Electroporation Basics -
Electrical Resistance and Conductance
By Michelle M. Ng, Ph. D.
In this post we will explore the relationship between electrical resistance and electrical conductance. We will also provide information on how resistance applies to electroporation applications, and finally we will answer a FAQ on the topic of sample resistance values in electroporation instrument log data.
Electrical Resistance is a measure of how difficult it is to pass an electric current through an object. The SI derived unit of electrical resistance is the Ohm (Ω). Electrical resistance (R) is defined as the Voltage (V) across the object divided by the current (I) through it.
Electrical Conductance (G) is the inverse quantity of resistance; conductance is the ease at which electrical current passes through. Conductance is defined as the Current (I) through an object divided by the voltage across it.
G= 1/R, or G=I/V
Electrical resistance applies to electroporation experiments in several ways:
1. Adjusting instrument internal resistance settings in exponential decay wave generators such as ECM 630 allows a user to modify the pulse length.
As you increase the resistance settings on your electroporation generator to a greater number of Ohms, you will increase the pulse length. Conversely, as you decrease the resistance settings on your generator to a lesser number of Ohms, you will decrease the pulse length. The effect of resistance on pulse length may be calculated with the equation T=RC, where the T is the Time Constant (time for the voltage to decay to 1/3 the set voltage), R is the Resistance* and C is the Capacitance.*Note: Sample resistance adds to instrument internal resistance to create the total resistance represented by R in T=RC, and thus the sample resistance contributes to pulse length.
2. Sample Resistance must be sufficiently high for an electroporation experiment to be successful. Sample resistance that is too low can cause problems with potential arcing and sample or instrument damage. Sample resistance is sometimes monitored as a safety feature by electroporators to protect the equipment and the sample from damage. For example, if the sample resistance is too low, you may experience an error from your instrument that will not allow you to proceed, to prevent the instrument from getting damaged. Similarly, during mid-pulse if the sample resistance is too low, the instrument may automatically shut off via an over-current pulse abort feature.
Some of the more advanced electroporators such as Gemini
also allow for the researcher to measure the resistance of the sample and record this data in the instrument’s experimental log data. For more information about sample resistance (Load) requirements for your electroporator, check the specifications section of your instrument manual.
FAQ on the topic of sample resistance:
Q: Since my electroporator measures the resistance of my sample, would you please tell me what this data means? Does a certain resistance value mean good efficiency of the electroporation or correlate with the viability of the cells, eggs, etc.?
A: Resistance measurements only provide information about the electrical resistance and conductivity of the sample as the electrical current passes through, and therefore are not a direct measure of transfection efficiency or cell viability. However, if the researcher is regularly electroporating the same type of cells, concentration of cells, buffer and transfectant concentrations, a change in resistance may provide indirect information about a difference in the sample overall. For example, a poor-quality DNA prep with salt contamination may cause the sample to have lower resistance than usual and may correlate with lower viability or efficiency. The best ways to assess transfection efficiency and cell viability however are directly:
• Assess transfection efficiency from experiment to experiment by running the same standard transfection positive control (for example, transfecting a construct expressing a fluorescent reporter protein) each experiment, side by side with experimental samples using the same electroporation protocol.
• Assess cell viability by running a viability assay (such as a stain of live versus dead cells) in each experiment, on a duplicate sample that has received the same transfectant and electroporation protocol.
Click here to visit our Electroporation Education Resource, for more basics.