Researchers use liquid metal and laser ablation to create stretchable micro antennas

Researchers have developed a new method of making micro stretchable antenna with water gel and liquid metal. These antennas can be used for wearable and flexible wireless electronic devices to provide links between devices and external systems for power transmission, data processing, and communication.

Using our new manufacturing method, we have demonstrated that the length of liquid metal antennas can be reduced by half, "said Chen Tao from Xi'an Jiaotong University in China. This may help reduce the size of wearable devices for health monitoring, human activity monitoring, wearable computing, and other applications, making them more compact and comfortable.

In the journal Optics Express, researchers described their new technology, which includes injecting eutectic gallium indium (a metal alloy that is liquid at room temperature) into microchannels created by a single step femtosecond laser ablation process. They used this method to create a size of 24 millimeters ×  0.6 mm ×  0.2mm antenna embedded into 70mm ×  12mm ×  7 mm in a water gel plate.

For example, stretchable and flexible antennas can be used for wearable medical devices that monitor temperature, blood pressure, and blood oxygen, "Chen said. Individual mobile devices can be connected to larger control units through flexible antennas to transmit data and other communication, forming a wireless body area network. Since the resonant frequency of flexible antennas varies with applied strain, they may also be used as wearable motion sensors.

More flexible metal
This work originates from the previous research carried out in cooperation with Jian Hu of King Abdullah University of Science and Technology in Saudi Arabia, in which the researchers developed a method of fabricating 3D silver structures embedded in hydrogels for strain sensing by femtosecond laser ablation (collaborative research) with Professor Hu Jian.

The tensile strength of silver structures is poor because they are very fragile, "Chen said. "Using liquid metal instead of solid metal structure not only makes it easier for metal to fill hydrogel microchannels, but also improves its tensile capacity."

In order to make liquid metal dipole antenna (the simplest and most widely used antenna type), researchers scan femtosecond laser to form a pair of symmetrical microchannels in the hydrogel without damaging the surface. The short pulse duration of the laser generates peak power, allowing for the ablation of transparent materials through nonlinear optical effects such as multiphoton absorption, thereby ensuring that ablation only occurs at the precise focus of the laser.

Then they inject liquid metal into microchannels to form hydrogel embedded wires that can be used as antennas.
They chose hydrogel as the substrate because it has more favorable dielectric properties than polydimethylsiloxane (PDMS) and other traditional polymer substrates, reducing the length of the antenna by half. The hydrogel based device can also be stretched to almost twice its original length.

However, liquid metal devices based on hydrogels are usually manufactured by using lasers to carve grooves on the top surface, filling them with liquid metal, and then bonding the patterned substrate with the non carved substrate.

"With our method, microchannels can be embedded into hydrogels using a single manufacturing step without layer bonding," Chen said. In addition, 3D microchannels and liquid metal structures can be easily formed through femtosecond laser 3D scanning, making it possible to manufacture 2D or 3D flexible antennas with complex structures to enhance performance and functionality.

Making stretchable antennas
To demonstrate the new manufacturing method, researchers prepared stretchable dipole antennas and measured their reflection coefficients at different frequencies. These experiments show that the pure water gel reflects almost all the energy of the incident electromagnetic waves, while the liquid metal dipole antenna embedded in the hydrogel effectively radiates most of the incident electromagnetic waves into the free space, and less than 10% are reflected at the resonant frequency.

They also showed that by changing the applied strain from 0% to 48%, the resonant frequency of the antenna can be tuned from 770.3 MHz to 927.0 MHz.
Researchers are currently working to improve the sealing technology used on laser induced microchannels to enhance the strength of flexible stretchable antennas and the threshold strain for liquid metal leakage. They also plan to explore how to apply this new method to develop fully flexible multidimensional strain and pressure sensors with complex 2D or 3D structures.

Source: Laser Network