Recent advances in piezoelectric textile materials: A brief literature review

Smart textiles (ST) can be defined as materials capable of detecting an external stimulus, responding, and adapting its behavior according to the stimulus obtained. The field of study and development of these materials is extensive, and ST can be seen in areas such as health, transport, security, civil construction, and sports. Piezoelectric textiles are part of the ST category and are characterized due the ability to generate electrical energy from mechanical stimulus, and vice versa. Therefore, the main objective of this review is to present the current research on piezoelectric ST. In addition, the study highlights the process of obtaining materials with piezoelectric properties and the challenges and limitations, seeking to understand the contribution of the development of these materials in the field of wearable electronic devices. Thus, the main challenge in developing piezoelectric textiles is in the ability to supply energy to electronic devices to be applied in various fields such as motion detection, acoustics, impact absorption, among others. Moreover, piezoelectric ST is remarkably promising for the development of wearable electronic textiles (e-textiles) that consequently impact the creation of new functional materials that enable renewable sources to offer a positive contribution in the daily society.


Introduction
Smart textiles (ST) differ from conventional textiles as they are sensitive to certain external stimuli, such as temperature change, pH change, mechanical action, chemical interaction, among others, [1][2][3][4] and react by generating a response and adapting their behavior to the environment. 5s an example of this behavior, we can mention textiles with thermal comfort property, which regulate the body temperature according to the external temperature. 6,7The ST materials are mostly developed with a predetermined functionality, 8 and can be obtained from two different paths: (i) surface modification of the conventional textile (yarn, flat fabric, knit fabric, or non-woven) with the application of additives conferring differentiated properties, or (ii) conceived with materials that provide special characteristics to the final product. 4,9,10he ST can be classified into three categories: passive, active, and very smart. 8Passives refer to the first category of ST and have functional characteristics that only detect the stimulus and do not depend on changes in the environment. 8ctive ST can sense and respond to a stimulus from the environment, mainly used as sensors and actuators (tactile sensors). 8The third category includes the very smart materials, which are capable of detecting external changes, reacting to them, and even adapting to desired conditions, being able to change their properties to adapt to the external environment. 4,8The very ST has a similar functioning to the human brain, in which it has cognitive capacity, reasoning and activation. 11The three previously described categories are illustrated in Figure 1.
3][14] Among the mentioned areas, the development of electronic textile materials (or e-textiles) is highlighted. 15This field of ST research addresses the development of textile materials capable of performing electronic functions.Hence, by adding features in conventional wearable textiles it is possible to build competitive advantages in the market.Furthermore, e-textiles have the potential to execute an important role in the development of the Internet of Things (IoT), connecting many electronic devices to each other within a cloud, for example.Thus, technological advances around this field can revolutionize the daily lives of many people in a near future. 16ne of the main challenges in the development of ST is providing energy for wearable and non-wearable textileelectronic devices, which has attracted the attention of research groups.Many efforts have been made to find renewable and green energy sources for these devices. 16In this scenario, textile materials stand out due to their low thickness and flexibility, especially those with intrinsic piezoelectric properties.Piezoelectricity is the ability to transform movement into electrical energy, and vice versa. 15][19][20] However, in addition to the function of collecting and generating energy, piezoelectric textiles can also perform other functions, such as motion sensors, 21,22 acoustic sensors, 23 and to absorb the impact. 24Textile materials with piezoelectric properties can be obtained in the form of yarns and filaments, woven fabric structures, knitted fabric structures, among other textile substrates. 25he literature review conducted by Rashid et al. 26 explains the application of piezoelectric coatings applied on textiles, with a focus on harvesting performance, piezoelectric performance, piezosensors and piezoelectric coating methods.Dolez 27 lead a state-of-the-art in energy harvesters for ST applications.The author focused on different paths for energy harvesting, such as photovoltaic and thermoelectric.The overview published by Scheffler and Poulin 28 highlights the processing of piezoelectric fibers and the challenges related to it.
Hence, this literature review presents a different approach, focused on the recent developments and applications of ST with piezoelectric property.Based on the above, this papers differs from the other reviews due that it's approach covers the application of piezoelectric ST in areas such as health, civil construction, clothing, footwear, accessories, sportswear, protection, and acoustic sensors.
The objective of the present work is to brief describe the piezoelectric effect, the main materials used, and to focus on the applications of piezoelectric ST.The aim is to contribute positively with the development of ST with piezoelectric property, as well as the improvement of the existents one.Moreover, the approach is relevant not only for the functional smart textiles area, but also for the sustainable product's field, which is currently a trend topic in the research worldwide.

The piezoelectric effect
The piezoelectric effect was discovered by Pierre and Jacques Curie brothers in France in 1880.Scientists looked at piezoelectricity in natural crystals and later, in 1910, the book by Woldemar Voigt (Lehrbuch der Kristallphysik) was published; it describes 20 classes of natural crystals with piezoelectric activity. 29The accomplishment was marked from the publication with the following sentence: "We found a new method for the development of electricity poles in these same crystals, consisting in subjecting them to pressure variations along their hemihedral axes."Since then, piezoelectric materials have stood out in comparison to electrostatic and electromagnetic ones, for example, due to the higher density of stored energy and ability to convert mechanical stimuli into electrical ones. 30The process for obtaining and using piezoelectric materials has been improved, being applied in areas that need to combine the goal of battery reduction (due to sustainable concerns) with the unique properties of the piezoelectric materials, such as the integration of devices with electronic systems.
Crystals with piezoelectric properties have a central asymmetric crystal structure. 29,31Some natural piezoelectric crystals are: tourmaline, topaz and quartz.Piezoelectricity is the electrical response to a mechanical stimulus in a given material, 32,33 which may be from ocean waves, vibrations, 34 pressure, 35 ultrasonic waves, human body movements, 36 the wind itself, 37 or other piezoelectric devices.9][40] In the inverse (or indirect) piezoelectric effect, an induction of mechanical deformation occurs when the material is subjected to an applied electric field. 16,30he piezoelectric property has drawn the attention of researchers because it is a practical way -since it is possible to capture even the movement of the wind itself -to obtain a mechanical stimulus from the external medium (environment) and convert it into electricity, in the case of direct piezoelectricity. 36,41The inverse piezoelectric effect is most applicable to acoustic emitters, vibration dampening, and actuators. 16n order to promote even more sensitivity to the piezoelectric material, the greatest choice for the raw material is the use of polymers, 42 due the reduced mass, 43 flexibility, easy processing, good compatibility, and elongation properties. 44iezoelectric results depend mainly on the angle at which the material moves, on the piezoelectric capacity of the raw material used, which are highly related to the lightness and flexibility of the developed material 45 and the macroscopic polarization applied to the material. 46

Piezoelectric ST developments and applications
The interest in research and development of piezoelectric ST is on the rise, mainly due to its ability to insert itself in different segments, such as the automotive, medical and biomedical, 47,48 sports, aviation, robotics, 49,50 clothing, protection, personal care, [51][52][53] civil construction, [54][55][56] shoes and accessories, 57 and aerospace 58 areas.Table 1 exemplifies some application areas of textile piezoelectric devices, the place of their use and the main results obtained.

Developments in the health field
The development of active polymeric fiber sensors, obtained through the melt spinning of piezoelectric polymer of PVDF has been extensively studied. 21Szewczyk et al. 60 investigated the influence of voltage polarity and ambient relative humidity in electrospinning for energyharvesting applications, controlling relative humidity improved the piezoelectric coefficient for PVDF fibers more than three times.These sensors can convert body movements, such as wrist rotation, breathing, vocal cord vibration, and lower limb movements, into electrical signals.In this context, the mentioned materials can provide reliable, continuous and accurate physiological data, which can be used, for example, for custom healthcare. 19earables and flexible sensors have the potential to act in a wide range of applications, as well as monitoring human conditions for medical purposes, such as respiratory rate, heart rate and body posture. 22An example of a textile-based piezoelectric material for application in the medical field is a sensor that monitors a patient's posture.It detects body movements such as bending the knees and rotating the hips, transferring these measurements to a computer via Bluetooth.From the development of a program to detect the patient's position through the electrical signals emitted by the patient's body, 88% success was obtained in tests performed with humans. 61n another study, Akerfeldt et al. developed a movement detection glove with the potential to be used in physical rehabilitation treatments.The sensor consists of a glove developed using warp knitting technology, a piezoelectric PVDF yarn and a printed conductor pattern, produced from a commercial formulation of textile finishing where conductive polymers, such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) were added.The glove with sensory properties was produced entirely from methods and materials commonly used to manufacture textile substrates, featuring high durability and comfort during its use. 21uture projections expect sensors with biocompatible, flexible and robust features to enable a change in healthcare, making it more personalized and focusing on disease prevention. 19With this perspective, the work of Su et al. demonstrates a cost-effective way to develop a self-powered, wearable bioelectronic (capable of generating energy for its own charging), and high-performance sensor.This device is inspired by human muscle, and the sensor was developed from electrospun piezoelectric nanofibers of barium titanate/poly(vinylidene fluoride) (BTO/PVDF).In order to improve properties, such as, interfacial adhesion, mechanical strength, and piezoelectricity, a chemical solution of polydopamine (PDA) was dispersed in the nonwoven formed by nanofibers.The sensor confirmed biocompatibility and mechanical durability, adapting to the skin's elastic modulus and can be attached to different parts of the body for continuous health monitoring, in addition to being self-powered. 19

Piezoelectric textile materials in the civil construction
Current methods of monitoring the integrity of concrete structures are only used sporadically (both in terms of time and location of measurements), so they are inefficient.As a result, research trends in this field are aimed at permanently incorporating transducers into structures.This makes it possible to continuously and in real time assess the structural integrity of buildings, for example. 62n this context, piezoelectric actuators have many advantages such as lightweight, fast response, high steering precision and easy integration.Consequently, there has been extensive research regarding the development of actuators in the form of composites reinforced with piezoelectric fibers. 63Dumoulin and Deraemaeker 62 explored the possibility of using composites with lead zirconate titanate (PZT) piezoelectric fibers as transducers (emitters and receivers) for monitoring concrete structures.The effective properties of piezoelectric composites were investigated using numerical and analytical models.Different materials were considered in the matrix phase: cement, epoxy resin, and polyurethane.As a result of the study, more classical materials, such as epoxy resin, are more promising for designing ultrasonic transducers from PZT fibers for monitoring the behavior of concrete.However, more studies are needed to understand the behavior of these transducer composites when effectively embedded in concrete. 62

Piezoelectricity in the clothing, footwear, and accessories area
The "wearable technology" and "wearable devices" are terms that describe computers and electronics that are integrated into clothing and other accessories that can be worn comfortably in everyday life. 16Wearable and ST devices aim to present multi functionalities such as  53 cognition, adaptation or integration. 64Zhu et al. developed a self-powered functional sock based on fabric-coated PEDOT:PSS containing an integrated PZT piezoelectric sensor.Additionally, to the basic ability to capture energy, the sock can identify a walking pattern for recognizing and tracking the movement of individuals.In addition, the functional sock is able to assess the wearer's sweat level.These developments allow for user monitoring and can be useful for home care applications. 64here are many studies on the development of wearable technologies with piezoelectric nanogenerators (NG) to capture energy from human movement, such as sensors developed with graphene nanoplatelets deposited in a textile through screen printing technique. 65Textile-based piezoelectric NGs are divided into three groups according to the formation structure.The first group comprises NG based on a single fiber, which has a coaxial structure with a core and external shell (electrodes), so that the flexible piezoelectric material is found in between the core and the external shell. 17Fabric-based piezoelectric NGs combine piezoelectric fibers into two-dimensional (2D) or threedimensional (3D) fabrics using different textile surface formation techniques.The last group are NG based on multilayer fabric structures, which can be obtained with layers of woven fabric or nonwoven fabrics, formed by stacked nanofibers. 17espite numerous research on the development of textile NG, there is still a considerable gap about what is currently used and what is expected in practical applications.This occurs mainly in the medical field, seeking to understand the interactions of devices such as stents, implants, and biomaterials in general with the human body.Thus, it is necessary to deepen and research in order to contribute to the knowledge of the subject in question. 66n another investigation, 53 a piezoelectric generator was constructed in the form of a filament from electrospun PVDF nanofibers involved in a conducting wire (internal electrode) and covered by two braided conductor wires (external electrode).Piezoelectric wires made from piezoelectric nanofibers and external electrodes were sewn directly into fabric, creating a wearable energy collection system.To investigate the performance of energy capture from body movements (pressure), sewing took place over strategic areas, such as on the insole of a shoe, obtaining good results regarding the efficiency of energy capture.
Another study 16 explored a new approach to developing textile energy harvesting devices that function as lastgeneration energy generators and sensors.In this study, textile spinning, knitting, weaving, and braiding technologies were used, using high production speed.Different textile substrates were developed from PVDF piezoelectric fibers with and without barium titanate (BT) nanoparticles (NPs) produced by the melt spinning process.The processing method demonstrated is potentially scalable for mass manufacturing ST with strain detection and energy harvesting.The developed devices showed high durability, lightness, and flexibility.

Piezoelectricity and the sportswear approach
A recent trend in the sportswear industry is piezoelectric fabrics. 67The possibility of developing strain sensors with textiles has been the focus of several research projects in recent years.This new generation of flexible devices is useful in applications where conventional sensors are not suitable due to their mechanical rigidity.The latest developments integrate sensing capabilities to provide instant information on athletes' physiological conditions, such as physical abilities and training status. 68iezoelectric fabrics have the potential to replace many extra devices used for performance measurements during physical activities, ensuring that the athlete wears the original garments intended for the specific sport.The main purposes of measurement systems are to contribute to improving the individual's performance during activity and to help prevent injuries, for professional athletes, dancers, or anyone who exercises. 67ao et al. 69 developed a system of piezoelectric textiles with biodetection and self-powered capacity.Fabrics produced from nanowires composed of Zinc Oxide in the shape of tetrapods are used as the central structure of the device (T-ZnO).The main mechanism of this device is based on enzymatic bonds and the piezoelectric effect of materials.The developed fabrics are adaptable and are now applied to the athlete's body like a conventional athletic uniform.The device can monitor in real-time speed, heart rate and joint angle, which refers to the impulse generated by an athlete's own body movement.In the same study, another functionality obtained occurs through the modification of nanofibers with lactate oxidase, which has the function of analyzing the concentration of lactate present in sweat in real time.This performance measurement technique can help determine the physiological demands and movement time of active athletes.In addition, it can be used in the scientific selection of outstanding athletes and in the development of individual sports training programs. 69he use of piezoelectric fibers can also be used for impact absorption.For example, in tennis rackets produced from piezoelectric fibers it is possible to reduce vibration and increase torsional stability, making them more comfortable for athletes.This is because the mechanical energy of the tennis ball impact is converted into electrical energy in less than 1 ms.Electric energy, unlike mechanical energy, does not produce vibrations.Piezoelectric fibers are inserted in the racket structure, reinforced by carbon fiber, while the electronics are hidden in the cable.In another example, piezoelectric fibers are integrated into a ski, consisting of an electronic management system.During sports activity, the mechanical stress generated in the fiber is converted into electrical energy.The electrical response changes the fiber structure increasing torsional stability, which gives the ski a better grip on snow, and consequently improves the skier's performance. 24

Innovations in the field of protection using piezoelectric textiles
An investigation reported a detailed technical development of the first aerospace electronic textile, with emphasis on piezoelectric fibers. 70The beta cloth simulant with piezoelectric fiber was introduced allowing an estimate for scaling the material across large area of spacecraft walls.This study also correlates the impactor speed and the energy contained in the resulting signature.

Acoustic sensors with piezoelectric textiles
Among several principles of transduction of acoustic sensors developed, the piezoelectric effect is the most used approach due to its high sensitivity to convert the vibration of acoustic waves into electrical energy and vice versa.Thus, piezoelectric materials have been adopted in underwater acoustic transducers.Another approach is the development of flexible acoustic devices used in the construction of acoustic detection networks for large areas with a high spatial and temporal resolution, such as in underwater fields.These acoustic sensors can be useful for monitoring ocean activities, such as offshore fish farms, autonomous underwater vehicle guidance, tidal/ earthquake identification, coastal surveillance, and underwater communications. 23lso in the same study, an acoustic detection device in the form of a bicomponent fiber produced using the melt spinning technique was presented with the aim of integrating different materials, such as a thin layer of piezoelectric polymer (PVDF-TrFE) and metal electrodes.The resulting piezoelectric fiber was able to efficiently detect underwater acoustic sources in 2-8 MHz frequency range with a noise rate above 20 dB, in addition to modulating multiple frequencies simultaneously. 23

Further research and challenges
The main interest in piezoelectric materials lies in the desire to develop portable devices that can combine a longer service life with a lower cost of the final product. 71,72In addition, reducing consumption or replacing batteries, especially chemical ones, become the main target for the use of these devices.
Piezoelectricity was initially detected in inorganic materials such as quartz crystal, discovered by the Curie brothers in 1880 and occurs mainly in semiconductor or nanostructured crystals. 71,73Other inorganic ceramics, such as perovskite and barium titanate (BaTiO 3 ) and aluminum nitride (AlN), for example, exhibit piezoelectricity due to their high piezoelectric coefficient in d 33 and d 31 directions and electromechanical coupling coefficient. 28,74These properties are important because it allows the evaluation of electric field generation under mechanical stress, as it quantifies the energy conversion efficiency. 74urrently, piezoelectric materials are classified into four main groups, crystals, ceramics, polymers, and composites. 71,75Crystals and ceramics have high piezoelectricity due to the organization of the crystalline structure and the greater arrangement of ions. 74Thus, inorganic materials present greater conversion of mechanical energy into electrical energy compared to organic ones.However, these materials have mechanical limitations due to the fragility of their structure.Scheffler and Poulin 28 details those inorganic materials, mainly crystals and ceramics, are brittle and fragile, especially in the form of fibers, which makes it difficult to apply mechanical stress to convert it into an electric field.
Polymeric materials stand out for their flexibility, low weight, easy processing and ability to be applied in different formats such as fibers, textiles, wires, and films, for example Zhang et al. 71 Piezoelectric polymers have been shown to be efficient acting as electronic textiles, as they are flexible and offer comfort to the user.Matsouka et al., 75 features polypropylene (PP) fibers acting in energy collection.Chakhchaoui et al. 76 describes PVDF nanofibers impregnated into fabric and flexible substrate as a replacement for sensors and wearable electronics.Nilsson et al. investigates energy capture properties using textile fibers in bicomponent PVDF format, combining fiber production with the weaving process.Finally, the author evaluated the ability to convert the mechanical deformation suffered by the material into electrical energy, obtaining a power of 0.7 mW. 77he advancement of technology and the need to develop more sustainable materials that are not dependent on limited energy resources and the miniaturization of devices to offer comfort to the user, piezoelectric textiles produced from polymers have stood out.In recent years, numerous researches have been carried out on piezoelectric textiles in the most diverse areas, especially material sciences, engineering, physics, astronomy, chemistry, and energy, as shown in Figure 2 ,78 for example, developed piezoelectric nanogenerators from coaxial wires of PVDF and flexible stainless steel.The main objective was to develop from two different techniques for coating the metallic electrode, by solution and torque spinning, a piezoelectric textile capable of generating an electric field. 78Jun Sim 79 and Peng et al. 80 invested in flexible nanogenerators from PVDF piezoelectric wires and silver-coated nylon fibers using techniques such as electrospinning to obtain nanofibers and nanowires with differentiated properties such as softness, flexibility, and constant piezoelectric voltage, with potential application as wearable sensors.
In all the researches presented, the biggest challenge is the limitation that polymeric materials have, which makes it difficult to achieve greater efficiency in energy conversion.Almusallam et al. 81 mentions that PVDF wires can be easily woven, but their piezoelectric coefficient is lower than those presented by ceramic materials, that is, the ability to convert mechanical stress into an electric field is very low.Zhang et al. reports that the piezoelectric properties of polymers are mainly affected by the semicrystalline orientation of their structure.PVDF, for example, requires greater alignment and orientation of its molecules for the most piezoelectric phase to occur, the β phase. 71nother determining factor for the functioning of piezoelectric polymers is in the processing in which the materials are submitted for the development of the properties.In some polymers, such as PVDF, it is necessary to stretch the chains, the properties are observed mainly when the polymer is in the form of fiber, nanofibers, and threads.Zhang et al. 71 described that to increase the β phase of the PVDF, processes of electrical polarization, mechanical stretching of the chains and production of composite with addition of charges were carried out, obtaining success in all attempts.Ramadan et al. complete that in other polymers, piezoelectricity can occur in polymeric films from the structure and molecular arrangement, in polymeric structures in the matrix and application of piezoelectric ceramic fillers.In addition, polymers with voids in their morphology due to air outlets, can present piezoelectric effects from the change in their polarization and semicrystalline and amorphous polymers, which have molecular structure with dipoles, display piezoelectricity from the reorientation of molecules. 74ue to the limitations between the integration of electronics and textiles, the application of new approaches to the use of fibers with different functionalities has been shown to be effective to be worked as piezoelectric sensors.In the work reported by Martins et al., 82 for example, a study was carried out on the development of new filament geometries from the process of co-extrusion of PVDF wires with conductive layers for application as piezoelectric textiles.
The idea of capturing mechanical body movements to generate energy has also been the subject of several studies.Boutaldat et al. used this approach for the development of a flexible textile substrate.The device obtained was integrated into the shoes to capture the movements of the feet during the steps and convert it into electrical energy. 83hveda et al. 84 also applied the smart textiles in shoes for energy harvesting while walking.The energy generated during the movement was able to power an arm lifting device, which acts as a robotic arm.
Textile materials with piezoelectric properties have been widely applied in clothing as sensors, but there is a constant concern about the need for washing after use.Ju et al. exposed that wash resistance is one of the biggest challenges faced to develop wearable piezoelectric devices.To reduce the problems presented, the authors created the device with plasma treatment, making the surface superhydrophobic and self-cleaning. 852][93] In addition, large-scale production is still industrially challenging due to the necessary specific conditions of the process, which can be: high temperature, gas control, vacuum, among others. 94Chen et al. reported that traditional piezoelectric polymers are processed from additive manufacturing, mainly from spinning, electrospinning and 3D printing.These processes offer advantages such as the possibility of developing complex structures, dimensional control, improved response of piezoelectric constants and electromechanical coupling, and finally enabling the integration of electronic devices. 95n this context, it is necessary to intensify studies related to the optimization of process conditions, in order to improve piezoelectric properties of textile polymeric devices.

Conclusion
This article contextualized the obtainment of textile materials with piezoelectric properties, highlighting some major areas of application of these materials and briefly presenting recent studies in the respective areas, namely: health, civil construction, clothing and accessories, sports, protection, and acoustic sensors.From the case studies presented, the extensive potential of ST, and the increasing relevance of monitoring and communication, regardless of the area of application, can be confirmed.Furthermore, it is noteworthy that the development of materials with piezoelectric property contributes to the generation of energy from renewable and green sources, contributing to a more sustainable product development and consequently with a positive environmental impact.

Figure 2 .
Figure 2. Main application of piezoelectric ST.

Table 1 .
Brief review of related studies on piezoelectric devices.