In recent years, activity trackers and other wearable electronic devices have gained popularity due to users’ desire to monitor, measure, and track using various real-time features related to their fitness or health, including the number of steps they take, their heart rate, their heart rate variability (HRV), the users’ temperature, their activity and/or stress levels, etc.

Fig. 1 - System block diagram.

One known technique for determining stress levels involves monitoring, measuring, and/or tracking electrodermal activity (EDA), which can be performed by measuring skin impedance or skin conductance. It has been shown in studies that in response to an environmental, a psychological, and/or a physiological arousal, users’ skin conductance would increase.

By measuring changes in the skin impedance or the skin conductance over time, metrics can be obtained relating to users’ activity level, stress level, pain level, and/or other factors associated with users’ present psychological and/or physiological condition, allowing users or physicians to take appropriate steps to address their condition based on the obtained metrics. This article provides a useful physical system to investigate and finally estimate/quantify the stress level of a person.

Measuring Stress

Stress is a physical, a mental, or an emotional factor that causes bodily or mental tension. Stresses can be external (environmental, psychological, or from social situations) or internal (illness or caused by a medical procedure). Stress can initiate the fight-or-flight response, a complex reaction of neurologic and endocrinologic systems.

The fight-or-flight response (also called the fight, flight, freeze, or fawn response in post-traumatic stress disorder, hyperarousal, or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival.

The reaction begins in the amygdala, which triggers a neural response in the hypothalamus. The initial reaction is followed by activation of the pituitary gland and secretion of the ACTH hormone. The adrenal gland is activated almost simultaneously and releases the epinephrine hormone.

The release of chemical messengers results in the production of the cortisol hormone, which increases blood pressure and blood sugar, and suppresses the immune system. The initial response and subsequent reactions are triggered in an effort to boost energy. This boost of energy is activated by epinephrine binding to liver cells and the subsequent production of glucose. Additionally, the circulation of cortisol functions to turn fatty acids into available energy, which prepares muscles throughout the body to respond. Catecholamine hormones, such as adrenaline (epinephrine) or noradrenaline (norepinephrine), facilitate immediate physical reactions associated with a preparation for violent muscular action.

However, under constant demand, the stress system becomes chronically active and can have damaging effects on the health of an individual. Many kinds of illnesses are caused by stress that affect both the body and the mind.1 These will be mentioned later in the article.

Methods

Different methods can detect and determine the stress level. The most important methods are measuring cortisol level, obtaining heart rate variability, or obtaining the electrodermal activity.

Measuring Cortisol Level. Cortisol is a steroid hormone in the glucocorticoid class of hormones and is produced in humans by the adrenal cortex, within the adrenal gland. It is released in response to stress. Thus, measuring the cortisol level is considered the gold standard method to quantify the stress level.2 However, this technique has two important issues.

One of the issues is the delay between the threat and the variation in the cortisol level, which may be up to 15 minutes. The second and most important issue is that stress levels should be obtained continuously to detect the threats and stress situations in the user’s daily life. Thus, this method is too complex, expensive, and unfriendly for anybody and, therefore, cortisol measurement is not a suitable method for general use.

Obtaining HRV. HRV is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval.3 Currently, many devices on the market can measure heart rate. The resolution of these devices is one beat per minute (bpm) in the best case. This resolution is good enough in many applications. However, the required resolution of HRV for stress assessment is 10 or 100 times higher. This means that the sampling frequency and algorithm complexity must be greater and, hence, the power consumption of the system can become too much for a wearable product or a 24/7 application.

Obtaining EDA. EDA is an indirect measure of neurally mediated effects on sweat glands’ permeability, observed as changes in the resistance of the skin to a small electrical current or as differences in the electrical potential between different parts of the skin.4 EDA presents more advantages than the other techniques in terms of power consumption, ergonomics, and circuit size.

System Description

The objective of this research is to develop a useful tool to investigate and estimate the stress level of a person. The stress level of a person is not constant, and it depends on the threats perceived by the person. Those threats are perceived differently by each person and many factors can make a simple event for a person an enormous threat for another. It is not useful to carry out a stress test in a hospital to determine the stress level of a person, since these threats appear in the normal life of the patient. Therefore, it is necessary to develop a system that allows us to estimate the stress level of a person during his or her normal life. Thus, this system must be noninvasive, user-friendly, and wearable. Finally, it must be able to work for several days without being recharged or replaced. The requirements for the final device imply the system must be:

  • Battery operated, since it must be wearable.

  • Low power, since the patient must be monitored for several days.

  • Reduced size, since it must be wearable and user-friendly.

  • Low cost, since if it is too expensive, the solution will not be implemented in any consumer device.

  • Compliant with safety regulations.

To ensure that the system is nonintrusive, the recording site must be taken into account. The best placement for the electrodes is the top of the wrist, since this results in the device being noninvasive, user-friendly, and simple from the mechanical point of view. However, the quality of the signal is not as good as the EDA signal obtained from other body locations, such as the medial phalanx in the index and middle fingers.5

Once the electrodes’ placement to obtain the EDA signal is decided, it becomes obvious that the final (target) system will present the form of a smart-watch or a similar device. At this point, the next specification to determine is the area that can be used by the EDA circuit. Several smartwatches were analyzed, and various vendors were consulted about this topic to determine this parameter. The conclusion was the maximum area of the EDA circuit should be less than 5 × 5 mm.

The power consumption of the EDA circuit is the third parameter to set. This parameter is key to ensure that the system will be able to record the EDA signal during several days without recharging or replacing the device. The battery’s capacity for different smart-watches and the power budget of some possible commercial systems are obtained. The target for the power consumption obtained after this investigation is fixed at a maximum average consumption of 200 μA.

Finally, the last specification to determine is the cost. However, this is not determined at this stage, since several factors can affect the final cost of the device. The circuit topology and components are selected to ensure a reasonable cost for the final solution.

Hardware Design

HRV is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval.3 This section describes how the circuit topology, range to measure, and resolution are determined.

One of the key decisions is to determine the topology of the circuit. Basically, there are two methods to measure an impedance. The system can apply current and measure the voltage across the impedance, or it can apply voltage and measure the current across the impedance. Besides, these signals can be dc or ac.6 It is important to analyze the advantages and disadvantages of each method.

Dozens of circuits can measure ac, and each one has its pros and cons. However, to accomplish the restrictions in performance, cost, and area, the following solution is considered as the best option.

The final decision is to use an ac voltage source as the excitation source and measure the current through the patient’s body to determine the skin conductivity. This solution avoids high voltages over single sweat glands, eliminating the danger of sweat gland damage and allowing the compliance of the IEC 60601 standard. The ac signals eliminate the problem of electrode polarization.7

The current that must be measured needs to be digitalized, stored, and analyzed. It means the circuit will require an analog-to-digital converter (ADC). As most ADCs convert voltage and not current, the current through the patient’s body needs to be translated to voltage. This is carried out by a transimpedance amplifier (TIA). The noise specifications, size, and power consumption are three critical characteristics to choose the best operational amplifier, which will be used to implement the TIA.

Once the topology of the system is decided, the next step is to determine the range and resolution of the system under development.

Problems for the amplification of the EDA signal mainly stem from its wide range and the required high resolution. Typically, a skin conductance device must cover a range from 0 to 100 µS, and it must also be able to detect 0.05 µS fluctuations. This resolution can be achieved using an ADC with at least 12-bit resolution. Regarding the resolution, the target in this project is 0.01 µS, and, therefore, an ADC with 14-bit or 16-bit resolution is required.8 To get a resolution of 0.05 µS within a range of 100 µS while complying safety regulations, these blocks are required:

  • An ac voltage source.

  • Protection elements to ensure compliance with IEC6060-1.

  • An electronic circuit to measure the current flowing through the patient’s body.

Variations in the ambient temperature and skin temperature can produce changes in the EDA signal.9 Thus, it would be interesting to acquire the ambient temperature and skin temperature, too. It can be carried out by a simple thermistor plus several discrete components and an ADC.

Finally, power consumption is critical in this circuit. To reduce it and ensure the system is only activated when a new measurement is minimum number of components and functionalities have been included required, a power management unit must be also integrated. This block must be easily controlled by the main microcontroller and it has to supply all the EDA measurement circuit. Figure 1 shows the complete block diagram. The following sections discusses how to determine the best components for this application.