# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 4 Sensors
### Biochemical Sensors  
Another category of the non-invasive sensor is the chemical analysis of ions, biomolecules, electrolytes, and proteins present in bodily fluids. The modern clinical examination usesblood samples for biochemical assessment for various diseases and conditions. But for non-invasive sensors, sweat and its ionic composition are the main targets for quantitative biochemical analysis. This almost completely removes the need for the invasive collection of samples that can cause psychological trauma or problematic infections. Such electrochemical sensors usually are composed of soft, stretchable, microfluidic systems with specialized receptors that interact with the target ions. These receptors are usually enzymes or antibodies that have a specific color or a change in pH when they react with the target ions. The characterization and biochemical analysis of biofluids, such as sweat, can be deemed to have high clinical value when complemented with other physical characteristics. Due to the relative ease of non-invasive sample collection, sweat is a representative biofluid that is full of intricate biomarkers. [PERSON] et al. demonstrated a soft, wearable microfluidic device that was capable of capturing and storing sweat for electrochemical analysis [128]. The device, which is shown in Figure 5g, allows the quantitative collection of sweat rates, pH variations, and changes in the concentrations of biomarkers. The changes in the biomarkers are analyzed by colorimetry, and the test is conducted during a controlled fitness cycle. The epidermal microfluidic device can evaluate the performances of athletes while monitoring various other markers of health status. Key markers, such as creatinine, lactate, chloride, glucose, and pH, are evaluated without the device losing its adhesion to failing to operate correctly. Another perspiration-based biochemical sensor was developed by [PERSON] et al., and it evaluates the variables related to cystic fibrosis for diagnostic purposes [139]. The electrolyte contents of the sweat of cystic fibrosis patients were obtained through non-invasive methods using sweat collected by the sensor. The device is composed of a miniaturized iontophoresis interface that delivers stimulating agonist to sweat glands via an electrical current. The correlation between the concentration of the drug and the rate of sweating has been demonstrated for clinical modeling with reliable stability of the device and user conformality. It also introduced a preliminary study of metabolic glucose content between sweat and blood. The culmination of the sweat profiles of individuals can enable future integration into large-scale clinical investigations for more extensive biomedical modeling. Another notable work that utilizes sweat for non-invasive recording is the textile-based potentiometric sensor developed by [PERSON] et al. for the analysis of multiple ions in sweat [140]. The incorporation of a polyurethane-based, ion-selective membrane with a stretchable ink electrode strengthens the conductivity and biocompatibility of the textile-based sensors. The integration of stretchable ink and components with serpentine design has greatly enhanced the durability of tensile stress. Despite the practicality of sweat-based sensors, there are issues related to the uncertainties associated with the device due to the presence of other biofluids and markers because they limit the impact of the biomedical applications for non-invasive sampling. To supplement the mentioned uncertainty associated with monitoring sweat, [PERSON] et al. developed a skin-like, non-invasive sensor for the highly-accurate monitoring of glucose in blood [129]. The device has integrated subcutaneous channels that drive intravascular blood glucose to the surface of the skin. The epidermal glucose detecting mechanism is shown in Figure 5h. This additional augmentation does not comprise the conformality of the device, and it provides prospects for high standard clinical application.