Interconnect Solutions for Medical Devices - The Ultimate Guide

As the energy-driven device market continues to grow, you may have noticed the term “interconnect solutions” rising in popularity.

Here are a few examples from around the web of how the term is being used:

• Provides extensive interconnect solutions in manufacturing and testing…
• Leading interconnect solutions in…
• Interconnection designs and manufacturing of customized interconnect solutions…

The lack of clarity from these descriptions may leave you wondering what “interconnect solutions” are, where they’re used and why you should care.

In this blog post, we’re going to clear up and help you understand what the term ”interconnect solutions” means, by:

• Defining interconnect solutions
• Exploring the role interconnect solutions play in the minimally invasive and electrophysiology industries
• Explaining interconnect drawings, schematics, and diagrams
• Discussing interconnect capacitance
• Diving into connectors and cables

Interconnect Solutions Definitions: What are “interconnect solutions?”

The term “interconnect solutions” is commonly associated with the components required to connect electrical signals from one point to another.

This could include connector with electrical pins, the wire with a specific design to transfer those signals, circuit boards (with or without electrical components), distal tips to deliver therapy or sense the environment, and all of the plastic housing that protects everything.

But true interconnect solutions consist of much more than a list of components.

Though the definition for “interconnect solutions” may vary according to the industry in which you operate, we define it as:

Interconnect Solutions – The designed integration of wire, cable, connectors, circuit board, and electrical components that enable an energy-driven (“electrical”) medical device to function properly, for its intended purpose, so that it can be used to improve the quality of one’s life.

Medical Interconnect: Minimally Invasive & Electrophysiology

Minimally Invasive

With an increasing number of minimally invasive devices requiring power and data-transmission capabilities, interconnect solutions are becoming a more significant consideration of the overall design.

As more energy-driven devices are brought to market, the role of the interconnect becomes even more crucial as it  makes it possible to connect and unify separate circuit devices into one, seamless device.

For example…

Robots used in the minimally invasive industry have the ability to save lives even when there are hundreds of miles that separate a skilled doctor from the patient who needs them to perform a life saving operation.

In this example…

The robot is made up of a myriad of interconnect solutions, some in the form of the printed circuit board connecting components at the circuitry level, others are the series of complex cables that are comprised of conductors housed in custom engineered connectors that are connected to multiple wires that have been specified to meet the exacting requirements of the devices end use.

Electrophysiology

Ever evolving interconnect solutions are enabling the advancement of electrophysiology (EP) technologies.

Consider an EP mapping catheter used to diagnose arrhythmias.

The electrodes at the distal tip of the device makes contact with the wall inside of the heart. They sense the microvolt electrical signals and deliver those signals to the computer so the doctor can analyze it.

The connection enables life-impacting decisions.

The multiple interconnect solutions throughout the device — from the distal tip interconnecting the sensors to the proximal end connector bringing all of the conductors together, must function seamlessly every time.

As electrophysiology technologies advance and require higher resolution, improved data-transmission and power in smaller spaces, interconnect solutions must keep pace, improving the ability to perform the required tasks while meeting decreasing parameters.

This is why interconnect solutions are crucial when it comes to improving people’s lives.

Interconnect Drawing: Diagrams, Schematics, & Symbols

What is an Interconnect Diagram?

Interconnection Diagram Definition

Interconnection Diagram (sometimes referred to as an “interconnect diagram” ) – An interconnect diagram is the visual structure of a circuit that displays components used in the circuit to show flow of power and signals between components.

Electrical Schematic Drawing Standards

Electrical schematic drawing standards are critical when there are multiple engineers working together on a project—especially when developing medical devices that impact people’s lives.

For example..

Imagine a minimally invasive device that is used to surgically repair the heart. If this device doesn’t perform its specific function every time it is used, there is a significant risk to the patient.

With no drawing standards in place, a misunderstanding between the engineering team and the manufacturing team can increase the chance of device failure.

Electrical schematic drawing standards help ensure information about the design of the interconnect solution is accurately communicated and understood—mitigating the risk that something might be wired or manufactured incorrectly.

Electrical Wiring Symbols

Electrical wiring symbols are used to represent the type of components that are being used in a circuit. The symbols are used to clearly and quickly communicate the function of the components and schematic or drawing to mitigate the chance of misunderstanding.

Electrical Drawing vs Schematic Drawing

One question we continually encounter is that of the difference between an electrical and a schematic drawing.

The answer to this question is: the terms are synonymous.

Electrical Interconnect Diagram & Schematic Interconnect Drawing Example

Interconnect Capacitance: Low Capacitance and Interconnect Cable

What is Interconnect Delay?

An interconnect delay describes the signal speed through a circuit. Electrical signals and power travel close to the speed of light.

There are some applications where this speed limit needs to be taken into account.

Additionally…

The speeds can differ depending upon the materials used in the interconnect solution. Another factor in interconnect delay is the capacitance of the cable and full interconnect system.

Capacitance

If you are not familiar with how a capacitor works, imagine two metal plates that are facing each other and are perfectly parallel. Consider what happens when electrons flow to one plate. The electrons have a negative charge.

Because opposite charges attract, the electrons on one plate will attract positive charges on the other plate. This is a capacitor. Let’s focus on the plate with the electrons.

Electrons all have the same negative charge, and just like two magnets of the same pole will push away or repel each other, electrons also repel and push away from each other.

So…

The more electrons that are pushed onto a plate the more force they push away from each other (this is voltage). If you have ever been on a subway train in Tokyo during rush hour, you have experienced what an electron on a charged capacitor feels like!

When a capacitor is in a circuit, the electrons flowing onto one of the plates takes time before it can’t force anymore electrons in.

While the capacitor is charging, only a portion of the signal will travel down the interconnect. Once the capacitor is “charged” or “full”, then the full signal will continue down the rest of the interconnect.

Why does this matter?

When two wires run next to each other in a cable or a printed circuit board, they form a small capacitor.

When signals are running fast, the cable or interconnect capacitance can cause problems meaning the signal that comes out is different from the signal that went in.

Low Capacitance Cable

Part of the solution is to design an interconnect system with low capacitance. To do this, analysis of the needed solution is conducted to determine proper voltages, currents and signal frequencies of the device.

This analysis enables design definitions of distance between conductors and which jacket insulating materials with low dielectric constants will lower the capacitance.

This intentional design process will provide for the most robust interconnect design to ensure the signal that comes out of the interconnect is as close as possible to the signal that goes into the interconnect.

Cable & Contact Resistance

What is the lowest resistance metal for an interconnect?

This is another question we commonly get asked.

The answer is: silver is the lowest resistance metal for interconnect device and when it oxidizes it is just as conductive. (One exception here is if the tarnish is due to sulfur, then the tarnish is not conductive.

So it is not recommended to use any silver if the environment is susceptible to sulfur.) Copper conductors perform exceptionally well and can be protected from oxidation by adding different cladding, insulations or grease-like materials during processing of the wire to provide protection from moisture.

 To help provide some context as to metal conductivity, if we set copper conductivity to a conductivity index-rating of 100, then other metal would have an approximate conductivity rating of:

Metal Conductor

Conductivity Rating

Silver

108

Copper

100

Gold

79

Aluminum

61

Brass

28

Zinc

27

Nickel

22

Iron

17

Tin

15

Phosphor Bronze

15

Cable Contact Resistance

Connector electrical contacts are typically made of metal alloys. Alloys like Phosphor-Bronze, Beryllium-Copper or Titanium-Copper are the most common alloys that provide spring properties when deflected and are used in contacts that move, (meaning contacts that are ‘compliant’).

Brass is often used for contacts that are held still, meaning contacts that are static.

Two components exist in most connector contact designs: a static contact (the side that does not move) and a compliant contact (the spring side that moves).

In most situation, the compliant contact pushes into the static contact.

To ensure low resistance, electrical contacts need to mate or push together with sufficient force such that the microscopic metal makes many points of contact.

Contacts that only lightly touch will result in a high contact resistance and can cause dangerous heating issues (if there is high current) or signal noise (if the signal is low voltage).

Proper force between electrical contacts for most disposable/limited-use medical device connectors is between 40 grams on the low end and 120 grams on the high end.

In medical devices, there is a balance to achieving sufficient contact normal force and meeting needed cost targets.

For example…

Some systems use components designed to achieve tens of thousands of mating cycles when the device is a disposable one-time use or limited use (discarded after ten or so uses) device.

This adds significant cost—as high-cycle connectors commonly utilize gold-plated contacts—yet does not provide any additional reliability or safety.

Inductance & Inductance Coupling

Another part of the solution is to design an interconnect system to minimize inductance coupling. To better understand what inductance is, let first talk through some basic physics that can be easily understood.

Imagine holding a magnet next to a wire. If the positive side of the magnet is next to the wire, the electrons in the wire are attracted to where the magnet is.

If the negative side of the magnet is next to the wire, the electrons are rappelled.

Now if I keep switching side of the magnet next to the wire, the electrons will be attracted and rappelled, pushed and pulled.

That movement of electrons in the wire is called electricity. A simple definition of electricity is electrons moving in a wire.

Another thing that is necessary to understand is when electrons are moving through a wire, a magnetic field is created around the wire.

The more electrons that move through the wire, the stronger the magnetic field. Current is a measurement of the number of electrons that are moving through a wire.

A simple definition of inductance is the creation of the magnetic field around a wire that has a current (or electrons) running through it.

Now imagine two wires running next to one another. If the first wire has a current running through it, it will create a magnetic field.

That magnetic field will create an attraction or repulsion of the electrons in the second wire.

As the current changes magnitude or direction (like a speaker wire will) in the first wire, a current will be created in the second wire.

This is inductive coupling.

Inductive Coupling is a serious consideration in interconnect design. Electrical circuits need a primary signal path and a return path. Imagine a speaker wire, one wire for the signal and one wire for ground. Ground is the return path.

To minimize inductive coupling interfering with the signals in a cable, it is common to twist two wire together so that any external magnetic field that would interfere with the signal will also interfere with the ground wire.

If both wires are affected the same, then the signal will be protected because the signal is always in reference to the ground wire. In other words, the signal voltage compared to the ground wire will be the same. This is called a twisted pair and is common practice in cable design.

A second way to minimize inductive coupling is to shield the wires. A shield is a conductive surface the surrounds the signal wire to prevent the magnetic field from reaching the signal wire.

A coax cable is a good example of this. Many medical device cables will have a shield surrounding all the wire in the cable to prevent external interference and reduce the chance of inductive coupling.

Additional resources.

For a custom interconnect solution to perform at the highest level, we recommend working with an interconnect expert.

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