Electrospinning is an old polymer processing technology that has been revitalized and may now be on the verge of revolutionizing the field of tissue engineering. This is evident from the rapid growth in and sheer number of publications in this area over the past several years. The enthusiasm surrounding this technology has arisen because it is a simple, scalable technology that allows the development of nanostructured scaffolds, which mimic the native extracellular matrix (ECM).
All of our tissues and organs are complex biocomposites composed of microenvironments that include the ECM, cells, and ground substance (a term describing everything else in the environment). The ‘structural’ portion of the ECM is composed of fibrillar collagens on the order of 50-300 nm in diameter. Originally, the function of the ECM was thought to be solely that of a structural element. Its role is now understood to be much more: it is the central part of a complex microenvironment that regulates every aspect of cellular function and activity through cell-ECM communications, which directly translate to tissue and organ function. Historically, tissue engineering has attempted to use a variety of synthetic and natural polymers and many processing methods to fabricate fibrous scaffolds, all of which have resulted in fiber diameters on the order of 10 μm. This is several orders of magnitude greater than the fibers in the native ECM. Until recently, a biomimicking fiber with a nanoscale diameter has been the missing element in scaffold design. This is a serious deficiency.
Synthetic polymers, e.g. poly(glycolic acid), are readily available and have been electrospun by many groups to develop partially biomimicking scaffolds in terms of fiber dimension. But these structures are ‘synthetic’ and lack the appropriate composition to allow the critical communication between the cell and ECM. To this end, the electrospinning of native ECM (collagen types I, II, and III, as well as elastin) and ‘provisional matrix’ (fibrinogen) proteins has been pioneered in my laboratory. In all of these cases, a ‘truly’ biomimicking scaffold is created in terms of both structure and composition. Upon electrospinning, the native proteins display not only the structural aspects of the ECM but also the ultrastructure, i.e. the composition of the building blocks within the fibers. This physiological ultrastructure allows proper integrin binding sites to be presented to the cells in the microenvironment. Integrin binding is key for the necessary cell-ECM communication that is lacking when synthetic polymers are used. Additionally, in vitro and in vivo studies reveal that fiber size and composition are important variables for cellular interactions, with smaller fiber diameters and native protein compositions enhancing adhesion and migration.
Electrospinning also enables fiber diameter to be modulated as a function of polymer concentration. As an example, collagen type I has been electrospun from concentrations of 30-100 mg/ml in 1,1,1,3,3,3-hexafluoro-2-propanol, resulting in scaffold fiber diameters from 100 nm to 5 μm with a linear relationship between collagen concentration and fiber diameter. This wide range of fiber diameters allows a great flexibility in scaffold production.
Electrospinning also allows some degree of control over fiber orientation within the scaffold. The motion of the mandrel and focusing of the electrical field during the process provide this control. In this way, electrospinning can be used to fabricate practically any composite structure desired in order to replicate and mimic the ECM of the tissue of interest. The thickness of the scaffolds or individual layers can also be controlled by simply adjusting the electrospinning time; scaffold thickness can range from a single fiber to several millimeters.
In terms of physiological acceptance, nano-fibrous scaffolds, regardless of composition, induce a reduced inflammatory response (versus micron-diameter fibers) and, in the case of native proteins, a minimal response is detected. Within days of implantation, nano-fiber collagen type I structures are incorporated into the surrounding tissue and are indistinguishable from native tissue. Within one week, nanofibrous collagen type I materials have developed a high degree of vascularization to maintain the ‘regenerated’ tissue. The nano-fiber scaffolds, especially those composed of native proteins, seem to have a ‘stealth’ nature with regard to the body's immune surveillance and a ‘natural’ acceptance by cellular components.
It may now be possible to engineer a truly biomimicking structural microenvironment for a variety of tissues using the process of electrospinning. Of particular interest is the replication of the fibrous architecture (ECM) of native tissue with control over fiber composition, orientation, and diameter to create scaffolds for cell seeding or acellular scaffold implantation. In conclusion, electrospinning may now allow the fabrication of ‘ideal’ scaffolds for any potential tissue engineering product.
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Gary L. Bowlin is an associate professor in the Department of Biomedical Engineering, Virginia Commonwealth University.