**4.4 Iontronic type**

Supercapacitive iontronic pressure sensors convert the pressure input into the output of constant-capacity change. This type of pressure sensor enhances the compression effect by utilizing the formation of an electron double layer (EDL) at the dielectric layer and contact electrode. In other words, ionic gel with numerous positive and negative ions are spatially trapped between the two electrodes. The positive and negative ions are attracted to the negative and positive respectively, forming two EDLs as an increase of the applied voltage. The operating mechanism of this type of sensor depends on changes in the area between the electrode and the active material, as shown in **Figure 5D**. Increasing the contact area under certain pressure, positive or negative ions are induced, resulting in increased capacitance values [98].

## **4.5 Organic field-effect transistor type**

Organic TFT-based sensors offer biosignals-sensing operation such as cell activity. In general, two major categories of organic TFTs are used for bio-sensing applications. Electrochemical doping and de-doping are the main reactions in an organic electrochemical transistor (OECT) to modulate ionic species to the active channel materials. Meanwhile, the capacitive field effect is the main reaction of electrolyte gate organic field effect transistor (EGOFET) at the interface of organic semiconductor and electrolyte [99].

*Advanced Materials and Assembly Strategies for Wearable Biosensors: A Review DOI: http://dx.doi.org/10.5772/intechopen.94451*

Gualandi et al. reported a fully textile, wearable biosensor based on an OECT (**Figure 12**) using PEDOT: PSS conducting polymer. The fabricated OECT sensors can detect three biomolecules (ascorbic acid, adrenaline, and dopamine) by the reduction-oxidation reaction. Their performance is similar to normal OECTs. These results demonstrate that OECT can be established on a 3D-networked fiber substrate [100].

#### **4.6 Photosensing type**

Near-infrared (IR)-response organic light detector (OPD) has been investigated due to its potential applications of health monitoring, remote control, artificial vision, optical communication, and night vision. Especially, the short exposure of near-IR on human skin is not toxic and near-IR can propagate under tissue at ranges of 4 mm. Thus, near-IR is proper for skin-mountable health monitoring devices [101]. Furthermore, narrowband detection of near IR light between ≈700 and 1300 nm is highly desirable for biomedical sensing. Park et al. demonstrated high-performance skin-mountable near-IR OPDs which are mechanically conformable for the application of health care electronics [101]. The OPD (thinner than 3 μm) exhibits stable operation under conditions of mechanical deformation (103 times bending). Thanks to its balanced properties of high responsivity and stable mechanical conformability, the IR sensor device showed superior sensitivity in the near-IR region when it is under operation in a skin-conformal photoplethysmogram sensor compared to that of an existing rigid substrate device of the glass.

#### **4.7 Chemical sensing type**

The biochemical signal of the human body significantly varies depending on the health condition of the subject. Biomarker concentrations range from complex patterns and other time scales, i.e., time-to-time fluctuations in metabolites, hormonal and inflammatory changes, from neuron synapses to millisecond spikes in ions and neurotransmitters.

Over the years, continuous wearable technology for non-invasive monitoring has been developed. Imani et al. developed wearable devices that could measure chemical and electrophysiological signals simultaneously in the single patch. The hybrid wearable, called Chem-Phys patch, consists of three-electrode ammeter lactate biosensors and two ECG electrodes printed on the screen, enabling simultaneous real-time measurements of lactate and ECG as shown in **Figure 13** [102].

Nightingale et al. demonstrated a fully integrated wearable microfluid sensor, which not only provides accurate, precise, and powerful fluid sampling and control but also provides on-site chemical tests that use water droplets as microreactors [103].

#### **Figure 12.**

*OECT working principle. Scheme of an OECT (A) operating in conditions of low (B) and high (C) conductivity of the channel. Adapted from Ref. [100].*

#### **Figure 13.**

*(A) Cyclic stretching test of near-IR-OPD. The stretching cycle test was conducted at 100% tensile strain for 103 cycles. Adapted from Ref. [101]. (B) (Left) Image of a Chem–Phys patch along with the wireless electronics. (Right) Schematic showing the lactate oxidase-based lactate biosensor along with the enzymatic and detection reactions. Adapted from Ref. [102].*

## **5. Device fabrication and assembly strategies**

As device-manufacturing technology is actively being developed, skin-mounted biosensors have attracted scientific and industrial attention to everyday applications such as e-skin, health testing, underwater sensing, and interaction between people and machines [104]. In this subsection, we review the novel fabrication strategies for wearable sensors.

Skin-mounted electronic devices should be established on flexible substrates with reasonable fabrication costs. In particular, the ability to realize flexible or wearable thin film-based sensors provides much freedom for target substrates. One of the most promising and powerful candidates to produce low-cost skin-mounted bio-sensors is ink-jet printing, and due to its ultra-low-cost, non-vibration, and environmentally friendly fairness, it is an appropriate strategy to implement commercial thin-film devices and systems [105]. Ink-jet printing originated from the graphic art industry for mass production of standard products including fabrics and papers. In addition, advanced printing machines and inks solutions can produce on large-area products with low-cost and high-printing-speeds on the order of 10 m s−1.

Holbery et al. demonstrated a demand (DOD) inkjet printing technique to fabricate touch sensors on polyethylene terephthalate (PET) substrates. The commercially available Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) solution and thermally curable methylsiloxane serves as transparent electrodes and dielectric, respectively. The resistance and transparency of PEDOT: PSS electrode gradually decreases from 20.8 to 6.9 kΩ and from 85 to 75%, respectively [106].

Inkjet printing supplies pressure pulses to the fluid-filled chamber, depleting ink drops on demand and triggering pulses through heat evaporation, sound perturbation, or piezoelectric operation. However, inkjet printing using thermal steam, acoustic perturbation, or piezoelectric operation lacks inkjet droplet control function of the inkjet nozzle in a controlled manner, making it challenging to print high resolution [107]. In particular, in electrohydrodynamic (EHD) jet printing, electric fields are applied between deposition nozzles and substrate to induce moving ions of ink to accumulate on the liquid surface. Because of the Coulombic repulsion of ions at the edge of the ink, the hemispherical meniscus turns into a conical shape (Taylor cone). Thus, a smaller diameter of inkjet resolution than that of the nozzle is obtained during EHD jet printing Compared to normal inkjet printing and aerosol printing which has limited resolution (~ tens of micrometer), few hundred resolutions (nearly 700 nm) can be realized using EHD jet printing [108].

Lee et al. reported three different transfer-printing (TP) methods for nanowire (NW) based devices [109]. Fundamentally, NWs on donor substrate is likely to be

*Advanced Materials and Assembly Strategies for Wearable Biosensors: A Review DOI: http://dx.doi.org/10.5772/intechopen.94451*

#### **Figure 14.**

*Various transfer printing technique. (A) STP method on PDMS (Prefabricated NW devices* → *Deposition of liquid PDMS and curing* → *Peel off PDMS*∕*NW devices* → *NW devices embedded inside PDMS). (B) DTP method (NWs on the growth substrate* → *HF etching to remove the native SiO2 of NWs* → *Pressing down a tape to the NWs* → *Peel off the tape with NW mesh* → *Pressing down the tape/NW mesh to the prefabricated electrodes* → *2nd Peel-off* → *NW device on the tape). (C) MTP method (1st column: pressing down a thermal release tape to the prefabricated electrodes* → *Peel off the thermal release tape with electrodes* → *Pressing down the thermal release tape*∕ *electrodes to a target substrate* → *The thermal release tape is thermally released at 90°C; 2nd column: NWs on the growth substrate* → *HF etching to remove the native SiO2 of NWs* → *Pressing down a tape to the NWs* → *Peel off the tape with NW mesh* → *Assembling of the tape/NW and the transferred electrodes). Adapted from Ref. [109].*

transferred onto acceptor substrates which have stronger adhesion force than that of donor substrate (**Figure 14**). (1) Single transfer printing (STP) enables fabricated NW devices on a Si wafer to be transferred onto a PDMS substrate through a single peel-off step. (2) Double transfer printing (DTP) required a two-times transfer process from NWs and electrodes to fabricate devices. (3) Multiple transfer printing (MTP) includes the transfer of multiple electrodes using thermal release tapes on both flexible and rigid substrates.

Inganäs et al. demonstrated the additive technique for producing all translucent and flexible polymer photodetectors with a wide area. PEDOT: PSS electrodes were printed on a flexible PET film substrate through roll-to-roll (RTR). The printed PEDOT: PSS electrodes were served as both cathode and anode by a coating of Polyethylenimine (PEI) on PEDOT: PSS. After the spin-coating of an active layer on top of PEDOT: PSS and PEDOT: PSS/PEI, the two multi films were laminated through a hot-pressing roller (120°C). The fabricated all-polymeric photodetector also demonstrated mechanical durability [110].

### **6. Future opportunities**

As part of the era of digital health, widespread use and deployment of wearable sensors should overcome specific technical challenges. One such challenge is the

biological receptor in wearable chemical biosensors. Since the signal transducer material and technologies of wearable chemical biosensors have already been considerably advanced, a major obstacle that hinders the development of the wearable chemical biosensors field may be not a signal transducer material, but a biological material. A diversity of target materials is narrow in current biological receptors and needs to be improved in terms of material stability, selectivity, bonding power, and production cost.

Ideal wearable sensors are physically small and can store important personal health data; therefore, biosensors and personal health data may be lost. The development of more secure and encryption technologies is desired to keep personal privacy and security.

The personal calibration of the wearable sensor is also one of the main challenges. Everybody has different personal health conditions (e.g. diet, family medical history and genetics). Therefore, symptoms of early diagnosis may vary from person-to-person. It is necessary to develop hardware and software that can comprehensively interpret human health through the development of artificial intelligence, as well as calibration of personal health status. Although artificial intelligence's big data could be used to interpret an individual's health, the patient's disease should not be immediately evaluated through other people's precedents using wearable sensors. In addition, since individual's body shape and skin surface condition are all different, research that combines 3D printing technology to create a wearable sensor according to the individual's condition might be an interesting direction for further research.
