Liquid Metal Supercooling for Low‐Temperature Thermoelectric Wearables

Abstract

Elastomers embedded with droplets of liquid metal (LM) alloy represent an emerging class of soft multifunctional composites that have potentially transformative impact in wearable electronics, biocompatible machines, and soft robotics. However, for these applications it is crucial for LM alloys to remain liquid during the entire service temperature range in order to maintain high mechanical compliance throughout the duration of operation. Here, we introduce LM-based functional composites that do not freeze and remain soft and stretchable at extremely low temperatures. We show that the confinement of LM droplets to micro-/nanometer length scales significantly suppresses their freezing temperature (down to-84.1 C from-5.9 C) and melting point (down to-25.6C from +17.8C) independent of the choice of matrix material and processing conditions. Such a supercooling effect allows the LM inclusions to preserve their fluidic nature at low temperatures and stretch with the surrounding polymer matrix without introducing significant mechanical resistance or inducing internal stress concentrations. These results indicate that LM composites with highly stabilized droplets are capable of operation over a wide temperature range and open up new possibilities for these emerging materials, which we demonstrate with self-powered wearable thermoelectric devices for bio-sensing and personal health monitoring at low temperatures.

Soft Electronics Manufacturing Using Microcontact Printing

“…For comparison, a theoretical prediction based on Ohm's law for the resistance of each design is also included in Figure 7A. [67] Referring to Figure 7A, the measured and predicted resistance values show a reasonable agreement for different wire sizes. The change in electrical resistivity of EGaIn with respect to temperature has been measured in the literature for temperatures below [64] and above the melting point.…”

Liquid metal nanocomposites

“…More recently, the researchers applied this method to synthesis of undercooled LM micro-/nanoparticles such as Field's metal (eutectic bismuth-indium-tin) and eutectic bismuth-tin droplets. 27,28 This synthesis method has been further extended by immersion mixing of LM droplets in polymers for LM polymer composites 29 and by incorporation of metallic llers for biphasic LM composites, as will be discussed later.…”

“…12,[37][38][39][40] Increasing the shear force and mechanical agitation with immersion mixer can further reduce the size of LM droplets down to 2 mm and smaller with a uniform distribution. 29 Breaking LM droplets into nanoparticles with shear mixing becomes challenging due to the dominant surface tension of LM at this scale. Recently, Pan et al have successfully synthesized so and stretchable LM-polymer nanocomposites with different droplet sizes.…”

“…Each of these polymers have different glass transition temperatures (T g ) and elastic moduli suitable for fabrication of LM-polymer nanocomposites with tunable structural and functional properties. 29,41,42 Atom transfer radical polymerization (ATRP) is one of the most powerful and robust controlled/living radical polymerization (CRP) techniques. 43 In ATRP, radicals are intermittently converted to dormant species to diminish contribution of radical termination.…”

“…Similar to other nanomaterials, connement of LM droplets to nanoscale inuences their thermal stability and crystallization temperature which opens up new possibilities for so LM composites as low-temperature wearables. 29,50 In a systematic study, it was shown that independent of presence of coating, the choice of polymer matrix, and processing conditions EGaIn micro-/nanodroplets show signicantly enhanced supercooling effect. 29 Their freezing temperature suppressed to À84.1 C from À5.9 C and their melting point reduced to À25.6 C from +17.8 C based on calorimetric measurements and dynamic mechanical analysis.…”