Toward Programmable Materials for Wearable Electronics: Electrical Welding Turns Sensors into Conductors

Abstract

again. [21,22] Thus, preserving material conductivity at large strains remains a real challenge. [23][24][25][26] One approach to maintaining conductivity is to better engineer the junctions among the conductive particles. Indeed, metallic nanowires are highly conductive, both electrically and thermally, [27] making it interesting nanomaterial for electronic and thermal applications. However, the junctions among the particles play a major role in both the electrical [28] and thermal [29] energy transfers within the network. Unlike the carbon-based nanoparticles, which are also often used for enhancing the electrical properties of polymers, the junction resistance of metal nanowires can be controlled by welding. Though several approaches have been developed for welding silver nanowire (AgNW) network, [28,[30][31][32] welding by joule heating with an electric voltage source (e-welding) is straightforward and economical. [28] In this way, the resistance of the junction can be efficiently controlled to transform an electrically conductive material from a stretchable sensor to a conductor. When polymeric sponge materials are made conductive by introducing metallic nanowires, the elevated and strain-dependent resistance of the initial interparticle junctions usually results in a sensor-like behavior. However, when subjected to an electrical current, the localized joule heating of the junctions enables the nanowires to weld together, resulting in a more conductor-like response.Here, we exemplify this using a conductive polymer sponge doped with a network of high-aspect-ratio AgNWs. The AgNWs are incorporated into the inner walls of the porous sponge to create an electrically conductive network. The piezoresistive response (measured by the gauge factor) of the conductive sponge can be controlled either by altering the initial percolated network of nanowires, or by progressive welding among the nanowires. In our experiments, the gauge factor progressively decreased as the level of e-welding increased. The material became progressively more conductive and less piezoresistive.In this paper, we demonstrate in detail the conversion of a sensor into a conductor by step-by-step welding of the AgNWs inside a polymer sponge using a DC-voltage source. In Section 2.1, we explain the fabrication and morphological characteristics of the soft, porous, stretchable, and conducting polymer sponge containing weakly connected AgNWs. Section 2.2 shows that the loosely connected AgNW network inside the sponge has high volume resistivity, depending on the degree of percolation. Poorly percolated AgNWs can slip one over another under linear strain (50% elongation) to produce sensor-like behavior, Stretchable electrical conductors and stretchable strain sensors are two key components in applications such as soft robotics, artificial soft skins, and wearable electronics. Very different approaches are usually implemented to design either conductors or sensors. Here, transformation of an electrically conductive material from a sensor to a conducto...

Nanoscale thermoplasmonic welding

“… Metal materials Complex operation, thermal damage. less applicable materials ( Chellattoan et al., 2019 ; Song et al., 2014 ; Tohmyoh et al., 2007 ) Electron beam welding It refers to the impact of high-speed beam on the material surface under vacuum, and most of its energy is converted into heat energy in a fraction of a microsecond. Metal/Semiconductor/Ceramic 2D materials, etc.…”

“…The heat supply is critically important to creating high-quality joints in nanoscale welding, as the high specific surface energy and increased surface volume ratio of materials are very sensitive to heat ( Wagle and Baker, 2015 ). For example, fusion welding, such as Joule heating ( Chellattoan et al., 2019 ; Song et al., 2014 ; Tohmyoh et al., 2007 ), electron beam welding ( Koleva, 2005 ; Terrones et al., 2002 ; Zhang et al., 2018 ), and ultrafast laser welding ( Gonzalez-Rubio et al., 2017 ; Huang et al., 2012 ; Itoh et al., 2013 ), have been applied for nano-connection, but these methods with high energy input unavoidably damage finer nano-objects. Therefore, it is highly necessary to develop accurate welding technology to solve these issues.…”

Flexible piezoelectric sensor based on polyacrylonitrile/MWCNT/MXene composites films for human physiological health detection

“…1,2 When pressure or vibration is applied to the piezoelectric sensor surface, polarization occurs inside the piezoelectric materials and leads to charge separation, an uneven distribution of charge and electric field occurs inside the sensors, thus the piezoelectric sensors have the ability to convert mechanical stimuli into electrical signals 3 ; compared to transistors, capacitive, resistive, and triboelectric sensors, piezoelectric sensors offer immediate and linear response, and it has excellent signal stability, variety of mechanical stimuli can be detected, including normal, shear, tensile, bend and torsional forces 4 ; the flexible piezoelectric sensors have low modulus, high sensitivity, and can be easily adhered to the skin, providing a comfortable experience for the people, when wore close to the skin in daily life, it can monitor the healthy in real-time without causing much discomfort, the piezoelectric sensors have been applied to disease diagnosis, touch display, wearable electronic device, and other related fields. [5][6][7][8] In various piezoelectric materials, PAN is cheap and has good piezoelectric properties, the dipole moment is higher than that of PVDF, it is suitable for flexible piezoelectric sensors; as shown in Figure 3, PAN exhibits two typical conformations: the 3 1 -helix conformation and the planar zigzag conformation; in the 3 1 -helix conformation, the hydrogen atoms and cyanide groups on two adjacent carbon atoms exhibit an alternating arrangement; in the planar zigzag conformation, these atoms are alternately arranged to form an interlock-like structure; additionally, there is a steric effect between adjacent hydrogen atoms and cyanide groups in the planar zigzag conformation, which gives the molecule a higher contraction capacity and better deformation ability, as a result, PAN possesses a larger dipole moment; while the field of sensors mainly focus on PDMS, PVDF, and its derivatives like P(VDF-HFP), P(VDF-TrFE), and PTFE [9][10][11][12][13][14] ; PVDF is known for its high piezoelectric properties, however, it should be noted that its piezoelectric behavior can be unstable and sensitive to temperature changes. In comparison, the dipole moment of PAN is 3.5 Debye, higher than PVDF, 15 furthermore, the stability of PAN is enhanced by the trivalent bonds (one σ bond and two π bonds) that link carbon atoms to nitrogen atoms, 16 which make PAN more stable; in summary, PAN is more cost-effective and thermally stable than PVDF, making it a more suitable option for mass production of high-performance piezoelectric sensors.…”

“…In the healthcare field, traditional sensors often have high modulus, high strength, and poor human compatibility, it will result in a negative experience for the people, while flexible piezoelectric sensors offer a suitable solution 1,2 . When pressure or vibration is applied to the piezoelectric sensor surface, polarization occurs inside the piezoelectric materials and leads to charge separation, an uneven distribution of charge and electric field occurs inside the sensors, thus the piezoelectric sensors have the ability to convert mechanical stimuli into electrical signals 3 ; compared to transistors, capacitive, resistive, and triboelectric sensors, piezoelectric sensors offer immediate and linear response, and it has excellent signal stability, variety of mechanical stimuli can be detected, including normal, shear, tensile, bend and torsional forces 4 ; the flexible piezoelectric sensors have low modulus, high sensitivity, and can be easily adhered to the skin, providing a comfortable experience for the people, when wore close to the skin in daily life, it can monitor the healthy in real‐time without causing much discomfort, the piezoelectric sensors have been applied to disease diagnosis, touch display, wearable electronic device, and other related fields 5–8 …”

Buckled Conductive Polymer Ribbons in Elastomer Channels as Stretchable Fiber Conductor

“…In the case the piezoresistivity results from the change in distance between particles in a network of nanoparticles, the morphology and density of the networks play a major role . The interface between the particles can also be engineered to tailor the intensity of the piezoresistivity to the desired value . On the other hand, resistance‐stable fibers can operate under large tensile strains without any significant change in the electrical resistance.…”