Voltera: Printing ECG Electrodes With Biocompatible Gold Ink On TPU

Electrocardiogram (ECG) electrodes are sensors attached to the skin that detect the electrical activity of the heart. They are critical components of ECG systems used for diagnosis and management of cardiovascular diseases. This project demonstrates the process of printing a set of dry ECG electrodes.

MATERIALS USED

• Creative Materials EXP 2613-40 gold ink
• Celanese Micromax™ Intexar™ PE874 stretchable silver paste
• Voltera Conductor 3 silver ink

SUBSTRATES USED

• Thermoplastic polyurethane (TPU)
• Polyethylene terephthalate (PET)
• FR1 board

TOOLS AND ACCESSORIES

• NOVA materials dispensing system
• Nordson EFD 7018424 dispensing tip
• Nordson EFD 7018395 dispensing tip
• Arduino Micro A000053
• SparkFun Heart Rate Monitor AD8232
• Heat press machine
• Dual asymmetric centrifugal mixer
• Bambu X1 Carbon 3D printer

Project Overview

Purpose

The purpose of this project was to demonstrate how we validated the effectiveness of printing ECG electrodes on TPU using biocompatible gold ink and stretchable silver ink. We used the Voltera NOVA materials dispensing system and the Voltera V-One PCB printer for this purpose.

Design 

We divided the project into three main parts:

1. The ECG electrodes to be attached to the skin 
2. The control unit with the heart rate monitor and the controller
3. An enclosure that protects the control unit from impact

The SparkFun Heart Rate Monitor AD8232 (SENS-12650) acts as a pre-amplifier, transforming the heart’s biopotentials picked up by the ECG electrodes into a usable voltage while also rejecting electrical noise inherent in the measurement. The Arduino Micro captures the voltage and interprets it as a graph of the heart waveform through a program that we custom-made for this project.

Figure 1: A graph showing beats per minute and ECG wave reading 

Desired outcome

The printed electrodes should be flexible enough to conform to body movement and different physiques. Once the gold ECG electrodes are attached to the skin and connected to the circuit, we connect the circuit to power. The Arduino Micro controller should accurately interpret heart rates and rhythm readings.

Functionality

Inspired by this study where researchers developed a hexagonal labyrinth pattern as an optimized dry electrode geometry, this design allowed for maximum sensitivity while eliminating the need for wet gel, which can cause skin irritation in some patients. 

For this project we printed a set of three electrodes, to be placed on the chest, as a proof of concept. Although this design was able to output data into meaningful graphs, commercial ECGs typically have 12 points of readings. As such, this project is not intended for diagnosis or treatment of any medical conditions.

Printing and post-processing the ECG electrodes

To ensure the electrodes could conform to body movement and accommodate different physiques, we designed the corners to be stretchable. We divided the layout into two layers:

1. Base silver layer for stretchability 
2. Top gold layer for biocompatibility 

Figure 2: Layer overview of the ECG electrodes 

This approach allowed us to use a relatively small amount of gold ink to minimize costs while achieving the desired outcome.

Printing the base silver layer

This layer consists of three circle patterns designed to connect to metal snaps, as well as traces that connect to the gold layer. For ease of alignment, we also included four sets of fiducials at each corner of the individual patterns.

Figure 3: Silver layer design

Figure 4: NOVA print settings for the silver layer

Printing the top gold layer

This layer consists of three hexagonal labyrinth patterns with traces that connect to the silver layer. 

Similar to the silver layer, we included four sets of fiducials for better alignment and precise cutting of the substrate.

Figure 5: Gold layer design

Figure 6: NOVA print settings for the gold layer

Figure 7: Finished print of the electrodes

Post-processing of the electrodes

After the electrodes were printed, we punched a hole in the middle of the silver circle on each electrode for the metal snaps. Next, we folded the electrodes in the middle and inserted the metal snaps into the punched holes.

Figure 8: One of the printed electrodes with a hole for a metal snap 

We cut three PET sheets proportional to the labyrinth pattern with the intention of inserting them between the layers of TPU. This added strength to the electrodes, preventing excessive stretching that could cause the gold ink to lose conductivity. We laminated them together using a T-shirt press machine. The electrodes were now ready to be connected to the heart rate monitor via the sensor cable.

Figure 9: Laminating the two sides of the TPU together

Printing the control unit

To mount the Arduino Micro controller and the SparkFun Heart Rate Monitor, we needed to drill a few holes on an FR1 board using V-One. After drilling, we used V-One to print silver traces that electrically connected the two components together. 

Figure 10: FR1 mounting board design           

Figure 11: FR1 mounting board 

We then inserted rivets, mounted the components in place, and connected the wires from the heart rate monitor to the electrodes. 

Figure 12: FR1 board with components

Printing the enclosure

To protect the control unit from impact, we designed and 3D printed an enclosure that consists of a top and a bottom cover.

Figure 13: Top cover of the enclosure        

Figure 14: Bottom cover of the enclosure

After placing the control unit inside, we bolted the top and bottom covers together.

Figure 15: Components and the enclosure

Challenges and advice

Minimizing material waste

One of the primary challenges we encountered was managing the gold ink. Given its high cost, we aimed to avoid waste. To mitigate this risk, we initially experimented with alternative inks to validate the electrode design. We tested silver ink first to ensure successful prints before proceeding with the gold ink. We also included overlapping fiducial marks for the two layers in our design to ensure precise alignment.

Optimizing ink flow

The gold particles settled at the bottom due to being left unused for extended periods, which initially resulted in the nozzle clogging. We resolved this issue by mixing the gold ink using a dual asymmetric centrifugal mixer before printing.

Conclusion

While working with gold ink presented new challenges, using NOVA allowed us to precisely control the amount of ink dispensed, a benefit particularly relevant for applications using expensive materials. As we continue to explore the possibilities of bioelectronics, we invite you to view the other application projects we’ve completed.