Application of Hydrogels in Cardiac Regeneration

Myocardial infarction (MI) is one of the most common cardiac diseases globally, contributing to significant morbidity and mortality. Ischemia during MI causes a significant loss of viable myocardial tissues, which impairs cardiac function. Because cardiomyocytes are terminally differentiated cells with extremely low ability for cardiac regeneration, collagen and extracellular matrix (ECM) will be synthesized to form scars and replace the infarcted myocardium. However, scars are less elastic and incompatible with cardiac contraction and dilation, leading to cardiac dysfunction and heart failure. In order to promote cardiac regeneration and mitigate cardiac remodeling after MI, numerous studies have attempted to deliver drugs, stem cells, cytokines, growth factors, and small molecules to infarcted cardiac tissues. Stem cell transplantation can promote cardiac regeneration and restore cardiac dysfunction after MI [1, 2]. However, low retention and survival rates of stem cells are two major issues limiting their efficacy. Moreover, proteins, growth factors, and small molecules have short half-lives, low bioavailability, short retention, and low stability, all of which limit their effective application as well.

Hydrogels are a category of networks with three-dimensional swollen structures. In recent years, hydrogels have been widely applied in biology and medicine. They have an incredible superabsorbent ability: when placed in a compatible liquid medium, they can absorb at least 20 times their own weight of water from the liquid media. These swelling structures can maintain specific volumes for a period of time, and they are quite stable and resistant to dissolving due to the cross-links. When an aqueous medium is absent, hydrogels will shrink [3]. Since hydrogels have high water content and share a similar flexibility as natural tissues, they are ideal biomaterials for use on natural tissues. Mounting evidence demonstrates that hydrogels not only can be applied to tissue engineering, but also can be utilized in regenerative medicine to deliver cells, drugs, and proteins due to their physical and chemical features, which can stabilize and retain those small molecules. Numerous studies have demonstrated that hydrogels coupled with growth factors, genes, drugs, and stem cells show great potential in cardiac regeneration. Single hydrogel use or combination with small molecules can significantly ameliorate cardiac dysfunction, enhance revascularization, and attenuate cardiac remodeling in animal models with MI or ischemic reperfusion (I/R) injury. Moreover, hydrogels can be easily modified and are injectable at body temperature, which facilitates their application in cardiac regeneration.

The aim of this review is to summarize current applications of hydrogels in tissue engineering and cardiac regeneration post-MI, providing readers with a general understanding of the importance of hydrogels in cardiac regeneration. This article is a review article which is based on previously conducted studies and does not contain any new studies with human participants or animals performed by the author.

Hydrogels suitable for cardiac regeneration must meet the following demands:

There are many ways to classify hydrogels. Based on their origin, hydrogels can be classified as natural or synthetic. Based on composition, hydrogels can be defined as homopolymeric, copolymeric, or multipolymeric. According to their chemical components and physical structure, hydrogels can be defined as crystalline, semicrystalline, or amorphous (noncrystalline). Based on interactions among polymer networks, hydrogels can be classified as permanent, chemical, or physical gels [4]. Natural hydrogels have many excellent properties, including good biodegradability, easy availability, and lower toxicity and manufacturing cost.

With the rapid development of technology, natural hydrogels are continuously being replaced by synthetic hydrogels because synthetic hydrogels have stronger water absorption, longer half-life, and greater stability against temperature fluctuations in different environments. Their degradability can be tuned by structural modification, and their functionality can be modified by adjusting the ratio of ingredients, molecular weights, and cross-linking methods [4]. Synthetic hydrogels have lower immune response, which facilitates their application as artificial biocompatible materials in regenerative medicine. A summary of the hydrogel features and applications can be seen in Fig. 1.

Based on the cross-linking methods, hydrogels can be divided into physical and chemical cross-linked hydrogels. The cross-linking methods are dependent on the application and function of hydrogels.

An ideal hydrogel for cardiac repair requires easy delivery, accurate targeting, and minimum injury to the local tissue. The natural injectable hydrogels currently approved by FDA are alginate, collagen, chitosan, chondroitin, dextran, fibrin, gelatin, HA, and silk. Synthetic hydrogels like PEG, PGA, PLA, PLGA, and PNIPAM are injectable as well. This feature facilitates endocardial, epicardial, and intracoronary delivery methods to treat MI. Injectable gels can be delivered as a single bolus or multiple injections. A single bolus is generally used for small animals, whereas multiple delivery can be applied to large animals. The volume for the injection is optimized by different needs and animal models. One study suggested injection volumes as follows: 10–50 µl for the mouse model, 50–250 µl for the rat model, 200 µl for the rabbit model, 1–4 ml for the porcine model, and 1.3–6 ml for the ovine model [57]. MI is followed by cardiomyocyte necrosis, inflammation, fibrosis and scar formation, and chamber dilation. Delivering hydrogels at different time points affects different pathological processes. MI cannot be reversed at late stages. Immediate injection of hydrogel after MI is not applicable in clinical practice since patients cannot be treated instantaneously after the MI. Thus, choosing the right time point for hydrogel delivery is crucial for therapeutic effectiveness in experiments and in clinical practice [57].

The application of natural and synthetic hydrogels in MI models has been well studied in recent years (Table 1). As a natural hydrogel, alginate is commonly used to test its roles in rodent MI models. Some studies have indicated that alginate can improve cardiac function, increase scar thickness, and attract myofibroblasts after intramyocardial injection, whereas other studies have demonstrated that alginate alone produces no improvement in cardiac function [5, 62, 63]. Collagen is another natural hydrogel that is used in rodent and swine models with MI. Studies found that collagen hydrogel is most effective when administered early after the onset of ischemia. Transplantation of collagen patches with ADSCs in rat and swine MI models was shown to significantly improve cardiac function, decrease fibrosis, and enhance angiogenesis and cardiac remodeling as well [11‐13]. Fibrin is another good natural hydrogel material. Studies indicate that hydrogel made with fibrin can enhance the survival of cell transplant, reduce infarct area, and promote blood flow to the ischemic myocardium [15]. Fibrin-rAAV9-CyclinA2 is effective in preventing cardiac remodeling and preserving cardiac function after MI [16]. BMSC-fibrin patch significantly ameliorated cardiac dysfunction, reduced cardiac remodeling, and increase angiogenesis after MI [64]. Gelatin-based natural hydrogel has been shown to be protective for the heart after MI in both rat and canine models [22, 27, 65]. HA-gelatin hydrogel has been demonstrated to be effective and protective in cell activity and improving cardiac dysfunction after MI in both mice and rats [19, 66]. Moreover, a biopolymer made with Matrigel, fibrin, and collagen has proved to be promising in treating MI in rat cardiac ischemia–reperfusion injury, enhancing the infiltration of myofibroblasts into the infarct area. In addition, Matrigel-based hydrogel shows good recruitment of stem cells and angiogenesis and improves iPS cell survival in ischemic cardiac tissue [30, 32, 67]. Silk fibroin or coupling with HA and BMSCs exhibits increased angiogenesis, elevated growth factors, decreased apoptosis, and reduced cardiac remodeling after MI in mice and rats [35, 68, 69]. Moreover, synthetic hydrogels or combination with natural hydrogels have been shown to be promising in rabbit, mouse, and rat MI models with BMSCs or tPA, or cell contractility inhibitors [70‐72]. Moreover, synthetic hydrogels combined with peptides, hydrazides, aldehydes, and siRNA have been shown to be protective for cardiac function and remodeling after MI in rat and mouse MI models [73, 74]. Other synthetic hydrogels such as PCL, PEG or PGCL, or PLGA, poly(NIPAAm-co-VPco-MAPLA-co-MATEMPO), coupled with BMSCs, VEGF, IGF-1, MSCs, and reactive oxygen species (ROS) scavengers is also protective for cardiac function, cardiac remodeling, and angiogenesis in mouse, rat, and rabbit MI models [8, 42, 54, 76‐80].

Different natural hydrogels each have their own advantages and disadvantages. Alginate offers the advantages of non-thrombogenic, biocompatible, and bio-inert characteristics. However, its cell adhesion is limited, which requires modification [5, 6, 81]. Chitosan is non-immunogenic, with low toxicity and good biocompatibility, and can be conjugated with other molecules for delivery. However, its mechanical properties are non-controllable [38]. Collagen is one of the most widely used natural hydrogels due to its low immunogenicity, good availability, remarkable biocompatibility, good biodegradability, and sufficient mechanical stability, and its tensile strength can be modified by varying the collagen amount and by cross-linking. Collagen enhances stem cell engraftment and requires fewer cells to be transplanted. Moreover, collagen can easily change shapes and can be used for 3D-printed tissues. The disadvantages are that its mechanical properties are not controllable, collagen lacks mechanical robustness, and cell migration is limited because of the high weight fraction of the gel [8, 13, 82]. Elastin is soft and stretchable, with good cell–matrix interactions, elasticity, and biocompatibility. However, elastin requires a specific temperature during aggregation or self-assembly, and its insolubility, easy calcification, and poor mechanical stability limit its applications [83, 84]. Fibrin has good biocompatibility and biodegradability; it can be easily cross-linked with cells and can be injected in vivo into tissues. Fibrin can be easily modified to many shapes and morphologies, and can thus provide a good drug delivery system with great extensibility. However, stiffness and rapid degradation are two major drawbacks of fibrin [85, 86]. Gelatin has good solubility and ideal biocompatibility and biodegradability. The isolation of gelatin is simple at very low cost. Since gelatin is nontoxic and has good compatibility in vivo, it can be used for 3D printing and in vivo experiments, and is ideal for drug delivery. The drawback for gelatin is that it has a prolonged existence in vivo [22]. Hyaluronic acid (HA) is non-immunogenic and has excellent biocompatibility and biodegradability. Its disadvantages are rapid degradation and poor retention in cells [20]. Matrigel is a soluble natural hydrogel and has good attachment for cells, which promotes cellular attachment, proliferation, differentiation, and angiogenesis. It is cytoprotective and less invasive, and can be used to deliver many growth factors. However, it undergoes a fast transition to a solid at 37 °C due to structural weakness, and can only be used for short-term analysis [29, 30]. Keratin has wide availability and can be easily obtained. Its primary advantages are its low cost, renewability, good biocompatibility and biodegradability, high stability, and no inflammatory or immunogenic response. However, it is insoluble, and the chemicals used to increase its solubility are toxic [87]. Silk fibroin is abundant in nature and thus low-cost. It has robust mechanical and elastic properties and is highly stable. Because it is nontoxic and highly biocompatible, it can be used for 3D printing. Its shortcomings include mild immune and inflammatory response, difficulty in chemical modification, and degradation rate dependent on processing methods [33, 34]. The pros and cons of natural hydrogels are summarized in Table 2.

Synthetic hydrogels have advantages and disadvantages as well (Table 3). PEG is a nontoxic synthetic hydrogel that can be modified by different methods and used to carry multiple drugs, ECMs, and growth factors due to its high biocompatibility and water solubility. Its synthesis and degradation are controllable and reproducible, which makes it an ideal material for drug delivery. However, its applications are limited because of poor degradation, which requires high temperature or modification [40, 88]. PGA is a biocompatible and biodegradable synthetic hydrogel, and is a commonly used biomaterial. However, the drawbacks of PGA are rapid degradation rate and insolubility [43, 44]. PLA has renewable resources, and it has many advantages, such as good biocompatibility, biodegradability, and bioabsorbability, low toxicity, proper mechanical strength, and mild inflammatory response, and it is transparent, low-cost, easily produced, and compressible, and can be injected in vivo. The disadvantages are low hydrophilicity and long-term degradation [4]. PLGA is a biodegradable and biocompatible biomaterial with a controllable degradation rate, and can be used as a nontoxic carrier for drugs and tissues. However, PLGA can induce inflammation [89]. PNIPAM is a thermosensitive biomaterial that is suitable for controlled drug release. The drawbacks for PNIPAM are its rapid aggregation and need for chemical modification [50]. PGCL is a biomaterial with good mechanical strength, biodegradability, and biocompatibility. Its elasticity and proper pore size are also suitable for drug and cell delivery [53].

Both natural and synthetic hydrogels have their advantages and disadvantages (Table 4). Natural hydrogels are biocompatible and can provide a similar microenvironment as native tissue. They are widely available, with easy processing at low cost. However, natural hydrogels have weak mechanical properties and sometimes are difficult to modify and purify, and variations can be seen from batch to batch. Synthetic hydrogels have good biocompatibility, biodegradability, and bioresorbability. They are easily modified and have good reproducibility, controlled degradation time, and ideal mechanical properties. The drawbacks are that synthetic hydrogels are costly and the manufacturing process is complicated.

As an ideal delivery system for small molecules and stem cells, hydrogels must be resistant to enzyme degradation, have the ability to target specific tissues, and allow the controllable release of relevant molecules. Below are different cellular and molecular ingredients which have been applied in animal models with MI and show promise for treating MI in humans.

A large number of biofunctional additives can be added to hydrogels in order to enhance their biological effects. These additives can be cytokines, nucleic acids, extracellular ligands, and short peptides [98], and can be performed as biochemical and biophysical signals, and provide an ideal microenvironment. In addition, they can determine and modulate cell fate and function during regeneration. More interestingly, hydrogels can modulate the microenvironment for tissue regeneration. A relevant study indicates that hydrogel together with antioxidants and anti-inflammatory factors can inhibit target tissue oxidation and inflammation [99, 100]. The heart has the ability to transduce orchestrated electrical activity during its normal function. After MI, collagen and fibrotic scar are often formed and severely affect electrical activity transduction. Hydrogels provide a good platform for electrical signal transduction after ischemic events. Many studies are engaged in developing novel hydrogels to lower the electrical impedance in the cellular environments and enhance the survival and maturation of engineered heart tissue. In this way, the excitability and attachment of the cardiac cells and tissue can be significantly improved. Some studies demonstrate that polymer hydrogels (polypyrrole (PPy)-chitosan hydrogel) can prominently enhance cell electrical signal transduction and ameliorate cardiac dysfunction when applied to the ischemic and infarct cardiac tissue. Collagen and fibrotic scar formation during MI severely affects impulse and electrical activity transduction. Some scientists have established hybrid properties with nano-electrics and cardiac tissues to sense the spatial electrical signal in the microenvironment, which is much more promising in drug testing in a real-time manner [101].

Hydrogels can also modulate endogenous cell response. During tissue regeneration, injured tissue response relies on the surrounding cell response, which is determined by the cell types and populations. As hydrogels can be utilized as a delivery platform for stem cells or cytokines, these combinations provide basic elements for cell survival and engraftment. They also enhance the cell–cell interaction, promoting communication of information between cells [98].

Hydrogels are promising biomaterials for cardiac regeneration. Additional biomaterials with ideal physical and chemical properties will be developed in the future. Numerous studies are needed to confirm the therapeutic efficacy of hydrogels in animals and humans with MI. Hydrogels play significant roles in cardiac regeneration and tissue engineering, and pave the way for future therapeutic treatments of MI.

Numerous newly developed hydrogels and applications of artificial intelligence in delivering hydrogels have not been mentioned. The signaling pathways and mechanisms of each hydrogel are not discussed in detail due to space limitations.

Hydrogels are promising biomaterials in tissue engineering and cardiac regeneration, which can be used in the future for treatment of MI.