**2. Types of wearable biosensors**

Several human biosignals and stimuli should be concerned to display user's health conditions. For example, the assessment of biochemistry in biofluids (e.g., sweat, interstitial fluid, and blood) can provide fruitful information about personal health status and disease progression. In addition, both electrical and non-electrical biosignals such as electrooculography (EOG), mechanomyogram (MMG), electrocardiogram (ECG), electromyogram (EMG), galvanic skin response (GSR), magnetoencephalogram (MEG), and electroencephalogram (EEG) are useful indicators to offer the biosignal and info of a special tissue, organ, brain, or cellular system, such as the nervous system.

In addition to these internal vital signs, the on-skin detection of external harmful stimuli with a biosensor is also a critical issue. When it comes to the, the realm of wearable biosensing applications can be extended to motion detection, hazardous gas monitoring, disease diagnosis, and harmful UV-light detection (**Figure 2**). The sensing mechanisms of these sensors are diverse. In this Section, we will discuss several examples of chemical, optoelectronic and mechanical biosensors.

#### **2.1 Wearable chemical biosensors**

Sweat and sebum are representative human skin secretions that originate from the sweat glands in the dermal layer of the epidermis and therefore in-situ detection of these secretions on the skin is critically important for health monitoring. Sweat, a physiological aid for regulating body temperature, is secreted from the external glands. For example, sweat is a particularly useful sensing target due to the ease and presence of biomarkers associated with critical health conditions such as dehydration, physical exhaustion, mental stress and illness [13–15]. Sebum is secreted to lubricate human skin from Sebaceous glands. When sebum spreads up along the hair shaft, it is distributed over the surface of the skin, lubricating, and waterproofing the stratum corneum, the outer layer of the skin. It consists mostly of lipids. Those secretions can reflect the human health condition indirectly. Sebum provides antioxidant and antimicrobial lipids to the skin surface. Thus, proper secretion of sebum on the human skin surface increases the skin permeable barrier function. However, excess sebum frequently results in acne vulgaris. The acidic state of the skin pH is an essential factor in retaining the integrity of the skin-permeable barrier. It is reported that skin pH can be changed in skin diseases such as atopic dermatitis [16].

Wearable chemical biosensors that non-invasively analyze biofluids such as sweat, sebum, saliva, tears, and intermediate fluids provide the potential to

**Figure 2.**

*A schematic diagram of wearable biosensors. (A) Wearable chemical biosensors. Adapted from Refs. [7, 8]. (B) Wearable photodetectors. Adapted from Refs. [9, 10]. (C) Wearable electromechanical biosensors. Adapted from Refs. [11, 12].*

dramatically improve health condition evaluations by tracking changes in metabolic processes [17]. The sweat sample collection can be performed using an absorbent pad or a plastic microtube [18]. However, these methods are not compatible with remote monitoring and on-site use because they depend on cumbersome multi-step expensive benchtop hardware for sample preparation procedures and analysis. Recent advances in soft microfluidics, stretchable/flexible chemical sensing technologies form a basis of a new wearable sensor system that overcomes the limitations of this conventional approach [19]. The wearable sweat sensors are promising tools for continuous health and physiological monitoring. Among the various types of sweat sensors, optical detection utilizes photo-transmission techniques such as chromaticity, fluorescence, and light detection to provide an attractive strategy for measuring integrated chemical sensors due to cost efficiency and simplicity. Colorimetric sensors are also widely adopted in wearable sensor platforms, especially in case of the integrated microfluidic systems. Koh et al. demonstrated that a soft, flexible microfluidic platform based on silicon elastomer was recently developed, using colorimetric dyes to detect lactate, chloride, glucose, pH and loss of sweat [7]. The dye was located on top of the filter paper inserted into the micro reservoir. The lactate and glucose measurements were achieved by integrating enzymes into chromaticity-inducing reagents to enable the colorimetric readout sensing.

In terms of the detection of hazardous gases to human, Lee et al. demonstrated a flexible and transparent biosensor on polyethylene terephthalate (PET) that can detect 255 ppb NH3 using spray-deposited single wall carbon nanotube (SWCNT) with Au nanoparticles in a reproducible manner [20]. The AuNP decoration of transparent SWCNT film through the use of an electron beam (e-beam)

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

evaporation enhanced the performance of the gas sensor, which exhibited a high uniformity of the sensing behavior. The enhancement of this sensing performance was resulted from the carrier depletion zone control enabled by the combination structure of the AuNPs and the SWCNTs. The proposed sensor had a fast response time regarding NH3 gas but did not fully recover at room temperature.

Lee et al. developed a patch-based, integrated system that combines noninvasive sweat glucose monitoring with microneedle-assisted therapy (**Figure 3**) [21]. The patch-based wearable/strip type single-use integrated systems include large-area porous metal based electrode, minimized sensor designs for stable sweat detection from small amounts of sweat, design patchable and disposable sensors to achieve practical application, multicycle operation of the sweat measuring control and absorption layer to collect efficiently sweat samples, a porous Au nanostructure film to maximize the surface-to-volume ratio to detect a tiny amount of glucose in sweat. This sweat sensor exhibited high performances in terms of sensitivity, multimodal sensing, and accuracy.

#### **Figure 3.**

*(A) Optical camera image (top; dotted line, edges of the patch) and schematic (bottom) of the wearable sweat monitoring patch. A porous sweat-uptake layer is placed on a Nafion layer and sensors. (B) Optical camera image (top) and schematic (bottom) of the disposable sweat monitoring strip. (C) Optical camera image (top; dotted line, edges of the patch) and schematic (bottom) of the transdermal drug delivery device. Replacement-type microneedles are assembled on a three-channel thermal actuator. Adapted from Ref. [21].*

#### **2.2 Wearable optoelectronic sensors**

Photodetector is a key device attached to the front end of the optical receiver that converts input optical signals into output electrical signals [22]. Especially in medical applications, optoelectronic devices are very useful as they are able to detect biometric signals and other clinical information non-invasively. Organic optoelectronic devices including organic light emitting diodes (OLEDs), organic photodetectors (OPDs), and organic phototransistors are in the spotlight in the area of medical devices as they provide a wide absorption spectrum and high photogeneration yield with easy-to-fabricate, lightweight, and flexible features [23].

Recently, OLEDs [24–30], polymer light-emitting diodes (PLEDs) [31–34], and OPDs [35–38] were fabricated on glass or plastic substrates, implementing a muscle contraction detector and transmission mode pulse oximeter. Moreover, OLEDs and organic photovoltaics were manufactured on 1-μm-thick ultra-thin films but were operated in N2 box atmosphere [39, 40]. Realizing ultra-flexible optical sensor with an extended stability of surrounding conditions, allowing the sensors to integrate intimately and unnoticed on the skin, and to enable the application's cornucopia [10].

A representative non-invasive measurement system using pulsed oximetry characterizes peripheral oxygen saturation evaluated by oxyhemoglobin in the blood is shown in **Figure 4** [10]. An organic pulse oximeter with polymer LEDs and Si-based photodetectors were implemented based on pulse sensing and display on the human skin. The reflective pulse oxygen system used green and red LEDs to generate two wavelengths through fingers, and a light detector later measured the change in absorbance on the same side of the LED to determine peripheral oxygenation [10].

Choi et al. demonstrated an array of ultra-high-density curved MoS2-graphene light detectors using a single lens optical unit [9]. The high-density MoS2-graphene curved structure of the photodetector array was fabricated using an ultra-thin, soft material and strain-isolating/-releasing device architectures. The photodetector array and ultrathin neural-interfacing electrodes were embedded onto the soft flexible printed circuit board, accomplishing a human eye-inspired soft implantable optoelectronic device. The proposed device offered minimal mechanical deformation to the eye model, which were demonstrated by both experiments and finite element analysis (FEA) simulations. The wearable photodetector array and ultrathin neurointerference system reduced mechanical distortion of the retina and effectively stimulated the retinal nerve responded by optical input signals.

Ng et al. developed organic bulk heterojunction photodetectors having dark current as low as <1 nA cm−2 and efficient charge collection behavior. The development of a 4 μm-thick sensor layer using amorphous silicon TFTs provided a highly flexible image sensor operation with excellent performance as high as > 35% external quantum efficiency and noise equivalent power of 30 pW cm−2 at the applied reverse bias voltage of −4 V [41].

#### **Figure 4.**

*Ultraflexible organic pulse oximeter. (A) Device structure of the pulse oximeter. (B) Operation principle of the reflective pulse oximeter. Adapted from Ref. [10].*
