Projects and studies which involve human electrophysiological measurements (eg. ECG/EKG, EMG, EEG) MUST conform to these specific safety protocols. Deviating from these protocols risks serious and fatal injuries. Even a few milli-amps of current can be fatal to the human heart, so take these warnings seriously.
- The human subject MUST be electrically isolated from the electrical grid (eg. Oscilloscopes, power supplies, desktop computers, USB devices connected to PCs), as well as high current output sources (eg. laptop computers).
- The Bioengineering Educational laboratories are equipped with Biopac units designed for electrophysiological studies. When used properly, they provide the necessary isolation. Signals can be measured, recorded, and the data exported for use in other applications.
- Most consumer products available through Amazon, Sparkfun, and other hobby stores do NOT provide electrical isolation. A rule of thumb is that if the device costs under $500, it is likely not isolated.
- When designing circuits to manipulate electrophysiological data, whenever possible, simulate the signal rather than connecting to a human subject. When ready for human subject trials, switch the power source to low-current capable power sources (eg. watch batteries), and disconnect anything connected to the electrical grid or other high-current output sources.
Isolating the Signal for use with external devices
- Open the Biopac software. On the toolbar, go to MP35 >> Output Control >> CH to Output.
- A rocker switch will appear on the Biopac main screen. Flip it to the “On” position
- Go to MP35 >> Set up Data Acquisition..., and set Channel 3 to acquire the appropriate signal. If you are fine with the raw unfiltered signal, just keep it at “Default”.
- Plug the electrode lead set into Channel 3 of the MP35 unit.
- Connect a DB9 cable from the rear output port of Biopac to a DB9-to-breadboard connector on a breadboard. Pin 2 will be the signal, and pin 3 will be ground.
- The signal is now isolated. You can connect it to other equipment (eg. myDAQ, Arduino, external filters, a PC, etc.). Do NOT connect any part of this isolated signal to any part of a circuit before the Biopac.
Background on Bioelectric Signals
Nerve and muscle cells generate bioelectric signals that are the result of electrochemical changes within and between cells. If a nerve or muscle cell is stimulated by a stimulus that is strong enough to reach a necessary threshold, the cell will generate an action potential. The action potential, which represents a brief flow of ions across the cell membrane, can be measured with intracellular or extracellular electrodes. Action potentials generated by an excited cell can be transmitted from one cell to adjacent cells via its axon. When many cells become activated, an electric field is generated that propagates through the biological tissue. These changes in extracellular potential can be measured on the surface of the tissue or organism by using surface electrodes. The electrocardiogram (ECG), electrogastrogram (EGG), electroencephalogram (EEG), and electromyogram (EMG) are all examples of this phenomenon.