Introduction

As wearable sensors become familiar, their sophistication increases, allowing these devices to do more than track activity levels. Wearable devices offer an alternative path to clinical diagnosis by monitoring vital signs like arterial oxygen saturation, breath rate, and body temperature in real time in a minimally invasive or non-invasive manner. In recent years, engineers achieved significant advancements in wearable sensor development based on materials having accuracy, flexibility, high sensitivity, and superior mechanical ability, heralding a new approach to remote and real-time health monitoring.

Sensors can be worn as glasses, jewellery, fitness bands, tattoo-like devices, bandages or other patches, face masks, wrist watches, and textiles. Wearables like smartwatches have proved their capability for early detection and monitoring of the progression and consequent treatment of several diseases, such as COVID-19, epileptic seizures, Parkinson's disease, cardiac arrests, etc. This article will explore the diverse healthcare sensors and their transduction mechanisms, showcasing their profound impact on patient health, clinical practices, and healthcare.

Wearable healthcare
Figure 1: Wearable healthcare devices are used in everyday life

Transduction mechanisms are used in various healthcare sensors

Sensors transform various forms of energy into readable electrical signals. These signals are proportional to the physical quantity being measured. Electromagnetic induction, capacitive sensing, piezoelectricity, and piezo resistivity are common sensor transduction mechanisms. Combining these essential single mechanisms could lead to the development of multi-transduction technology. (Sensors & transducers).

Accelerometers

Accelerometers are available in various sensitivities, weights, sizes, and shapes. They are considered for both shock and vibration measurements.

Accelerometer
Figure 2: (a) Principle of operation of an accelerometer (b) LIS2DW12 micro electro mechanical systems accelerometer

Figure 2(a) shows the acceleration of the frame (a), which is to be sensed and is indicated as the inertial force (ma) on the mass. The equations show the derivation of the accelerometer's sensitivity using the mechanics and vibration theory fundamentals. The movement of the frame is denoted by x and that of the mass by y. The net extension of the spring and the damper will thus be, z = y −x.

NXP LIS2DW12 is an ultra-low-power, high-performance, three-axis linear accelerometer. The key features of the LIS2DW12 include ultra-low power consumption and minimal noise (down to 1.3 mg RMS in a low-power mode with a supply voltage of 1.62 V to 3.6 V). The functionality of the sensors includes a high-performance mode to focus on low noise and low power modes, which is a trade-off between power consumption and noise. The self-test allows examination of the sensor operation without displacement. The activity/inactivity function recognizes the device's sleep state and enables a reduction in system power consumption.

The LIS2DW12 has a dedicated internal engine to process motion and acceleration detection, including free-fall, wakeup, activity/inactivity, stationary/motion detection, portrait/landscape detection, and 6D/4D orientation. Other accelerometers, such as BRKTSTBC-A8471 and LIS2DUX12TR, are also available.

Heart rate sensors

Photoplethysmography (PPG) is a non-invasive method of calculating the heart rate. The heart rate sensor works on the principle of photoplethysmography.

There are two types of photoplethysmography:

  1. Transmission: Light is transmitted through the vascular region.
  2. Reflection: Light is reflected by tissues. Figure 3(b).

Heart Rate sensors consist of a light emitter and a detector. The detector output forms a DC signal proportional to the heartbeat rate. The isolation of the AC component is of prime importance as this component is synchronized with the heartbeat, caused by pulsatile changes in arterial blood volume, and is superimposed on the DC signal.

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Figure 3: (a) Basic circuit topology of photoplethysmography. (b) Transmission mode in the ear and reflection mode in the finger

To acquire this AC signal, the output from the detector is first filtered using a 2-stage HP-LP circuit (figure 3. a) and is then converted to digital pulses utilizing a comparator circuit or simple ADC. The digital pulses are then given to a microcontroller for calculating the heartbeat rate, provided by the formula-

BPM (Beats per minute) = 60*f
Where f is the pulse frequency

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Figure 4: Analog Devices MAX86141ENP+T

Figure 4, MAX86141 is an ultra-low power, wholly integrated optical data acquisition system. On the transmitter side, this device has three programmable high-current LED drivers that engineers can configure to drive up to six LEDs utilizing an external 3x2:1 mux. On the receiver side, the device has two optical readout channels that can operate simultaneously. It has an optimized architecture for transmissive and reflective heart rate or SpO2 monitoring.

Biosensor array

Biological sensing elements interact with the analyte of interest to produce a signal. The list of sensing elements includes materials like tissues, microorganisms, organelles, cell receptors, enzymes, antibodies, and nucleic acids. Figure 5a shows how the signal produced through the interaction of the analyte of interest and the sensing element transforms into a measurable and quantifiable electrical signal via the transducer.

biosener
Figure 5: (a)The basic scheme of a biosensor. (b) Biosensor design illustrating the various components necessary for generating a signal
VCNL4020C
Figure 5: (c) VCNL4020C (High-resolution digital biosensor for wearable applications) )( VCNL4020C-GS08)

The VCNL4020C is a fully integrated biosensor and ambient light sensor with an infrared emitter (IRED), ambient light sensor (ALS), photodiode (PD), and signal conditioning IC modules included in the package. The I2C communication interface contains seventeen 8-bit registers for operation control, parameter setup, and result buffering. Built-in infrared emitter and broader sensitivity photodiode allow it to work with green and red LEDs. The good resolution of 16-bit makes it resistant to noise and ensures excellent cross-talk immunity. Signal modulation is done in such a way as to suppress the ambient light. The application areas of these sensors are wearables, health monitoring, and pulse oximetry.

Temperature sensors

Temperature sensors for body temperature measurements can be affected by accuracy, response time, power consumption, and the specific application's requirements.

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Figure 6: MAX30205 Temperature sensor for body temperature monitoring (MAX30205MTA+)

The MAX30205 temperature sensor is extremely accurate when measuring temperature and provides an overtemperature alarm/ interrupt/shutdown output. This device uses a high-resolution, sigma-delta, analog-to-digital converter (ADC) for converting the temperature measurements to digital form. The I2C serial interface accepts standard commands such as write byte, read byte, send byte, and receive byte commands to receive and read the temperature data and also configure the behavior of the open drain overtemperature shutdown output. The MAX30205 features three address select lines with 32 available addresses. The sensor has a 2.7V to 3.3V supply voltage range, a low 600µA supply current, and a lockup-protected I2C-compatible interface, making it ideal for wearable fitness and medical applications. Some other temperature sensors available are STTS751-0DP3F and MAX30205.

Gyroscopes

MEMS gyroscopes use Coriolis acceleration to measure the angular rate. Figure 2a shows that when the resonating mass goes toward the outer edge of the rotation, it accelerates to the right and also exerts on the frame a reaction force towards the left. When it moves toward the center of the rotation, it then exerts a force to the right, as indicated by the green arrows.

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Figure 7: (a)Demonstration of Coriolis effect in response to a resonating silicon masas suspended inside a frame. The green arrows show the force applied to the structure based on the status of the resonating mass. (b) Schematic of the gyroscope's mechanical design

The Coriolis acceleration can be measured using a specific technique. The frame that contains the resonating mass is tethered by springs to the substrate. These springs are 90° relative to the resonating motion (Figure B). This figure also shows the Coriolis sense fingers that are used to sense displacement of the frame through capacitive transduction in response to the force exerted by the mass.

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Figure 7: (c) ADIS16260 (Programmable Digital Gyroscope Sensor)

Analog Devices ADIS16260 and ADIS16265 integrate a MEMS gyroscope with data sampling, signal processing, calibration functions, and a simple user interface. This sensing system collects data autonomously and makes it available to any processor system that supports a 4-wire serial peripheral interface (SPI). They provide accurate performance requiring full motion calibration with any other MEMS gyroscope in their class. An addressable register structure and a standard serial peripheral interface (SPI) provide simple access to sensor data and configuration settings. Data processing in the embedded controller includes correction formulas, filtering, and checking for preset alarm conditions. They provide additional benefits for wearable devices, including improved rep counting in gym activities and improved distance and direction calculations when the GPS is switched off.

Blood oxygen sensors

Pulse oximetry uses red and infrared (IR) lights to estimate hemoglobin oxygen saturation of arterial blood. Figure 8 shows the blood oxygen sensors mounted on an evaluation board. Oxyhemoglobin (HbO2) absorbs visible and infrared light differently than deoxyhemoglobin (Hb). Absorption in the arterial blood is represented by an AC signal that is superimposed on a DC signal, representing absorptions in other substances like pigmentation in tissue, venous, capillary, bone, and so forth. The cardiac-synchronized AC signal is approximately 1% of the DC level. This value is referred to as the perfusion index percentage.

Equation 1 approximates the ratio of ratios, R, and percent Saturation of Peripheral Oxygen (SpO2) as follows:

equation 1

Equation 2 gives the standard model of computing SpO2.

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Figure 8: ADPD105, a highly configurable photometric front-end (AFE) device from analog devices

The main component of the oximeter is the ADPD105, a highly configurable photometric front-end (AFE) device from Analog Devices. This IC consists of three current sinks LED drivers with four AFE input channels and the common cathode. The photodiode (PD) elements connect to these channels and drivers. Depending on needs, the IC has eight PD inputs. These inputs can be routed to AFE input channels. Three possible PD configuration settings are possible, and they are programmable via the I2C interface. Oximeter click uses two LEDs suitable for measuring blood oxygen saturation: a red color LED and an infrared LED. Also, a single PD element is used to sense the reflected light. Engineers can use it to develop oximetry algorithms, heart rate measurement applications, and ambient light applications. This Click board can be used in virtually any photometric application by offering expandability with external LED and PD elements.

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Figure 9: (a) Blood oxygen sensors (b) Blood oxygen sensors mounted on the evaluation board

Latest technology trends in healthcare sensors: Wearable and implantable sensors

Users can access quality, affordable, and accessible healthcare using implantable and wearable devices. Recent progress in microfabrication has stimulated implantable miniaturization. The pacemaker, cardiac defibrillator, insulin pump, neurostimulator, cochlear implant, and retinal implants are a few examples of clinically well-established precision healthcare implantable devices, as shown in Figure 10.

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Figure 10: Different wearable and implantable healthcare devices are typically used in precision healthcare

The energy system of an implantable device has three components: energy source, storage, and power management unit. The power consumption may vary from a few microwatts to milliwatts. The operational life of an implant depends on its uninterrupted power supply.

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Figure 11: Engineering level block diagram of the mid-level wearable device

Figure 11 shows a network of sensors and connectivity solutions used in wearable devices. The selected LCD is a 1.28-inch, 128 x 128 resolution monochrome HR-TFT transflective panel. Other components include the STTS751 and MAX30205 temperature sensors from NXP and Analog Devices. The accelerometers are FXLN83xxQ from NXP and LIS2DUX12 from STMicroelectronics. These two combine to make an inertial measurement unit and an accelerometer fitted with a low-power, low-noise sensor for mobile and indoor applications. The MAG3110 and LIS2MDL Magnetometers from NXP and STMicroelectronics have low-power, low-noise 3-axis digital geomagnetic sensors for compass applications. The heart rate monitor (HRM) has a capacitive touch sensor. User interface navigation is made possible via capacitive touch buttons and a complementary controller. These devices communicate through near-field communication (NFC), a short-range wireless connectivity technology.

Conclusion

Healthcare sensors are used in various medical applications and have saved countless lives. Hospital wards, intensive care units, GP offices, dental practices, in-home care, and laboratories use these healthcare sensor solutions for prevention, treatment, and monitoring applications. The significant constraints while designing a miniature wearable sensor include material engineering, power management, and fabrication. Accelerometers, temperature, gyrometers, heart rate, and blood oxygen sensors can measure vital body signals. Sensors, along with connectivity solutions, can be configured as wearable devices.

element14 has partnered with many different suppliers catering to a wide range of sensors such as MEMS accelerometers, pulse oximeter & heart-rate sensor, COB, temperature sensors, mikroelektronika mikroe-3102- blood oxygen sensors evaluation board.

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