Printed rGO-Based Temperature Sensor for Wireless Body Area Network Applications

Abstract

This chapter discusses a Printed Reduced Graphene Oxide (rGO)-based temperature sensor for Wireless Body Area Network (WBAN) applications. The sensor is designed to monitor body temperature and facilitate real-time health monitoring. A Metaheuristic (MH) algorithm is implemented to optimize the sensor readings. MH algorithms, known for their global optimization capabilities, are employed to enhance the accuracy and energy efficiency of the temperature sensor. The integration of the rGO-based sensor and MH algorithm holds great potential for optimizing temperature monitoring in WBANs, enabling precise healthcare applications while conserving energy resources.

1 Introduction

A wireless body area network (WBAN) is a type of wireless network that connects small, low-power, and lightweight devices worn on the body to monitor various physiological parameters, such as heart rate, blood pressure, temperature, and other health-related information. The devices communicate with each other using wireless communication technologies such as Bluetooth, Zigbee, and Wi-Fi. WBANs are primarily used for healthcare applications and can provide real-time monitoring and analysis of a patient’s health status [32, 35]. They are often used for remote patient monitoring, where patients can be monitored from their homes or other remote locations, allowing healthcare providers to make timely decisions and provide appropriate care. WBANs can also be used for sports and fitness monitoring to track an individual’s physical activity, heart rate, and other physiological parameters (Fig. 1).

Fig. 1

WBAN Architecture [29]

Wireless Body Area Networks (WBANs) offer several advantages for healthcare and wellness monitoring [33]. They are:

• 1. Continuous Monitoring: WBANs provide continuous and real-time monitoring of various physiological parameters, allowing healthcare providers to detect early warning signs of health problems and prevent complications. WBANs can also be used for sports and fitness monitoring, providing individuals with real-time feedback on their physical activity and health status.

• 2. Non-Invasive Monitoring: WBAN devices are typically small, lightweight, and non-invasive, making them easy and comfortable to wear. This can improve patient compliance with monitoring protocols, allowing for longer and more accurate monitoring periods.

• 3. Mobility: WBAN devices are designed for mobility, allowing patients to move around freely while still being monitored. This can be particularly useful for patients who require continuous monitoring but want to avoid being confined to a hospital bed.

• 4. Personalized Care: WBANs can provide personalized care by monitoring an individual’s specific health parameters and providing tailored feedback and recommendations. This can help individuals make informed decisions about their health and wellness.

• 5. Reduced Healthcare Costs: WBANs can help reduce healthcare costs by enabling remote patient monitoring and reducing the need for hospital visits. This can be particularly useful for individuals with chronic conditions who require frequent monitoring and care.

While Wireless Body Area Networks (WBANs) offer several advantages for healthcare and wellness monitoring, there are also some potential disadvantages to consider:

• 1. Security and Privacy: WBAN networks can collect sensitive personal data, which raises concerns about security and privacy. Data breaches and unauthorized access to sensitive data can compromise patient privacy and lead to identity theft or other forms of harm.

• 2. Interference: WBAN networks can be susceptible to interference from other wireless devices, which can cause disruptions in the data transmission and affect the accuracy of the monitoring.

• 3. Compatibility: The compatibility of different WBAN devices and systems can be a challenge, as different devices may use different communication protocols and standards.

• 4. Limited Range: The range of WBAN networks is limited, which can make it difficult to monitor patients who are far away from the receiver or base station.

• 5. Battery Life: WBAN devices are typically powered by batteries, which can limit the duration of monitoring and require frequent recharging or replacement.

• 6. Interoperability is another challenge, as different WBAN devices may use different communication protocols and standards.

A wide range of sensors is used to monitor various physiological parameters of the body. These sensors are networked using wireless sensing protocols as shown in Fig. 2. The different types of sensors include:

• 1. Temperature sensors: These sensors are used to monitor body temperature and can be used to detect fever, hypothermia, or other changes in body temperature. They are commonly used in medical applications, such as monitoring patients with infectious diseases or post-operative care.

• 2. Heart rate sensors: These sensors measure the electrical signals generated by the heart and can be used to monitor heart rate, heart rhythm, and other cardiac parameters. They are commonly used in sports and fitness applications to track an individual’s physical activity and monitor their cardiovascular health.

• 3. Blood pressure sensors: These sensors measure the pressure of blood flowing through the arteries and can be used to monitor blood pressure and other cardiovascular parameters. They are commonly used in medical applications to diagnose and manage hypertension and other cardiovascular conditions.

• 4. Oxygen saturation sensors: These sensors measure the oxygen saturation level in the blood and can be used to monitor respiratory function. They are commonly used in medical applications to diagnose and manage respiratory conditions, such as chronic obstructive pulmonary disease (COPD) or sleep apnea.

• 5. Glucose sensors: These sensors measure the glucose level in the blood and can be used to monitor blood sugar levels in patients with diabetes. They are commonly used in medical applications to monitor diabetes management and prevent complications associated with high or low blood sugar levels.

• 6. Accelerometers: These sensors measure the acceleration of the body and can be used to monitor physical activity and detect falls or other accidents. They are commonly used in sports and fitness applications to track an individual’s physical activity and monitor their overall health and wellness.

Fig. 2

Patient monitoring sensors in WBAN network [30]

2 Background and Motivation

Temperature sensors are an essential type of sensor used in Wireless Body Area Networks (WBANs) for monitoring body temperature. WBANs are a type of wireless sensor network that enables real-time and continuous monitoring of an individual’s physiological parameters, such as heart rate, blood pressure, and temperature, among others. WBANs have numerous applications in medical, sports, and fitness settings, and can help individuals and healthcare providers make informed decisions about their health and wellness [31]. In WBANs, temperature sensors are used to monitor body temperature, which is an important physiological parameter that can indicate various health conditions. Changes in body temperature can be a sign of fever, hypothermia, or other health conditions, and continuous monitoring of body temperature can help in the early detection and management of these conditions.

Temperature sensors used in WBANs are typically small and low power, allowing for continuous monitoring of body temperature without causing discomfort to the patient. They can be attached to the skin using adhesive patches or incorporated into clothing or wearable devices, providing real-time temperature readings that can be transmitted wirelessly to a central monitoring system. The commonly used temperature monitoring devices in WBAN are:

• 1. Thermistors: Thermistors are a type of resistor whose resistance changes with temperature. They are commonly used in medical applications to measure body temperature and can provide accurate temperature readings in a small form factor [26].

• 2. Thermocouples: Thermocouples are made of two dissimilar metals that generate a voltage proportional to the temperature difference between the two junctions. They are commonly used in industrial applications to measure temperature but can also be used in WBANs for body temperature monitoring.

• 3. Infrared sensors: Infrared sensors measure the temperature of an object by detecting the infrared radiation emitted by the object. They are commonly used in non-contact temperature measurements and can be used in WBANs for monitoring skin temperature.

• 4. Fibre optic sensors: fibre optic sensors use optical fibre’s to measure temperature changes. They are commonly used in industrial and aerospace applications but can also be used in WBANs for monitoring body temperature.

• 5. Integrated circuits (ICs): Temperature sensors based on integrated circuits can provide accurate temperature readings in a small form factor. They are commonly used in consumer electronics and can be used in WBANs for monitoring body temperature.

These sensors are available in the market in the form of digital thermometers, Infrared thermometers, mercury thermometers, disposable thermometers, chemical thermometers and smart thermometers [38]. Digital thermometers use electronic sensors to measure body temperature. They are available in various forms, including oral, axillary, and rectal thermometers. They typically provide a fast and accurate reading and can display the temperature in Fahrenheit or Celsius. Infrared thermometers use infrared technology to measure body temperature without making contact with the skin. They can be used on the forehead or the ear and provide a quick and non-invasive reading. Mercury thermometers are a traditional type of thermometer that uses mercury to measure temperature. They are usually used for rectal temperature measurements and have largely been replaced by digital thermometers due to safety concerns related to the handling of mercury. Disposable thermometers are single-use thermometers that are often used in clinical settings to reduce the risk of cross-contamination. They are usually made from plastic or paper and are designed for use in the mouth or under the armpit. Chemical thermometers use chemical reactions to measure temperature changes. They are not commonly used for measuring human body temperature but are often used in laboratory settings for other applications. Smart thermometers are digital thermometers that are connected to a smartphone app. They can record and track temperature readings over time, and some models can provide additional health insights and recommendations.

A wearable thermometer is a type of wearable device that measures and monitors a person’s body temperature. Unlike traditional thermometers that require physical contact with the body, wearable thermometers can be worn on the body, allowing for continuous temperature monitoring without the need for constant manual measurements. Wearable thermometers can come in various forms, including patches, wristbands, and smartwatches. They use sensors to detect changes in body temperature and transmit the data to a mobile device or other connected devices for further analysis. Wearable thermometers have become increasingly popular in recent years, especially during the COVID-19 pandemic, as they allow for remote temperature monitoring and can help detect potential infections early on. They are also useful for monitoring the temperature of individuals with chronic illnesses or for tracking the temperature of infants and young children. Wearable thermometers are a convenient and effective way to monitor body temperature continuously and remotely, making them a valuable tool for both medical professionals and individuals concerned about their health. Some of the wearable type of sensors used in market include:

• 1. Smartwatches: Many modern smartwatches, such as Apple Watch and Samsung Galaxy Watch, include sensors that can monitor body temperature. These sensors can measure skin temperature on the wrist and can provide continuous temperature tracking.

• 2. Thermometer patches: Thermometer patches are small, flexible adhesive patches that can be applied to the skin to monitor body temperature. They typically use a wireless transmitter to send data to a smartphone or other device for monitoring and analysis.

• 3. Wireless temperature monitoring systems: These are systems that can be worn on the body, such as a bracelet or necklace, and provide continuous temperature monitoring. They typically use wireless technology, such as Bluetooth or Wi-Fi, to transmit data to a smartphone or other device.

• 4. Smart clothing: Smart clothing includes clothing with embedded sensors that can monitor various physiological parameters, including body temperature. These sensors can be integrated into fabrics and can provide continuous temperature tracking.

• 5. Earbuds: Some earbuds, such as the Jabra Elite Sport earbuds, include sensors that can monitor body temperature through the ear canal. These sensors can provide continuous temperature tracking and can be used for sports and fitness tracking.

3 Literature Review

Monitoring body temperature is critical for identifying human health problems and reflecting physiological activities. In conventional temperature sensing, rigid temperature detectors, such as thermometers, are commonly utilised. The main weakness of this approach is its inability to mix with curved surfaces, such as human or animal bodies. Also, thermometers cannot be placed under the arm as required, making them inappropriate for children [2]. Furthermore, when inflexible temperature detectors come into contact with uneven surfaces, they become uncomfortable. As a result, a wearable, flexible temperature sensor capable of making direct contact with the human body is critical. The mechanical robustness, biocompatibility, multifunctionality, comfort, and comfort of flexible sensors enable next-generation wearable technologies. Two examples are RTDs and Thermistors [4]. Significant advances in the development and upgrading of flexible temperature sensors have been made to replace standard sensors on rigid substrates and provide a viable alternative for wearable applications [3]. Temperature sensors of various types include thermocouples and resistive temperature detectors (RTDs) [4].

The resistance of the conductor material in RTDs varies with temperature [1, 5, 6]. Thermistors, on the other hand, utilise variations in electric conductivity of a sensor material to compensate for temperature variation [7]. The main advantages of RTD temperature sensors are their low cost, good linearity, and simple fabrication technique [1, 6]. However, RTD has a slow response time and a low sensitivity. Thermistors, on the other hand, have advantages due to their qualities such as inexpensive cost, wide sensing range, quick response, and extremely accurate temperature measurements [7, 8]. The two thermometer (PTC) types are the negative temperature coefficient (NTC) Thermistor and the positive temperature coefficient. Resistance increases with resistance in the NTC, whereas resistance decreases with temperature in the PTC.The temperature coefficient of resistance (TCR), which represents the sensor‘s sensitivity, has attracted the most attention for all temperature sensors [6, 8]. The sensor becomes more sensitive to small temperature variations as the TCR value increases [9]. Equation 1 [1, 5, 6, 8, 9] drives the TCR.

$$ TCR=\upalpha =\left(\mathrm{R}1\hbox{--} Ro/ Ro\ \left(\mathrm{T}1-\mathrm{To}\right)\right) $$

(1)

where R1 is the thermistor’s resistance at absolute temperature T1[C] and R0 is the tested sample’s initial resistance at [C]. The two most common processes utilised to make flexible and wearable sensors are photolithography and printing [10]. Photolithography is used to create high-performance gadgets. Unfortunately, due to the need for cleanroom facilities, significant material waste, and high fabrication costs, these procedures are unfortunately prohibitively expensive [10, 11]. As a result, low-cost printing processes that are simple to use and have substantially lower fabrication costs are in demand. Because of inkjet printing, the use of a stencil mask or clean-room lithography is no longer required [12]. It offers precise and continuous deposition of micro- and nanomaterials onto a range of surfaces under ambient conditions [13]. Inkjet also provides simple, flexible print patterns with high resolution, low material usage, and cost savings [10, 13].

Temperature sensors are used in a wide range of applications, from industrial control to biological monitoring. Several types of temperature sensors have been developed and intensively explored in response to the growing demand for high precision and dependability. The following are the most popular temperature sensor technologies, which range in responsiveness and accuracy from excellent to poor:

• Negative Temperature Coefficient (NTC) Thermistors.

• Resistance Temperature Detectors (RTDs)

• Thermocouples.

• Semiconductor-Based Sensors.

• Infrared (IR) temperature sensors

• Fiber optic temperature sensors

A thermistor is a type of thermally sensitive resistor that changes resistance as a function of temperature continuously, slowly, and incrementally. An NTC thermistor has higher resistance at low temperatures. The resistance gradually reduces as the temperature rises, according to the R-T table. Resistance rises per °C allow for exact reflection of minor variations [19]. The output of an NTC thermistor cannot be linearized due to its exponential nature; but, depending on the application, it can be. The effective operating range of regular thermistors or glass-encapsulated thermistors is 150 °C [18]. With a resistance temperature detector, or RTD, the resistance of the RTD element changes with temperature. To make an RTD, a wire or, for greater precision, a film is wrapped around a ceramic or glass core. Although nickel and copper are less expensive, they are not as stable or reproducible as platinum, which generates the most exact RTDs. Platinum RTDs, while more expensive than copper or nickel RTDs, produce a highly precise linear output over a temperature range of −200 to 600 °C [17]. A thermocouple is made up of two wires of different metals that are electrically connected at two points. The changing voltage generated between these two metals reflects proportional temperature changes [14]. Because thermocouples are nonlinear devices, they must be converted using a table when utilised for temperature control and correction. A lookup table is widely used for this. Thermocouples operate throughout the widest temperature range, from −200 to 1750 °C, although their accuracy is limited (0.5–5 °C) [15].

A semiconductor-based temperature sensor is commonly seen in integrated circuits (ICs). To track temperature changes, these sensors use two identical diodes with temperature-sensitive voltage vs. current characteristics. Although having the lowest accuracy of the basic sensor types, they provide a linear response. Furthermore, these temperature sensors respond slowly over the entire temperature range (−70 to 150 °C) [22]. Infrared (IR) temperature sensors enable accurate non-contact temperature measurement in medical applications. The most common applications for this type of temperature sensor are to measure skin, forehead, or ear temperatures. Fiber optic temperature sensors have gained popularity in recent years due to its high precision, resistance to electromagnetic interference, and capacity to detect temperature at multiple locations at the same time. They require an expert for signal processing, though, and are more expensive than conventional sensors [22].

In general, the temperature sensor to be used is determined by the specific application and needs. Despite the ubiquitous use of thermocouples and RTDs, recent advances in fibre optic temperature sensors provide a potential opportunity for highly accurate and scattered temperature readings. The four most common types of temperature sensors in modern electronics are thermocouples, RTDs (resistance temperature detectors), thermistors, and semiconductor-based integrated circuits (IC). A thermocouple is a type of temperature sensor. This sensor is made up of two distinct metal wires that are linked at one end and coupled to a thermocouple thermometer or other thermocouple-capable device at the other [15]. When properly configured, thermocouples may provide temperature measurements over a wide temperature range. A thermocouple is a sensor that measures temperature. It is constructed of two distinct metals that are joined at one end. When the junction of the two metals is heated or cooled, a voltage that is proportional to temperature is created [16]. Thermocouples are a popular temperature sensor due to its simplicity, wide temperature range, and low cost. Unfortunately, they have measurement errors built in, and factors such as temperature gradients and electromagnetic interference can affect their precision. Because of their versatility as temperature sensors, thermocouples are widely used in a wide range of applications, ranging from household appliances and utilities to industrial thermocouples [15]. Because thermocouples come in such a wide range of models and technical specifications, understanding their fundamental construction, operation, and ranges is critical in order to select the optimal kind and material for your application [14].

Thermocouple sensors work on the following principle: A continuous current flows through the thermoelectric circuit when two wires of different metals are joined at both ends and one of the ends is heated. The most common thermocouple types are “Base Metal” thermocouples, which include Types J, K, T, and E. In high-temperature applications, noble metal thermocouples of types R, S, and B are used [16]. The capacity of thermocouple sensors to provide exact measurement values for industrial applications requiring high temperatures is its primary advantage [14]. They are long-lasting, inexpensive, fast to respond, and incredibly effective for many years to come. Straightforward vast range of temperatures selection Rugged It is “self-sufficient.”

The resistance temperature detector is another typical type of temperature sensor that relies on the idea of temperature-dependent resistance (RTD). They provide more accuracy and repeatability than thermocouples, making them excellent for applications requiring highly accurate temperature measurements [18]. Yet, they only work within a small temperature range and require more expensive signal conditioning and processing equipment [17]. Identifying RTDs. The resistance of a sensor known as an RTD (Resistance Temperature Detector) fluctuates with temperature. As the temperature of the sensor rises, so does the resistance. Temperature and resistance have a well-established and constant relationship [18]. Temperature is measured with an RTD, which stands for “Resistance Temperature Detector,” because its resistance varies with temperature. The resistance of the RTD increases linearly with temperature. RTDs are commonly referred to be wire wrapped. They’re manufactured with a glass or ceramic core wrapped in microscopic wire. The wire was made of platinum [18]. The RTD elements are often housed in a protective probe to protect them from the environment and to boost their robustness, which is another noteworthy aspect [17].

Thin film RTDs are less expensive. They are comprised of a basic ceramic that has been coated with a tiny layer of platinum. Let us now look at how an RTD works. As previously stated, an RTD is made up of insulated Platinum wires and a resistance element [17]. RTDs may occasionally incorporate three or even four wires to increase accuracy and minimise connection lead resistance concerns [18]. Platinum is utilised as the resistance element because it is chemically inert, has a wide temperature range, and is exceptionally long-term stable [17]. The RTD functions on a fundamental principle. When a metal’s temperature rises, so does its resistance to the flow of electricity. When an electrical current is conducted through the sensor, the resistance element measures the resistance of the current passing through it [18]. Electrical resistance increases as the temperature of the resistance element rises. The electrical resistance is measured in Ohms. The resistance value can then be converted into temperature using the element’s characteristics. An RTD usually reacts between 0.5 s and 5 s. As a result, they are well suited for a wide range of applications [17]. RTD sensors have numerous benefits. Long-term steadiness, extreme accuracy, repeatability, consistency, and ability to deliver precise measurement even in severe settings. Platinum RTDs can sustain a wider range of temperatures. When compared to thermocouples and thermistors, they are the most dependable and consistent throughout time [16, 17]. RTD disadvantages: A more sophisticated measurement circuit is required due to low sensitivity and a high starting cost. Larger than typical bulbs; low absolute resistance; a current source is required; less rugged in a high vibration environment; a bridge circuit with power supply is required [17]. Thermistors are temperature sensors that operate on the premise that the resistance of semiconductor materials changes with temperature. They have high sensitivity, short reaction times, and low pricing. Self-heating and nonlinearity, however, can reduce their accuracy [18].

A thermistor is a resistor with temperature-dependent resistance or a resistance thermometer. The term is made up of the words “thermal” and “resistor.” It is made of metallic oxides that are shaped into a bead, disc, or cylinder shape before being covered in an impermeable substance such as epoxy or glass [19]. The temperature sensor in this case is an inkjet printer thermistor. The resistance of thermos witches, also known as temperature-dependent resistors, fluctuates with temperature. They are sensitive to even minor temperature swings, making them suitable for temperature control. They can measure liquids, gases, or solids depending on the type of thermistor. The body temperatures of infants and children are measured with thermometers [19]. To determine if the sensor is a thermistor or an RTD, measure the resistance between the two dissimilarly coloured wires: An RTD PT100 has a resistance of 100 ohms at 0 °C. The resistance of an RTD PT1000 at 0 °C is 1000 ohms [20]. The thermistor’s key advantages are its high sensitivity, small heat capacity, and quick reaction; nevertheless, its main disadvantages are its low interchangeability and non-linear thermoelectric properties, which broaden the measurement range [19].

Thermistors can have errors as low as less than 0.001 °C when utilized over a restricted temperature range of 0–50 °C. Thermistors have more sensitivity than other temperature sensors, which enables them to function well over a narrow temperature range [20].

• They respond quickly

• They are inexpensive and therefore inexpensive to replace.

• Simple to use.

• Compact dimensions that enable them to occupy the tiniest areas.

Semiconductor-based temperature sensors, also known as IC sensors, are made up of two identical diodes on a dual integrated circuit (IC). Diodes and temperature-sensitive voltage are used to measure temperature. Although these sensors produce a relatively linear output, their accuracy falls between 1 °C and 5 °C [21]. An IC temperature sensor is a two-terminal integrated circuit temperature transducer that creates an output current proportional to the absolute temperature. The sensor package is small, has a fast response time, and has a low thermal mass [21]. Temperatures typically vary from −58 to 302 °F (55 to 150 °C). The output of a solid-state sensor can be either analog or digital [20]. The IC transducer is a small device with a fast response time and low thermal mass. Ideally suited for circuit board temperature management, computer CPU temperature control, and telecommunications (cell phones) applications [22]. The advantages and disadvantages of IC sensors, as well as the possibility of analogue or digital outputs and inexpensive pricing, are strengths.

• Direct temperature measurement (1.000 = 100C and 298A = 298 K or 25 °C for some analog devices)

• Linear output, no curve fitting

• Direct voltage, current, or digital output needing no additional circuitry

• Wider interchangeability than most RTDs and thermistors

Limited temperature range: −55 to 150 °C Max

• Large variance in accuracy across models.

• With some immersion systems, small package sizes might be a barrier to low-cost applications (Fig. 3).

Fig. 3

The comparative chart for various temperature sensors [23]

4 Materials and Methods

A printed temperature sensor is a type of sensor that is fabricated using printing techniques such as screen printing, inkjet printing, or flexography. These sensors are designed to measure temperature changes in various environments and are typically made from materials such as conductive inks, polymers, or carbon nanotubes.

The design of a printed temperature sensor depends on the specific application for which it will be used. Some sensors are designed for use in wearable devices, while others are intended for use in industrial or automotive applications. The sensor‘s design must take into account factors such as temperature range, sensitivity, accuracy, and response time. The fabrication process for printed temperature sensors involves depositing the sensor‘s material onto a substrate using printing techniques. This process allows for the creation of sensors with complex shapes and patterns, making them suitable for a wide range of applications. Printed temperature sensors have several advantages over traditional temperature sensors, including low cost, flexibility, and ease of integration with other electronic components. They are also more environmentally friendly than traditional sensors, as they use fewer materials and generate less waste during the manufacturing process. The design and fabrication of printed temperature sensors have opened up new possibilities for temperature sensing applications, particularly in the fields of wearables, smart homes, and the Internet of Things (IoT).

4.1 Materials

The choice of material for a temperature sensor that can be used for WBAN application includes the selection of material for temperature sensing as well as the material choice for the substrate on top of which the sensor is fabricated.

4.1.1 Sensing materials

There are several materials that can be used for the fabrication of a printed temperature sensor. These materials are typically chosen based on their thermal and electrical properties, as well as their ability to be printed using various printing techniques. Some common materials used for printed temperature sensors include:

• 1. Conductive inks: These are inks that contain conductive particles such as silver, copper, or carbon. They are commonly used to print the sensing element and the interconnects of the sensor.

• 2. Polymers: Polymers such as polyimide, polycarbonate, or polyethylene terephthalate (PET) are commonly used as substrates for printed temperature sensors. These materials offer good thermal stability and flexibility.

• 3. Carbon nanotubes: Carbon nanotubes are an emerging material for printed temperature sensors. They offer high thermal conductivity, low thermal mass, and excellent mechanical properties [27].

• 4. Graphene: Graphene is another emerging material for printed temperature sensors. It offers high thermal conductivity and excellent electrical properties [24].

• 5. Thermocouples: Thermocouples are often used as temperature sensors in industrial applications. They are made from two dissimilar metals such as copper and constantan, and generate a voltage proportional to the temperature difference between the two junctions.

• 6. Metal oxides: Metal oxide materials such as zinc oxide, tin oxide, and indium oxide are commonly used in printed temperature sensors. These materials have high thermal stability and can be printed using inkjet or screen printing techniques.

• 7. Conductive polymers: Conductive polymers such as polyaniline, polypyrrole, and polythiophene are another class of materials that can be used for printed temperature sensors. These materials offer high electrical conductivity and can be printed using inkjet or spray coating techniques.

• 8. Nanoparticles: Nanoparticles of metals such as gold, silver, or copper can also be used for printed temperature sensors. These materials offer high thermal conductivity and can be printed using inkjet or aerosol-jet printing techniques.

• 9. Ceramic materials: Ceramic materials such as alumina, zirconia, or silicon carbide can be used as substrates for printed temperature sensors. These materials offer excellent thermal stability and can be printed using screen printing or inkjet printing techniques.

• 10. Organic materials: Organic materials such as organic semiconductors, organic dyes, or organic molecules can be used for printed temperature sensors. These materials offer good flexibility and can be printed using inkjet or spray coating techniques.

• 11. Metal oxides: Metal oxide materials such as zinc oxide, tin oxide, and indium oxide are commonly used in printed temperature sensors. These materials have high thermal stability and can be printed using inkjet or screen printing techniques.

• 12. Conductive polymers: Conductive polymers such as polyaniline, polypyrrole, and polythiophene are another class of materials that can be used for printed temperature sensors. These materials offer high electrical conductivity and can be printed using inkjet or spray coating techniques.

• 13. Nanoparticles: Nanoparticles of metals such as gold, silver, or copper can also be used for printed temperature sensors. These materials offer high thermal conductivity and can be printed using inkjet or aerosol-jet printing techniques.

• 14. Ceramic materials: Ceramic materials such as alumina, zirconia, or silicon carbide can be used as substrates for printed temperature sensors. These materials offer excellent thermal stability and can be printed using screen printing or inkjet printing techniques.

• 15. Organic materials: Organic materials such as organic semiconductors, organic dyes, or organic molecules can be used for printed temperature sensors. These materials offer good flexibility and can be printed using inkjet or spray coating techniques [25].

The choice of material for a printed temperature sensor will depend on several factors, including the required temperature range, sensitivity, accuracy, and cost.

4.1.2 Substrate Material

The choice of substrate for a printed temperature sensor will depend on several factors, such as the required temperature range, the desired accuracy and sensitivity, the manufacturing process, and the cost. Some common substrates used for printed temperature sensors include:

• 1. Flexible polymeric substrates: Polymeric substrates, such as polyimide, polyethylene terephthalate (PET), and polydimethylsiloxane (PDMS), are widely used in printed temperature sensors because they offer excellent flexibility, low thermal mass, and good thermal stability. These substrates are suitable for printing using techniques such as screen printing, inkjet printing, and roll-to-roll printing.

• 2. Rigid substrates: Rigid substrates such as glass, silicon, or ceramic materials can be used in printed temperature sensors where high accuracy and stability are required. These substrates are suitable for printing using techniques such as photolithography or inkjet printing.

• 3. Paper-based substrates: Paper-based substrates such as cellulose-based paper or coated paper are low-cost options for printed temperature sensors. These substrates are suitable for printing using techniques such as screen printing or inkjet printing.

• 4. Metal foils: Metal foils such as copper, aluminum, or stainless steel can be used as a substrate for printed temperature sensors. These substrates offer good thermal conductivity and stability but may not be as flexible as polymeric substrates. Metal foils are suitable for printing using techniques such as screen printing or flexography.

• 5. Textiles: Textiles such as cotton, silk, or polyester can be used as a substrate for printed temperature sensors in wearable applications. These substrates offer excellent flexibility, breathability, and comfort. Textiles are suitable for printing using techniques such as inkjet printing or screen printing.

• 6. Polyimide (PI): Polyimide is a popular substrate material for printed temperature sensors because it offers good thermal stability, flexibility, and durability. It can withstand high temperatures and is commonly used in harsh environments.

• 7. Polycarbonate (PC): Polycarbonate is another popular substrate material for printed temperature sensors because it offers good thermal stability and is easy to print on. It is commonly used in low-temperature applications.

• 8. Polyethylene terephthalate (PET): PET is a widely used substrate material for printed temperature sensors because it is flexible, transparent, and offers good thermal stability. It is commonly used in consumer electronics applications.

• 9. Ceramic: Ceramic materials such as alumina, zirconia, or silicon carbide can be used as substrates for printed temperature sensors because they offer excellent thermal stability and can withstand high temperatures. They are commonly used in high-temperature applications.

• 10. Metal foils: Metal foils such as copper or aluminum can be used as substrates for printed temperature sensors because they offer good thermal conductivity and can withstand high temperatures. They are commonly used in high-temperature applications.

• 11. Flexible printed circuit boards (FPCBs): FPCBs are a type of substrate material that can be used for printed temperature sensors. They offer good flexibility, durability, and can be easily integrated into other electronic systems.

• 12. Silicone rubber: Silicone rubber is a flexible and durable substrate material that can be used for printed temperature sensors. It is commonly used in medical and wearable applications.

• 13. Metals: Metals such as stainless steel or titanium can be used as substrate materials for printed temperature sensors. They offer high thermal conductivity and can withstand harsh environments. However, they are typically more expensive than other substrate materials.

• 14. Glass: Glass is a substrate material that offers high thermal stability, chemical resistance, and can be easily printed on. It is commonly used in high-temperature applications and in harsh chemical environments.

The choice of substrate material will depend on the specific requirements of the sensor and the printing technique used to fabricate the sensor. For example, some substrates may be better suited for certain printing techniques than others. Additionally, the substrate material may affect the performance of the sensor, such as its sensitivity and response time, so careful consideration is required during substrate selection. It is important to select a substrate material that can withstand the operating temperature range of the sensor, is compatible with the printing method used, and provides the desired level of durability and flexibility.

4.2 Methods

A temperature sensor can be simulated using COMSOL Multiphysics. It uses finite element analysis (FEA) to simulate the behavior of a temperature sensor under different conditions. This allows designers to optimize the performance of the sensor and identify potential issues before it is fabricated. To design a temperature sensor using COMSOL, the model of the sensor should be first drawn. This model typically includes the geometry of the sensor, the materials used, and the boundary conditions. The heat transfer equations that govern the behavior of the sensor can then be defined and COMSOL’s built-in solvers can be used to simulate the behavior of the sensor under different conditions. COMSOL also offers several features that make it ideal for designing temperature sensors. These include:

• 1. Multi-physics simulation capabilities: COMSOL can simulate the behavior of temperature sensors under different physical conditions, such as fluid flow and electromagnetic fields.

• 2. User-friendly interface: COMSOL has a user-friendly interface that makes it easy to set up and run simulations.

• 3. High-performance computing: COMSOL can take advantage of high-performance computing resources to speed up simulations and handle large datasets.

• 4. Comprehensive documentation and support: COMSOL offers comprehensive documentation and support resources to help users design and simulate temperature sensors.

The structure of the temperature sensor using COMSOL Multiphysics can be achieved by using the following steps

• 1. Defining the geometry: The first step is to define the geometry of the sensor. This can be done using the COMSOL CAD tools, or by importing a CAD model of the sensor.

• 2. Setting up the model: The next step is to set up the model by defining the materials and properties of the sensor. This includes specifying the thermal conductivity, specific heat capacity, and density of the materials used in the sensor.

• 3. Defining the boundary conditions: The boundary conditions of the model must be defined, including the heat flux and temperature distribution on the surface of the sensor.

• 4. Solving the model: Once the model is set up, it can be solved using COMSOL’s built-in solvers. This will provide a temperature distribution throughout the sensor.

• 5. Analysing the results: The final step is to analyse the results of the simulation. This includes analysing the temperature distribution, heat transfer rate, and any other relevant parameters.

• 6. Table 1 shows the recent temperature sensors with the sensing material, substrate material, fabrication techniques and encapsulation methods.

Table 1 Materials and methods used for temperature sensors [28]

5 Fabrication

5.1 Fabrication

This session describes the fabrication of a temperature sensor that can be used for WBAN application. The sensor fabrication is performed in two steps

• 1. Fabrication of the electrodes

• 2. Fabrication of the sensing material

The electrode is fabricated on a cladded FR4 substrate using a photolithography process. The process includes:

• 1. Cleaning and preparation: The copper-clad FR4 substrate is cleaned and prepared for the photoresist coating.

• 2. Photoresist coating: A thin layer of photoresist is applied to the copper-clad FR4 substrate using a spin coater.

• 3. Mask alignment and exposure: The photomask, which contains the desired circuit pattern, is aligned over the photoresist-coated substrate using a UV light aligner. The substrate and the photomask are then exposed to UV light, which causes the photoresist to harden in the areas that are exposed to light.

• 4. Developing: The substrate is then washed with a developer solution that dissolves the unexposed photoresist, leaving only the hardened areas.

• 5. Etching: The exposed areas of the copper are etched away using a chemical solution. This creates the desired circuit pattern on the copper-clad FR4 substrate.

• 6. Photoresist removal: Finally, the remaining photoresist is removed using a solvent, leaving the patterned copper-clad FR4 substrate ready for further processing.

The fabrication of the sensing material which is chosen to be reduced graphene oxide on top of the electrode include:

• 1. Preparation of the reduced graphene oxide ink:

  • (a) Synthesize graphene oxide: Graphene oxide (GO) is synthesized from graphite through a modified Hummers method.

  • (b) Reduce graphene oxide: Graphene oxide is then reduced to rGO using a reducing agent, such as hydrazine, sodium borohydride, or hydrogen.

  • (c) Prepare the ink: The rGO can then be dispersed in a suitable solvent, such as water, ethanol, or isopropyl alcohol, to make the ink. You can add a small amount of surfactant, such as Triton X-100, to improve the stability of the ink.

  • (d) Characterize the ink: You should characterize the ink to ensure that it has the desired properties, such as viscosity, surface tension, and conductivity. You can use techniques such as rheology, contact angle measurement, and four-point probe measurement to characterize the ink.

• 2. Preparation of the substrate: The printed substrate was cleaned using de ionized water of resistivity 15MΩm for 5 minutes. The cleaned substrate was dried in sunlight for 10 minutes

• 3. Printing the structure:

  • (a) Print the temperature sensor: The rGO ink can be printed onto a suitable substrate such as paper, glass, or plastic using various printing techniques such as inkjet, screen, or flexographic printing. The printed rGO pattern can be designed as a resistor, thermistor, or another temperature-sensing element.

  • (b) Sinter the rGO: To improve the electrical conductivity of the printed rGO pattern, it must be annealed sensor at a high temperature, exposing it to UV light, or treating it with a suitable chemical agent.

  • (c) Characterize the temperature sensor: The printed and sintered rGO temperature sensor can be characterized by measuring its electrical resistance at different temperatures using a multimeter or other measurement instrument. The sensor‘s response time, sensitivity, and accuracy can also be evaluated to ensure it meets the desired performance specifications.

Reduced graphene oxide (rGO) has excellent temperature-sensing properties due to its high thermal conductivity and sensitivity to temperature changes. When exposed to different temperatures, the electrical conductivity of rGO changes, making it a good candidate for use in temperature sensors. The temperature sensing property of rGO is based on the fact that as the temperature increases, the thermal energy causes the rGO sheets to vibrate more rapidly, which increases the resistance of the rGO material. Conversely, when the temperature decreases, the rGO sheets vibrate less, which decreases the resistance of the rGO material. This change in resistance can be measured and correlated with changes in temperature, allowing rGO to be used as a sensitive temperature sensor. The high thermal conductivity of rGO also helps to improve its temperature sensing accuracy and response time.

6 Generic Metaheuristic Algorithm for Optimization

The sensor thus created was integrated with an IoT device which is used to display the measured temperature value. For the sensor to measure and provide a meaningful temperature value the output obtained from the sensor is mapped to specific values and is optimized using the generic metaheuristic algorithm [34, 36, 37]. The steps involved include:

• 1. Initialization: The initialization step in a generic metaheuristic algorithm for optimizing a temperature sensor involves defining the initial state of the algorithm and the search space. Here are the steps to perform the initialization step:

  • (a) Define the search space: The first step in initializing a metaheuristic algorithm is to define the search space, which represents the set of all possible solutions to the problem. In the case of temperature sensor optimization, the search space would consist of all possible configurations of the sensor‘s parameters, such as the placement of the sensor, the type of material used, and the sensitivity of the sensor.

  • (b) Generate an initial solution: The next step is to generate an initial solution within the defined search space. This can be done randomly or using a heuristic method. For example, the initial solution could be a randomly selected configuration of the sensor‘s parameters.

  • (c) Evaluate the initial solution: Once the initial solution is generated, it needs to be evaluated to determine its quality. In the case of temperature sensor optimization, the quality of the solution would be measured by its ability to accurately measure temperature. This can be done using simulation or experimental testing.

  • (d) Set the initial temperature: For metaheuristic algorithms such as simulated annealing, the initial temperature is an important parameter that determines the degree of exploration in the search space. The initial temperature should be set high enough to allow the algorithm to explore the search space, but not so high that it causes the algorithm to become stuck in a local minimum.

  • (e) Repeat the process: If the initial solution is not satisfactory, the initialization step can be repeated with a new initial solution or search space until a suitable starting point is found.

• 2. Evaluation: The evaluation step involves measuring the quality of a candidate solution generated by the algorithm. Here are the steps to perform the evaluation step:

  • (a) Define the evaluation metric: The first step in evaluating a candidate solution is to define an evaluation metric that measures its quality. In the case of temperature sensor optimization, the evaluation metric could be the accuracy or precision of the sensor in measuring temperature.

  • (b) Simulate or measure the solution: Once the evaluation metric is defined, the candidate solution needs to be simulated or measured to determine its quality. This could involve running simulations or experiments to measure the accuracy of the sensor in measuring temperature.

  • (c) Calculate the fitness value: Based on the results of the simulation or experiment, a fitness value can be calculated for the candidate solution. The fitness value should be based on the evaluation metric defined in step 1 and should reflect the quality of the solution relative to other solutions in the search space.

  • (d) Compare the fitness value: Once the fitness value is calculated, it can be compared to the fitness values of other candidate solutions generated by the algorithm. The candidate solution with the highest fitness value is considered the best solution so far.

  • (e) Repeat the process: The evaluation step is repeated for each candidate solution generated by the algorithm until the termination criterion is met.

• 3. Search: The search step involves generating new candidate solutions in the search space in an attempt to find the optimal solution. Here are the steps to perform the search step:

  • (a) Choose a search operator: The first step in the search step is to choose a search operator that will generate new candidate solutions. There are several search operators that can be used, such as mutation, crossover, or perturbation.

  • (b) Apply the search operator: Once the search operator is chosen, it is applied to the current solution to generate a new candidate solution in the search space. This new solution should be feasible, meaning that it should satisfy any constraints imposed by the problem.

  • (c) Evaluate the new solution: Once the new solution is generated, it is evaluated using the evaluation metric defined in the evaluation step. The fitness value of the new solution is calculated and compared to the fitness value of the current solution.

  • (d) Accept or reject the new solution: Based on the fitness value of the new solution and the current solution, the algorithm decides whether to accept or reject the new solution. If the new solution has a higher fitness value, it is accepted as the new current solution. If the new solution has a lower fitness value, it may still be accepted with a certain probability determined by the current temperature value in the case of simulated annealing.

  • (e) Repeat the process: The search step is repeated with the new current solution generated in step 4. The search continues until a stopping criterion is met, such as reaching a maximum number of iterations, or when the improvement in the fitness value is below a certain threshold.

• 4. Selection: The selection step involves selecting candidate solutions for the next iteration of the search process. Here are the steps to perform the selection step:

  • (a) Sort the candidate solutions: Sort the candidate solutions generated in the search step based on their fitness values with respect to the accuracy or precision of the temperature sensor.

  • (b) Select the best solutions: Select the best solutions from the sorted candidate solutions for the next iteration of the search process. The number of solutions selected depends on the specific metaheuristic algorithm used.

  • (c) Perform elitism: In some metaheuristic algorithms, such as genetic algorithms, the best solution found so far is preserved and carried forward to the next iteration. This is important for a temperature sensor optimization problem to ensure that the best sensor calibration settings are not lost.

  • (d) Generate new solutions: Generate new candidate solutions from the selected best solutions using search operators such as mutation or perturbation. These new solutions should be feasible and satisfy any constraints imposed by the problem.

  • (e) Repeat the process: Repeat the selection step for each iteration of the search process until a stopping criterion is met, such as reaching a maximum number of iterations or when the improvement in the fitness value is below a certain threshold.

• 5. Termination: The termination step involves deciding when to stop the search process. Here are the steps to perform the termination step:

  • (a) Define a stopping criterion: The first step in the termination step is to define a stopping criterion, which is a condition that indicates when the search process should stop. The stopping criterion could be a maximum number of iterations, a specific target accuracy or precision, or when the improvement in the fitness value is below a certain threshold.

  • (b) Check if stopping criterion is met: At the end of each iteration, the stopping criterion is checked to see if it has been met. If the stopping criterion has been met, the search process stops and the best solution found so far is returned as the optimal solution.

  • (c) Repeat until stopping criterion is met: The search process is repeated until the stopping criterion is met. If the stopping criterion is not met, the algorithm proceeds to the next iteration of the search process.

  • (d) Return the optimal solution: Once the stopping criterion is met, the best solution found so far is returned as the optimal solution. This solution should provide the best calibration settings for the temperature sensor.

• 6. Solution: The solution step involves returning the optimal solution found by the search process. Here are the steps to perform the solution step:

  • (a) Check if stopping criterion is met: At the end of each iteration, the stopping criterion is checked to see if it has been met. If the stopping criterion has been met, the search process stops and the best solution found so far is returned as the optimal solution.

  • (b) Return the optimal solution: Once the stopping criterion is met, the best solution found so far is returned as the optimal solution. This solution should provide the best calibration settings for the temperature sensor.

  • (c) Validate the optimal solution: Before using the optimal solution in a real-world scenario, it is important to validate the solution. This involves testing the temperature sensor with the optimal calibration settings and comparing the sensor readings with the actual temperature values.

  • (d) Implement the optimal solution: Once the optimal solution has been validated, it can be implemented in the temperature sensor. This involves programming the sensor with the optimal calibration settings.

7 Conclusion and Future Works

This chapter discusses on temperature sensors with a special consideration for temperature sensors with rGO as the sensing material. The fabrication procedure for the sensor is also mentioned. The resulting sensor acts as a negative temperature coefficient resistor. The voltage drop across the resistor is mapped to the temperature in celcius. The resultant values are optimized using the generic meta heuristic algorithm to obtain a more reliable result. The chapter also discusses the various sensors available in the literature and the materials that are available in the market which will help to build a temperature sensor. Different types of temperature sensors were also discussed.