Heart is one among the least regenerative organ in the body as it cannot repair by itself. Myocardial infarction (MI) remains the foremost threat to the heart failure round the world (Hansson etal., 2009).
Traditional approaches like drugs, heart transplantation and LV assist devices are the current forms of remedy to MI. Tissue engineering affords some promising solutions for treating myocardial infarction and moving forward in a rapid pace. From the last two decades various attempts were made in treating MI which includes stem cell therapy, LV restraints, cardiac patches and injectable scaffolds (Christman KL, lee RJ. 2006, Aboli A eta., 2011).
Hydrogels for tissue repair.
Heart is a very complex organ which is prompted by electrical signals in contraction and relaxation of auricles and ventricles to pump the blood throughout the body. The systolic and diastolic movements of the heart create stress which is stabilized by the proteins like collagen, titn and the extracellular components like fibronectin, laminin, vitronectin, and elastin (Godier-furnemont AFG, vunjak-Novakovic G 2012). For repairing MI the most common type of bio-material used in trials are hydrogels for of their unique ability in allowing nutrients, oxygen, and diffuse metabolic wastes easily from the matrices. Hydrogels are 3D structures which can withhold considerable amount of liquid components by several thousand folds, (So Yeon Kim 2002) enriches cell attachments, provides physical strength and which can be easily degraded and replaced by extracellular cell-secreted proteins(vunjak novakovic et al., ). Cross linking monomers and polymers are the basic approach in the hydrogel preparation. According to their material of origin hydrogels can be classified into natural, synthetic, semi-synthetic (table).
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Hydrogel can be prepared by physical, chemical, irradiation crosslinking and by free radical polymerization. Physical cross linking includes polyelectrolyte complexation, hydrogen bonding and hydrophobic associations (Ibrahim M. El-Sherbiny 2013)
Biomaterials in repairing myocardial infarction.
Unlike the synthetic materials, natural biomaterials enhances the interaction through the cell surface receptors which facilitates cell adhesion, degradation etc., natural materials are much efficient for invivo because of its mechanical and biological properties can be customized to its native organs (Marsano etal., 2010). All over the world many groups were actively working in developing hydrogel which can potentially fix the myocardial damage. There are different kinds of approaches are tried in treating the MI (figure)
Limitations in tissue engineered constructs and hazards.
Though tissue engineering is an emerging field with some promising stories like tracheal segments, skin grafts and tissue engineered bladders several challenges should be fixed for other complete organs like liver, heart and kidney(Geckil H et al.,) eventually using scaffold in repairing the organs becomes inevitable. Organs and tissues inside the body are connected with blood vessel and its capillaries with a maximum distance of 200 µm distance for the supplementation of oxygen, nutrients and exclusion of metabolic wastes.( Folkman etal., 1973). The tissue constructs which exceeds 400 µm failed to succeed hence because of insufficient supply of oxygen to the cells, consequently there is a need for vascularization at tissue constructs which are grown in-vitro and also when they are implanted in-vivo (Francois A. Auger etal.,2013). Consequently, the scaffolds should be made in a sizable thickness to facilitate the vascularization.
Cardiac tissues scaffolds should be made with the molecules mimicking from the cardiac cells which also includes structure and mechanical properties (Junmin Zhu and Roger E Marchant 2011). Biocompatibility is the important factor in the engineered tissue hydrogels which includes with minimal immune response and toxicity. The main challenges are toxic moieties and chemicals used in crosslinking the monomers, for instance the monomers, initiators, organic solvents, stabilizers which are used during the process (Bryant sj et al.,). On the other hand the limiting factor to the advancement of tissue engineering is its clinical inability to perfuse tissue engineered constructs with advanced vascularization (Jennifer C.-Y Chung et al., 2012)
Advanced hydrogels are now in trial that facilitates cell adhesion, cell orientation, improves perfusion channels with mechanical properties, and better degradation ability to maximize the functional tissues. Nonetheless, storing and adapting the hPSC to the in vivo is still lacking in excellence of scaffolds. More over the hydrogel construction with cell encapsulation is at its preliminary stages, high level of perfection is needed for the constructs. Besides, other techniques like contractile loops, electrically oriented scaffolds are also substantially involved in developing functional scaffolds. However, much facile technique is needed for vascularized implantation deprived of immune rejection.
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Approaches for vascularizing the scaffolds
Biomaterials are used in tissue engineering because for their beneficial interaction with cells like migration, proliferation differentiation and apoptosis. (Grikscheit and vacanti, 2002). Vascularization of a engineered scaffold can be achieved upon grafting, activating the surrounding tissue to permeate the blood vessels into the scaffold enriched with growth promoters (Kannan etal 2005) and embedding the biomaterial with endothelial cells in which they joins with the host tissue and combines to form the capillaries. This will evade the tissue being ischemic and enrich the scaffolds. (kannan etal 2005, Sheridan etal 2000)
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