New battery-free and wirelessly powered electronic devices find applications in advanced medicine and healthcare. Yet, such electronic devices interfaced with biological tissues need to possess an antenna for wireless communication and energy transfer.
To position devices on biological tissues, the mechanical mismatch at the tissue-device interface needs to be resolved. This can be done using flexible and stretchable electronics technologies. To mechanically trace and match the curvature of organs, such systems tend to make use of a thin-film design comprising elastomers to guarantee stretchability and bendability.
The researchers at the Singapore University of Technology and Design (SUTD) used Galinstan, a low-toxicity liquid metal, to make stretchable and conductive traces for the coil. This is to enhance the electrical and mechanical performance of the antennas for the wireless devices interfaced with tissues.
The study has been published in the journal Advanced Materials.
The newly developed Galinstan-based microfluidic antenna maintains increased wireless powering efficiency even when pushed to extreme deformations like bending, stretching and twisting. The device is also capable of conformally binding to dynamically moving, moist and soft biological tissues, acting as a thin wireless base for implantable devices.
According to the lead author, Dr. Kento Yamagishi, this technology, “can advance implantable medical applications in hard-to-reach lesions with conventional devices.”
Direct-Ink-Writing 3D Printing of Microfluidic Antennas
The currently available liquid metal antennas feature monolithic structures with a thickness of >100 μm but thicker than the required proportion to be flexible enough to conformally follow the surface of biological tissues. The research team suggested employing direct ink writing (DIW) 3D printing to develop a softer and non-monolithic structure to resolve this issue.
A fast-curing silicone sealant was pneumatically extruded onto a 7-µm thick elastomeric substrate (Ecoflex micro sheet) to pattern the outline of the microchannel. DIW 3D printing is capable of controlling the height, space and width of the antenna.
Following the embedment of light-emitting diodes (LEDs) and jumper wiring, the outline was closed with a free-standing Ecoflex micro-sheet to make microfluidic channels.
A sacrificial layer of polyvinyl alcohol (PVA), which is a water-soluble polymer, was employed to offer mechanical support and facilitated the liquid metal to flow in the thin-film microchannel to create the stretchable coil.
The fluidic antenna works at a frequency of about 13.56 MHz, which is a standard near-field communication (NFC) frequency. The liquid metal antenna provides a high quality (Q)-factor (>20), illustrating the efficiency of wireless powering.
Flexibility, Stretchability and Conformal Adhesion to Moist and Soft Biological Tissues
In general, serpentine and wavy patterns of solid metallic circuits are used in stretchable electronics to guarantee the elongation needed to obtain the electrical connections under strain. Yet, the elongation is bound to a limit, and the wiring eventually fractures. However, liquid metal provides unlimited stretchability, which makes it an attractive option for enduring large deformations.
According to the research team, the Galinstan antenna can endure up to 200% tensile strain, equals a 3 mm radius of curvature and resists an 180° twisting angle as well as maintains a high Q factor.
Repetitive tensile strain tests revealed no degradation in the Q factor or significant shift in the operating frequency, pointing out the device’s mechanical stability.
Finally, to improve the adhesiveness of the Galinstan antenna to the moist and soft biological tissues, a mussel-inspired bioadhesive known as polydopamine was utilized to improve the adhesion strength to prevent sutures that resulted in cell damage.
Demonstrations with an explanted porcine small intestine, heart and inside a chicken leg showed the stable adhesion and dependable wireless operation of the antenna even under the deformation of the tissues.
Dr. Kento Yamagishi, the lead author of the study, stated, “Our liquid metal antenna offers a new capability for the design and fabrication of wireless biodevices, which require conformal tissue-device integration. We believe this technology paves 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.
Michinao Hashimoto, Study Principal Investigator and Associate Professor, Singapore University of Technology and Design
This study was headed by SUTD’s Soft Fluidics Laboratory in association with Associate Professor Shao Ying Huang’s group from SUTD.
Yamagishi, K., et al. (2021) Ultra-Deformable and Tissue-Adhesive Liquid Metal Antennas with High Wireless Powering Efficiency. Advanced Materials. doi.org/10.1002/adma.202008062.