Direct Ink Writing 3D Printing for Fabricating Ultra-Deformable Microfluidic Antennas

<b><i>Background. </i></b>Three-dimensional (3D) printing has become the new paradigm for the fabrication of microfluidic devices and microchannel-based electronic devices. Although stereolithography (SL) printing has been increasingly used to fabricate microfluidic channels, the function of fabricated devices is largely restricted by the property of photoresins. In particular, most commercially-available photocurable resins used in SL printing are not flexible and stretchable, limiting their use to create flexible and mechanically compliant devices. Besides, SL printing fabricates the final structure as a unibody with continuous printing, which makes the integration of additional components (such as electronic parts) with the fabricated device challenging.<br/><b><i>Contribution. </i></b>To overcome this limitation, we present direct ink writing (DIW) 3D printing as an alternative route to fabricate microchannels. DIW 3D printing allows extrusion-based patterning of silicone-based elastomeric materials (<i>e.g.</i>, room-temperature-vulcanizing (RTV) silicone and addition-curing two-part silicone) on any flat substrates. This capability permits fabricating flexible microchannels on highly elastomeric sheets. In addition, due to the mechanism of DIW 3D printing, electronic components (<i>e.g.</i>, light-emitting diode chips and near field communication (NFC) tags) can be readily included in the device during the layer-by-layer fabrication to enhance the functionality of the device. To highlight the advantage of DIW-based fabrication of microchannels, an ultra-deformable film-based microfluidic device with embedded liquid metals was fabricated to demonstrate the device conformability that addresses mechanical mismatch at the tissue-device interface. The demonstrated method based on DIW 3D printing offers unique opportunities to fabricate microfluidic devices with advanced functions to develop flexible sensors and actuators.<br/><b><i>Technical achievement. </i></b>We demonstrated fabricating microchannels by direct ink writing on a 7 μm-thick elastomeric substrate, referred to as Ecoflex microsheet. The fabricated microchannels were filled with liquid metal (e.g., galinstan) to demonstrate microfluidic antennas that exhibited unprecedented deformability and stretchability. We selected a fast-curing silicone sealant as 3D-printed ink to regulate the printing condition and realize the self-supporting patterns of microchannels. In addition, we demonstrated a simple methodology to integrate small electronic components interfaced with the microchannel. We embedded an integrated circuit (IC) chip and LED chips in the silicone sealant that forms the outline of microchannels; these components were readily fixed in the outline of microchannels between the multiple printed layers. The injection of liquid metal into the multilayered microchannels with embedded electronic components offered the fabrication of 3D electrical circuits aligned with electronic components. As a demonstration, we fabricated a wireless light-emitting device that can be powered by a standard near-field-communication system (13.56 MHz) that operated consistently under various deformations such as stretching (&gt; 200% uniaxial strain), twisting (180^○ twist), and bending (3.0-mm radius of curvature), where the Q factor of such antenna remained high (&gt; 20) under deformation. To demonstrate the advantage of such film-based devices, suture-free conformable adhesion to <i>ex vivo</i> animal tissues under mechanical deformations was demonstrated.<br/><b><i>Significance. </i></b>Fabricating flexible and stretchable devices by DIW 3D printing offers a new capability for the design and fabrication of wireless biodevices. DIW 3D printing technology can also offer integration of other functional components such as physical sensors, electrochemical sensors, and drug delivery systems. This technology paves the way towards minimally invasive, imperceptible medical treatments.