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How Stretchable Liquid Metal Antennas Can Match Deformations of Moving Organs
New flexible liquid metal antenna can conform to soft biological tissues and withstand extreme deformability, paving the way for better biomedical antenna implants.

With the development of information technologies, smart healthcare has finally come to the fore. From smart wound dressing to wearable sweat biosensors, smart healthcare is bringing in a wide array of new-generation infrastructure technologies that support healing and streamline treatments in ways never done before. But what makes smart healthcare smart?

In line with the intention of making patient care timelier and more personalised, smart devices are developed to be battery-free and wireless to allow for remote communication and rapid information transfer. However, when such devices are interfaced with biological tissues, they need to have an antenna for wireless communication and energy transfer. Antennas can be implanted into human bodies or mounted over the torso, but they need to be flexible and stretchable to be mechanically compatible at the tissue-device interface. To mechanically follow and match the curvature of organs, scientists have incorporated a thin-film design composed of elastomers to ensure bendability and stretchability. But antennas constructed to date have limited deformability and tissue-adhesiveness.

Now, researchers from the Singapore University of Technology and Design have innovated a stretchable and conductive Galinstan-based microfluid antenna that surpasses the electrical and mechanical performance of previously developed antennas. Galinstan is a low-toxicity liquid metal. Their Galinstan-based antenna can maintain high wireless powering efficiency even when subjected to extreme deformations like stretching, bending, and twisting. It can also conformally adhere to dynamically moving moist and soft biological tissues, serving as a thin and wireless platform for implantable technologies. Dr. Kento Yamagishi, the lead author of the study, believes that their novel technology “can advance implantable medical applications in hard-to-reach lesions with conventional devices.”

Up until now, liquid metal antennas have been too thick to be flexible enough to conformally follow the surface of biological tissues. Existing antennas have monolithic structures that are over 100 micrometres thick. In a bid to create thinner and more flexible antennas, Yamagishi and colleagues proposed to use direct ink writing 3D printing to create softer, non-monolithic structures.

The team pneumatically extruded a fast-curing silicone sealant onto a 7-micrometre thick elastomeric substrate known as Ecoflex microsheet. The substrate was to be used to pattern the outline of the microchannel or antenna. They then embedded light-emitting diodes and jumper wiring onto the antenna and sealed the outline with a free-standing Ecoflex microsheet. Together, these parts formed the microfluidic channels to serve as the antenna.

After constructing the microchannels, the researchers had to find a way to enable the liquid metal to flow in the thin-film microchannel to form the stretchable coil. To this, they used a sacrificial layer of polyvinyl alcohol, a water-soluble polymer, which also better equipped their antenna with mechanical support. Because the researchers used direct ink writing 3D printing, they were able easily to control the width, space, and height of the antenna during the process of design and development.

To achieve flexibility, stretchability, and conformal adhesion to biological tissues, the researchers borrowed the powers of Galinstan’s tensile strength. Previously, wavy and serpentine patterns of solid metallic circuits were used in stretchable electronics to meet the requirements of length and electrical strength. However, there is a limit to elongation as most wirings eventually fracture, thus calling the need for more stretchable and deformable materials – like that of liquid metals.

Fortunately, the research group has reported that their Galinstan antenna can experience up to 200 per cent tensile strain, maintain a 3-millimetre radius of curvature, and even withstand a 180-degree twisting angle while maintaining a high quality (Q)-factor, which demonstrates its efficiency for wireless powering. A series of repetitive tensile strain tests revealed no degradation in the Q factor or meaningful shift in the operating frequency, hence demonstrating the mechanical stability of the device.

Since the antenna is purposed for biomedical uses, the researchers had to ensure that their Galinstan antenna can adhere to soft tissues. Borrowing the powers of polydopamine, a mussel-inspired bio-adhesive, they were not only successful in enhancing the adhesive strength of the antenna but also were able to avoid tissue-damaging sutures. They gave evidence of the antenna’s biocompatibility through demonstrations using an explanted porcine small intestine, heart, and the inside of a chicken leg. Their tests revealed that the antenna could stably adhere as well as function wirelessly and reliably even when the tissues were deformed.

Having shown the microfluid antenna’s promising strengths, Dr. Yamagishi said that their “liquid metal antenna offers a new capability for the design and fabrication of wireless biodevices, which require conformal tissue-device integration.” The team believes that the technology can pave “the way towards minimally invasive, imperceptible medical treatments.”

“While we demonstrated the direct fabrication of microchannels on ultrathin films in this work, direct 3D printing of microchannels enables the creation of microchannels and other fluidic components on different types of surfaces, including biological surfaces. We believe that such capabilities will bring new opportunities for biological sensing, communication, and therapeutics,” added Principal Investigator, Associate Professor Michinao Hashimoto.

Source: Yamagishi et al. (2021). Ultra-Deformable and Tissue-Adhesive Liquid Metal Antennas with High Wireless Powering Efficiency. Advanced Materials.

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