Cardiovascular disease is a pervasive health issue, ranking among the leading causes of death worldwide ^[1][2][1,2]. According to the World Health Organization, it is estimated that cardiovascular diseases accounted for approximately 17.9 million deaths in 2019, representing 32% of all global deaths (World Health Organization, Geneva, Switzerland (2021)). Cardiovascular Diseases (CVDs). Retrieved from (https://www.who.int/news-room/fact-sheets/detail/cardio, accessed on 3 April 2023) [3]. Its detrimental impact on individuals and communities necessitates urgent efforts to develop novel treatments. In this regard, cardiac regeneration has emerged as a promising area of research, aiming to pioneer innovative approaches for repairing or replacing damaged heart tissue [4]. The ultimate goal of cardiac regeneration is to provide a long-term solution for the treatment of various cardiovascular diseases, including heart failure, heart attacks, and strokes. As the global population continues to age and the available treatment options remain limited, the field of cardiac regeneration has assumed significant importance ^{[5][6][7][8][9]}[5,6,7,8,9].

2. Stem Cell Therapy

Stem cell therapy has garnered significant attention in recent years for its potential to revolutionize the treatment of cardiac diseases. Stem cells possess a remarkable capability to divide and transform into specialized cells, making them ideal for repairing damaged tissues. Specifically, in case of heart failure and other cardiac diseases, stem cells can regenerate cardiac tissues, offering a promising avenue for treatment ^{[10][11][12][13]}[58,59,60,61]. One of the primary advantages of utilizing stem cell therapy for cardiac regeneration lies in its potential to restore lost cardiac function ^{[12][14][15][16]}[60,62,63,64]. Recent studies have demonstrated that stem cells can enhance heart function in patients suffering from heart failure by regenerating tissue and reinstating blood flow. For instance, studies published in The Lancet ^[17][18][65,66] and in Physiological Review ^[19][67] showed that cardiac stem cells improved heart function and reduced scar tissue in patients with heart failure. Furthermore, stem cell therapy has the potential to reduce or even reverse the progression of cardiac diseases. A study published in Circulation Research ^[20][68] reported that injecting bone marrow-derived stem cells into the hearts of patients with severe heart failure resulted in a significant reduction in major adverse cardiovascular events. Another notable benefit of stem cell therapy for cardiac regeneration is the relative safety of the procedure. Stem cells can be derived from various sources, including the patients’ own body, and carry a low risk of triggering immunological reactions ^[21][22][69,70]. Additionally, stem cell therapy is minimally invasive and could be performed as an outpatient procedure, minimizing the burden on patients ^[23][71]. While the potential of stem cell therapy for cardiac regeneration is immense, there are still certain limitations to be addressed. Several studies have revealed drawbacks in current stem cell-based therapies. These limitations include significant cell death and apoptosis, insufficient cell engraftment, limited cardiac regeneration post cell transplantation, and the need for careful monitoring of potential autoimmune adverse reactions ^[24][72]. To overcome these limitations, various strategies have been developed. These strategies aim to improve cell survival and engraftment, as well as induce transdifferentiation of somatic cells directly into functional cardiomyocytes to stimulate endogenous cardiac regeneration ^[25][73]. One such study, published in Nature ^[26][74], demonstrated successful reprogramming of fibroblasts into cardiomyocyte-like cells using a combination of transcription factors. Cell types used for cardiac regeneration can be broadly categorized into two groups: adult stem cells and pluripotent stem cells ^[27][75]. Adult stem cells, such as skeletal myoblasts (SMs), hematopoietic stem cells (HSCs), endothelial progenitor cells (EPCs), mesenchymal stem cells (MSCs), and cardiac progenitor cells (CPCs), can be isolated from different tissues, including skeletal muscle, adipose tissue, peripheral blood, bone marrow, and heart tissue ^[28][29][76,77]. In addition, pluripotent stem cells, such as embryonic stem cells and induced pluripotent stem cells (reprogrammed somatic cells), possess a clear potential to differentiate into functional cardiomyocytes, setting them apart from adult stem cells ^[17][30][31][65,78,79]. Notably, an important difference between these two categories of cells is that while adult stem cells have a variable cardiogenic transdifferentiation capacity, pluripotent stem cells have an unequivocal potential to differentiate into functional cardiomyocytes ^[32][80]. Extensive research has been conducted utilizing these cell types in cardiac regeneration studies, demonstrating their tremendous potential for treating cardiovascular diseases. Adult stem cells and pluripotent stem cells are typically administered via intravascular injections, injections into the pericardium, or infusion of cardiac patches ^[33][81]. Moreover, these cells can be employed to repopulate synthetic and natural scaffolds, allowing for various recellularization methods ranging from simple manual cell seeding to sophisticated controllable systems ^[34][35][82,83]. Although stem cell therapies hold considerable potential for cardiac regeneration, their utilization comes with a host of noteworthy risks and challenges that must be comprehensively addressed before embarking on stem cell-based treatments for heart repair. A profound understanding and meticulous mitigation of these challenges are imperative for the successful realization of stem cell-based heart regeneration. A prime concern associated with stem cell therapies revolves around the potential for transplanted cells to undergo malignant transformation and form tumors ^[36][84]. This is particularly relevant for pluripotent stem cells, which possess the remarkable capability to differentiate into diverse cell types, including cancerous ones ^[37][85]. Orchestrating the precise differentiation of stem cells into desired cardiac cell types while averting uncontrolled proliferation poses a formidable obstacle. Furthermore, the immune system may perceive transplanted stem cells as foreign intruders, instigating immune responses that could impede the survival and function of the introduced cells ^[36][84]. Addressing this issue might necessitate immune suppression, which introduces its own set of intricate complications and potential side effects. The achievement of effective heart regeneration extends beyond mere tissue replacement; it entails establishing harmonious electrical integration with the existing heart structure. Disharmony in electrical signaling between transplanted cells and native tissue can lead to arrhythmias and other electrical irregularities that jeopardize cardiac function. Additionally, the complete maturation and functional integration of transplanted stem cells into mature cardiac cells can be elusive. Inadequate integration or functional capacity may undermine the anticipated contribution of these cells to heart repair. Crucial to the survival of transplanted cells is their ability to access oxygen and nutrients from the bloodstream. Ensuring proper vascularization and blood supply to these cells presents a formidable challenge, which could potentially limit the efficacy of the therapy. The durability and long-term effects of stem cell-based cardiac regeneration therapies remain subjects of ongoing investigation. Determining the duration of treatment effects and the potential need for repeat interventions is paramount ^[38][86]. Transitioning from preclinical studies to clinical applications involves a multitude of clinical translation challenges ^[7][39][7,87]. These encompass scaling up production of consistent, high-quality stem cell products, tackling safety concerns, and substantiating efficacy in human trials. Patient heterogeneity is another pertinent consideration, as individual responses to stem cell therapies may differ based on factors such as age, overall health, genetics, and the severity of cardiac conditions. Tailoring treatments to suit individual patient profiles remains intricate ^[40][88]. A significant obstacle lies in the absence of standardized protocols within the evolving field of stem cell therapies. The lack of uniformity in cell types, delivery methods, and dosages hampers reliability and comparability across studies and trials ^[41][42][89,90]. In light of these complex challenges and risks, meticulous preclinical assessment, well-designed clinical trials, and ongoing vigilance are crucial to guarantee the safety and effectiveness of stem cell-based cardiac regeneration therapies.

3. Tissue Engineering

The field of regenerative medicine has witnessed remarkable strides in recent years, with engineered cardiac constructs emerging as a promising approach for addressing the pressing need for effective treatments for heart diseases. These constructs, meticulously designed to mimic the intricacies of native cardiac tissue, hold the potential to revolutionize therapeutic approaches ^[43][44][165,166]. Preclinical studies have laid the foundation for the development and optimization of engineered cardiac constructs. In vitro models, such as 3D cell cultures and tissue-engineered platforms, have provided invaluable insights into the behavior of these constructs under controlled conditions. These investigations have enabled researchers to fine-tune factors such as cell source, scaffold materials, and biomechanical cues to enhance construct functionality and integration. Moreover, animal models have served as vital testing grounds for engineered cardiac constructs, offering a bridge between in vitro experiments and clinical trials. Preclinical studies have demonstrated the constructs’ potential for restoring damaged cardiac tissue, improving contractility, and promoting angiogenesis. These findings underscore the feasibility of translating these technologies into human therapies ^[45][46][167,168]. The transition from preclinical success to clinical trials represents a critical juncture in the journey of engineered cardiac constructs. Initial clinical trials have primarily focused on safety and feasibility, showcasing the constructs’ ability to engraft into the host tissue without eliciting adverse immune responses. Early-phase trials have provided glimpses of improved cardiac function, affirming the constructs’ potential to impact patients’ lives. As clinical trials advance, researchers are addressing challenges such as scalability, long-term efficacy, and optimal patient selection. These trials also offer a platform for refining delivery methods, minimizing off-target effects, and tailoring treatment regimens to individual patient needs ^[47][48][49][169,170,171]. The promise of engineered cardiac constructs extends far beyond the confines of cardiac repair. These constructs hold potential for diverse therapeutic applications, including myocardial infarction treatment, heart failure management, and drug screening ^[50][51][172,173]. Engineered cardiac tissues provide an invaluable tool for understanding disease mechanisms, enabling researchers to unravel complex pathways and test novel therapeutic interventions. Furthermore, the synergy between engineered cardiac constructs and other regenerative strategies, such as gene therapy and stem cell treatments, opens new horizons for personalized medicine ^[52][174]. The ability to harness the regenerative potential of these constructs, combined with their capacity for tailored interventions, heralds a new era in cardiovascular care.

4. Biomaterials

Biomaterials play a crucial role in cardiac regeneration by providing a supportive environment for cell growth, facilitating tissue repair, and promoting functional recovery of the heart. The unique properties of biomaterials make them suitable for cardiac regeneration applications. They can be engineered to mimic the structure and function of native cardiac tissue, allowing them to integrate seamlessly with the surrounding heart tissue. Biomaterials can be designed to possess specific mechanical properties, such as elasticity and stiffness, which are essential for proper cardiac function ^[53][54][175,176]. In the context of cardiac regeneration, biomaterials can serve various functions. They can act as scaffolds, providing a framework for the attachment, proliferation, and differentiation of cardiac cells. These scaffolds can be biodegradable, meaning they are gradually broken down and replaced by new tissue as the heart heals. Additionally, biomaterials can release bioactive molecules or drugs to enhance cell survival, promote angiogenesis, and modulate the immune response ^[55][56][177,178]. Various types of biomaterials have undergone extensive research for their application in cardiac regeneration. These include a diverse range of natural polymers, such as collagen, fibrin, gelatin, alginate, chitosan, silk, hyaluronic acid, and decellularized extracellular matrix ^[57][179]. Furthermore, synthetic polymers like polyurethane, polyethylene glycol, poly(ε-caprolactone), poly(lactic-co-glycolic acid), poly(l-lactide), and poly(glycerol sebacate) have also been meticulously investigated for their potential in this field. These materials can be processed into various forms, such as hydrogels, films, or fibres, depending on the specific application ^[58][180]. In recent years, advances in biomaterial science and tissue engineering have shown promising results in preclinical and clinical studies of cardiac regeneration. The use of biomaterials has demonstrated improvements in cardiac function, scar reduction, and tissue remodeling, leading to a better quality of life for patients with heart disease ^[59][60][181,182]. While challenges remain, such as achieving long-term functionality and ensuring proper integration of regenerated tissue with the host heart, the field of biomaterials for cardiac regeneration holds great promise. Continued research and innovation in this area have the potential to revolutionize the treatment of heart disease, providing new opportunities for restoring damaged hearts and improving patient outcomes. Biomaterials offer exciting opportunities for cardiac regeneration by providing structural support, promoting cell growth, and guiding tissue repair. With ongoing advancements in biomaterial science, the development of innovative strategies, and collaboration between scientists, engineers, and clinicians.

5. Bioreactors

Bioreactors are essential tools for tissue engineering, as they allow to grow cells and tissues in a controlled environment and study their behavior. Bioreactors provide the necessary nutrients and oxygen to the cells, while controlling the temperature and pH levels. Bioreactors come in a variety of shapes and sizes, from small benchtop models to larger, more complex systems. Depending on the type of research being conducted, researchers can choose from a range of technologies, including rotating wall vessels, stirred tanks, rocking platforms, and perfused systems. Bioreactors are not only used in tissue engineering, but also in stem cell research, drug development, and other areas of biomedicine. Their most important function, though, is to provide a safe and controllable environment for growing tissue and organ in culture. Bioreactors have been extensively studied in the field of tissue engineering, and numerous published studies have highlighted their critical role in creating optimal environments for cell and tissue growth. For example, a study conducted by Smith et al. demonstrated the importance of nutrient delivery in bioreactors for promoting cell proliferation and maintaining cell viability. The researchers found that by carefully formulating the culture media, they were able to provide cells with optimal nutrition, leading to enhanced growth and maintenance ^[61][62][277,278]. In another study by Jomezadeh Kheibary et al. (2020), the researchers focused on the regulation of oxygen levels within bioreactors. They showed that precise control of oxygenation in the bioreactor environment mimicked physiological conditions and supported cellular metabolism. This study emphasized the significance of maintaining sufficient oxygen levels for promoting proper cellular function and tissue development ^[63][279]. Temperature and pH regulation within bioreactors have also been extensively investigated. Research by Chen et al. (2020) demonstrated the impact of temperature control on cell growth and function. The study revealed that maintaining the desired temperature within the bioreactor, matching the physiological conditions of the targeted tissue, contributed to cell growth resembling their natural environment ^[64][280]. Similarly, pH levels have been investigated, as highlighted in the study conducted by Lee and colleagues in 2017. They found that carefully monitoring and adjusting the pH to the optimal range within the bioreactor helped cell growth and function ^[65][281]. Numerous studies have utilized bioreactor systems to investigate the behavior of cells and tissues in response to different stimuli. For example, a study by Ginai et al. (2013) utilized a bioreactor to study the effects of therapeutic agents on cells. By introducing drugs into the bioreactor system, the researchers were able to observe cellular responses in a controlled environment, providing valuable insights into the efficacy and potential side effects of novel treatments ^[66][282]. Various types of bioreactors have been developed to meet specific research needs, as highlighted in the study conducted by Williams et al. (2019). The researchers compared the advantages of different bioreactor designs and demonstrated how these different bioreactor systems offered unique benefits for cell and tissue culture, such as three-dimensional growth, large-scale production, fluid flow simulation, and nutrient exchange ^[67][283].

6. Gene Therapy

Gene therapy has emerged as a cutting-edge and increasingly popular method for treating a wide range of diseases, revolutionizing the field of medicine ^[68][300]. Among its vast potential applications, researchers have directed their attention toward exploring the use of gene therapy in cardiac tissue engineering, aiming to overcome the limitations of conventional treatments and provide new options for addressing cardiac diseases ^[69][301]. Cardiac tissue engineering involves the intricate process of utilizing gene therapy techniques to create new heart tissue or regenerate damaged tissue, thereby compensating for the effects of various cardiac disorders or injuries. This approach typically involves introducing therapeutic genes into the affected area, either by directly delivering them to the tissue or by using vectors such as viral particles to transport and integrate the genes into the cells ^[70][302]. To assess the effectiveness of gene therapy in cardiac tissue engineering, numerous studies have been conducted, spanning both preclinical and clinical settings ^[71][72][303,304]. These investigations have utilized various animal models and in vitro experiments to elucidate the potential of gene therapy interventions in repairing and regenerating cardiac tissue ^[73][74][75][305,306,307]. Additional studies have demonstrated the capacity of gene therapy to stimulate the regeneration of cardiac muscle cells, which are vital for proper heart function. For instance, researchers have used gene therapy to deliver genes responsible for cell proliferation, differentiation, and survival to the damaged cardiac tissue. These genes trigger the activation of specific cellular pathways, leading to the replication and maturation of cardiac muscle cells and ultimately improving the contractility and overall performance of the heart ^{[76][77][78][79]}[309,310,311,312]. Furthermore, gene therapy has shown promise in addressing heart failure, a condition characterized by the loss of cardiac muscle cells and diminished heart function. Studies have investigated the use of gene therapy to stimulate the proliferation and differentiation of CPCs into mature cardiac muscle cells. By delivering genes that enhance the growth and maturation of these progenitor cells, researchers have observed the restoration of cardiac tissue integrity and function, offering hope for individuals suffering from heart failure ^[79][312]. Amidst its bright prospects, gene therapy for cardiac regeneration also unveils a spectrum of risks that demand accurate management ^[80][313]. This therapeutic modality involves the introduction of foreign genetic material into cells, a process that carries the potential for unintended interactions with the host genome ^[81][314]. The repercussions of such off-target effects might manifest as genetic mutations, disruption of normal cellular processes, or, in extreme cases, the initiation of cancerous growth ^[72][82][83][304,315,316]. The utilization of viral vectors or alternative gene delivery systems in this context can incite immune responses within the body. The immune system’s identification of these viral vectors as intruders might trigger a defensive reaction, potentially neutralizing the intended therapeutic impact and inducing inflammatory reactions ^[84][317]. Moreover, the expression of therapeutic genes or proteins could trigger inflammatory responses, culminating in tissue impairment or untoward reactions ^[85][318]. Assuring the prolonged safety of gene therapy constitutes a formidable task. The enduring effects of gene therapy might display a transient nature, with the therapeutic benefits diminishing over time as the introduced genetic material becomes diluted or degrades ^[86][319]. The utilization of viral vectors to facilitate the delivery of therapeutic genes bears certain risks, including the potential integration of vector DNA into the host genome ^[84][317]. This eventuality could disrupt normal gene function or pave the way for oncogenic repercussions. Striking an optimal balance in gene expression assumes pivotal significance. Excessive or deficient expression may precipitate unforeseen consequences, affecting therapeutic efficacy or instigating deleterious effects ^[87][51]. In individuals harboring inherited or genetic heart conditions, gene therapy has the potential to interact with existing genetic mutations, thereby complicating therapeutic outcomes or introducing novel risks. Patient responses to gene therapy invariably exhibit variability, influenced by factors spanning genetics, age, and overall health ^[68][88][300,320]. Crafting treatments attuned to the unique exigencies of individual patients while minimizing risks necessitates a personalized approach. The transition from preclinical studies to clinical applications introduces multifaceted complexities. This journey entails refining gene delivery methods, optimizing dosages, and establishing meticulous monitoring protocols ^[89][90][91][321,322,323]. The intricate interplay between genes, characterized by complexity and incomplete comprehension, introduces an additional layer of intricacy. Modifying one gene could trigger cascading effects on other genes and pathways, potentially engendering unforeseen outcomes.

7. Conclusions

In conclusion, cardiovascular disease remains a significant global health issue, and the development of novel treatments is crucial to address this burden. The field of cardiac regeneration holds great promise in providing long-term solutions for treating cardiovascular diseases. Recent advancements in stem cell therapy, biomaterials, bioreactors, and gene therapy have significantly contributed to the progress of cardiac tissue engineering. Stem cell therapy has shown promising results in differentiating stem cells into heart cells and repairing damaged cardiac tissue. Biomaterials have enabled the creation of scaffolds that support the growth and organization of heart cells, facilitating the formation of functional cardiac tissue. Bioreactors have provided controlled environments to promote the maturation of cardiac tissue, closely resembling native heart tissue. Gene therapy has offered strategies to enhance stem cell differentiation or improve the survival and function of existing heart cells. Clinical applications of tissue engineering are already being used to repair damaged heart tissue and improve the lifespan of these medical interventions. Furthermore, the development of lab-grown functional heart cells opens the possibility of personalized treatments that surpass traditional methods in effectiveness. These advancements in cardiac tissue engineering have the potential to revolutionize cardiovascular medicine, offering new treatments and more durable replacement organs. However, further research and clinical trials are necessary to fully evaluate the safety and efficacy of these therapies. Each approach here described has shown potential based on current data, albeit no single technology or strategy has definitively emerged as the most promising. The common assumption among the scientific community is that the right combination of different approaches may hold the most potential for cardiac regeneration. Combining stem cell therapies with tissue engineering, gene therapy, or other techniques could enhance the overall regenerative effect, as the complexity of the regeneration process needs a multipronged approach focused on diverse and fundamental aspects related to cardiac regeneration, such as cells, biological signaling and microenvironment.