**Biomaterials for Cardiac Tissue Engineering**

M. Arnal-Pastor, J. C. Chachques, M. Monleón Pradas and A. Vallés-Lluch

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/56076

**1. Introduction**

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#### **1.1. Cardiovascular diseases**

Cardiovascular diseases (CVD) are a leading death cause in developed countries (1 of every 3 deaths in the United States in 2008) [1]. Changes in diet and habits are causing CVD to become major mortality pathologies in developing countries too [2] (they are already responsible for a 30% of the world deaths). This group of diseases constitutes a great burden for the national health systems, consuming great percentages of the health systems budgets. In the particular case of the coronary heart diseases (CHD), 3,8 million men and 3,4 million women die a year worldwide because of them [3]. In the United States 1 of every 6 deaths in 2008 was caused by CHD [1].

The heart is a complex organ that pumps 7000 liters of blood to all the tissues in the body per day [4]. This pumping function precisely determines its anatomy. Heart tissue basically is formed by cardiac myocytes (contractile elements) [5], smooth muscle cells, fibroblasts, blood vessels, nerves and the extracellular matrix components (cardiac interstitium and collagen) [6] organized in a very particular way. Myocytes form muscular fibers with changing orientation across the ventricular wall up to 180º [7]. At the same time, muscular fibers are organized into myocardial laminas 4-6 myocytes thick separated from neighboring laminas by extracellular collagen [8]. The particular arrangement of the ventricular myocytes influence the mechanical and electrical function of the heart and small changes in it can lead to severe changes in these functions [9].

The extracellular matrix (ECM) connects the cells into a 3D architecture allowing the coupling of the forces produced by the myocytes. The anatomical model proposed by Torrent-Guasp [8], which considers the heart one muscle band plied in a double helical loop, explains how the

© 2013 Arnal-Pastor et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ventricles contract and get an efficient pumping in every heart beat, achieving an ejection fraction of the 60% when sarcomeres individually contract 15% only [10].

Myocytes are intimately connected, forming a functional syncytium [8]. Each myocardial cell is coupled in average to 9,1 ± 2,2 [11] myocytes, by 99 [12] gap junctions where the transfer of ionic currents takes place. Gap junctions are a specialized form of cell connection; they are formed by a cluster of ionic channels essential to the rapid propagation of the action potential. The action potential is the electrical impulse responsible for the contraction of the cells [13]. A proper electrical coupling of the cells is critical to avoid arrhythmias and reentries and essential for the contraction to spread as a wave front.

Acute myocardial infarction (AMI) occurs when a coronary artery is clogged, in 80% of the cases, by coronary atherosclerosis with superimposed luminal thrombus [14]. This occlusion leaves the downstream zone of the heart without blood supply, what means lack of oxygen, nutrients and metabolites wash for the affected zone. As a consequence, the aerobic metabo‐ lism changes to anaerobic glycolysis [14], leading to a decrease in the pH and reduction in the contractile function. Within 20 to 40 minutes without blood supply cells start to die and as times passes more myocardial tissue is compromised. There is also a zone of the heart affected by the infarction, where myocytes remain viable but lower their activity to reduce the metab‐ olism and oxygen consumption to survive under hypoxic conditions; they can recover their contractibility after revascularization [15].

Clinical practices aim to limit the severity and extension of the AMI by rapidly restoring the blood flow (reperfusion), alleviating the oxygen demand [16] and reducing reperfusion injury. This can be done with different treatments or combinations of them. Pharmacological ap‐ proaches involve the use of anticoagulant therapies and thrombolytic drugs to eliminate the clot. Vasodilatators like nitrates are also used to favor the dilation of the vessels, aspirin to avoid platelet aggregation, betabloqueants to reduce the heart pace, as well as morphine to reduce the pain are employed. Another group of therapies are the percutaneous coronary interventions; they physically reopen the vessel via catheterization. There are different techniques: the regular angioplasty uses a catheter with a balloon that is inflated in the place of the thrombus to reopen the lumen [17], or allows the permanent implantation of a stent in the vessel to keep it open. There is a wide variety of these devices depending on their compo‐ sition, whether they release drugs or are biodegradable or not, etc [18, 19].

These therapies restore the blood flow to the infarcted zone; but reperfusion therapy is not exempt of risks: it is a complex process that can induce apoptosis by the microenvironmental changes that the recovery of the blood supply induces (formation of free radicals, calcium release, neutrophils, etc.) [20]. So it has to be done carefully and there is always a compromise between limiting the infarction extension due to the time without oxygen and the induced apoptosis due to the reperfusion. Reperfusion done soon after the onset of the ischemia is very advantageous, saving more tissue by restoring the blood flow than the tissue that will be lost because of the toxic substances released in the reperfusion. All the aforementioned treatments basically limit the damage of the acute episode but do not regenerate the damaged tissue and do not avoid the subsequent ventricular remodeling following an AMI.

In the infarcted area there is a great number of dead myocytes, and the host response to the injury consists in activating the inflammatory response and producing cytokines [21]. There‐ upon neutrophils, monocytes and macrophages migrate into this area to remove the necrotic tissue [22]. Then, matrix metalloproteases (MMPs) are activated, which have a deleterious effect on the collagen matrix of the heart and in the surrounding coronary vasculature by degrading them [23]. The weakening of the collagen leads to wall thinning and ventricular dilation, as well as mural realignment of myocytes bundles [24]. After the inflammatory phase and the resorption of the necrotic tissue, there is an increase in the deposition of cross-linked collagen in the infarcted area that leads to scar tissue formation. During the remodelling process a change in the collagen composition occurs, the type I collagen fraction is reduced from 80% to 40% and the collagen III is increased [25].

Against what it was thought, this scar is a living tissue with a fibroblast-like cell population nourished by a neovasculature; these cells regulate the collagen turnover of the scar tissue [22]. The scar tissue has a reduced or absent contractility as compared with the original healthy myocardium [26], what leads to a reduction in the overall cardiac function [27].

The remodeling process initially is a compensatory mechanism to overcome the loss of contractile tissue. But with time this adaptative process of overload becomes maladaptative [15]. To compensate the additional effort, the remaining beating tissue hypertrophies trying to overcome the reduction in the cardiac function. This overload leads to myocyte slippage and fibrotic interstitial growth and to a degenerating process that may end in heart failure. The heart remodeling produces in the ventricles a set of anatomical and functional changes, including increased wall stress, slimming of the wall, chamber dilation, increase of the sphericity, and a significant loss of cardiac function.

The ventricular shape change from elliptical to spherical reduces its ejection fraction, because of a change in the apical loop fiber orientation [28]. Another problem caused by the shape change is that the papillary muscles are separated, what leads to regurgitation, contributing to the overload of the heart [24]. Besides, remodeled hearts are more prone to suffer arrhyth‐ mias as the membrane potential is altered and because of the interstitial fibrotic growth that may affect conductivity [15].

The end stage of the degeneration is the heart failure, when the heart is unable to pump enough blood to match the metabolic needs of the tissues. Current treatments aim to avoid reaching this point. Pharmacological treatments aspire to reduce the work load and to protect the cardiac tissues from the accumulated harmful substances [29]. Surgical therapy involves different techniques with different objectives: to restore a proper blood flow in areas that lack it (bypass surgery), to restore the normal elliptical geometry (Dor and Batista procedures), to restore the wall stress to normal (Dynamic Cardiomyoplasty), to limit the pathologic dilation, etc [10].

#### **1.2. Cell therapy and cardiac tissue engineering**

ventricles contract and get an efficient pumping in every heart beat, achieving an ejection

Myocytes are intimately connected, forming a functional syncytium [8]. Each myocardial cell is coupled in average to 9,1 ± 2,2 [11] myocytes, by 99 [12] gap junctions where the transfer of ionic currents takes place. Gap junctions are a specialized form of cell connection; they are formed by a cluster of ionic channels essential to the rapid propagation of the action potential. The action potential is the electrical impulse responsible for the contraction of the cells [13]. A proper electrical coupling of the cells is critical to avoid arrhythmias and reentries and essential

Acute myocardial infarction (AMI) occurs when a coronary artery is clogged, in 80% of the cases, by coronary atherosclerosis with superimposed luminal thrombus [14]. This occlusion leaves the downstream zone of the heart without blood supply, what means lack of oxygen, nutrients and metabolites wash for the affected zone. As a consequence, the aerobic metabo‐ lism changes to anaerobic glycolysis [14], leading to a decrease in the pH and reduction in the contractile function. Within 20 to 40 minutes without blood supply cells start to die and as times passes more myocardial tissue is compromised. There is also a zone of the heart affected by the infarction, where myocytes remain viable but lower their activity to reduce the metab‐ olism and oxygen consumption to survive under hypoxic conditions; they can recover their

Clinical practices aim to limit the severity and extension of the AMI by rapidly restoring the blood flow (reperfusion), alleviating the oxygen demand [16] and reducing reperfusion injury. This can be done with different treatments or combinations of them. Pharmacological ap‐ proaches involve the use of anticoagulant therapies and thrombolytic drugs to eliminate the clot. Vasodilatators like nitrates are also used to favor the dilation of the vessels, aspirin to avoid platelet aggregation, betabloqueants to reduce the heart pace, as well as morphine to reduce the pain are employed. Another group of therapies are the percutaneous coronary interventions; they physically reopen the vessel via catheterization. There are different techniques: the regular angioplasty uses a catheter with a balloon that is inflated in the place of the thrombus to reopen the lumen [17], or allows the permanent implantation of a stent in the vessel to keep it open. There is a wide variety of these devices depending on their compo‐

These therapies restore the blood flow to the infarcted zone; but reperfusion therapy is not exempt of risks: it is a complex process that can induce apoptosis by the microenvironmental changes that the recovery of the blood supply induces (formation of free radicals, calcium release, neutrophils, etc.) [20]. So it has to be done carefully and there is always a compromise between limiting the infarction extension due to the time without oxygen and the induced apoptosis due to the reperfusion. Reperfusion done soon after the onset of the ischemia is very advantageous, saving more tissue by restoring the blood flow than the tissue that will be lost because of the toxic substances released in the reperfusion. All the aforementioned treatments basically limit the damage of the acute episode but do not regenerate the damaged tissue and

sition, whether they release drugs or are biodegradable or not, etc [18, 19].

do not avoid the subsequent ventricular remodeling following an AMI.

fraction of the 60% when sarcomeres individually contract 15% only [10].

for the contraction to spread as a wave front.

276 Regenerative Medicine and Tissue Engineering

contractibility after revascularization [15].

For many years, the heart has been considered a fully differentiated organ, with no myocyte regeneration after birth [30]. Recently it has been proved that myocytes have a limited regenerative capacity, around 1% of the cells per year at the age of 20 and it is reduced to 0,3% at the age of 75 [31]. This regenerative capacity is achieved thanks to a small population of cardiac stem cells [32]. Nevertheless, their regenerative capacity is limited and in any case it is not enough to regenerate the heart if it suffers severe damage, like the one provoked by a myocardial infarction. New therapies under development like cell therapy or tissue engineer‐ ing, aim to boost this limited regenerative potential of the native tissue by employing cells, drugs, factors or patches.

The aim of cardiac cell therapy is to heal the damaged infarcted tissue by the implantation of cells into or onto the pathologic myocardium by different techniques (figure 1 a). In tissue engineering strategies, different types of cells have been combined with materials and with bioactive molecules if necessary to again try to recover the injured tissue. The employed materials will support cells, provide them 3D organization, protect them, stimulate and guide its growth, maintain them in the site of interest, etc.; in sum, they will act as an artificial extracellular matrix during the regeneration process. But the use of materials either injectable, or *ex vivo* conformed (gels –patches- or scaffolds) (figure 1 b) has an additional and important effect: the implantation of a material in the scarred ventricular wall, increases its thickness and by Laplace's law, this increase leads to a reduction in the wall stress. This side-effect could be by itself very positive, even although regeneration did not arrive to happen, to limit ventricular remodeling and improve the quality of life of cardiac patients [29].

**Figure 1.** (a) Classical cell therapy in the heart (*freely inspired* in Strauer BE, Kornowski R, *Circulation* 2003; 107: 929-934). (b) Tissue engineering approaches with cell sheets, scaffolds or injectable materials (*freely inspired in* Masu‐ da S *et al*, *Adv. Drug Del. Revs* 2008; 60(2): 277-85). (c) Ventricular restrain device.
