Polyimide Surface Research Articles

Combining the good tribological properties with the high adhesion strength of the amorphous carbon films in situ grown on PI

Polyimide (PI) is widely utilized in the modern industry because of its excellent heat resistance, dielectric properties, radiation resistance and chemical stability. However, PI will appear wear failure due to different working environments in the application of various friction components. In order to ensure the normal function of Pi-based parts and reduce the wear of PI materials, it is necessary to deposit amorphous carbon film with high adhesion strength and wear resistance on PI substrate. In this experiment, we treated polyimide surfaces with carbon plasma to prepare the amorphous carbon films with in situ transition layers. The microstructure, mechanical properties and tribological properties of the amorphous carbon films were studied. The results showed that the hardness and wear resistance of the PI surface were greatly improved by the amorphous carbon film with in situ transition layer. More importantly, the in situ transition endowed the film high adhesion strength on the PI substrate. The roles of the carbon plasma in the deposition process of the amorphous carbon, namely, the deposition effect, and the induction effect, referring to the effect on the top layer of PI substrate, were systematically analyzed. This work realizes the purpose of protecting the PI surface with the amorphous carbon film with high adhesion strength, and also provides a new idea for improving the adhesion between hard coating and flexible substrate.

Basic amino acid-mediated cationic amphiphilic surfaces for antimicrobial pH monitoring sensor with wound healing effects

BackgroundThe wound healing process is a complex cascade of physiological events, which are vulnerable to both our body status and external factors and whose impairment could lead to chronic wounds or wound healing impediments. Conventional wound healing materials are widely used in clinical management, however, they do not usually prevent wounds from being infected by bacteria or viruses. Therefore, simultaneous wound status monitoring and prevention of microbial infection are required to promote healing in clinical wound management.MethodsBasic amino acid-modified surfaces were fabricated in a water-based process via a peptide coupling reaction. Specimens were analyzed and characterized by X-ray photoelectron spectroscopy, Kelvin probe force microscopy, atomic force microscopy, contact angle, and molecular electrostatic potential via Gaussian 09. Antimicrobial and biofilm inhibition tests were conducted on Escherichia coli and Staphylococcus epidermidis. Biocompatibility was determined through cytotoxicity tests on human epithelial keratinocytes and human dermal fibroblasts. Wound healing efficacy was confirmed by mouse wound healing and cell staining tests. Workability of the pH sensor on basic amino acid-modified surfaces was evaluated on normal human skin and Staphylococcus epidermidis suspension, and in vivo conditions.ResultsBasic amino acids (lysine and arginine) have pH-dependent zwitterionic functional groups. The basic amino acid-modified surfaces had antifouling and antimicrobial properties similar to those of cationic antimicrobial peptides because zwitterionic functional groups have intrinsic cationic amphiphilic characteristics. Compared with untreated polyimide and modified anionic acid (leucine), basic amino acid-modified polyimide surfaces displayed excellent bactericidal, antifouling (reduction ~ 99.6%) and biofilm inhibition performance. The basic amino acid-modified polyimide surfaces also exhibited wound healing efficacy and excellent biocompatibility, confirmed by cytotoxicity and ICR mouse wound healing tests. The basic amino acid-modified surface-based pH monitoring sensor was workable (sensitivity 20 mV pH−1) under various pH and bacterial contamination conditions.ConclusionHere, we developed a biocompatible and pH-monitorable wound healing dressing with antimicrobial activity via basic amino acid-mediated surface modification, creating cationic amphiphilic surfaces. Basic amino acid-modified polyimide is promising for monitoring wounds, protecting them from microbial infection, and promoting their healing. Our findings are expected to contribute to wound management and could be expanded to various wearable healthcare devices for clinical, biomedical, and healthcare applications.

Optimization of Alkyl Side Chain Length in Polyimide for Gate Dielectrics to Achieve High Mobility and Outstanding Operational Stability in Organic Transistors

Alkyl chain modification strategies in both organic semiconductors and inorganic dielectrics play a crucial role in improving the performance of organic thin-film transistors (OTFTs). Polyimide (PI) and its derivatives have received extensive attention as dielectrics for application in OTFTs because of flexibility, high-temperature resistance, and low cost. However, low-temperature solution processing PI-based gate dielectric for flexible OTFTs with high mobility, low operating voltage, and high operational stability remains an enormous challenge. Furthermore, even though di-n-decyldinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (C10-DNTT) is known to have very high mobility as an air-stable and high-performance organic semiconductor, the C10-DNTT-based TFTs on the PI gate dielectrics still showed relatively low mobility. Here, inspired by alkyl side chain engineering, we design and synthesize a series of PI materials with different alkyl side chain lengths and systematically investigate the PI surface properties and the evolution of organic semiconductor morphology deposited on PI surfaces during the variation of alkyl side chain lengths. It is found that the alkyl side chain length has a critical influence on the PI surface properties, as well as the grain size and molecular orientation of semiconductors. Good field-effect characteristics are obtained with high mobilities (up to 1.05 and 5.22 cm2/Vs, which are some of the best values reported to date), relatively low operating voltage, hysteresis-free behavior, and high operational stability in OTFTs. These results suggest that the strategy of optimizing alkyl side-chain lengths opens up a new research avenue for tuning semiconductor growth to enable high mobility and outstanding operational stability of PI-based OTFTs.

Template free preparation of graphene tubes from polyimide catalyzed by calcium carbonate.

This work reports a new means of preparing graphene tubes (GTs) without relying on chemical vapor deposition (CVD) and it's template-free. Surprisingly, we found that under the action of calcium oxide (CaO) and after 1500 °C heat treatment, a large amount of GTs grew on the surface of polyimide (PI). These nanotubes have a maximum diameter of about 600 nm and a length of up to millimeters, and some nanotubes even have a branching structure. We propose a simple, effective and green method which exhibits prospects for large-scale production of GTs using polymeric materials.

Temperature-Dependent n-to-p-Type Transition of 2D Mn Oxide Nanosheets toward NO2 for Flexible Gas Sensor Application.

Rapid and on-site detection of nitrogen dioxide (NO2) is important for environmental monitoring as NO2 is a highly toxic chemical emitted from automobiles and power plants. In this study, we proposed atomically thin two-dimensional (2D) Mn oxide nanosheets (NSs) assembled on a flexible heating substrate for application in flexible and wearable NO2 sensors. A liquid-phase exfoliation technique was used to obtain individual Mn oxide layers that formed a homogeneous suspension. A flexible heater was fabricated by partially embedding Ag nanotubes (NTs) on the surface of a colorless polyimide (CPI) film for use as a sensor substrate. Temperature-dependent NO2 sensing properties were investigated via control of the operating temperature using a Ag NT-embedded CPI heating film. As a result, the n-type sensing behavior of the Mn oxide NSs exhibited a response [(Rgas - Rair)/Rair × 100 (%)] of 1.20 ± 0.21% for 20 ppm NO2 at room temperature (25 °C). Meanwhile, n-p transition occurred with p-type sensing property as the operating temperature increased to 150 °C with an improved response [(Rair - Rgas)/Rair × 100 (%)] of 4.10 ± 0.42% for 20 ppm NO2. The characteristic n-p transition of Mn oxide NSs at different operating temperatures was evidenced by the surface-adsorbed oxygen ions (i.e., O2- and O-) and nitrate species (NO3- and NO32-).

Localized Surface Hydrophilicity Tailoring of Polyimide Film for Flexible Electronics Manufacturing Using an Atmospheric Pressure Ar/H2O Microplasma Jet.

The poor hydrophilicity of polyimide (PI) films limits their applications in flexible electronics, such as in wearable and implantable bio-MEMS devices. In this paper, an atmospheric pressure Ar/H2O microplasma jet (μAPPJ) with a nozzle diameter of 100 μm was utilized to site-selectively tune the surface hydrophilicity of a PI film. The electrical and optical characteristics of the μAPPJ were firstly investigated, and the results showed that multi-spikes occurred during the plasma discharge and that diverse reactive species, such as O atoms and OH radicals, were generated in the plasma plume. The physical and chemical properties of pristine and microplasma-modified PI surfaces were characterized by the water contact angle (WCA), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The wettability of the PI surface was significantly enhanced after microplasma modification, and the WCA could be adjusted by varying the applied voltage, water vapor content, plasma treatment time and storage time. The AFM images indicated that the surface roughness increased after the plasma treatment, which partially contributed to an improvement in the surface hydrophilicity. The XPS results showed a reduction in the C content and an increase in the O content, and abundant hydrophilic polar oxygen-containing functional groups were also grafted onto the PI film surface. Finally, the interaction mechanism between the PI molecular chains and the microplasma is discussed. The breaking of C-N and C-O bonds and the grafting of OH radicals were the key pathways to dominate the reaction process.

Scalable in-situ surface-coated polymer dielectrics with significantly enhanced high-temperature breakdown strength

Wide bandgap inorganic layers exhibit the effect to improve the insulating properties as charge barrier layers between the electrode and polymer dielectrics. However, their influence mechanisms on the high-temperature breakdown behavior still need to be further revealed. Herein, the in-situ grown strontium oxide (SrO) layers were coated on the surfaces of polyimide (PI) film to conduct this research. Compared with neat PI, the breakdown strength and discharge energy density (efficiency >90%) of the coated film was significantly elevated up 45% and 1000% at 200 °C, respectively. The experimental results show that SrO greatly reduces the entropy of surface molecular conformations and increases the energy level of localized states of PI film. The SrO coating effectively improves the injection barrier and interface trap levels simultaneously, undertakes a higher electric field, and hinders the destruction of PI film's surface molecules by high-energy electrons, thus reducing the conduction loss and constraining the generation and development of electrical trees at high fields. This work not only provides a comprehensive insight into the in-situ oxide coating on the insulating properties in polymer dielectrics but also offers a paradigm for developing scalable dielectric materials with high breakdown strength used in harsh-condition electrification.

Plasma-Modified PI Substrate for Highly Reliable Laser-Sintered Copper Films Using Cu2O Nanoparticles.

Plasma modification of polyimide (PI) substrates upon which electrical circuits are fabricated by the laser sintering of cuprous oxide nanoparticle pastes was investigated systematically in this study. Surface properties of the PI substrate were investigated by carrying out atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Experimental results show that surface characteristics of PI substrates, including surface energy, surface roughness, and surface binding significantly affected the mechanical reliability of the sintered copper structure. Among the plasma gases tested (air, O2, Ar-5%H2, and N2-30%H2), O2 plasma caused the roughest PI surface as well as the most C=O and C–OH surface binding resulting in an increased polar component of the surface energy. The combination of all those factors caused superior bending fatigue resistance.

Porous silica coating with excellent atomic oxygen protection performance and flexibility

With the rapid development of spacecrafts in the low earth orbit (LEO), the protection requirements of polyimide (PI) surface against atomic oxygen (AO) erosion had been highlighted. Inorganic coatings trended to crack and peel off due to stress, which limited their application on flexible polymer surfaces. Different from the conventional idea of using dense inorganic coatings to develop AO resistance, in this work, APTES ((3-aminopropyl) triethoxysilane) -modified silica coatings with porous structure have successfully removed the influence of stress and increased the adsorption of AO and adhesion. As a result, the porous silica coating exhibited excellent AO protection performance (low erosion of 1.18 × 10−26 cm3 atom−1), good flexibility (no damage or crack on the surface after bending), and enhanced PI substrate-adhesion. The results of first-principles calculations and experiments indicate that the structure with large surface area and small pore volume is able to adsorb AO and reduce the erosion. This study proposed a new solution of adopting non-dense structure and a novel AO protection mechanism of porous coatings, which expands the application of inorganic coatings for AO protection.

Controlling the triboelectric properties and tribological behavior of polyimide materials via plasma treatment

Surface charge density greatly influences triboelectric properties and tribological behavior. Given the limitations of current techniques for regulating surface charge, a novel plasma-irradiation-based surface treatment was used to enhance the triboelectric properties and optimize the tribological behavior of polyimide (PI) films. An in-situ two-step Ar+O2 reactive ion etching (RIE) plasma treatment was used to etch the surface of the PI films to enhance the triboelectric performance of PI-PA11-based triboelectric nanogenerators (TENGs). The generation of granular nanostructures on the PI film surface enhanced the effective contact area between triboelectric layers. The C-O and CO content was also greatly increased, improving the electron affinity of the PI films. At the same time, to obtain a fully antistatic PI surface in a PI-PVDF-based TENG, the optimal irradiation time was calculated. In addition, to control the tribological behavior, the surface charge of PI film was successfully eliminated by a one-step Ar plasma irradiation, and the regulation of the coefficient of friction was realized, which significantly improved the friction stability and wear resistance of the material. This new approach can find wide applications in energy harvesting, industrial production, electronic circuit, and other applications.

Research on Surface Metallization of Polyimide Film

In order to make the Polyimide (PI) film material have good surface conductivity and meet its application in aerospace fields, for example, radar antenna, in this paper, the highly chemically inert PI material on the surface was modified by alkaline etching, and the metal layer on the surface of PI film was then deposited by electroless copper plating technology, to give PI good surface conductivity and meet its application in aerospace fields, for example, radar antenna. The microstructure and properties of surface metal layer of PI film before and after surface modification were characterized by SEM, contact angle tester and resistance tester. After alkaline etching at room temperature, there is a staggered and evenly distributed protrusion structure with dendritic and rivet structures on the surface of PI. However, the PI film surface etched at 60 °C presents pits with different sizes and depths of etching holes, and the hydrophilicity of the PI film surface is enhanced after etching. The coating on the PI is uniform and dense, and has good conductivity. This research realizes the preparation of high conductivity and high bonding force metal layer on the surface of PI film under alkaline etching, which provides technical support for the application of polyimide in aerospace fields.

Efficacy of hierarchical pore structure in enhancing the tribological and recyclable smart lubrication performance of porous polyimide

Herein, a porous oil-containing material with hierarchical pore structure was successfully prepared through microtexturing large pores on the surface of porous polyimide (PPI) with single-level small pores. Compared to the conventional oil-containing material, the hierarchically porous oil-containing material exhibited high oil-content, and retained excellent mechanical properties and high oil-retention because of the synergistic effects of large pores and small pores. Furthermore, the lubricant stored in the hierarchically porous polyimide could release to the interface under thermal-and-mechano-stimuli, and the released lubricant could be reabsorbed into the hierarchically porous polyimide via the capillary-force offered by the porous channel. Based on the high oil-content and recyclable oil release/reabsorption, the hierarchically porous oil-containing polyimide exhibited excellent lubrication performance (coefficient of friction was 0.057). Furthermore, the composite could perform 1,069 cycles of smart lubrication (1 h per cycle), which significantly extended the service life of the hierarchically porous oil-containing smart lubrication material.

Flexible Copper Metal Circuits via Desktop Laser Printed Masks

AbstractA novel process is demonstrated that produces patterns of electrically conductive copper on a flexible polyimide film substrate using standard desktop laser printed toner and near room temperature aqueous chemistry. The laser toner acts as a mask to selectively block the ion exchange self‐metallization (IESM) reduction reaction that forms nanoscale silver or palladium coatings at the polyimide surface. The silver or palladium IESM coating is then used as a catalyst for electroless deposition of copper. Under appropriate conditions, the copper is deposited selectively on top of the catalyst layer. The resulting copper layer has a measured sheet resistance as low as 0.3 Ohms/sq. Electrical isolation is measured between copper traces spaced as close as 300 microns, and high conductivity is measured along traces with widths as low as 200 microns. The minimum pattern size appears to be limited primarily by the resolution of the laser toner pattern, as the IESM metal layer is observed to follow the contours of individual toner particles. The process avoids the use of high temperature, vacuum, and organic solvents and is thus suitable for very low cost prototyping or distributed manufacturing of simple electronic devices.

Ultraviolet nanosecond laser ablation of polyimide with thermal and nonthermal effects near threshold fluence

Typically, a 355 nm laser is not sufficient for inducing photochemical ablation and leads to photothermal effects such as melting and recasting. Consequently, most previous studies on the irradiation of polyimide (PI) with such lasers have focused on temperature distribution on the surface and variations in the absorption properties of PI depending on the temperature and time. Herein, the authors demonstrate ablation without thermal effects on the surface of PI using a 355 nm pulse nanosecond laser near the threshold fluence. For ablation without thermal effects, the ablated and nonablated areas can be clearly distinguished in the area exposed to the laser. The interface between these areas, that is, the ablation shape, became wider with increasing laser fluence, even when the spot size remained unchanged; also, the shape of the ablated area was related to the laser intensity distribution. Ablation with thermal effects began to appear in the nonablated areas at higher laser fluence. This study shows that PI, which is physically stable and relatively inexpensive, can be used to develop cation exchange membranes, which can be applied to organic flow cells as well as microbial fuel cells.

Roll-to-roll fabrication of large-scale polyorgansiloxane thin film with high flexibility and ultra-efficient atomic oxygen resistance

One of the most widely used and well-established atomic oxygen (AO) protection solutions for low Earth orbit (LEO) satellites is the deposition of protective coatings on polymeric materials. However, manufacturing extensive expanses of these coating materials with good transparency, flexibility, smoothness, ultra-thinness, and exceptional AO resistance remains a critical issue. Herein, we successfully deposited a 400 nm thick polyorgansiloxane (SiO x C y H z ) coating with high optical transparency and uniform good adherence on to a 1.2 m wide polyimide surface, by optimizing the distribution of hexamethyldisiloxane and oxygen as precursors in the roll-to-roll compatible plasma-enhanced chemical vapor deposition process. After AO irradiation with the fluence of 7.9 × 1020 atoms·cm–2, the erosion yield of the SiO x C y H z -coated Kapton was less than 2.30 × 10–26 cm3·atom–1, which was less than 0.77% of that of the Kapton. It indicates that the SiO x C y H z coating can well prevent the erosion of Kapton by AO. In addition, it was also clarified that a SiO2 passivation layer was formed on the surface of the SiO x C y H z coating during AO irradiation, which exhibited a ‘self-reinforcing’ defense mechanism. The entire preparation process of the SiO x C y H z coating was highly efficient and low-cost, and it has shown great potential for applications in LEO.

A Soft Zwitterionic Hydrogel as Potential Coating on a Polyimide Surface to Reduce Foreign Body Reaction to Intraneural Electrodes.

Invasive intraneural electrodes can control advanced neural-interfaced prostheses in human amputees. Nevertheless, in chronic implants, the progressive formation of a fibrotic capsule can gradually isolate the electrode surface from the surrounding tissue leading to loss of functionality. This is due to a nonspecific inflammatory response called foreign-body reaction (FBR). The commonly used poly(ethylene glycol) (PEG)-based low-fouling coatings of implantable devices can be easily encapsulated and are susceptible to oxidative damage in long-term in vivo applications. Recently, sulfobetaine-based zwitterionic hydrogels have emerged as an important class of robust ultra-low fouling biomaterials, holding great potential to mitigate FBR. The aim of this proof-of-principle in vitro work was to assess whether the organic zwitterionic—poly(sulfobetaine methacrylate) [poly(SBMA)]—hydrogel could be a suitable coating for Polyimide (PI)-based intraneural electrodes to reduce FBR. We first synthesized and analyzed the hydrogel through a mechanical characterization (i.e., Young’s modulus). Then, we demonstrated reduced adhesion and activation of fibrogenic and pro-inflammatory cells (i.e., human myofibroblasts and macrophages) on the hydrogel compared with PEG-coated and polystyrene surfaces using cell viability assays, confocal fluorescence microscopy and high-content analysis of oxidative stress production. Interestingly, we successfully coated PI surfaces with a thin film of the hydrogel through covalent bond and demonstrated its high hydrophilicity via water contact angle measurement. Importantly, we showed the long-term release of an anti-fibrotic drug (i.e., Everolimus) from the hydrogel. Because of the low stiffness, biocompatibility, high hydration and ultra-low fouling characteristics, our zwitterionic hydrogel could be envisioned as long-term diffusion-based delivery system for slow and controlled anti-inflammatory and anti-fibrotic drug release in vivo.

Mass-Producible 2D Nanocomposite-Based Temperature-Independent All-Printed Relative Humidity Sensor.

Relative humidity sensors are widely studied under the categories of both environmental and biosensors owing to their vast reaching applications. The research on humidity sensors is mainly divided into two concentration areas including novel material development and novel device structure. Another approach focuses on the development of printed sensors with performance comparable to the sensors fabricated via conventional techniques. The major challenges in the research on relative humidity sensors include the range of detection, sensitivity (especially at lower %RH), transient response time, and dependence on temperature. Temperature dependence is one of the least studied parameters in relative humidity sensor development. In this work, relative humidity sensors were fabricated using all-printed approaches that are also compatible with mass production, resulting in low cost and easy development. Laser-induced graphene (LIG)-based printed electrodes were used as the transducers, while the 2D MoS2 and graphene nanocomposite was used as the active layer material with the built-in property of temperature independence. The exfoliation process of 2D MoS2 was based on wet grinding, while graphene for the active layer was obtained by scratching the graphene grown on the polyimide (PI) surface via laser ablation. The resulting sensors showed an excellent output response for a full range of 0%RH to 100%RH, having no dependence on the surrounding temperature, and excellent response and recovery times of 4 and 2 s, respectively. The developed sensors can be confidently employed for a wide range of humidity sensing applications where the temperature of the surrounding environment is not constant.

Polysaccharide Layer-by-Layer Coating for Polyimide-Based Neural Interfaces.

Implantable flexible neural interfaces (IfNIs) are capable of directly modulating signals of the central and peripheral nervous system by stimulating or recording the action potential. Despite outstanding results in acute experiments on animals and humans, their long-term biocompatibility is hampered by the effects of foreign body reactions that worsen electrical performance and cause tissue damage. We report on the fabrication of a polysaccharide nanostructured thin film as a coating of polyimide (PI)-based IfNIs. The layer-by-layer technique was used to coat the PI surface due to its versatility and ease of manufacturing. Two different LbL deposition techniques were tested and compared: dip coating and spin coating. Morphological and physiochemical characterization showed the presence of a very smooth and nanostructured thin film coating on the PI surface that remarkably enhanced surface hydrophilicity with respect to the bare PI surface for both the deposition techniques. However, spin coating offered more control over the fabrication properties, with the possibility to tune the coating’s physiochemical and morphological properties. Overall, the proposed coating strategies allowed the deposition of a biocompatible nanostructured film onto the PI surface and could represent a valid tool to enhance long-term IfNI biocompatibility by improving tissue/electrode integration.

Laser Direct Activation of Polyimide for Selective Electroless Plating of Flexible Conductive Patterns

Flexible conductive patterns on polyimide substrates are attracting increasing research interest in the areas of wearables, biomedicals, automotives, and energy harvesters. This paper reports a laser-induced selective activation (LISA) process for electroless metallization and, therefore, the creation of complex conductive patterns on polyimide substrates. In this process, a Q-switched pulsed laser is utilized for scanning the surface and forming the catalytic layer consisting of microporous structures, which enhances the stability of electroless plated metallic structures on the polyimide surface and exhibits superior mechanical stability under repeated bending and harsh environments. The high resolution of the metallic patterning enables the demonstration of a copper microgrid pattern with a line width down to 50 μm. Moreover, flexible light-emitting diode displays and electromagnetic interference shielding films are successfully manufactured to indicate the promising capability of our LISA process.

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.