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theory:safety:safety

Electrical Safety and the Ethics of Experimenting

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.

Safety

There are several potential risks when using electronics close to the human body:

  • An electric current through the body may cause (permanent) damage. This will be specifically addressed in a section below
  • Exposure to radiation may be harmful depending on the time and intensity. For the electronics presented on this website this is less likely. Most people use existing Bluetooth modules of low power for communication.
  • Vibrations can be harmful for organs and joints. Not explained on this page, but search for information when applicable
  • Heating of tissue. When an electronic circuit is in contact with the body, we may get burn effects or over-heating effect. This is also important and likely to occur and is therefore described in a section below.
  • Intense light like power LEDs or lasers can cause blinding effects that may be permanent.

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:

  • Ensure patient safety (protect against macroshock/microshock)
  • Define test conditions for normal operation and anomalies (internal electrical breakdown, damage to power cord, leakage currents)
  • Meet codes & standards
  • Protect against legal liability in case of a patient incident

The effect of electrical currents through the body

The physiological effects of electricity flowing through the human body that may have negative effects are:1)

  • Electric stimulation of excitable tissue like nerves and muscles,
  • Resistive heating of tissue,
  • Electrochemical burn due to DC currents.

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.

Current Effect
$0.001 A (1 mA)$ up to $0.010 A (10 mA)$ Tinteling: threshold of perception
$0.006 A (6 mA)$ up to $0.020 A (20 mA)$ Muscle spasm: hard to release hand
$0.020 A (20 mA)$ up to $0.050 A (50 mA)$ Pain, fainting, difficulty to breathe
$0.075 A (75 mA)$ up to $0.450 mA (450mA)$ Atrium or ventricular fibrillation
$> 5 A$ Breathing spasm, burn wounds, muscle cramp, denaturalisation of proteins (tissue damage)
Tab. 1: Electrical currents and the safety issue for the human body

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:

  • $12V$ in wet environments,
  • $25V$ in humid environments, and
  • $50V$ in dry environments.

The origin of electrical shocks

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:

  • Macroshock: Contact with a current carrying wire results into a direct current through the body or parts of the body
  • Microshock: A grounding problem in combination with a medical device in direct contact with the myocardium may already give severe problems at low leakage currents

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.

Fig. 1: Three phases on a power socket

Fig. 2: Macroshock: A current directly through the body to ground

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:

  • Equipment that is not intended to contact patients shall not have leakage currents above $500 \mu A$ while
  • Patient contacted systems have an upper leakage limit of $100 \mu A$.

How to prevent electric shocks in the circuit design?

  • Make the outside of the cabinet of the electronic circuit of insulating material (plastic) and/or ground the conducting parts
  • Make sure the low-voltage circuit (battery operated) is not in galvanic contact with the fixed world. Normally, we are only interested to transmit signals containing data. Therefore and optocoupler or a wireless transmission can make the circuit safer.
  • Use voltage limiters on the electrodes. Two opposing diodes will short circuit the electrodes in case of a high electrode voltage. This can be done with two parallel opposing diodes, or with two series zener-diodes placed back-to-back3).
  • Limit the largest possible current by series resistors. Most biopotential measurements are probing voltages, and can have a high impedance. With $30 k \Omega$ resistors in series with the electrodes, and a power supply voltage of $9V$, the current can never exceed $0.3 mA$.
  • In case you connect your electronics to a computer (for example: electrode - Arduino - USB - PC), then use a laptop and use it battery operated.

To be added: how to test?

Heat

To be added: Thermal damage 4)

Ethics

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:

  • “Informed Consent”
  • Medical Ethical Committees6)

Regulations

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:

  • Hardware: The IEC 60101-1 recommendations by the International Electrotechnical Commission7)
  • Software: IEC62304
  • Risk Management: ISO14971
  • Usability: IEC62366-1
  • Quality Systems: ISO13485

While the first one is very broad, the last four are mainly process standards.

1) , 2)
John G. Webster (editor), Medical instrumentation, application and design, second edition, Houghton Mifflin Company, 1992
3)
Ákos Jobbágy, Sándor Varga, Biomedical Instrumentation, 2014, http://www.tankonyvtar.hu/en/tartalom/tamop412A/2011_0079_jobbagy_biomedical/ch03s04.html
4)
Yarmolenko, P. S., Moon, E. J., Landon, C., Manzoor, A., Hochman, D. W., Viglianti, B. L., & Dewhirst, M. W. (2011). Thresholds for thermal damage to normal tissues: An update. International Journal of Hyperthermia : The Official Journal of European Society for Hyperthermic Oncology, North American Hyperthermia Group, 27(4), 320–343. Available via NIH
5)
World Medical Association, WMA Declaration of Helsinki - Ethical Principles for Medical Research Involving Human Subjects, http://www.wma.net/en/30publications/10policies/b3/
7)
International Electrotechnical Commission, Medical electrical equipment - Part 1: General requirements for basic safety and essential performance, https://webstore.iec.ch/publication/2612
theory/safety/safety.txt · Last modified: 2017/11/01 10:53 by glangereis