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Over the last decades, the attention to biomaterials which have a biodegradation behavior has increased significantly according to the dramatic increase of the demand for biodegradable materials (Cao and Wang, 2009). Biomaterial defines as any substances or compound of substances original or synthetic, which has certain properties to be used any time partly or fully for the body treatment purposes such as augmenting or replacing any organ, tissue, or function (Von Recum and Laberge, 1995). If any natural or man-made material serves the surgical, pharmaceutical and medical applications, it can be investigated as biomaterial .Particularly; one of the crucial characteristics of the biomaterials is biodegradability (Cao and Wang, 2009).
Silk has been considered a biomaterial; this is attributed to its excellent mechanical properties such as biocompatibility, flexibility, biodegradability, non-inflammatory, non-cytotoxicity etc (Kaplan et al., 1994). In general, silk fiber contains two kinds of self-assembled proteins, fibroin which Constitute a major component of the silk fiber serving as a core, and the sericin which Constitute the minority of the silk fiber and serving as coating protein (Von Recum and Laberge, 1995).Silk is fibrous protein is a natural protein, is commonly produced by silk worm, Bombyx mori, members of the Arachnidan class (more than thirty thousand kinds of spiders) and by many of worms of the order Lepidoptera, which contains moths, mites and butterflies ( Kaplan et al.,1998).
In addition to the silk traditional usages, Silk has been explored as biomaterial for tissue engineering and cell cultural, and also has achieved FDA authority for expanded utility. (Altman et al., 2003) (Wang et al., 2006). Recently, silk widely used as a biomaterial in various form, such as gels, sponges, films (Vepari and Kaplan, 2007), powder(Hino et al .,2003) and scaffolds (Wang et al .,2006). As well as medical applications include nets (Unger et al., 2004), burn-wound dressings (Santin et al., 1999) and structural implants (Minel et al., 2005). The chemical and physical advantages of the silk can be controlled easily via manipulating (genetic engineering) the secondary structure to suit desirable characteristics and mechanicals and to create selective features which helps in the biomaterials designing.(Huemmerich et al., 2004).(Li et al.,2003).
Due to the silk successful properties, silk is one of preferred candidates to develop therapeutic devices such as three-dimensional porous structures as tissue engineering scaffolds and controlled release drug delivery vehicles. (Von Recum and Laberge, 1995).This assignment will focus on these two main biomedical applications and review silk structures and properties.
1. Structure of silk biomaterials:
Silks represent a unique group of structural proteins that are biocompatible, degradable and mechanically superior, offer a wide range of properties are amenable to aqueous or organic solvent processing Naturally, native silk fiber consists of two types of proteins: fibroin and sericin which both contain the same 18 amino acids such as glycine, alanine and serine in different quantity (Zhou et al., 2000; Chitrangada et al., 2009). The authors described that the core fibroins are covered by sericin is a family of hydrophilic proteins that holds the fibroin fibers together (Inoue et al., 2000; Altman et al., 2003). Furthermore, Vepar and Kaplan (2007) remarked the native silk fibers can be regenerated in aqueous solution. The fibroin has a great molecule comprising a crystalline portion about two-thirds and an amorphous region of about one-third. The crystalline portion contains along sequence from amino acids for instance (-Gly-Ala-Gly-Ala-Gly-Ser-) which plays an important role in the stability and mechanical properties of the fiber (Hoslot, 1998; He et al., 1999; Kim et al., 2005). Moreover, Tsuboi et al. (2001) found that the main secondary structures of fibroin are of the random-coil or unordered type and the antiparallel Î²-sheet type, which is formed through hydrogen bonds between adjacent peptide chains.
2. Properties of silk biomaterials:
Silk fibroins have excellent mechanical properties such as remarkable strength and toughness., Huang et al. (2007) discovered that the hydrophobic domains have a significant role into silk fibers because of that the high strength of fibers and the thermal stability . Furthermore, Bini el al. (2004) concluded that properties of silk fibroins as biodegradability and biocompatibility based on their special molecular structure.
2.1. Biodegradation behaviour of silk biomaterial:
The characteristics of silk biodegradation behaviours vary with different of proteolytic enzymes such as chymotrypsin, actinase and carboxylase which play an essential role in the degradation of silk fibroins (Chen et al., 1991; Chen et al., 1996; Li et al., 2003).Naira and Laurencina, (2007) observed that the enzymatic degradation of silk has two-step process, the first step is absorption of the enzyme on the surface of binding domain and the second step is hydrolysis of the ester bond. The degradation behavior of biomaterials is important in the medical application in vivo and control over the rate of the silk scaffold form, that matches with the rate of tissue growth lead to significant feature of function tissue design.
Silk fibers have been used as medical application such as surgical sutures since the end of the 19th century and have proved to be effective biomaterial. Gupta, (2007) demonstrated that fibroin supports cell attachment and proliferation for a variety of cell. In addition to this, Minoura et al. (1995) reported that silk from wild silkworm, Autheraea pernyi, contains the RGD sequences which support cell attachment and growth much greater than B. mori silk. Silks have similar structural characteristics to amyloid which is in the body conjunction with neuro-degenerative diseases such as Alzheimer and Parkinson (Li et al., 2001; Chen et al., 2007). Recent studies trying to use silk -based biomaterials in human body. (Altman et al., 2003; Wang et al., 2006; Hakimi et al., 2007).
3. Medical application of silk fibroin protein:
The silk fibroin one of the most demanded biomaterial because of practical application that can create opportunities for medical advancement. According to its unique structure and mechanical properties, the silk fibroin utilized as scaffold in tissue engineering of surface for cell attachment in cardiac tissue, skeletal tissue and drug delivery. This material is promising for use in nerve generation; further researches need to be conducted in this area (Yen et al., 2009).
3.1 Cardiac tissue engineering:
The human heart cannot regenerate after injury. Lost cardiomyocytes are replaced by scar tissue, but that reduce cardiac function causing high morbidity and mortality. The strategies to repair the heart include regenerating through stem cell, induction of cardiomyocyte proliferation, xenotransplantation and cardiac tissue engineering based on silk fibroin scaffold. The scaffold play critical role in tissue engineering. The function of scaffold is to direct the growth of cell and it must provide suitable substrate for cell attachment, cell proliferation, differentiated function (Yen et al., 2009). In order to engineer 3D tissue aims to fabricate a biodegradable scaffold that mimics in vivo. Petra et al. (2012) investigated suitability of silk fibroin protein from Antheraea mylitta as a scaffold for cardiac engineering. Their data support that silk fibroin protein suitable for cardiac engineering and have a great potential to solve a wide range of biomedical application.
3.2. Drug delivery system:
Silk fibroin uses as 3D scaffold in form of drug delivery system embedded in calcium alginate beads to control drug releasing. Mandal and kundu (2009) conducted study with aim to develop drug releasing system in form of 3D scaffold, bead embedded-silk 3D scaffold were fabricated by embedding calcium alginate. The authors reported that calcium alginate beads are basically hydrogel in nature and due to high concentration gradient of entrapped molecules inside the bead diffusion to outside solvent is very rapid and further release of compound was assisted by degradation of calcium beads resulting an enhance delivery. This characteristic of initial release from any drug vehicle is undesirable, this leads to loss of drug and may leads to harmful effect. The author highlighted that when calcium alginate beads were layered within silk fibroin the drug releasing was effectively restricted and slower degradation was observed. Thy attributed this finding to crystalline nature of silk fibroin protein. Also due to highly polymer density per unit mass of beads, as well as fibrous nature of silk fibroin protein. Recently, silk fibroin was utilized to coat alginate microspheres for drug release application (Wang et al., 2007). The silk fibroin coating stabilized calcium alginate from degradation and also leads to sustained protein drug release by providing diffusion barrier (Mandal and Kundu, 2009).
As has been shown, the unique structure of silk and its mechanical properties make silk a promising material for many clinical functions. This assignment has been only possible to focus on two main silk biomedical applications. However, more research should be carried out to cover more needed applications for achieving the medical demands, and for extending the silk uses.