The biomedical application of electrospun fibers is mainly in four aspects: drug-controlled release, biological dressing, tissue repair and enzyme immobilization.
Composite materials with multiple functions are the hotspots in the field of biomedicine .
Combination of 3D printing and traditional electrospinning techniques could bring a breakthrough for fabrication of considerably complex nanofiber-based 3D scaffolds/tissue constructs with appropriate chemical, mechanical, and biological cues in a spatially controlled manner .
Electrospinning has gained considerable attention in many fields due to its ability to produce continuous fibers from a variety of polymers and composites in a simple way. Electrospun nano fibers have many merits such as diverse chemical composition, easily adjustable structure, adjustable diameter, high surface area, high porosity, and good pore connectivity, which give them broad application prospects in the biomedical field.
Biological and medical applications
Many human tissues have limited regenerative capacity, thus, the injury and degeneration usually cause significant disability. In an attempt to improve the clinical outcome, drug delivery, tissue engineering, regenerative medicine, and regenerative engineering concepts have emerged aiming to regenerate damaged tissues and restore their functions in the past decades. Owing to the advances in material sciences, engineering, and life sciences, enormous progress has been made for tissue regeneration with better recapitulation of anatomical structures and functional recovery.
Great potencial for drug delivery applications
In drug delivery and regenerative medicine, scaffolds/substrates play a crucial role in controlling drug release rate, regulating cell behavior, and regenerating injured tissues. Scaffolds composed of electrospun fibers with diameters ranging from several nanometers to tens of micrometers are ideal owing to the biomimicry of extracellular matrix (ECM). In addition, electrospun fiber scaffolds offer distinct features including large surface area-to-volume ratio, high porosity, uniformity in fiber size, diversity in composition, flexibility in assembled structure, and ease of functionalization with bioactive molecules, making them promising candidates as carriers for drug delivery and scaffolds for regenerative medicine.
The drug delivery systems made of electrospun nanofibers have been new horizons in drug delivery. Up to date, a number of therapeutic agents including small molecular drugs and biological substances such as antibiotics, proteins, DNA, RNA, growth factors, and living cells have been incorporated to electrospun fibers through either encapsulation during electrospinning such as blended electrospinning, emulsion electrospinning, and coaxial electrospinning or surface modification post-electrospinning.
The electrospun nanofiber scaffolds are suitable for topical drug delivery, transdermal drug delivery, and oral drug delivery, while short nanofibers/fragments are feasible for local injection to diseased sites in a minimal invasive way. In addition, electrospun nanofibers afford remarkable advantages in controlling drug release rate by varying the composition (e.g. hydrophobic and hydrophilic materials), microstructure (e.g. homogenous structure, core-sheath structure and multilayered structure), or macrostructure (e.g. stacked meshes). Recently, the emergence of stimuli-responsive nanofibers provided a novel strategy of controlled drug delivery and release temporally and spatially. 
Tissue regeneration and other applications
The biomimetic nanofiber scaffolds capable of imitating fiber morphology, ECM architecture, and biochemical cues may be ideal for tissue regeneration. Therefore, the use of electrospun nanofibers as scaffold in regenerative medicine has drawn a great deal of attention. The introduction of 3D printing technology holds tremendous promise for development of shape-specific fiber scaffolds to repair damaged tissues. In addition, the breakthroughs in stem cell biology, in particular for human induced pluripotent stem cells, and genome editing may facilitate tissue regeneration. 
Electrospun nanofibers have good application prospects in tissue repair, biological dressing, drug-controlled release and enzyme immobilization. Tissue repair is the main research direction in the field of biomedical applications, and electrospun fiber materials were first applied to vascular repair tissue engineering in 1978. They were then used for bone tissue, nerve tissue, tendon tissue, heart valves, urethral stents, skin, and cartilage. In recent years, electrospun nano-fibers have been widely used in the pharmaceutical field. A variety of drugs including antibiotics, proteins, DNA, RNA, growth factors and anticancer drugs have been loaded into electrospun nanofibers for disease treatment. In recent years, many kinds of enzyme have also been successfully immobilized in fibers by electrospinning. 
1. Tissue repair
Electrospinning enables the preparation of continuous ultrafine fibers with diameters similar to those of natural extracellular matrices (ECMs). Electrospun nanofiber scaffolds can thus mimic the structure of ECMs in humans to a great extent. Electrospun fiber membranes or fiber mats have high porosity, good pore connectivity, a large specific surface area and biocompatibility, which provide a good microenvironment for cell survival to help cells adhere, differentiate, and proliferate.
The thickness, the three-dimensional structure, and mechanical properties of the electrospun nanofiber scaffold can be regulated by adjusting the electrospinning parameters. Moreover, bioactive molecules (such as growth factors, cell regulators, and even living cells) and inorganic particles (such as hydroxyapatite) could be added into to nano-scaffolds in electrospinning, which give electrospun fiber scaffolds a variety of functions. Electrospun nanofiber materials are widely used in the field of tissue repair such as trachea, nerves, skin, cartilage, bones, blood vessels, tendons, and ligaments.
Electrospun fibers have been used to repair a variety of tissues as shown above and summarized in Table 5. Current studies have shown that electrospun fibers mainly repair tissue by directly interfering with cell orientation and proliferation, affecting cell mobility, changing cell morphology, interfering with cell differentiation. However, these studies are far from enough. As a substitute for functional tissue, it is necessary to further deepen its action mechanism. .
More details on Tissue Engineering applications here.
2. Drug delivery
Electrospun nanofibers have many advantages as drug delivery systems, and have been studied for many years as transdermal, oral, injection dosage forms. Examples of electrospun fibers used to deliver multiple types of drugs are also given above and summarized in the Table 1. However, the study of drugs release mechanism and pharmacokinetics of drug-loaded electrospun nanofibers is still very few, which would limit the practical application. 
More on drug delivery here.
3. Biological dressings
Electrospun fibers have good gas permeability and can provide a biomimetic environment for skin cell regeneration during wound healing. The small pore size distribution of the fiber membrane can also effectively block the invasion of bacteria, which is suitable for skin dressings and artificial imitation skin (Fig. 8). In addition, the electrospun fiber membrane has a large specific surface area and can be loaded with drugs to promote the rapid recovery of damaged skin.
Electrospun fibers have made great progress in wound dressings, but there are still many areas need to be improved. For example, the fabrication of new structure nanofibers for wound healing, and the combination of electrospinning and 3D printing technology. The electrospun fibers can also be combined with electrical stimulation, pressure, magnetic force and other external forces to play a better role. 
Find out more on wound dressing here.
4. Enzyme immobilization
The high specific surface area and porosity of the electrospun fibers can effectively alleviate the diffusion resistance of the matrix and greatly improve the catalytic ability of the loaded enzyme. The method of enzyme immobilization in electrospun fibers is mainly the surface loading method (such as the active functional group loading method, chemical crosslinking, and surface modification).
At present, the enzyme immobilization in electrospun fibers is mainly by surface loading method. Different loading modes lead to different binding capacities between enzymes and fibers. Considering new loading modes will be of great help to explore the applications of enzymes. 
More information on enzime immobilization here.
5. Conclusions and Future Perspectives
Electrospinning offers a versatile and robust method for producing 1D nanofiber fragments, 1D nanofiber bundles/yarns, 2D nanofiber membranes with patterned structure, and 3D fiber scaffolds with highly porous structures. Owing to the biomimicry, these electrospun nanofiber materials have opened up numerous opportunities for the biomedical applications including drug delivery, control of cell behavior, and regenerative medicine.
Combination of 3D printing and traditional electrospinning techniques could bring a breakthrough for fabrication of considerably complex nanofiber-based 3D scaffolds/tissue constructs with appropriate chemical, mechanical, and biological cues in a spatially controlled manner.
Regarding to regenerative medicine, the innovation in electrospun fiber-based scaffolds has provided better mimicry of the structure and effective drug/growth factor/cell delivery, resulting in robust tissue regeneration. However, full repair and regeneration has not been achieved. In the future, introduction of programmed delivery system into nanofiber-based tissue engineering scaffolds would create exciting opportunities for tissue regeneration. In recent years, the stem cell-based therapy holds great potential in regenerative medicine. The success of the stem cell transplantation strategy still needs long-term preclinical studies. Use of nanofiber scaffolds to optimize the microenvironment of stem cells is promising to favor stem cell therapy. 
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