Magnetic ink that can be used to make electrochemical sensors, self-healing batteries and wearable, textile based electrical circuits, has been developed by a team of engineers at the University of California San Diego.
Micro-particles that are oriented in an exact configuration by a magnetic field are the key ingredient for the ink. Due to the particle’s orientation, particles on both sides of a tear are magnetically attracted to one another. This enables a device printed with the ink to heal itself. The device is able to repair tears up to 3 millimeters wide, which is a new record in the field of self-healing systems.
Joseph Wang is the director of the Center for Wearable Sensors and chair of the nano-engineering department at UC San Diego. He believes this work holds substantial promise for many practical applications for long lasting printed electronic devices.
Existing self-healing materials need an external trigger to start the healing process and the process takes between a few minutes to several days to work. The system developed by Wang and his colleagues repairs damage within about 50 milliseconds and does not require any outside catalyst to work.
After using the ink to print electrochemical sensors, batteries and wearable, textile based electrical circuits, the engineers set out to damage these devices by pulling them apart and cutting them to create increasingly wide gaps. Even after being damaged nine times at the same location and sustaining damage in four different places, the devices were still able to heal themselves. Full function was recovered while only a minimum amount of conductivity was lost.
In one experiment, nano-engineers printed a self-healing circuit on the sleeve of a T-shirt. The circuit was then connected to a LED and a coin battery. When the circuit and the fabric it was printed on was cut, the LED turned off. Within a few seconds however, it turned back on as the two sides of the circuit came together again and healed themselves.
Amay Bandodkar, one of the papers’ first authors, earned his Ph.D. in Wang’s lab and is now a postdoctoral researcher at Northwestern University. He explained that the purpose of the study was to develop a smart system with impressive self-healing abilities. One of the criteria however was that inexpensive materials that are easy to find had to be used in the construction.
Wang’s research group has a wealth of experience in the field of printed wearable sensors, so it was logical for them to turn to ink as a starting point for their self-healing system.
The ink was loaded with micro-particles from a magnet made of neodymium, a soft, silvery metal. Neodymium magnets are commonly used in research. One characteristic of neodymium is that the particles’ magnetic field is much larger than their individual size. This is the key to the ink’s self-healing properties, as the strong attraction between the particles allows for the closing of tears that are millimeters wide.
The particles are inexpensive and conduct electricity. They are however difficult to use in the electrochemical devices on their own as they have poor electrochemical properties. Researchers overcame this obstacle by adding carbon black to the ink. Carbon black is a material commonly used to make batteries and sensors.
The next challenge was that the micro-particles’ magnetic fields canceled each other out when in their natural configuration. This eliminated their healing properties. This was solved by printing the ink in the presence of an external magnetic field, causing the particles to orient themselves to behave as a permanent magnet. The two opposite poles are then at the end of each printed device. When the device is cut, the two damaged pieces act as different magnets that attract each other and are thus able to self-heal.
The engineers plan to make different inks with different ingredients for a wide range of applications in the future. They also envisage developing computer simulations to test different self-healing ink recipes before actually trying them out in the lab.
The full study was published in the journal Science Advances.