# Advancements and Prospects of Electronic Nose in Various Applications: A Comprehensive Review
## 3 Application Areas
### Disease Diagnosis
#### 3.3.1 Human Clinical Pathology  
In recent years, the field of human clinical pathology and diagnostic medicine has witnessed significant strides in the development of innovative electronic methods and approaches aimed at enhancing the accuracy and speed of early diagnoses for human diseases [155]. A notable achievement in the realm of medical diagnostics was realized by [PERSON] et al. in 1995, when they successfully identified beta-streptococcal infections in leg ulcers. This groundbreaking advancement involved the comprehensive analysis of bacterial cultures obtained from a group of 21 patients [156]. Intriguingly, researchers delved into the metabolic processes of fungi and bacteria, revealing the emission of VOCs known as microbial volatile organic compounds (MVOCS) [110]. These compounds, formed as byproducts of microbial metabolism, contributed to unique aromatic profiles for different microorganisms. With each bacterium and fungus releasing its distinct composition of MVOCS, these emissions emerged as olfactory signatures with great potential for detection applications. Taking this knowledge forward, [PERSON] et al. [157] expanded the horizons of this field in 1998. They achieved the differentiation of six pathogenic bacterial species: Clostridium perfringens, Proteus, Haemophilus influenzae, Bacteroides fragilis, Oxford staphylococcus, and Pseudomonas aeruginosa. This remarkable feat was made possible by analyzing the headspace surrounding bacterial cultures. Utilizing an electronic nose armed with four commercially available metal oxide sensors, the researchers effectively captured the distinct VOC profiles of these bacterial entities [157]. Moving on to gastrointestinal (GI tract) diseases [155], the focus on improving diagnostics for inflammatory bowel disease (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), stands out as a prominent research area. IBD manifests as a chronic, relapsing condition characterized by gut inflammation, leading to symptoms such as abdominal cramping, diarrhea, vomiting, fatigue, and weight loss [2]. It is worth noting that while UC predominantly affects the colon, CD can impact the entire digestive system, highlighting the need for early and precise detection to mitigate complications [2]. However, the diagnostic landscape for IBD is not devoid of challenges. Current diagnostic methods entail invasive and costly procedures, such as endoscopic investigations, imaging of the lower GI tract, histological examination, and analysis of fecal inflammatory biomarkers [2]. Despite its invasiveness and discomfort for patients, colonoscopy with histology remains the gold standard, involving multiple biopsies and associated morbidity [2]. Amid these challenges, recent research has explored promising avenues for enhanced IBD diagnostics, particularly through the analysis of VOCs present in exhaled breath. [PERSON] et al. [155] conducted a study on breath analysis utilizing both an experimental electronic nose (Wolf eNose) and a commercial gas chromatograph-ion mobility spectrometer (G.A.S. BreathSpec GC-IMS). Their findings underscored the significance of specific VOCs, such as butanoic acid and acetic acid, in differentiating IBD patients from healthy controls [155]. These VOCs, arising from the fermentation of cellulosic fiber in the colon, emerged as potential biomarkers for IBD. Notably, this technology exhibited advantages over conventional methods, being cost-effective, portable, non-invasive, and suitable for various patient demographics [155], including vulnerable patients such as children and the elderly. In the context of urinary tract infections, electronic noses (e-noses) have been utilized to analyze urine samples either directly or after brief incubations in test tube systems with complex media [158; 159; 160]. Various data reduction and pattern recognition techniques have been proposed to differentiate between bacterial species and their distinct metabolic states [161; 162].