## 3 Electrospinning
### Biomimetic Applications of Electrospun Nano Structures for Tissue Engineering Purposes
However, the fibers produced from these materials are not as strong as synthetic counterparts. Furthermore, they also exhibit uncontrollable degradation rates [60]. On the other hand, the mechanical properties and degradation rates of synthetic polymers can be easily modified [60]. Hence, currently, a blend of both natural and synthetic polymers is largely preferred to fabricate electrospun nanofibers to exploit their respective strengths [60]. [PERSON] et al. co-electrospun collagen type I with poly (L-lactic acid)-co-poly(\(\varepsilon\)-caprolactone) (PLLA-CL) to produce nanofiber scaffolds, which enhanced the growth of human artery endothelial cells [63]. Another method to use multiple polymers is by layering individual fiber meshes one on top of another by sequential electrospinning. This was performed by [PERSON] et al. by making a tri-layered scaffold composed of nano- and microfibrous mesh of syrenated gelatin, type I collagen and segmented polyurethane which mimicked the native environment of arteries [64]. PCL/collagen composites are one of the most extensively studied polymer blends that can be easily electrospun into nanosized fibers (210-225 nm), known to withstand high-pressure flow and proven to have in vivo stability [64] A significant increase in cell proliferation was also observed with these systems when compared to that of pure PCL nanofibers [64]. Gelatin is a cheaper alternative to collagen which has similar biochemical property, but higher tensile moduli than collagen [60]. It has also been electrospun with PCL to produce nanofibers and was observed that with an increase in total polymer concentration there is a proportional increase in fiber diameter [65; 66; 67; 60]. Table 1 lists the recent examples of different electrospun scaffolds made of collagen, gelatin or laminin. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  
Electrospinning of BM protein laminin-I was first performed by [PERSON] et al. to produce nanofibers about 90-300 nm diameter [73]. They observed that, unlike collagen, laminin fibers were stable in culture without any fixation by chemical crosslinking, and human adipose stem cells showed superior adhesion quality compared to laminin films [76]. A subsequent study by this group using PCL/laminin blend also showed similar results [73]. BM mimics are considered as the major requirement to produce a successful tissue engineered skin, vascular graft and invitro alveolar models [77; 78; 79; 80]. [PERSON] et al. electrospun trilayer BM mimic, made of a single layer of nanofibrous polyhydroxyvalerate, interwoven at both sides by microfibrous poly L lactic acid. This BM mimic not only increased the attachment and proliferation of keratinocytes and fibroblasts but also encouraged the self-sorting of these cells into epithelial and dermal layers [77]. Similarly, a BM substitute produced by coelectrospinning a tripeptide derived from BM type IV collagen, along with PCL had an innate ability to control the ratio of endothelial to smooth muscle cells, a characteristic feature required to produce a successful vascular graft [78]. In vitro alveolar models to study the lung physiology and lung inflammatory reactions have also used electrospun BM mimics [79; 80]. It was observed that epithelial and endothelial cells grown on these BM mimic better replicate the alveolar environment and its functions [80]. Recent studies were successful in producing poloxamer 188 based nanofibers by co-electrospinning it with other polymers (polyacrylamideonitrile, poly(vinyl alcohol)/poly(vinyl pyrrolone)-iodine) [81; 82]. Poloxamer 188 is a non-ionic surfactant, which can increase the viability and proliferation of cells that were damaged due to multiple sub-culturing, pipetting, cryostorage and thawing [83]. To impart complex spatial arrangements and dimensions into the scaffold, different types of electrospinning techniques have evolved, which could relatively fabricate the structural complexity of natural ECM compared to conventional electrospinning [84].