## 3 Results and Discussion
Figure 5: Distribution of clusters for exhaled breath samples from patients with Alzheimer’s disease (AD), Parkinson’s disease (PD), and from healthy individuals as a control group using system D. (**A**) Principle component analysis (PCA) pattern analysis of AD, PD, and control groups; (**B**) Sammon’s mapping of AD, PD, and control groups; (**C**) Combination of PCA and Sammon’s mapping for healthy individuals, AD patients, and PD patients. Oxidative stress is also one of the vital key processes influencing the chemical composition of exhaled breath [24; 25]. Oxidative damage to the cells, protein, nucleic acids, sugars, and even lipids has been previously reported in patients with AD [26; 27]. An elevation of specific biomolecule markers associated with oxidative stress has also been intensively studied in patients with AD. It has been reported dyshomeostasis contributes to elevated zinc, copper, and iron in patients with AD [28; 29; 30]. Subsequently, the dysregulation and imbalance of metal homeostasis in AD could change the metabolite by-products in the body [28]. These changes may contribute to the variation of chemical compounds in exhaled breath samples. We also correlated the chemical compounds from the exhaled breath with potential metabolic pathways based on the chemical structures of the compounds. Chemical compounds such as 1-phenanthrol and ethyl 3-cyano-2,3-bis (2,5-dimethyl-3-thienyl)-acrylate belong to the group of hydrocarbons. Hydrocarbons are mostly produced by lipid peroxidation, and are in the form of secondary metabolites or intermediate products in the body [31]. One study has shown that silicon can be secreted in bile, and subsequently it affects the lipid metabolism of the body. As a result, lipid metabolism disorders may contribute to the increase of exhaled siloxane in patients with PD. The results from the GC-MS and exhaled breath sensor system analyses of exhaled breath samples suggest that the chemical compounds present in the patient groups are significantly different from those of healthy individuals. Based on these findings, patients in the AD group could be distinguished from those in the PD and control groups based on the clustering analysis using the exhaled breath sensor systems. The distribution pattern is associated with the variations in chemical compositions of the exhaled samples, which result from pathophysiological changes in the body. Furthermore, it is also possible that the accumulated peroxididative damage and pollutants previously exposed to the body are released gradually in the exhaled breath when the defense system of a body becomes weak [32]. The distribution of clusters by Principle component analysis (PCA) pattern analysis for exhaled breath samples from patients with Alzheimer's disease (AD), Parkinson's disease (PD), and from healthy individuals as a control group using sensor array system A is expressed in Figure 6. Because there is no definitive evidence of the mechanisms and origin of the chemical components in exhaled breath, further study of such factors in patients with AD is necessary. Additionally, a longitudinal study should be carried out to verify the consistency and reliability of composition variation in the exhaled breath of patients with AD. Despite this, the findings from our exhaled breath study using GC-MS and sensor systems suggest that the exhaled breath system could potentially be used as a diagnostic tool for AD in the near future. The current development of chemical sensors is restricted to the external sensing of the compound of interest. In the near future, a chemical sensor ought to be explored as an ingestible sensor to improve health services and ensure a better quality of life [33]. Figure 6: Distribution of clusters by Principle component analysis (PCA) pattern analysis for exhaled breath samples from patients with Alzheimer’s disease (AD), Parkinson’s disease (PD), and from healthy individuals as a control group using system A.