Interconnect Materials for Medical Devices [A Technical Guide]
Selecting the correct materials for your interconnect solution is vital to ensuring that your device functions correctly and meets the requirements of the application.
However, finding the interconnect materials best suited for your device can be particularly challenging because the interconnect solution consists (most likely) of many different components that can be made of many different types of materials.
In this post, we’ll discuss the typical types of materials that can be used to create different interconnect components.
Cables, wires, and assemblies.
In our Cable 101 series, we discussed the four basic components of most cables: the conductor, the insulation, the shield, and the jacket.
Each of these four components utilizes different types of materials to achieve different outcomes (e.g., improve conductivity, reduce interference, etc.).
Before we get started talking about the different types of material that can be used for each component, it’s important to note that you should not choose a specific material for a specific component in isolation.
Instead, you should approach the cable as a mix of mutually interactive pieces and analyze all four components together to determine which material should be used for each specific part.
That being said, let’s dive in.
If you’ve read our blog post What is the Conductor? [Cable 101] then you know that the conductor is the heart of a cable—it’s the component that actually transports the electrical current from its source to its load.
Though a conductor can theoretically be made from any material that allows the flow of electricity, the most common category of material used to create a wire is metal.
Metals like silver, copper, gold, aluminum, brass, and nickel can be used to create wire conductors.
However, within the medical device field, the most common type of conductor material is copper plated with a different type of metal (e.g., tin) to prevent corrosion.
Though silver is the most conductive interconnect material, copper is commonly used because it costs less than silver while still offering strong conductivity (though silver can be found in shorter cables where achieving the highest conductivity possible is required).
Insulation is used to physically separate conductors and minimize electrical interference.
As you can imagine, this job requires non/minimally-conductive material that can stop (or minimize) the flow of electricity.
Though there are a variety of materials that can be used to insulate a conductor, the most common insulators within the medical field are DEHP-free polyvinyl chloride (“PVC”), polyethylene (“PE”), polytetrafluoroethylene (“PTFE”), ethylene propylene rubber (“EPR”), and silicone rubber.
Selecting the right material for your application depends on many factors.
For low cost, disposable devices in a non-harsh environment, DEHP-free PVC may be a wise option.
For high-temperature applications, PTFE may be a viable option.
For high-cost, reusable devices that must survive multiple autoclave cycles, silicone rubber may be the only option you have.
A shield protects the conductors from external signal noise and reduce the conductors’ electromagnetic radiation.
The material used for the shield depends on the type of shield being used.
In general, medical device cables typically utilize either foil shielding or braid shielding (or a combination of both).
Foil shielding, as the name implies, consists of a thin metal sheet that is wrapped around the insulated conductors.
Within the medical device realm, foil shielding is commonly made of either copper, aluminum, or metallized biaxially-oriented polyethylene terephthalate (“BoPET”).
Compared to braid shielding, foil shielding provides rigidity.
Braid shielding consists of thin metallic wires bound together around the insulated conductors.
When it comes to medical device cables, the most common type of material used for braid shielding is copper.
Depending on the application and target costs, the copper can be plated with either tin or silver.
The cable jacket is used to pull all of the aforementioned components together and protect them from the external environment.
So, beyond insulation capability and price point, some of the key considerations when selecting the appropriate material for the cable jacket are look, feel, and flexibility.
That being said, the most popular jacketing materials used in the medical device world today are DEHP-free PVC, thermoplastic elastomers (“TPEs”), thermoplastic polyurethanes (“TPUs”), and silicone rubbers.
For low-cost applications, PVC is a common choice as it is relatively low-cost and easier to manufacture than other plastics.
For applications where a device may be reused several times and/or the cable is coming into contact with, or going into the body of, a patient for extended periods of time, TPEs and TPUs are commonly found.
Finally, for reuseable cables that must withstand dozens or hundreds of autoclave cycles, silicone rubbers are the most common.
Connectors, contacts, and housings.
Attached to the end of each cable is the connector that enables the cable to function.
If you’ve read our blog post about interconnect connectors, you know there are two primary pieces that make up a connector: contacts and housings.
Contacts enable the transfer of data and/or power from the conductor(s) to the intended endpoint.
As such, contacts are predominately made of metal.
When it comes to medical devices, contacts are typically made of copper-based alloy or nickel and plated with copper, gold, nickel, or aluminum.
Housings, on the other hand, are used to separate the contacts and make the connector easier to grasp.
Given this information, housings are typically made of insulative, low-conductive material like plastic or rubber.
In the medical device space, housings are commonly made using hard-shore TPUs and/or TPEs.
However, for devices that must go through many autoclave cycles, silicone rubber is the common material of choice.
Printed circuit boards (“PCBs”) have become common components of energy-driven medical devices.
In general, PCBs are categorized as either rigid (e.g., the stiff “gum stick” in your TV remote) or flexible (e.g, one of the thin, film-like PCBs that can be found connecting components in your computer).
Regardless of the categorization all circuit boards can be broken down into three general pieces: the substrate, the copper layer, and the solder mask.
The substrate provides the main foundation for the PCB.
As such, it is typically made from a low/non-conductive material.
In the medical device world, most rigid PCBs utilize fiberglass (“FR4”) as the substrate, while flex PCBs use polyimide.
The copper layer consists of the complex network of conductive tracks that enable the transmission of data and power.
As the name suggests, the copper layer is typically made of copper.
Finally, the solder mask is used to prevent oxidation and the formation of solder bridges (which could ruin the signal within the PCB).
Within the medical device world, the solder mask is typically made from a low-conductive polymer like epoxy liquid or liquid photo imageable epoxy.
Beyond these three components, the different connection points on the PCB may consist of gold or copper-plated metal.
Selecting the correct material for your interconnect solution requires a knowledge of how all the different components—from the conductor to the PCB substrate—will interact with each other.
This can be an extremely difficult task—especially if you’ve never done it before—which is why we recommend speaking with an expert before getting started.
If you’d like to dive in and learn more about interconnect solutions within the medical device space, download our free ebook.