Associate Professor Binghamton University, United States
Introduction: Liquid metals (LM), like eutectic Gallium-Indium (EGaIn), are noteworthy for their liquid state at or near room temperature, offering unique properties distinct from most metals. They possess exceptional thermal and electrical conductivity, compatibility with biological systems, and the ability to undergo shape transformations with stimuli, making them ideal for flexible electronics, addressing the limitations of rigid conductors. LM finds applications in wearable technology and medical devices, including wound healing and sensors. Recent research combines LM with nonwoven nanofibers, enabling complex designs. To overcome challenges in creating LM composite fibers, we propose a solution: a non-woven fiber mat with a thermoplastic elastomeric sheath encasing an LM-based ink core. The conductive network of LM particles in the ink core allows for screen printing or coating while maintaining continuity, even under stretching. Our hypothesis is that embedding the percolation network of LM particles within the polyurethane fiber core will sustain continuity.
Materials and
Methods: Tecoflex SG-80A Thermoplastic Polyurethane (PU) was purchased from Lubrizol (Wickliffe, OH). The LM ink (ELMNT SL Ink) was obtained through collaboration with UES inc. affiliated with the Air Force Research Laboratory (Dayton, OH). Diacetoxyhexane (DHA) and Dimethylformamide (DMF) were purchased from VWR (Randor, PA) Tetrahydrofuran (THF) were purchased from ThermoFisher Scientific (Waltham, MA). PU-SL Ink fibers were created using the MSK-NFES-3 benchtop electrospinning unit. PU solutions were made at 16 wt% in a cosolvent ratio of 3:1 using THF and DMF, respectively. SL Ink solutions were created by using the SL Ink and further diluting the ink in DHA 20% by weight to reduce viscosity. PU and SL Ink solutions were placed into 15.65 mL syringes and fed through a Polytetrafluoroethylene (PTFE) tubing. To form the coaxial structure, a 21-gauge needle and 16-gauge needle were used for core and sheath respectively, with a accelerating voltage of 11 kV applied to create the fibrous network of PU-SL Ink nanofibers.
Results, Conclusions, and Discussions: Our preliminary results of coaxial electrospinning the ELMNT Ink core with the PU sheath show the fiber morphology of the LM integrated into the fibers. In Figure 1A, the percolation network from the EGaIn particles embedded into the SL Ink can be seen in the PU sheath and throughout numerous fibers (Fig. 1B), allowing for a continuous network of LM particles for conductive fibers. EDS analysis confirms the presence of the liquid metal pathway with the mapping demonstrating the structure of randomly aligned nanofibers indicating coaxial encapsulation into the PU sheath fibers (Fig. 1C). Furthermore, PU-SL Ink fiber mats were placed in liquid nitrogen and split to expose the cross-section of fibers through freeze fracture to observe the coaxial alignment of SL Ink particles in the PU fibers. Figure 1D shows the particles embedded in the PU fibers, which can be further optimized by adjusting electrospinning parameters. A crucial criterion for this conductive fiber mat to be realized for stretchable electronics is the sintering procedure responsible for fusing LM particles, creating continuity through the fiber mat. Literature discusses that most methods of sintering for particles to fuse together involve a form of mechanical strain or applied force. Figure 1E shows the PU-SL Ink fiber after striking the surface of the fiber mat with a sharp-tipped object with enough pressure to cause mechanical sintering of the particles. Further optimization of electrospinning parameters will be fine-tuned to achieve greater than 60% of fibers having continuous LM particles encapsulated within the PU sheath. Upon having continuous particles, the liquid metal particles will then be fused through a sintering process to form a conductive pathway, allowing for a liquid metal core fiber. With the core-sheath fiber structure formed, the cytotoxicity of the materials as a function of immersion time will be tested along with the quantification of leaching micro/nanoparticles. We will fully characterize chemical, mechanical, electrical, and physical properties.
