When connecting electronics to the human body, and doing experiments with humans, it is very important to understand how to do this safely and what are good habits of experimenting on humans.
The tutorial and code below is our best attempt to minimise risks, and is optimised for the specific student projects of the Fontys University of Applied Sciences. Even when the guidelines are followed, any electronic equipment and experiment still has risks and should be approached with common sense.
There are several potential risks when using electronics close to the human body:
Now you have an idea of technology that may be harmful to the body, you understand that the concept of “safety” has to be described. This is for the following reasons and from the following perspectives:
The physiological effects of electricity flowing through the human body that may have negative effects are:1)
From a low magnitude to a higher magnitude, the effects are summarized in table 12). When the current is large enough to excite some nerve endings at the skin of the entry point, the person can feel a tinteling, or slight warming when above. The threshold of perception is the minimal current a person can feel and varies strongly with the person and situation. In wet conditions on a bare metal, the lowest thresholds are around $0.5mA$ for $50Hz$. For DC currents, the thresholds are $2mA$ up to $10mA$.
For higher currents, nerves and muscles are stimulated and can contract involuntary. This may already cause pain and fatigue. As a result, a person may faint, fall, or make other movements that cause a secondary accident. When increasing the current, the muscle contraction in combination with the situation, the person may not be able to open the hand and withdraw from the electric source. This is the let-go current.
When increasing the current further, even without physiological damage, there may be a serious situation where muscles involved with the respiratory system are contracted. The result is respiratory paralysis, pain and fatigue. This has been observed starting at currents of $20mA$.
For currents up to $75mA$ and $450mA$ where the current pathways can go through the heart, the electrical activity of the heart may be interrupted. This results into ventricular fibrillation and may not stop when the cause of the fibrillation is removed. This is the major cause of death due to electric shock.
Even currents up to $1A$ up to $6A$ do not necessarily result into permanent damage, especially not for short periods. However, there is a strong risk starting at these current levels. Proteins are electrochemically denaturalised causing tissue damage. Tissue may burn due to the resistive heating or due to direct puncture of the skin at the entry point. The nervous system including the brain stops functioning because of the current. Excessive muscle contractions may cause rupture of ligaments from the bones.
The description above is based on electric currents. The potentials that may cause such currents depend on the contact area, humidity and bodily and electric resistances involved.
In case the bare dry skin touches a conducting surface of one square centimeter, the electrical contact resistance may range from $15k \Omega$ to $1M\Omega$. For a wet skin, the resistance can drop by a factor of 100! Inside the body, the resistance is much lower. We can assume a resistance of a $100 \Omega$ for the trunk and $200 \Omega$ for each limb (arm or leg).
As a rule of thumb, hazardous current levels can already be reached with:
The cause of electric shocks (or any damage due to electricity) is a potential difference between the subject and the experimentation equipment that results into an electric current. Strictly speaking, when all surfaces are at the same potential, there is no hazard. Such potential differences may arise either from the electronics itself or from an unexpected difference in the grounding.
The entry point of the current can give some classification of the situation:
In case of microshock, there is a high current density through the heart, and small currents may already cause muscle contractions in the heart tissue. This happens with intercardiac catheters or other electrodes that are very close to the heart with a low impedance.
The most likely situation with equipment around the body is the macroshock. The European and US grounding system is drawn in figure 1. With an ungrounded chassis, and a fault in the wiring (hot line contacts accidentally the cabinet), a current will flow through the human body to the grounded feet of the person. This is illustrated in figure 2.
When there is more than one ground point, there may be a potential difference between the ground-points, and a current will flow when a subject accidentally short-circuits these two different grounds with his body. As a result, a shock may be experienced, when a person in a hospital bed touches his TV-set or the metal side of the bed. The reason is that the subject may be grounded already by a medical electric equipment. Such a situation is called a ground fault. There are guidelines for the maximum allowed leakage currents of equipment:
To be added: how to test?
To be added: Thermal damage 4)
The Declaration of Helsinki (DoH) is a set of ethical principles regarding human experimentation developed for the medical community by the World Medical Association (WMA)5).
To be added:
Regulations can help you to develop safe products, and to create evidence that all risks have been identified and anticipated. For medical products, the following five standards describe the basic foundations to meet the Medical Device Directive essential requirements:
While the first one is very broad, the last four are mainly process standards.