@article{erlenbach_mondal_ma_neumann_ma_holbery_dickey_2023, title={Flexible-to-Stretchable Mechanical and Electrical Interconnects}, volume={1}, ISSN={["1944-8252"]}, url={https://doi.org/10.1021/acsami.2c14260}, DOI={10.1021/acsami.2c14260}, abstractNote={Stretchable electronic devices that maintain electrical function when subjected to stress or strain are useful for enabling new applications for electronics, such as wearable devices, human-machine interfaces, and components for soft robotics. Powering and communicating with these devices is a challenge. NFC (near-field communication) coils solve this challenge but only work efficiently when they are in close proximity to the device. Alternatively, electrical signals and power can arrive via physical connections between the stretchable device and an external source, such as a battery. The ability to create a robust physical and electrical connection between mechanically disparate components may enable new types of hybrid devices in which at least a portion is stretchable or deformable, such as hinges. This paper presents a simple method to make mechanical and electrical connections between elastomeric conductors and flexible (or rigid) conductors. The adhesion at the interface between these disparate materials arises from surface chemistry that forms strong covalent bonds. The utilization of liquid metals as the conductor provides stretchable interconnects between stretchable and non-stretchable electrical traces. The liquid metal can be printed or injected into vias to create interconnects. We characterized the mechanical and electrical properties of these hybrid devices to demonstrate the concept and identify geometric design criteria to maximize mechanical strength. The work here provides a simple and general strategy for creating mechanical and electrical connections that may find use in a variety of stretchable and soft electronic devices.}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Erlenbach, Steven and Mondal, Kunal and Ma, Jinwoo and Neumann, Taylor V and Ma, Siyuan and Holbery, James D. and Dickey, Michael D.}, year={2023}, month={Jan} }
 @article{neumann_kara_sargolzaeiaval_im_ma_yang_ozturk_dickey_2021, title={Aerosol Spray Deposition of Liquid Metal and Elastomer Coatings for Rapid Processing of Stretchable Electronics}, volume={12}, ISSN={["2072-666X"]}, url={https://doi.org/10.3390/mi12020146}, DOI={10.3390/mi12020146}, abstractNote={We report a spray deposition technique for patterning liquid metal alloys to form stretchable conductors, which can then be encapsulated in silicone elastomers via the same spraying procedure. While spraying has been used previously to deposit many materials, including liquid metals, this work focuses on quantifying the spraying process and combining it with silicones. Spraying generates liquid metal microparticles (~5 μm diameter) that pass through openings in a stencil to produce traces with high resolution (~300 µm resolution using stencils from a craft cutter) on a substrate. The spraying produces sufficient kinetic energy (~14 m/s) to distort the particles on impact, which allows them to merge together. This merging process depends on both particle size and velocity. Particles of similar size do not merge when cast as a film. Likewise, smaller particles (<1 µm) moving at the same speed do not rupture on impact either, though calculations suggest that such particles could rupture at higher velocities. The liquid metal features can be encased by spraying uncured silicone elastomer from a volatile solvent to form a conformal coating that does not disrupt the liquid metal features during spraying. Alternating layers of liquid metal and elastomer may be patterned sequentially to build multilayer devices, such as soft and stretchable sensors.}, number={2}, journal={MICROMACHINES}, publisher={MDPI AG}, author={Neumann, Taylor V. and Kara, Berra and Sargolzaeiaval, Yasaman and Im, Sooik and Ma, Jinwoo and Yang, Jiayi and Ozturk, Mehmet C. and Dickey, Michael D.}, year={2021}, month={Feb} }
 @article{padmanabhan ramesh_sargolzaeiaval_neumann_misra_vashaee_dickey_ozturk_2021, title={Flexible thermoelectric generator with liquid metal interconnects and low thermal conductivity silicone filler}, volume={5}, ISSN={["2397-4621"]}, DOI={10.1038/s41528-021-00101-3}, abstractNote={AbstractHarvesting body heat using thermoelectricity provides a promising path to realizing self-powered, wearable electronics that can achieve continuous, long-term, uninterrupted health monitoring. This paper reports a flexible thermoelectric generator (TEG) that provides efficient conversion of body heat to electrical energy. The device relies on a low thermal conductivity aerogel–silicone composite that secures and thermally isolates the individual semiconductor elements that are connected in series using stretchable eutectic gallium-indium (EGaIn) liquid metal interconnects. The composite consists of aerogel particulates mixed into polydimethylsiloxane (PDMS) providing as much as 50% reduction in the thermal conductivity of the silicone elastomer. Worn on the wrist, the flexible TEGs present output power density figures approaching 35 μWcm−2 at an air velocity of 1.2 ms−1, equivalent to walking speed. The results suggest that these flexible TEGs can serve as the main energy source for low-power wearable electronics.}, number={1}, journal={NPJ FLEXIBLE ELECTRONICS}, author={Padmanabhan Ramesh, Viswanath and Sargolzaeiaval, Yasaman and Neumann, Taylor and Misra, Veena and Vashaee, Daryoosh and Dickey, Michael D. and Ozturk, Mehmet C.}, year={2021}, month={Mar} }
 @article{park_shintake_piskarev_wei_joshipura_frey_neumann_floreano_dickey_2021, title={Stretchable and Soft Electroadhesion Using Liquid-Metal Subsurface Microelectrodes}, volume={6}, ISSN={["2365-709X"]}, DOI={10.1002/admt.202100263}, abstractNote={AbstractElectroadhesion is an attractive mechanism to electrically modulate adhesion to surfaces. Electroadhesion arises from the interaction of electric fields with conductive or dielectric materials. Electroadhesion devices consist of in‐plane, interdigitated electrodes that generate out‐of‐plane electric fields, which increase adhesion with target surfaces. To date, these electrodes have predominantly been composed of carbonaceous materials. Here, liquid metal is utilized to create the electrodes in silicone substrates. Liquid metal can be patterned in a variety of unique ways, including microfluidic injection, spray deposition, or printing. These electrodes have nearly unlimited deformation in soft and stretchable substrates while maintaining metallic conductivity. The experimental results show that stretching improves electroadhesion performance due to the changes in geometry of the electrodes and insulation layer, whose behaviors are theoretically predictable. The use of liquid‐filled, sub‐surface microchannels can help to maintain contact between the elastomer and substrate during peeling due to the surface stresses caused by the capillary pressure. This approach to electroadhesion can be implemented in ultra‐stretchable and soft substrates, including those used in soft robotics, due to the inherently compliant and deformable electrical conductivity of the liquid metal electrodes.}, journal={ADVANCED MATERIALS TECHNOLOGIES}, author={Park, Sungjune and Shintake, Jun and Piskarev, Egor and Wei, Yuwen and Joshipura, Ishan and Frey, Ethan and Neumann, Taylor and Floreano, Dario and Dickey, Michael D.}, year={2021}, month={Jun} }
 @article{sargolzaeiaval_ramesh_neumann_misra_vashaee_dickey_ozturk_2020, title={Flexible thermoelectric generators for body heat harvesting - Enhanced device performance using high thermal conductivity elastomer encapsulation on liquid metal interconnects}, volume={262}, ISSN={["1872-9118"]}, DOI={10.1016/j.apenergy.2019.114370}, abstractNote={This paper reports flexible thermoelectric generators (TEGs) employing eutectic gallium indium (EGaIn) liquid metal interconnects encased in a novel, high thermal conductivity (HTC) elastomer. These TEGs are part of a broader effort to harvest thermal energy from the body and convert it into electrical energy to power wearable electronics. The flexible TEGs reported in this paper employ the same thermoelectric legs' used in rigid TEGs, thus eliminating the need to develop new materials specifically for flexible TEGs that often sacrifice the so-called figure of merit' for flexibility. Flexible TEGs reported here embed rigid thermoelectric legs' in soft and flexible packaging, using stretchable EGaIn interconnects. The use of liquid metal interconnects provides ultimate stretchability and low electrical resistance between the thermoelectric legs. The liquid metal lines are encased in a new stretchable silicone elastomer doped with both graphene nano-platelets and EGaIn to increase its thermal conductivity. This high thermal conductivity elastomer not only reduces the parasitic thermal resistance of the encapsulation layer but it also serves as a heat spreader, leading to 1.7X improvement in the output power density of TEGs compared to devices fabricated with a conventional elastomer. The device performance is further improved by a thin Cu layer acting as a heat spreader providing an additional 1.3X enhancement in the output power at 1.2 m/s air velocity (typical walking speed). Worn on the wrist, our best devices achieve power levels in excess of 30 μW/cm2 at an air velocity of 1.2 m/s outperforming previously reported flexible TEGs.}, journal={APPLIED ENERGY}, author={Sargolzaeiaval, Yasaman and Ramesh, Viswanath Padmanabhan and Neumann, Taylor V and Misra, Veena and Vashaee, Daryoosh and Dickey, Michael D. and Ozturk, Mehmet C.}, year={2020}, month={Mar} }
 @article{yang_tang_ao_ghosh_neumann_zhang_piskarev_yu_truong_xie_et al._2020, title={Ultrasoft Liquid Metal Elastomer Foams with Positive and Negative Piezopermittivity for Tactile Sensing}, volume={30}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.202002611}, abstractNote={AbstractSoft, capacitive tactile (pressure) sensors are important for applications including human–machine interfaces, soft robots, and electronic skins. Such capacitors consist of two electrodes separated by a soft dielectric. Pressing the capacitor brings the electrodes closer together and thereby increases capacitance. Thus, sensitivity to a given force is maximized by using dielectric materials that are soft and have a high dielectric constant, yet such properties are often in conflict with each other. Here, a liquid metal elastomer foam (LMEF) is introduced that is extremely soft (elastic modulus 7.8 kPa), highly compressible (70% strain), and has a high permittivity. Compressing the LMEF displaces the air in the foam structure, increasing the permittivity over a large range (5.6–11.7). This is called “positive piezopermittivity.” Interestingly, it is discovered that the permittivity of such materials decreases (“negative piezopermittivity”) when compressed to large strain due to the geometric deformation of the liquid metal droplets. This mechanism is theoretically confirmed via electromagnetic theory, and finite element simulation. Using these materials, a soft tactile sensor with high sensitivity, high initial capacitance, and large capacitance change is demonstrated. In addition, a tactile sensor powered wirelessly (from 3 m away) with high power conversion efficiency (84%) is demonstrated.}, number={36}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Yang, Jiayi and Tang, David and Ao, Jinping and Ghosh, Tushar and Neumann, Taylor V. and Zhang, Dongguang and Piskarev, Yegor and Yu, Tingting and Truong, Vi Khanh and Xie, Kai and et al.}, year={2020}, month={Sep} }
 @article{sargolzaeiaval_ramesh_neumann_miles_dickey_ozturk_2019, title={High Thermal Conductivity Silicone Elastomer Doped with Graphene Nanoplatelets and Eutectic GaIn Liquid Metal Alloy}, volume={8}, ISSN={["2162-8769"]}, DOI={10.1149/2.0271906jss}, abstractNote={This paper reports the thermal conductivity and mechanical properties of Sylgard 184 polydimethylsiloxane (PDMS) elastomer loaded with graphene nano-platelets (GnPs) and eutectic Ga-In (EGaIn) liquid metal droplets. We fabricated samples with different GnP and EGaIn concentrations and measured their thermal conductivity using the steady-state absolute technique. The results show that the thermal conductivity of the elastomer can be enhanced up to 5.6X when both GNP and EGaIn are included in the elastomer. Without EGaIn, the enhancement is limited to 4.4X. The results suggest that EGaIn inclusion did not change the viscosity of the uncured material significantly at any GnP loading level. We also observed that addition of just EGaIn to PDMS did not have a significant impact on the material's stiffness while lowering its ultimate tensile strength by a factor of 2X and the maximum elongation at the break point by a factor of 1.6X. On the other hand, it was demonstrated that GnP addition to pure PDMS or EGaIn doped PDMS made the elastomer stiffer and less tear resistant with lower elongation at the break point.}, number={6}, journal={ECS JOURNAL OF SOLID STATE SCIENCE AND TECHNOLOGY}, author={Sargolzaeiaval, Yasaman and Ramesh, Viswanath Padmanabhan and Neumann, Taylor V. and Miles, Rebecca and Dickey, Michael D. and Ozturk, Mehmet C.}, year={2019}, month={Jun}, pages={P357–P362} }
 @article{barbee_mondal_deng_bharambe_neumann_adams_boechler_dickey_craig_2018, title={Mechanochromic Stretchable Electronics}, volume={10}, ISSN={["1944-8252"]}, url={https://doi.org/10.1021/acsami.8b09130}, DOI={10.1021/acsami.8b09130}, abstractNote={Soft and stretchable electronics are promising for a variety of applications such as wearable electronics, human-machine interfaces, and soft robotics. These devices, which are often encased in elastomeric materials, maintain or adjust their functionality during deformation, but can fail catastrophically if extended too far. Here, we report new functional composites in which stretchable electronic properties are coupled to molecular mechanochromic function, enabling at-a-glance visual cues that inform user control. These properties are realized by covalently incorporating a spiropyran mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that a strain threshold has been reached. The resulting colorimetric elastomers can be molded and patterned so that, for example, the word "STOP" appears when a critical strain is reached, indicating to the user that further strain risks device failure. We also show that the strain at color onset can be controlled by layering silicones with different moduli into a composite. As a demonstration, we show how color onset can be tailored to indicate a when a specified frequency of a stretchable liquid metal antenna has been reached. The multiscale combination of mechanochromism and soft electronics offers a new avenue to empower user control of strain-dependent properties for future stretchable devices.}, number={35}, journal={ACS APPLIED MATERIALS & INTERFACES}, publisher={American Chemical Society (ACS)}, author={Barbee, Meredith H. and Mondal, Kunal and Deng, John Z. and Bharambe, Vivek and Neumann, Taylor V. and Adams, Jacob J. and Boechler, Nicholas and Dickey, Michael D. and Craig, Stephen L.}, year={2018}, month={Sep}, pages={29918–29924} }
 @article{andrews_mondal_neumann_cardenas_wang_parekh_lin_ballentine_dickey_franklin_et al._2018, title={Patterned Liquid Metal Contacts for Printed Carbon Nanotube Transistors}, volume={12}, ISSN={["1936-086X"]}, url={https://doi.org/10.1021/acsnano.8b00909}, DOI={10.1021/acsnano.8b00909}, abstractNote={Flexible and stretchable electronics are poised to enable many applications that cannot be realized with traditional, rigid devices. One of the most promising options for low-cost stretchable transistors are printed carbon nanotubes (CNTs). However, a major limiting factor in stretchable CNT devices is the lack of a stable and versatile contact material that forms both the interconnects and contact electrodes. In this work, we introduce the use of eutectic gallium-indium (EGaIn) liquid metal for electrical contacts to printed CNT channels. We analyze thin-film transistors (TFTs) fabricated using two different liquid metal deposition techniques-vacuum-filling polydimethylsiloxane (PDMS) microchannel structures and direct-writing liquid metals on the CNTs. The highest performing CNT-TFT was realized using vacuum-filled microchannel deposition with an in situ annealing temperature of 150 °C. This device exhibited an on/off ratio of more than 104 and on-currents as high as 150 μA/mm-metrics that are on par with other printed CNT-TFTs. Additionally, we observed that at room temperature the contact resistances of the vacuum-filled microchannel structures were 50% lower than those of the direct-write structures, likely due to the poor adhesion between the materials observed during the direct-writing process. The insights gained in this study show that stretchable electronics can be realized using low-cost and solely solution processing techniques. Furthermore, we demonstrate methods that can be used to electrically characterize semiconducting materials as transistors without requiring elevated temperatures or cleanroom processes.}, number={6}, journal={ACS NANO}, publisher={American Chemical Society (ACS)}, author={Andrews, Joseph B. and Mondal, Kunal and Neumann, Taylor V. and Cardenas, Jorge A. and Wang, Justin and Parekh, Dishit P. and Lin, Yiliang and Ballentine, Peter and Dickey, Michael and Franklin, Aaron D. and et al.}, year={2018}, month={Jun}, pages={5482–5488} }
 @article{neumann_dickey_2016, title={Recent Applications of Liquid Metals Featuring Nanoscale Surface Oxides}, volume={9871}, ISSN={["1996-756X"]}, DOI={10.1117/12.2229255}, abstractNote={This proceeding describes recent efforts from our group to control the shape and actuation of liquid metal. The liquid metal is an alloy of gallium and indium which is non-toxic, has negligible vapor pressure, and develops a thin, passivating surface oxide layer. The surface oxide allows the liquid metal to be patterned and shaped into structures that do not minimize interfacial energy. The surface oxide can be selectively removed by changes in pH or by applying a voltage. The surface oxide allows the liquid metal to be 3D printed to form free-standing structures. It also allows for the liquid metal to be injected into microfluidic channels and to maintain its shape within the channels. The selective removal of the oxide results in drastic changes in surface tension that can be used to control the flow behavior of the liquid metal. The metal can also wet thin, solid films of metal that accelerates droplets of the liquid along the metal traces .Here we discuss the properties and applications of liquid metal to make soft, reconfigurable electronics.}, journal={SENSING AND ANALYSIS TECHNOLOGIES FOR BIOMEDICAL AND COGNITIVE APPLICATIONS 2016}, author={Neumann, Taylor V. and Dickey, Michael D.}, year={2016} }