Silver Flakes Research Articles

Gold and silver nanocomposite-based biostable and biocompatible electronic textile for wearable electromyographic biosensors

Wearable biosensors have received significant attention due to the possibility of measuring physiological signals on demand. Particularly, the monitoring of electromyographic (EMG) signals on demand by wearable platforms has significant potential to revolutionize the diagnostics and treatment of neuromuscular diseases and for advancing human–computer interfaces. Electronic textile-based biosensors have several advantages, including the simple scale-up process and the ease of fabricating multiple large area electrodes over the whole body to obtain precise measurements. Hence, the electronic textile production requires an affordable approach to fabricate biocompatible and biostable electronic circuits on textile materials. This work explores the possibility of combining screen printing and electrodeposition techniques to produce a biostable nanocomposite-based EMG biosensor on textile. Screen printing was selected to fabricate conductive fabrics that would ultimately be a highly durable textile-based sensor. Silver paste, including microscale silver flakes, was printed on PET/cotton blended fabrics. However, the microscale silver surface was limited for EMG sensors due to low surface area and toxicity, causing low signal detection performance and skin irritation. Gold nanoparticles (Au NPs) were deposited on silver flakes to address the requirements of high-performance and biocompatible biosensors. We confirmed that the gold functionalization improved electrical and electrochemical performance. In addition, various tests were performed to determine electrochemical and biological stability under physiological conditions. The test results proved that Au NPs have successfully encapsulated the surface of silver flakes, preventing the exposure of the silver to the physiological environment. EMG signal recording was performed to confirm the functionalization effect that improved the signal to noise ratio (SNR) of 12.5 with 120 nm Au NPs. Moreover, EMG sensing from bicep workouts and finger movements showed the high sensitivity of the electronic fabrics. Although the SNR of EMG signals dropped to 7.2 after a 15-time washing test, the stabilized SNR after 5 washing cycles indicated that the Au/Ag biosensors showed washing durability. The study demonstrates that this affordable approach can be considered for large-scale production of wearable EMG biosensors.

Hybrid silver pastes with synergistic effect of multi-scale silver fillers and the application in flexible circuits

Silver circuits prepared by new hybrid pastes exhibit high electrical conductivity and mechanical properties, which are ideal for modern flexible electronics. Herein, the hybrid silver pastes composed of different silver nanoparticle content (0, 10, 20, 30 wt%), micron-sized silver flakes and epoxy-based binders were prepared. The corresponding electrical conductivity and bending resistance of the screen-printed silver circuits were studied. The experimental results demonstrated that the synergistic effect of micro-size silver flakes and nanoparticles can greatly improve the electrical conductivity and bending resistance of flexible circuits. Specifically, the silver circuits with 46 wt% micro-size silver flakes and 20 wt% silver nanoparticles incorporation exhibit a lower electrical resistivity of 8.1 × 10−5 Ω·cm. Moreover, 10 wt% silver nanoparticles can be applied to significantly reduce the resistance change of flexible circuit, indicting a superior bending property. Our designed hybrid silver pastes with excellent performance might enable valuable applications in advanced electronic devices.

Thermosensitive and Conductive Hybrid Polymer for Real-Time Monitoring of Spheroid Growth and Drug Responses.

Three-dimensional (3D) cell culture based on polymer scaffold provides a promising tool to mimic a physiological microenvironment for drug testing; however, the next-generation cell activity monitoring technology for 3D cell culture is still challenging. Conventionally, drug efficacy evaluation and cell growth heavily rely on cell staining assays, using optical devices or flow cytometry. Here, we report a dual-function polymer scaffold (DFPS) composed of thermosensitive, silver flake- and gold nanoparticle-decorated polymers, enabling conductance change upon cell proliferation or death for in situ cell activity monitoring and drug screening. The cell activity can be quantitatively monitored via measuring the conductance change induced by polymeric network swelling or shrinkage. This novel dual-function system (1) provides a 3D microenvironment to enable the formation and growth of tumor spheroid in vitro and streamlines the harvesting of tumor spheroids through the thermosensitive scaffold and (2) offers a simple and direct quantitative method to monitor 3D cell culture in situ for drug responses. As a proof of concept, we demonstrated that a breast cancer stem cell line MDA-MB-436 was able to form cell spheroids in the scaffold, and the conductance change of the sensor exhibited a linear relationship with cell concentration. To examine its potential in drug screening, cancer spheroids in the cell sensor were treated with paclitaxel (PTX) and docetaxel (DTX), and predicted quantitative evaluation of the cytotoxic effect of drugs was established. Our results indicated that this cell sensing system may hold promising potential in expanding into an array device for high-throughput drug screening.

Stretchable Carbon and Silver Inks for Wearable Applications.

For wearable electronic devices to be fully integrated into garments, without restricting or impeding movement, requires flexible and stretchable inks and coatings, which must have consistent performance and recover from mechanical strain. Combining Carbon Black (CB) and ammonia plasma functionalized Graphite Nanoplatelets (GNPs) in a Thermoplastic Polyurethane (TPU) resin created a conductive ink that could stretch to substrate failure (>300% nominal strain) and cyclic strains of up to 100% while maintaining an electrical network. This highly stretchable, conductive screen-printable ink was developed using relatively low-cost carbon materials and scalable processes making it a candidate for future wearable developments. The electromechanical performance of the carbon ink for wearable technology is compared to a screen-printable silver as a control. After initial plastic deformation and the alignment of the nano carbons in the matrix, the electrical performance was consistent under cycling to 100% nominal strain. Although the GNP flakes are pulled further apart a consistent, but less conductive path remains through the CB/TPU matrix. In contrast to the nano carbon ink, a more conductive ink made using silver flakes lost conductivity at 166% nominal strain falling short of the substrate failure strain. This was attributed to the failure of direct contact between the silver flakes.

Neuroprosthetics: Durable and Fatigue‐Resistant Soft Peripheral Neuroprosthetics for In Vivo Bidirectional Signaling (Adv. Mater. 20/2021)

In article number 2007346, Dae-Hyeong Kim, Taeghwan Hyeon, Donghee Son, and co-workers develop a durable and fatigue-resistant soft bidirectional neuroprosthetic device composed of a biocompatible nanocomposite of gold-nanoshell-coated silver flakes dispersed in a tough, stretchable, and self-healing polymer. The electrical conductivity of our neuroprosthetics spontaneously recovers even after repetitive degradations by irregular and severe mechanical deformations.

Highly Stretchable and Durable Nanocomposite Bow-Tie Antenna for Wearable Application.

We present a highly stretchable and compact bow-tie antenna which operates at 5 GHz for wearable applications. The dimensions of the bow-tie antenna were 7.9 mm×17.8 mm. The stretchable antenna was fabricated with a composite mixture of silver flake and polymer binder. The composite paste was printed on polyurethane and textile using the screen printing technique. The RF performances, stretchability, bendability, and durability of the antennas were evaluated, which are critical requirements in wearable electronics. The stretchable bow-tie antennas showed excellent RF performances and stretchability up to a stretching strain of 40%. The antennas could be bent up to a bending radius of 20 mm without degrading RF performance. The stretchable antennas also exhibited outstanding mechanical endurance after 10,000 cyclic stretching tests. The antennas were not affected by the presence of the body and showed very stable RF performances, exhibiting promising results for mobile and wearable applications.

Tuning the electrical resistivity of conductive silver paste prepared by blending multi-morphologies and micro-nanometers silver powder

As an important interconnection material in electronics, conductive silver paste has attracted much research interest in chip packaging and printed circuit board due to its predominant properties like high conductivity and flexible interconnection. In this paper, the silver nanoparticles, the silver sphere particles, and flake silver powders are fabricated by various methods. Different proportions of silver powder are selected to prepare micro-silver paste and micro-nano (mn) silver paste (fabricated by silver nanoparticles:sphere particles:flake silver powder = 1:1:1). Compared to traditional silver paste with high silver content (containing 80 wt% silver), paste in this work contains only 66.67 wt% silver. When the ratio of sphere particle to flake silver powder is 1:1, the electrical resistivity of micro-silver paste cured at 200 °C for 45 min in air is about (3.31 ± 0.73) × 10−5 Ω cm, the resistance of silver paste rises a little bit after being folded by ten times. The addition of nano-silver particles can reduce the resistivity in lower temperature curing. The folding endurance of mn silver paste is comparable to that of micro-silver paste with sphere:flake = 1:1.

Highly stretchable conductors comprising composites of silver nanowires and silver flakes

Stretchable conductors are receiving increasing attention due to their potential applications in wearable electronics. Highly stretchable conductors are fabricated using composites of silver nanowires (Ag NWs) and flakes on elastic polydimethylsiloxane (PDMS) substrates. The resistance of the Ag NW/flake composite conductor decreases to 1.9 × 10−4 Ohm·cm and increases by only 1.8 times after 100 stretch/release cycles at 100% peak strain. The improved conductivity and stretchability of the composite conductors are due to the synergy between the Ag NWs and Ag flakes because the Ag NWs connect the Ag flakes as conductive cables and the Ag flakes fill the gaps among the Ag NWs. Robust Ag NW/flake conductors are promising candidate for wearable electronics.

Durable and Fatigue-Resistant Soft Peripheral Neuroprosthetics for In Vivo Bidirectional Signaling.

Soft neuroprosthetics that monitor signals from sensory neurons and deliver motor information can potentially replace damaged nerves. However, achieving long-term stability of devices interfacing peripheral nerves is challenging, since dynamic mechanical deformations in peripheral nerves cause material degradation in devices. Here, a durable and fatigue-resistant soft neuroprosthetic device is reported for bidirectional signaling on peripheral nerves. The neuroprosthetic device is made of a nanocomposite of gold nanoshell (AuNS)-coated silver (Ag) flakes dispersed in a tough, stretchable, and self-healing polymer (SHP). The dynamic self-healing property of the nanocomposite allows the percolation network of AuNS-coated flakes to rebuild after degradation. Therefore, its degraded electrical and mechanical performance by repetitive, irregular, and intense deformations at the device-nerve interface can be spontaneously self-recovered. When the device is implanted on a rat sciatic nerve, stable bidirectional signaling is obtained for over 5 weeks. Neural signals collected from a live walking rat using these neuroprosthetics are analyzed by a deep neural network to predict the joint position precisely. This result demonstrates that durable soft neuroprosthetics can facilitate collection and analysis of large-sized in vivo data for solving challenges in neurological disorders.

Research on the Working Mechanism of Melt Metallurgical Effect

This paper takes rectangular tinned silver flake as the research object to explore the working mechanism of melt metallurgical effect. Firstly, the paper designs and processes rectangular tinned silver flake through the electroplating technology, then conducts low-overload pre-arc experiment with the metallurgical samples. According to the experiment results, it is proposed that “Under low overload current, the formation of silver-tin alloy leads to the increase of current density of the metallic silver layer, which significantly advances the break time of the silver flake. At the same time, the silver-tin interpenetration speed is a linear function of the temperature.” Finally, the self-programming software is used to establish the transient electric heating field model of the metallurgical tinned silver flake. By comparing simulation and experiment, the results verify the accuracy of the hypothesis, and obtain the working mechanism of the melt metallurgical effect, which provides a reliable theoretical basis for the design and application of the metallurgical trigger.

Dynamic electro‐mechanical analysis of highly conductive particle‐elastomer composites

AbstractThe availability of stretchable conductive materials is a key requirement for the development of soft and wearable electronics. Although there are many promising materials, the characterization of these materials under realistic conditions is complex and a standardized and reliable procedure has not been etablished yet. We therefore introduce a comprehensive protocol for the practice‐oriented dynamic electro‐mechanical analysis of elastomer‐particle composites. In addition to strain dependence (0–100% strain) and fatigue strength (10,000 cycles), this protocol aims in particular to clarify the influence of strain rate (0–100% s−1) on conductivity. Samples with the commonly used filler representatives carbon black and silver flakes with 20 vol% each were prepared and investigated. Silicone elastomers of different stiffness were used as matrix in order to determine its influence. We found that while the conductivity of the carbon black composites of about 1 × 102 S m−1 proved to be fatigue resistant and largely independent of the strain rate, the silver flake composites lost their initially higher conductivity of 1 × 104 S m−1 at high strain rates and increasing numbers of cycles. In addition, the use of a softer silicone matrix improved the performance of both particle composites, which was also demonstrated on an exemplary wearable electronic device.

Study on the mechanism of forming silver nanoparticles on micron-scale flake silver powder

Due to the high conductivity and stability of silver, silver ink has been widely used in the field of printed electronics. However, since mechanical strain would cause the contact between key particles to be broken. Recently, higher requirements have been put on the bending resistance of silver paste. In this experiment, by preparing Ag NPs: Ag Flakes composite materials of 20:80 and 30:70 respectively, a new method for preparing stable modified flake silver powder is proposed, and the attachment and growth methods of Ag NPs reduced on micron flake silver powder is discussed. Flake silver powder, dispersant and reducing agent are added to the silver-ammonia complex, and the nano-silver particles are reduced on the flake silver powder by controlling the reaction conditions. When Ag+: Ag Flakes in the solution is 30:70, there is a clear interface between the silver particles and the silver flakes, and the size distribution range is between 10 and 100 nm; when Ag+: Ag Flakes is 20:80, the distance between the nano-silver particles and the silver plate crystal plane is about 0.222 nm and 0.260 nm, these spacings represent the (1–12) and (004) planes of Ag, respectively. The reduced silver particles grow on the surface of the silver sheet, the size is about 5 ∼ 60 nm, and form a sheet-point combination of shaped silver powder, which can be used to make conductive silver paste with cost and performance competitiveness. The reaction can be completed at room temperature (5 ∼ 25 °C) and atmospheric pressure (80 ∼ 100 kPa), the operation is simple, the reaction time is short, and it has a positive effect on the commercial production of printable highly conductive silver paste.

Mechanically interlocked stretchable nanofibers for multifunctional wearable triboelectric nanogenerator

Abstract Nanofibers with good softness and high surface specific area are excellent choices for wearable triboelectric nanogenerator (TENG), despite that the deformability and durability remain challenging in seamless integration with daily textiles/clothes. Herein, we propose a physical interlocking strategy to realize a self-interlocked stretchable, breathable and waterproof nanofibers-membrane by simultaneous electrospinning of poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and electrospraying of styrene-ethylene-butylene-styrene (SEBS). The electrosprayed SEBS microspheres serve as the elastic binders and hydrophobic modifiers for enhancing stretchability and waterproofness of the electrospun PVDF-HFP fibers network. By means of a printable electrode consisted of liquid metal (gallium indium tin particles) and silver flakes, a stretchable nanofibers-based TENG (SNF-TENG) with high triboelectric output (85 V, 219.66 mW m−2) and electrical durability was demonstrated, capable of harvesting energy from human motions and flowing water, powering 200 commercial LEDs and an electronic watch. The stretchable nanofibers membrane shows favorable mechanical compliance, which can be facilely integrated onto stretchable textile to fabricate textile-TENG, promising for comfortable wearable applications for power sources, smart raincoat, self-powered e-skin and tactile interactive interfaces.

Fabrication of high thermal conductive epoxy composite by adding hybrid of expanded graphite, iron (III) oxide, and silver flakes

Thermally conductive epoxy adhesive composites have been fabricated by adding a tri-filler mixture of expanded graphite (EG), iron (III) oxide (Fe3O4), and silver (Ag) flakes with increasing concentration from 20 to 50 weight percent. The epoxy composite containing mixed filler of EG/Fe/Ag at 50 wt% has demonstrated diminutive higher thermal conductivity (TC) (4.85 W/mK) when compared to composite filled with EG/Fe hybrid (4.82 W/mK) at equivalent filler % which was about 25 fold improvement than epoxy unaided. The crystallography of the graphite flakes (GF), EG, neat epoxy, EG/Fe/Ag-epoxy, and EG/Fe-epoxy adhesive composites has been analyzed by X-ray diffraction (XRD) technique. The morphology of filler (EG and EG/Fe/Ag) and fractured surface analysis of corresponding epoxy composite has been carried out by scanning electron microscopy. The bonding strength through lap shear test of epoxy adhesives with the addition of 50 wt% of EG/Fe/Ag mixed and EG/Fe hybrid fillers has been displayed a decreased value as compared to neat epoxy. At the primary phase analysis of thermal dilapidation, the thermo-gravimetric analysis (TGA) concluded a negative effect on the thermal constancy of epoxy composites at higher assimilation (> 35 wt%) of EG, Fe3O4, and Ag mixed fillers. The damping factor (tan δ) (0.44 and 0.43) and glass transition temperature of composite demonstrated lower value (70 and 64 °C) when 50 wt% of EG/Fe and EG/Fe/Ag were added.

Highly Conductive and Highly Dispersed Polythiophene Nanoparticles for Fabricating High-Performance Conductive Adhesives

The continuous advancement of modern electronic equipment requires increasing the conductivity of electrically conductive adhesives (ECAs), but the gap between discrete silver flakes in conventiona...

Electromagnetic Shielding Effectiveness and Conductivity of PTFE/Ag/MWCNT Conductive Fabrics Using the Screen Printing Method

The management of the electromagnetic interference (EMI) of thin, light, and inexpensive materials is important for consumer electronics and human health. This paper describes the development of conductive films that contain a silver (Ag) flake powder and multiwall carbon nanotube (MWCNT) hybrid grid on a polytetrafluoroethylene (PTFE) film for applications that require electromagnetic shielding (EMS) and a conductive film. The Ag and MWCNT hybrid grid was constructed with a wire diameter and spacing of 0.5 mm. The results indicated that the proposed conductive films with 0.4 wt% MWCNTs had higher electromagnetic shielding effectiveness (EMSE) and electrical conductivity than those with other MWCNT loading amounts. The results also showed that the film with 0.4 wt% MWCNT loading had a high 62.4 dB EMSE in the 1800 MHz frequency and 1.81 × 104 S/cm electrical conductivity. This combination improved stretchability, with 10% elongation at a 29% resistivity change rate. Conductive films with Ag/MWCNT electronic printing or lamination technologies could be used for EMI shielding and electrically conductive applications.

Wearable and Comfortable e-Textile Headband for Long-Term Acquisition of Forehead EEG Signals

Electroencephalography (EEG) has a wide range of applications in medical diagnosis, and novel form of Human Machine Interfaces (HMI) for controlling prosthetic implants, wheelchairs, and home appliances in various forms of paralysis. However, the current EEG setups are composed of many wires hanging down from the system, and individual electrodes that must be set manually, which is time-consuming. As a result, the overall system is neither comfortable, nor aesthetically appealing. Here, we introduce for the first time, a comfortable textile-based EEG headband system that is soft, conformal to the skin, and comfortable. We present materials and methods for fabrication of multi-layer stretchable e-textile, that interfaces the human epidermis from one side through printed electrodes, and interfaces a rigid PCB island on the second layer. We as well demonstrate a method that allows creation of VIAs (vertical interconnect access) between the layers, using a CO2 laser. All Electrodes are integrated into the headband, and thus there is no need for individual electrode placement, and individual wiring. By screen printing a home-made conductive stretchable ink, patient-specific EEG headbands can be tailor made considering the optimal positioning of the electrodes for each patient. We show that these printed electrodes benefit from a very low skin-electrode impedance, comparable to gold standard Ag/AgCl, or gold cup electrodes, thanks to the high surface area silver flakes used in this work. The e-textile headband interfaces with an EEG acquisition device that captures, amplifies, and transmits the data to an external mobile phone or a PC. Furthermore, the integrated amplification in the textile and the use of an EMF rejection layer on top of the electrodes were shown to reduce the unwanted EM noise that is picked up by the system. We as well show application of the developed headband for usage in Human Machine Interfaces and Sleep Data Acquisition. Altogether, this device is step toward wider use of EEG acquisition devices for daily-use applications.

Screen-printing process of electromagnetic interference (EMI) shielding materials on mulberry paper

ABSTRACT A mulberry paper-based electromagnetic interference shielding material with screen-printing process was realized for high shielding effect and mechanical strength. With various film thickness of silver flakes achieved by controlling squeeze depth in the screen printer, the performance of the material was systematically investigated. By adaption of silver flakes on mulberry paper, the shielding effect of the screen-printed electromagnetic interference shielding material revealed over 70 decibels, which is the highest value of shielding effect for X-band. Moreover, after rupture test, 63% higher mechanical strength of 3.26 Mpa on the printed sample was observed compared to commercially available copy paper. As far as we know, this report is the first one related to the printed mulberry paper for electromagnetic interference shielding materials that require high mechanical strength with high shielding effect.

Direct writing of stretchable metal flake conductors: improved stretchability and conductivity by combining differently sintered materials

Stretchable and flexible electronics with elastic interconnects is critical for electronics to be compatible with wearable applications. Especially fabricating such interconnects with printing technology promises rapid, low-cost, and mass manufacturing. Here, a novel direct-write manufacturing process is presented for a stretchable conductor using an extrusion printer and a micron size silver flake ink as the conductive material without expensive nanomaterials. Conductivity increases as the sintering temperature is increased but at the cost of limited stretchability. To overcome this trade-off, we place one mildly sintered line on top of a fully sintered line to create a hybrid serpentine feature. These two lines of a hybrid feature have two different stretchability-conductivity profiles, synergistically improving both the stretchability and conductivity. With this method, a maximum stretchability of 120% strain is obtained with a minimal change in resistance by a factor of 2.8. This stretchable conductor can endure more than 250 cycles of stretching and releasing under 40% strain. The aggregate electrical conductivity is in the range of 4.74*104 S cm−1, which is far superior to any carbon filler based printable composites.

Electron tunneling of hierarchically structured silver nanosatellite particles for highly conductive healable nanocomposites

Healable conductive materials have received considerable attention. However, their practical applications are impeded by low electrical conductivity and irreversible degradation after breaking/healing cycles. Here we report a highly conductive completely reversible electron tunneling-assisted percolation network of silver nanosatellite particles for putty-like moldable and healable nanocomposites. The densely and uniformly distributed silver nanosatellite particles with a bimodal size distribution are generated by the radical and reactive oxygen species-mediated vigorous etching and reduction reaction of silver flakes using tetrahydrofuran peroxide in a silicone rubber matrix. The close work function match between silicone and silver enables electron tunneling between nanosatellite particles, increasing electrical conductivity by ~5 orders of magnitude (1.02×103 Scm−1) without coalescence of fillers. This results in ~100% electrical healing efficiency after 1000 breaking/healing cycles and stability under water immersion and 6-month exposure to ambient air. The highly conductive moldable nanocomposite may find applications in improvising and healing electrical parts.