This study utilizes cold water consumption as a method of vagus nerve stimulation, investigating how body mass index (BMI) and repeated stimulation affect heart rate variability (HRV) in both time and frequency domains. HRV was measured in 18 subjects (mean age 21.3 ± 1.6 years) over 90 minutes, divided into three equal phases: resting (baseline), first stimulation (FSP), and second stimulation (SSP). Fourier and Short-Time Fourier Transform (STFT) analyses were used to extract the static and time-varying frequency components of HRV. For the normal body mass index (BMI) group, stimulation resulted in statistically significant changes in frequency band power densities compared to their baseline measurements (p

Melanin nanoparticles (MNPs) have attracted growing attention in biomedical fields due to their wide bioavailability, high biocompatibility, biodegradability, and multifunctionality. Key features of MNPs are broad-spectrum light absorption, high drug-loading capacity, near-infrared light-triggered drug release and reactive oxygen species generation, efficient hepatobiliary, renal clearance, and superior binding affinity towards metal ions that enhance MNPs’ traceability in magnetic resonance imaging (MRI). Despite challenges such as agglomeration in time, lack of extraction standardization, and a limited number of animal and clinical studies, the strong photothermal conversion and reactive species production abilities of MNPs make them effective photosensitizers and photothermal agents, while their dual capability for molecular binding and metal chelation positions them as promising agents for theranostic applications. This review provides a comprehensive overview of MNPs, covering their structural features, manufacturing methods and associated limitations, and recent advances in their physicochemical characterization. Special focus is given to MRI-guided biomedical applications, including drug delivery, photothermal therapy, and photodynamic therapy, along with critical challenges related to the clinical translation and commercialization of MNP-based platforms. By consolidating recent advances, this review highlights key structure–function relationships that underpin emerging melanin-based nano-systems.