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The detection and management of humidity plays a vital role in human life and the operation of electronic products. Humidity sensors are used in various fields, including atmospheric and soil treatment, industrial manufacturing, agricultural production, and physiological health management. In addition, the human body continuously loses water through explicit and implicit sweating. As a biomarker, the ubiquitous humidity in the human body provides support for the development of non-contact sensor networks for emerging wearable digital medical technologies. Therefore, the creation of high-performance humidity sensors designed for non-contact and respiratory monitoring has attracted great attention. Cellulose is considered an ideal candidate for green humidity sensors because of the abundant hydrophilic groups on its molecular chain, which gives it good water absorption and swelling properties. Due to the flexibility and versatility of cellulose in material design, cellulose-based sensors can be used to achieve sensing through a variety of pathways, including resistance, capacitance, colorimetry, gravity, microwave, etc. Among them, resistance sensors are considered to have the most development potential due to their simple structure, convenient reading, and high accuracy. However, since cellulose is not conductive, many conductive materials, including CNT, PPY, PANI, Ag, RGO, NaCl, KOH, etc., are used to composite with cellulose to improve the conductivity of cellulose, and its conductive mechanism is divided into electronic conductivity and ionic conductivity.
The main point of this paper
Coaxial electrospinning device:
Similar to the conventional electrospinning device, but the spinneret is modified by embedding a more compact inner tube mounted parallel to the outer tube, forming a coaxial arrangement
Solution feed control:
The solution feed can be controlled by a metering pump or air pressure. Unlike previous studies where the shell liquid is exposed to external pressure, the coaxial spinning method requires a heating system covering the reservoir
Coaxial electrospinning process:
The study by Wang et al. eliminated the need to embed the inner capillary into the outer capillary, using a double reservoir with tubes of different volumes, with the smaller tube embedded in the outer Taylor cone when extracting the larger capillary
Charge accumulation and jet formation:
The high-pressure polymer solution is charged by single-tube electrostatic spinning, and the accumulated charge mainly occurs in the outer layer of the sheath liquid, coming from the coaxial outer capillary. The charge repulsion increases the droplet area and changes the shape into a cone. When the charge accumulates to a certain extent, a jet diffuses outward from the cone.
Shearing of the core liquid and cone formation:
The pressure generated in the sheath is the basic reason for the shearing of the core solution through contact friction and viscous drag. The core liquid is twisted into a cone and a coaxial jet is generated on the tip of the needle.
Preparation of nanofibers:
As long as the cone is well balanced, the core will be evenly integrated into the sheath to achieve the development of core/shell materials. Traditional electrospinning systems use only one fluid. When the jet flows to the collector, the jet will experience bending instability and play a role of bidirectional oscillation. At this time, both the core and shell solvents evaporate, thereby obtaining the desired core nanofibers.
Technical advantages:
Coaxial electrospinning technology can prepare nanofibers with core-shell structure, which is not possible in traditional single-nozzle electrospinning. This technology can break through the limitations of the single-nozzle system and prepare composite materials with a variety of functional structures, greatly expanding the scope of application
Preparation of functional and hollow fibers:
Coaxial electrospinning technology can be used to prepare functional and hollow fibers from non-spinnable precursors. By removing the core layer material by heating or dissolving, a hollow fiber structure can be obtained
Drug degradability problem:
A common disadvantage observed in single-nozzle electrospinning is the problem of drug degradability, which can be avoided by coaxial electrospinning
Application in the biomedical field:
Coaxial electrospinning technology has a wide range of applications in the biomedical field, including wound dressings, periodontal regeneration, neural tissue engineering, bone tissue engineering, dual drug delivery, cancer treatment, etc.
Technical comparison:
The comparison between coaxial electrospinning and single-nozzle conventional electrospinning has been established and supported by the latest literature. Coaxial electrospinning provides a more efficient and sustainable approach to develop functional core-sheath nanofibers
Design principles and applications:
Coaxial electrospinning has been intensively studied for its design principles, comprehensive characterization methods, mechanistic insights, and thriving applications in the field of solar cells. This technique is known for its unique ability to produce core-shell nanofiber structures
This paper reviews the progress in the synthesis of nanofibers by coaxial electrospinning. This synthesis process is of great significance for the preparation of nanofibers with advanced structural features. This is because coaxial electrospinning can produce nanofibers exclusively from precursors that are usually considered "unspinnable". This process can enable society to fully exploit all possible uses of nanoscale fibers. It provides a simple technology for the production of hollow fibers and/or the encapsulation of functional compounds such as drugs, self-healing substances and nanoparticles into durable textiles. When drugs are encapsulated by coaxial electrospinning, the synthesized fibers have a sustained release effect. This is preferable to the burst release that is usually obtained by combining drugs with single-nozzle electrospinning alone. Coaxial electrospun fibers containing self-healing agents release healing compounds when needed, extending the service life of structural components and preventing surface corrosion. Core-sheath electrospun nanofibers play an important role in the biomedical field, tissue engineering, wound dressings, drug delivery, periodontal regeneration, cancer treatment, bone tissue engineering and neural tissue engineering.
