**6. Ventricular restraints**

**Figure 2.** (a) Scanning electron microscopy (SEM) image of poly(ethyl acrylate), PEA, elastomeric membranes with in‐ terconnected spherical pores. (b) CryoSEM image (cross section) of a PEA scaffold whose pores are filled with the selfassembling peptide (SAP) gel RAD16-I. (c) Adipose stem cells (nuclei stained in blue and actin cytoskeleton stained in green) seeded in a PEA scaffold with a SAP gel filling. Confocal laser scanning microscopy image of a 50 μm thick in‐

External electrical fields have been shown to contribute to the differentiation towards cardio‐ myocytes of different cell types, such as embryonic stem cells (ESC) [200] or BMSC [201] seeded in collagen scaffolds, and to the development of conductive and contractile properties of neonatal cardiac cells, in this case seeded with Matrigel in a collagen porous scaffold [202]. It has been proposed that the intracellular endogenous reactive oxygen species (ROS) produced

In an attempt of optimizing the electrical stimulation parameters [204], it has been determined that the electrode material is very important, and best results have been obtained for carbon electrodes. Amplitude and frequency of the stimulation have also a great influence in the cultured cardiac tissue. Micropatterned electrodes can be of interest as they allow spatial

Polymeric scaffolds limit cardiomyocytes electric communication, what restricts the synchro‐ nous beating of the engineered tissue. To improve it, gold nanowires were incorporated to a porous alginate scaffold [206]. Another approach to obtain elastic and electrical conductive scaffolds consisted in impregnating thiol-HEMA/HEMA scaffolds with gold nanoparticles [207]. In both cases even without electrical stimulation the improvement in the scaffold

Mechanical stress has a great impact on cell proliferation, ECM formation and hypertrophy (increased cell size), and has been intensively studied in the field of cardiac tissue engineering. Embryonic chick and neonatal rat cardiac myocytes mixed with collagen and mechanically stimulated exhibited hypertrophy and improvement of contractile function [208]. Cardiac myocytes from neonatal rats mixed with collagen I and Matrigel and casted in rings subjected to mechanical stretch [209] showed histological characteristics of adult cardiac tissue. Action potential measurements indicated electrophysiological behavior akin to cardiac tissue.

when an electric field is applied contribute to the hESC differentiation [203].

ternal slice.

*Electrical stimulation*

control of the electric field [205].

*Mechanical stimulation*

conductivity had positive physiological effects.

**5.6. Electrical and mechanical stimulation**

300 Regenerative Medicine and Tissue Engineering

After Chachques and Carpentier work [213], it was found that wrapping the heart even with a passive muscle flap had beneficial effects; this finding led to the development of the ven‐ tricular restraint therapy [214]. In this approach the aim is not to regenerate the ischemic tissue, but to avoid the progress of the adverse remodeling following a myocardial infarction. It is based on the application of a mechanical restraint (schematized in figure 1 c), which should limit or revert ventricular dilation. A variety of synthetic meshes have been proposed to achieve this goal.

A bilayer membrane with polypropylene in one side to promote tissue ingrowth (or at least limit the ventricular dilation) and with polytetrafluoroethylene in the other side to prevent pericardial adhesions was studied in a chronic infarction model of pig as a restraint [215]. The use of this patch induced improvements once the remodeling process following an infarction had started. The use of a non-biodegradable material is intentional as authors considered that a permanent mechanical reinforcement would be necessary to limit the remodeling.

To determine the extent at which a mechanical restraint is beneficial, a comparative study of two types of restrain was carried out in sheep: a patch over the infarct (non-biodegradable Marlex mesh) or a wrap (non-biodegradable Merseline mesh) [216]. The use of the mesh wrapping the ventricle reduced the remodeling whereas the patch applied over the infarct did not yield considerable improvements when compared with controls (untreated infarcted animals).

Paracor heartnet is a nitinol mesh proposed as a restrain device that is under clinical study in patients with severe dilated cardiomyopathy. In a study, six months after the implantation in 51 patients, results obtained suggested clinical benefits tending to reverse remodeling and that it could consequently be reliably implanted [217]. The PEERLESS-HF trial is the last carried out with this device so far [218]. It proved to be safe and improved patient's quality of life and ventricular dilation; however, no improvement in the peak of VO2 was produced (which was an end-point of the trial), what led to stop enrollment in the trial. Nevertheless, a new clinical trial is planned. In another study in an animal model, it was shown that the heartnet can alter myocardial blood flow patterns in dilated cardiomyopathy, although it remains unclear if these changes are clinically relevant [219].

Another left ventricular restraint proposed is Acorn Corcap, a polyester mesh that is also being assessed in clinical trials after the positive results obtained in animal models [220]. 5 years after implantation it exhibited safety, a sustained reverse remodeling with a significant reduction in the left ventricular end diastolic volume and a slight increase in the sphericity index [221]. However, in an echocardiographic study using tissue velocity imaging, no improvement in cardiac output was achieved [222].

Limited results obtained with the ventricular restraint therapy can be, among other reasons, because of the absence of tissue regeneration. A more advanced approach combines the ventricular restraint therapy with a regenerative strategy such as patches or scaffolds loaded with cells. For instance, the Acorn Corcap and a collagen matrix loaded with MSC has been implanted in sheeps, and the combination was found to limit the fibrosis produced as foreign body reaction against the Corcap and improve the systolic and diastolic function [223].
