## 1 Introduction  
Transcranial electrical stimulation is a treatment that delivers weak electrical stimulation to the brain surfaces through electrodes located above the scalp for the spontaneous activation of neuronal cells to repair brain function and relieve symptoms [1; 2; 3; 4; 5]. Transcranial electrical stimulation can be classified as transcranial direct current stimulation (tDCS), which delivers a direct current without frequency modulation to the brain; transcranial alternating current stimulation (tACS), which delivers a modulated electrical current at a constant frequency; and transcranial random noise stimulation (tRNS), which delivers a current with randomly changing frequencies and intensities [6; 7]. In tDCS, depending on the characteristics of the direct current flowing from the anode to the cathode, brain function is controlled by placing an anode on the head to induce depolarization, which increases the neuronal excitability (anodal tDCS), or by placing a cathode on the head to induce hyperpolarization, which reduces the neural excitability (cathodal tDCS). In contrast, in tACS, which delivers alternating currents without a constant current direction, brain functions are adjusted by causing brain oscillatory activities at a specific frequency. Recently, results of preclinical in vivo studies have been reported to understand a mechanism of transcranial electrical stimulations and explore the possibility of the application for the treatment of brain diseases and symptom relief [8; 9; 10; 11; 12; 13]. In addition, based on these results, clinical studies to use tDCS and tACS for clinical brain disease treatment, symptom alleviation, and brain function improvement have been actively progressed [14; 15; 16; 17; 18; 19]. In particular, the results of several studies indicated that \(\alpha\)-band (10 Hz) tACS can be applied to an improvement of sensory/motor function and an up-regulation of the default mode network [20; 21; 22; 23]. Ensuring the safety of medical devices is important for the diagnosis and treatment of diseases through the application of such devices to the human body [24]. In particular, ensuring the safety of transcranial electrical stimulation devices, which contact the scalp and apply electrical stimulation to the brain tissue, is essential for the prevention of unexpected brain damage caused by electromagnetic fields. In a safety evaluation study on tDCS, [PERSON] et al. investigated whether brain tissue damage was caused by tDCS with three different currents (I = 0.1, 0.5, and 1.0 mA) and different electrical-stimulation treatment times (T = 30 to 270 min) [25]. The results indicated that if the charge per unit area exceeds a certain threshold (lesion threshold), brain tissue damage occurs, and the extent of damage that occurs when the charge per unit area is above the threshold is proportional to the unit area charge. [PERSON] et al. investigated the safety of 500-uA cathodal tDCS via a brain-function test involving behavioral experiments and histological staining [26]. However, there is a lack of research on the safety of tACS, and safety studies on tACS are needed for the application of tACS to treat brain diseases and the development of transcranial electrical-stimulation-based medical devices. In this study, we investigated the safety of \(\alpha\)-band (10 Hz) tACS using selected parameters (stimulation intensity of 1.0-2.0 mA, frequency of 10 Hz, and treatment time of 20 min). To examine the safety of tACS, alternating current stimulation was applied to rat brains, and the effects and safety of the alternating current stimulation on the brain tissue were evaluated via three methods: (1) macroscale analysis of the damage to the brain tissues through magnetic resonance imaging (MRI), (2) observation of the tissue composition and variations in neuronal cells via hematoxylin and eosin (H&E) and Cresyl violet (Nissl) staining, and (3) measurement of the temperatures between the electrodes and skin in tACS to determine whether the medical device is safe in contact with the skin.