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. One known technique to determine stress levels involves monitoring, measuring, and/or tracking electrodermal activity (EDA), which can be performed by measuring skin impedance or skin conductance.
Studies have 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 factor(s) associated with users’ present psychological and/or physiological condition. This would allow users or physicians to take appropriate steps to address their condition based on the obtained metrics.
Introduction
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) 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.
There are many kinds of illnesses caused by stress that affect both the body and the mind.1
There are different methods to 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 issue 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 in order 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, there are many devices on the market that 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 became 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 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 there are many factors that 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 in order 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 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 smartwatch 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 in order to determine this parameter. The conclusion was the maximum area of the EDA circuit should be less than 5 mm × 5 mm.
The power consumption of the EDA circuit is the third parameter to set. This parameter is key to ensure 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 smartwatches 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 there are several factors that can affect the final cost of the device. The circuit topology and components are selected to ensure a reasonable cost for the final solution.
References
1 Melissa Conrad Stöppler. “Stress.” MedicineNet.com, 2016.
2 Danmin Miao, Li Luo, Lijun Xioa, and Xiaomin Luo. “The Relationship Between Mental Stress Induced Changes in Cortisol Levels and Vascular Responses Quantified by Waveform Analysis: Investigating Stress-Dependent Indices of Vascular Changes.” Biomedical Engineering and Biotechnology, July, 2012.
3 Chu Kiong Loo, Einly Lim, Manjeevan Seera, Naoyuki Kubota, and Wei Shiung Liew. “Classifying Stress From Heart Rate Variability Using Salivary Biomarkers as Reference.” IEEE Transactions on Neural Networks and Learning Systems, October, 2016.
4 Wolfram Boucsein. “Principles of Electrodermal Phenomena.” Electrodermal Activity, pp 1-86, Springer U.S., 2012.
5 Wolfram Boucsein. “Recording Sites.” Electrodermal Activity, pp 104-109, Springer U.S., 2012.
About the Authors
Javier Calpe [[email protected]] received his B.Sc. in 1989 and his Ph.D. in physics in 1993, both from the Universitat de Valencia, Spain. Javier is the site manager of ADI’s development center in Valencia, Spain.
Jose Carlos Conchell [[email protected]] is a product applications engineer in the Industrial and Healthcare Business Group, based in Valencia, Spain. He focuses on bioimpedance applications. José Carlos Conchell joined ADI in 2011. He received his B.Sc. and M.Sc. degree in electrical engineering and M.Sc. degree in biomedical engineering from Universitat de Valencia, Spain, in 2007, 2010, and 2016 respectively.
