Wearables as second galvanic skins
I will start off with a recent visit that I had to Maxim Integrated. I was taken on a tour of their demo lab. During the tour I was pleasantly surprised that among the many demos on display, two of them were very important health and fitness wearables with which designers could shorten their design time and get their product to market quickly.
Before we discuss the tools for the designer, let’s get a grasp on what some of the latest important techniques, theory and electronics technologies that have made these wearable devices possible in our everyday life.
Galvanic Skin Response (GSR)
Galvanic Skin Response (GSR)1 is one electrodermal response (EDR) among many. An EDR is a change in the electrical properties of human skin caused by the interaction of environmental events and the person’s psychological state. The body’s skin is a pretty good electrical conductor. When a small electrical current is administered to the skin, there are changes in its conduction which can be measured. We can measure several variables such as skin resistance or skin conductance, its reciprocal. So if we use Ohm’s Law, the R is skin resistance which equals the voltage (V) applied between two electrodes on the skin divided by the electrical current (I) that passes through the skin or R= V/I as we learned in EE101. See Figure 1 for a typical GSR skin response to a stimulation.
GSR also has applications in medical treatment, lie detection as well as wellness monitoring.
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A key to GSR is the Féré effect, which is the change in skin conductivity when a subject is stimulated.
The role of the GSR amplifier is to apply a constant voltage to the skin, which not even perceived by the wearer since it is so small. This is done through tiny electrodes. Current then flows through the skin and is detected by a receiver, processed and used to display various parameters to the user.
The amplifier’s output voltage needs to be constant and is known so that once the current flow in the skin is measured, the skin’s conductance in µS or microSiemens is determined by the GSR amplifier.
There are two kinds of skin conductance: tonic and phasic.
Tonic is the skin conductance baseline level without any environmental stimuli and is also called Skin Conductance Level (SCL). We all have differing SCL ranging from 10 to 50 µS. These levels vary with time and depend upon our autonomic nervous system regulation and physiological state.
Phasic will change with events and are also called GSRs. Environmental stimuli such as smell, sound, sight and more will cause time-related changes in our skin conductance. These are known as Skin Conductance Responses (SCRs). These SCRs are increases in skin conductance that can last for 10 or 20 seconds and are followed by a return to the baseline level of SCL, which is also the tonic level.
The parameters of event-related GSRs that can be measured and used are amplitude (in microSiemens) and latency, rise-time and half-recovery times in seconds.
GSR and the cardiovascular system
Let’s look at how the circulation can be modeled by Ohm’s Law. Blood Pressure (BP), cardiac output (CO), and total peripheral resistance (TPR) are analogous to voltage (V), current (I), and resistance respectively. See Figure 3 below. The blood pressure curve of a normal young person shows the relation between systolic BP, diastolic BP, pulse pressure and mean arterial pressure. 1A shows the relationship between BP, CO, and TPR. 1B shows how systolic BP (SBP) and diastolic BP (DBP) are the peak and minimum values of BP in each heart beat cycle. The Pulse Pressure (PP) is the difference between SBP and DBP. The mean arterial pressure (MAP) is about equal to DBP plus 1/3 of the PP.
At INDICON, engineers showed an example of how acute hypertension can be predicted.2 Health and fitness devices can be developed to monitor any and all of the parameters in Figure 4 and then the designing company can add their other “special functionalities” depending upon their target markets.