Biological skin inspired temperature/pressure bimodal tactile sensing: materials, device and systems

Abstract

As significant components of intelligent sensing technology, flexible tactile sensors have attracted widespread attention in recent years. In particular, bimodal sensors that can mimic the sensing functions of biological skin have been used to monitor external pressure and temperature in real time, showing promising potential in fields such as the Internet of Things (IoT) and artificial intelligence (AI). The recent progress of biological skin inspired temperature/pressure bimodal sensors is introduced and discussed in terms of bionic design principles, material selection, device design, system integration and applications. Additionally, through an in-depth analysis of the development of temperature/pressure bimodal sensors, the remaining challenges and opportunities for temperature/pressure bimodal sensors are discussed in detail.

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Qi-Jun Sun

Qi-Jun Sun is currently a Professor at the School of Physics and Optoelectronic Engineering & Guangdong Provincial Key Laboratory of Sensing Physics and System Integration Applications, Guangdong University of Technology (GDUT). He received his PhD degree in physics and materials science from City University of Hong Kong (CityU) in 2017. After graduation, he worked in CityU as a research fellow until he joined GDUT in 2021. His main research interests include flexible electronics and wearable sensors. He has published more than 100 articles in journals of Adv. Mater., Adv. Funct. Mater., Nano Energy, Small, J. Mater. Chem. C, etc.

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1. Introduction

With the rapid development of science and technology, tactile sensing technology has indispensable importance in many fields.¹ Tactile sensing is one of the most important ways in which human beings perceive the outside world, covering a wide range of information such as pressure, temperature, vibration and texture.² In recent years, inspired by the sensing ability of human skin, haptic sensing technology has been gradually applied to robotics,³ virtual reality, medical monitoring and human–computer interaction, thus promoting the development of intelligent systems. Biomimetic tactile devices can effectively mimic and reproduce the functions of the human tactile system,⁴ and biomimetic devices based on memristors and temperature/pressure bimodal tactile sensors has great potential in the field of next-generation wearable electronics.⁵

As a natural multi-functional tactile sensor, biological skin has demonstrated excellent performance in temperature and stress sensing by virtue of its fine receptor distribution and unique physiological structure.⁶ The epidermis and dermis of the skin together form a complex sensory network, with the epidermis mainly playing a protective role, while the dermis is rich in blood vessels and nerve endings, providing a physiological basis for temperature and stress sensing.⁷ In addition, the elasticity and adaptability of skin allow it to maintain good sensory abilities under different environmental conditions. Whether it is extreme cold or heat, or a gentle touch versus intense pressure, the skin can sense it effectively.⁸

In recent years, bionic sensors inspired by the intricate mechanics and sensing mechanisms of the human tactile system have emerged as a pivotal technological innovation, exerting profound impacts across diverse fields including biomedicine, environmental monitoring, and robotics.⁹ These sensors are not only capable of sensing temperature and pressure information simultaneously,¹⁰ thus realizing multi-dimensional sensing of the external environment,¹¹ but also show great potential in diverse application areas. However, there are remaining challenges in the development of high-performance bionic temperature/pressure bimodal tactile sensors.¹² For practical applications, temperature and pressure sensing with excellent sensitivity, flexibility and durability are highly desired,¹³ which requires materials with both flexibility and excellent sensing properties.¹⁴ The tactile sensitive materials not only need to be able to withstand repeated mechanical stress,¹⁵ but also need to ensure performance stability under a variety of environmental conditions, with higher requirements in terms of material heat resistance and corrosion resistance. Secondly, sensor integration and signal processing are also major challenges.¹⁶ The sensed signals of temperature and pressure often have complex characteristics, and how to efficiently integrate these two signals and achieve accurate data analysis is the key to realizing the intelligence of bimodal sensors.¹⁷ In addition, the design of the sensor needs to take into account the integration of multiple sensing functions,¹⁸ and how to efficiently integrate the temperature and pressure sensors in a compact device is also a major challenge in the realization of the technology.¹⁹

This review article provides a comprehensive overview of flexible pressure and temperature sensors, examining them from four distinct perspectives, bionic design principles, material selection, device design and systems integration and applications, as depicted in Fig. 1. This paper introduces temperature/pressure bimodal tactile sensors from the perspectives of device structure and signal coupling, and compares in detail the advantages and disadvantages of different output modes as well as their applicability. The main contents include the following aspects, structural design of pressure and temperature sensors, material selection for pressure and temperature sensors, structural design of pressure and temperature sensors, system integration of pressure and temperature sensors and their applications.

Fig. 1 Overview of pressure and temperature sensors in terms of bionic design principles, material selection, device design and systems integration and applications. Reproduced with permission.²⁰ Copyright 2023, Elsevier. Reproduced with permission.²¹ Copyright 2022, Springer Nature. Reproduced with permission.²² Copyright 2023, OAE Publishing.

2. Design principles for temperature/pressure bimodal tactile sensors

2.1 Sensing mechanisms of human skin

As shown in Fig. 2(a), the human skin is a complex and sophisticated sensory system that can not only protect the body from external damage,²³ but also plays a key role in tactile perception. The mechanism of tactile perception relies on a variety of receptors in the skin, especially temperature receptors and pressure receptors,²⁴ which work together to enable human beings to acutely perceive changes in the external environment.

Fig. 2 Bionic principle of flexible sensors. (a) Schematic of skin under stimulation. Reproduced with permission (Shang et al. 2023). Copyright 2023, Springer Nature. (b) Structure of human skin. (c) Tactile receptors in human skin. (d) Sensors that mimic the human sense of touch. Reproduced with permission (Zou et al. 2024a). Copyright 2024, WILEY.

The temperature sensing mechanism in human skin relies on thermoreceptors in the skin that sense temperature changes and transmit the information to the brain.²⁵ There are two main types of temperature receptors in the skin, cold receptors and heat receptors, as illustrated in Fig. 2(b).²⁶ Cold receptors are mainly located in the superficial layer of the skin, especially in the fingers, toes and face, and are sensitive to low-temperature stimuli, while heat receptors are mainly distributed in the deeper layers of the skin and are able to perceive stimuli that increase in temperature.²⁷ When the ambient temperature is low, cold receptors are activated, generating nerve impulses and transmitting them via sensory nerves to the spinal cord and brain.²⁸ Conversely, when exposed to high temperature, heat receptors are activated and signals are similarly transmitted to the central nervous system.²⁹ The nerve signals transmitted by these receptors are sent through the spinal cord to the thalamus in the brain and are then sent to the sensory areas of the cerebral cortex.³⁰ In the human brain, these signals are processed to produce a perception of temperature, feeling cold or hot. In order to maintain stable body temperature, the brain activates different physiological responses based on temperature perception.³¹

The pressure sensing mechanism in human skin relies on a variety of mechanoreceptors in the skin that sense externally applied pressure and touch, as shown in Fig. 2(c).³² When pressure is applied to the surface of the skin, receptors such as Meissner's vesicles, Pachyonychia vesicles, and Ruffini endings are activated. Meisner's vesicles primarily sense light touch and mild pressure and adapt quickly, pachyonychia are sensitive to deep pressure and vibration and can sense more intense pressure, and Ruffini endings primarily sense stretching and sustained pressure on the skin.³³ These receptors convert pressure signals into nerve impulses that travel through sensory nerves to the spinal cord and then to the brain.³⁴ In the brain, these signals are processed to produce a sense of pressure that helps us recognize the properties of objects and interact with our environment, which enables humans skin to be sensitive to pressure change and perform a variety of daily activities.³⁵

The bionic principle of temperature/pressure bimodal sensors focuses on designing and manufacturing sensors capable of detecting both physical quantities by mimicking the functions of biological systems in tactile sensing.³⁶ These sensors typically combine different materials and fabrication technologies to enable sensing of both temperature and pressure.³⁷ The bionic principle of temperature sensing mimics the thermoreceptors in human skin, utilizing thermoelectric materials,³⁸ and thermistors or other temperature sensing elements that are sensitive to changes in temperature and are able to convert changes in temperature into electrical signals.³⁹ The bionic principle of pressure sensing, on the other hand, is based on mechanoreceptors in the skin, such as Meissner's vesicles and Pacini's vesicles.⁴⁰ For pressure perception, piezoelectric, capacitive or piezoresistive materials are utilized, which can convert the external pressure stimuli into electrical signals, realizing the pressure sensing,⁴¹ as shown in Fig. 2(d). In order to achieve both temperature and pressure sensing, modern sensors often integrate both functions in a single system.⁴² For example, sensors can be designed with different levels of structure,⁴³ where the temperature sensing element and the pressure sensing element can be arranged in a specific way to ensure that they can work independently while responding efficiently to changes in the external environment.⁴⁴ This integrated design allows the sensor to vividly mimic the function of biological skin and achieve satisfying sensitivity and accuracy.⁴⁵

2.2 Key metrics for bionic tactile sensors

For practical applications, pressure and temperature sensors should exhibit excellent performances, which are estimated using specific parameters.⁴⁶ In the following section, key parameters will be introduced and discussed.

Sensitivity is one of the most important performance metrics for temperature/pressure sensors, describing the ability of the sensor to respond to changes in the input signal.⁴⁷ Specifically, sensitivity is usually defined as the ratio between the amount of change in the output signal and the amount of change in the input signal, and this parameter is critical to the characterization of the entire sensing system.⁴⁸ In the case of temperature sensors, sensitivity is primarily expressed as the rate of change of the signal output due to a change in temperature. Typically expressed in units of percent per degree Celsius (%/°C) or millivolts per degree Celsius (mV °C⁻¹), these units help to quantify the effect of a temperature change on the sensor's output signal.⁴⁹ For example, a temperature sensor with high sensitivity can produce significant electrical signal changes for relatively small temperature changes, thereby improving the system's responsiveness to temperature changes and measurement accuracy.⁵⁰ In pressure sensors, sensitivity is expressed as the rate of change in signal output per unit change in pressure. (For capacitive pressure sensors, this rate of change is usually expressed in units of kPa⁻¹, and for piezoelectric pressure sensors, in units of mV N⁻¹).⁵¹ This means that when the pressure changes, the sensor can accurately reflect the change in pressure by a corresponding change in the electrical signal.⁵²

Response time and recovery time are also indispensable evaluation parameters, typically measured in milliseconds (ms). Response time refers to the rate at which a sensor's output signal changes in response to an external stimulus.⁵³ To accurately measure the response time, it is usually calculated by comparing the difference between the input time of the external stimulus and the time it takes for the sensor's output signal to reach a steady state (or to reach 90% of its steady state value).⁵⁴ Response time indicates the sensor's ability to respond instantly to changes in the external environment. Recovery time refers to the duration needed for the sensor to revert to its original state once the applied pressure or temperature stimulus is removed.⁵⁵ This means how quickly the sensor can return to an unaffected state when the external stimulus is removed. The brevity of the recovery time affects not only the accuracy of the sensor's measurements, but also its use in dynamic environments.⁵⁶ These two parameters reflect the sensor's ability to respond and recover quickly to changes in pressure or temperature. Short response and recovery times enable the sensor to sense dynamic changes in the outside world promptly and accurately.⁵⁷

Resolution is a critical performance indicator for temperature/pressure sensors, typically defined as the smallest amount of change that the sensor can detect.⁵⁸ It reflects the fineness of measurement detail that the sensor can provide within its measurement range. Resolution is usually expressed as a unit change, e.g. °C or °F for temperature sensing or kPa or psi for pressure sensing.⁵⁹

Stability is typically characterized by evaluating the variation in the system's output response after subjecting it to thousands of cycles of external stimuli.⁶⁰ In this process, the sensor produces a certain response to each external stimulus, and as the number of cycles increases, the sensor may experience changes in various physical and environmental conditions.⁶¹ Ideally, a high-quality sensor should be able to maintain the accuracy and consistency of its output after many cycles.⁶²

3. Material selection

3.1 Temperature-sensitive materials

The performance and accuracy of temperature sensors is highly dependent on the temperature sensitive materials used.⁶³ Thanks to materials science and engineering, the temperature sensitive materials can be engineered with physical and chemical properties, which enables them for a variety of applications.⁶⁴Table 1 summarizes the performance of flexible temperature sensors made of different materials.

Table 1 Summary of the main performances of flexible temperature sensors based on different materials

Material	Sensor type	Temperature range	Sensitivity	Response time	Ref.
Graphene nanowall	Thermoresistive (PTC)	25–120 °C	0.21 °C^−1	1.6 s	73
rGO	Thermoresistive (NTC)	25–85 °C	−0.0148 °C^−1	0.5 s	65
Polyurethane (PU)	Thermoresistive (NTC)	25–50 °C	−0.01185 °C^−1	7 s	72
CuCo2O4	Thermoresistive (NTC)	25–85 °C	−1.04% °C^−1	20 s	68
ZnO	Thermoresistive (NTC)	29–60 °C	−1.06% °C^−1	5 s	66
Ti3C2Tx	Thermoresistive (NTC)	27–140 °C	0.00222 °C^−1	10 s	67
PEDOT:PSS	Thermoresistive (NTC)	0–90 °C	−0.63% °C^−1	4.8 s	94
SWCNTs	Thermoelectric	47–197 °C	−54.5 μV K^−1	0.6 s	87
In2O3/Pt	Thermoelectric	25–193.5 °C	204.35 μV °C^−1	2 s	80
Bi2Te3	Thermoelectric	4–50 °C	266.4 μV K^−1	1 s	78

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Thermistor materials are widely employed in temperature sensors because of their temperature dependent resistance changes.⁶⁵ Thermistors can be divided into two main types, negative temperature coefficient (NTC) thermistors and positive temperature coefficient (PTC) thermistors.⁶⁶ NTC thermistors are semiconductor materials based on metal oxides, whose resistance decreases with increasing temperature. The flow chart of the flexible temperature sensor fabricated based on NTC is shown in Fig. 3(a). Common NTC thermistor materials include metal oxides such as NiO, CoO, CuO and MuO. Among them, NiO is the most widely used material in NTC thermistors with good temperature sensitivity and stability, showing promising potential as candidates for high precision temperature sensing.⁶⁷ As a metal oxide, CuO exhibits excellent performance in medium and low-temperature measurements due to its stable structure, regular thermal excitation, and carrier movement.⁶⁸ PTC thermistor materials are usually polymers or ceramics whose resistance increases with temperature.⁶⁹ BaTiO₃ is a widely used PTC thermistor material that exhibits a steep resistance jump near the Curie temperature and is extremely sensitive to specific temperature points.⁷⁰ This threshold response enables precise detection of specific temperatures and facilitates temperature alarms.⁷¹ In comparison to ceramic PTC materials such as BaTiO₃, polymer materials with simpler preparation processes, such as polyethylene (PE) and polyvinyl chloride (PVC), have also been utilized as PTC thermistors.⁷² The polymer matrix has significant thermal expansion properties. As the temperature rises, the movement of polymer molecular chains intensifies, causing volumetric expansion that increases the spacing between filler particles and disrupts conductive pathways, thereby leading to changes in resistance.⁷³ The characteristics of polymer materials, such as flexibility, flexural tolerance, and non-fragility, make them more suitable for fabricating flexible or wearable temperature sensors.⁷⁴

Fig. 3 Temperature sensors prepared based on thermal materials. (a) Fully inkjet-printed bimodal sensor based on GO. Reproduced with permission.²⁰ Copyright 2023, Elsevier. (b) Flexible temperature sensor made based on PEDOTPSS. (c) Schematic of layered preparation of flexible temperature sensors. Reproduced with permission.⁹⁹ Copyright 2024, American Chemical Society.

Based on the Seebeck effect, thermoelectric materials can detect the temperature by converting the temperature difference into a voltage signal.⁷⁵ When a temperature gradient exists across the material, the heat causes carriers (electrons or holes) to migrate from the high temperature region to the low temperature region, thus creating a voltage difference between the two endpoints.⁷⁶ The magnitude of this voltage difference is proportional to the temperature difference, which in turn can be used to accurately measure temperature.⁷⁷ Bi₂Te₃ is one of the most widely used thermoelectric materials for the room to medium temperature (−40 °C to 150 °C) range.⁷⁸ It has Seebeck coefficients as high as 200 μV K⁻¹ and is capable of generating significant voltage signals during temperature changes.⁷⁹ In addition, its moderate electrical conductivity and low thermal conductivity make it an excellent candidate for thermoelectric energy conversion.⁸⁰ Bismuth telluride alloys are widely used in devices such as thermoelectric thermometers, portable temperature sensors, and thermoelectric coolers,⁸¹ and their high responsiveness and accuracy make them the material of choice for many high-end applications.⁸² Se₂Te₃ is another important thermoelectric material, often used in combination with bismuth–telluride alloys, and is particularly suited to the mid-temperature range (200 °C to 400 °C).⁸³ Bismuth antimonide has a Seebeck coefficient similar to that of bismuth–telluride alloys, which is effective in increasing the efficiency of thermoelectric energy conversion.⁸⁴ Its stability and oxidation resistance at high temperatures make it an ideal choice for industrial temperature sensors.⁸⁵ In the low to medium temperature range, PbS exhibits good performance as a thermoelectric material, suitable for applications from −50 °C to 250 °C.⁸⁶ Lead sulfide has a relatively high Seebeck coefficient and is able to efficiently generate voltage signals at low temperatures.⁸⁷ Its relatively low cost and good processability give it an advantage in mass production.⁸⁸

In recent years, oxide ceramics have also been gaining interest. These materials are typically adapted to high temperatures, withstanding temperatures of up to 1000 °C. Their good chemical stability and mechanical strength allow them to operate stably under extreme conditions.⁸⁹ The principle of operation of oxide ceramics is also based on the Seebeck effect, where the migration of carriers caused by temperature differences generates a voltage signal at both ends of the material. Applications for these materials are mainly focused on high temperature sensors and environmental monitoring equipment. Meanwhile, fibers can also serve as core components of temperature sensors. Ionic liquid-based scalable fiber temperature sensors exhibit a high sensitivity of 2.61%/°C, as well as the ability to resist compression and bending interference.⁹⁰

Numerous published studies have described the role of organic thermoelectric materials in temperature measurement.⁹¹ Although their thermoelectric properties are slightly lower than those of inorganic materials, their good flexibility and light weight give them an advantage in portable devices and wearable technology.⁹² Organic thermoelectric materials also rely on the Seebeck effect,⁹³ which generates a voltage signal through a temperature gradient. PEDOT:PSS is a conductive polymer whose electrical conductivity is largely dependent on the migration of carriers.⁹⁴ The preparation flow chart and structure of the flexible temperature sensor fabricated from PEDOT:PSS are depicted in Fig. 3(b) and (c). When the temperature rises, the thermal excitation of carriers increases, leading to a rise in carrier concentration, which improves the conductivity of the material, conversely, at low temperatures,⁹⁵ the thermal excitation of carriers decreases and the carrier concentration decreases, leading to a decrease in conductivity.⁹⁶ In addition the polymer chains of PEDOT:PSS also deform and rearrange at different temperatures, a process that further affects the conductivity. At high temperatures, the increased mobility of the polymer chains may lead to an increase in conductivity,⁹⁷ whereas the opposite is true at low temperatures, where the mobility of the polymer chains is reduced, thus affecting the conductivity. As another class of organic materials, hydrogen-bonded organic frameworks (HOFs) have rapidly advanced their development in the field of temperature sensors due to their characteristics such as structural tunability, abundant pores, excellent thermal stability, and sensitive responsiveness.⁹⁸

3.2 Pressure sensitive materials

Pressure-sensitive materials are primarily classified by sensing principle into piezoresistive, piezoelectric, capacitive, optical sensitive materials, and flexible sensitive materials.⁹⁹Table 2 summarizes the performance of flexible pressure sensors made based on different materials.

Table 2 Summary of the main performances of flexible pressure sensors based on different materials

Material	Sensor type	Sensitivity	Pressure range	Response time	Ref.
CNTs/PDMS	Piezoresistive	242.4 kPa^−1 (50.7–185.5 kPa)	0–185.5 kPa	44 ms	100
Graphene/PDMS	Piezoresistive	0.122 ± 0.002 (0–5 kPa)	0–20 kPa	70 ms	103
MXene/PDMS	Piezoresistive	95.26–1104.35 kPa^−1	0–800 kPa	100 ms	101
AgNWs/PVDF	Piezoresistive	0.014 kPa^−1	0–100 kPa	64 ms	102
Silicon rubber (SR)/NaCl/carbon black (CB)	Capacitive	3.15 kPa^−1	0–200 kPa	120 ms	109
Porous PDMS/Nacl	Capacitive	0.132 kPa^−1 (0–21.6 kPa) 0.031 kPa^−1 (21.6–81.3 kPa), 0.0058 kPa^−1 (81.3–200 kPa)	0–200 kPa	120 ms	111
GaN	Piezoelectric	0.0758 mV kPa^−1	5–200 kPa	4 ms	105
PVDF/MXene/rGO	Piezoelectric	8.84 kPa^−1	1.1–6.3 kPa	18.2 ms	107
Fused silica	Optical	3.18 nm kPa^−1	0–500 kPa	NA	114
Silica diaphragm	Optical	3.54 μm MPa^−1	0–1000 kPa	NA	117

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Resistive sensing materials are the most commonly used type in pressure sensors, and typical resistive pressure-sensitive materials mainly include metals, semiconductor materials, and conductive polymers.¹⁰⁰ A key characteristic of these materials is that their resistance values change with the applied pressure.¹⁰¹ When pressure is applied to the surface of a metal, the deformation caused by the pressure leads to changes in the internal lattice structure of the metal, thereby altering its resistance.¹⁰² Conductive polymers such as polyaniline (PANI) and polypyrrole (PPy) exhibit promising applications in flexible pressure sensors due to their excellent mechanical flexibility and electrical conductivity. In addition, conductive composites, such as carbon black and polyurethane composites, also exhibit good pressure sensing properties.¹⁰³ These composites change the relative position of the carbon black particles when pressure is applied, resulting in a change in the overall resistance, which enables pressure sensing.¹⁰⁴

Piezoelectric sensing materials are another important class of pressure sensitive materials, which are widely used in pressure sensors because of their ability for self-powered pressure sensing.¹⁰⁵ Common piezoelectric ceramic materials, such as lead titanate (PZT), are widely used for precision pressure sensors because of their high sensitivity and excellent electro-mechanical coupling coefficient.¹⁰⁶ In addition, piezoelectric polymers, such as polyvinylidene fluoride (PVDF), also have excellent pressure sensing properties, as shown in Fig. 4(c).¹⁰⁷ These materials change their internal polarity when pressure is applied, which in turn generates an electrical signal proportional to the pressure.¹⁰⁸

Fig. 4 Pressure sensors prepared from pressure sensitive materials. (a) Bimodal sensor made of GO and PDMS. Reproduced with permission.¹²⁷ Copyright 2024, Elsevier. (b) Dielectric layer with porous microstructure. Reproduced with permission.¹²⁸ Copyright 2024, MDPI. (c) Piezoelectric and piezoresistive flexible pressure sensors based on PVDF and reduced graphene oxide (rGO). Reproduced with permission.¹²⁹ Copyright 2025, Elsevier.

Capacitive sensing materials are also an important pressure-sensitive material for E-skin.¹⁰⁹ Capacitive sensors typically consist of an insulating material sandwiched between two conductive layers.¹¹⁰ When pressure is applied, the thickness or surface area of the insulating layer change, resulting in a change in capacitance.¹¹¹ Commonly used capacitive membrane materials include polyester and polyimide membranes,¹¹² which have demonstrated good sensitivity and stability for use in flexible E-skin.¹¹³

Optical pressure sensors utilize changes in the optical properties of a material when pressure is applied for pressure sensing.¹¹⁴ Fiber optic sensors are a typical type of optical sensor, and their principle of operation is based on changes in the refractive index of an optical fiber.¹¹⁵ When pressure is applied, the geometry or length of the optical fiber changes, which causes a change in the optical signal, and pressure measurement can be achieved by analyzing the change in the optical signal.¹¹⁶ In addition, certain photosensitive materials change their optical properties (e.g., light absorption and reflection under pressure and can also be used in the design of optical pressure sensors).¹¹⁷ This contactless detection method shows advantages in some applications with strict environmental requirements.¹¹⁸

Friction electric materials received widespread attention in recent years in sensor technology, especially in the applications of electronic skin and self-powered sensors.¹¹⁹ Common friction electric materials include polyethylene (PE), polydimethylsiloxane (PDMS) and polyvinylidene fluoride (PVDF).¹²⁰ PDMS exhibits excellent elasticity and deformation recovery capabilities, making it suitable for fabricating flexible and stretchable sensor substrates and sensitive layers, as shown in Fig. 5(a) and (b). The principle of operation of friction electric materials is based on contact charge transfer.¹²¹ When two distinct substances make contact and then separate, electrons move from one substance to the other, leading to the accumulation of opposite charges on their respective surfaces,¹²² thus creating an electric field.¹²³ In pressure sensors, the applied pressure affects the contact area and contact time between the materials,¹²⁴ thus changing the amount of charge generated.¹²⁵

Fig. 5 Dual mode sensor prepared based on DEOS. (a) Sensor structure combining piezocapacitive and thermosensitive effects. Reproduced with permission.²⁰ Copyright 2023, Elsevier. (b) Sensor structure consisting of thermosensitive material and bilayer capacitor. Reproduced with permission.¹³¹ Copyright 2023, Royal Society of Chemistry. (c) Sensor based on piezoresistive and thermoelectric effects. Reproduced with permission.¹³⁴ Copyright 2022, American Chemical Society. (d) Schematic of the fabrication of a PCPP sensor. Reproduced with permission.¹³⁵ Copyright 2022, Royal Society of Chemistry.

4. Construction of temperature/pressure bimodal sensors

Systematic design and construction are undoubtedly important in realizing temperature/pressure bimodal sensing, and there are normally two strategies used to design highly integrated pressure and temperature sensors for pressure and temperature sensing.¹³⁰

4.1 Temperature/pressure sensors with different electrical outputs

The different electric output signals (DEOS) system refers to the realization of separate output for pressure and temperature signals by means of different sensing mechanisms, which fundamentally avoids signal interference problems. One scheme to realize pressure and temperature sensing is to combine piezocapacitor and thermistor effects to realize different output capacitors and resistors representing pressure and temperature, respectively. For example, Yuan et al. proposed a sensing structure consisting of porous PDMS and rGO membranes by mimicking the structure of human skin (Fig. 5(a)).²⁰ Since the porous PDMS has a lower modulus of elasticity than the solid PDMS membrane, the compressibility of the flexible dielectric layer can be greatly improved, which in turn enhances the pressure sensitivity of the device. rGO has good electrical conductivity and negative temperature coefficient (NTC), and there is a potential barrier between neighboring rGO nanosheets, which hinders the transport of charge carriers. As the ambient temperature increases, the probability of carrier hopping increases, leading to a decrease in the overall system resistance. The resistance–temperature sensitivity of the rGO membrane is utilized to achieve temperature sensing. Fan et al. demonstrated a flexible dual-parameter sensor based on the thermoresistive and piezocapacitive effects, which is composed of a PEDOT:PSS/MWCNT serpentine electrode and a PVA/H₃PO₄ ionic film dielectric layer, as shown in Fig. 5(b).¹³¹ With increasing temperature, the enhanced random motion of PEDOT:PSS particles improves interparticle electron hopping, thereby enhancing electrical conductivity and leading to a decrease in resistance. When external pressure is applied, the contact area between the ionic membrane fiber and the electrode is changed, and the density of positive and negative ion distribution in the double layer changes, which in turn affects the interface capacitance value. Sensors integrating piezocapacitive and thermoresistive effects feature rapid response kinetics, rendering them ideal for real-time dynamic measurements while offering high-precision temperature sensing capabilities.¹³² However, the output signals of these sensors can be unstable at static pressure, and additional signal processing and compensation mechanisms may be required in applications where static pressure needs to be continuously monitored.¹²⁶ Secondly, the performance of such sensors may be somewhat limited at high temperatures, and the measurement accuracy may degrade when the temperature exceeds the range that their materials can withstand.¹³³

Sensors utilizing the piezoresistive and thermoelectric effects for pressure and temperature measurement are another typical example of DEOS. For example, Li et al. proposed a structure based on a conductive metal–organic framework with dual-modal sensing functions of pressure and temperature, which mainly consists of an MSMC substrate, Ni₃(HiTP)₂, and PDMS, as illustrated in Fig. 5(c).¹³⁴ When the sensor is subjected to pressure, the microstructure of the composite film changes, altering the conductive pathways within the film and thus changing the resistance to enable pressure detection. Under thermal stimulation, when a temperature gradient is created across one side of the sensor incorporating conductive Ni₃(HiTP)₂, charge carriers exhibit directional migration from the hotter region to the cooler region, leveraging the Seebeck effect to generate a thermoelectric voltage for temperature detection. Gao et al. demonstrated an integrated temperature and pressure dual-mode sensor for monitoring temperature (with an ultra-low detection limit of 0.05 K) and pressure (with a wide detection range of 0–100 kPa) based on PDMS foam decorated with thermoelectric PEDOT:PSS and CNT, as shown in Fig. 5(d).¹³⁵ Sensors integrating piezoresistive and thermoelectric effects exhibit high sensitivity, enabling high-precision pressure measurement, rapid detection of pressure changes, and excellent performance under extreme temperatures.

Integrating the triboelectric effect and the thermoelectric effect provides another efficient method for pressure and temperature sensing. For example, Chen et al. demonstrated a self-powered temperature-pressure dual-function E-skin (STPES) based on the coupling of triboelectric and thermoelectric effects, which realizes self-powered monitoring of pressure and temperature through a conical microstructure PDMS membrane and a PEDOT:PSS/MWCNTs composite film (Fig. 6(a)).¹³⁶ In the initial state, PDMS and PET come into contact, causing them to carry positive and negative charges, while the upper and lower electrodes also charged due to electrostatic induction. When pressure is applied and causes the device to deform, the contact area of the triboelectric materials increases, leading to an increase in charges and the generation of a potential difference, thereby enabling pressure sensing, as shown in Fig. 6(c). When the temperature changes, the thermocouple membrane composed of a PEDOT:PSS (MWCNTs) composite membrane and sputtered Cu membrane will generate a potential difference due to the different electron transport rates in different materials, and the thermoelectric effect will be utilized to convert thermal energy into voltage output. The temperature sensing mechanism is mentioned in Fig. 6(b). A distinct advantage of this sensor design is the triboelectric effect's ability to serve as an autonomous AC power source for the entire sensing system, drastically reducing dependency on external power grids.¹³⁷

Fig. 6 Multifunctional sensing mechanism of the STPES. (a) Voltages in different directions for monitoring temperature and pressure dynamic signals. (b) Mechanism of thermoelectric effect in temperature sensing under heating conditions. (c) Friction electric effect mechanism in pressure sensing under finger-touch conditions. Reproduced with permission.¹³⁶ Copyright 2022, Elsevier.

The DEOS system is able to realize independent outputs of pressure and temperature signals based on different sensing mechanisms, thus effectively avoiding the problem of signal interference.¹³⁸ For example, in the sensor based on piezoresistive and thermoelectric effects, the piezoresistive effect mainly responds to the resistance change caused by the pressure change, while the thermoelectric effect responds to the thermal voltage generated by the temperature gradient. As the physical mechanisms for signal generation of the two sensing modalities are entirely distinct, their inherent separation at the source significantly simplifies signal processing requirements, eliminating the need for complex decoupling algorithms. However, the hardware design is more complex and costly, and it is suitable for scenes that require high signal accuracy and real-time performance, such as precise physiological monitoring and environmental monitoring.

4.2 Same electrical output modes

The same electrical output signal (SEOS) system means that the signal output adopts the same electrical output mode, and in order to detect pressure and temperature simultaneously, a resistive mechanism can be utilized. For instance, Yuan et al. demonstrated an all-resistive dual-mode sensor based on Ti₃C₂T_x MXene and PDMS, which realizes crosstalk-free detection of pressure (120 Pa–100 kPa) and temperature (25–50 °C) through a sandwich structure and two-phase contact mechanism (Fig. 7(a)).¹³⁹ Zhou et al. demonstrated a dual-mode flexible sensor (DMFS) based on a multilayer structure, which enables intelligent monitoring of pressure (7 Pa–70 kPa) and temperature (20–50 °C) through an elastic nanofiber skeleton and “urchin”-like conductive materials (SBS/PDA/CNT and SBS/PS@PANI composites), as shown in Fig. 7(b). Under different pressure intensities, the contact sites and areas between the layers vary, leading to changes in resistance. The detection of pressure is realized based on the changes in contact resistance and volume resistance. When the temperature changes, high temperature promotes electron migration and network reorganization between CNT tubes, accelerates charge carrier movement and reduces resistance.

Fig. 7 Dual mode sensor prepared based on a SEOS system. (a) Fully resistive bimodal sensor based on MXenes. Reproduced with permission.¹³⁹ Copyright 2024, Elsevier. (b) Ionic skin based on a capacitive bimodal sensor. Reproduced with permission.¹⁴⁰ Copyright 2017, Royal Society of Chemistry. (c) Multilayer bimodal resistive flexible sensor structure. Reproduced with permission¹⁴² Copyright 2023, Elsevier.

Additionally, piezoelectricity and pyroelectricity can also be used to construct pressure and temperature sensors with SEOS systems. For example, Song et al. proposed a self-powered bimodal sensor for temperature and pressure based on the ferroelectric material BaTiO₃.¹⁴⁰ When the temperature changes, the positive and negative charges inside the BTO material are displaced relative to each other, leading to a change in spontaneous polarization. This change in turn triggers the generation of pyroelectric current and voltage signals. Due to the non-centrosymmetric crystal lattice of BTO materials, the application of external pressure induces a deformation-driven change in spontaneous polarization, leading to the generation of a piezoelectric voltage signal through the direct piezoelectric effect. Through the synergistic effect of the pyroelectric and piezoelectric effects, the sensor can simultaneously output voltage signals to perceive temperature and pressure changes.

Capacitive response sensing is also an effective method for achieving simultaneous temperature and pressure monitoring. Lei et al. introduced a multifunctional bionic skin sensor prepared by integrating a 3D-printed thermo-responsive hydrogel into a capacitive circuit, as illustrated in Fig. 7(c).¹⁴¹ With increasing temperature, the hydrogel undergoes swelling and transparency transition, expanding the ionic conductive layer area and consequently elevating capacitance. Under pressure, the grid-structured hydrogel deforms, altering the contact area between the ionic conductive layer and dielectric layer to modulate capacitance, leveraging the mechanical–ionic coupling of the hydrogel network.

As previously mentioned, the DEOS system avoids crosstalk and decoupling processes between pressure and temperature signals. However, due to the identical integrated mechanisms, the SEOS system inevitably generates interference in order to accurately obtain pressure and temperature information, and additional complex signal decoupling algorithms and circuits need to be designed. Despite the challenge of signal interference, this approach still finds applications in scenarios where cost sensitivity is high or the requirement for signal precision is not extremely stringent. For example, in some simple temperature and pressure monitoring scenarios, such as common indoor environment monitoring, low-cost industrial equipment condition monitoring, etc., the SEOS system can meet the basic monitoring needs through appropriate signal processing and calibration.Tables 3 and 4 summarize the pressure and temperature performance of sensors fabricated from different materials.

Table 3 Summary of pressure performance of different temperature/pressure sensors

Material	Sensitivity (kPa^−1)	Pressure range	Response time (ms)	Durability (cycles)	Ref.
PDMS/rGO	0.12% (0–250 kPa)	0–1000 kPa	40	10 000	20
	3% (250–1000 kPa)
Ni3(HiTP)2/MSMC	61.61 (0–32 kPa)	0–300 kPa	20	16 000	134
	10.9 (32–70 kPa)
	1.09 (70–300 kPa)
PEDOT:PSS/CNT	1.97% (0–25 kPa)	0–100 kPa	170	5000	135
	1.09%kPa^−1 (25–40 kPa)
	0.39% (40–75 kPa)
	−0.04% (75–100 kPa)
PEDOT:PSS/MWCNTs/PVA/H3PO4	1249.34 (0–10 kPa)	0–600 kPa	9	150	131
	169.27 (10–200 kPa)
	64.31 (200–600 kPa)
SBS/PDA/CNT/PDMS	111.96 kPa^−1 (0–10 kPa)	0–70 kPa	40	5000	142
	23.62 kPa^−1 (10–40 kPa)
	5.03 kPa^−1 (40–70 kPa)
PDMS/PEDOT:PSS/MWCNTs	1.394 V kPa^−1 (1 Pa–2 kPa)	1 Pa–100 kPa	57.6	3000	136
	0.379 V kPa^−1 (2–100 kPa)
MOF-MXene/PU/PVDF	1.02 (0–10 kPa)	0–100 kPa	10	20 000	159
	0.31 (10–100 kPa)
PEDOT:PSS/Ti3C2Tx/PDMS	0–40 kPa: 3.05 kPa^−1	0–120 kPa	157	200	168
	40–80 kPa: 2.16 kPa^−1
	80–120 kPa: 1.07 kPa^−1

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Table 4 Summary of temperature performance of different temperature/pressure sensors

Material	Sensitivity	Temperature range (°C)	Response time (s)	Durability (cycles)	Ref.
PDMS/rGO	−0.0033% °C^−1	20–60	0.45	100	20
Ni3(HiTP)2/MSMC	57.1 μV K^−1	25–65	3	100	134
PEDOT:PSS/CNT	40.5 μV K^−1	25–55	NA	50	135
PEDOT:PSS/MWCNTs	0.032 °C^−1 (15–50 °C)	15–80	0.8	25	131
	0.004 °C^−1 (50–80 °C)
SBS/PDA/CNT/PDMS	2.23% °C^−1	20–50	6.83	400	142
PDMS/PEDOT:PSS/MWCNTs	220 μV K^−1	−3 to 87	1.8	2000	136
MOF-MXene/PU/PVDF	−3.6% °C^−1	20–50	NA	100	159
PEDOT:PSS/Ti3C2Tx/PDMS	22.7 μV K^−1	0–40	NA	100	168

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4.3 Signal decoupling

As mentioned earlier, sensors utilizing the SEOS system feature a simple structure but suffer from the issue of signal decoupling. In contrast, sensors employing the DEOS system, though relatively complex in structure, can physically separate the two types of signals, thereby avoiding the problem of signal crosstalk. In temperature/pressure bimodal sensors, the issue of signal crosstalk represents a significant technical challenge, with its formation mechanism being complex and involving multiple factors.¹⁴³ Such interference not only affects the output accuracy of the sensor, but may also lead to erroneous measurement results, thus causing a series of problems in practical applications.¹⁴⁴ The sensing mechanisms for temperature and pressure each rely on different physical effects, with temperature sensors typically employing pyroelectric or thermoelectric effects, while pressure sensors rely primarily on piezoelectric effects.¹⁴⁵ Although these effects are fundamentally independent in physical principles, temperature variations in real-world applications can induce alterations in material properties—such as piezoelectric coefficients and elastic moduli.¹⁴⁶ When the temperature increases, certain materials may exhibit a non-linear electrical response, resulting in an electrical signal generated at the same pressure that is not consistent with the signal at room temperature. This interplay causes the pressure signal to be affected when measuring temperature changes and vice versa.¹⁴⁷ The selection and implementation of signal processing algorithms is also a key factor affecting the signal crosstalk problem.¹⁴⁸ In the subsequent signal interpretation process, the temperature and pressure signals usually need to be processed to extract independent measurements.¹⁴⁹ If the algorithms used are not sufficiently robust to crosstalk or fail to effectively account for the interaction between temperature and pressure, this may lead to inaccurate interpretation results.¹⁵⁰ Simple linear models may not be able to capture the non-linear effects of temperature changes on the pressure signal, especially in the case of rapid changes, and the signal processing may not be adjusted in time, leading to unstable measurements.¹⁵¹

Among the two sensing approaches discussed previously, the DEOS system exhibits notable advantages in mitigating signal crosstalk, primarily due to its decoupled sensing mechanisms and optimized structural design that minimize inter-parameter interference.¹⁵² By using different types of signals to collect temperature and pressure data separately, DEOS effectively avoids interference between the two types of signals, making the measurement of each parameter more independent and accurate.¹⁵³ In this model, the choice of materials is particularly important, as the physical properties of different materials will directly affect the performance of the sensor. Taking PDMS as an example, PDMS has been widely used in pressure sensing layers due to its good elasticity and flexibility.¹⁵⁴ Notably, PDMS's thermal expansion-induced signal drift becomes particularly pronounced in high-temperature conditions, potentially compromising pressure measurement accuracy.¹⁵⁵ In the SEOS system, the output signals for temperature and pressure are of the same type, which inevitably leads to the occurrence of signal crosstalk.¹⁵⁶ In practice, a change in temperature may result in a change in resistance or capacitance value, thus affecting the pressure measurement and vice versa. This interaction can trigger significant errors in high-precision measurement tasks, so effective methods are necessary to minimize interference between the two signals.¹⁵⁷

In the initial stage of sensor construction, adopting structural design to avoid crosstalk between the two signals is an effective and feasible approach.¹⁵⁸ This separate design of temperature and pressure sensing elements into a hierarchical architecture not only enhances the sensor's comprehensive performance but also physically decouples the two types of signals, eliminating the need for complex post-processing algorithms to distinguish temperature and pressure inputs. For example, Lu et al. prepared an all-MXene flexible pressure–temperature bimodal sensor, as shown in Fig. 8(a).¹⁵⁹ The sensitive layer of this sensor is divided into two layers, one layer composed of MOF-MXene/PU is responsible for temperature monitoring, while the other layer made of S-MXene/PVDF composite material achieves pressure monitoring. When the temperature changes, only MOF-MXene/PU produces resistance changes due to the semiconductor properties of MOF and the synergistic effect of MXene, while the S-MXene/PVDF composite does not respond to temperature changes. Under pressure variation, the S-MXene/PVDF composite exhibits pressure-dependent capacitance changes via structural deformation, ensuring pure pressure sensitivity without thermal crosstalk. The hierarchical design not only enhances the sensor's structural decoupling but also streamlines signal processing and data acquisition workflows. By enabling the data system to distinguish temperature and pressure signals with minimal interference, this approach facilitates real-time high-precision measurements with sub-millisecond response speeds, positioning it for applications in dynamic multi-physical-field monitoring.¹⁶⁰

Fig. 8 Methods to achieve signal decoupling. (a) Fully MXene flexible pressure–temperature bimodal sensor with a layered structure. Reproduced with permission.¹⁵⁹ Copyright 2024, IEEE. (b) Pressure sensing mechanism of MXene. (c) Schematic diagram of the compensating effect of PDMS on the negative TCR of MXene. Reproduced with permission.¹⁴² Copyright 2024, Elsevier.

Another strategy for signal decoupling involves exploiting material-specific characteristics for compensation. For example, Yuan et al. leveraged the distinct properties of materials to realize signal decoupling in MXene-based all-resistive sensors.¹⁴² As previously discussed, PDMS has a high thermal expansion coefficient, causing pressure monitoring to be affected by temperature. In contrast, MXene's negative temperature coefficient of resistance (NTC) makes it an ideal material for temperature sensing, allowing for simultaneous compensation of thermal effects in sensor designs.¹⁶¹ Under pressure, the volumetric expansion of PDMS induces mechanical stretching of embedded MXene nanosheets, increasing inter-nanosheet spacing and disrupting the conductive network, as depicted in Fig. 8(b). This structural deformation reduces electron transport pathways, leading to resistance increase and a positive temperature coefficient of resistance (TCR).¹⁶² By precise tuning of PDMS layer thickness, the thermal expansion-induced positive TCR of the composite can be calibrated to numerically cancel out MXene's inherent negative TCR, thereby achieving temperature-independent pressure detection.¹⁶³ After PDMS compensation, the TCR of the pressure sensor is as low as −2.1 × 10⁻³ %°C⁻¹ (25–50 °C), which is much lower than that of uncompensated MXene (−0.22%°C⁻¹), demonstrating that temperature has minimal impact on the pressure signal. For temperature monitoring, the MXene film is not fully covered by PDMS, with the corresponding area hollowed out, as shown in Fig. 8(c). When pressure is applied (0–120 kPa), the sensor exhibits a resistance change of less than 0.5%, demonstrating that temperature sensing is not affected by the thermal expansion of PDMS and that pressure causes no significant interference with the temperature signal.

In terms of eliminating signal crosstalk, this can be achieved by building a mathematical model and performing theoretical calculations.¹⁶⁴ First, constructing a correction formula is essential to quantify the independent impacts of temperature and pressure on output signals. This process entails fitting experimental datasets with theoretical models to identify dominant factors affecting signal behavior, thereby enabling the derivation of a refined mathematical expression that decouples thermal and mechanical contributions.¹⁶⁵ Subsequently, substituting the calibrated parameters into the final correction formula allows for the derivation of corresponding actual temperature or pressure values. Furthermore, integrating machine learning algorithms into the modeling framework can further enhance signal decoupling accuracy by dynamically adapting to complex nonlinear relationships between multi-physical parameters, outperforming traditional correction formulas in noisy environments.¹⁶⁶ Specifically, a convolutional neural network (CNN) can be used to extract the features of the signal, and the output can be further optimized by integrating information from different sources through feature fusion techniques.¹⁶⁷

5. Systems integration and applications

5.1 System integration

The integration of temperature and pressure sensing elements into a unified system allows for real-time, synchronous monitoring of multi-physical parameters,¹⁶⁸ thereby providing more comprehensive environmental perception capabilities for various applications.¹⁶⁹ To achieve such system integration, constructing a hierarchical modular architecture is essential, typically employing a sandwich structure or a stacked configuration, as shown in Fig. 9(a) and (b).¹⁷⁰ The sandwich structure usually consists of an upper and lower electrode layer and a dielectric layer in the middle.¹⁷¹ The electrode layer is responsible for signal acquisition and transmission, while the dielectric layer acts as an isolation element to provide the necessary electrical and mechanical support for the temperature and pressure sensors.¹⁷² Li et al. demonstrated a flexible dual-mode sensor based on the “neutral surface” structural design, which is used to realize the decoupled monitoring of temperature and pressure through a PDMS/BaTiO₃ dielectric layer (temperature-sensitive) and a Ni₈₀Cr₂₀ thin film (pressure-sensitive), as illustrated in Fig. 9(c). This approach ensures that each sensing layer operates independently with minimal mutual interference, thereby enhancing the sensor's long-term stability and measurement reliability in dynamic environments. Meanwhile, low-dimensional nanostructures (e.g., 0D quantum dots, 1D nanowires, 2D MoS₂) can enable monolithic 3D integration of flexible electronic devices, achieving high-density device stacking via vertical interconnections and layer-by-layer transfer printing, thereby improving system performance while maintaining mechanical flexibility.¹⁷³

Fig. 9 Approach to dual mode sensor system integration. (a) Friction electric bimodal sensor with a sandwich structure. Reproduced with permission.¹³⁶ Copyright 2022, Elsevier. (b) Bimodal electronic skin with a layered structure. (c) Bimodal sensor with neutral surface structural design. Reproduced with permission.¹⁷⁰ Copyright 2020, American Chemical Society. (d) Schematic flow of layered preparation of composite membranes. Reproduced with permission.¹⁶⁴ Copyright 2024, Royal Society of Chemistry.

In the system integration of sensors, it is crucial to leverage the characteristics of different materials to develop new composite films for realizing the integration of multifunctional materials.¹⁷³ This hybrid design integrates the conductive network of electron-transporting materials with the insulating properties of dielectric matrices, creating a composite film that balances charge transport efficiency and inter-signal isolation.¹⁷⁴ In addition, the use of flexible materials ensures that the composite membrane maintains its performance when subjected to external forces, enhancing the durability and adaptability of the sensor.¹⁷⁵ For example, PDMS/BaTiO₃ composite films are commonly used in temperature sensing, as depicted in Fig. 9(d). By dispersing BaTiO₃ nanoparticles in PDMS, a “dielectric-geometry” dual-effect sensitive unit is formed using the negative temperature coefficient of BaTiO₃ and the thermal expansion characteristics of PDMS, amplifying the capacitance changes caused by temperature.¹⁷⁶ Through the integration of diverse material categories to form composite films, the synergistic combination of these properties enables the realization of multifunctional integration, where each constituent material contributes uniquely to the overall functionality of the composite. Integrating stretchable synaptic transistors—capable of mimicking synaptic behaviors like EPSC and LTP under 50% strain—into layered modular architectures enables neuromorphic computing in bimodal sensing systems, enhancing real-time information processing and memory capabilities.¹⁷⁶ Advanced fabrication technologies, such as printed electronics (spray coating, screen printing), soft transfer, 3D printing, and deformation fabrication (kirigami), enable high-precision integration of flexible components into layered systems, enhancing the conformability and scalability of sensors.¹⁷⁷ Through the integration of structures and composite films, temperature/pressure bimodal sensors can be made thinner and lighter, rendering them more suitable for applications in health monitoring and robotic electronic skin.

5.2 Health monitoring

An array of temperature/pressure bimodal sensors enables perception of the surrounding environment by monitoring changes in capacitance and resistance to achieve real-time monitoring of environmental and object states, thus fulfilling health monitoring functions.¹⁷⁸ Changes in capacitance reflect deformation and displacement of materials, while changes in resistance can indicate the pressure exerted on an object when it comes into contact with the E-skin.¹⁷⁹ These data allow the sensor to independently monitor temperature and pressure, thus accurately obtaining information about the temperature state and dimensions of the object.¹⁸⁰ To achieve human health monitoring, Li et al. utilized the microstructured fibers (24.8 μm) and high surface roughness (197.255 nm) of the MOF–MSMC composite film to enable a pressure detection limit as low as 1 Pa, and by optimizing the thickness of the MOF film (245.4 nm), both the pressure response time and recovery time were reduced to 20 ms.¹³⁴ By measuring the output voltage and resistance changes of each pixel, 2D temperature thermal maps and 3D pressure histograms can be obtained, enabling accurate observation of temperature and pressure distributions under external stimuli, as shown in Fig. 10(a).¹⁸¹ Additionally, integrating self-power generation with pressure-sensing capabilities enables the development of flexible wearable devices.¹⁸² This system relies on the sensor's ability to fully convert low-temperature waste heat between human skin and the environment, combined with its superior pressure-sensing performance, to continuously monitor vital signs without an external power source.¹⁸³ Temperature/pressure bimodal sensors can also achieve precise capture of motion postures.¹⁸⁴ When an electronic skin embedded with sensors is attached to a finger joint, the built-in pressure sensors exhibit responses to the pressure exerted on their surface as the bending angle of the finger changes.¹⁸⁵ This pressure change will cause the pressure sensor to output a different voltage signal, thus realizing the accurate capture of real-time changes in the bending angle of the finger.¹⁸⁶ Different bending angles generate different voltage outputs, with the help of which the system can accurately analyze and evaluate the dynamic posture of the finger.¹⁸⁷

Fig. 10 Application of bimodal sensors in health monitoring. (a) Application of bimodal sensors in environmental sensing. Reproduced with permission.¹⁸⁸ Copyright 2025, American Chemical Society. (b) Application of flexible dual-sided sensors in self-powered systems. (c) Motion posture capture using bimodal sensors. Reproduced with permission.¹³⁴ Copyright 2022, American Chemical Society.

5.3 E-skin for robots

The application of temperature/pressure bimodal sensors in robotic E-skins aims to give robots the ability to sense and interact with each other, enabling them to perform tasks more naturally in a wide range of environments.¹⁸⁹ Gao et al. enhanced the pressure response using a 3D porous structure, achieving a pressure sensitivity of 1.97% kPa⁻¹ within the range of 0–25 kPa, while the low thermal conductivity of PDMS foam (0.15 W (m K)⁻¹) and the high Seebeck coefficient of PEDOT:PSS/CNT (40.5 μV K⁻¹) ensures the stability of temperature sensing.¹³⁵ By combining the dual detection functions of temperature and pressure, such sensors enable robots to mimic human tactile and temperature sensing, thereby enabling a variety of complex functions.¹⁹⁰ First, the integration of the bimodal temperature/pressure sensor enables the robot to provide a precise tactile response to external objects.¹⁹¹ When the robot touches an object, the sensor is able to monitor the pressure applied to its surface in real time to determine the shape, hardness, and texture of the object.¹⁹² At the same time, this sensor is able to sense the temperature of the object, enabling the robot to recognize whether the object is warm, cold or at room temperature.¹⁹³ This dual sensing allows the robot to not only avoid applying too much force when grasping an object and causing damage, but also to react quickly when coming into contact with a heat source, thus safeguarding its own safety.¹⁹⁴ The detection capabilities provided by temperature/pressure bimodal sensors can also be extended to the environmental adaptability of robots.¹⁹⁵ When the robot works under different environmental conditions, the temperature sensor can monitor the surrounding temperature changes in real time, so that the robot can adjust its work strategy according to the changes in the environment.¹⁹⁶ For example, in a hot environment, the robot can reduce its work intensity or change its work path to ensure stable and safe operation.¹⁹⁷ This adaptability not only improves the robot's work efficiency, but also enhances its ability to survive in complex environments.¹⁹⁸ Notably, a skin-integrated haptic interface system based on a stretchable pressure sensor array with ultrahigh sensitivity (96.1 kPa⁻¹) and wide pressure range (up to 123.1 kPa) enables wireless tactile visualization, enhancing immersive interactions in VR/AR and robotic human–machine interactions.¹⁹⁹ By sensing different pressures and temperatures, the robot can adjust its response according to the strength and location of the touch to provide personalized services.²⁰⁰

6. Conclusion and prospects

In this review, recent advances in temperature/pressure bimodal sensors in terms of bionic design principles, material selection, device design, system integration and applications are summarized. Bionic design provides the theoretical basis for the preparation of multifunctional tactile sensors, the correct material selection can ensure the stability and effectiveness of the devices, and different integration methods lead to different structural designs to realize different functions.

Although some progress has been demonstrated in the applications of temperature/pressure bimodal sensors, multiple challenges that may affect the performance, accuracy and reliability of the sensors remain (Fig. 11).²⁰¹ The sensing performance of the bimodal temperature/pressure tactile sensors is highly related to the structural design, the sensing materials, and the manufacturing process. Therefore, in order to improve the sensitivity and accuracy of the sensor, it is necessary to select high-performance materials, such as composite materials with excellent thermal and electrical conductivity, and optimize the structural design of the sensor.²⁰² The environmental interference problem is also a major challenge for temperature/pressure bimodal sensors. The surrounding environmental factors, such as vibration, airflow changes and temperature fluctuations, may significantly affect the measurement results and lead to data instability.²⁰³ To solve this problem, laboratories should adopt vibration isolation measures to reduce the interference of external vibrations on sensor performance. Meanwhile, the use of insulating materials and shielding techniques can also effectively reduce the impact of surrounding electromagnetic interference. In the process of data acquisition, advanced data processing algorithms can be considered to eliminate outliers and noise, thus improving the reliability of the final results. The issue of long-term stability is another challenge that needs to be addressed.²⁰⁴ Over time, the performance of temperature/pressure bimodal sensors may degrade, resulting in their insufficient long-term stability, especially under intense experimental conditions. To improve long-term stability, it is crucial to select materials with high durability, which will help enhance the aging resistance of the sensors. In terms of integration and compatibility, the application of temperature/pressure bimodal sensors faces the same challenges. When integrating the sensors with other devices or sensors, inconsistencies in signal processing methods and communication protocols may arise, which can lead to difficulties in data integration. Therefore, when selecting sensors, it is important to ensure that they have uniform interface standards and communication protocols to facilitate seamless integration with other devices. In addition, the use of a modular design can allow different sensors to work independently or cooperatively in the same system, which will enhance the flexibility and compatibility of the system. In the manufacturing process of sensors, numerous technical difficulties and challenges also exist, primarily stemming from the multifunctionality of materials, the complexity of microstructures, and the constraints of production processes. Over decades, a wide range of manufacturing techniques have been employed, integrated, and advanced to produce high-performance flexible pressure and temperature sensors. These methods encompass coating processes (such as spray-coating and spin-coating), photolithography, etching, self-assembly, doctor-blading, electrospinning, chemical vapor deposition, additive manufacturing, and various printing technologies (including screen printing, inkjet printing, gravure printing, and flexography printing). Among all manufacturing methods, printing processes, including both contact and non-contact printing, are most suitable for large-scale production. Compared with other methods, printing techniques can achieve high-resolution and complex patterns required for sensors while avoiding cumbersome processing steps.

Fig. 11 Challenges and future perspectives of temperature/pressure bimodal sensors.

Emerging technologies currently show promise for addressing the challenges faced by flexible temperature/pressure bimodal sensors. For instance, nanoimprint lithography enables the fabrication of micro-nano structures required for sensors with high precision and low cost. Meanwhile, machine learning and deep learning algorithms leverage big data and algorithmic models to automatically interpret multi-parameter signals, thereby resolving issues of signal cross-interference and decoupling. Novel multifunctional high-performance material technologies enable long-term stable operation of sensors. For example, the introduction of self-healing polymeric materials allows automatic repair of microcracks through self-healing mechanisms based on microcapsule or porous structures when microcracks or damages occur, thereby prolonging the service life of devices. Furthermore, multi-signal fusion and intelligent decoupling technologies, as an effective solution to environmental interference, can achieve joint detection of multiple physical quantities such as temperature and pressure by integrating multi-parameter sensing and intelligent decoupling algorithms. They utilize advanced algorithms like machine learning for signal self-calibration, significantly enhancing the anti-environmental interference capability. Specifically, by incorporating complementary information from multi-sensing signals, the system can extract valid information from signal deviations caused by environmental changes and automatically correct sensing errors.

Overall, temperature/pressure bimodal sensors have shown good momentum in technological advances, market demand and the expansion of application areas. In brief, a new era of intelligent sensing society is expected through the efforts of researchers from this research area.

Conflicts of interest

The authors declare no conflicts of interest, financial or otherwise.

Data availability

No primary research results, software or codes have been included and no new data were generated or analyzed as part of this review.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant no. of 52305598), the Guangzhou Basic and Applied Basic Research Foundation (grant no. 2025A04J5202), and the Foundation of “The Pearl River Talent Plan” of Guangdong Province (2023QN10C596).

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