Mechanical heart valves (MHVs) have been in use for more than 50 years. The advantage of MHVs is their durability, since these valves are made from materials such as pyrolytic carbon, titanium and other metallic alloys, and they last for 20 years or more [8, 9]. They have long-term stability, which minimizes the risk of reoperation, which is especially important for young patients or those who have a longer life expectancy. Individuals with mechanical valves, on the other hand, require lifelong anticoagulation therapy because these valves increase the risk for blood clot formation. Moreover, this anticoagulation requirement is associated with increased bleeding hazard, making these valves less suitable for patients with bleeding disorders or those who cannot strictly follow anticoagulation protocols [8, 9, 10].

Recent developments have improved the hemodynamic characteristics and biocompatibility of mechanical valves, and advancements in valve structures, such as bileaflet and tilting-disc models, have enhanced flow characteristics and decreased turbulence, thus lowering thrombogenicity and structural deterioration [11, 12, 13]. Scientists are also working on the development of composite valves that are made of both metals and polymers to mimic the natural movement of native valves and eliminate the need for anticoagulants [12, 14, 15].

Bioprosthetic heart valves (BHVs) are categorized by source: autografts (from the patient’s own tissue, for example, the pulmonary artery), allografts (from donors) and xenografts. Owing to limitations such as material sourcing, ethical considerations and complications, the use of autograft and allograft valve transplants remains restricted. Clinically, xenograft valves are the primary type of BHV, and most research improvements have focused on these valves. Xenogeneic BHVs are sourced from animal tissues, usually bovine pericardium or porcine aortic valve tissue, and are treated with cross-linking agents such as glutaraldehyde (GLUT) to increase the sturdiness of the tissue [16, 17, 18]. These valves exhibit more physiological characteristics of the native valves and are not usually associated with long-term anticoagulation therapy, making them ideal for elderly patients or those at risk for bleeding. However, BHVs exhibit reduced durability compared with mechanical valves primarily because of calcification and structural degradation over time [3].

To address these durability issues, new biomaterials, such as decellularized fish bladder tissue, have been developed. Fish bladder tissue has a natural collagen matrix with anti-calcification properties that can improve the biocompatibility of the valve and potentially increase its durability [19, 20, 21]. Research is also being conducted on enhanced cross-linking strategies that minimize immune reactions while maintaining the mechanical characteristics of the valve [21]. Nevertheless, the problem of calcification of bioprosthetic valves still persists and results in lower durability compared to mechanical valves.

Polymer heart valves (PHVs) provide excellent mechanical strength and fatigue resistance, along with the required flexibility, biostability and durability. PHVs can theoretically be implanted in patients of any age. Polymer valves are available in a wide range of biocompatible and biostable polymers. These valves are superior to biologic valves in terms of freedom from antigens (e.g., galactose $\alpha$-1,3-galactose and N-acetylneuraminic acid) [2]. The earliest PHV materials include polysiloxanes, polytetrafluoroethylene, and polyurethane, but these materials do not effectively prevent prosthetic degeneration and complications given limitations in their chemical properties and surface structure. With improvements in polymer manufacturing methods and advances in nanotechnology, new polymer materials, such as polyhedral oligomeric silsesquioxane poly (carbonate-urea) urethane (POSS-PCU), nanocomposite based on the functionalized graphene oxide and poly (carbonate-urea) urethane (FGO-PCU, Hastalex), nanocomposite polyvinyl alcohol (PVA) and bacterial cellulose (PVA-BC), exhibit improved mechanical properties and biocompatibility [15]. Some valves based on new polymer materials have already been used in in vitro and animal studies as well as clinical trials and have shown promising results.

Tissue-engineered heart valves (TEHVs) constitute the next generation of heart valve prosthetics and are designed to develop living, self-repairing heart valves through tissue engineering. These valves employ biodegradable meshes populated with autologous or allogeneic cells that, in due course, form new valve tissue [22, 23, 24]. TEHVs have the potential to transform valve replacement therapy since the valve can grow and change in size in the body, especially for children [25, 26, 27].

TEHV scaffolds are generally categorized into acellular and synthetic scaffolds. Acellular scaffolds are obtained from decellularized human or animal tissues and have a structure that mimics the native valve structure [28, 29]. Scaffolds made from synthetic materials, including biodegradable polymers, allow for the fine-tuning of mechanical characteristics and degradation profiles [14, 30]. Nevertheless, TEHV has several limitations, including scaffold degradation, calcification, and cell incorporation [22, 31]. Current research is being conducted to optimize scaffold materials and promote endothelialization to obtain better long-term results. Notably, despite the universal appeal of TEHV, the technique has not been used clinically.

The endothelial layer is the first barrier to calcification [32]. Shear forces and mechanical stress on the valve surface can disrupt endothelial cells and make the underlying tissue susceptible to infiltration by lipids and immune cells [33, 34, 35]. When endothelial cells are damaged, they are no longer able to prevent clot formation and become proinflammatory, promoting calcification [36]. Damaged endothelial cells release adhesion molecules that allow immune cells to attach to the tissue, increasing inflammation and accelerating calcification [37, 38].

Lipid accumulation, especially low-density lipoprotein (LDL) accumulation, is involved in the process of calcification of natural valves [39, 40]. When LDL is oxidized, it forms ox-LDL, which causes inflammation that attracts macrophages, and these macrophages take up ox-LDL and become foam cells, which cause atherosclerosis-like lesions on the valve [41]. Foam cells secrete cytokines and growth factors that induce the osteoblastic phenotype of valve interstitial cells (VICs) and lead to mineralization of the valve matrix [42]. Lipids may also be involved in amyloid deposition in valve calcification through the formation of amyloids from misfolded apolipoproteins, thereby altering ion concentrations, providing templates for mineral deposition, and promoting apoptosis in valve interstitial cells [43].

The involvement of the immune system in calcification is now well appreciated, with T cells and macrophages representing key players in the inflammatory process that leads to calcification [37, 44]. Immune cells secrete matrix metalloproteinases (MMPs) and proinflammatory cytokines that degrade the extracellular matrix (ECM) and promote calcification [44, 45]. In addition, immune cells release cytokines that stimulate osteogenic processes in VICs. As a result, calcium nodules are formed, and the valve becomes hardened [41, 46].

In addition to the immune inflammation, cell infiltration, and cytokine secretion observed in natural valve calcification, nonspecific plasma protein adsorption can activate the complement system, platelets, coagulation cascade, and cell adhesion [49, 50]. Infiltrating immune cells release proteases, degrading the ECM, which contributes to calcification by releasing calcium ions and providing binding sites [51]. In particular, the xenoantigens galactose-$\alpha$1,3-galactose ($\alpha$-Gal) and N-glycolylneuraminic acid (Neu5Gc) are thought to be important in triggering the immune response to mediate calcification [52]. Anti-Gal antibodies are the most abundant natural antibodies in humans and constitute the main immune barrier in xenotransplantation [53]. Anti-Gal in human circulation binds to $\alpha$-gal epitopes on the endothelial cells of xenografts and induces complement-mediated cytolysis, followed by platelet aggregation, small-vessel occlusion, vascular bed collapse, and hyperacute rejection of the xenografts [54]. Currently, Gal knockout (KO) pigs created using gene knockout are better able to avoid hyperacute rejection. However, the problem of calcification due to immunogenicity is not completely resolved in this model given the presence of other immune epitopes. Neu5Gc is another key xenoantigen found primarily in glycoproteins and gangliosides in most mammals, but humans do not synthesize Neu5Gc. In humans, Neu5Gc obtained through dietary intake induces natural immunity and produces anti-Neu5Gc antibodies in the serum [55]. Antibody-antigen binding promotes valve calcification, and the levels of Neu5Gc, anti-Neu5Gc immunoglobulin G (IgG), and complement deposition are much greater in calcified BHVs compared with calcified natural aortic valves [52]. In addition to the two important antigenic epitopes described above, other antigenic epitopes, such as Sda, may also participate in the process of valvular calcification, and these antigenic species and the safety of methods of elimination need to be revealed by further research.

Decellularization reduces BHV immunogenicity by removing cells and nucleic acids from the ECM using physical (freeze-thaw, high hydrostatic pressure, supercritical fluid) and chemical methods (surfactants, acids, bases, and enzymes such as trypsin) [56]. Physical methods preserve ECM integrity but are less effective immunogenically. Chemical methods are widely used but may damage ECM proteins [57]. Combination methods optimize results, but residual immunogenicity and ECM changes can affect immune responses and calcification. Some in vitro experiments have shown a higher rate of calcification in decellularized porcine aortic valves than in those fixed with GLUT, and this difference may be attributed to tissue surface modification and residual cellular debris during decellularization [58].

Natural collagen undergoes intramolecular and intermolecular crosslinking via enzymatic processes or nonspecific glucose interactions, resulting in the formation of advanced glycation end products. These crosslinks protect proteins from degradation and maintain their stability [59]. BHV cross-linking aims to enhance these crosslinks using physical and chemical techniques, influencing calcification.

Currently, GLUT is the most commonly used cross-linking agent because of its highwater solubility, fast reaction rate, superior cross-linking properties, and cost effectiveness. However, GLUT cross-linking can induce valve calcification through several mechanisms. For example, GLUT treatment causes cell death, stops membrane ion pumps, increases the level of intracellular calcium, and promotes nucleation and calcification [3]. Glycosaminoglycans in the ECM are not cross-linked by GLUT, leading to degradation under mechanical stress or proteases, exposing calcification-prone areas and facilitating collagen mineralization [3]. Unlike collagen, GLUT cannot stabilize elastin because of insufficient active amino groups, making elastin susceptible to mechanical and enzymatic degradation, resulting in calcification [60]. GLUTs form polymers through aldol condensation in water, with free aldehyde groups persisting and causing cytotoxicity [61]. The study has shown that aldehyde content is correlated with increased tissue calcium levels [62]. GLUT cross-linked biomaterials carry a negative charge, attracting positively charged calcium ions from host plasma and leading to calcification [61].

After decades of development, transcatheter aortic valve replacement (TAVR) has demonstrated the feasibility of transcatheter interventions for the treatment of heart valve disease. The bioprosthetic valves used in TAVR need to be folded and then unfolded after being accessed through a catheter in the appropriate position. The impact of different procedures on future valve calcification remains inconclusive. However, TAVR may lead to calcification or even SVD due to the use of a thinner pericardium and biomaterial microinjury during the curling process. The Nordic Aortic Valve Intervention (NOTION) trial randomized patients at low surgical risk for TAVR or surgical aortic valve replacement (SAVR) and reported their 10-year clinical and bioprosthesis prognoses, revealing that the risk of severe bioprosthesis SVD after TAVR was lower than that of SVD after TAVR. Compared with SAVR, prosthesis SVD after TAVR has a lower risk [63]. Another retrospective study reported similar findings [64]. However, to varying degrees, the aforementioned studies involved small sample sizes, survival bias, and multiple influencing factors, and the relationship between implanted valve calcification and the surgical approach needs to be further investigated.

Multiple genes involved in the process of calcification in the cardiovascular system, such as Runt-related transcription factor 2 (RUNX2), are essential for osteogenic differentiation and can cause calcification when overexpressed [67]. Patients with RUNX2 gene mutations may undergo more severe calcification in both bioprosthetic valves and native heart valves, and this gene is normally active in bone formation but can be abnormally switched on in heart tissue, leading to mineralization [68].

Another important signaling pathway is the bone morphogenetic protein (BMP) pathway, in which BMPs, especially BMP-2, are known to be strong promoters of osteogenic differentiation in vascular tissues [69]. Alleles that increase BMP expression are involved in the development of calcific aortic stenosis and may promote calcification of prosthetic valves [70]. In addition, mutations in SMAD (homolog of Caenorhabditis elegans Sma and the Drosophila mad, mothers against decapentaplegic) proteins that are involved in BMP signaling are associated with abnormal mineralization and how the body controls calcification responses in implanted valves [71].

The immune response to implanted valves is one of the major contributors to calcification, especially in xenogeneic bioprosthetic valves, in which polymorphisms in proinflammatory cytokines, including tumor necrosis factor alpha (TNF-$\alpha$), interleukin-1 beta (IL-1$\beta$), and interleukin-6 (IL-6), are associated with an enhanced immune response leading to local inflammation and calcification [72, 73]. These cytokines activate macrophages, which subsequently create osteogenic precursor cells within the valve tissue, a process that increases calcification.

Genes involved in lipid metabolism are also involved in calcification, mainly through the oxidation of LDL, such as the apolipoprotein E (APOE) gene, which is involved in lipid transport and metabolism in the body. The APOE $\epsilon$4 allele increases the risk of LDL oxidation in patients, which leads to inflammatory reactions in valve tissues and calcification, and oxidized LDLs penetrate the valve scaffold, activate macrophages, and promote the osteoblastic transformation of VICs [74, 75].

Hyperphosphatemia is a common feature in patients with CKD and contributes to vascular and valvular calcification, as phosphate combines with calcium to form hydroxyapatite crystals that precipitate in valve tissues. This condition encourages calcification through osteogenic signaling pathways, including the activation of genes that are involved in bone formation, such as alkaline phosphatase (ALP) and osteopontin [76, 77].

Furthermore, CKD patients have disordered calcium metabolism, and high serum calcium leads to mineral deposition on the valve surface. Phosphate binders, commonly prescribed to manage hyperphosphatemia, can worsen these problems by increasing serum calcium concentrations and thereby increasing the risk of calcification [76, 78].

Diabetes mellitus is associated with increased calcification risk, primarily because of the formation of AGEs at high glucose concentration [79]. AGEs are proteins or lipids that are glycated because of high blood sugar levels and cause tissue hardening and calcification, including prosthetic valves, and they interact with receptors on immune cells, especially macrophages, to stimulate inflammatory signaling that leads to calcification [71, 80]. AGEs alter the mechanical properties of valve tissues by cross-linking collagen and increase the susceptibility of valves to calcification [81]. A literature review revealed that diabetic patients undergo calcification of bioprosthetic valves at a faster rate than nondiabetic patients do; thus, they require reoperations more frequently [80]. AGE inhibitors are among the preventive measures that are being researched for their ability to slow calcification in diabetic patients.

Dyslipidemia, which is defined by increased LDL and triglyceride levels, is involved in prosthetic valve calcification through lipid infiltration and oxidation. As discussed in Section 3.2, oxidized lipids activate inflammatory processes and attract immune cells to the valve site that subsequently become foam cells. These cells enhance the differentiation of VICs into osteoblast-like cells and increase the rate of mineralization [41]. In the case of statins, which are used to treat dyslipidemia, the effects on calcification have been inconclusive. However, newer drugs in the lipid-lowering family, such as proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, may provide benefits in preventing lipid-induced calcification [82].

Calcification is also regulated by hormonal factors, especially parathyroid hormone (PTH). PTH is involved in calcium and phosphate balance, and hyperparathyroidism may cause an excess of calcium phosphate, which increases the calcification potential [83, 84]. Dietary and pharmacological interventions may help reduce calcification risk in high-risk populations, but more research is needed to fine-tune these strategies.

Currently, local strategies for preventing calcification primarily target the physical and chemical properties of the valve materials themselves, aiming to create prosthetic valves that are resistant to calcification while maintaining biocompatibility and mechanical integrity. These strategies include improving the decellularization process, improving cross-linking chemistry, eliminating xenoantigens, adding coatings to the valve surface, polyphenol-based treatment and drying biological valve techniques.

Echocardiography remains the gold standard for prosthetic valve imaging, as it provides real-time information on the function and structure of the valve. Procedures such as three-dimensional transesophageal echocardiography (3D TEE) allow for detailed imaging of the valve leaflets and calcifications [135, 136]. Doppler echocardiography quantifies flow across the valve and can identify any hemodynamic changes due to calcification [137]. Transesophageal and intracardiac echocardiography are used during the implantation of the valve to visualize the valve and check for early signs of calcification and proper positioning of the valve [138, 139]. Intraoperative imaging is very important for minimizing postoperative complications and for obtaining an instant assessment of the procedure’s effectiveness.

CT is a highly sensitive technique that provides detailed images of the calcification process and enables accurate measurement of calcified plaque, and it is also applied to assess calcification in aortic valves, providing an opportunity to quantify the calcification process and its dynamics in time [140]. Quantitative CT can monitor changes in calcification density, which can help in early diagnosis and evaluations of the efficacy of anti-calcification medications, including statins or phosphate binders [141]. CT imaging is also useful in preoperative evaluation, where surgeons can determine the degree of calcification in native and prosthetic valves. Recent developments in 3D reconstruction of CT images have enabled precise visualization of the valve morphology, which is essential for choosing the right type of valve and its placement during the operation [142, 143].

PET imaging, especially when integrated with CT (PET-CT), is helpful in evaluating metabolic activity related to early calcification. Here, radiotracers such as 18F-sodium fluoride (18F-NaF) are taken up in areas of active calcification and allow the clinician to identify early mineralization processes that are not visible on CT alone [144, 145, 146]. 18F-NaF PET imaging has high sensitivity for detecting microcalcifications and is a valuable tool in the assessment of early calcification in bioprosthetic valves [146, 147]. PET imaging is also used to assess the effectiveness of anti-calcification therapies, as decreased radiotracer uptake suggests decreased metabolic activity and possible calcification. Thus, PET imaging may assist clinicians in modifying treatment plans according to patients’ response to calcification in real time [148].

MRI is generally less sensitive to calcific deposits compared with CT. However, it offers important information about the mechanical properties and materials used in prosthetic valves, helping to distinguish between calcified and noncalcified tissues. This information helps to understand the mechanical characteristics of the valve and identify potential zones of degeneration. T1 and T2 mapping are two of the most recent MRI techniques that can be used to quantify tissue stiffness, which is associated with early calcification and fibrosis [149, 150]. MRI is especially valuable in the assessment of tissue-engineered heart valves because it offers high soft tissue contrast and does not involve the use of ionizing radiation. This makes MRI suitable for use in pediatric and younger patients over long periods because frequent imaging does not contribute to the development of cancer due to radiation [151].