# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 6 Wearable Integration  
Wearable sensors have been integrated with various platforms, such as contact lenses, tattoos, patches, watches, mouth guards as summarized in Table 2[1, 158, 159]. The integrated devices are required to be portable, body-compliant, and able to provide reliable data under dynamic conditions for a prolonged period of time. To fulfill those requirements, all of the components must be configured appropriately and efficiently. This section describes some of the latest research that has demonstrated highly-integrated, wearable, healthcare electronics. [PERSON] et al. demonstrated a soft, smart contact lens that enables real-time, wireless tacking of cortisol levels in tears using smartphones, as shown in Figure 7a [14]. The lens platform was integrated with a cortisol sensor, transparent antenna, resistor, NFC chip, capacitor, and 3D printed interconnects. The cortisol sensor was fabricated by immobilizing cortisol monoclonal antibodies (C-Mabs) onto the graphene channel of a graphene-field effect transistor (FET). The antigen-antibody interaction between cortisol and the C-Mab induced variations in the drain current, thereby enabling the quantification of the cortisol level. The antenna coil was made of a transparent and stretchable AgNW-AgNF hybrid to ensure that the smart contact lens did not obstruct the wearer's view. The rigid-soft hybrid geometry also used on the substrate enabled efficient arrangement of the components depending on their rigidity and hardness, rendering the entire device stress-resistant. The 3D interconnects were made by direct printing of liquid metal through a nozzle. After the integration of all the components, the resulting flat sample was molded with a PDMS precursor to form the curved shape of a lens. The graphene channel of the sensor was kept uncovered in the process of molding such that tears can make contact with the channel when the resulting smart contact lens was worn on the eye. The wireless power supply from an NFC-enabled smartphone allowed the logic chip to operate and control the operation of the sensor, the recording of data, and the real-time transmission of the data. The data transmitted to a smartphone over NFC was calibrated after which the quantified cortisol levels were displayed on custom software. The resulting data were highly correlated with the actual cortisol concentrations of the tested solutions. [PERSON] et al. developed an electronic patch capable of battery-free, wireless sensing of lactate, glucose, chloride, pH, and sweat rate/loss, as shown in Figure 7b [16]. The patch consisted of a microfluidic and NFC electronic subsystems that were combined magnetically. The microfluidic subsystem was composed of a microfluidic patch and colorimetric and biofuel cell sensors. Chloride levels and pH were monitored by colorimetric assays, while sweat rates were measured by volumetric analysis. The concentrations of lactose and glucose were measured by the biofuel cell sensor. Notably, the biofuel cell sensor was able to generate electric currents in proportion to the glucose and lactate levels, which eliminates the need for a potentiostat. The NFC electronic subsystem that consisted of NFC chips, amplifiers, antenna, resistors, and capacitors enabled the wireless transmission of the electrochemical sensing data to mobile electronics without a battery. The skin-adhesive membrane that was bonded strongly to the patch enabled robust and conformal attachment of the patch to the skin, even during physical exercise. Since the two subsystems were attached magnetically, they could be separated easily, allowing users to reuse the NFC electronic subsystem and dispose of the microfluidic patch after a single use, which makes the entire system more cost-effective. In rigorous 2-day field studies that involved  
Figure 7: **(a)** Smart contact lens for wireless cortisol monitoring integrated with a cortisol sensor, antennas, and wireless communication circuits operated via NFC (reprinted with permission from [14].