Advances in the design, generation, and application of tissue-engineered myocardial equivalents

Abstract

Due to the limited regenerative ability of cardiomyocytes, the disabling irreversible condition of myocardial failure can only be treated with conservative and temporary therapeutic approaches, not able to repair the damage directly, or with organ transplantation. Among the regenerative strategies, intramyocardial cell injection or intravascular cell infusion should attenuate damage to the myocardium and reduce the risk of heart failure. However, these cell delivery-based therapies suffer from significant drawbacks and have a low success rate. Indeed, cardiac tissue engineering efforts are directed to repair, replace, and regenerate native myocardial tissue function. In a regenerative strategy, biomaterials and biomimetic stimuli play a key role in promoting cell adhesion, proliferation, differentiation, and neo-tissue formation. Thus, appropriate biochemical and biophysical cues should be combined with scaffolds emulating extracellular matrix in order to support cell growth and prompt favorable cardiac microenvironment and tissue regeneration. In this review, we provide an overview of recent developments that occurred in the biomimetic design and fabrication of cardiac scaffolds and patches. Furthermore, we sift in vitro and in situ strategies in several preclinical and clinical applications. Finally, we evaluate the possible use of bioengineered cardiac tissue equivalents as in vitro models for disease studies and drug tests.

Keywords: cardiac tissue engineering; cardiovascular diseases; disease modeling; myocardium; regenerative medicine.

FIGURE 1

Native myocardium in physiological conditions. (A) Transversal section of the cardiac ventricles. (B) Longitudinal distribution of myocardial fibers. (C) Organization of myocardial fibers. (D) Structure of a sarcomere with Z-discs at both extremities, longitudinally distributed A-bands composed of myosin and actin, I-bands rich in actin and titin and disposed between Z-discs and A-bands, in the middle, H-zones with only myosin and cross-connected centrally by M-lines. During contraction, the filaments of myosin and actin slide.

FIGURE 2

Scaffolds for in vitro engineering and in situ regeneration of myocardial tissues. (A) Cardiac organs can be a source of natural scaffolds after decellularization and isolation. (B) Cell sheets submitted to decellularization provide a cardiac-like extracellular matrix synthetized by cardiomyocytes in vitro. (C) Scaffolds can be also realized with many different biomaterials, both natural and synthetic, and techniques (bioprinting, electrospinning, molding, etc.) to tune biomechanical characteristics (elasticity, porosity, etc.). (D) Scaffolds can be also available as hydrogels, obtained gelatinizing a specific biomaterial. They can assume a solid transition liquid-like state to be injected, also with cells, and afterwards be polymerized in the site of final application.

FIGURE 3

Cell sources for myocardial tissue engineering (A): mesenchymal stem cells from bone marrow (BM-MSCs), stem cells from umbilical cord (UC-SCs), adipocytes and adipose-derived mesenchymal cells from adipose tissue (AD-MSCs), cardiomyocytes (CMs) either differentiated by cardiac progenitor cells (CPCs) or induced pluripotent stem cells (iPS). Once isolated, these cells can be cultured in vitro (B) in order to realize a cell-sheet, a sufficient number for an in situ heart injection (C), to populate a scaffold creating a viable myocardial tissue patch (D) for in vivo implantation or in vitro disease modeling and drug testing.

FIGURE 4

Current modalities of scaffold engineering. (A) Electrospinning uses electric force to draw charged threads of polymer solutions or melts up to fiber diameters in the order of hundred nanometers. (B) 3D (bio)printing is the construction of a 3D scaffold from a digital 3D model. The chosen material is deposited by an extruder or syringe needle, joined, and solidified typically in layer-by-layer way. (C) A hydrogel scaffold is a biphasic material, a mixture of porous permeable solids and insoluble water, characterized by insoluble 3D network of natural or synthetic polymers with an intrinsic hydrophilic character due to their functional groups. (D) An injectable hydrogel is generally based on the possibility of some biomaterials to be injected as liquid into the human body, and then, in situ transformed into solid gels. Generally, a mixture of the polymer/monomer solution (called precursor) and therapeutic agents in the syringe is feasibly administrated to a desired site in the body since its viscosity is low enough to be injected through a syringe needle. Then, the therapeutic agent-loaded hydrogel is formed by crosslinking reaction where its viscosity drastically increases during the sol-to-gel transition phase. The gelation typically occurs by forming the crosslinks via chemical or physical reactions. (E) Decellularization is a procedure that removes cells and non-ECM proteins from a native tissue using physical, chemicals and/or enzymatic treatments. In opportune conditions, decellularized scaffolds preserve the original ECM of tissues and organs, including the network supporting vasculature and innervation. (F) Functionalization is a process aiming at adding new features, capabilities, or properties to a material by changing its surface chemistry. It is performed by decorating the material surface with molecules or nanoparticles, either with a chemical bond or just through adsorption. (G) Temperature-responsive polymers are a class of “smart” materials that exhibit drastic and discontinuous changes in their physical properties in response to temperature. Thanks to this property, thermoresponsive materials find application into drug delivery, gene delivery, and in situ tissue engineering. (H) Electroconductive materials have the capacity to carry electric current, thus conveying bioelectronic signals. Artificial electroconductive materials, as conductive peptides (composites containing gold or silver nanoparticles and nanowires), carbon-based materials (like graphene and carbon nanotubes), and synthetic conductive polymers (originating from the field of organic electronics) can be mixed with different synthetic/natural polymers and used to realize electroconductive scaffolds.