Consecutive large-scale fabrication of surface-silvered polyimide fibers via an integrated direct ion-exchange self-metallization strategy

Improving Interfacial and Compressive Properties of Polyimide Fiber by Constructing Inorganic Layers on Fiber Surface

The compressive performance of organic fiber has always been a key problem, limiting its development. In this paper, silicon oxide, alumina, and titanium oxide particles were separately deposited on the surface of high-strength and high-modulus polyimide (PI) fibers to form a structural supporting shell by using a magnetron sputtering method. The theoretical thickness was calculated by thermogravimetric analysis in good agreement with the actual thickness determined from scanning electron microscopy. The mechanics, surface, and interface properties of the measured fibers were analyzed mainly from the aspects of surface energy, interfacial shear strength (IFSS), and compression strength. The results showed that after magnetron sputtering, the inorganic shells were uniformly deposited on the surface of PI fiber, resulting in an increase in the content of inorganic elements as well as the roughness. As a result, the surface energy and IFSS of silica-coated fiber was increased by 174 and 85.6%, respectively, and compression strength was increased by 45.7%. This study provides a new approach for improving the interface property and compression strength of high-strength and high-modulus PI-fiber-reinforced composites.

Stretchable One-Dimensional Conductors for Wearable Applications

Continuous, one-dimensional (1D) stretchable conductors have attracted significant attention for the development of wearables and soft-matter electronics. Through the use of advanced spinning, printing, and textile technologies, 1D stretchable conductors in the forms of fibers, wires, and yarns can be designed and engineered to meet the demanding requirements for different wearable applications. Several crucial parameters, such as microarchitecture, conductivity, stretchability, and scalability, play essential roles in designing and developing wearable devices and intelligent textiles. Methodologies and fabrication processes have successfully realized 1D conductors that are highly conductive, strong, lightweight, stretchable, and conformable and can be readily integrated with common fabrics and soft matter. This review summarizes the latest advances in continuous, 1D stretchable conductors and emphasizes recent developments in materials, methodologies, fabrication processes, and strategies geared toward applications in electrical interconnects, mechanical sensors, actuators, and heaters. This review classifies 1D conductors into three categories on the basis of their electrical responses: (1) rigid 1D conductors, (2) piezoresistive 1D conductors, and (3) resistance-stable 1D conductors. This review also evaluates the present challenges in these areas and presents perspectives for improving the performance of stretchable 1D conductors for wearable textile and flexible electronic applications.

Patterned Ag/PI RFID Tag Integrated with Humidity Sensing by In Situ Metallization

This article presents a cost-efficient flexible chipless radio frequency identification (RFID) tag with wireless humidity sensing, which is fabricated by in situ metallization and inkjet printing techniques. The inkjet printing technique is applied to print the mask for RFID antenna, which is designed with frequency-encoding simulation by a high-frequency structure simulator (HFSS). A high-quality patterned Ag antenna is realized by the in situ metallization of a polyimide (PI) film, leading to strong adhesion between the Ag antenna and PI substrate. The patterned Ag antenna of the chipless RFID tag consists three parallel dipole resonators, one of which is sensitive to humidity, while the other two are utilized to encode and store data. As a result, a 2-bit chipless RFID with high humidity sensitivity based on a Ag/PI film is developed, which displays excellent flexibility and good mechanical stability. The performance of the fabricated tag shows good agreement with the simulation results. Moreover, the tag is applied to detect the water source, where the resonance frequency shows good linearity versus the distance to the water source. These results demonstrate that the proposed chipless RFID tag with humidity sensing has a 2-bit storage capacity, high humidity sensitivity, excellent mechanical properties, and long-term stability, confirming a cost-efficient preparation process for flexible electronics.

Molecular mechanisms of the antibacterial activity of polyimide fibers in a skin-wound model with Gram-positive and Gram-negative bacterial infection in vivo

Recently, the need for antibacterial dressings has amplified because of the increase of traumatic injuries. However, there is still a lack of ideal, natural antibacterial dressings that show an efficient antibacterial property with no toxicity. Polyimide (PI) used as an implantable and flexible material has been recently reported as a mixture of particles showing more desirable antibacterial properties. However, we have identified a novel type of natural polyimide (PI) fiber that revealed antibacterial properties by itself for the first time. The PI fiber material is mainly composed of C, N, and O, and contains a small amount of Ca and Cl; the characteristic peaks of polyimide appear at 1774 cm-1, 1713 cm-1, 1370 cm-1, 1087 cm-1, and 722 cm-1. PI fibers displayed significant antibacterial activities against Escherichia coli (as a Gram-negative bacteria model) and methicillin-resistant Staphylococcus aureus (MRSA, as a Gram-positive bacteria model) according to the time-kill kinetics in vitro, and PI fibers damaged both bacterial cell walls directly. PI fibers efficiently ameliorated a local infection in vivo, inhibited the bacterial burden, decreased infiltrating macrophages, and accelerated wound healing in an E. coli- or MRSA-infected wound model. In conclusion, PI fibers used in the present study may act as potent antibacterial dressings protecting from MRSA or E. coli infections and as promising candidates for antimicrobial materials for trauma and surgical applications.

Surface Engineering on Polyimide-Silver Films in Low-Cost, Flexible Humidity Sensors

In this work, surface engineering is applied to polyimide (PI) films to fabricate low-cost Ag/PI wireless humidity sensors with a resonant frequency of 2.45 GHz. The sensors were obtained by in situ metallization technique coupled with inkjet printing, where PI plays triple roles as a flexible substrate, ion-exchange surface, and sensing material to moisture. Moreover, the humidity sensitivity can be enhanced by the improvement of hydrophilicity via loading with different ions on the PI surface, which has been demonstrated by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The wireless humidity sensor loaded with K⁺ ions has the maximum sensitivity of 97.7 kHz/% RH at a low relative humidity range of 20-65% and 359.7 kHz/% RH at a high relative humidity of 65-90%, respectively. Accordingly, a sensing mechanism of the fabricated humidity sensor has been discussed in detail. On the other hand, the characteristics of the humidity sensor such as response and recovery speed and stability are analyzed. The mechanical performance tests show that the humidity sensor displays excellent flexibility and good mechanical stability. A strong adhesion between the Ag antenna and PI substrate can be found as well. The passive wireless humidity sensor described in this work has the advantages of having a simple structure, low cost, high sensitivity, long-term stability, and good mechanical properties, which has potential applications in automated industry and healthcare with real-time humidity monitoring.

In Situ Growth of Silver Film on Polyimide with Tuned Morphologies for Flexible Electronics

In this work, the silver films with tuned morphologies have been fabricated on flexible polyimide substrate by in situ direct-ion-exchange technique. The morphology of Ag films with loose nanoparticles, dense polyhedrons, aggregated nanoparticle clouds, and dendrite structure can be obtained by a controlled reduced process as illustrated by scanning electron microscopy (SEM) and optical microscopy, respectively. All of the Ag films show good crystalline and high conductivity, which is confirmed by X-ray diffraction (XRD) and four-point probe resistance measurements. Infrared (IR) spectra demonstrate the occurrence of the polyimide surface metallization, which favors good adhesion between the Ag films and the flexible substrate. The adhesion test proves the strong adhesion of these Ag films, especially for the Ag films with the dendritic structure. Moreover, the mechanical properties of these Ag/PI films have been investigated as well. It can be found that all of the Ag/PI films exhibit low sensitivity to the bending test. However, the strain sensitivity strongly depends on the morphology of the Ag films, which can be applied for diverse flexible electronics.

Fabrication of Porous Polyimide Membrane with Through-Hole via Multiple Solvent Displacement Method

Porous polyimide (PI) membranes are widely used in separation processes because of their excellent thermal and mechanical properties. However, the applications of porous PI membranes are limited in the nanofiltration range. In this study, porous PI membranes with through-holes have been successfully fabricated by the novel multiple solvent displacement method. This new method requires only a porous polyamic acid (PAA) membrane, which was prepared by immersing PAA film in N-methylpyrrolidoneebk; (NMP) prior to immersing it in a mixed solvent consisting of NMP and a poor solvent, followed by immersion only in poor solvent. The pore size, morphology, porosity, and air permeability demonstrated that the fabricated PI membranes had a uniformly porous structure with through-holes over their surface. This new method enabled control of pore size (3-11 μm) by selecting a suitable poor solvent. This multiple solvent displacement method is highly versatile and promising for the fabrication of porous PI membranes.

Ion-Selective Polyamide Acid Nanofiber Separators for High-Rate and Stable Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have attracted great attention because of their high energy density and high theoretical capacity. However, the "shuttle effect" caused by the dissolution of polysulfides in liquid electrolytes severely hinders their practical applications. Herein, we originally propose a carboxyl functional polyamide acid (PAA) nanofiber separator with dual functions for inhibiting polysulfide transfer and promoting Li⁺ migration via a one-step electrospinning synthesis method. Especially, the functional groups of -COOH in PAA separators provide an electronegative environment, which promotes the transport of Li⁺ but suppresses the migration of negative polysulfide anions. Therefore, the PAA nanofiber separator can act as an efficient electrostatic shield to restrict the polysulfide on the cathode side, while efficiently promoting Li⁺ transfer across the separator. As a result, an ultralow decay rate of only 0.12% per cycle is achieved for the PAA nanofiber separator after 200 cycles at 0.2 C, which is less than half that (0.26% per cycle) of the commercial Celgard separator.

Preparation and characterization of a copper@polyimide core–shell structure via an in situ induction/imidization route

Based on the combination of an in situ induction and imidization method for improving the interface bonding of an inorganic material and a polymer, copper@polyimide (Cu@PI) core–shell composite particles have been successfully prepared from poly(amic acid) ammonium salts (PAAS) and a Cu complex via a simple solvothermal process. The structures and the morphologies of the samples were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM), respectively. It was found that PAAS formed PI via a thermal imidization and subsequently precipitated in the solvent. Through crystallization induction, it then successfully coated on the surface of the formed Cu particles. Based on thermo gravimetric analyses curves and due to no Cu oxidation reactions taking place in the core coated with high-temperature-resistant PI, the weight increase was determined to be 106.4%, instead of up to 124.0% in samples consisting of pure Cu.