Table 1.

Suitable sensors for SF design.

Sl. No	Sensor Type, Its Operation Principle, and Possible Applications in Smart Footwear
1	Ultrasonic sensors [3]: These sensors utilise ultrasonic waves to measure distance and detect objects. They are one of the most commonly used sensors in footwear, specifically for aiding people with visual disability. They can detect the presence or absence of objects within a specific range. Further, these sensors can measure insole thickness and footwear wear and tear and suggest replacement schedules.
2	LiDAR sensors [4]: Light detection and ranging (LiDAR) based Time of Flight (ToF) sensors are currently the preferred technology for automotive and drone applications. ToF sensors have the emitter, receiver, and processor system on the same PCB/package for easy, cost-effective, and small-footprint integration. They offer high-speed, precise distance measurement independent of target size, colour, and reflectance. A LiDAR sensor can be integrated into footwear to replace the traditional ultrasonic sensor or added as an additional sensor to support features such as pothole detection, obstacle warning, etc.
3	Pressure sensors [5]: Pressure sensors measure pressure by converting the applied pressure into an electrical signal that can be measured and utilised for various applications. Strain Gauge Pressure Sensors measure the strain or deformation caused by pressure. Capacitive Pressure Sensors use changes in capacitance. Changes in capacitance are used to measure pressure. Piezoelectric Pressure Sensors utilise the piezoelectric effect, where pressure generates an electric charge in certain materials. Resonant Pressure Sensors use change in the resonant frequency of a vibrating element under pressure to determine the applied force. They measure pressure at different points on the foot (placed in soles). Additionally, strain gauge sensors installed in the footwear can detect sudden bends and movements in the footwear. Pressure sensors can also be used to measure weight.
4	Accelerometers and Gyroscopes [6]: An accelerometer measures linear acceleration and can detect the movement of an object in terms of acceleration, deceleration, or changes in direction. On the other hand, a gyroscope measures angular velocity around a particular axis. It detects changes in orientation or rotational movements, such as tilting, rotating, or twisting. Inertial measurement units (IMUs) incorporate accelerometers and gyroscopes into a single sensor package, providing a more compact and integrated solution for motion-sensing applications. IMUs are integrated into wearable devices to monitor and analyse physical activities. They can measure steps, distance, speed, and calories burned and provide feedback on movement patterns and exercise techniques.
5	Sweat Sensors [3]: Skin-worn biosensors can analyse the wearer’s sweat to monitor various physiological conditions. Biomarkers in the sweat can be used to detect certain genetic conditions. Also, using the glucose-level correlation between sweat and blood leads to potential applications in the continuous monitoring of diabetes.
6	Temperature Sensors: Temperature sensors detect and measure the heat and coolness of air, liquids, or solid surfaces and convert them into electrical signals. Types of temperature sensor include: Resistance Temperature Detector (RTD) sensors, which measure temperature by changing resistance proportional to the temperature. These are available as individual sensors or fully packaged assemblies consisting of a sensor element, a covering, an epoxy or filler, extension leads, and sometimes connectors, thus allowing for flexibility in design [7]. Negative Temperature Coefficient (NTC) thermistors, which use the properties of ceramic/metal composites that have an inverse relation between resistance and temperature to measure temperature. NTC sensors have a small size, excellent long-term stability, high accuracy, and precision [8]. A thermocouple sensor is formed by joining two individually insulated dissimilar metals at one end. The temperature is measured at this joint. When the joint is placed in a high-temperature environment, a small voltage is produced at the open ends of the two metals, which can be measured and interpreted [9]. Thermopile infrared (IR) temperature sensors, which generally provide non-contact temperature measurements. These are composed of small thermocouples on a silicon chip that absorb the incident IR energy and produce an equivalent output signal. Modern sensors also have an integrated reference sensor for calibration and compensation. The output can be an analogue voltage or a digital value [10]. Digital Temperature Sensors, which are IC temperature measurement systems. The miniature packages are designed specifically for tight spaces. The integrated microcircuit design allows quick response to changes in process temperature, fast conversion times, and very low power consumption [11]. Temperature sensors can be used in smart footwear to keep track of foot temperature and act as surrogate activity markers. Temperature changes in specific foot areas can indicate inflammation, injury, or conditions like diabetic foot ulcers. Elevated temperatures in localised regions might signify inflammation or infection. They can also assist in monitoring blood circulation in the feet. Changes in foot temperature might indicate poor circulation.
7	Gas sensors can be used to detect foot odour, and they can detect Bromodosis, possibly caused by fungal infection [12]. Bromodosis is smelly feet, and it is often caused by the interaction of sweat with bacteria on the skin’s surface. Fungal infections like athlete’s foot or other dermatophyte infections can also contribute to foot odour. Gas sensors can detect the specific gases emitted by these fungi, aiding in the early identification of such infections.