**3. Cellulose**

68 Cellulose – Medical, Pharmaceutical and Electronic Applications

structures. In this way, biosensors can be described as integrated receptor-transducer devices which provide selective quantitative or semi-quantitative analytical information using biological recognition elements. The main advantages of biosensors, over traditional analytical detection techniques, are their cost-effectiveness, fast and portable detection, which makes *in situ* and real time monitoring possible. Implantable biosensors can made a continuous monitoring of metabolites providing an early signal of metabolic balances and assist in the prevention and cure of various disorders, for instance diabetes and obesity [5].

Enzymes are well-known biological sensing materials used in the development of biosensors due to their specificity. However, since they have poor stability in solution, enzymes need to be stabilized by immobilization. Enzyme immobilization can be made by covalent linkage, physical adsorption, cross-linking, encapsulation or entrapment [6, 7]. The choice of the immobilization method depends on the nature of the biological element, the type of transducer used, the physicochemical properties of the analyte and the conditions in which the biosensor should operate [8, 9]. Moreover, it is essential that the biological

As a result, the development of a sensing device based on enzymes is in a good agreement with the present concerns of Green Chemistry due to inherently being a clean process. Notwithstanding some shortcomings such as high sensitivity to environmental factors (like pH, ionic strength and temperature), dependence on some cofactors and limited lifetime

To overcome the drawbacks, enzyme-free biosensors have been actively developed owing to their simple fabrication, stability and reproducible characteristics. Novel nanoparticle (NP) modified electrodes and other functionalized electrodes have been tested in the design of enzyme-free biosensors [10, 11]. Nanostructured materials have the advantage to be easily functionalized exhibiting high electrocatalytic activity and stability. For instance, carbonbased nanostructures have been widely studied as a platform which can hybridize with other functionalized materials, such as metal and metal oxides, forming nanocomposites with improved electrochemical properties [12]. Overall, these nanostructures can provide

optimal composite electrode materials for high-performance enzyme-free biosensors.

The rising interest in Micro Electrical Mechanical Systems (MEMS) due to expanding application areas and new products opportunities, gave rise to the need for reliable and cost effective MEMS, especially in areas such as biosensors, energy harvesting, and drug delivery

Biomedical technology usually requires various portable, wearable, easy-to-use, and implantable devices that can interface with biological systems. Currently, implantable medical microsystems are powered by small batteries with limited lifetime. Although, the scientific progress in this area has enabled a decrease in the electrical requirements of the miniaturized devices, the development of a suitable power source remains a major challenge

element exhibit maximum activity in its immobilized environment.

hinder the utilization of enzymes in some specific situations.

**2.2. Implantable energy harvesting devices** 

[13, 14].

The demand for products made from renewable and sustainable resources, non-petroleum based, and with low environmental safety risk is persistently increasing. For that reason, renewable materials have been widely explored by consumers, industry, and government. Half of the biomass produced by photosynthetic organisms such as plants, algae, and some bacteria is made up of cellulose, which is the most abundant molecule on the planet. Natural cellulose-based materials, such as wood and cotton, have been used by our society as engineering materials for thousands of years. Cellulose exhibit excellent characteristics, which include hydrophilicity, chirality, biodegradability, capacity for broad chemical modication, and ability to form semicrystalline ber morphologies, which drawn considerably increased interest and encouraged interdisciplinary research on cellulose-based materials.

#### **3.1. Cellulose source materials**

Cellulose plays a significant role in the structural support of wood, plants, and composites because of its high mechanical properties. Wood remains the most important raw material

source of cellulose. The structure of wood is highly complex due to the presence of lignin, a three-dimensional polymer network that binds to carbohydrates (hemicellulose and cellulose) to form a tight and compact structure. The compact structure of wood biomass is particularly challenging because in its native state is impossible to dissolve it in conventional solvents. Traditionally, cellulose is extracted from wood through the Kraft pulping process [22] which involves toxic chemicals and the intensive processing conditions. Recently, research studies focused on a "greener" process which uses Ionic Liquids (ILs) for wood dissolution [23]. A wide variety of plant materials have been studied for the extraction of cellulose including cotton, potato tubers, sugar beet pulp, soybean stock, and banana rachis[24, 25]. Furthermore, cellulose microfibrils can be produced by several species of algae, such as green, gray, red, and yellow-green. Among the algae species, differences in cellulose microfibrils structures can be obtained due to the different biosynthesis process [26]. The cellulose obtained from algal species contains porous or spongy like structure, which is substantially different from the higher plant cellulose. Cellulose microfibrils can also be segregate by bacteria under special culturing conditions. Bacteria can produce a thick gel composed by cellulose microfibrils and water (97% of water content). The major advantage found in bacterial cellulose is the possibility to modify microfibrils structure by changing the culture conditions [27].

Cellulose-Based Bioelectronic Devices 71

Moreover, cellulose can be chemically modified to yield cellulose derivatives. The cellulose derivatives were designed and fine-tuned to obtain certain desired properties and the chemical functionalization of cellulose is done by changing the inherent hydrogen bond network and by introducing different substituents (Figure 2). Indeed, the properties of cellulose derivatives are mainly determined by the group of substituents and the degree of substitution. These substituents can prevent spontaneous formation of hydrogen bonding or even create new interactions between the cellulose chains. With this insight, recent progress has been made in cellulose chemical modification achieving new routes that are now

**Figure 2.** The most relevant cellulose derivatives and their synthesis pathways.

#### **3.2. Cellulose functionalization**

The solubility of cellulose depends on many factors especially on its structure, molecular weight and source. Polysaccharides are well-known to manifest a strong tendency to aggregate or to incomplete solubilization due to the formation of hydrogen bonds. The hydrogen bonding patterns in cellulose are considered as one of the most relevant factors on its physical and chemical properties. The solubility, crystalinity and hydroxyl reactivity can be directed affected by intra- and intermolecular bond formation (Figure 1) [28].

**Figure 1.** The structure and intra- (1) and interchain (2) hydrogen bonding pattern in cellulose.

Moreover, cellulose can be chemically modified to yield cellulose derivatives. The cellulose derivatives were designed and fine-tuned to obtain certain desired properties and the chemical functionalization of cellulose is done by changing the inherent hydrogen bond network and by introducing different substituents (Figure 2). Indeed, the properties of cellulose derivatives are mainly determined by the group of substituents and the degree of substitution. These substituents can prevent spontaneous formation of hydrogen bonding or even create new interactions between the cellulose chains. With this insight, recent progress has been made in cellulose chemical modification achieving new routes that are now

70 Cellulose – Medical, Pharmaceutical and Electronic Applications

changing the culture conditions [27].

**3.2. Cellulose functionalization** 

source of cellulose. The structure of wood is highly complex due to the presence of lignin, a three-dimensional polymer network that binds to carbohydrates (hemicellulose and cellulose) to form a tight and compact structure. The compact structure of wood biomass is particularly challenging because in its native state is impossible to dissolve it in conventional solvents. Traditionally, cellulose is extracted from wood through the Kraft pulping process [22] which involves toxic chemicals and the intensive processing conditions. Recently, research studies focused on a "greener" process which uses Ionic Liquids (ILs) for wood dissolution [23]. A wide variety of plant materials have been studied for the extraction of cellulose including cotton, potato tubers, sugar beet pulp, soybean stock, and banana rachis[24, 25]. Furthermore, cellulose microfibrils can be produced by several species of algae, such as green, gray, red, and yellow-green. Among the algae species, differences in cellulose microfibrils structures can be obtained due to the different biosynthesis process [26]. The cellulose obtained from algal species contains porous or spongy like structure, which is substantially different from the higher plant cellulose. Cellulose microfibrils can also be segregate by bacteria under special culturing conditions. Bacteria can produce a thick gel composed by cellulose microfibrils and water (97% of water content). The major advantage found in bacterial cellulose is the possibility to modify microfibrils structure by

The solubility of cellulose depends on many factors especially on its structure, molecular weight and source. Polysaccharides are well-known to manifest a strong tendency to aggregate or to incomplete solubilization due to the formation of hydrogen bonds. The hydrogen bonding patterns in cellulose are considered as one of the most relevant factors on its physical and chemical properties. The solubility, crystalinity and hydroxyl reactivity can

be directed affected by intra- and intermolecular bond formation (Figure 1) [28].

**Figure 1.** The structure and intra- (1) and interchain (2) hydrogen bonding pattern in cellulose.

**Figure 2.** The most relevant cellulose derivatives and their synthesis pathways.

available for the production of functional and sustainable cellulose–based materials [29]. The chemical modification of cellulose surface is a classical approach to transform the polar hydroxyl groups sitting at the surface of cellulose into moieties able to enhance interactions with the matrix. Indeed, the high density of free hydroxyl groups in cellulose makes it a helpful solid substrate that can undergo functionalization to come into novel advanced applications. Owing to cellulose chain rigidity, some cellulose derivatives can form thermotropic or lyotropic mesophases (in suitable solvents). Among cellulose ethers, hydroxypropylcellulose (HPC) have encouraged the scientific community due to its cholesteric liquid crystalline organization at high concentration [30]. These liquid crystalline phases, with an internal periodic modulation of the refractive index, exhibit many remarkable optical properties as a result of their photonic band structure, which have applications such as polarized light sources, information displays, and storage devices [31]. These phases may also mimic the structural organization of type I collagen and are good analogues of the extracellular matrix, with a structure close to that of biological tissues. These materials can be used either in tissue repair or as models for the culture of cells in 3D, the study of their migration and signaling activities, in a manner close to physiological conditions [32].

Cellulose-Based Bioelectronic Devices 73

biocompatibility [36]. When addressed to biosensor applications, the well dened dendritic structures generate surfaces with increased reproducibility and high affinity for biomolecular immobilization. This is due to the extraordinary control over the architecture coupled to the possibility of designing a large number of accessible active sites at the

A further approach is the modification of cellulose-based structures with ionic liquids (ILs). Ionic liquids are often used in the preparation of functional materials by its covalent attachment to the support surface forming a stable composite. Moccelini [37] have reported the development of a novel polymeric support based on cellulose acetate and 1-n-butyl-3 methylimidazolium bis(trifluoromethylsulfonyl)imide-based IL, BMI.N(Tf)2 IL, for enzyme immobilization. The introduction of the IL probably causes an increase in the distance between the cellulose chains due to the interactions of the anion of the IL and the hydrogen bond networks of the cellulose acetate. Thus, the enzyme can be entrapped within the interstitial space of the formed composite, which results in a considerable stabilization of the enzyme structure, and consequently increases its activity. The study performed demonstrates that this material was able to immobilize Laccase, leading to high efficient and

robust biocatalysts thus improving the electrochemical performance of the biosensor.

bioelectrochemical activity, enhanced biological affinity as well as good stability.

The simple electrode fabrication methodology and the biocompatibility of the cellulose– MWCNT matrix mean that the immobilization matrix can be extended to diverse proteins, thus providing a promising platform for further research and development of biosensors

The use of ILs as an intermediary solvent to facilitate the combination of cellulose and CNTs has been suggested by Jun Wan [39]. A cellulose and single wall carbon nanotube (SWNTs) composite was utilized to immobilize leukemia K562 cells on a gold electrode to form a cell

Envisaging the immobilization of other biomolecules, Alpat and Telefoncu [40] describes the development of a novel biosensor based on the co-immobilization of TBO (Toluidine Blue O), NADH (Nicotinamide adenine dinucleotide) and ADH (alcohol dehydrogenase) on a

The use of ILs is an alternative either for cellulose dissolution or to facilitate the dispersion of carbon nanotubes. For that reason, Xuee Wu [38] describes a method to immobilize enzymes in a cellulose-multiwalled carbon nanotube (MWCNT) matrix via the IL reconstitution process. This method consists in the dissolution of cellulose in the IL, followed by dispersion of MWCNT in the solution and enzyme addition. Subsequently, the IL is removed by dissolution, leaving the cellulose-MWCNT matrix with the enzyme encapsulated on the surface. The cellulose–MWCNT matrix possesses a porous structure which allows the immobilization of a large amount of enzyme close to the electrode surface, where direct electron communication between active site of enzyme and the electrode is enabled. The –OH groups of cellulose can also provide a good environment for the encapsulation of the enzyme. The authors have employed the resulting porous matrix in the immobilization of Glucose oxidase (GOx). The encapsulated GOx showed good

periphery of the dendritic scaffolds.

and other bioelectronics devices.

impedance sensor.

In the next section, the functionalization of cellulose will be addressed in detail. Novel functionalized cellulose-based materials have been developed for biosensors and energy storage devices. Some approaches for enzyme immobilization methods including covalent attachment of enzymes by reaction with chemically modified cellulose as well as by adsorption of proteins will be described.
