High Flex Wearable Cable Assemblies: Case Study
The manufacturer of a class III wearable medical device approached Winchester regarding a failing cable assembly design. The customer was experiencing a greater than 10% fallout during bend tests and the points of termination were being ripped apart. The assembly was used for a medical device that was expected to be worn on patients at all times so flexibility was crucial. Furthermore, the current design was not waterproof which left patients exposed to medical emergencies when the device was removed to avoid water damage.
Winchester’s engineers identified several challenges to be addressed – flexibility, biocompatibility, strength, and waterproofing. Solving these key hurdles would mean success in the field and better patient quality of life.
A cable’s ability to achieve a high flexibility count (> 10M cycles) is determined by the construction of the cable. The IEC 60228 standard presented by the customer could not yield breakage greater than 10%. By strategically laying the individual conductors during the extrusion process, a cable’s flexibility dramatically increases. Equally important is the selection of raw components. A solid center conductor is not suitable for high flex applications; therefore, using alternate conductor constructions will improve a cable life over flexure. Pure copper center conductors can be replaced with alloys such as copper, aluminum, or copper-clad steel.
In selecting the raw components, special care had to be taken to ensure all material that could come in contact with patients was biocompatible as defined by ISO 10993. The device would need to withstand harsh chemicals as it would be subjected to multiple cleanings throughout its lifecycle. Along with biocompatibility and withstanding a rigorous cleaning process, the cable needed to be “non-tacky” to prevent catching of skin or clothing.
The strength of the cable was also an important consideration. The natural movement of patients caused unexpected strain at all termination points. A strain relief could be incorporated into the cable along with a custom overmold at the termination points so that the energy of these unexpected strains would be transferred from the termination points to the connector shell, board, and cable.
As mentioned earlier, the current design of the class III device was not waterproof. Due to the critical nature of the application, Winchester’s engineers determined that an IP67 rating for the termination points and connectors would be required. Working to the IP67 rating would allow patients to wear the device in a bath or shower and reduce the chance of a patient emergency due to the removal of the device. Increasing the rating to IP67 required a redesigned connector and a specific construction of the strain relief at the cable-connector transition.
Winchester’s engineers designed a custom multi-conductor bulk cable using a stranded alloy with a specialized lay to maximize flexibility and minimize size that was “non-tacky” and biocompatible. To satisfy IP67 requirements, a custom overmold for all termination points was designed that would double as a strain relief. For added strength, a kevlar strand was included in the cable during the extrusion process. With the overmold and kevlar strand combined, the cable assembly was able to withstand the strains and harsh cleanings throughout the device’s lifecycle as defined by the customer.
Working closely with the customer, Winchester was able to fully understand the issues and provide a turnkey, tested cable assembly that included termination of the bulk cable to the customer’s PCB’s with an overmold. The new design provided a more reliable product that exceeded the customer’s flexibility, environmental, and strength requirements.