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By Steve Baker | Aug 24, 2018
Printed electrodes by GMN

A US-based medical company was developing a wearable cardiac telemetry device to monitor the ECG signals of a patients’ heart. They approached GM Nameplate (GMN) to develop the electrode (circuit) that would be embedded in the low-profile, wireless device that measures, records, and transmits physiological data. In addition to the specific needs regarding the circuit’s slender shape and size, the customer had three crucial requirements:

1) The electrode should have a tight tail pitch - Given the slim shape of the electrode, the customer wanted to use a small connector to the motherboard, and therefore, required the circuit to be fine pitch. Although printing tight pitches can be extremely challenging, the experts at GMN met this requirement by meticulously designing the 0.5 mil tail for the circuit.

2) The silver conductive inks and dielectric inks couldn’t touch the patient’s skin - To meet this requirement, GMN designed the circuit on a polyester substrate. The silver silver chloride pads were printed on the side that would directly stay in contact with the patient’s skin, and the dielectric inks and silver conductive inks were printed on the other side of the substrate. With the polyester substrate acting as a divider, the inks on both sides were connected through via-holes.

3) The electrode needed to meet a target resistance - As a device that would be picking up even the weakest ECG signals, the electrode’s ability to meet a targeted resistance from the silver silver chloride pads to the tail end was extremely critical. This necessity primarily dictated the design of the circuit pattern. Since the traces had to be of a certain length to meet the targeted resistance, GMN elongated the circuit by creating utility loops. The final circuit design was a perfect amalgamation of the customer’s specifications for the part’s shape and GMN’s ability to meet the resistance requirements. During the design development phase, multiple other factors including sheet size, sheet layout, print volumes, tolerances, and ink layers were also taken into account.

Later, the project went through qualification procedures to ensure quality and repeatability throughout production. After a few rounds of prototyping and testing, the parameters for Operational Qualification (OQ), including the ink type, drying temperature, drying capacity, curing process, and print speed, were defined. First part development and manufacturing efficiencies were taken into consideration to develop different print pattern layouts, which eventually caused the most optimum solution to make its way to production. The nominal settings from OQ were run with different material lots on three different setups through the entire production process. All three outputs were measured and evaluated to ensure that all the critical performance parameters were within the window of minimum and maximum levels of variances. After the electrodes were printed and sheeted, they were precisely die-cut with a steel-rule die on a clam shell press.

The high volume of production quantities and the flimsy shape of the electrode made it extremely difficult to handle and test each individual part manually. To address this issue, GMN automated the inspection process by employing a robotic arm. The arm would pick up one circuit at a time and place it in a test fixture where it was tested for continuity and broken traces. The resistance was tested from the end of the fine pitch tail to every individual silver silver chloride pad. The pieces that passed the inspection were sorted to one tray and the defective parts were delivered to another. Automating the final inspection process not only led to faster turnaround, but also resulted in improved consistency and reliability. To dive deeper into the robotic automation at GMN, watch our short video here.

With decades of technical printing experience in highly regulated industries, GMN partnered with this medical customer from the early development phase through multiple design cycles to manufacture a product that met all of their technical specifications. Our rapid prototyping capabilities also enabled the customer to seamlessly transition from concept to high-volume production. Since functional printing projects require extremely tight tolerances, rigorous printing and testing procedures were set in place to ensure that every element of the electrode was carefully controlled. To learn more about the printed electrodes at GMN, read our capabilities page here