Smart bioactive hydrogels for myocardial infarction repair: a multifunctional approach integrating stimuli-responsive drug delivery, electroconductivity, and real-time biosensing

Collagen, a primary structural protein, contributes significantly to the mechanical strength of tissues. Among the various collagen types present in the human body, type I collagen is the most abundant and widely utilized in tissue engineering due to its excellent biocompatibility. Type I collagen hydrogel can be synthesized by dissolving collagen in a 0.3% glacial acetic acid solution, followed by neutralization to a physiological pH of 7.4. However, injecting acidic solutions into infarcted cardiac tissue may induce localized inflammation, while slow gelation time poses additional challenges in cardiac tissue engineering(Table). [] ⁷ [] ⁷ 1

Fibrin, another natural hydrogel, is formed through the interaction of fibrinogen and thrombin during the coagulation process, catalyzed by calcium. As a nontoxic and biodegradable biomaterial, fibrin holds significant potential for tissue engineering applications. Birlareported the implantation of hollow fibrin gel tubes filled with neonatal CMs into adult rat femoral arteries, highlighting its feasibility for myocardial repair. Additionally, Huangimplanted rat CMs in fibrin gel and observed sustained contractility and normal pacing ability for up to 2 months. Blackfurther explored fibrin/CM constructs under conditions of radial contraction, demonstrating their potential utility in myocardial tissue engineering(Table). [] ⁸ [] ⁸ [] ⁹ [] ¹⁰ et al et al et al 1

Synthetic polymers, including poly(ethylene glycol) (PEG), polylactide (PLA), polylactide-glycolic acid (PLGA), polycaprolactone (PCL), polyacrylamide (PAA), and polyurethane (PU), are widely utilized in cardiac tissue engineering due to their customizable physical and chemical properties, such as modulus, water affinity, and degradation rate, making them well-suited for myocardial repair. However, a key concern with synthetic polymers is their potential cytotoxicity. Among the commonly used polymers in drug delivery, PEG, PLA, and PLGA are extensively studied, while PAA and PU have demonstrated biocompatibility in bothandstudies. [] ¹¹ [,] ^{12 13} in vitro in vivo

PEG, a water-soluble polymer synthesized via ring-opening polymerization of ethylene oxide, is widely employed in drug delivery and tissue engineering due to its biocompatibility. Photopolymerization of diacrylate-modified PEG in the presence of UV light leads to the formation of PEG hydrogels, which serve as supportive matrices in various tissue engineering applications, including the liver, pancreas, bladder, skin, and cartilage. However, its low protein affinity limits cell adhesion, necessitating modifications such as protein or peptide conjugation and the incorporation of growth factors. PEG hydrogels have also been used to study three-dimensional (3D) CM–matrix interactions. [] ¹³ [] ¹³

Poly(2-hydroxyethyl methacrylate) (PHEMA) is another hydrophilic polymer with pendant hydroxyl groups that has been explored for cardiac applications. Walkerinvestigated its use in the canine epicardium by embedding a poly(ethylene terephthalate) (PET) mesh-reinforced PHEMA gel, highlighting its potential for myocardial repair. PAA gels, owing to their amide structure-mimicking properties, hold promise for cardiac tissue engineering. Similar to PEG, PAA hydrogels benefit from crosslinking techniques and exhibit a thermo-responsive sol–gel transition, allowing them to remain in liquid form at lower temperatures and transition into a soft gel at physiological temperatures. This unique property makes PAA hydrogels ideal for injectable applications, enabling minimally invasive delivery. Poly(-isopropylacrylamide) (PNIPAM) is a widely studied thermosensitive polymer with a lower critical solution temperature of 32°C, remaining in a liquid state at room temperature and forming a gel at physiological temperatures. [] ¹⁴ [] ¹⁵ [] ¹³ [] ¹⁶ et al N

Hydrogels have emerged as versatile platforms for myocardial repair due to their ability to deliver bioactive molecules, stem cells, and extracellular vesicles (EVs) in a controlled manner, thereby promoting angiogenesis, CM survival, and immunomodulation. Angiogenesis, essential for restoring blood supply to ischemic myocardium, is tightly regulated by growth factors such as VEGF, PDGF-BB, bFGF, HGF, IGF, and TGF-β. The therapeutic efficacy of these factors depends on precise release kinetics, as insufficient or excessive exposure can result in unstable or aberrant vessel formation. Hydrogel scaffolds have been engineered to modulate growth factor delivery through strategies, such as covalent crosslinking with heparin or fibrin, incorporation of proteolytically degradable microspheres, functionalization with bioactive peptides (e.g., RGD, REDV), and layer-by-layer polymer assembly. These approaches enhance endothelial cell attachment, migration, and proliferation, facilitating the formation of stable vascular networks while mitigating burst release. Notably, multicomponent delivery systems – such as heparin-immobilized scaffolds or fibronectin-functionalized hydrogels – demonstrate superior angiogenic outcomes compared to single-factor delivery, emphasizing the importance of recapitulating the complex signaling environment of the extracellular matrix. [] ¹⁸ [,] ^{19 20}

Beyond growth factor delivery, hydrogels serve as carriers for stem cells and EVs, offering additional regenerative and immunomodulatory benefits. Stem cell-derived EVs exhibit higher bioactive cargo content than somatic cell EVs, and their composition can be influenced by tissue origin, donor age, sex, and environmental conditions. To circumvent ethical and scalability limitations of cardiac stem cells, alternative sources such as mesenchymal stem cells, endothelial progenitor cells (EPCs), and induced pluripotent stem cell (iPSC)-derived CMs have been employed. Chendemonstrated that EPC-derived EVs encapsulated in a shear-thinning HA hydrogel reduced infarct size and enhanced angiogenesis in a rat MI model, as indicated by increased vWF, α-SMA, and CD11b-positive cells in the peri-infarct region. Similarly, self-assembling peptide amphiphile hydrogels incorporating cardiac protective peptides and MSC-derived EVs promoted vascularization, attenuated fibrosis, suppressed apoptosis, and modulated TGF-β1 signaling. [] ²¹ [,] ^{22 23} [] ²⁴ et al

iPSC-derived CM EVs delivered via collagen Gelfoam hydrogel patches further illustrate the critical interplay between EV type and hydrogel properties. Liureported sustained EV release, reduced infarct size, mitigated pathological hypertrophy, and improved ejection fraction in athymic rats, while CM-derived EV patches outperformed iPSC-EV patches in reducing Caspase activity and apoptosis. Immunomodulatory strategies using dendritic cell-derived EVs (DEXs) encapsulated in alginate hydrogels have also been shown to prolong EV retention, reduce M1 macrophage infiltration, increase CD206+ macrophages and regulatory T cells, and enhance vascularization, collectively contributing to improved myocardial repair. et al [] ²⁵ [] ²⁶

Replicating the structural complexity of native cardiac tissue is crucial for engineering functional myocardial constructs. Scaffold porosity and microarchitecture significantly influence tissue formation by affecting matrix stiffness, extracellular matrix (ECM) secretion, and cellular behavior, including survival, proliferation, and migration. Traditional scaffold fabrication techniques, such as solvent casting, freeze-drying, and gas foaming, allow for control over pore size and porosity but lack precision in creating complex geometries or guiding cell distribution necessary for biomimetic cardiac tissue engineering. Advancements in biofabrication, including microfabrication tools, fiber-based technologies, and 3D bioprinting, have overcome these limitations by enabling precise control over scaffold mechanical properties, pore interconnectivity, and cellular organization. These technologies have been widely applied in cardiac tissue engineering due to their ability to generate highly structured, cell-laden constructs. Among these, photopatterning and micromolding have emerged as leading microfabrication approaches for hydrogel-based cardiac scaffolds, allowing fine-tuned cell–material interactions and improved myocardial tissue integration. [] ²⁷ [] ²⁷ [] ²⁸ [,] ^{27 28}

Photopatterning, or photolithography, employs light exposure to create micropatterned hydrogels by selectively crosslinking regions of a photocrosslinkable polymer, such as gelatin methacryloyl (GelMA) or methacrylated HA. This method provides spatial control over the cellular microenvironment and enables the incorporation of multiple cell types within a single construct. However, challenges such as optimal UV exposure time and resolution limitations at higher wavelengths remain. Studies have shown that photopatterned GelMA hydrogels loaded with endothelial cells and cardiac progenitors exhibit high cell viability, with microconstruct width influencing cellular alignment and elongation – critical factors for CM organization and function. [] ²⁹ [] ²⁹

Electroconductive hydrogels (ECHs) have emerged as a transformative strategy in cardiac tissue engineering by enhancing electrical integration and promoting myocardial remodeling. These hydrogels combine a biocompatible matrix with conductive components to mimic the electrophysiological properties of native myocardium, supporting CM alignment, differentiation, and functional coupling. By restoring electrical signaling, ECHs facilitate synchronized contraction, gap junction formation, and extracellular matrix (ECM) remodeling, which are critical for reducing fibrosis and supporting cardiac regeneration. [] ³² [,] ^{32 33}

ECHs can be broadly categorized into ionic, metallic, and polymer-based systems. Ionic conductive hydrogels, containing positively and negatively charged groups within a porous 3D structure, support ion transport, a key determinant of electrophysiological function. However, their applications are often limited by suboptimal biocompatibility and insufficient self-healing properties. Metallic-based hydrogels offer superior electrical conductivity and mechanical stability, yet crosslinking between metal particles and polymer chains can compromise their performance in biological environments. To overcome these limitations, conductive polymers such as polypyrrole (PPy), polyaniline (PANI), and poly-(3,4-ethylene dioxythiophene) (PEDOT) have been incorporated into hydrogels, providing a balance of elasticity, conductivity, and biocompatibility. The hydrophilic polymer network ensures structural stability, while the conductive components facilitate charge transport and dynamic swelling behavior, enhancing responsiveness to the cardiac microenvironment. [] ³³ [] ³⁴ [] ³⁵ [,] ^{33 34}

Fabrication strategies for ECHs include integrating conductive polymers, introducing ionic carriers, and embedding conductive nanomaterials such as GO, CNTs, and gold nanowires (GNWs). Conductive polymers offer efficient charge transport via conjugated π-electron structures, while ionic carriers optimize electrophysiological responses by facilitating ion flow. Nanomaterial incorporation further enhances conductivity, mechanical resilience, and CM maturation, although potential cytotoxicity and long-term biocompatibility remain concerns. [,] ^{35 36}

Beyond electrical signaling, ECHs provide platforms for controlled, electrically-responsive drug delivery. Mechanisms such as electrically-triggered diffusion, redox-switching, and electro-responsive erosion enable precise, on-demand release of bioactive molecules, antifibrotic agents, or pro-survival factors. These properties allow ECHs to function not only as structural scaffolds but also as dynamic therapeutic reservoirs, integrating tissue engineering and regenerative pharmacology. [] ³³

Carbon-based ECHs are increasingly investigated in cardiac tissue engineering for their ability to replicate the electrical conductivity of native myocardium, promote CM maturation, and facilitate synchronized contraction. Among these materials, graphene and its derivatives have shown particular promise. Graphene, a two-dimensional (2D) hexagonal carbon lattice, offers exceptional electrical conductivity, mechanical strength, and thermal stability, making it well-suited for myocardial tissue engineering. Incorporation of GO or reduced graphene oxide (rGO) into hydrogels enables fine-tuning of mechanical stiffness and bioactivity, supporting CM attachment, electrical signal propagation, and integration with host tissue. However, the hydrophobicity and aggregation tendency of graphene present challenges, requiring advanced dispersion strategies such as wet spinning orredox processes to maintain uniform conductivity and hydrogel stability. Biocompatibility assessment remains a critical step for clinical translation. [] ³⁷ [] ³⁸ [] ³⁷ in situ

CNTs, characterized by their high aspect ratio and exceptional electrical and mechanical properties, have also been incorporated into ECHs to enhance CM contractility and synchronous beating. CNT-based hydrogels facilitate electrical coupling and mechanical reinforcement, which are essential for functional cardiac patches and engineered heart tissues. Nonetheless, CNTs' strong van der Waals forces, hydrophobicity, and aggregation tendency complicate homogeneous hydrogel formulation. Surface functionalization and dispersing agents, including microgel particles, have been employed to improve integration. Despite these strategies, potential cytotoxicity and inflammatory responses require thorough biocompatibility validation prior to clinical application. [] ³⁸ [,] ^{38 39}

MXenes, a class of 2D transition metal carbides, nitrides, or carbonitrides, have emerged as promising conductive nanomaterials for cardiac ECHs due to their high electrical conductivity, hydrophilicity, and inherent biocompatibility. MXene-integrated hydrogels support CM adhesion, proliferation, and electrical signaling, making them attractive for injectable cardiac patches aimed at restoring conductivity in infarcted myocardium. However, their low dispersibility in aqueous environments necessitates surface modifications and self-assembled hydrogel platforms to enhance stability and uniform performance in cardiac applications. [] ⁴⁰ [] ⁴¹ [] ⁴⁰

ECHs have evolved from niche biomaterials into multifunctional therapeutic platforms, offering a convergence of properties that extend well beyond isolated applications in cardiac tissue engineering. Their intrinsic ability to restore myocardial electrical continuity – by facilitating action potential propagation, synchronized cardiomyocyte contraction, and enhanced gap junction formation – establishes a foundation for functional cardiac recovery. Importantly, these electroactive matrices are increasingly being engineered to couple electrical stimulation with localized, controllable drug delivery. Through mechanisms such as electrically triggered diffusion, redox-mediated conformational switching, and electro-responsive erosion, ECHs can release regenerative growth factors, antifibrotic compounds, or pro-survival molecules in a spatially and temporally precise manner, thereby integrating electrophysiological restoration with targeted biochemical modulation. [–] ^{32 34} [] ³³

Parallel advances in 3D and 4D bioprinting have expanded the potential of ECH-based technologies by enabling the fabrication of complex, biomimetic constructs that faithfully replicate myocardial architecture. Bioprinted hydrogels incorporating CMs, endothelial cells, and stem cell populations can be engineered with conductive polymers, carbon nanostructures, or metallic nanowires to achieve concurrent electroconductivity and drug-loading capacity. The advent of 4D printing adds a further dimension of functionality, allowing constructs to undergo programmed transformations or modulate therapeutic release in response to physiological stimuli, thus approximating the dynamic adaptability of native cardiac tissue. [–] ^{59 63} [] ⁶³

The deliberate integration of electroconductivity, controlled pharmacological delivery, and advanced bioprinting strategies establishes a translationally relevant paradigm for next-generation cardiac therapies. Rather than functioning as discrete technologies, these modalities converge to yield bioengineered constructs that are electrically active, biologically instructive, and pharmacologically responsive. Such systems have the potential to not only restore electrophysiological integrity but also deliver therapeutics in a highly controlled and adaptive fashion, ultimately reducing fibrosis, enhancing myocardial regeneration, and improving long-term functional outcomes.

The clinical translation of hydrogel-based 3D/4D bioprinting and AI-assisted delivery systems for cardiac regeneration, including MI therapy, is hindered by several interrelated limitations. From a materials standpoint, many hydrogels lack the mechanical resilience and elasticity needed to withstand repetitive myocardial contractions, resulting in premature degradation or displacement, while their inherently poor electrical conductivity limits synchronized contraction unless conductive additives are incorporated, sometimes at the expense of biocompatibility. Mechanical weaknesses are compounded by challenges in tuning degradation rates to match tissue regeneration, as excessive breakdown leads to transient effects, whereas prolonged persistence can interfere with remodeling. In drug delivery applications, growth factor denaturation during gelation and uncontrolled burst release due to high porosity reduce therapeutic efficacy, while inadequate retention in highly vascularized myocardium further diminishes outcomes. Biologically, the generation of mature, patient-specific CMs at clinically relevant scales remains limited, and hydrogel-delivered stem cells often exhibit low survival and differentiation rates. Vascularization within engineered constructs is still suboptimal, with long-term patency and integrationposing unresolved challenges. Immune responses to certain synthetic or modified hydrogels may induce inflammation or fibrosis, undermining biocompatibility. Technically, although high-resolution printing is essential for microvascular patterning, it prolongs fabrication times and may compromise cell viability, while controlling postprinting shape transformations in 4D constructs under complex physiological conditions remains imperfect. AI integration adds further barriers, including dependence on large, high-quality datasets that are scarce in personalized cardiac regeneration, the high cost and limited accessibility of robotic delivery platforms, and complex regulatory approval pathways for hybrid biologic device systems. Translation to clinical practice is additionally constrained by complex manufacturing processes, storage and scalability limitations, limited large-animal and human trial data, and ethical concerns surrounding stem cell sourcing, AI-driven decision-making, and patient data privacy. Overcoming these obstacles will require improvements in mechanical stability, controlled release kinetics, immune compatibility, and manufacturing scalability, alongside rigorous preclinical and clinical validation to facilitate safe and effective adoption in cardiac repair. [] ⁴⁶ [] ²³ [] ³³ [] ⁴² [,,] ^{12 47 65} [] ⁶⁵ in vivo

While hydrogel-based strategies for cardiac repair, including electroconductive, carbon- and MXene-enhanced, stimuli-responsive, and 3D/4D-bioprinted systems, show considerable promise, the quality of the underlying studies varies widely. Most evidence derives from preclinicalor small-animal models, often with limited sample sizes and short follow-up periods, which may not fully capture long-term safety, efficacy, or functional integration in human myocardium. Variations in hydrogel composition, fabrication methods, and cellular components further complicate comparisons across studies. Few investigations rigorously address reproducibility, blinding, or standardized outcome measures, highlighting potential biases. Consequently, while the findings suggest significant therapeutic potential, cautious interpretation is warranted, and future work should emphasize robust study design, reproducibility, and translational relevance to strengthen confidence in clinical applicability. in vitro