**1. Introduction - Physiological background for implantable medical device**

#### **1.1 Cardiac pacemaker**

Cardiovascular diseases (CVDs) are the major cause of death globally, which takes an estimated 17.9 million lives per year based on the World Health Organization (WHO) statistics. Especially, arrhythmia has a strong clinical correlation with sudden cardiac death (SCD) [1]. The irregular heart rhythm called arrhythmia can mainly be divided into two main types: tachycardia arrhythmia and bradycardia arrhythmia. As shown in **Figure 1**, the tachycardia and bradycardia arrhythmia represent the heart beats too fast and slow, respectively. The physical treatment for tachycardia and bradycardia syndrome required a regulator to suppress the abnormal heart rhythm. The implantable cardiac pacemaker is commonly applied in the cardiac modulation that generates the electrical stimulation pulse to regulate the heart's sinoatrial node, thus obtaining the normal rhythm.

As shown in **Figure 2**, the cardiac pacemaker would be activated once the heart detector measures the abnormal cardiac rhythm. Thus, the stable operation of a cardiac pacemaker is important for adverse patients, thus providing prompt treatment in arrhythmia.

**Figure 1.**

*Heart rhythm is divided into normal rhythm, tachycardia arrhythmia, and bradycardia arrhythmia.*

**Figure 2.**

*Schematic of the detection and treatment system for cardiac arrhythmia.*

### **1.2 Vagus nerve stimulator**

Epilepsy is a neurological disorder that can induce the brain activities abnormal, causing seizures and further loss of awareness suddenly [2–4]. According to WHO statistics, epilepsy is a chronic non-communicable brain disease that affects humans of all ages, around 50 million globally, becoming one of the most common neurological diseases worldwide. The risk in the death probability of epilepsy patients is up to three times higher than healthy people. In epilepsy diagnosis, Electroencephalogram (EEG) is the most common non-invasive approach to record the brain's electrical activity and identify the measured signals, whether it is epilepsy or not [5]. In addition, the imaging-based diagnosis of computed tomography (CT) [6], positron emission tomography (PET) [7], and MRI [8] can help further examining brain-tumor-induced epilepsy. In epilepsy treatment, an vagus nerve stimulator is the common physical approach implanted near the left chest area, as shown in **Figure 3**. The electrode is attached around the vagus nerve in the neck to generates the electrical pulses to the brain via the vagus nerve. The delay time more than 10 minutes from seizure onset would increase mortality, which emphasizes the importance of timely treatment and time and medical emergency [9]. Therefore, high reliability is crucial for vagus nerve stimulation (VNS) during incidental neurological disease. Moreover, VNS is also widely applied in significant disease treatment, including cardiac function [10], depression [11], anxiety [12], Parkinson's disease [13], and Alzheimer's disease [14].

**Figure 3.** *Schematic of vagus nerve stimulation and its installations.*

**Figure 4.**

*EMI in medical devices from external sources with time-varying electrical and magnetic fields such as base station, radar, mobile device, and microwave oven [16].*
