Medical Power Cables

Actively Humidified Breathing Circuits require electricity to heat the tubes and get the patient air to 37degrees (as explained in my previous article ‘The Current State of Active Humidification’. This comes from the Humidifier itself, allowing it to adjust the temperature utilising its software control. The objective of this project is it design the cable which connects the tubes to the Humidifier.

This cable will need three connectors, one into the Humidifier, one into the Inspiratory Tube, and one into the Expiratory Tube. Setup must be quick and simple so the connectors should be easily distinguishable from one another.

Humidifiers are classified as an ‘Applied Part’ because of the wet air passing down the tubes, a break in any of the insulation could connect the patient to the electrical heating circuit, therefore the cable must protect against this scenario. The simplest way to achieve this is to produce a cable without an earth connection and allow the Humidifier to handle a short circuit safely and correctly.

Unlike the single use breathing tubes, these cables are reusable and will be cleaned after each use with antibacterial wipes. These cables will need to last for several years so any design measures which can be introduced to extend their life should be included. The aim is to produce a premium product, therefore the higher cost of these measures is acceptable.

Design Specification

• MDD (Medical Device Directive) and ‘Applied Part’ compliant

• Include Strain relief to increase the longevity of cables

• Match aesthetics of other cables in the product line

• Provide power to the patient’s breathing tubes

• Reusable and suitable for repeated cleaning

• Three easily distinguishable connectors

• Suitable for batch manufacturing

Final Product

The final product turned out great, featuring injection moulded components with an over-moulding assembly process that allowed for the insertion of electrical contacts, wiring, and strain relief into the mould tool before injection to create a one-piece cable for increased longevity.

I attended the manufacturer’s site to discuss the design and made changes based on their suggestions to ensure it was optimised for them, building a relationship that allowed us to overcome any issues faced during the early production runs.

The three connectors are easy to tell apart thanks to their differing shapes (Oval, Cloverleaf, PCB) but are also ‘keyed’ to ensure an incorrect connection cannot be made. The subtle upwards curve on the PCB connector matches the shape of the accompanying Humidifier, providing a hint to where the connector is placed.

The shell of the PCB connector displays the Applied Part symbol, moulded into the plastic itself to remove the need for the application of additional labels which would peel over time leaving a residue which would collect germs.

Lessons Learned

• Utilising a manufacturer’s process expertise early in the design phase leads to a vastly better product which is optimised for their facilities.

• Over-moulding connectors leads to a cleaner and longer-lasting cable, but it results in inefficient manufacturing routing. This increased complexity and cost should be weighed-up against the design requirements early on and included in the Design Specification.