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Advancing healthcare through piezoresistive pressure sensors: a comprehensive review of biomedical applications and performance metrics

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Published 27 September 2024 © 2024 The Author(s). Published by IOP Publishing Ltd
, , Citation Mokhalad Alghrairi et al 2024 J. Phys. Commun. 8 092001DOI 10.1088/2399-6528/ad7d5d

2399-6528/8/9/092001

Abstract

Piezoresistive pressure sensors have transformed biomedical applications, enabling precise diagnostics and monitoring. This concise review explores the fundamental principles, key components, and fabrication techniques of piezoresistive pressure sensors, focusing on critical performance metrics such as sensitivity, accuracy, and response time. Biomedical design challenges, including biocompatibility and long-term stability, are examined, offering insights into solutions for optimal sensor integration. In diverse biomedical applications, piezoresistive pressure sensors play pivotal roles, from blood pressure monitoring to implantable medical devices. The paper emphasizes their versatility in enhancing patient care through continuous and accurate monitoring. Looking forward, the review discusses emerging trends and potential research directions, positioning piezoresistive pressure sensors as central contributors to the future of biomedical technology, promising improved patient outcomes and advanced healthcare delivery through precise and continuous monitoring.

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1. Introduction

Pressure sensors play a pivotal role in various applications, ranging from industrial processes to biomedical diagnostics [14]. Many similar structures were developed in the field of pressure sensing, wherein every structure displayed distinctive properties that were suited for particular applications. Some of these common pressure sensor structures included piezoresistive [5], capacitive [6], optical [7], piezoelectric [8], and resonant [9], which were related to the micro-electro-mechanical systems (MEMS) technology. The MEMS devices incorporated the actuators, sensors, mechanical components, and electronics on one chip, usually at the microscopic scale [4].

Every above-mentioned pressure sensor structure is applied by using the MEMS fabrication procedures, which improves their use in the highly-integrated, compact, and miniaturised applications. The Piezoresistive pressure sensors display several advantages like a low power consumption, small size, and better sensitivity, which makes them suitable for many industrial applications, especially in the biomedical field [10].

One of the most popular types of pressure sensor structures, i.e., the piezoresistive pressure sensors function on the principle that states that the electrical resistance of a few materials can change when they are subjected to a mechanical stress. This difference in the resistance is seen to be directly proportional to the pressure that is applied, which helps in estimating their precise pressure values. The piezoresistive sensors are reliable, simple, and inexpensive, which makes them popular in different fields [5].

In this study, the researchers have described the primary piezoresistive effect and its historical application in semiconductors. This study also presents the conventional utilisation of rigid Piezoresistive Sensors (PSs), like those constructed using single-crystal silicon, and their development for fabricating better and flexible designs. The researchers also reviewed a few published studies that used PSs to monitor many physiological factors such as blood pressure, followed by other vital signs like heart and respiratory rates. This study has further investigated the use of different materials to enhance device sensitivity, which includes materials like carbon and graphene nanotubes, in addition to the application of functionalized materials for selective sensing using biofluids. This is a comprehensive review that highlights the relevance of pressure-based PSs to improve personalized healthcare via accurate and continuous physiological monitoring. The findings of this review will help develop biomedical technology in the future.

2. Traditional and modern piezoresistive pressure sensors

Many biomedical applications use the pressure sensing technology for monitoring, diagnosing, and controlling the different health conditions. Researchers have used piezoresistive pressure sensors in different biomedical applications because of their basic advantages, like higher sensitivity, miniature size, and better compatibility after being integrated with other medical devices. The piezoresistive pressure sensors function on the principle that states that the electrical resistance of any material changes after the application of a mechanical stress. In biomedical applications, these sensors are crucial for measuring physiological pressures [11, 12]. The concept of the piezoresistive effect, where the electrical resistance of a material changes in response to applied mechanical stress, was fundamental to the development of piezoresistive pressure sensors as illustrate in figure 1. This effect was first observed in semiconductors, such as silicon, where changes in resistance were noted when the material was subjected to mechanical deformation.

Figure 1. Refer to the following caption and surrounding text.

Figure 1. Illustrate piezoresistive pressure sensor.

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Traditional piezoresistive strain/pressure sensors utilize materials like single-crystal silicon, metals, and metal oxide films, which are sensitive yet lack flexibility and can only endure minimal strain due to their rigid and brittle characteristics [13]. To address this, innovative approaches have been developed for creating flexible and stretchable sensors. The work by Rogers and Huang has been notable in designing stretchable electronics using inorganic materials like silicon and metal films, implementing mechanics-driven methods and specific buckling patterns to achieve flexibility [14]. Additionally, the use of serpentine and fractal patterns in metal films enables the creation of stretchable connections for practical applications [15]. Another method involves using flexible electrodes with unique microstructures to form sensors that detect pressure changes through the interaction of these electrodes. Despite these advancements, such strategies often involve complex micro/nano fabrication processes and offer limited strain detection capabilities [16, 17]. Emerging solutions focus on three-dimensional and porous conductive materials, presenting a promising avenue for flexible piezoresistive sensors. These materials boast benefits like being lightweight, capable of handling large strains, and offering cost-effective and simple manufacturing. The material science and engineering community benefits from integrating nanomaterials and functional soft materials, such as conducting polymers and carbon nanotubes, into porous structures. These materials have several applications, e.g., as electrode materials in energy storage devices [18]. In the past, a few studies investigated the probability of using the piezoresistive materials for physiological pressure sensing, especially measuring blood pressure [19]. In the earlier studies, researchers evaluated the use of pressure-based PS devices as primary devices to assess the vital physiological parameters and develop real-time personalized medicines. The popularity of these PS devices has increased because of their simple fabrication procedure, low energy usage, and high sensitivity, particularly while determining minor pressure changes. The properties of the PS devices help in constantly tracking and determining the physiological parameters, which could help in developing patient-specific medical interventions [20, 21].

3. Advances in sensor sensitivity and design

Developments made in the sensor technology have improved the design and sensitivity of sensors that could be used in different applications. These developments were specifically noted in the PS devices, which have garnered a lot of popularity owing to their increased reliability, compatibility with Micro and Nanoelectromechanical systems (MEMS/NEMS), and simple integration into different systems [22]. A few recent studies have highlighted the advantages of utilizing the p-type piezoresistive materials compared to n-type materials because of their improved performance in many strain-sensing-based applications [23]. Furthermore, several researchers noted the promising use of other nanomaterials like graphene, Carbon Nanotubes (CNTs), and few additional 2-dimensional materials because of their better mechanical properties and high gauge factors [24]. When these nanomaterials were integrated into the PS designs, it led to the development of novel PSs that could detect very minute variations in force or pressure and displayed a very high sensitivity.

Advancements noted in the structural design could also improve the PS sensitivity of the devices. Furthermore, the accuracy and responsiveness of the PS devices increased when the PS constituents were miniaturized and complex geometries were developed. For example, the integration of nanoscale components within the PS design and optimisation of the piezoresistive network could improve the signal output and strain distribution of the device. In the past, the researchers noted that structural designs maximizing the surface-to-volume ratio of the sensing components could improve device sensitivity [25].

The combination of many advanced technologies like MEMS and NEMS has significantly improved the PS function and design. These techniques help in fabricating compact and sensitive PSs that could be used in several environmental applications. Additionally, the combination of advanced signal processing methods and the construction of advanced readout circuits have improved PS performance. The above integrated systems offer high-resolution data while minimising noise interference, which makes them excellent devices to be used in dynamic and harsh situations.

Earlier studies have employed high-sensitivity PS devices in different fields such as structural health monitoring, biomedical monitoring, wearable technology. For example, in the case of biomedical applications, the PSs can be used to monitor vital signs like pulse rate and blood pressure, which can present important data to enhance patient care. In the case of structural health monitoring, PS devices are utilised to identify minor variations in the construction of bridges, buildings, and other infrastructures, which increases reliability and safety of the people. Additional development in the PS technology can increase the performance and sensitivity of the PS devices by combining different technologies, new materials, and novel design strategies. The development of the next-gen novel and sensitive PS devices depends on the constant investigation of nanomaterials and modifications of the MEMS/NEMS fabrication processes.

Advancements made in the design and sensitivity of the PS devices have offered reliable, accurate, and flexible sensing solutions that improve their applications in different industries. The performance of the PS devices has improved considerably owing to the strategic utilisation of novel structural designs, advanced materials, and cutting-edge integration processes. The functionality, sensitivity, and the future application of the PS devices would be further improved with significant developments made in the field of PS technology.

4. Structural of piezoresistive pressure sensor

Micro-pressure-based PSs detect micro pressure changes by deforming a diaphragm under pressure, which are then converted into electrical signals using piezo-resistors. The simplest and most traditional structure is the flat diaphragm, where piezo-resistors are placed at the edges or the centre. While this structure provides a baseline for performance, it often struggles with the trade-off between sensitivity and linearity, as the central region tends to deflect more than the edges, leading to non-linear outputs. Therefore, more sophisticated structural designs have been developed to address these limitations as shown in figure 2.

Figure 2. Refer to the following caption and surrounding text.

Figure 2. Presents three common configurations for beam islands in piezoresistive micro pressure sensors: (a) A peninsula structure, (b) A cross-beam membrane combined with a peninsula structure, and (c) An enhanced design featuring four petals. Reproduced from [26]. CC BY 4.0.

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To enhance performance, structural engineering techniques such as beam–membrane and groove structures have been employed. The beam–membrane structure incorporates stiffening beams within the diaphragm, concentrating stress around the piezo-resistors and thereby improving sensitivity while maintaining linearity. Variants like the Cross Beam–Membrane (CBM) structure use these beams to ensure that the diaphragm remains stiff in critical regions, reducing excessive deflection. Similarly, groove structures, which involve etching grooves near the piezo-resistors, create stress concentration points that boost sensitivity. These grooves can be local or annular, depending on their placement and intended function as shown in figure 3.

Figure 3. Refer to the following caption and surrounding text.

Figure 3. Depicts different groove structures: (a) A localized groove and (b) An annular groove. Reproduced from [26]. CC BY 4.0.

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5. Applications in physiological monitoring

Numerous studies have demonstrated the effectiveness of piezoresistive sensors in monitoring various physiological parameters, including blood pressure, and other vital signs. Here is an overview of some notable research in each area:

5.1. Blood pressure monitoring

Blood pressure (BP) is a critical physiological parameter that plays a pivotal role in the quantitative assessment of cardiovascular health [27]. Notably, cardiovascular diseases stand as the leading cause of mortality worldwide, accounting for approximately 18% of all deaths [28]. Early detection and ongoing monitoring of blood pressure are essential strategies that could prevent a significant number of these fatalities. High blood pressure, commonly referred to as hypertension, is a key factor in the onset of life-threatening cardiovascular conditions, including cardiac arrest, coronary artery disease, and stroke [29, 30].

Blood pressure (BP) is defined as the force exerted by circulating blood on the walls of blood vessels. It can be assessed through two primary methods: invasive and non-invasive techniques [31, 32]. The non-invasive approach has become particularly popular for its ease of use, eliminating the necessity for professional medical skills and enabling individuals to measure their BP in the comfort of their own homes. This method encompasses various techniques such as oscillometry, tonometry, auscultatory, and photoplethysmography as shown in figure 4(b) [34, 3739]. Traditionally, the auscultatory method, which involves detecting the Korotkoff sounds using a stethoscope, was the standard practice for blood pressure measurement [40].

Figure 4. Refer to the following caption and surrounding text.

Figure 4. Illustrate different type of pressure sensor (a) The APBP pressure sensor. Reprinted with permission from [33]. Copyright (2019) American Chemical Society, (b) Commonly used calibration tools (cuff BP device, sphygmomanometer, volume clamp). Reproduced from [34]. CC BY 4.0, (c) Current mapping on two different pressure sensors for different magnitudes of pressure. Reprinted with permission from [35]. Copyright (2015) American Chemical Society, (d) A flexible multifunctional sensor combining piezoelectric, triboelectric, and pyroelectric effects. [36] John Wiley & Sons. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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The introduction of piezoresistive pressure sensors to biomedicine represented a major advancement, especially in the field of non-invasive blood pressure (NIBP) monitoring. These sensors' ability to be miniaturized has allowed for their incorporation into catheters and other medical devices, transforming the practice of precise and localized pressure measurement within the human body [19].

Recently, there has been a burgeoning interest in flexible electronic devices utilizing micro- and nanostructured paper as shown in figures 4(c), (d) [41, 42]. This type of paper, characterized by its porous and textured nature, becomes a viable active material for such devices following specific treatments [43, 44]. Notably, nanocellulose paper (NCP), known for its high transparency and smooth surface, has emerged as a popular substrate for flexible electronics due to its ability to maintain electrode conductivity as shown in figure 4(a) [45, 46], which is often compromised in papers with high surface roughness [47]. While various paper-based pressure sensors have been documented, their reliance on petroleum-based polymers for encapsulation and the complex, costly manufacturing of interdigitated electrodes and sensing elements pose challenges to creating disposable, affordable, and environmentally friendly sensors [48]. However, progress has been made with the development of an all-paper-based piezoresistive (APBP) pressure sensor that offers affordability, simplicity in manufacturing, and quick production times.

These sensors have demonstrated exceptional efficacy, featuring high sensitivity like (1.5 kPa−1 within a range of 0.03 to 30.2 kPa), extremely low energy consumption (approximately 10−8 W), minimal operating voltage (0.1 V), and swift response time (90 ms) [33]. Various topographical microstructures, such as pillars [35], hemispheres, [49] triangular pyramids, [36] and interlocked configurations [50], have been incorporated into the design of flexible piezoresistive sensors to enhance their sensitivity, detection limit, and response ti. These unique microstructures have been proven to significantly improve the sensor's performance [16, 5153]. However, both natural and synthetic micro/nanostructured flexible materials struggle to retain high sensitivity under pressures exceeding 100 kPa, [54, 55], mainly due to the limited contact area under relatively low pressures [56]. The structural integrity of a piezoresistive three-dimensional (3D) conductive sponge, created through freeze-drying and coated with conductive nanomaterials, is compromised under high pressure, attributed to its inherent rigidity and fragility and poor bonding between the conductive layer and the sponge substrate [57, 58]. Other sponge-based sensors, like those using template-based, porous reverse-micelle-induced, or laser-scribed graphene, can withstand high pressures but suffer from reduced sensitivity [56, 59]. The goal is to design sensors capable of detecting pressure changes across a broad spectrum, from a few pascals to several megapascals, without compromising on sensitivity [60]. However, the high cost of raw materials and complex manufacturing processes pose obstacles to the widespread adoption of advanced soft electronics [61, 62]. Nonetheless, successful integration of thoughtful structural design with appropriate materials has led to the creation of highly sensitive pressure sensors with extensive operational ranges. Inspiration from nature, particularly the complex porous architectures seen in honeycombs, spider webs, and ant nests, has played a pivotal role in advancing sensor design, showcasing the potential of biomimicry in technological innovation [63, 64]. Upon closer examination of ant nests, researchers have discovered that their structure comprises a three-dimensional, interlaced hierarchical porous framework, distinguished by superior mechanical properties that safeguard against destruction [65]. These hierarchical porous structures excel in maximizing contact area and facilitating intermolecular interactions, while reducing diffusive resistance, a stark contrast to conventional porous frameworks with uniform pore sizes. Leveraging these unique structural attributes has proven beneficial in applications such as sensing, energy storage, and catalysis [66]. To enhance the mechanical stability and functional performance of pressure sensors, adopting the complex morphologies and structures found in nature seems a prudent approach. A case in point is the development of a flexible piezoresistive sensor inspired by nest-like architectures. This design, in conjunction with a carbon black percolation network, significantly boosts the sensor's sensitivity and broadens its pressure detection range. The resulting sensor not only exhibits an impressive sensitivity of 1.12 kPa−1 but also boasts a low detection threshold of 20 Pa and an extensive measuring range that extends to 1.2 MPa [67].

The incorporation of a boss diaphragm in sensor design has been observed to linearly enhance sensitivity and affect non-linearity. Studies involving the cross-beam membrane structure have reported high sensitivity with linear response characteristics, reaching 7.018 mV kPa−1 [68, 69]. For altimetry applications, the beam and island design in piezoresistive micro pressure sensors demonstrated a notable sensitivity of 17.795 μV V/Pa within a 500 Pa operational range at room temperature [70]. Further innovation was seen with a novel boss diaphragm integrated with a peninsula-island structure, aimed at minimizing strain energy outside the focal stress area, achieving a sensitivity of 0.066 mV/V/Pa and a non-linearity of 0.33% FS for low-pressure measurements [71]. Nonetheless, the beam and island combination has been associated with increased linearity errors, posing limitations on the sensor's applicability across different domains. Study [72] utilized a peninsula-structured membrane to measure pressure, achieving a low non-linearity of 0.36% FSS at a pressure of 5 kPa and a sensitivity of 18.4 mV/V for the full scale output. Research [73] described a nanoelectromechanical system (NEMS) piezoresistive pressure sensor with angular grooves on the diaphragm, capable of operating within a pressure range of 120 mmHg. This angularly grooved membrane acted as a stiffened structure, resulting in a normalized sensitivity of 28.4 9 10–10(DR/R)/mmHg/lm2) alongside a very low non-linearity error. In an earlier study [74], the researchers used a rod beam and grooved membrane for fabricating a sensor, which improves its performance and structural integrity. They employed the finite element and curve fitting analysis for estimating the pressure sensor dimensions, which showed a non-linearity of 0.21% full scale span (FSS) and sensitivity of 30.9 mV/V/psi for low-pressure values. The earlier studies have stated that the integration of a stiffened membrane (SM) in the structure of the pressure sensor could help to improve its sensitivity and decrease its non-linearity. This SM technique selectively strengthens this membrane, which ensures that the pressure that is applied can optimise the sensitive areas within the sensor without affecting its linearity and sensitivity. Material selection for temperature compatibility is crucial, allowing the sensor to operate efficiently at elevated temperatures. A rod beam structure, recommended for the graphene piezoresistive pressure sensor, capitalizes on SM and thermal adaptability. This structure, arranged diagonally across the sensor's central square surface, provides localized stiffening at critical strain points. At room temperature (25 °C), a sensor with this rod beam structure exhibits a sensitivity of 6.28 mV psi−1, which increases by 58% at 30 °C compared to sensors lacking the rod beam design, especially in low-pressure scenarios [75].

Studies [76, 77] have enhanced both the sensitivity and linearity of sensors by implementing an island/beam structure atop a flat diaphragm as shown in figure 5(d). Research [81] further refined the flat pressure sensor's performance by modulating the doping concentration of the piezoresistive material and altering the diaphragm's thickness. Additionally, sensor miniaturization, which involves thinning the sensor to prevent device fragmentation during subsequent processes, is crucial. However, reducing the wafer's thickness too much can lead to increased chip fragmentation due to decreased structural integrity, subsequently affecting the production yield adversely.

Figure 5. Refer to the following caption and surrounding text.

Figure 5. Illustrate (A) Schematic diagram of the 4H-SiC piezoresistive sensor. Reproduced from [78]. CC BY 4.0, (B) Schematic of one piezo-resistors and configuration of the sensing membrane and the layout of interconnection. Reproduced from [79]. CC BY 4.0, (C) Pictorial representation of the hybrid film. Reproduced from [80]. CC BY 4.0, (D) Partial section view and detailed structure around the rib place of the membrane. Reproduced from [77]. CC BY 4.0.

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A novel fabrication process for miniature piezoresistive pressure sensors, utilizing temporary bonding technology, has been proposed to maintain device thickness at a minimal 100 μm, thereby preventing fragmentation. This technique involves the processes like chemical mechanical polishing (CMP), temporary bonding, and deep reactive-ion etching (DRIE). They carried out several theoretical analyses and simulations, and established the optimal location of piezoresistive elements, dimensions of a sensing diaphragm, and doping levels, which led to the designing of a half-bridge pressure sensor [77]. Another advancement includes the development of a technique that uses an air chamber and MEMS piezoresistive cantilevers for determining the pressure changes, which can improve the reliability and accuracy of the readings [82]. Furthermore, in another study, the researchers developed another cutting-edge free-standing graphene hybrid material, which included functionalised gold nanoparticles (AuNPs) sandwiched between 2 graphene layers as shown in figure 5(c). This sensor, which was fabricated using a vacuum filtration and layer-by-layer deposition technique, displays a rapid response, high stability, higher conductivity, very high sensitivity, and an extended durability, which indicates a significant development of the sensor technology [80].

In the recent past, the researchers developed the silicon-on-insulator (SOI) technology for designing the piezoresistive pressure sensors displaying a super high-temperature resilience as shown in figure 5(a). The results indicated that this technology successfully overcame the operational temperature limitations presented by the conventional bulk-silicon piezoresistive sensors, which allowed these sensors to operate under extreme conditions, i.e., they could withstand temperatures of ≥250 °C [78, 83, 84]. Furthermore, the SOI-based pressure sensors display better process control under mass production, particularly in comparison to new materials such as silicon nanowires, silicon carbide, and graphene. In a research study Li and colleagues [8587], created a pressure sensor using SOI technology featuring a width of 1000 μm. This sensor demonstrated the capability to operate effectively at temperatures exceeding 350 °C showcasing its resilience in conditions. Additionally, they developed another pressure sensor utilizing SI (silicon insulator) with a similar 1000 μm diaphragm width that could function at 350 °C. They also designed an SI (silicon insulator) pressure sensor with a 1000 μm diaphragm width, which could operate at temperatures of 350 °C [88]. created a pressure sensor using SOI technology featuring a width of 1000 μm. This sensor demonstrated the capability to operate effectively at temperatures exceeding 350 °C showcasing its resilience in conditions. Additionally they developed another pressure sensor utilizing SI (silicon insulator) with a similar 1000 μm diaphragm width that could function at 350 °C [77]. Meng et al designed a piezoresistive pressure sensor where, following numerical simulation for optimization, the piezo-resistor was strategically positioned at the centre and edge of the diaphragm with a thickness of 2 μm. This design achieved a sensitivity of 37.79 mV V−1 MPa−1, with a low hysteresis of 0.09% Full Scale (FS) and excellent repeatability of 0.03%FS, although the sensor's dimensions were relatively large, measuring 5 mm by 5 mm by 0.9 mm [89]. Yao et al examined a high-temperature SOI piezoresistive pressure sensor with integrated signal conditioning, designed for consistent performance within a temperature range of 50 °C to 220 °C. However, the sensitivity recorded for this sensor was modest at 0.42 mV V−1 KPa−1 [84]. Gao et al created a C-structure SOI (Silicon on Insulator) piezoresistive pressure sensor capable of measuring pressures from 0 to 45 bar, albeit with a relatively low sensitivity of 9.21 mV bar−1 [90]. Song et al developed an SOI pressure sensor incorporating a Wheatstone bridge on the diaphragm's lower surface to minimize exposure to external factors, achieving a wide pressure range but with a sensitivity of 20 mV V−1 KPa−1 [91]. Companies like Goodrich, Gefran, and Kulite have introduced SOI high-temperature pressure sensors designed for operation in medium- and high-temperature environments [92]. The challenge remains in enhancing the high-temperature capabilities of these sensors while ensuring high accuracy. Extensive research in MEMS piezoresistive pressure sensors has been dedicated to enhancing sensitivity and temperature endurance. A high-performance SOI-based piezoresistive differential pressure sensor was engineered for air pressure measurement ranging from 0 to 30 kPa, with an optimized sensing structure through stress distribution simulation as shown in figure 5(b), ensuring concentrated stress and controlled diaphragm dimensions for optimal performance [79]. The trade-off between compactness and superior sensing ability in high-temperature environments warrants further investigation.

Additionally, a novel piezoresistive sensor was developed by [93], employing a PDMS (polydimethylsiloxane) sponge as the elastic medium and rGO (reduced graphene oxide) for conductivity, resulting in a flexible and highly sensitive pressure sensor. This innovation leverages the properties of rGO and PDMS to achieve flexibility and enhanced sensitivity in pressure detection.

In summary, Engineering piezoresistive devices that balance sensitivity and sensing range involves overcoming significant challenges. This balance is crucial because high sensitivity often leads to a limited sensing range, while a broad range can compromise sensitivity. The task is to design materials and structures that respond to minute changes in pressure (indicating high sensitivity) without sacrificing the ability to detect changes over a wide range of pressures (indicating a broad sensing range). Advanced materials engineering, precision fabrication techniques, and innovative design strategies are often required to optimize these properties in piezoresistive sensors.

5.2. Multi-vital sign monitoring

Piezoresistive pressure sensors are commonly utilized for monitoring signs expanding their usefulness beyond just pressure measurements to encompass a wide range of health monitoring functions. Several researchers have begun exploring the potential of these sensors, in capturing signals. For instance, Meng and colleagues employed nanoparticle-based strain sensors to detect expressions in a study. This application showcased the ability of sensors to detect physical changes demonstrating their versatility in analysing complex physiological signals [94]; In a study Zhixiang Li and team developed a strain sensor using thermoplastic polyurethane (TPU) and multiwalled carbon nanotubes for tracking human movements [95].

In a research project scientists created a sandwich structured paper-based sensor. The sensor featured CSA as a pressure film sandwiched between two paper layers with electrodes applied using screen printing. The sensor was evaluated for tracking signals, like vocal cord vibrations, finger movements and wrist pulses. These demonstrations showcase the use of the sensor, in technology and healthcare applications indicating its capability to monitor and evaluate different physical activities and bodily functions [96].

The transcranial doppler (TCD) method is commonly employed in neuro care to track the variations, in blood flow (CBF) particularly in relation, to vasospasm associated with subarachnoid haemorrhage. The models that use the TCD-derived data have shown a correlation with the invasively-measured ICP, since they make use of factors like arterial blood pressure, flow velocity (FV) in the mid-cerebral artery, and pulsatility index (PI) [97103]. Recently, studies have been conducted that involve combined models for estimating the ICP. These studies have demonstrated a good relationship with the invasive estimations [100]. Though TCD has shown a significant potential, it has a few disadvantages, which include intra- and inter-observer variability, offering a single-time measurement, and many challenges in the interpretation of values displayed by a group of patients [104]. The applicability of TCD is more related to neuro-monitoring adjuncts instead of a primary ICP sensor [105].

The metabolic activity and the respiratory health of a person is reflected by his respiratory rate, i.e., no. of breaths every minute [106]. The respiratory rate can be monitored by using piezoresistive pressure sensors as it detects the minor pressure changes resulting due to the expansion and the contraction of the abdomen or chest during breathing [107]. When these sensors are placed into bedding materials or inserted within the wearable devices, they can constantly monitor the respiratory patterns of an individual, thereby offering essential data for studying their respiratory health in the home and clinical settings. This application helps to monitor some conditions where the patient's respiratory function gets affected, such as asthma, chronic obstructive pulmonary disease (COPD), and sleep apnoea [108].

The piezoresistive pressure sensors also help in monitoring heart rates [11, 19]. They are able to determine the pressure changes linked to the patient's heartbeat, which assists in constantly monitoring the heart rate. This ability of the sensor is essential to assess the cardiac health in heart patients or those who undergo rigorous physical activities. When these sensors are integrated into the wearable devices like fitness bands and smartwatches, the individuals are able to measure their real-time heart rates, which further enables them to proactively manage their wellness and health [1, 109]. The constant data that is presented by the above sensors helps in early identification of cardiac anomalies and guides in medical intervention.

The pulse oximetry depends on optical techniques for measuring the blood oxygen levels [110], however, the piezoresistive pressure sensors are seen to play a complementary role as they offer pressure data for calibrating and stabilising the devices against the physiological and environmental pressure changes. This combination improves the preciseness of the oxygen saturation values, which help to monitor the cardiac and respiratory state of the patients, particularly those with respiratory issues or during the surgical processes.

All the above studies present the reliability and versatility of the piezoresistive sensors in determining the physiological factors. The piezoresistive sensors can be used to determine different pressure changes, ranging from blood pressure to intracranial pressure or even more, thus proving their efficiency in different medical applications. Thus, they can be used for further improving the assessment of personalized healthcare.

6. Materials for sensitivity improvement

Several studies have investigated the use of novel materials and fabrication processes for improving the performance of the piezoresistive pressure sensors, especially their selectivity, sensitivity, and biocompatibility for different biomedical applications. The major findings and important areas of study have been presented below:

6.1. Novel materials for sensitivity improvement

It has been shown that the use of nanostructured materials, like graphene and carbon nanotubes [111], and a polycrystalline MoS2 thin film [112] can be used for improving the sensitivity of the piezoresistive pressure sensors. Moreover, exploring inspired manufacturing methods has enabled the creation of flexible biocompatible sensors that mimic the mechanical characteristics of living tissues. Techniques, like patterning and soft lithography are employed to produce sensors that seamlessly blend into the body minimizing discrepancies and lowering the chances of immune rejection or inflammation. These sensors are designed to mirror structures enhancing their precision and sensitivity, in detecting physiological changes.

In a similar vein, the researchers have also investigated the ability of conductive filler-embedded polymeric composites to improve the piezoresistive sensitivity [113]. This research has contributed to the progress of pressure sensor technology leading to the creation of specialized, responsive and biocompatible sensors. By combining these materials and innovative manufacturing methods it has facilitated physiological tracking making them suitable, for various biomedical and clinical uses. Summary details from literature on sensors are outlined in table 1 along, with the parameters and advancements observed in recent years.

6.2. Selective materials for specialized applications

Simultaneously, the research into the use of smart coatings for PSs involved in different biomedical applications helps in improving their selectivity in difficult environments. It was noted that these coatings respond to the specific biochemical conditions, which allows the PS to differentiate between the interfering and the essential stimuli present in the PS environment. It was further noted that these coatings were responsive which retained the PS's selectivity even in the presence of potentially-interfering compounds. This property could increase their implementation in several biomedical environments, which need the precise detection of physiological changes and chemical molecules. This improves the usability and dependability of PS devices in a variety of complicated biological systems, enabling the development of better diagnostic and medical monitoring equipment.

6.3. Biocompatible materials and fabrication techniques

The latest developments noted in the production of biocompatible thin films have changed the future of implanted PS devices [114]. The thin-film-based PS devices are made of materials that are compatible with the body. This can further decrease the probability of adverse reactions. In this study, the researchers fabricated biocompatible thin film-based PS devices that can maintain their functionality and integrity for a long duration after their implantation. This can reduce the likelihood of tissue rejection or response. Furthermore, it was noted that such materials are further used to accurately monitor the body's physiological factors. This, in turn, can improve the treatment strategies, finally contributing to medical research.

Additional investigation in the bio-inspired fabrication processes has led to the fabrication of flexible biocompatible PSs that replicate the mechanical properties displayed by biological tissues. A few of these processes including soft lithography and biomimetic patterning help in developing PSs that can be easily integrated in the body, which reduce the mechanical mismatch and lower the risk of inflammation or immune rejection [115]. The structure of the PSs resembles complex biological structures, which can increase their sensitivity and accuracy while detecting physiological changes.

This paper has contributed to the development of a pressure-based PS technology, which led to the fabrication of sensitive, selective, and biocompatible PSs. The integration of these materials and novel fabrication processes has helped in the constant, real-time physiological monitoring, which paves their application in different biomedical and clinical applications. Table 1 presents a summary of the recent literature study conducted on PSs, in addition to their parameters and developments that have occurred in the past few years.

Table 1. Summary of recent literature survey on piezoresistive sensors.

Study/AuthorYearDiaphragmSensitivity (mV/kPa)Operating range (kPa)Response time (ms)
[49]2014CNTs/PDMS interlocked microdome15.1 kPa−10.2 Pa- 59 kPaN\A
[70]2015BMQI17.8(mV/(V.kPa))0.5N\A
[81]2015SI0.328 mV/kPa0.5–40 kPaN\A
[116]2016PEDOT: PSS851 kPa−10–200.15
[117]2016Graphene8.5 kPa−10–1270
[118]2016Shuriken-structured4.72(mV/(V.kPa))3N\A
[119]2017Carbonized Silk Nanofiber34.47 kPa−10–516.7
[120]2017Vertically aligned CNT/PDMS0.3 kPa−12Pa-10 kPaN\A
[121]2017ACNT/G/PDMS19.8 kPa−10.6 Pa- 0.3 kPaN\A
[76]2017Modified beam–island3.654(mV/(V.kPa))1.2N\A
[122]2017CBMP5.16(mV/(V.kPa))5N\A
[123]2018MXene, PVA nanospacers147 kPa−10–18.56138
[48]2018Carbonized paper5.67 kPa−10–2080
[124]2018rGO, PVDF47.7 kPa−10–40060
[125]2018Graphene110 kPa−10–7580
[126]2018CNT47.062 kPa−10–2624
[127]2018Porous Carbon15.630–2135
[74]2018Top groove and bottom rood beam4.48(mV/(V.kPa))6.89N\A
[128]2018CBMP+ petal6.934(mV/(V.kPa))5N\A
[129]2019GNP1875.30–401.3
[130]2019PEDOT: PSS62.560–6N\A
[33]2019AgNWs, NCP1.5 kPa−10.03−30.290
[131]2020MXene151.40–15229
[132]2020Graphite-flakes modified nonwoven fabrics6.40–8004
[133]2020SOI, Borofloat 33, Cu-Cu36 μV/(V·kPa)0–180N\A
[80]2020Gr, AuNPs× 10–4 kPa−1 <15
[134]2020ITBM1.928(mV/(V.kPa))3N\A
[89]2021SOI0.037 mV/V/kPa0–2.5 MPaN\A
[79]2022SOI2.255 mV/V/kPa0 to 30N\A
[87]2022SOI0.308 mV/V/bar0–140 barN\A
[135]2022NPSD with groove4.54(mV/(V.kPa))5N\A
[90]2022SOI9.21(mV/V/bar)40 barN\A
[136]2022SI34(mV/V/kPa)1–5kpaN\A
[137]2023SOI7.75(mV/V)0–250(kPa)N\A
[138]2023SI47.928 mV/V/kPa0–10kpaN\A
[93]2024PDMS, rGO,0.27 mmHg−1400 mmHgN\A

7. Challenges in designing sensors for biomedical use

The use of MEMS pressure sensors, in settings relies on addressing issues related to the trustworthiness, precision and durability of the sensors. These aspects are crucial, for ensuring that the sensors can provide accurate pressure readings over a prolonged duration especially in demanding healthcare settings.

7.1. Biocompatibility

It's crucial to make sure that the materials utilized in creating MEMS pressure sensors are biocompatible because these sensors directly interact with tissues. Biocompatibility is defined as the capability of the above materials to interact effectively with the body, without giving rise to any adverse reactions, like inflammation, toxicity, or immune responses. It is crucial for sensors to be seamlessly integrated into systems ensuring their efficient functioning, over an extended period. To achieve this goal, thorough optimization and testing of all materials through a range of in vivo and in vitro studies are necessary. This process aims to confirm that the materials are biologically inert, non-toxic and exhibit compatibility, with biological tissues. The primary objective is to fabricate sensors that are used in the body or can be easily implanted, and are able to reliably transmit correct physiological data without affecting the function or health of the surrounding tissues. Thus, biocompatibility is regarded as the basic criterion for designing and fabricating MEMS pressure sensors that are used for biomedical applications. Hence, biocompatibility supports their reliability, functionality, and acceptance in many medical activities.

7.2. Long-term stability

Long-term stability is a critical factor that can affect the sensor performance, especially if the sensors are used in biomedical applications related to implantable devices. It is defined as the capability of the sensor to retain its accuracy and functionality over long periods without getting degraded. It is important to achieve a long-term stability as it involves a proper selection of materials that are durable and can resist the environmental and biological stresses. Hence, this factor comprises the application of advanced materials that can tolerate the dynamic and corrosive environment in the human body, which includes their resistance to thermal fluctuations, biofouling, and mechanical stress.

Furthermore, the use of protective layers or advanced coating materials is essential as it protects the sensor components from the negative effects displayed by the reactive chemicals in the body such as body fluids and enzymes. These protective coatings prevent the further degradation of the sensor materials and loss of signal integrity.

Also, it is essential to develop complete protocols for the calibration and maintenance of the devices, to determine their decreasing performance and sensor aging. This involves scheduled adjustments and assessments for ensuring that the sensor device displays an accurate and reliable output. This further helps in monitoring and collecting accurate data.

To summarise, the long-term stability of the sensors involved in biomedical applications can be attained by using a multifaceted technique that involves the selection of durable materials, effective engineering designs, and proactive maintenance protocols. All these strategies ensure that the external and the implantable sensors can effectively function over a long time. This assists in the continuous and accurate monitoring of the patient's health which is essential for improving patient care and can aid in medical research in the future.

7.3. Miniaturization and power consumption

The structure of the sensors that are used for biomedical applications is concentrated on miniaturisation and effective power consumption which fulfils the requirements of the wearable, minimally invasive, and implantable devices. Miniaturisation comprises the engineering of sensors, which are so small that they can be unobtrusive and are effortlessly integrated within or on the human body. This can improve the patient's comfort level and decrease the risk of discomfort or tissue damage. This involves the use of advanced material science and microfabrication processes for producing functional but compact sensors.

At the same time, it is essential to manage the power consumption of these devices for ensuring that the above devices can be constantly operational without the need for frequent recharging or replacing the battery. This can be especially challenging in the case of implantable devices. Hence, the development of miniature and low-power sensors includes the optimisation of their circuit design, by using power-efficient components in addition to energy harvesting technologies. These techniques help in extending the operational life of the device and decreasing their maintenance, which can improve the efficacy of the device in constant health monitoring and also enhance the overall user experience.

Thus, when the researchers focus on the miniaturisation and lower power consumption of the modern biomedical sensing device, it helps them address the following essential factors, i.e., (1) device needs to be minimally invasive which improves patient's autonomy and comfort, and (2) device must be energy-efficient for improving its reliable and long-term functionality. This advancement in sensor technology can significantly improve the functionality and feasibility of the implantable and wearable biomedical devices, which further helps to improve the patient care and medical monitoring procedures.

7.4. Harsh environments

For ensuring accuracy and reliability of the PSs under the above demanding conditions, robust PSs need to be fabricated. This involves the selection of materials that resist thermal degradation, corrosion, and mechanical wear. Also, the design of the PSs needs to include the features which protect them against environmental stressors, like encapsulation for preventing fluid ingress, resilient and flexible structures for absorbing mechanical stress, and thermal insulation for decreasing the negative temperature effects. All these design parameters need to be considered to maintain the functionality of a PS and ensure precise and constant measurements over a long time period, even under the harsh biomedical environments.

7.5. Calibration and drift

PSs, particularly the piezoresistive pressure-based PSs are utilised in several biomedical devices and hence, must be calibrated for improving their long-term accuracy. Many factors such as PS ageing, material deterioration, and environmental variations can lead to the issue of calibration drift in these devices, which affects the accuracy and reliability of their measurements. Additional factors contributing to calibration drift include thermal fluctuations, mechanical stress, and exposure of PS devices to chemicals or biofluids, which change their properties and response characteristics.

To resolve these issues, the researchers have to employ reliable and constant calibration processes to monitor and alter the PS device's performance factors. These processes involve the use of sophisticated algorithms and reference standards to measure the deviations occurring in the performance of the PS device from the baseline, and then apply corrective strategies. The researchers must constantly calibrate the devices that require a high accuracy and are used in biomedical applications, which guarantees the reliability of diagnostic and monitoring functions.

The development of piezoresistive pressure PSs requires addressing the problems related to calibration drift and optimising the performance parameters. These processes are required for the integration of PSs into biomedical devices, where they have to display accuracy and reliable performance under varying circumstances. The researchers should focus on these factors so that technology can increase pressure estimation accuracy in essential healthcare applications. These steps help in improving the patient outcomes and increase the efficiency of medical interventions.

7.6. Accuracy

Any change in pressure must be accurately determined in biomedical applications since data precision could help the researchers make better diagnostic and therapeutic decisions. Researchers have to implement strict calibration protocols to ensure data accuracy. These procedures enable the researchers to consider the impact of environmental variables, such as temperature and humidity, on PS performance, along with the probable PS drift over time. Furthermore, strict quality control must be maintained throughout the PS fabrication procedure, to decrease the performance differences that could probably occur between the various PS batches. This results in consistent and accurate reading. This procedure also involves a strict compliance of the manufacturing standards and the frequent assessment of PS results under different circumstances. Additionally, the researchers stated that the investigation of advanced sensing processes can increase the accuracy of the PS devices. For this purpose, they have emphasised the development of better transduction mechanisms that accurately and effectively translate the physical pressure changes into electrical signals. The integration of these technologies improved the specificity and sensitivity of the fabricated PS devices, which led to accurate and reliable measurements.

In conclusion, the integration of calibration processes, quality control, and technological developments could help in the fabrication of pressure PS devices that can satisfy the needs of the biomedical application. This could further help in the reliable monitoring and delivery of an excellent healthcare facility.

7.7. Reliability

In the past, researchers have stated that piezoresistive pressure-based PS devices should display a higher reliability, especially as they are implemented in the healthcare industry. This could be attributed to the fact that erroneous values or irregularity noted in the data can cause wrong diagnosis or implementation of wrong treatment plans, which can cause serious problems. To improve the reliability of the PS devices, the researchers developed a new strategy that combined redundancy with the PS array. This included the integration of several PS devices to validate the data to ensure that a single point of failure cannot affect the system's total accuracy. According to the researchers, using a stable encapsulation strategy protects PS devices from harsh environmental conditions such as mechanical stress, humidity, and extreme temperatures. This may improve the long-term operation and reliability of PS devices.

7.8. Dynamic range

The intrinsic properties of piezoresistive materials often dictate a trade-off between sensitivity and dynamic range [139, 140]. High sensitivity materials may exhibit limited dynamic range and vice versa. This inherent limitation poses a significant challenge in sensor design. Furthermore, the mechanical structure of piezoresistive sensors, including the diaphragm or cantilever, plays a crucial role in determining both sensitivity and dynamic range. Designing structures that can deform significantly under pressure without losing the ability to detect small pressure changes is complex. In addition, High sensitivity sensors are more susceptible to electrical noise and signal interference, which can reduce the effective dynamic range. Balancing the sensor's ability to detect small signals while minimizing noise is a technical challenge. Piezoresistive sensors' performance is often affected by temperature variations. Temperature-induced changes in resistance can impact both sensitivity and dynamic range, complicating the design process.

8. Conclusion

To conclude, in this review, the researchers have highlighted the significant role played by the piezoresistive pressure sensors in restructuring biomedical applications. They have mentioned the basic principles, important components, and the different fabrication techniques used for designing the sensors. This helps in laying the foundation of their seamless incorporation into different medical devices. They examined some important performance metrics such as accuracy, sensitivity, and response times, which highlights the relevance of their precision within the healthcare sector. Despite some of the challenges such as long-term stability and biocompatibility, piezoresistive pressure sensors were seen to display a high versatility in the applications that ranged from implantable medical devices to blood pressure monitoring. The researchers have highlighted their role in improving personalised healthcare via constant monitoring. In summary, this review has identified the emerging trends in the field of pressure sensors, which indicate that the precise and constant monitoring of these sensors can play a vital role in the development of the biomedical technology, better healthcare delivery, and improved patient outcomes. Finally, the piezoresistive pressure sensors act as a bridge that can bring together the healthcare needs and technological advancements, thus transforming the monitoring and medical diagnostics systems.

Data availability statement

No new data were created or analysed in this study.

Conflicts of interest

The authors declare no conflict of interest.

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