The failure of a single electrical connection within an operating room can be fatal.
Unfortunately, there are many different ways an electrical connection can fail.
Understanding where a failure may happen and what could potentially cause it can help you develop better energy-driven medical devices.
One of the potential causes of an electrical connection failure is unwanted/unknown levels of contact resistance.
In this blog post, we’ll discuss contact resistance, what causes it, how you can find it, and some of the ways you can prevent it.
What is contact resistance?
Before diving into contact resistance, it’s important to note what “resistance” is.
When talking about electrical components and circuits, resistance refers to the opposition of the flow of an electrical current.
In other words, resistance describes the ability of a material to prevent/slow the flow of electricity.
That being said, elements that make good conductors (e.g., silver, copper, etc.) have low resistance, while elements that make good insulators (e.g., rubber, glass, etc.) tend to have high resistance.
In talking about interconnect solutions, there are generally two broad categories of resistance: resistivity and contact resistance.
Resistivity (sometimes referred to as “intrinsic resistance” or “material resistance”) describes the resistance of the material itself (at a given size and area).
When someone says copper has low resistance, they’re referring to the resistivity of copper.
Contact resistance, on the other hand, describes the resistance faced when two conductors come into contact with each other.
When two cables are connected and the voltage of the cables changes as an (mostly) unintended result of this connection, it is commonly caused by contact resistance.
What is the cause of contact resistance?
To be clear, the existence of resistance is not a bad thing.
In fact, many energy-driven products (medical devices included) utilize mechanisms to control or introduce resistance.
Resistors, which Merriam Webster defines as “a device that has electrical resistance and that is used in an electric circuit for protection, operation, or current control,” are a perfect example of this concept.
Contact resistance, however, is a different story.
In general, contact resistance is not designed into a device, connector, or circuit.
Instead, devices, connectors, and circuits are designed around the existence of contact resistance and—in most cases—are designed to prevent or mitigate the impact it might have.
That being said, contact resistance is typically caused by three culprits: constriction, film resistance, and stress relaxation.
Constriction describes the narrowing of the true contact surface when two materials come into contact.
When two materials make contact, it is often assumed that electrons are flowing across the entire surface area of the materials.
In reality, due to the behavior of electrons beyond the scope of this blog post, even when two flat materials are perfectly pressed together, true contact will only be made at certain points.
Film resistance describes the resistance created by the occurrence of unwanted resistive material on the surface (“film”) of the contact.
Film resistance has two sources: oxidation and contamination.
Broadly speaking, oxidation describes the loss of electrons.
Unfortunately, most metals are typically classified as “easily oxidized.”
When it comes to electrical connectors, oxidation occurs when the layer of oxides on a contact absorb moisture.
Oxidation causes connector contacts to corrode—increasing contact resistance and causing a negative impact on the voltage passed through the connection.
Contamination happens when foreign debris finds its way onto the contact of a connector.
If contacts are covered in a bonding agent, dirt, or anything else that could disrupt the connection, contact resistance increases.
Over time and/or with excessive use, material can become deformed and the shape of your contact can vary from what was originally intended.
This deformation and change is often referred to as “stress relaxation.”
Stress relaxation has the potential to increase resistance because it can reduce contact force and the ability of the contact to “wipe” itself (both concepts are discussed in the next section).
For high-temperature, high-cycle environments, stress relaxation can be a major cause of contact resistance.
How can you mitigate the risk of unintended connector contact resistance?
Though there is no way to completely eliminate the chance of unintended contact resistance appearing, there are several precautions engineers can take during device development to mitigate the risks that contact resistance might pose.
First, select the correct material for your contacts.
Having an in-depth understanding of how the contacts will be manufactured (e.g., will they require a process that could potentially introduce contamination?) and the application environment (e.g., will the connector come into contact with fluid like blood or saline?) can help you select a cost-effective material that offers the preventative characteristics you need.
Second, ensure you have the proper contact force.
A current can only travel from one conductor to another if they are in contact.
If the contact points are loosely touching—that is, if the contacts aren’t being forced together—the connector will experience contact resistance.
Ensuring that there is enough contact force to create a sturdy connection between points goes a long way toward reducing potential contact resistance.
Third, engineer a “wipe” into the contact system.
Most contacts today are engineered to “wipe” contaminants when connected.
With sufficient contact force and a strong edge, the contact can push contaminants from itself and its mate when being connected.
Finally, ensure that connector contacts are clean before use.
For disposable devices, ensuring that there is no debris, contamination, or oxidization on the contacts before they leave the manufacturing facility is a key point of concern.
For reusable devices that must survive an autoclave, you should not only consider the manufacturing process, but also the post-sterilization process.
Will your contact points oxidize or corrode after an autoclave cycle?
Is there a potential point during/after an autoclave cycle where debris may find itself onto the contact?
These four considerations are by no means an exhaustive list of ways to mitigate the risk of unintended contact resistance, but they provide a useful starting point.
For help selecting the right material, designing a connection with the right contact force, or developing a reusable connector, we recommend speaking with an expert.
Contact resistance is only one consideration of an interconnect project.
To learn more about interconnect solutions, download our ultimate guide.