**3.4 Elastomer**

What is another important material in wearable sensor are elastomers which are polymers with viscoelasticity (i.e., both viscosity and elasticity). Elastomers have weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. Main strategy to construct a conductive elastomer is to combine elastomers and metallic nanomaterials for stretchability and conductivity. By optimizing the material design, elastomer-to-metallic nanomaterial composition ratio and fabrication processes of the metallic nanocomposites, stretchable conductors with exceptionally conductive and stretchable properties can be realized [73].

Maintaining electrical conductivity of the stretchable conductor during certain strain or tension is a critical parameter in the application of the wearable device. Usually, the electrical pathway in the composite materials should be formed in the composite. Various techniques, such as structural optimization of the nanomaterial, welding between the metallic nanomaterials (**Figure 10A**) [74], and additives [75], have been adopted In order to enhance the conductivity, (**Figure 10B**).

As the thickness decreases, the bending stiffness decreases at a rate of 3 times faster when the thickness of the elastomer is reduced to obtain a highly flexible wearable device. Elasticity can be obtained through various strategies for example fractal interconnect design, free deformation wavy configuration, bridge structure and serpentine structure [50, 76–84].

#### **Figure 9.**

*(A) Schematic illustrations of a sensor (left) and a TFT (right), consisting of AuNP-AgNW conductors, P3HTNF/PDMS semiconductor composite, and ion gel dielectric vertically stacked on a PDMS substrate. Adapted from Ref. [71]. (B) Schematic of the DoS transistor based on the Ag-PEDOT:PSS ink as the conductor, P3HT-NF ink as the semiconductor, and ionic gel ink as the dielectric. Adapted from Ref. [72].*

#### **Figure 10.**

*(A) (Left) Plan-view SEM image of silver nanowire junctions before illumination. Scale bar is 200 nm. (Right) Plan-view SEM image of silver nanowire junctions after optical welding with a tungsten halogen lamp. Scale bar is 500 nm. Adapted from Ref. [74]. (B) Fabrication process of elastic conductor ink. Upper picture, elastic conductor ink. Scale bar, 10* μ*m. Lower picture, printed elastic conductor with high resolution. Scale bar, 100 mm. Adapted from Ref. [75].*

The representative elastomers for the stretchability of wearable sensors are poly(styrene-butadiene-styrene) (SBS), polyurethane, polydimethylsiloxane (PDMS) and hydrogel polymer materials. Those elastomers are usually used as substrates or a matrix while embedding nanoparticles, nanowires, nanosheets in wearable electronics. Liquid conductors such as gallium metal alloys are less reactive and non-toxic, thus, they are utilized in microfluidic channels of wearable sensors [52, 85]. Hydrogel and polymer have high biocompatibility but relatively low conductivity, limiting performance, and many hydrogels suffer from stiffness over time due to drying. Conventional biocompatible materials usually combine hydrogel and polymer with nanomaterials and were constructed as composite materials to enhance performance and stability. When using nanomaterials such as CNT, care should be taken by encapsulating inside the elastic system to prevent potential health problems [86, 87]. Advances of biocompatible materials was required to improve breathability and stability for pragmatic use [62].

Especially, hydrogel is a 3D structure of hydrophilic polymers, which has been widely applied as a biomaterial due to their biocompatibility with human skin surface. Conductive hydrogel can be synthesized by combining hydrogel polymer with nanostructured metal or conductive polymer (**Figure 11A**) [88]. The synthesis of hydrogel involves physical and chemical crosslinking at a molecular level. Two type of cross-linked hydrogel, physically or chemically cross-linked hydrogels can be considered. Physically cross-linked one usually has self-healing properties but poor mechanical properties (**Figure 11B**) [89]. Chemically cross-linked one has high mechanical properties that withstands physical deformation but no mechanical self-healing properties [90]. Several recent approaches of structure fabrication include a dual network and sliding cross-link, which adjusts the Young's moduli from kilopascals to megapascals [91–93]. However, hydrogels suffer from dehydration and debonding. Encapsulation using elastomers on the hydrogels or the surface modification using supramolecules can be applied to avoid the dehydration and debonding [94].

Reliable signal acquisition from biophysical activity is paramount in the assessment of wearable sensors. Continuous physical movement of human body and dynamic skin surface condition facilitate detachment of wearable sensors during the signal acquisition. Thus, facile chemical and physical approaches have been adapted to achieve for the robust attachment of wearable sensor on arbitrary human skin surface under investigation. Yuk et al. investigated a simple yet effective strategy of inserting a cross-linked hydrophilic polymer (hydrogel skin) into various polymer

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

#### **Figure 11.**

*(A) Schematic of the bioelectronic interface between a peripheral nerve and soft conductor electrodes and insulation materials. (B) Schematic of the stepwise PEDOT:PSS ECH synthesis process and SEM images showing morphological changes in each step during the synthesis of an ECH. An ionic liquid, 4-(3-butyl-1-imidazolio)-1-butanesulfonic acid triflate, was blended with the PEDOT:PSS solution and subsequently dried to form an ion gel (i); ionic liquid is exchanged with water and then dried at room temperature (ii); the dried sample exhibits aligned and interconnected microstructures that swell in water to form the ECH (iii). The interconnected PEDOT polymer network in the ECH results in a continuous electronic conductive pathway. Scale bar, 1mm for the inset of (iii), which is an optical image of a hydrated ECH. Adapted from Ref. [88]. (C) Design concept for a transparent, self-healing, highly stretchable ionic conductor using ion–dipole interaction as the dynamic motif and demonstration of healing process and chemical structure of polymer and imidazolium cation. Adapted from Ref. [89].*

surfaces, including silicon rubber, polyurethane, PVC, nitrile rubber and natural rubber, to facilitate the robust attachment of hydrogel and device interfaces. Due to the unique combination of soluble initiators absorbed on the polymer surface and hydrogel pregel initiators dissolved in hydrogel solutions, hydrogel skin is placed on the surface and adapted to the complex and fine geometry of the polymer substrate [95]. Hydrogel skins provided tissue-like softness with excellent mechanical rigidity, low friction, anti-easing performance and ion conductivity.

Yuk et al. also reported fabrication of bio-compatible dry double-sided tape (DST). The bio-compatible DST consists of biopolymer and crosslinked poly(acrylic acid) [96]. The authors attached an elastic strain sensor to a beating porcine heart to evaluate thermal motion that can serve as a versatile platform for wearables and implantable devices. In addition, they demonstrated possible applications using ex vivo models and the combination of DST and biosensors. This DST offered advantages over conventional tissue adhesives and sealants, such as fast adhesive formation, strong adhesion performance, flexibility, storage, and ease of use.

### **4. A variety of transduction systems for Wearable biosensors**

To obtain physiological information or signals from the human body using skinmounted bio-sensors, they are mainly composed of stretchable or flexible materials. Generally, flexible substrates, electrodes, and sensing materials are three essential parts of skin-mounted biosensors. More importantly, appropriate device systems,

architectures, and sensing mechanisms should be combined with relevant electronic materials. In this Section, we mainly review widely used sensing mechanisms and architectures. [43, 97].
