**2. Green electronic materials: conducting polymers**

Conducting polymers (CPs) have occurred as competitive sensing materials for biological sensing applications. Their convenience of synthesis by chemical or electrochemical routes at ambient conditions, functionalization with monomer, dopant, monomer/dopant ratios and oxidation state to enhance the conductivities over 15 orders of magnitude, biocompatibility and low energy optical transitions have caused a significant concern. CPs have been synthesized by differing procedures, namely, electrochemical dip-pen lithography, mechanical stretching, electrospinning and template-directed electrochemical synthesis [6].

Since CPs were discovered, they have found many utilization. The swift progress in conductive polymer technologies is a significant motivating force for utilization of these materials as alternatives [7, 8] to conventional conductors, such as copper and gold [9, 10], as elements in the construction of electromagnetic devices. Regardless of that, the conductivity of polymers is lower than in metals, it has been presented to be adequate to construct antennas [11, 12]. Simultaneously, appearing green materials are contemplated to achieve a more aspiring purpose, designated by the integration of biocompatible, biodegradable and cost-reasonable electronics, such as the monitoring or the diagnosis of humans with environmentally benign technologies. Examples of the most common conductive polymers are shown in **Tables 1** and **2** [11–14] and **Figure 1**. Among the different conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most encouraging materials for bioelectronics due to its relatively high conductivity, stability and more importantly its organic nature and good compatibility with bioorganic molecules such as enzymes compared to other CPs. Nevertheless, because of

Conducting Polymers as Elements of Miniature Biocompatible Sensor http://dx.doi.org/10.5772/intechopen.75715 55

**Figure 1.** The most common CPs as sensor elements.

entered the research field in the 1970s holding the high promise of delivering cost-reasonable and energy-efficient materials and devices. Despite intense effort of the scientific community during the past 30 years, the efficiency and stability of organic semiconductors endure at current times' major obstacles in their development as solid challengers of the inorganic materials [1–3]. Consequently, the large-scale rapid replacement of hard core inorganic counterparts, like the ones active in high-speed processors, integrated circuits, and solar cell modules, with organic components is not immediately expected [1–3]. However, the "soft" nature of carbonbased components confers them a serious benefit over the inorganic materials, enabling pro-

Conducting polymers, the so-called "fourth generation of polymeric materials", can provide effective methods for the diagnosis and treatment of different disorders, that is, diabetes. Conducting polymers have often excellent biocompatibility. They can provide favorable interfaces for bioelectrodes owing to their hybrid conducting processes, combining both electron and ionic charge carriers. Many (i.e. glucose) biosensors use immobilized enzymes to construct a selective layer on CP structure. Miniaturization of sensors is a new demand. Mini sensors are portable and wearable with low utilization of sample. New biosensors with a market size of a US\$13 billion annual turnover have quickly become valuable instruments in the healthcare. Actually, glucose biosensors (accounting for 85% of the total biosensor market)

Conducting polymers (CPs) have occurred as competitive sensing materials for biological sensing applications. Their convenience of synthesis by chemical or electrochemical routes at ambient conditions, functionalization with monomer, dopant, monomer/dopant ratios and oxidation state to enhance the conductivities over 15 orders of magnitude, biocompatibility and low energy optical transitions have caused a significant concern. CPs have been synthesized by differing procedures, namely, electrochemical dip-pen lithography, mechanical

Since CPs were discovered, they have found many utilization. The swift progress in conductive polymer technologies is a significant motivating force for utilization of these materials as alternatives [7, 8] to conventional conductors, such as copper and gold [9, 10], as elements in the construction of electromagnetic devices. Regardless of that, the conductivity of polymers is lower than in metals, it has been presented to be adequate to construct antennas [11, 12]. Simultaneously, appearing green materials are contemplated to achieve a more aspiring purpose, designated by the integration of biocompatible, biodegradable and cost-reasonable electronics, such as the monitoring or the diagnosis of humans with environmentally benign technologies. Examples of the most common conductive polymers are shown in **Tables 1** and **2** [11–14] and **Figure 1**. Among the different conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most encouraging materials for bioelectronics due to its relatively high conductivity, stability and more importantly its organic nature and good compatibility with bioorganic molecules such as enzymes compared to other CPs. Nevertheless, because of

stretching, electrospinning and template-directed electrochemical synthesis [6].

duction of flexible, conformable and even extremely thin electronic equipment [4].

have notably mended the quality of life of diabetics [5].

54 Green Electronics

**2. Green electronic materials: conducting polymers**

its poor solubility and processability, PEDOT is often mixed with PSS to generate water soluble poly(3,4-ethylenedioxythiophene):poly(styrene sulphonate)—PEDOT:PSS, which was developed and patented in 1988 by Bayer AG [15]. Doping of PSS could disrupt the combination of the PEDOT chains and lower their electrochemical efficiency [16]. Despite the fact that water soluble PEDOT:PSS can be applied to generate conducting thin films, the resulting planar layer lacks morphological benefit, having only restricted accessible surface in comparison with processable colloidal interface materials [16].

To date, some electrical conductors have been applied in implantable biological interfaces such as cardiac patches [17], neural interfaces [18], electroceuticals [19] and on-command drug delivery platforms [20]. Although classic inorganic electrical materials (i.e. metals and semiconductors) are not appropriate for a seamless biointerface because of the need for extracellular functionalities, the semiconducting polymers seem to be valuable alternatives for these applications [21]. Controlling this limited biocompatibility in flexible electronic materials endures a challenge, and is recently drawing fair research efforts in the bioelectronics group.


**Table 1.** The conductivity of the most common CPs.


Physical modification has been investigated by enlarging surface roughness by different processes such as generating microporous layers using polystyrene sphere templates, creating composites of nanoparticles and polylactide [25], growing CPs within hydrogels [18] and

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57

The electrode coatings used are rather soft [26] and can be tailored at the micrometer, nanometer and molecular scale to have fibrillary, nodular, fuzzy, tubular [27] or porous surface morphologies [25]. As a consequence, most tissue- and device-compatible surface tempering of the electrode would be bringing electrical activity, bioactivity, mechanical softness and architectural properties on a similar scale to that of cells in tissues. The surface roughness character of the conducting polymer layers can be tailored by modification of the conducting polymer synthesis temperature [22]. Exceptional adaptation of the surface roughness characteristics is important because rougher topology corresponds with increasing of surface area, which would expand the signal conduction by growing the interface with neurons. For example, polypyrrole films synthesized at a lower temperature (418°C) were rougher than the same layers achieved at 2518°C [28]. Kmecko et al. [29] presented that the introduction of carbon nanotubes as dopants to PPy and PEDOT prefers the creation of bumps and grows the surface roughness. Functionalization of CPs with biomolecules has permitted engineers to modify CPs with biological sensing elements, and to turn ON and OFF different signaling routes demanded for several cellular mechanisms to form conducting polymers that extend cell proliferation/differentiation. Moreover, dopants can be applied as intermediates to allow further modification of CPs, that is, doping with poly(glutamic acid) supplies a carboxylic acid pendent group, which can be functionalized further by covalent binding to any amino group, such as those found in polylysine and

The electrochemical character of the CPs can be varied by modifying the dopant concentration. Electrical conductivities can be varied by as much as 15 orders of magnitude by changing the dopant concentrations so that control is feasible over the all ranging from insulator to semiconductor and then to metal [31]. The usually used dopants contain aromatic sulphonate variants such as *p*-toluenesulphonate, styrene, sodium benzenesulphonate to dope the polymers [32]. Other appropriate dopants for oxidation polymerization contain buffer

sequences, hydroxyapatite or a silk-like polymer with fibronectin units and polysaccharides [22]. Nevertheless, a main disadvantage of introducing dopants is the possible diffusion of the dopant into the culture medium with effects on cytotoxicity and deterioration of the electrical characteristics of the CP layer itself. For example, dodecyl sulphate-doped PPy layers

The scope of possible dopants is huge as long as the selected dopant is charged. On the other hand, covalent methods can be used to more constantly functionalize CPs. The monomer can be synthesized with required functional groups and then polymerized, or post-polymerization covalent modification is also possible. It is crucial to note that the steric effects of any introduced functional group may interrupt the planarity of the conjugated arrangement,

undergo structural modifications after 7 days of soaking in deionized water [23].

(**Table 3**). Biological dopants include laminin peptide

mixing with biomolecules to yield "fuzzy" structures.

laminin [30].

salts, I<sup>2</sup>

, BF<sup>4</sup>

, perchlorates and FeCl3

which could in turn lower the conductivity [22].

**Table 2.** Properties of the CPs.
