Design of asymmetric-adhesion lignin reinforced hydrogels with anti-interference for strain sensing and moist air induced electricity generator

A bio-based Janus hydrogel from cellulose and lignin with bilayer structure and asymmetric adhesion for accurate and sensitive human motion monitoring

Janus hydrogels for human motion monitoring are thriving due to their conductivity, flexibility, anisotropy and self-adhesion, etc. However, most of them face challenges such as complex processes, interlayer detachment, and surface contamination, which degrade their sensing accuracy and sensitivity. Hence, this study proposes a facile strategy using the cellulose and lignin as building blocks to construct a Janus hydrogel for accurate and sensitive sensing. The proposed process involves sequential pouring and in-situ thermal-induced polymerization. Initially, the cellulose dissolved in ZnCl2 solution, along with acrylic acid (Cel/ZnCl2-AA), forms the antifouling and conductive precursors in the top layer, while lignosulfonate and acrylamide (LS-AM) function as the adhesive precursor in the bottom layer. The viscosity difference of the two precursors allows local diffusion and polymerization at the interlayer interface, thereby developing a bilayer structure with strong interface bonds. Consequently, the Janus hydrogel exhibits high conductivity (1.2 S/m), excellent asymmetric adhesion, good stretchability (520 %) and compressive strain (∼70 %). These properties enable the hydrogel to accurately monitor both large (elbow and wrist bending) and small (swallowing and frowning) human movements with accurate sensitivity (gauge factor up to 2). This work offers new insight and synthesis strategy to design the bilayer hydrogels for practical applications in electronic skin and flexible sensing.

A facile strategy to prepare fibrous and water resistant moist-electric generator with adjustable response speed

This work presents a polyvinyl alcohol (PVA) and cellulose nanofiber (CNF) based fiber structure moist-electric generator (FMEG), which demonstrates enhanced suitability for smart wearable electronics compared to traditional MEGs. The resulting PVA FMEG generated a relatively high output voltage of 0.50 V and a current of 4 μA per 2 cm fiber. The improved performance stems from the efficient directional ion movement enabled by the fiber structure (from outer to inner layers). Water resistance and response speed of FMEG were further improved by crosslinking PVA with boric acid (BA) and the introduction of CNF. After five washing cycles, the crosslinked FMEG retained 86 % of its initial weight. Additionally, the response time (time to reach 0.40 V) of the CNF-enhanced FMEG was reduced to 140 s, significantly shorter than that of pure PVA FMEG (600 s) and BA-crosslinked FMEG (1300 s). By tuning the crosslinking degree and CNF content, the response speed could be precisely regulated for applications such as breath sensing or powering a red LED bulb. This study demonstrates a promising FMEG with high output, water resistance, and tunable sensitivity, offering superior applicability for smart wearable devices compared to conventional MEGs.

A novel self-ratiometric fluorescent sensor of sodium alginate hydrogel bead doping with coumarin derivative with extremely acidic pH visual monitoring of fruit juice freshness

The slight acidic pH changes during food storage and transportation are often difficult to detect in a timely and accurate manner. There is also an urgent need to establish a portable, accurate, and low interference detection method for real-time monitoring of the quality changes of fresh juice that already has its own color. In this paper, a self-ratiometric fluorescent sensor (DCCA/SA bead) was prepared by combining a modified coumarin and sodium alginate (SA), and was used for the extreme pH monitoring of fluorescent by naked eye colorimetry. The derivative of natural active ingredient coumarin (DCCA) exhibited excellent self-fluorescence properties in the ratio of orange red (630 nm) to blue-green (480 nm) under the excitation, and exhibited good linear changes at extreme acid pH range (2.5-5.0), biocompatibility high sensitivity, and anti-interference ability. Based on the multiple interactions between DCCA and SA, DCCA/SA bead could be used for the monitoring small pH changes of fruit juice and storage stage in a fast response time. In addition, analyzing the RGB values of DCCA/SA bead images under ultraviolet light also successfully quantitatively determined changes in pH values. These results indicated that the self-ratiometric fluorescent hydrogel sensor had great user convenience and point-of-care testing (POCT) to monitor extreme acidic pH for freshness of foods.

Highly adhesive conductive hydrogels fabricated by catechol lignin/liquid metal-initiated polymerization of acrylic acid for strain sensors

Conductive hydrogels have emerged as promising candidates for next-generation flexible electronics owing to their unique combination of electrical conductivity and mechanical compliance. However, the development of an eco-friendly and efficient polymerization strategy to simultaneously achieve robust adhesion and superior functionality remains a challenge. In the work, catechol lignin (DAL)/liquid metal (LM) were utilized as initiators for the polymerization of acrylic monomers (PAA), resulting in the preparation of conductive hydrogels (PAA-DAL-LM). The engineered DAL component serves dual functions of establishing an interfacial stabilization layer for LM nanoparticles while participating in radical generation for polymerization initiation, and this synthesis protocol eliminates conventional toxic initiators through LM-mediated radical generation mechanisms. The resultant PAA-DAL-1.6 %LM hydrogel demonstrated remarkable performance characteristics, including exceptional compressive strength (688.5 KPa), good self-healing properties, and high electrical conductivity (0.24 S/m). Structural modification of alkali lignin through catechol incorporation significantly improved both the water solubility and interfacial adhesion strength (16.23 KPa). Systematic characterization revealed stable strain-responsive electrical behavior with high strain sensing accuracy as well as stable electrical output. These multifunctional hydrogels not only hold significant potential for advancing flexible sensor technologies but also pave the way for sustainable valorization of lignin biopolymers in advanced material applications.

Lignin-enabled ultra-stretchable eutectic gels for multifunctional sensors

Eutectic gels as important conductive polymers have promising practical applications in wearable electronic devices. However, the development of the ultra-stretchable and self-adhesive eutectic gel for multifunctional flexible sensors remains a challenge. Here, a lignin-enabled ultra-stretchable eutectic gel (LEG) integrating with excellent self-adhesion and high conductivity is prepared through polymerizable deep eutectic solvents (PDES) treated lignin followed by in-situ polymerization. In this LEG, the lignin macromolecules are utilized as important mediators to build dynamic crosslinking points in the polyacrylic acid (PAA) networks via hydrogen bond interactions. The dynamic disruption and reconstruction of the hydrogen bonds between the mobile PAA chain and dynamic crosslinking points ensure the high integrity of the crosslinking network to realize the ultra-stretchability (about 4845 %). Additionally, the abundant phenol groups of lignin endow the LEG with robust self-adhesion, which allows the LEG to seamlessly adhere to the different substrates. Based on these features, the LEGs are assembled as wearable strain sensors with high sensitivity, fast response time, and long-term sensing stability, and this wearable strain sensor demonstrates promising applications in human motion monitoring and information encryption systems. This work develops an effective pathway to design lignin-enabled ultra-stretchable eutectic gels for multifunctional sensors.

Recent advances in moisture-induced electricity generation based on wood lignocellulose: Preparation, properties, and applications

Moisture-induced electricity generation (MEG), which can directly harvest electricity from moisture, is considered as an effective strategy for alleviating the growing energy crisis. Recently, tremendous efforts have been devoted to developing MEG active materials from wood lignocellulose (WLC) due to its excellent properties including environmental friendliness, sustainability, and biodegradability. This review comprehensively summarizes the recent advances in MEG based on WLC (wood, cellulose, lignin, and woody biochar), covering its principles, preparation, performances, and applications. In detail, the basic working mechanisms of MEG are discussed, and the natural features of WLC and their significant advantages in the fabrication of MEG active materials are emphasized. Furthermore, the recent advances in WLC-based MEG for harvesting electrical energy from moisture are specifically discussed, together with their potential applications (sensors and power sources). Finally, the main challenges of current WLC-based MEG are presented, as well as the potential solutions or directions to develop highly efficient MEG from WLC.

Lignin-induced rapid polymerization of asymmetrical adhesion Janus gel for strain sensor

Functional hydrogel sensors have shown explosive growth in the health and medical fields. However, the uniform adhesion and the complicated polymerization process of hydrogels seriously hinder their further development. Herein, inspired by the layered structure of human skin, we prepare a Janus gel using in-situ polymerization. Based on the lignin-Fe3+ dual catalytic system, the rapid polymerization of the gel was achieved at room temperature. By tailoring the mass ratio of lignin and Fe3+ in the precursor, the adhesion of the upper and bottom layers can be easily adjusted. In addition, hydrophobic association is introduced into the upper layer to improve the gel's mechanical properties. The obtained asymmetric bilayer gel has a significant difference in adhesion (7 times), and exhibits excellent mechanical properties in the elongation at break (1437 %) and the breaking strength (463.2 kPa). Moreover, the bilayer gel also has good freezing and UV resistance. We use the bilayer gel as a wearable strain sensor, which shows a wide strain detection range of 0-800 % (maximum gauge factor = 5.3). The proposed simple strategy avoids UV irradiation and heating processes, which provides a new idea for the rapid polymerization of multifunctional Janus hydrogels with adjustable performances.

Mussel-Bioinspired Lignin Adhesive for Wearable Bioelectrodes

As a natural "binder," lignin fixes cellulose in plants to foster growth and longevity. However, isolated lignin has a poor binding ability, which limits its biomedical applications. In this study, inspired by mussel adhesive proteins, acidic/basic amino acids (AAs) are introduced in alkali lignin (AL) to form ionic-π/spatial correlation interactions, followed by demethylation to create catechol residues for enhanced adhesion activity. Atomic force microscopy reveals that catechol residues are the primary adhesion structures, with basic AAs exhibiting superior synergistic effects compared to acidic AAs. Demethylated lysine-grafted AL exhibits the strongest adhesion force toward skin tissue. Molecular dynamic simulation and density functional theory calculations indicate that adhesion against skin tissue mainly results from hydrogen bonds and cation-π interactions, with the adhesion mechanism being based on the Gibbs free energy of the Schiff base reaction. In summary, a biomimetic electrode based on lignin inspired by mussel adhesive proteins is prepared; the presented method offers a straightforward strategy for the development of biomimetic adhesives. Furthermore, this mussel-inspired adhesive can be used as a wearable bioelectrode in biomedical applications.

Development of lignin hydrogel reinforced polypyrrole rich electrode material for supercapacitor and sensing applications

The preparation of natural polymer-based highly conductive hydrogels with reliable durability for applications in supercapacitors (SCs) is still challenging. Herein, a facile method to prepare alkaline lignin (AL)-based polypyrrole (PPy)-rich, high-conductive PPy@AL/PEGDGE gel was reported, where AL was used as a dopant, polyethylene glycol diglycidyl ether (PEGDGE) as a cross-linking agent, and PPy as a conducting polymer. The PPy@AL/PEGDGE gel electrode materials with hollow structures were prepared by electrochemical deposition and chemical etching method and then assembled into sandwich-shaped SCs. Cyclic voltammetry (CV), galvanotactic charge discharge (GCD), electrochemical impedance spectroscopy (EIS) and cycling stability tests of the PPy@AL/PEGDGE SCs were performed. The results demonstrated that the SCs can achieve a conductivity of 25.9 S·m-1 and a specific capacitance of 175 F·g-1, which was 127.4 % higher compared to pure PPy (77 F·g-1) electrode. The highest energy density and power density for the SCs were obtained at 23.06 Wh·kg-1 and 5376 W·kg-1, respectively. In addition, the cycling performance was also higher than that of pure PPy assembled SCs (50 %), and the capacitance retention rate can reach 72.3 % after 1000 cycles. The electrode materials are expected to be used as sensor and SCs devices.

Lignin reinforced tough, adhesive, and recoverable protein organohydrogels for wearable strain sensing under sub-zero temperatures

Protein-based hydrogels with promising biocompatibility and biodegradability have attracted considerable interest in areas of epidermal sensing, whereas, which are still difficult to synchronously possess high mechanical strength, self-adhesion, and recoverability. Hence, the bio-polymer lignosulfonate-reinforced gluten organohydrogels (GOHLx) are fabricated through green and simple food-making processes and the following solvent exchange with glycerol/water binary solution. Ascribing to the uniform distribution of lignosulfonate in gluten networks, as well as the noncovalent interactions (e.g., H-bond) between them, the resultant GOHLx exhibit favorable conductivity (∼14.3 × 10-4 S m-1), toughness (∼711.0 kJ m-3), self-adhesion (a maximal lap-shear strength of ∼33.5 kPa), high sensitivity (GF up to ∼3.04), and durability (∼3000 cycles) toward shape deformation, which are suitable for the detection of both drastic (e.g., elbow and wrist bending) and subtle (e.g., swallowing and speaking) human movements even under -20 °C. Furthermore, the GOHLx is also biocompatible, degradable, and recoverable (by a simple kneading process). Thus, this work may pave a simple, green, and cheap way to prepare all-biomass-based, tough, sticky, and recoverable protein-based organohydrogels for epidermal strain sensing even in harsh environments.

Lignin derivatives-based hydrogels for biomedical applications

Recently, numerous studies have been conducted on renewable polymers derived from different natural sources, exploring their suitability for diverse biomedical applications. Lignin as one of the main components of lignocellulosic has garnered significant attention as a promising alternative to petroleum-based polymers. This interest is primarily due to its cost-effectiveness, biocompatibility, eco-friendly nature, as well as its antioxidant and antimicrobial properties. These characteristics could be more beneficial when incorporating lignin into the formulation of value-added products. Although lignin has a chemical structure that is suitable for various applications, these characteristics require modifications to guarantee that the resultant materials display the desired biological, chemical, and physical properties when applied in the creation of biodegradable hydrogels, particularly for biomedical purposes. This study delineates the recent modification approaches that have been employed in the creation of lignin-based hydrogels. These strategies encompass both chemical and physical interactions with other polymers. Additionally, this review encompasses an examination of the current applications of lignin hydrogels, spanning their use as scaffolds for tissue engineering, carriers for pharmaceuticals, materials for wound dressings and biosensors, and elements in flexible and wearable electronics. Finally, we delve into the challenges and constraints associated with these materials, discuss the necessary steps required to attain the appropriate properties for the development of innovative lignin-based hydrogels, and derive conclusions based on the presented findings.

Novel flexible hydrogels based on carboxymethyl guar gum and polyacrylic acid for ultra-highly sensitive and reliable strain and pressure sensors

To realize on stable and real-time monitoring of human activities, novel hydrogels using polyacrylic acid (PAA) and carboxymethyl guar gum (CMGG) were fabricated as wearable and flexible strain or pressure sensors in the presence of lignosulfonate (LS) and Al3+. Based on the co-existence of metal coordination bonds, hydrogen bonds and ionic interaction in this system, the obtained hydrogels exhibited desirable mechanical properties with good self-recovery ability. The hydrogels displayed good self-adhesion behavior and an ultra-high tensile sensitivity (gauge factor (GF) = 24.30), therefore, they could precisely detect human joints movements such as elbow, wrist, and finger bending as well as tiny movements and external stimuli such as swallowing, smile, frown, pulse, speaking, writing, and even the falling of different liquid drops. Additionally, the hydrogels showed excellent self-healing ability with the healing efficiency as high as 100 % after 30 h. Most importantly, the healed hydrogel could perform the same sensing performance as before. Based on these distinguished characteristics, this hydrogel represents great potentials in wearable and flexible sensors for long-term and stable health monitoring application.

Conductive and Eco-friendly Biomaterials-based Hydrogels for Noninvasive Epidermal Sensors: A Review

As noninvasive wearable electronic devices, epidermal sensors enable continuous, real-time, and remote monitoring of various human physiological parameters. Conductive biomaterials-based hydrogels as sensor matrix materials have good biocompatibility, biodegradability, and efficient stimulus response capabilities and are widely applied in motion monitoring, healthcare, and human-machine interaction. However, biomass hydrogel-based epidermal sensing devices still need excellent mechanical properties, prolonged stability, multifunctionality, and extensive practicality. Therefore, this paper reviews the common biomass hydrogel materials for epidermal sensing (proteins, polysaccharides, polyphenols, etc.) and the various types of noninvasive sensing devices (strain/pressure sensors, temperature sensors, glucose sensors, electrocardiograms, etc.). Moreover, this review focuses on the strategies of scholars to enhance sensor properties, such as strength, conductivity, stability, adhesion, and self-healing ability. This work will guide the preparation and optimization of high-performance biomaterials-based hydrogel epidermal sensors.

Design of cationic surfactant reinforced carrageenan waterproof composite films and applied as water induced electricity generator

Carrageenan (CR) is a renewable polysaccharide material for packaging application due to its good film-forming property, but its use can be limited by the water solubility. In this research, CR hydrogels were modified by quaternary ammonium surfactants with different length of hydrocarbon tails (n, 8≦n≦16) by adsorption method and waterproof films were obtained after drying. The composition and charge interaction of composite films was confirmed by FTIR. Both thermogravimetric analysis and energy dispersive spectrometer indicated that the surfactant ions replaced K+ to form complexes with CR. The X-ray diffraction revealed the decreased amorphous nature of composite films compared to neat CR film. Water-related physical properties, such as water content, weight percentage change after contact with water, water vapor transmission, and water contact angle were intimately related to n. When 8≦n≦14, the waterproof properties were enhanced with the increase of n. Meanwhile, the waterproof property of composite film was ascertained by the no leakage result in the boiling water packaging experiment. When n = 16, sandwich structure was found in the sectional micromorphology images, and water bag structure formed after immersed into water. By comparing the mechanical properties of the composite films in different condition, we found that quaternary ammonium surfactants improved significantly the tensile strength in water and increased elongation at break in dry state. The composite films can be used as water induced voltage generator for their polyelectrolyte nature. Benefiting from the high stability of the composite films in water, their water-induced voltage generation process had good recyclability. Due to the antimicrobial activity of the quaternary ammonium salts and the waterproof property, composite films were more stable and degraded more slowly than neat CR film in nature environment.

Janus-type ionic conductive gels based on poly(N,N-dimethyl)acrylamide for strain/pressure sensors

Strain/pressure sensors with high sensitivity and a wide operation range have broad application prospects in wearable medical equipment, human-computer interactions, electronic skin, and so on. In this work, based on the different solubilities of Zr4+ in the aqueous phase and the hydrophobic ionic liquid [BMIM][Tf2N], we used N,N-dimethylacrylamide (DMA) as a vinyl monomer to prepare a Janus-type ionic conductive gel with one-sided adhesion through "one-step" UV irradiation polymerization. The Janus-type gel has satisfactory mechanical properties (tensile strength: 217.06 kPa, elongation at break: 1121.01%), electrical conductivity (conductivity: 0.10 S m-1), one-sided adhesion (adhesion strength to glass: 72.35 kPa) and antibacterial properties. The sensor based on the Janus gel can be used not only for real-time monitoring of strain changes caused by various movements of the human body (such as finger bending, muscle contraction, smiling, and swallowing) but also for real-time monitoring of pressure changes (such as pressing, water droplets, and writing movements). Therefore, based on the simplicity of this method for constructing Janus-type ionic conductive gels and the excellent electromechanical properties of the prepared gel, we believe that the method provided in this study has broad application prospects in the field of multifunctional wearable sensors.

A self-healing magneto-responsive nanocellulose ferrogel and flexible soft strain sensor

Crosslinks are the building blocks of hydrogels and play an important role in their overall properties. They may either be reversible and dynamic allowing for autonomous self-healing properties, or permanent and static resulting in robustness and mechanical strength. Hence, a combination of crosslinks is often required to engineer the 3D network of hydrogels with both autonomous self-healing and required robustness for strain sensing application; however, this complicates the fabrication of such hydrogels. The facile, yet versatile, approach used in this study is to forgo the use of extra crosslinks and instead rely solely on the properties of magnetic nanocellulose to fabricate a tough, stretchy, yet magneto-responsive, ionic conductive ferrogel for strain sensing. The ferrogel also gives stimuli-free and autonomous self-healing capacity, as well as the ability to monitor real-time strain under external magnetic actuation. The ferrogel also functions as a touch-screen pen. Based on our findings, this study has the potential to advance the rational design of multifunctional hydrogels, with applications in soft and flexible strain sensors, health monitoring and soft robotics.

Multifunctional conductive hydrogels based on the alkali lignin-Fe3+-mediated Fenton reaction for bioelectronics

Requirements for sustainable development have led to the urgent need for low cost, green, and reproducible resources. Lignin is one of the resources meeting this requirement. Herein, an alkali lignin (AL)-Fe³⁺-H₂O₂ autocatalytic system was introduced to assemble multifunctional AL-Fe³⁺/polyacrylic acid (PAA) hydrogels. The AL-Fe³⁺ pair-mediated Fenton reaction can generate a large number of free radicals to accelerate gelation. Owing to the abundant hydrogen bonds and metal coordination bonds, the AL-Fe³⁺/PAA hydrogels possessed excellent mechanical properties (tensile strength of 38 kPa), adhesion properties (18 kPa for pigskin), and self-healing ability (78 % for tensile strength and 88 % for tensile modulus). In addition, hydrogel-based sensors with high durability, strain sensitivity, and fast response times were employed to accurately monitor motion or electrophysiological signals. Subsequently, a portable sensing device for the wireless and remote monitoring of a user's motion status was integrated. As a result, an AL-Fe³⁺-H₂O₂ autocatalytic system has great potential for use in hydrogel preparation in flexible bioelectronics and wearable sensors. It can promote the sustainable development of flexible bioelectronics.

Starch-g-Acrylic Acid/Magnetic Nanochitin Self-Healing Ferrogels as Flexible Soft Strain Sensors

Mechanically robust ferrogels with high self-healing ability might change the design of soft materials used in strain sensing. Herein, a robust, stretchable, magneto-responsive, notch insensitive, ionic conductive nanochitin ferrogel was fabricated with both autonomous self-healing and needed resilience for strain sensing application without the need for additional irreversible static chemical crosslinks. For this purpose, ferric (III) chloride hexahydrate and ferrous (II) chloride as the iron source were initially co-precipitated to create magnetic nanochitin and the co-precipitation was confirmed by FTIR and microscopic images. After that, the ferrogels were fabricated by graft copolymerisation of acrylic acid-g-starch with a monomer/starch weight ratio of 1.5. Ammonium persulfate and magnetic nanochitin were employed as the initiator and crosslinking/nano-reinforcing agents, respectively. The ensuing magnetic nanochitin ferrogel provided not only the ability to measure strain in real-time under external magnetic actuation but also the ability to heal itself without any external stimulus. The ferrogel may also be used as a stylus for a touch-screen device. Based on our findings, our research has promising implications for the rational design of multifunctional hydrogels, which might be used in applications such as flexible and soft strain sensors, health monitoring, and soft robotics.

Bioinspired Hydrogels as Platforms for Life-Science Applications: Challenges and Opportunities

Hydrogels, as interconnected networks (polymer mesh; physically, chemically, or dynamic crosslinked networks) incorporating a high amount of water, present structural characteristics similar to soft natural tissue. They enable the diffusion of different molecules (ions, drugs, and grow factors) and have the ability to take over the action of external factors. Their nature provides a wide variety of raw materials and inspiration for functional soft matter obtained by complex mechanisms and hierarchical self-assembly. Over the last decade, many studies focused on developing innovative and high-performance materials, with new or improved functions, by mimicking biological structures at different length scales. Hydrogels with natural or synthetic origin can be engineered as bulk materials, micro- or nanoparticles, patches, membranes, supramolecular pathways, bio-inks, etc. The specific features of hydrogels make them suitable for a wide variety of applications, including tissue engineering scaffolds (repair/regeneration), wound healing, drug delivery carriers, bio-inks, soft robotics, sensors, actuators, catalysis, food safety, and hygiene products. This review is focused on recent advances in the field of bioinspired hydrogels that can serve as platforms for life-science applications. A brief outlook on the actual trends and future directions is also presented.

Design of asymmetric-adhesion lignin-reinforced hydrogels based on disulfide bond crosslinking for strain sensing application

Soft and elastic polymer hydrogel materials are booming in the fields of wearable biomimetic skin, sensors, robotics, and bioelectrodes. Currently, many researchers are exploring new chemistries for the preparation of hydrogels to improve their performance. In the present study, we design and develop a strategy to prepare lignin reinforced hydrogels based on disulfide bond crosslinking mechanisms, and resultant hydrogels exhibit excellent stretchability, with tensile strain of up to 1085.4%, and high adhesion (with the highest T-peel strength of up to 432.2 N/m to pigskin). The underlying mechanism is based on the disulfide bonds that act as crosslinkers in the as-prepared hydrogel, and they can be easily cleaved and re-formed under mild conditions. Thanks to the presence of lignin, the as-obtained hydrogels also have excellent UV shielding effect. When assembled into a strain sensor, they can output stable and sensitive sensing signals, with gauge factor (GF) of 2.72 (strain: 0-72.8%). Furthermore, a simple and effective strategy to construct asymmetric adhesive hydrogels was adopted, which is based on directional soaking of the top portion of the hydrogel in a high-concentrated calcium chloride solution. The asymmetric hydrogel strain sensor transmits accurate and stable signals without the interference of various contaminants.