1 Introduction

Smart wearable devices are classified into four main groups, (i) near-body electronics, (ii) on-body electronics, (iii) in-body electronics, and (iv) electronic textiles. This definition is in accordance with the international electrotechnical committee (IEC) and more details can be found in Fig. 1a. Furthermore, the evolution of wearable technologies as a function of materials and devices types is illustrated in Fig. 1b. Considering the used materials, wearable devices can be classified in three categories (Zulqarnain et al. 2020): (1) rigid devices, where all active components are rigid silicon integrated circuits (ICs), (2) hybrid devices, where sensors and/or front end elements are based on flexible materials/substrates while the rest of the system is implemented with rigid silicon ICs, and (3) flexible devices, where all components are built using flexible electronics. Currently, there is commercial availability of conventional wearable devices that are used as accessories for different applications, such as smart glasses, smart watches, smart shoes, medical devices, among others. Figure 2a illustrates some examples of such devices. There is also a lot of research and development focused on the design, fabrication and characterization of innovative wearable sensors that can be used as implants, for example, those shown in Fig. 2b. These advances in wearable sensors were only possible due to the recent progress in the synthesis and characterization of new emerging smart materials in micro/nanoscale, which detect and communicate changes in temperature, touch, pressure, movement and humidity in real time.

Fig. 1
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a Classification of wearable smart devices by the IEC (The International Electrotechnical Committee), b progress of wearable device technologies

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Fig. 2
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Examples of types of wearable devices, a accessories, and b implantable

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Therefore, flexible wearable devices (FWDs) are essential components of the flexible electronic technology because they have great potential for health monitoring and medical applications (Liu et al. 2017a; Wang et al. 2017a). The commercial scale-up of FWDs will depend on the successful development of flexible sensors, including pressure and temperature sensors (Graz et al. 2009; Gao et al. 2019), strain sensors (Zhou et al. 2008; Jung et al. 2019), and humidity sensors (Xuan et al. 2015). However, wearable technologies will become real progress from the point where components based on flexible materials are well enough integrated into items that can be used daily, such as our clothes. With the growing interest and rapid development of industrial and academic active research in flexible and stretchable strain/pressure sensors, wearable technologies tend to consolidate in a few years.

As seen in recent years, several flexible sensors with excellent performance have been reported. At the same time, flexible sensors have been applied to achieve self-healing (Huynh et al. 2017; Zhu et al. 2020), for drug monitoring and screening, for self-powered wearable health and biomedical sensing devices (Wang et al. 2019a), and has also been integrated with several other types of sensors. Several reviews have been published on flexible electronics, flexible sensors and their applications on wearable devices. Table 1 summarizes the recent review papers. As can be seen, the core of these articles is in the self-healing, biomedical sensors, soft electronics based on liquid metals, piezoelectric and triboelectric energy harvesting, actuators and sensors, piezoresistive and capacitive pressure sensors, electronic skin sensors, and electrochemical sensors for drug monitoring. However, few studies explicitly focused on the discussion of materials used, fabrication processes, and integration of piezoresistive sensors. It can be noted in Table 1 that 2021 was the year of review articles. This effect is a consequence of the pandemic caused by COVID-19 that generated a global lockdown, preventing the sequence of face-to-face scientific research. Thus, the beating mind of most researchers focused their efforts on publishing review articles. With this initiative, academic productivity was assured, and the thoughts were occupied to alleviate all the harmful consequences of the pandemic. Piezoresistive sensors (PRSs) are electromechanical sensors based on transduction mechanism called piezoresistivity (Santos et al. 2020; Li et al. 2020a). In addition to it, there are two other well-known electromechanical transduction mechanisms: piezocapacitance and piezoelectricity/triboelectricity (Santos et al. 2020; Li et al. 2020a).

Table 1 Summary of the recent review papers on wearable technologies
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It is worth mentioning that the electromechanical sensors convert mechanical signals into electrical ones by generating electrical charges under pressure/strain loads, which change the resistance or capacitance of electromechanical sensors. Among them, the PRSs are low-cost, robust, durable, easy to construct with precise reading circuits besides have a high sensitivity and fast reaction time in response to pressure/mechanical strain applied. All the qualities mentioned above make the piezoresistive materials attractive and bring tremendous attention for broad applications (Santos et al. 2020; Li et al. 2020a; Zheng et al. 2020). Table 2 compares the pressure sensitivity and reaction time for piezoresistive, piezoelectric, and piezocapacitive sensors constructed using different materials. It is noted that the devices operated in piezoresistive effect demonstrated excellent pressure sensitivity and time response in comparison with the values obtained by piezocapacitive and piezoelectric sensors (PESs). This behavior puts piezoresistive technology (PRT) in advantage for applications in monitoring devices of physical stimuli in real time. It is worth highlighting that Table 2 presents high-performance PRSs fabricated with a variety of raw materials. For example, Gunasekara et al. (2020) used micropatterned polypyrrole film as piezoresistive material (PRM) and achieved a sensitivity highest sensitivity was 10.69 ± 1.53 kPa−1 with 1 kPa and its lowest and highest limits of pressure detection were recorded as ranging from 0.05 to 119 kPa. On the other hand, Pan et al. (2014) used 3D hybrid porous Mxene-sponge to reach values of 133 kPa−1 and 50 ms.

Table 2 Summary of the sensing performance of piezoresistive, piezocapacitive and piezoelectric sensors constructed using different materials
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The best sensitivity results for piezocapacitive sensors, 15 kPa−1 and 4.2 ms, were achieved by Li et al. (2017) using iontronic nano-fabric. Optimized sensitivity for PESs was reported by Zhang et al. (2019a), 5.8 kPa−1 and 190 ms, using lead zirconate-titanate (PZT) thin films. Also, as can be seen in Table 2, the PRT offers sensing performance strain over a wide range (< 1–147 kPa−1), in addition to short response times (0.2–138 ms), allowing for reliable sensing performance.

Due to the lack of manufacturing complications, high sensitivity, fast reaction time, low cost, and durability, flexible and elastic PRSs have become crucial for smart wearable electronics. With the advent of the Internet of Things (IoT) applied to Industry 4.0, sensors based on piezoresistive effects have become vital for monitoring people’s health and controlling machines. Therefore, wearable piezoresistive sensors are a current trend and a relevant research topic. This fact can be seen in Fig. 3, which illustrates the number of publications with the term “Wearable Piezoresistive.” CrossRef Metadata platform was used to carry out this survey, and as can be seen, in 2012, there were only 366 publications with this focus. When it looks at it ten years later, there is an increase of approximately ten-folds the number of publications. This increase year after year shows the importance of wearable piezoresistive sensors in our daily lives.

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Number of publications each year on wearable piezoresistive sensors

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Based on this introduction, this review paper aims to show a comprehensive status and perspectives of the flexible and stretchable PRSs. This was divided into three main parts: (i) First, it focused on the distinct types of materials used in the construction of these sensors and, to make easy the compression, it was divided into four categories organized by dimensional structures, namely, zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D), (ii) Second, the fabrication processes and integration of PRSs are discussed and (iii) Finally, the last part presents some examples of wearable PRSs and their applications.

2 Nanomaterials for wearable piezoresistive sensors

This section exhibits the current status and challenges of the bottom-up-synthesized and top-down-processed nanomaterials for wearable piezoresistive sensors (WPS). For a better understanding of the synthesized and processed nanomaterials for WPS, this section was divided into four categories organized by dimensional structures, i.e., 0D, 1D, 2D, and 3D. The main example of 0D nanomaterials are the nanoparticles (NPs) based on metal compounds. In turn, 1D nanomaterials are represented by nanotubes such as carbon nanotubes (CNTs), 2D nanomaterials can be produced by dichalcogenide nanosheets or an allotrope of carbon consisting of two-dimensional honeycomb lattice called graphene, and 3D nanomaterials are based on conductive foam, for example.

2.1 Zero-dimensional piezoresistive nanomaterials

Metallic NPs have drawn tremendous attention in the fields of nanobiotechnology and nanotechnology due to their exceptional properties stemming from the nano-thickness, nano-width, and nano-length of the particles. In WPS applications, these NPs are often incorporated into a polymeric matrix or flexible membranes to maintain good flexibility, a good Young’s Modulus, while maintaining an excellent degree of piezoresistivity (Li et al. 2020b; Chen et al. 2019). Thus, 0D nanomaterials have acquired growing interest due to their capability to achieve greater sensitivity in WPS, holding the flexibility and electromechanical stretchability of the flexible matrices (polymers or membranes), enhancing the measurement of electrical signals through physical stimuli (Li et al. 2020b). When the external pressure (physical stimuli) varies slightly, the piezoresistive process does not sense the breaking of the ohmic contact or new defects formed in the material (Ramuz et al. 2012; Someya et al. 2004). Due to this fact, it can be supposed that 0D nanomaterials help the WPS to sense the least internal resistance caused by external stress, allowing devices to reach very high resolutions (Chen et al. 2019). Therefore, this mechanism that occurs between small distances that separate the NPs is responsible for controlling the tunneling and hopping processes among adjacent electrons into the PRSs (Sheng and Abeles 1972, 1973).

Figure 4 presents the most used NPs on piezoresistive, capacitive, and piezoelectric sensors. In PRSs, gold nanoparticles (Au-NPs) are widely used in flexible matrices. Farcau et al. (2011) integrated Au-NPs in flexible polyethylene terephthalate (PET) and used electron tunneling to fabricate highly sensitive resistive strain gauges without any lithographic pre-patterning. Therefore, the simplicity of fabrication through convective self-assembly makes this approach potentially useful for developing other types of PRSs. Jiang et al. (2015) used Au-NPs in thin film strain sensors on flexible polyimide substrates. The PRSs presented disorderly interparticle tunneling that introduced a microscopic deviation of the charge conduction. They observed a considerable change in resistance when the current flow is perpendicular to a unidirectional strain. For daily usages, the strength and stability of these PRSs were validated with 70,000 cycles of bending. Lee et al. (2015) embedded Au-NPs and silver nanoparticles (Ag-NPs) in flexible insulating polyurethane (PU) elastomer. They combined Au-NPs and Ag-NPs to make piezoresistive pressure sensors composed of sea-urchin-shaped metal nanoparticles (SSNPs). The sensor has a sensitivity of 2.46 kPa−1 with a response time of 30 ms with a quantum tunneling effect between SSNPs and flexible PU.

Fig. 4
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Main metallic nanoparticles used on wearable sensors

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Recently, Wang et al. (2020a) reported the fabrication of PRSs based on Au-NPs assembled and interconnected with graphene microchannel-embedded PDMS film. The 3D coating consists of porous nickel foam/Au-NPs/graphene/embedded PDMS, being a useful and sensitive pressure and strain sensor. The sensors show a broad detection range (50 kPa) and a sensitivity of 5.37 kPa−1 in the range from 10 to 50 kPa with a response time equal to 20 ms and strain factor up to 15. Compared with 1D films (porous nickel foam/Au-NPs/graphene/embedded PDMS), the sensitivity improved 40-fold between 0 and 1 kPa pressure range. These characteristics put this device as a candidate for wearable biomedical and traditional sensors.

In addition to Au-NPs, the Ag-NPs on flexible substrates are also widely used. As gold, the silver is a transition metal with exciting properties to apply on piezoresistive devices. Lee et al. (2014a) reported a combination of a roll-to-roll (R2R) slot-die coating and laser direct-writing process to fabricate uniform Ag films embedded with Ag-NPs over a broad area of flexible substrates of polyimide (PI) and PET with low sheet resistance. They showed that the gauge factor for these PRSs is 40% higher than the tested commercial product, in addition to demonstrating excellent linearity over a range of static loads regardless of the initial resistance. Hoang et al. (2016) fabricated a simple, low-cost, and easy to scale up PRSs based on Ag-NPs/CNTs involved in PI/polydimethylsiloxane (PDMS) substrates, resulting in durable sensors that remain active after 500 stretching cycles (0.5% of maximum strain) without sensitivity degradation. As an impressive result, they showed that the Ag-NPs/carbon deposited on the PDMS/PI substrate presented a substantially broad resistance change about 22-fold greater than film on the PI substrate. Shen et al. (2020) improved sensitivity of PRSs based on a traditional polyanion hydrogel-poly(acrylic acid) (PAAc) introducing a pre-compressed double-cross-linked (DC) lignocellulosic hydrogel with uniformly and regularly distributed big pores coated with polydopamine (pDA) followed by in situ deposition of Ag-NPs. They reported excellent properties, such as water content of about 82% with abundant ions, superior mechanical properties, maximum strain from 76.8 to 94.8%, and sensitivity up to 9.34 MPa−1. Ko et al. (2021) fabricated a sensitive PDMS-based WPS based on Ag-NPs/CNT films prepared by a low-cost, environmentally-friendly, and simple scalability at an industrial level approach of tip-sonification spray process. They observed a sensitivity approximately from 0.02 kPa−1 at low-pressure (< 11.67 kPa) and 0.004 kPa−1 at high-pressure (~ 11.67–33.3 kPa) better than the sensitivity of polymer-based pressure sensors. Another critical point is the mechanical reliability exhibited under loading–unloading pressures of 6.67 kPa and 16.67 kPa tested on wearable sensors.

Besides Au-NPs and Ag-NPs, another two transition metals, namely, platinum (Pt) and Palladium (Pd), emerges as 0D nanomaterials applied on flexible piezoresistive sensors. Hwang et al. (2018) used Pt-NPs embedded on hydrogels based on ethylenediamine (EDA) and functionalized graphene oxide (GO) to improve the piezoresistive effect of the material. Pt-NPs incorporated into EDA-GO hydrogel improved the linear strain range by 52.8% and raised the compressive modulus by 873% due to the multi-scale reinforcing. This mechanism of reinforcement was attributed to the powerful interaction between Pt-NPs and graphene platelets. Hence, the piezoresistivity of the hydrogels can have a great appeal in the field of high-performance pressure sensors. On the other hand, Radha et al. (2011) fabricated flexible semitransparent PRSs based on Pd-NPs on micro molded carbon-micro-stripes deposited on PI substrates. They showed a direct relationship among Pd-NPs size, base resistance, and gauge factor. When the nanoparticle size varied from 2 to 25 nm, the base resistance increases drastically from 19.3 Ω to 15.68 × 106 Ω. Another parameter that grows with a decrease of nanoparticle size is the gauge factor, which varied from 0 to 176. Therefore, they report a method that produced flexible PRSs and, on the other hand, flexible conduits with unchanging resistance, adjusting only the size of the nanoparticles.

It is worth mentioning the work of Rizvi and Naguib (2014), where carbon nanoparticles (C-NPs) were studied in polyethylene (PE) composites. The C-NPs fillers investigated were single-wall carbon nanotube (SWCNT), multi-wall carbon nanotube (MWCNT), and graphene nanoplatelets (GNP). They demonstrated that the piezoresistance response to MWCNT had a strong dependence on stress and filler content. In contrast, the reaction of SWCNT was stable with a networked state of nanoparticles.

As a perspective of application in PRSs, we can mention the zinc oxide nanoparticles (ZnO-NPs) and cadmium sulphide nanoparticles (CdS-NPs), which were reported by Prashanthi et al. (2011) and Goud et al. (2016), respectively. In a theoretical article, Clayton et al. (2020) explored a new numerical approach to explain an unexplained physical behavior that can be used to design PRSs with specific characteristics. They used a basic model using quantum tunneling, and the percolation theory focusing on a simple analytical model for isotropic orientation distribution and subsequent Poisson-based gap evolution. The model used a nanocomposite sensor comprised of a silicone matrix with nickel nanoparticles (Ni-NPs), which demonstrated a great potential for application in PRSs.

2.2 One-dimensional piezoresistive nanomaterials

1D nanomaterials can be divided into (i) nanowires based on metallic, metal oxide, and conducting polymer; and (ii) nanotubes based on carbon, called carbon nanotubes (CNTs). Due to the high-aspect-ratio (HAR), exceptional optical, electronic, and mechanical properties, 1D nanomaterials can contribute to the development of wearable devices by raising the optical and electronic performance. They also have a lower fabrication cost and can expand the possibilities of integration with other nanomaterials (Gong and Cheng 2016). Another unique property that can be highlighted is the high persistence length that provides direct transport of the charge and reduces the grain boundaries. This property decreases the considerable dependence of resistance on the strain and overcomes the issues caused by gaps and cracks in the material (Qi et al. 2020). Based on these properties, this subsection focuses on CNTs which are the most 1D nanomaterials used as conductive filler incorporated in conductive polymeric matrices used in PRSs.

Table 3 summarizes the main articles that used CNTs in PRSs published recently. As can be seen, the most used matrices include polymers and elastomers, namely, poly(vinylidene fluoride) (PVDF), polymethyl methacrylate (PMMA), acrylonitrile butadiene styrene (ABS), PDMS, Ecoflex, PU, elastomer–styrene–butadiene–styrene copolymer (SBS), and Epoxy. However, recent progress in the synthesis, architecture and characterization of novel conductive polymeric matrices shows new perspectives for CNTs applications. For example, Qi et al. (2020) used CNTs-embedded PU nanofiber (PU-NF) to fabricate PRSs. They took advantage of the coaxial structure (CNTs/PU-NF) to promote the retention of intimate contact between the CNTs/PU-NF layer and the core electrode during the strain and fabricated a multimodal textile sensor, which is woven by smart nanofiber sensing yarns based onto a stretchable fiber-shaped core electrode through a simple electrospinning process. Tran et al. (2020) developed a hybrid nanocomposite based on graphene combined with carbon nanotubes to build a robust hierarchical conducting architecture. This nanocomposite was used to fabricate quantum resistive pressure sensors (QRPS). The proof of concept of pressure monitoring and mapping, with a flexible and integrable array of four hybrid QRPS, demonstrated real-time sensing capabilities. An example of novel conductive matrices was showed by Kumar et al. (2020) who applied CNTs to reinforce nanofiller in high-temperature vulcanized (HTV) and room-temperature vulcanized (RTV) silicone rubber. Their results showed a higher gauge factor with RTV silicone rubber compared to HTV silicone rubber, and both showed stretchability > 100%. Sencadas et al. (2020) reported how biodegradation improved the sensitivity and electromechanical performance of piezoresistive sensors. They used the poly(glycerol sebacate) (PGS) elastomer as a matrix, and the porous sensor was synthesized and blended with CNTs and sodium chloride (NaCl). As a result, a linear behavior over an extensive range of pressures was reached. An increase in their sensitivity from 0.12 to 8.00 kPa−1 was achieved after eight weeks in a simulated body fluid media.

Table 3 Summary of recent publications on PRSs using CNTs and their, respectively conductive polymeric matrices
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2.3 Two-dimensional piezoresistive nanomaterials

The 2D nanomaterials applied in PRSs can be divided into (i) dichalcogenide nanosheets and (ii) a carbon allotrope that consists of a two-dimensional honeycomb structure called graphene. In particular, graphene has attracted attention since the segregation of its single layer in 2004, through the novel mechanical exfoliation technique developed to synthesize it (Novoselov et al. 2004). Due to the 2D structure with carbon atoms (monolayer) arranged in hexagonal network, particularly suitable and extraordinary characteristics have emerged for flexible PRS applications, including exceptional mechanical, electrical and thermal properties, besides tunable 2D assembly, lightweight, flexibility, and high transport properties (Zheng et al. 2011; Niu et al. 2018; Mohan et al. 2018).

As occur to CNTs, the most used conductive matrices for PRSs developed with 2D nanomaterials including polymers and elastomers, namely, PVDF (Costa et al. 2017), PDMS (Rajitha and Dash 2018), Ecoflex (Zhang et al. 2019b) (Joo et al. 2015), Silly Putty (Boland et al. 2016), poly (lactic acid) (PLA) (Scaffaro et al. 2018), and other blends of elastomers. In most cases, PRSs based on graphene/polymer composite are more sensitive due to the large surface area of 2D graphene and its high carrier mobility (Chen et al. 2017b).

Recently, to promote the improvement of graphene-based PRS, Cao et al. (2020a) framed a loofah sponge and coated it with graphene-modified by carbon black nanoparticles (CB-NPs) using a facile and environment friendly solvothermal method. They showed that CB-NPs decreased the contact resistance between the graphene sheets and improved the sensitivity and conductivity of 3D PRS. Sankar et al. (2020) used hydrogen-exfoliated graphene (HEG) to fabricated PRSs. They used a waterproof and flexible PVDF polymer as conductive material and incorporated the polystyrene sulfonate sodium salt (PSS) polymer-functionalized HEG in a PVDF-based flexible PRSs. The electrical conductivity of PVDF/PSS-HEG-based is two-fold higher than the PVDF/HEG-based, and the PSS-HEG promotes stable dispersion in the hydrophobic polymer matrix without harming the intrinsic characteristics of the device. Cao et al. (2020b) developed a novel nanofiber reinforced graphene aerogel (aPANF/GA). They showed that PRSs based on aPANF/GA have an excellent sensitivity of 28.62 kPa−1 with fast response time (37 ms), and the structural stability and durability were present after 2600 cycles. Zhu et al. (2019) prepared a PRSs based on hollow MXene spheres/reduced graphene composite aerogel and flexible interdigital electrodes. This process exhibits 609 kPa−1 of sensitivity in the range of 6.4–10 kPa, a short response time of 232 ms, and durability of 6000 cycles. An interesting multi-application was shown by Jing et al. (2020); the PRSs were applied to detect blood pressure, heartbeat and human movements, including finger bending, elbow movement, and knee squatting. They fabricated a PRS based on graphene-PDMS sponge with high elasticity (strain up to 85%), high sensitivity (0.075 kPa−1), a wide responding range (0–50 kPa), and increased stability (2000 cycles pressure test). These results showed the right consistency and stability of the as-prepared sensor through the sponge, PDMS, and graphene synergy. Khalifa et al. (2020) manufactured a PRS and fast response humidity sensor based on graphene-coated cellulose paper. The humidity sensor obtained a response 290% greater than the uncoated cellulose paper and the piezoresistive response was 250% greater than the uncoated one. The low-cost, lightweight and biocompatible cellulose paper combined with graphene shows to be a promising material for smart and wearable electronic devices (Chen et al. 2020).

2.4 Three-dimensional piezoresistive nanomaterials

3D nanomaterials are based on conductive foam with a pore surface decorated with 0D, 1D, and 2D nanomaterials. The advantages of foams are low-cost, fast and scalable features, and easy fabrication methods. For example, Wu et al. (2017) reported a new and easy strategy for making highly elastic channel crack-based Au-NPs/PU foam piezoresistive material. They used a flexible 3D conductive network prepared by gold ion sputtering, being the channel cracks are designed on the 3D sponge skeletons. The PRSs had excellent elasticity, a response time of 9 ms and an ultra-low detection limit of 0.568 Pa, with good reproducibility up to 1000 cycles. Owing to the new structure, these PRSs can monitor both short and long human movements that derive from the nano crack joint sensing mechanism and physical contact of conductive interconnected networks. De Meo et al. (2020) used CNT embedded into a polymeric foam (PU) that showed an increase in electrical and mechanical properties. They manufactured three kinds of PRSs based on PU-CNT foam, (i) PU-CNT 1.5%, (ii) PU-CNT-COOH 1.0%, and (iii) PU-CNT-COOH 1.5%. The samples with COOH presented an insulating-conductive transition phenomenon at 80% of its compression strain with a gauge factor of 30. Instead, the samples without COOH showed conductivity at 1.5% of filler concentration and a steady piezoresistive behavior with a gauge factor of 80. Fortunato et al. (2020) fabricated PRSs bases on novel 3D Ecoflex open-cell foams using different concentrations of graphene nanoplatelets (GNPs). As a result, it was noted that PRSs infiltrated with 1 mg of ethanol/GNP solution with a GNP concentration of 3 mg/mL was more sensitive than those implanted with the same amount of ethanol/GNP solution and a lower concentration of GNP. It was observed that the PRSs were sensitive at very low deformations with a maximum sensitivity of 0.28 kPa−1 for pressures lower than 10 kPa.

3 Fabrication processes for wearable piezoresistive sensors

In addition to the nanostructured materials used in wearable piezoresistive sensors, there are still challenging issues for the implementation of wearable sensors on an industrial scale. For example, many of the flexible and extensible wearable sensors so far involve complicated or cumbersome manufacturing processes, such as multiple coating cycles, printing or photolithographic techniques and deposition of conductive materials. Moreover, since most wearable sensors are composed of metal-based micro/nano structures and hybrid composites with nanomaterials (such as metallic NWs and graphene sheets), the fabrication process requires high temperature (such as chemical and physical vapor deposition), electroplating, and delicate transfer processes (Huang et al. 2019). In this topic we will present some of the main techniques used in the manufacturing process that were broken down as: Surface decoration methods and Porous architecture methods. This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.

3.1 Surface coating (decoration) methods

3.1.1 Electroplating

Electroplating, also known as the electrodeposition, is the process of depositing elemental or compound metals or semiconductors on a conducting substrate by passing an electric current through an ionic electrolyte in which metal or semiconductor ions are present. In sum, it is a galvanic or electrochemical cell that acts in reverse (Yang et al. 2018; Ojo and Dharmadasa 2018).

Recently, Li et al. (2020b) used electroplating PPy decorated sponge with multiscale porous structures to extend the pressure sensing range of PPy-based piezoresistive sensors. In other recent study, Wang et al. (2020c) presented a flexible device integrated with piezo-resistive sensing element and voltage-sensing element to simultaneously measure human respiration and electro-cardiac signal. When applied to the measurement of respiratory pressure, the piezo-resistive performance of the device was enhanced by electrodeposited nano-copper.

3.1.2 Electropraying

Electrospraying, also known as electrodynamic spraying, is capable of producing diminutive droplets with submicron sizes by means of an electric field. For the purpose of nanoparticle production, the common setup of electrospraying consists of a high-voltage power supply, a plastic/glass syringe capped by a metallic capillary to hold a polymer solution, a syringe pump to control the flow of the solutions, and a grounded collector. When a high electric field is applied at the needle, a charged liquid jet will break up into droplets, which form small particles with generally narrow size distribution on the collector (Wang et al. 2019b).

Recently, Masa et al. (2020) investigate the electrospray technique for the preparation of graphene layers for use in chemiresistive gas sensors. They dispersed reduced graphene oxide (rGO) in isopropyl alcohol (0.1 mg/mL) that was electrosprayed and the rGO flakes are deposited onto a polymeric substrate with printed interdigitated electrodes.

3.1.3 Vapor deposition techniques

The majority of Physical or Chemical Vapor Deposition methods (PVD or CVD) has been tailored to create 1D nanoscale materials such as nanotubes, nanorods or nanowires, using the original geometric shape of specific nanomaterials as templates or scaffolds (Rodrigues et al. 2019). This field is still evolving, with several fabrication methods reported in the literature for the synthesis of nanostructures with great potential for technological applications. For the purpose of freestanding materials, the vapor deposition technique must give excellent conformality and deposition control at the nanoscale level (Dias et al. 2020; Chiappim et al. 2020a, b). Among them, the atomic layer deposition (ALD) proved to be the key to this specific problem. ALD is a versatile technique that can be used to coat and functionalize surfaces with single or multiple components (Pessoa et al. 2019). The foremost advantage of ALD is its ability to control the coating thickness down to the subnanometer range as a function of the number of reaction cycles (Chiappim et al. 2021). ALD does not depend on the line of view to reach secluded spots; thus, it can be applied to almost any kind of high aspect ratio structures producing pinhole-free conformal coatings and chemically bonded layers by the use of sequential self-limiting surface reactions and low temperatures (Rodrigues et al. 2019). Several metals, metallic oxides, nitrides and sulfides can be produced by ALD, however, there is a limitation of the technique for the synthesis of carbides and carbonaceous materials (Fraga and Pessoa 2020, 2021; Chiappim Junior et al. 2021; Pessoa et al. 2018). Such difficulties are believed to be overcome in the next decade with the advancement of technology and chemical routes between precursors.

Finally, although CNTs and graphene could be generally fabricated with CVD method, the complexity of experimental conditions limited their mass production and practical application.

3.2 Porous architectures methods

3.2.1 Electrospinning

In an electrospinning process, high voltages are applied to the droplets at the nozzle; when the electric force becomes larger than the surface tension force, a jet shoots out from the Taylor cone. The diameter of the fiber is usually much smaller than ones made from 3D printing, i.e., of the order or lower than micrometers (Huang et al. 2019). The experimental setup for this process is similar to that of electrospraying, the difference is concerning the concentration of the polymer solution, where it dictates the structures of the resulting materials. Spherical particles are observed at lower solution concentrations. As the concentration approaches a critical value, column strand morphologies are usually generated, that is, electrically spun fibers with particles. Above the critical concentration of the solution, uniform fibers can be produced.

Electrospinning process can be used in the fabrication of nanofibers (NFs) or microfibers composed of numerous materials, such as elastic polymers, conductive materials, and graphene (Kweon et al. 2018). In addition, electrospinning is suitable for rapidly producing large-area membranes with adjustable properties (high surface area, pore volume, and material type) under ambient conditions (Lou et al. 2016). Flexible and porous polymer mesh fabricated through electrospinning can function as a layer of dielectric or piezoelectric materials with tunable properties. The nonconductive polymer mesh can be converted to piezoresistive materials through carbonization (Wang et al. 2017b) or surface coating (Cheng et al. 2017). A recent review highlighted that the electrospinning technique is a promising method for constructing 3D graphene-based structures for wearable piezoresistive pressure sensors (Cao et al. 2021).

3.2.2 Roll-to-roll inkjet printing and screen printing

The roll-to-roll (R2R) inkjet printing (non-contact) and screen-printing (contact) processes are particularly suitable approaches for fabricating 2D patterns, such as flexible sensor array (Riza and Du 2020) and sensing electrodes (Chang et al. 2009).

Inkjet printing uses a digital image signal to print the design by propelling ink droplets onto the substrate instead of any physical image carriers. Typically, in inkjet printing, the inks will be in a liquid state with low viscosities and can be directly printed onto the substrate (Maddipatla et al. 2020).

Screen-printing is typically done either by hand or using a semi or fully automated system. The screen printer consists of a squeegee, stencil, and a screen with the design on it. Typically, screen fibers are made of plastic, natural silk, or metal fibers, and the squeeze is made of rubber. A section in the screen will be imposed by the design of the desired print and the ink is allowed to pass through it, and the desired design is created on the substrate being used. The ink is transferred either by pushing or forcing the ink by squeezing through the screen, and thus, this printing process is also referred as push through process. The print quality mainly depends on the mesh count, wire diameter, emulsion thickness, off-set height, and deflection angle of the screen (Maddipatla et al. 2020).

The advantage of the screen-printing technique over inkjet is mainly the ability to print in a large area, which facilitates its implementation at an industrial level. However, the screen-printing technique still has relatively low resolution, which makes it difficult to miniaturize devices.

Concern the use of R2R printing technology in the synthesis of wearable piezoresistive sensor onto polymeric substrate, Lee et al. fabricated all-printed high-density SWCNT thin film transistor (TFT) arrays, where the pressure sensing function was achieved by integrating with piezoresistive rubbers (Lee et al. 2016). The recent review of Maddipatla et al. (2020) presents the recent progresses in manufacturing techniques of printed and flexible sensors.

Recently, a few ultrahigh-resolution R2R systems have been developed based on flexures and advanced control, which may be used to produce next-generation high-resolution pressure sensor arrays (Huang et al. 2019).

3.2.3 Hydrogel

Hydrogels show a strong capability to absorb a high amount of water, and possess biological and elastic (i.e., softness) compatibility: these are desirable features for biological applications in wearable chemical (bio)sensors, which require mechanical flexibility to result comfortable to the body (Scarpa et al. 2020). Several designs of resonating MEMSs, in particular piezoresistive-based MEMSs, have been proposed and realized in combination with hydrogels (Millet et al. 2012).

4 Examples of wearable piezoresistive sensors and their applications

An interesting application of wearable piezoresistive sensors has been reported by Jeong et al. (2009) that evaluated the accurate performance of this device type for monitoring biosignal. In this study, the piezoresistive fabric sensors were prepared by knitting a single conductive yarn on the shirt and tested by detecting the respiratory effort signal through the thorax movement. It was observed that the results of the respiratory rate are in a quite good agreement with the reference values even in the fast running movement. In a previous publication, Huang et al. (2008) demonstrated the feasibility wearable yarn-based piezoresistive sensor to track the respiratory signals precisely. Besides, they report the performance testing of yarn-based piezoresistive sensors, the fabrication process of these devices using piezoresistive fibers, elastic, and regular polyester fibers is discussed in detail. Still on the use of wearable piezoresistive sensors in biosignals monitoring, another interesting example is in automobile safety belt as reported by Hamdani and Fernando (2015). These authors proposed a cardiorespiratory sensor system prototype constructed in an automobile safety belt using a copper ink based nonwoven material, which works based on the piezoresistive effect due to the pressure exerted on the sensor as a result of expansion of the thorax/abdomen area of the body for respiration and due to the principle of ballistocardiography (BCG) in heart signal sensing. More recent, Wang et al. developed a wearable piezoresistive sensor capable of capturing cardiorespiratory signals fabricated by sandwiching a nano silver electro-conductive ink impregnated nonwoven material between two layers of knitted silver fabric that function as sensor electrodes. The analysis of the performance of this sensor, together with a comparison against a commercially available, portable, cardiorespiratory signal monitoring device, shows that it is a highly sensitive, accurate and reliable device for capturing cardiorespiratory signals (Wang et al. 2018b).

Recent publications in the monitoring of physiological parameters using wearable piezoresistive sensors are numerous, as for example the article “Smart Textile Based on Piezoresistive Sensing Elements for Respiratory Monitoring” (Massaroni et al. 2019), which discussed the use of piezoresistive elements to develop a smart textile for respiratory monitoring concluding that it is suitable for respiratory frequency monitoring in a wide range of values. There is also a recent study on the application of knitted piezoresistive smart chest band for respiration patterns assessment (Raji et al. 2019). Experiments were conducted under several test conditions and results showed a successful application of this chest band with result patterns being in congruence with studies conducted using other related respiratory measurement technologies and instruments. Moreover, Choundry et al. (2020) have proposed to develop wearable piezoresistive sensors based on e-textiles fabricated using a new easy scalable stitching technique, which allows to create free-form sensor designs. After the extensive characterization, the developed sensor was embedded inside a garment and used to measure small pressure changes exerted by human muscles. The tests showed that this low cost piezoresistive sensor is capable of monitoring breathing rate through ribcage and step counting through hamstring muscle. One last example of wearable piezoresistive devices for human physiological signals monitoring is a flexible piezoresistive sensor based on conducting polymer-coated fabric developed by Lin et al. (2020). This sensor can be attached on different positions of body and applied to recording physiological signals. Its advantages include rapid response, recovery speed and excellent durability.

Another important field of application for wearable piezoresistive sensors is human body movement monitoring. Orengo et al. (2010) reported wearable devices instrumented with commercial piezoresistive sensors for human body movement tracking. They developed a simple and accurate RLC model fitted on sensor electrical response under fast deformation and relaxation movements, which allowed to predict the actual device behavior in tracking body fast movements. A highly cited article in this area, published in 2014, explores the use of graphene woven fabrics (GWFs) based piezoresistive strain sensors for human motion monitoring. The sensor exhibited relatively good sensitivity, high reversibility, superior physical robustness, easy fabrication and ease to follow human skin deformation. These characteristics demonstrate its potential not only for body monitoring but also for displays, robotics, fatigue detection, among other (Wang et al. 2014). Another important article of this period discussed a new type of stretchable strain sensor for human motion detection fabricated. This flexible piezoresistive pressure sensor uses a thin PDMS membrane as a deformable diaphragm and an Ag NP based serpentine metallic pattern as the piezoresistive element. It was noted that the developed sensor can be applied to detect multiple human motions such as finger and wrist motions and for measuring the pressure with high sensitivity to a low pressure which demonstrates which it can provide an efficient and economic approach to such applications (Lee et al. 2014b). A pressure sensor based on irregular microhump patterns has been also proposed for body motion sensing (Wang et al. 2016). The active layer of this sensor is fabricated by spin coating PEDOT: PSS on top of the patterned PDMS. This flexible device exhibited high sensitivity, showing its great potential in the next generation of smart sensors for robotics, real‐time health monitoring, and biomedical applications. More recent Mai et al. (2019) reported a self-standing piezoresistive sensors fabricated using a simple method based on blending multiwall carbon nanotubes (MWCNTs) with a stretchable elastomeric matrix (Ecoflex).

These sensors exhibited remarkable mechanical and electromechanical response features which are essential for wearable applications to monitor human motion with minimal discomfort, e.g., finger bending, walking or speaking, and even detecting a hot liquid poured into a cup. Also, Rezaei et al. (2019) developed a core–sheath fiber strain sensor from readily available materials in a one-step dip-coating process and demonstrate the development of a smart sleeveless shirt for measuring the kinematic angles of the trunk relative to the pelvis in complicated three-dimensional movements. This year, the design of a flexible piezoresistive sensor based on interlocked graphene microarrays on a flexible substrate was reported by Cheng et al. (2020). This sensor exhibits good performance and fast-response characteristic of this sensor, which allows meticulous detection of complicated stimuli from various human motions. In addition, Zheng et al. (2021) explored the use of a MXene/cotton fabric (MCF) in wearable pressure sensor for human motion detection and E-skin applications. It was observed that MCF based pressure sensor can be employed to detect and distinguish various human health signals, including finger motion, early-stage Parkinson’s static tremor and wrist pulse.

Other applications of wearable piezoresistive sensors include human facial expression detection and assessing remission in mental health. The potential of a low-cost stretchable piezoresistive strain sensor based on stretchable silver ink to distinguish three facial expressions (happy, sad and disgust) was explored by Ramli et al. (2020). The obtained results indicate that this sensor is promising for applications in the field of facial expression detection, human body monitoring and robotics. Regarding to wearable monitoring system based on piezoresistive sensors, Lanata et al. (2015) proposed a system called PSYCHE, which comprises a comfortable t-shirt with embedded sensors, such as textile electrodes, to monitor electrocardiogram heart rate variability (HRV) series, piezoresistive sensors for respiration activity, and triaxial accelerometers for activity recognition. The authors presented experimental results gathered from ten bipolar patients, wearing the PSYCHE system, with severe symptoms who exhibited mood states among depression, hypomania, mixed state, and euthymia, which confirm that the developed system is suitable for this application type. Finally, Table 4 shows some medical parameters measured by these sensors.

Table 4 Examples of measured parameters by piezoresistive devices based on different materials
Full size table

5 Conclusions

In recent years, the efforts to gain a better understanding of the dynamics of fabricating foldable, flexible, and elastic wearable sensors has increased and gained an interdisciplinary approach that enables fabricating miniaturized, low-cost sensors via high-throughput techniques, which can be used as monitoring of the health, e-skins, flexible displays, and epidermal sensors. The foldable, elastic, and miniaturized wearable sensors can potentially have applications in aerospace, automotive, environmental, packaging, biomedical, and defense. Thanks to the revolutionary progress of nanomaterials processing methods and an understanding of novel sensing mechanisms, nanomaterials-based flexible piezoresistive physical sensors, such as pressure sensors, strain sensors, and temperature sensors, have been very quickly developed with excellent optical, physical, mechanical, and transport properties. In this paper, we showed a global review of recent advances in nanomaterials-based piezoresistive wearable sensors. The synthesis of nanomaterials used on sensing structures, including Zero-dimensional, One-dimensional, Two-dimensional, and Three-dimensional piezoresistive nanomaterial, and interconnected nanoarchitectures, are summarized.

The piezoresistive sensor manufacturing involves two major fabrication processes, including the (i) surface coating (decoration) methods, which can be divided into electroplating, electrospraying, vapor deposition techniques, and (ii) porous architectures methods that can be divided into electrospinning, roll-to-roll inkjet printing and screen printing, and hydrogel. The structures and performance of nanomaterials-based pressure, strain, and temperature sensors are presented. Practical applications are showed of artificial e-skins, human activity detection, health monitoring (respiration rate, heart rate, blood pressure, facial expression recognition, intracranial pressure (ICP), and intraocular pressure (IOP)), and wearable entertainment. Although admirable progress has been made in developing nanomaterials-based piezoresistive sensors, considerable challenges need to be overcome for future applications. The main advantages and disadvantages are presented as follows:

Among the wearable sensors, those based on piezoresistive effect are the earliest and most widely used type. The piezoresistive sensors are robust with excellent resistance to vibration, shock, and dynamic pressure changes due to used materials. The circuits are straightforward and allow higher resolution in the measurement. The output is quasi-linear, and the response time is below one millisecond. They can be used for a broad range of pressure measurements from 21 kPa to about 150 MPa. The output is also stable over time. Alternatively, thin-film resistors can be created directly on the membrane. The chief disadvantage is related to the necessity of power that makes them unsuitable for battery operated or low power systems. The size is directly proportional to the resistance, and a decrease in the size increases the power consumption. Another disadvantage is related to the limitations on scaling due to strain averaging reduces the piezoresistive sensor's sensitivity. Another disadvantage is that the sensor output is temperature-dependent and becomes significant harm for applications such as tire pressure measurement. There are large temperature changes over the operating cycle.