the applied strain, leading to alterations in the electric field. An essential consideration in fabricating flexible strain sensors is the selection of the appropriate materials and manufacturing methods. The fabrication process should facilitate low-cost, large-scale production and convenience. To date, stretchable strain sensors have been created by incorporating materials such as carbon black (CB) [54-56], carbon nanotubes (CNTs), NWs, nanoparticles (NPs) [57, 58], silicone-based elastomers (e.g., polydimethylsiloxane (PDMS) [11, 26, 59-61], polyurethane (PU) [62-64], or ecoflex [30, 48, 65]), and rubbers (natural rubber and thermoplastic elastomers) [66-69]. The impact of the choice of the polymer type on the strain-sensing performance, although relatively minor, is explored in detail in another section. Various methods for fabricating stretchable strain sensors, including filtration [70], printing [27, 37, 71, 72], transfer [73, 74], coating [75, 76], and chemical synthesis [77], have been reported. For example, a highly stretchable and sensitive strain sensor was produced by filtering silver nanowire (AgNW)-PDMS through a filtration process. This involves coating the AgNWs onto a silicon substrate, followed by PDMS filtration to create a film by curing and peeling. In addition, a flexible 754 H. HA, N. QAISER, T. G. YUN, J. Y. CHEONG, S. LIM, B. HWANG and sensitive strain sensor was fabricated by spray coating graphene flakes onto a PET substrate [73]. Numerous other methods have also been explored, such as strain sensor fabrication through printing and solution mixing to create nanocomposites (Fig. 2). Fig. 2 Fabrication processes of stretchable strain sensors. a) Printing technology. Reprinted with permission from [71], copyright 2019 American Chemical Society. b) Solution mixing. Reprinted with permission from [64], copyright 2021 American Chemical Society. c) Spinning and Transfer method. Reprinted with permission from [74], copyright 2019 American Chemical Society. In this paper, we review the fabrication methods, mechanisms, strain-sensing performance, and applications of stretchable strain sensors. We begin by describing the general mechanism of these sensors, followed by an explanation of the responsive behavior mechanisms of both resistive and capacitive sensors. In addition, we discuss critical aspects, such as stretchability, sensitivity, linearity, response time, durability, and other factors that validate the performance of stretchable strain sensors. Finally, we provide relevant examples and offer insight into the future of this technology. This review enhances the crucial understanding of the presentation of novel ideas to resolve the limitations related to these sensors. 2. STRAIN-RESPONSE MECHANISM 2.1. Geometrical Effect The response of a strain sensor varies depending on factors such as material, micro/nanostructure, and manufacturing process. The traditional strain resistance response results from the geometry and piezoresistivity of a material [78]. However, unlike Sensing Mechanism and Application of Mechanical Strain Sensor: a Mini-Review 755 conventional strain gauges, modern stretchable strain sensors utilize mechanisms such as separation between sensing elements, crack propagation in films, and tunneling effects. When a material undergoes stretching, it tends to contract laterally owing to its Poisson ratio. In the case of a solid, its resistance (R) can be calculated using the formula: 𝑅 = 𝜑𝐿 𝐴 (1) where φ represents the resistance, L is the length, and A is the cross-sectional area of the solid. As the length increases and the cross-sectional area decreases owing to contraction, the resistance of the solid increases. However, when a capacitive sensor is subjected to stretching, an increase in capacitance arises from changes in the thickness of the dielectric layer and alterations in the capacitor area [79]. The formula for calculating the capacitance (C) is based on the overlap area between the two electrodes (A) and thickness (d) of the dielectric layer. This is expressed as follows: 𝐶 = 𝜀0𝜀𝑟 𝐴 𝑑 (2) where 0 and r are the permittivity of vacuum and relative permittivity of the dielectric, respectively. As the sensor experiences strain, its capacitance of the capacitive sensor increases by preserving the capacitive region while reducing the thickness of the dielectric layer. Assuming that the stretchable electrode and the dielectric layer of the stretchable strain sensor share the same Poisson ratio, calculating the capacitance is possible during stretching. In this scenario, the capacitance of the capacitive sensors increased linearly with applied strain. However, importantly, if the applied strain becomes excessively large, the linear relationship dictated by Poisson's ratio may not hold for polymers. Therefore, recognizing that this linear function is valid within a specific range of strains and may not be applicable beyond that range is essential. 2.2. Piezoresistive Effect Piezoresistance refers to the change in the resistance of a material owing to structural deformation [80-83]. This piezoresistance can be expressed as following Eq. (3): ∆𝑅 𝑅 = (1 + 2𝑣)𝜏 + ∆𝑝 𝑝 (3) where ν is the Poisson’s ratios for the stretchable layer, τ is the applied strain, R is the measured resistance and p is the applied pressure. Semiconductor materials such as Si and Ge can significantly increase the sensor resistance by altering the bandgap between atoms [84]. Conversely, some metals and metal alloys exhibit piezoresistive properties that can substantially increase strain gauge resistance. Moreover, materials at the nanoscale, such as carbon nanotubes (CNTs) and nanowires (NWs), can exhibit ultrahigh resistance depending on factors such as chirality and barrier height [85, 86]. This makes semiconductors and nanoscale materials promising for fabricating highly sensitive strain gauges. However, despite their potential, the flexibility required for human attachment and wearable strain sensors surpasses that of conventional strain gauges fabricated using metals and semiconductors. These limitations are attributed to technical challenges, inconsistent 756 H. HA, N. QAISER, T. G. YUN, J. Y. CHEONG, S. LIM, B. HWANG electrical and mechanical properties, and low elasticity. Consequently, SWCNTs (single- walled carbon nanotubes) and NW-based sensors have not been widely adopted for strain- sensing applications. To address these limitations, novel approaches were introduced to create more flexible and sensitive deformation sensors by incorporating nanomaterials into elastic polymer composites. Unlike previous methods, these approaches aimed to enhance the flexibility and sensitivity of strain sensors. However, notably, the contribution of piezoresistive nanomaterials to the overall resistance and strain behavior of elastic strain sensors is expected to be limited. This limitation arises from the weak interfacial bonding strength between the nanomaterials and polymers, as well as the anticipated significant elastic mismatch between the two components [87]. 2.3. Disconnection Mechanism In a conductive film comprising conductive nanomaterials, electrons traverse the percolation network formed by stacked nanomaterials [16, 88]. As the film underwent stretching, the area of overlap between the connected nanomaterials decreased, increasing the resistance. At the microstructural level, the disconnection of overlapping nanomaterials during stretching occurs because of weak interfacial bonding between the materials and significant elastic mismatch between the nanomaterials and polymer. Open circuits within conductive nanomaterials obstruct electrical pathways, resulting in exceptionally high resistance and diminished performance of the strain sensors. As the strain level increases, the disconnection mechanism becomes more pronounced, and the performance of the sensor becomes heavily reliant on this process of disconnection and reconnection, leading to irreversible sensitivity degradation (Fig. 3). Elastomer composite sensors incorporating nanomaterials, such as graphene flakes [89], AgNWs [42], and ZnONWs [90], have been reported. Similar mechanisms have been observed where disconnections occur at the junctions between NWs after expansion, causing increased resistance and decreased performance. Stretchable and sensitive strain sensors were developed using thin films of NW networks and graphene flakes, focusing on disconnection mechanisms. For instance [11], in the case of AgNWs embedded in PDMS, where the AgNWs possess Young's modulus of 81–176 GPa, and PDMS exhibits a modulus of 2–3.5 MPa, during stretching cycles, the AgNWs within the PDMS slip into the PDMS matrix, leading to disconnections at the NW– W junctions. Another example involves sensors using AgNW, graphene and PDMS that exhibit a GF of 9156 under strain of >36% [91]. Even with various temperature range, these sensors rely on a disconnection mechanism to detect changes in resistance. In this context, the rigid nature of graphene sheets result in microcrack junctions allowing resistance changes to be observed. Similar mechanisms have been identified for sensors that use the rolling resistances of ZnO NWs and graphene flakes. Sensing Mechanism and Application of Mechanical Strain Sensor: a Mini-Review 757 Fig. 3 (a) Schematic of the sensing mechanism and SEM images of the strain sensors at various stretching states: (b,c) 0%, (d, e) 20%, (f, g) 40%, (h, i) and back to 0%. Scale bars in (b), (d), (f) and (h) are 20 μm. Scale bars in (c), (e), (g) and (i) are 5 μm. Reprinted with permission from [91], copyright 2021 American Chemical Society. 2.4. Crack Propagation Cracks tend to form in thin and delicate coating layers on the film and can be easily initiated and propagated when subjected to stretching. These cracks typically originate from regions where the stress is concentrated, thereby effectively releasing localized stress. Fine cracks developed in the CNTs, NWs, NPs, and graphene films coated on stretched substrates [58, 92, 93]. As the stretching process continued, these cracks expanded within the thin film, significantly increasing the resistance. This mechanism is used to create highly sensitive and flexible strain sensors. Moreover, several methods have been explored to enhance the strain sensitivity and GF further. One approach involves peeling off the AgNWs in conjunction with the PDMS and cutting the composite material mechanically [11]. As shown in Fig. 4, owing to the structural alterations induced by mechanical cutting, certain areas experienced partial cutting, allowing the strain to reach up to 30% and consequently yielding a GF value of 290.1. Notably, the resistance returned to its original state, indicating complete recovery. 758 H. HA, N. QAISER, T. G. YUN, J. Y. CHEONG, S. LIM, B. HWANG Fig. 4 (a) Schematic of the sensor and electrical performances of strain sensor under applied strain. (b) Optical images of the top surface of the sensor under applied strain showing the crack opening. Reprinted with permission from [11]; copyright 2023 American Chemical Society. 3. PERFORMANCE OF STRETCHABLE STRAIN SENSORS 3.1. Stretchability The performance of strain sensors depends on various critical factors, including stretchability, sensitivity, GF, linearity, hysteresis, response and recovery times, and durability. These factors are significant for wearable strain sensors that can be exposed to prolonged, repeated strain. The stretchability of a strain sensor exhibits substantial variability, contingent on factors such as the sensor type, utilized materials, structural design, and manufacturing techniques. For instance [94], a resistive sensor composed of an aligned CNT-PDMS composite achieved an impressive stretchability of ~280%. This exceptional stretchability was attributed to the consistent propagation of microcracks within the CNT film and interconnection of the aligned CNTs. Similarly, capacitive sensors demonstrated ~300% stretchability using CNT–silicone elastomer composites owing to the remarkable stretchability of the dielectric material and robust nature of the CNT-based strain electrodes. The integration of 1D nanomaterial-polymer composites plays a pivotal role in significantly enhancing the stretchability of strain sensors [95]. Owing to the high aspect ratio of 1D nanomaterials, an effective network can be readily established, ensuring stable electrical and mechanical properties, even under conditions of substantial stretchability. Strain sensors employing CB, NPs, and graphene films exhibit stretchabilities >70% [96- 98]. Examples include graphene–cellulose composites, fragmentized graphene foam Sensing Mechanism and Application of Mechanical Strain Sensor: a Mini-Review 759 (FPF)–PDMS composites, and graphene–rubber composites. Conversely, the limited stretchability observed in CB-, NP-, and graphene-based strain sensors can be attributed to the absence of a well-connected network of nanomaterials with lower aspect ratios [99]. 3.2. Sensitivity or Gauge Factor The sensitivity or GF of the strain sensors was determined by the relative change in the electrical signal and slope of the applied strain. The GF can be calculated for both resistive and capacitive sensors using the following equation: 𝐺𝐹 = ∆𝑅 𝑅0𝜌 , 𝐺𝐹 = ∆𝐶 𝐶0𝜌 (3) where C0, R0, and ρ, are the initial capacitance, resistance and the dielectric constant. Traditional metal film-based strain gauges typically offer a GF in the range of 2–5, while semiconductor-based strain sensors can provide GF values >100 [16]. By contrast, stretchable strain sensors can exhibit various GF values depending on factors such as the sensing mechanism, nanomaterial used, and structural design of the sensor. Among resistive sensors, a broad range of GFs can be achieved by adjusting the density of the AgNWs. Some studies have also aimed to attain high GF values and stretchability by reducing the AgNW concentration and employing a disconnection mechanism. However, these approaches have limited reliability. Highly sensitive sensors using cracks and disconnection mechanisms with graphene have been reported; however, they tend to have very low stretchability [100]. By contrast, capacitive-type sensors offer high stretchability but typically possess very low GF values. Most capacitive sensors have GF values <1. Theoretically, a GF of 1 represents the upper limit for capacitive sensors, making achieving a GF higher than this value impossible [101]. 3.3. Linearity Linearity is a crucial factor affecting the performance of stretchable strain sensors. This is particularly relevant for resistivity-based sensors, where nonlinearity can complicate data processing and adversely affect data collection. Nonlinearity often arises when the microstructure of a thin film changes from homogeneous to nonhomogeneous owing to stretching, causing the strain sensor to respond nonlinearly to the applied strain. Nonuniform generation and propagation of microcracks during deformation contribute to this nonlinear response in the strain sensor data calculation process. Efforts have been made to address these limitations, including enhancing CNTs through hybrid structures that incorporate metals or other nanomaterials. However, these efforts mostly resulted in a trade-off between high linearity, sensitivity, and stretchability. Conversely, capacitive sensors typically exhibit excellent linearity but lower sensitivity than sensors with a GF of 1 or higher. High-sensitivity strain sensors often exhibit high nonlinearity and low stretchability. Consequently, the development of a strain sensor with high stretchability and sensitivity remains a significant challenge. 760 H. HA, N. QAISER, T. G. YUN, J. Y. CHEONG, S. LIM, B. HWANG 3.4. Hysteresis In the context of using strain sensors for body attachment and wearable applications, hysteresis is a critical consideration, particularly when the sensor is subjected to dynamic loading. Significant hysteresis can lead to irreversible sensing performance under dynamic loads. Capacitive sensors generally exhibit a better hysteresis performance than resistive sensors [53]. The performance of capacitive sensors relies on maintaining a stable overlapping area between the electrodes rather than on the conductivity of the electrodes. However, resistive sensors are more susceptible to hysteresis because they are affected by changes in the conductivity of the thin films, which can contribute to greater hysteresis effects. In addition, hysteresis can stem from interactions between the polymer and nanomaterial fillers as well as the viscoelastic properties of the polymer [94]. Strong interfaces between the polymer and filler materials improve strain sensor performance. Conversely, weak connections between nanomaterials and the polymers can lead to slipping and increased disconnections during stretching, thereby causing high hysteresis [102]. Researchers have endeavored to develop applications that can be attached to the body while significantly reducing hysteresis. For example [103], one approach involved creating a hydrogel using materials like PEDOT:PSS and polyvinyl alcohol (PVA), demonstrating a remarkably low hysteresis of ~1.44%, even in a strain range of 300%. However, room for improvement exists in terms of sensitivity, as this particular hydrogel exhibited a relatively low GF value of 4.07. 3.5. Response and Recovery Time The response time is a crucial parameter that indicates how quickly a strain sensor returns to its normal state after experiencing a strain. Because of the viscoelastic properties of polymers, all polymer-based strain sensors inherently exhibit some degree of response delay. Typically, the standard response time of stretchable sensors is defined using a 90% time constant. For resistivity sensors based on materials such as AgNW/PDMS [104] and AgNW/AgNP/PDMS [43], response times of 200 and 51 ms, respectively, were reported. By contrast, capacitive sensors exhibit faster response times. Capacitive sensors based on Mxene/PVP [105] and AgNW/PDMS [106] show response times of 30 ms. The recovery time is another important performance factor for stretchable strain sensors. Capacitive sensors generally have recovery times in milliseconds, whereas resistive sensors require few hundred milliseconds to recover. For instance, capacitive strain sensor exhibit a 15 ms recovery time [105], while resistive strain sensors of AuNP/PDMS [107] and CNT/PDMS [108] composites require 250 and 500 ms to recover. The recovery time of nanocomposite-based strain sensors depends on the interactions between the fillers and polymers and varies based on the type and ratio of the fillers used. Capacitive sensors offer advantages such as high linearity, stretchability, and low hysteresis. However, they have a very low GF and can exhibit unpredictable responses owing to unstable overlapping areas and interactions. Conversely, resistive sensors provide high GF values but often respond to applied strains with high hysteresis and nonlinearity. Sensing Mechanism and Application of Mechanical Strain Sensor: a Mini-Review 761 4. APPLICATIONS 4.1. Human motion detection and healthcare Flexible strain sensors can continuously record a wide spectrum of bodily movements ranging from significant joint actions to subtle deformations associated with physiological signals. Consequently, these strain sensors have garnered considerable attention as essential components in healthcare monitoring devices, serving purposes such as human motion detection and diverse healthcare applications. Developing highly sensitive sensors capable of generating high-resolution and accurate signals is important to meet the demands of detecting minute variations such as pulses or respiration [20, 25, 43, 44, 64]. Fig. 5A illustrates the application of strain sensors employing AgNW/PDMS composites to monitor pulse rates by attaching them to the brachial and radial arteries. After undergoing structural transformation via partial cutting, the sensors exhibited a strain of up to 30% and an impressive GF value of 290.1. In addition, the sensors demonstrated a rapid increase in resistance through a disconnection mechanism and exhibited exceptional performance stability even after 1000 reliability test cycles. The sensors respond to changes in blood flow pressure resulting from the pulse, enabling the tracking of various pulse characteristics. Furthermore, these sensors can be affixed to the back of a person to discern body movements in different directions, thereby facilitating human motion detection and healthcare monitoring, particularly for spinal issues [11]. In addition to the pulse and major body motions, wearable strain sensors can discern alterations in facial expressions and emotional states [64]. Capacitive sensors strategically placed around the eyes and mouth enable the detection of actions such as laughter, crying, and frowning with high sensitivity. Monitoring diverse emotional expressions is valuable in assessing conditions such as Alzheimer's disease. Fig. 5B further illustrates the application of these sensors to the neck, capturing changes in resistance corresponding to vocal cord movements, including phonetic sounds like “I,” “A,” and “M.” Moreover, wearable sensors can gauge vital parameters, such as respiratory rate, oxygen saturation, and heart rate, and transmit these data wirelessly. These sensors can connect to emergency service centers to dispatch alarm messages during emergencies. This strain sensor was engineered to concurrently measure respiratory volume and rate using polystyrene, Au, and Pt [20]. These sensors were attached to the rib cage and abdomen to measure volume variations during respiration and to calculate the respiratory rate and volume. The precision was confirmed by comparing the resistance changes due to respiration with spirometer readings, yielding a high-precision level with an R2 of 0.92. Unlike conventional wearable sensors, which typically measure only the respiratory rate, this sensor can also estimate the respiratory volume. It can monitor breathing across different activities, is not limited to stationary positions, and is invaluable for identifying individuals with chronic respiratory conditions through everyday wireless bluetooth devices. These versatile sensors, which are suitable for placement on various parts of the human body, hold promise for applications in diverse fields, including exercise, health monitoring, and disease management. 762 H. HA, N. QAISER, T. G. YUN, J. Y. CHEONG, S. LIM, B. HWANG Fig. 5 Demonstrations of the strain sensor for personal health monitoring. (A) Blood pulse on the radial artery and brachial artery and spine for monitoring lower back strain. Reprinted with permission from [11], copyright 2023 American Chemical Society. (B) Detection of human joint movements and the vocal cord vibration. Reprinted with permission from [64], copyright 2021 American Chemical Society. Sensing Mechanism and Application of Mechanical Strain Sensor: a Mini-Review 763 4.2. Soft robotics Wearable strain sensors play a pivotal role in soft robotics, a field of technology characterized by robots crafted from pliable materials. These sensors are instrumental in overseeing and directing the shape, movement, and interactions of soft robots in their environment. Unlike their traditional counterparts, soft robots exhibit natural fluidity of motion and adaptability to their surroundings. They are typically affixed to soft grippers and detect the force and positioning required for secure object manipulation and lifting. The sensors transmit these data to the robot control system, enabling real-time control and interaction. Applications of soft robots span diverse fields, including medicine, structural inspections, environmental exploration, and disaster relief [8, 19, 24, 109]. Fig. 6 Recognition of finger dexterity, gestures, and temperatures. (a) Relationship between the ΔR/R0 of MWNTs/PDMS fibers recorded by a tablet oscilloscope and the angle of finger joint bending. Metacarpophalangeal point (MP), proximal interphalangeal point (PIP), and distal interphalangeal point (DIP) are three important joints in the hand. (b–e) Photograph of the smart glove for recognition of (b) the bending angles of the finger, (c) numbers, and (d) alphabets. Reprinted with permission from [21], copyright 2020 American Chemical Society. For example, a hydrogel-type strain sensor comprising PEDOT:PSS and PVA was applied in soft robotics [103]. This strain sensor was attached to the gripper where alterations in the gripper angle corresponding to the size of the grasped object were observed, changing 764 H. HA, N. QAISER, T. G. YUN, J. Y. CHEONG, S. LIM, B. HWANG the resistance. This relationship was linearly corrected by analyzing the bending angle and resistance variations. Furthermore, this study demonstrates that an industrial robot can be remotely guided by hand manipulation by integrating these sensors. The strain sensors continuously monitor the bending motion of each finger, allowing the controller to direct the movements of the robot based on automatically recognized signals. The experiment underscores that industrial robots can be precisely, reliably, and immediately controlled with minimal hysteresis, thereby ensuring stable performance and high resilience. In addition, other research explored the creation of a smart glove equipped with a strain sensor using MWCNT/PDMS fibers [21]. (Fig. 6) When worn, this smart glove detects resistance changes when the fingers are bent or flexed. Numbers and alphabets can be recognized according to the bending of the finger joints, and the corresponding information can be checked through the tablet on the back of the hand. Remarkably, this system exhibited an impressive high linear coefficient R2 of >0.995 within the 0-120% strain range. Moreover, it demonstrated excellent durability, enduring 20,000 cycles of stretching and release with a 50% strain applied. Nevertheless, it shows the limitation of having very low GF values from 0.45 to 2.08 with different. 4.3. Virtual reality Another compelling application of wearable strain sensors is as motion sensors for virtual reality (VR) and augmented reality (AR). By affixing these sensors to various parts of the human body, such as gloves, stockings, and clothing, users can engage in immersive gaming experiences that leverage motion detection, pressure sensing, and hand movements, all of which are facilitated by these motion sensors. Moreover, wearable strain sensors not only detect the user's movements and touches but also govern interactions within virtual environments, playing a pivotal role in gaming, educational simulations, and other forms of entertainment [110, 111]. As shown in Fig. 7A, this research [19] introduced a sensor embedded in a smart glove designed to track the intricate movements of an individual's fingers. These smart gloves, equipped with printed PCB chips, established Bluetooth connections with the robot’s hands, enabling them to respond to an array of gestures. The strain sensor using liquid metal and PDMS exhibited an impressive stretch capacity of up to 105% strain and a remarkable detection limit as low as 0.05%. By demonstrating remarkable durability over 8000 cycles at 50% strain, these smart gloves proved adept at tracking the movements of various body parts in the virtual reality realm. Fig. 7B shows the implementation of a strain sensor based on the MXene/SWCNT/PVA composite [18]. For human–machine interaction purposes, strain sensors were strategically positioned on the elbow, knee and other joints. These sensors were linked to a data acquisition circuit board that was meticulously designed to interpret finger movements, gather data, and transmit them to a computer via Bluetooth. This setup allows users to control VR movements by interacting with the strain sensors. Furthermore, these sensors highlight their potential as versatile motion sensors for both real-world movements and immersive realms of virtual and augmented reality. Notably, they offer several advantages over traditional camera setups and motion detection systems, including less power consumption, high sensitivity of > 1000, cost-effectiveness, and high accuracy. Sensing Mechanism and Application of Mechanical Strain Sensor: a Mini-Review 765 Fig. 7 (A) Demonstration of robotic hand operated by remote system. Reprinted with permission from [19], copyright 2021 Wiley. (B) Photos of the remote control system and snapshots of playing the game with fingers to control all the actions. Reprinted with permission from [18], copyright 2022 Springer Nature. 766 H. HA, N. QAISER, T. G. YUN, J. Y. CHEONG, S. LIM, B. HWANG 5. CONCLUSION AND FUTURE WORK Table 1 summarizes the strain sensor types, materials, and main features of their applications. This study provides an overview of recent developments in the production methods, mechanisms, strain-sensing capabilities, and practical applications of stretchable and wearable strain sensors. Wearable sensors using various materials, including carbon- based materials, 1D and 2D materials, and metals, have exhibited remarkable performance enhancements. This paper also discusses several prominent applications of wearable sensors. However, further performance improvements are vital for real-world applications. Within the scope of wearable sensors, capacitive sensors offer advantages such as stretchability, linearity, and low hysteresis but exhibit limited sensitivity. By contrast, resistive sensors provide significantly higher sensitivity and stretchability but suffer from drawbacks such as high hysteresis and nonlinear responses. Table 1 Summary of the strain sensor types, materials, and main features Materials Type Key performance Pros and cons [Ref] AgNW@PDMS Resistive GF : 290.1 (30% strain) ▪ High GF and multifunctional ▪ Low stretchability and still limited sensing area [11] Au/Pt@polystyrene Resistive High linearity ▪ Multifunctional ▪ Low GF (2.64) [20] PEDOT:PSS@PVA Resistive Ultralow hysteresis (<1.5%) Linearity (R2 = 0.98) ▪ Ultralow hysteresis, high linearity (R2 = 0.98) and high stretchability (~300%) ▪ Low GF (4.07) and limited sensing area [103] Graphene nanosheet@PU fiber Resistive GF : 661.59 (0%–75% strain) ▪ High stretchability(~500%), GF and reliability (>10,000 cycles) ▪ High hysteresis and limitations depending on strain area [112] CNT/MXene@TPU Resistive GF : 363 Light weight ▪ High GF and wide working strain range ▪ Nonlinearity depending on strain area [64] LM@PDMS Capacitive Ultralow detection limit (<0.05%) ▪ Excellent mechanical robustness (>8,000 cycles) and fast response time (58 ms) ▪ Limited sensitivity [19] AgNW@PMMA Capacitive GF : 1 ▪ Transparent and low detection limit (<12 mg) ▪ Limited sensitivity [113] Recent advancements in material science and structural research led to the development of composite materials and functional structures to address these challenges. However, issues such as durability degradation over time, sensitivity concerns, and varying GF values persist as strain sensors exhibit nonlinear responses and variable GF values depending on the strain applied, making them suitable only for specific applications. Furthermore, to make these sensors practically viable, they must be evaluated under diverse conditions, Sensing Mechanism and Application of Mechanical Strain Sensor: a Mini-Review 767 including temperature and humidity variations, which can influence the sensor performance. Precise measurements are crucial for elastomers, which are commonly used elastic materials, because their physical properties can change with temperature. In addition, when fabricating wearable sensors from elastomers, they must be designed to ensure breathability and comfort for end users. Despite the proposals for fiber and nanowire composites, performance enhancements are still required. Furthermore, research is needed into new microstructures and nanostructures to enable multimodal sensing capabilities. Addressing these challenges holds the potential for the application of strain sensors in physical stimulation, wearable medical devices, smart surgical equipment, drug delivery monitoring, and artificial organ research. Furthermore, the development of multifunctional strain sensors may open doors for various research and applications, including those involving biological components. Acknowledgement: This research was supported by GRDC (Global Research Development Center) Cooperative Hub Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science and ICT(MSIT) (RS-2023-00257595). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (2021R1A2C1011248). REFERENCES 1. 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