Ultrasoft Liquid Metal Elastomer Foams with Positive and Negative Piezopermittivity for Tactile Sensing

Abstract

Soft, 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.

Aerosol Spray Deposition of Liquid Metal and Elastomer Coatings for Rapid Processing of Stretchable Electronics

“…The probe used for this experiment is the same shown in Figure 5 e and the applied force was measured using the Instron mechanical testing system in compression mode. The sensor may be made more sensitive by increasing the area of the electrodes, using a softer elastomer, or by using a material with a higher dielectric constant [ 87 , 88 , 89 ]. Future works may consider further characterization of sensors such as the one reported here.…”

All-soft multiaxial force sensor based on liquid metal for electronic skin

“…All-soft liquid metal-based multiaxial force sensor detects the forces by the change of resistances of multi LM microchannel array. Galinstan, which is an eutectic alloy of gallium, indium, and tin attracts increasing attentions for its excellent mechanical and electrical properties as a component of stretchable electronic applications [23,24]. Due to its intrinsic properties as a liquid metal, the microchannel filled with Galinstan can easily deformed into 3D dome shape without any mechanical or electrical failure.…”

Liquid Metal Based Flexible and Implantable Biosensors

“…Ecoflex is a better candidate. Yang et al introduced a liquid metal-based composite consisting of EGaIn and Ecoflex for pressure sensing, called liquid metal elastomer foam [ 75 ]. By creating a foam architecture, composite obtains large permittivity, large permittivity change, and is ultra-soft.…”

“…After the air had been fully pressed out, the change in permittivity would become negative due to the changed shape of EGaIn droplets. These properties gave the composite the ability to be applied in tactile sensing and motion sensing ( Figure 5 h) [ 75 ]. Interestingly, they also achieved to power tactile sensor wirelessly from 3 m away with high power conversion efficiency.…”

“…Copyright (2019) WILEY-VCH. ( h ) The tactile sensor array is based on liquid metal elastomer foam and its capacitance change ratio [ 75 ]. Copyright (2020) WILEY-VCH.…”