Sr-Site Substitution in SrTiO₃ Perovskite from First-Principles: Bio-Safe Photo-Responsive Materials for Wearable Cardiac Health Monitoring

Abstract Background Cardiovascular diseases remain the leading cause of mortality globally, necessitating continuous, non-invasive monitoring technologies. SrTiO₃-based perovskites have emerged as promising candidate materials for next-generation wearable cardiac monitoring devices due to their non-toxicity, high chemical stability, and controllable electronic structure. Their potential application in photoplethysmography (PPG) sensors and photocatalytic sterilization of monitoring devices is particularly attractive for cardiac patients who require long-term implantable or wearable systems. Purpose This study aims to address the critical challenge of narrowing SrTiO₃ band gap through element substitution while maintaining biosafety for cardiovascular applications. The goal is to develop a theoretical framework for designing perovskite materials that can efficiently respond to visible-near infrared light suitable for continuous cardiac monitoring, eliminating bio-toxic substances that could trigger inflammation or arrhythmias in cardiac patients. Methods Based on the CASTEP module and GGA-PBE functional, we conducted first-principles calculations on Sr-site substitution in SrTiO₃ using ten common elements (Zr, Nb, Ru, Y, Ca, Ba, Mo, La, Ce, Cd). A 2×2×2 supercell with 15-Å vacuum layer was constructed using 489.8 eV cutoff energy and 4×4×4 k-grid. We employed GGA+U correction (Ti3d: U=4 eV) and HSE06 cross-validation. Health-oriented screening indices included Ti-3d/O-2p orbital hybridization intensity, substituted ion valence states (+2 to +6), and ionic radius mismatch, with special consideration for cardiac-specific biosafety parameters including hemocompatibility and myocardial cell cytotoxicity thresholds. Results Transition metals like Ru significantly enhanced d-orbital hybridization, reducing band gap to 0.3566 eV (RuTiO₃), enabling near-infrared light conversion to ROS. Alkaline earth and rare-earth elements widened the band gap due to ionic radius mismatch (e.g., ZrTiO₃: 2.934 eV). The study reveals a strong linear negative correlation (R²=0.91) between Ti-3d–O-2p orbital overlap and substituted ion valence state, providing quantitative criteria for designing "valence state-health windows" specifically tailored for cardiovascular device applications, including the critical 620-920 nm wavelength range optimal for transdermal cardiac monitoring. Conclusion This first-principles study elucidates the microscopic mechanism by which Sr-site substitution precisely regulates SrTiO₃ band gap (0.36-2.93 eV) through dual "orbital hybridization-charge compensation" pathways. Mapping computational results to cardiac health requirements reveals: Ru-systems are suitable for near-infrared wearable phototherapy patches preventing pacemaker pocket infections; La/Ce-systems enable hospital-grade UV air purification for cardiac catheterization labs; Ca/Ba-systems are ideal for daily antibacterial coatings on smartwatch-based cardiac monitors. This work establishes a theoretical foundation for environmentally friendly perovskite materials that integrate "design-health" principles specifically for cardiovascular applications, paving the way for next-generation bio-safe, light-responsive cardiac monitoring platforms.