Coaxial electrospinning

Coaxial electrospinning can prepare nanofibers of various morphologies with a core-shell structure, such as ribbon fibers, hollow fibers, tube-sleeved nanofibers, etc. If the inner fibers are removed by extraction or calcination, fibers with a porous structure, a hollow structure, and even a multi-channel structure can be obtained.
Compared with single-component fibers, coaxial fibers have many advantages and have better application prospects. First of all, the preparation of some non-spinable polymer nanofibers is solved. The hard-to-spin and non-spinable polymers are used as the core layer, and the material with good spinning performance is used as the shell layer. The core layer is under the action of the shell solution. Hard-to-spin polymers can be spun into nanofibers, such as polyaniline PANi-polyvinyl alcohol co-spinning; secondly, incompatible two-phase or multi-phase solutions are injected into the coaxial nozzles, which can combine different physical properties and chemistry. Two or more raw materials of the properties produce an integrated fiber, which can be adjusted, modified, reconstructed, doped and functionalized as the core material. These will give the terminal products more abundant applications.
1. Preparation of hollow fibers: Nanofibers with a shell-core structure are prepared by coaxial electrospinning, and then the core layer material is removed by solvent extraction or high-temperature calcination to obtain nanofibers with a hollow structure. The hollow fiber prepared by the electrostatic spinning method has a controllable wall thickness, a lot of suitable materials, and a wide range of applications. Commonly used materials for the inner sacrificial layer include mineral oil, methyl silicone oil, and some materials that can be removed by calcination or solvent extraction.
2. The mutual solubility of the inner and outer layer materials: Some researchers believe that the inner and outer layers are immiscible or have low mutual solubility. For example, the inner and outer layer liquids can form an oil-in-water or water-in-oil structure to form coaxial fibers. However, experiments have shown that the two compatible solutions can still be coaxially spinning. For example, when DMF is used for both the inner and outer layer solvents, coaxial fibers with good internal and external bonding can be formed. If one of the materials is removed by calcination or extraction, it is easy to form coaxial fibers with a porous structure.
3. Production of coaxial nozzle: In order to make the nozzle of the nozzle more conducive to the formation of Taylor cones, according to the experience of relevant users, the length of the inner needle and the outer needle can be set to a certain length difference, such as 1-2mm. According to the needs of use, KONE Micronano can provide different length differences between the inner and outer layers, and the specific values can be specified.
4. The flow rate of the inner and outer layers of liquid: the flow of the inner and outer layers of the solution are controlled by independently connected micro pumps, and the inner and outer layers can be set with different flow rates and flow rates. The flow rate setting is mainly determined according to different solution viscosities. Generally speaking, the flow rate of the outer layer solution should be greater than the flow rate of the inner layer solution to easily form a coating structure.
5. Determination of the molecular weight and concentration of the inner and outer layer polymers: Generally speaking, the outer spinning solution as the driving fluid must have sufficient viscosity to easily drive the flow of the inner spinning solution. The viscous stress must be sufficient to overcome the interfacial tension between the two solutions so that fibers can be formed simultaneously inside and outside. Therefore, the inner polymer should use a polymer with a relatively small molecular weight as much as possible, and use a relatively low concentration; while for the outer polymer, select a polymer with a relatively large molecular weight, and a higher concentration and viscosity can be easily formed. Shaft structure.
6. Common coaxial fiber combinations in the literature: On the basis of the following basic formulas, other functional materials can be added to the inner or outer spinning solution at will to obtain composite materials with different application directions.

Note: The contents of the following table are derived from different literatures and are for reference only.

Inner layer	Solvent	Outer layer	Solvent
PAN	DMF	Methyl silicone oil
PVP	DMF	mineral oil
PAN	DMF	PVP	DMF
PAN	DMF	PMMA	DMF
PAN	DMF	Graphene oxide	DMF
PVA	water	Collagen acetic acid solution	water
PVA	water	PVA+Chitosan	water
PCL	DMF	PVP	Ethanol
PCL		PEO+PVA
PCL		PVDF-HFP Polyvinylidene fluoride-hexafluoroethylene
Hyaluronic acid HA	Formic acid: hexafluoroisopropanol = 1:2	PLA+PCL	Hexafluoroisopropanol
PLGA	Tetrahydrofuran：DMF=3:2	PEO	Chitosan + water
PLGA	Chloroform: DMF=4:1	PVP	DMF:EtOH=1:4
PVDF, 18%		PEI, 22%
Polylactic acid-glycolic acid copolymer，PLGA		PVP+PLGA
Degradable polyhydroxybutyrate PHB		Poly DL-lactic acid
PS/DMF		PAN/DMF
PLLA, Mw 20000	Hexafluoroisopropanol	PA6	Hexafluoroisopropanol
Collagen 8%wt		TPU
PLA		PVA
collagen		Polycaprolactone
Polystyrene PS	DMF
Regenerated silk fibroin aqueous solution		Deionized water

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