**7. Factors affecting superabsorbent hydrogel**

## **7.1. Density of cross-linking**

of both growths of polymerization reaction. As shown, the polymerization occurs which by chain growth would proceed by random dimerization of the neighboring monomers and then oligomerization formation until the cross-linked network is obtained. Step growth polymerization reaction has formed dimers, trimers, tetramers, etc., until short chains are formed

Another attractive feature is the development of new complex hydrogel films with targeted architecture. Porous materials are materials having different pore size structures (from nanometer to millimeter). Hydrogel has a porous structure in size of the micrometer called superporous structure. **Figure 11** shows the pores with different shapes with different accessibility. Almost, the surfaces of pores are hydrophilic, and the void begins to open due to group's repletion of the same charge. The swelling process shows variability of pore structure obtained as (a) closed pores, (b) one side opened pores like cylinder, (c) two sides opened pores like tunnel, (d) one side opened pores like ink bottle shaped, (e) two side opened pores like funnel shaped, (f) pores with rough surface, (g) separated closed pores, (h) interconnected pores, (k) collected pores or density pores, and (p) pores like internal tunnel. In maximum swelling we reach to superporous hydrogel when the pores predomi-

Further compression properties of the superporous hydrogel are α-elastin fabricated under 60

of the hydrogels which was enhanced 20-fold when the pressure was increased from 1 to 60 bar.

**Figure 11.** Given the different type of pores where white region is hydrophilic region filled with water and blue region is hydrophobic region or covalent bond of cross-linkage. **Shape**: a, k, g, and h (closed pores: (b) cylindrical open shaped, (c and p) tunnel shaped, (d) ink bottle shaped, (e) funnel shaped, and (f) roughness). **Accessibility**: a, p, h, g, and k (closed

pressure which was comparable with 1 bar. SEM image in **Figure 12** shows the pore size

within the combination of them to form cross-linked hydrogel.

**6. Types of porous structure obtained**

nate than the solid network.

bar CO<sup>2</sup>

56 Hydrogels

pores: c, e, d, f, and p).

Increasing the ratio of cross-linked portion leads to slow down the movement of chains, resulting in the decrease in free volume, the pore sizes, and the swelling degree which are also decreasing. This can be observed by SEM analysis or DSC where increased cross-linking causes increase of glassy temperature (*Tg* ) of the polymer [54, 55]. However, in some cases, a decreased cross-linking leads to a decrease of *Tg* , where nonfreezing (bounded) water molecules are attached to function groups causing a decrease of *Tg* [56].

#### **7.2. The ratio of hydrophobic/hydrophilic surface area of the hydrogels**

The ionization power and number of hydrophilic functional groups along the hydrogel chains and its counterion type play an important role in the degree of swelling. A high proportion of superabsorbent hydrogels are present as acrylates with carboxylic acid functional groups, which in the salt form undergo dissociation upon contact with water. In the dissociated state, the hydrogel network will have a series of functional groups that have the same electric charge and thus repel each other. This leads to expansion of the hydrogel network structure with the further absorption of water molecules. Furthermore, the number of hydrophilic moieties when increasing the swelling could be increased and vice versa.

#### **7.3. Applications of superabsorbent hydrogel**

According to the required application, the hydrogels have been tailored and designed to achieve the purpose of applications. The presented section demonstrates the research concerning the characterization of hydrogels on various bases, physical and concoction qualities of these items, and specialized practicality of their usage.

#### **7.4. Internal curing agent for cement**

Cross-linking of superabsorbent hydrogels based on poly(acrylamide-co-sodium alginate) by γ-radiation shows higher swelling capacities in basic than in acidic media [57]. This property proposes the use of (PAM-*co*-NaAlg) hydrogel as an internal curing agent for cement. The cement with (PAM-co-NaAlg) hydrogel mixture improves the compressive strength of cement at 0.1 and 0.2 wt%. Intermediate values are found when using 0.1, 0.3, 0.4, and 0.5 wt% of hydrogel. Thus, the maximal improvement percentage on the compressive strength is 0.2% with respect to the hydrogel. This would indicate that using 0.2 wt% of hydrogel is a critical value. Under this value, the hydrogel shows less ability for water retention than what is needed for cement curing. Above this value, the higher amount of hydrogel is contributing to increase voids that cause decreased compressive strength. **Figure 13** demonstrates the procedure for mixing of cement-hydrogel samples in the laboratory for compressive strength test. Mixing of cement and hydrogel samples was carried out with a known w/c ratio at 0.4. The setup of preparation steps is as follows: (1) The known weight of dried PAM-co-NaAlg is added to 100 ml water, and (2) during a specified time period, the hydrogel is allowed to swell, and water uptake is liberated slowly during the hydration of the cement. Then, 40 g of cement (3) is added to the swollen hydrogel, and the slurry of cement is obtained (4). The slurry is stirred well for 3 min and then poured in the aluminum mold (4 × 4 × 4) cm3 (5). After a period of time (24 hr) which corresponds to the time required obtaining dried cement samples, the mold is removed.

**Figure 14a** and **b** shows the SEM images of the cement-hydrated (cured) product particles. It is clear that the particles in **Figure 14b** for 0.3 wt% hydrogel are smaller than the particles in **Figure 14a** (without hydrogel). This indicates that the slow absorption of water during cement curing helps the formation of small particles and leads to decreased permeability. Moreover, without hydrogels, the poorly hydrated system is reflecting the

Many studies report the use of superabsorbent hydrogel (SAH) in agricultural field (**Figure 15**). Using superabsorbent hydrogel increases soil ability to hold water, so plant growth increases and it can resist drought for a long time [58–60]. Also, adding superabsorbent hydrogel (SAH) to soil improves plants through supplying the plant roots with water,

and components of polymers held the ions of salt in the drying soil [61]. Superabsorbent hydrogels (PVP/CMC) based on polyvinylpyrrolidone (PVP)/carboxymethyl cellulose (CMC) were prepared by using gamma radiation as an initiator of polymerization reaction [62]. Water and fertilizers are vital factors for producing high-yield agricultural crops. Fertilizers are composed of phosphate (P), potassium (K), and nitrogen (N) nutrients for plants in the form of water-soluble salts which are loaded into (PVP/CMC) hydrogel. The hydrogels show adsorption desorption of the three fertilizers; also, the results revealed that the presence of CMC in the hydrogel improves their water retention capability with high swelling ratio. This indicates that the addition of PVP/CMC hydrogels to soil could improve the water-holding capacity of the soil which has the excellent water absorbing capacity. So, the soil could hold much more water during the irrigation period or raining time than the soil without it and could efficiently reduce irrigation water consumption. It can be concluded that PVP/CMC hydrogel had good water retention capability suggesting their possible use of the prepared superabsorbent hydrogel as a soil conditioner in

**Figure 14.** SEM image of a fracture surface of cement: (a) blank sample without hydrogel and (b) 0.3 wt% of hydrogel.

/Na+

http://dx.doi.org/10.5772/intechopen.74698

homeostasis,

Superabsorbent

59

providing soil with potassium ion which is important to retain a K<sup>+</sup>

compressive strength results.

**7.5. Agricultural proposal**

agriculture applications.

**Figure 13.** The procedures of preparing cement hydrogel mixture for compressive strength test.

**Figure 14a** and **b** shows the SEM images of the cement-hydrated (cured) product particles. It is clear that the particles in **Figure 14b** for 0.3 wt% hydrogel are smaller than the particles in **Figure 14a** (without hydrogel). This indicates that the slow absorption of water during cement curing helps the formation of small particles and leads to decreased permeability. Moreover, without hydrogels, the poorly hydrated system is reflecting the compressive strength results.

#### **7.5. Agricultural proposal**

**7.4. Internal curing agent for cement**

58 Hydrogels

3 min and then poured in the aluminum mold (4 × 4 × 4) cm3

**Figure 13.** The procedures of preparing cement hydrogel mixture for compressive strength test.

Cross-linking of superabsorbent hydrogels based on poly(acrylamide-co-sodium alginate) by γ-radiation shows higher swelling capacities in basic than in acidic media [57]. This property proposes the use of (PAM-*co*-NaAlg) hydrogel as an internal curing agent for cement. The cement with (PAM-co-NaAlg) hydrogel mixture improves the compressive strength of cement at 0.1 and 0.2 wt%. Intermediate values are found when using 0.1, 0.3, 0.4, and 0.5 wt% of hydrogel. Thus, the maximal improvement percentage on the compressive strength is 0.2% with respect to the hydrogel. This would indicate that using 0.2 wt% of hydrogel is a critical value. Under this value, the hydrogel shows less ability for water retention than what is needed for cement curing. Above this value, the higher amount of hydrogel is contributing to increase voids that cause decreased compressive strength. **Figure 13** demonstrates the procedure for mixing of cement-hydrogel samples in the laboratory for compressive strength test. Mixing of cement and hydrogel samples was carried out with a known w/c ratio at 0.4. The setup of preparation steps is as follows: (1) The known weight of dried PAM-co-NaAlg is added to 100 ml water, and (2) during a specified time period, the hydrogel is allowed to swell, and water uptake is liberated slowly during the hydration of the cement. Then, 40 g of cement (3) is added to the swollen hydrogel, and the slurry of cement is obtained (4). The slurry is stirred well for

which corresponds to the time required obtaining dried cement samples, the mold is removed.

(5). After a period of time (24 hr)

Many studies report the use of superabsorbent hydrogel (SAH) in agricultural field (**Figure 15**). Using superabsorbent hydrogel increases soil ability to hold water, so plant growth increases and it can resist drought for a long time [58–60]. Also, adding superabsorbent hydrogel (SAH) to soil improves plants through supplying the plant roots with water, providing soil with potassium ion which is important to retain a K<sup>+</sup> /Na+ homeostasis, and components of polymers held the ions of salt in the drying soil [61]. Superabsorbent hydrogels (PVP/CMC) based on polyvinylpyrrolidone (PVP)/carboxymethyl cellulose (CMC) were prepared by using gamma radiation as an initiator of polymerization reaction [62]. Water and fertilizers are vital factors for producing high-yield agricultural crops. Fertilizers are composed of phosphate (P), potassium (K), and nitrogen (N) nutrients for plants in the form of water-soluble salts which are loaded into (PVP/CMC) hydrogel. The hydrogels show adsorption desorption of the three fertilizers; also, the results revealed that the presence of CMC in the hydrogel improves their water retention capability with high swelling ratio. This indicates that the addition of PVP/CMC hydrogels to soil could improve the water-holding capacity of the soil which has the excellent water absorbing capacity. So, the soil could hold much more water during the irrigation period or raining time than the soil without it and could efficiently reduce irrigation water consumption. It can be concluded that PVP/CMC hydrogel had good water retention capability suggesting their possible use of the prepared superabsorbent hydrogel as a soil conditioner in agriculture applications.

**Figure 14.** SEM image of a fracture surface of cement: (a) blank sample without hydrogel and (b) 0.3 wt% of hydrogel.

**Figure 15.** Used SAH as soil conditioner to support plant growth safely.

#### **7.6. Biomedical proposal**

Hydrogels have become very popular due to their unique properties such as high water content, biocompatibility, flexibility, and softness. Natural and synthetic polymers can be a resemblance to the living tissue that opens up several opportunities for applications in biomedical and medicine fields. Currently, hydrogels are used for manufacturing hygiene products, contact lenses, scaffolds, tissue engineering, wound dressings, and drug delivery systems. More developments are expected in drug delivery and tissue engineering. A hydrogel based on poly-2-hydroxyethylmethacrylate (PHEMA) as a synthetic biocompatible material is used as contact lens applications [63, 64]. Wound dressing is an effective hydrogel dressing that relies on an understanding of the healing process. Healing can be hindered by many factors such as infection, abnormal bacterial presence or desiccation, maceration, necrosis, pressure, edema, and trauma [65]. The "ideal" wound management product should absorb excess toxins and exudate, keep a good moisture between the wound and the dressing with increasing collagen production, preserve the wound from external sources of infection, prevent excess heat at the wound, have good permeability to gases, be supplied completely sterile, and be easy to remove without further trauma to the wound [66]. Their high water content allows vapor and oxygen transmission to the wounds such as pressure sores, leg ulcers, surgical and necrotic wounds, lacerations, and burns. They seem to play an important role as emergency burn treatment alone or in combination with other products, thanks to their cooling and hydrating effect [67]. Hydrogels have attracted noticeable interest for their use in drug delivery due to their unique physical properties [68]. The high porosity that characterizes SAH can easily absorb and desorb drugs easily by adjusting the density of cross-links in their matrix and the affinity to water.

Tissue engineering is the application of the principles and methods of engineering and life sciences toward fundamental understanding of the structure–function relationship in normal and pathological mammalian tissues and the development of biological substitutes for the repair or regeneration of tissue or organ function [69]. Tissue engineering is a more recent application of hydrogels, in which they can be applied as space-filling agents, as delivery vehicles for bioactive substances or as three-dimensional structures that organize cells and present stimuli to ensure the development of a required tissue. Space-filling agents are the most commonly used group of scaffolds, and they are employed for bulking, to prevent

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**Figure 16.** SEM image of hydrogel in funny situation.

**Figure 16.** SEM image of hydrogel in funny situation.

**7.6. Biomedical proposal**

60 Hydrogels

**Figure 15.** Used SAH as soil conditioner to support plant growth safely.

Hydrogels have become very popular due to their unique properties such as high water content, biocompatibility, flexibility, and softness. Natural and synthetic polymers can be a resemblance to the living tissue that opens up several opportunities for applications in biomedical and medicine fields. Currently, hydrogels are used for manufacturing hygiene products, contact lenses, scaffolds, tissue engineering, wound dressings, and drug delivery systems. More developments are expected in drug delivery and tissue engineering. A hydrogel based on poly-2-hydroxyethylmethacrylate (PHEMA) as a synthetic biocompatible material is used as contact lens applications [63, 64]. Wound dressing is an effective hydrogel dressing that relies on an understanding of the healing process. Healing can be hindered by many factors such as infection, abnormal bacterial presence or desiccation, maceration, necrosis, pressure, edema, and trauma [65]. The "ideal" wound management product should absorb excess toxins and exudate, keep a good moisture between the wound and the dressing with increasing collagen production, preserve the wound from external sources of infection, prevent excess heat at the wound, have good permeability to gases, be supplied completely sterile, and be easy to remove without further trauma to the wound [66]. Their high water content allows vapor and oxygen transmission to the wounds such as pressure sores, leg ulcers, surgical and necrotic wounds, lacerations, and burns. They seem to play an important role as emergency burn treatment alone or in combination with other products, thanks to their cooling and hydrating effect [67]. Hydrogels have attracted noticeable interest for their use in drug delivery due to their unique physical properties [68]. The high porosity that characterizes SAH can easily absorb and desorb drugs easily by adjusting the density of cross-links in their matrix and the affinity to water.

Tissue engineering is the application of the principles and methods of engineering and life sciences toward fundamental understanding of the structure–function relationship in normal and pathological mammalian tissues and the development of biological substitutes for the repair or regeneration of tissue or organ function [69]. Tissue engineering is a more recent application of hydrogels, in which they can be applied as space-filling agents, as delivery vehicles for bioactive substances or as three-dimensional structures that organize cells and present stimuli to ensure the development of a required tissue. Space-filling agents are the most commonly used group of scaffolds, and they are employed for bulking, to prevent adhesion, and as biological "glue." Drugs can be delivered from hydrogel scaffolds in numerous applications including promotion of angiogenesis and encapsulation of secretory cells. Additionally, hydrogel scaffolds have also been applied to transplant cells and to engineer many tissues in the body, including the cartilage, bone, and smooth muscle [11].

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#### *7.6.1. Environmental proposal*

Hydrogel is an eco-friendly material that has many uses as water purification and air purification. Hydrogel as a new type of adsorbent for water purification is a composite with graphene oxide [70]. These materials usually exhibit high-capacity adsorption toward water pollutants and air pollutants [71]. Due to their highly developed porous structure, hydrogel fabricated with other materials such as graphene oxide and zeolite exhibits a high capacity for gas adsorption and selectivity for gas separation. Researches are still on developing the efficiency of hydrogel gas adsorbents with a good stability, recyclability, and substantial capacity. Because of their porous structure, and their environment stability, hydrogel is a good candidate for gas adsorption.

#### *7.6.2. Gallery of hydrogel*

**Figure 16** shows funny imaginary SEM pictures of swollen hydrogel (a) PVP/PAAc, (b) PVP/ PAAc after swelled in 1 M KCl, and (c) CMC/PVP loaded with three kinds of salts.
