Some nanofibers are bioresorbable so they can be applied as wound dressing and , is another important structure in the electrospinning field. . exchange, freeze and drying processes to get the final foam like structure . ..  showed an inverse relationship between the surface area of carbon. Electrospinning process is commercially used to form nanofibers as scaffolds in a relationship between cellular response and molecular structure [16, 17]. Electrospinning is a process that produces continuous polymer fibers The morphology of the electrospun nanofibers was analyzed by .. fibers, blobs and flattened web-like structures. . process relationship of electrospun bioabsorbable.
The development of nanofibers has enhanced the scope for fabricating scaffolds that can potentially mimic the architecture of natural human tissue at the nanometer scale. The high surface area to volume ratio of the nanofibers combined with their microporous structure favors cell adhesion, proliferation, migration, and differentiation, all of which are highly desired properties for tissue engineering applications Bhattari et al ; Ma et al a.
Therefore, current research in this area is driven towards the fabrication, characterization, and applications of nanofibrous systems as scaffolds for tissue engineering. Due to their potential, the nanofiber-based systems are also being pursued for a variety of other biological and non-biological applications Li et al ; Wang et al ab ; Nair et al This review summarizes the currently available approaches for the fabrication of nanofibers and discusses their application in the engineering of a variety of tissue types.
Methods for nanofiber synthesis Currently, there are three techniques available for the synthesis of nanofibers: Of these, electrospinning is the most widely studied technique and also seems to exhibit the most promising results for tissue engineering applications. Nanofibers synthesized by self-assembly and phase separation have had relatively limited studies that explored their application as scaffolds for tissue engineering.
Although there are a number of techniques for the synthesis of carbon nanofibers, such as chemical vapor deposition using a template method Che et alcatalytic synthesis catalytic deposition, floating catalyst method Teo et alsynthesis using radiofrequency-supported microwave plasmas Cui et althe description of each of these techniques is beyond the scope of this review.
Therefore, for carbon and alumina nanofibers, the discussion is restricted to their applications in tissue engineering. Electrospinning Electrospinning represents an attractive technique for the processing of polymeric biomaterials into nanofibers. This technique also offers the opportunity for control over thickness and composition of the nanofibers along with porosity of the nanofiber meshes using a relatively simple experimental setup Doshi and Reneker ; Reneker and Chun ; Dzenis ; Jayaraman et al Although the concept of electrospinning or electro-spraying has been known for more than a century, polymeric nanofibers produced by electrospinning have become a topic of great interest only in the past decade Rayleigh ; Doshi and Reneker The high surface area and high porosity of electrospun nanofibers allow favorable cell interactions and hence make them potential candidates for tissue engineering applications Li et al ; Smith and Ma ; Khil et al ; Ma et al a.
In the electrospinning process, fibers ranging from 50 nm to nm or greater Reneker and Chun ; Shin et al a ; Fridrikh et al can be produced by applying an electric potential to a polymeric solution Hohman et al ab see Figure 1a. The solution is held at the tip of a capillary tube by virtue of its surface tension. The electrical potential applied provides a charge to the polymer solution. Mutual charge repulsion in the polymer solution induces a force that is directly opposite to the surface tension of the polymer solution.
An increase in the electrical potential initially leads to the elongation of the hemispherical surface of the solution at the tip of the capillary tube to form a conical shape known as the Taylor cone Doshi and Reneker ; Yarin et al A further increase causes the electric potential to reach a critical value, at which it overcomes the surface tension forces to cause the formation of a jet that is ejected from the tip of the Taylor cone.
The charged jet undergoes instabilities and gradually thins in air primarily due to elongation and solvent evaporation Zeleny ; Reneker et al ; Shin et al ab ; Frenot et al Crystallinity was also diversified by different types of solvents.
Thermal analyses, such as differential scanning calorimetry DSC and Raman spectroscopy, revealed lower crystallinity of PCL fibers electrospun from chloroform than from hexafluoroisopropanol HFIP [ 19 ]. The diameter of electrospun fibers, determined by properties of solvents, was not taken into account.
Influence of average Mw on molecular orientation was characterized using PVA as an example [ 77 ]. WAXS data indicate higher molecular orientation of PVA fibers electrospun from polymers characterized by higher molecular weight.
It correlates with increase of crystallinity of fibers electrospun from PVA with higher Mw. By optical microscopy dark fieldoptical birefringence of PEO electrospun fibers was determined [ 78 ]. Degree of birefringence was not analyzed. In another publication, ordering of polymer macromolecules in the direction of the electric field was found in the needle [ 79 ].
Morphology and molecular structure of a single electrospun fiber influences mechanical properties. It was observed that thin fibers electrsopun from solution with low concentrations were characterized by higher crystallinity than submicron fibers. It is opposite to publications which suggest increase in crystallinity with polymer concentration fiber diameter.
Authors suspect increase in molecular orientation with fiber diameter. Fibrils consist of staggered crystal and amorphous phases; lamellas consist of densely located lamellas and non-oriented amorphous phase. It can be related to the higher order of chains in thin fibers and smooth surface.
In the same publication, significant elongation at break increase is observed with an increase in polymer concentration. These observations are confirmed on PVA [ 84 ]. Setup parameters One of the most significant parameters in electrospinning process is applied voltage.
Applied voltage regulates charges to the solution droplet. Density of the applied voltage, as value of applied charges on the surface of the droplet is described by applied voltage, distance between the needle and collector, and solution conductivity. It has to be related to the solvent type and polymer concentration. Fiber diameter increases with same maximum value for fibers electrospun with different applied voltages in range of 7—20 kV [ 40 ].
Description of the applied voltage influence on crystallinity was not found for polyesters. Crystallinity as a function of applied voltage was found for cellulose fibers electrospun on plate [ 11 ]. Analyses indicate an increase of crystallinity with voltage, followed by a decrease of crystallinity after achieving a certain maximum value.
It is explained as a result of powerful dynamics of crystallinity in high voltage which falters after the critical voltage is exceeded by longer distance fibers falling down. Volume expansion of the solution is regulated by the flow rate settled by a medical pump.
Flow rate is critical in Taylor cone formation; as a consequence of too fast flow rate, beads are formed, in the case of too slow flow rate, the needle clogs [ 1570 ]. Length nozzle optimization was also described. With the length of needles, polymer chains orientation increases in the jet and standard deviation of diameter of electrospun fibers decreases [ 87 ]. An appropriate distance between the needle and collector enables solvent evaporation which also strongly influences the fiber morphology single, not agglomerated fibers.
Nanofibers and their applications in tissue engineering
In both PV and NV, it was found that the fiber diameter generally increased with increasing the solution concentration and increasing inner diameter but decreased with increasing the working distance, while the solution feeding rate did not significantly affect the fiber diameter.
However, the fiber diameter increased with increasing PV but decreased with increasing NV. Polarity of the applied voltage did not significantly affect the water contact angle, which suggests no influence on the chemical structure of fibers.
In the case of polyamide 11, changes of surface chemistry were observed while comparing fibers electrospun using NV and PV [ 76 ]. The architecture of electrospun patches depends on the collector type. Usually, fibers are electrospun on plates, drums, wires, and grids.
Structure and process relationship of electrospun bioabsorbable nanofiber membranes – ScienceOpen
Porosity of the patches may be changed by using different types of collectors [ 18 ]. Fibers electrospun on plate are of a random architecture, fibers electrospun on a drum are orientated in the direction of the drum rotation.
Degree of fiber polarization is regulated by the speed of drum rotation and applied voltage [ 989 ]. Fibers collected on drum have lower diameter than those collected on plate because of stretching during collecting provided other parameters remain constant.
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Intensity of peaks registered by WAXS of PLLA fibers change depending on polymer concentration and degree of stretching; increase in molecular orientation and crystallinity was found as a function of degree of stretching comparison: Peaks intensity increases with concentration, additionally with fibers annealing.
Molecular orientation of PAN electrspun fibers increases with the speed of drum rotation of 0— Tailoring the rotation speed makes it possible to electrospin fibers of demanded molecular orientation [ 91 ]. High-speed rotation also guarantees paralyzation of electrospun fibers.
The latter is more related to whole patches than single fibers. Differences in calculation arise from consulting real fibers amount on the sample cross-section. In general, fibers collected on a drum display higher strength than those collected on a plate, which is related to molecular orientation of fibers collected on a drum [ 40 ]. Ambient conditions Temperature plays a significant role during solvent evaporation. Time of crystallization decreases with the rising temperature which correlates with a weaker degree of crystallinity in the shorter time [ 340 ].
Changes of temperature also lead to fiber porosity [ 15 ]. Air humidity has an effect on electrical conductivity of the ambient environment.
Changes of humidity also influence surface porosity of a single fiber [ 92 ]. Usually, the electrospinning process takes place in air atmosphere; however, electrospinning in atmospheres of different gases is possible. Temperature and humidity are not strictly controlled, as a consequence not so many researches describe influence of these parameters on fiber morphology and structure. Bicomponent fibers with Col, Ge, and Chit In the last few years, some literature has confirmed positive influence of biopolymer additive on the functionality of electrospun patches in tissue engineering applications.
Most common biopolymers added to synthetic polymers are Col, Ge, and Chit. Fibers with Col and Ge Biopolymer additives to polyester influence the morphology as well as molecular and supermolecular structures of electrospun fibers.
This part of description concentrates on blends with Ge and Col. Additionally, smaller fiber diameter and pore size of the patches electrospun on a drum in comparison to fibers collected on a plate were seen.
From the viewpoint of tissue engineering application in-vitro and in-vivo studymiscibility compatibility of both polymers or interactions of their chemical groups seem to be important. Literature about miscibility of biopolymer and polyesters is sketchy and inconsistent.
Phase separation leads to electropsun fibers splashing, bonding, and distribution in fiber diameter. Small addition of acetic acid to solution was found to lead to the opaque solution becoming transparent immediately without occurrence of precipitation for over 1 week.
Nanofibers formed from solution with acetic acid are thinner, smoother, and more uniform. Increase in tensile strength and decrease in strain at break in fibers electrospun without acetic acid was observed [ 96 ].
Structure and process relationship of electrospun bioabsorbable nanofiber membranes_百度文库
Miscibility of polymer depends on glass temperature Tg. It is known that Tg value of miscible blends is between the values of those for both components [ 97 ]. In immiscible blends, two separate Tg values are observed [ 98 ]. Tg is usually determined from DSC data. As a consequence of leaching, pores on the surface of electrospun fibers appeared [ ].
Observation by transmission electron microscopy TEM illustrated Ge phase located along fiber axes Figure 8 [ 86 ]. In the article, Electro-spinning of pure collagen nano-fibers — Just an expensive way to make Ge? Molecular orientation also changes with addition of Ge to PCL, as was observed on drum-collected fibers [ 79 ].
PCL crystallinity increases in the presence of a small amount of Ge. In patches functioning as 3D cells scaffolds, wettability plays a significant role. Lack of additives of this parameter has not been clearly explained in literature.
In some publications, it is described as a result of ordering nonpolar groups of polymer in proximity of synthetic polymer; in some others, ionization of Ge is considered to play a significant role.