Keywords

1 Introduction

Polymer gels (PGs) are defined as viscoelastic cross-linked hydrophilic three-dimensional polymeric networks with dissociated ionic functional groups. PGs can absorb large amount of water or other biological fluids in short time and release them under certain conditions (Thakur and Thakur 2014a, b, 2015). Smart polymer gels (SPGs) are one class of polymer gels with a special characteristic which is the response to the environment change such as temperature, electric, light, sound field, magnetic fields, and pH. The response to the environment change gives this class of materials wide range of application in many fields such as medical and water treatment. SPGs are polymerized via normal polymerization methods such as solution, suspension, and emulsion polymerization. Cross-linking process of SPGs occurs via chemical (using cross-linking agent) or physical method (entanglements or crystallites).

2 Classification of Smart Polymer Gels

SPGs are classified depending on their response to surrounding environment as illustrated in Fig. 1.

Fig. 1
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Classification of SPGs according to their response to the external medium

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2.1 Thermosensitive Gels

This class of SPGs is defined by its ability to swell and shrink when the temperature of surrounding fluid is change (Hongyan He 2006). This kind of PGs can be divided into three classes: negative temperature sensitive, positive temperature sensitive, and thermoreversible.

2.1.1 Negative Temperature Sensitive

This class of SPGs has critical parameter called low critical solution temperature (LCST), which means that SPGs will shrink when the temperature increases above LCST and showing swelling behavior at temperature less LCST. LCST of this class of SPGs can be controlled using different ways such as changing the molar ratio of ionic copolymer or by changing the solvent composition. In general, LCST of polymer of more hydrophobic constituent shifts to low degree which means changing the percentage ratio of hydrophobic to hydrophilic contents of SPGs leads to change LCST (Schild 1992; Hongyan He 2006). Figure 2 shows the structures of some of these polymers.

Fig. 2
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Negatively pH-sensitive polymer gels

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From the structure of negative thermosensitive SPGs, it can be seen that the polymers have two parts; the first is hydrophilic part –CONH–, and the second is hydrophobic part –R– (Zhang et al. 2003). At temperatures lower than LCST water or fluid interact with the hydrophilic part by forming hydrogen bonds which leads to dissolution and swelling behavior. At temperature higher than LCST hydrophobic interaction among hydrophobic part will be stronger and hydrogen bonds will become weaker; therefore, network shrinking occurs due to inter-polymer chain association (Qiu and Park 2001), and the absorbed fluid will go out of the structure (de-swelling process).

2.1.2 Positive Temperature Sensitive

Positive temperature SPGs are known with their upper critical solution temperature (UCST) (Peppas et al. 2000). When the temperature is below UCST network structure contract and release absorbed fluids (dehydration). Network relaxation occurs at temperature higher than UCST (swelling). This type of PGs is retrogressive with negatively temperature sensitive gels. This class of SPGs is shrinking at low temperature due to the formation of complex structure via hydrogen bonds. Network structure dissociates at high temperature due to breaking of hydrogen bonds and SPGs swelling to maximum possible extent rapidly above the UCST. There are a wide range of polymers and copolymers that are positive temperature such as poly(AAm-co-BMA), and poly(AA-co-AAm-co-BMA) (Hongyan He 2006).

2.1.3 Thermoreversible

This class SPGs has similar structure and contents of negative and positive temperature SPGs with deference in kind of bonds. Polymer chains in this class are not covalently cross-linked, and SPG undergoes sol–gel phase transitions instead of swelling–shrinking transition (Hongyan He 2006). Sol–gel phase transformation depends on glucose concentration in the surrounding medium. Sol–gel reversible SPGs require glucose response cross-linking. The most commonly used thermoreversible gels are pluronic and tetronic compounds as illustrated in Fig. 3 (Qiu and Park 2001).

Fig. 3
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Polymer structures of pluronic and tetronic

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2.2 pH-Sensitive Polymer Gels

pH-sensitive gels are materials that respond to pH values of surrounding medium. Swelling behavior occurs due to changes in hydrophobic–hydrophilic nature of chains or due to hydrogen bonds and the complexation of inter- and intramolecular or electrostatic repulsion. Depending on nature of polymer pendant group, pH-sensitive PGs are classified into anionic and cationic polymer gels (Hongyan He 2006).

2.2.1 Anionic Polymer Gels

This class of SPGs often has carboxylic or sulfonic acid groups (Qiu and Park 2001). The more important parameter of anionic gels is the relation between pKa of the polymer and pH of surrounding environment, which means that, when pKa is higher than pH of surrounding medium, the ionized structure will increase the electrostatic repulsion of the network and enhance the swelling properties (Hongyan He 2006). Examples of anionic pH-sensitive PGs are shown in Fig. 4.

Fig. 4
figure 4

Anionic pH-sensitive polymer gels

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2.2.2 Cationic Polymer Gels

Cationic SPGs usually have pendant group such as amine (Qiu and Park 2001). The more important parameter in these materials is the relation between polymer pKb and pH of surrounding medium. When pH of environment is lower than polymer pKb amine group is changing from NH2 to NH3+ which subsequently increase the hydrophilicity, electrostatic repulsion, and absorption capacity (Peppas et al. 2000). Polyacrylamide (PAAm) is the most used polymer as cationic polymer gels.

2.3 Electric Signal Sensitive

Raw matrix of this class is polyelectrolyte which is similar to pH-sensitive SPGs. Electric-sensitive SPG undergoes swelling and de-swelling as a response of applied electric signal. Electric-sensitive PGs have three kinds of transition phases: swelling, shrinking or de-swelling, and bending. These phases depend on a number of conditions such as the system that has been used to apply electric field (contact system or spread system) (Kim et al. 2003; Hongyan He 2006). In the last decades, new class of smart gels has been emerged called core–shell gels. Core–shell gels are structured composite particles consisting of at least two different components: one in principle forms the core and the other the shell of the particles (Ha et al. 2002). This class of materials has attracted much attention because of the combination of superior properties not possessed by the individual components. The systems might combine the characteristics and properties of both shell and core (Hendrickson et al. 2010). Core–shell gels might be pH and thermosensitive at same time. This characteristic granted this class of gels numbers of applications as impact modifiers, surface coatings, printing, catalysis, pollution control, sensing, and drug delivery in biomedical application (Jones and Lyon 2003; Li and Stöver 2000; Sasa and Yamaoka 1994; Khan et al. 2008). Core–shell gels are usually prepared in spherical form, implying a particle structure with the initially polymerized polymer located at the center of the particle, and the later-formed polymers becoming incorporated into the outer shell layer as illustrated in Fig. 5 (Dimonie et al. 1997). The core part may be a solid or a liquid or a gas; however, the shell materials usually a solid but its nature depends on the type of applications (Jones and Lyon 2003). Core–shell polymer gels can be in nanosize and microsize according to preparation methods and targeted applications (Blackburn and Lyon 2008; Berndt et al. 2006).

Fig. 5
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Core–shell polymers (CSPs) consisting of central part may be a solid, liquid, or a gas and shell part usually a solid

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3 Composites of Smart Polymer Gels

Composite of different materials can be possible solution to get special desired properties for particular applications or to gain the desired properties from each material in the compound. SPG composite has been carried out using different kind of polymers, fibers, and fillers of different sizes from micro- to nanosize. Many minerals have been used to act as filler for production of SPG composite such as kaolin, diatomite, potassium humate, glass, smectite, kaolinite, sodium silicate, bentonite, and montmorillonite (Kabiri et al. 2011). Natural fibers also have been used in some areas such as study by Rui Liang et al. used wheat straw (WS) to improve PGs properties and investigate the effect of WS on absorption capacity of PAA superabsorbent hydrogel in distilled water and 0.9% sodium chloride (NaCl). The result showed that the maximum absorbency attained at 20% of WS and absorbency started decreasing after further increase in WS % as shown in Fig. 6 (Liang et al. 2009). Composite of SPGs was prepared to improve mechanical properties, swelling behavior, and reduce cost. In fact, composites of polymer gels were successfully prepared and utilized in many filed when mechanical properties is needed. More information on polymer gel composites can be found in very good review by Kabiri et al. (2011).

Fig. 6
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Influence of wheat straw content on the absorbency of WS/APP superabsorbent composites

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4 Properties of Smart Polymer Gels

4.1 Absorption Properties

Swelling behavior and absorption capacity (AC) are the most important properties that give SPGs wide applications. In general, swelling and absorption properties are attributed to the presence of hydrophilic groups such as –OH–, CONH–, –CONH2–, and –SO3H in the chemical structure of the network (Hamidi et al. 2008). The ratio of sample weight at swelling and de-swelling actions is called swelling ratio (SR) (Tang et al. 2008; Kim et al. 2003; Zuo Chun et al. 2007). There are many factors affecting SR and AC of SPG such as chemical compositions (Karadag et al. 2005; Guilherme et al. 2005), network structure (Patachia et al. 2007), cross-linking ratio, specific stimuli or the surrounding medium (Peppas et al. 2000; Hongyan He 2006), polymers molecular weight (Raj et al. 2009), and chemical structure of polymer repeating unit (Chun et al. 2009). The swelling process of SPGs can be explained as follows: The solvent tries to penetrate the polymer networks to produce 3D-molecular network at the same time expanding the molecular chains between the cross-linked points, thus decreasing the configuration enthalpy value. The molecular network has an elastic contractive force which tries to make the networks contract. When these opposite forces reach equilibrium, the expansion and contraction also reach a balance. In this process, the osmotic pressure is the driving force for expansion of swelling and network elastic force is the driving force of the contraction of gel. AC is estimated using volumetric, gravimetric, spectroscopic, and microwave method. The volumetric method measures the sample volume changes or absorbed fluid before and after absorption. The gravimetric method depends on measuring sample weight changes. Spectrometric method uses UV-spectrum of sample change, and the microwave method measures microwave absorption by energy changes. Absorbed water in network structure of PAA can exist in three states: bound, half-bound, and free water. Free water shows freezing point when the environment temperature is around 0 °C. However, this freezing point cannot be seen with bound water. The half-bound water shows property between them. Bounded water is usually 0.39–1.18 g/g. Absorption capacity of SPG mainly depends on osmotic pressure, network affinity, and network cross-linking density.

4.1.1 Cross-Linker and Cross-Linking Density

Cross-linker plays major role to adjust SPGs properties in terms of absorption and mechanical properties. Cross-linking density of SPGs is controlled by the fraction of cross-linking agent present in the polymerization and double bond conversion. Therefore, smaller amount of cross-linking agent leads to different cross-linking degree and change in AC of PGs. At low percentage of cross-linking agent, the three-dimensional PGs do not form effectively and the water molecules cannot be held resulting in SR and AC decrease. At high concentration of cross-linker a large number of growing polymer chains are involved to produce an additional network structure. High tied network does not allow water to enter the network and decrease AC (Turkington and Paradise 2005; Peppas et al. 2000). Effect of cross-liker content on SR of copolymer of carboxymethyl chitosan-g-polyacrylic acid is shown in Fig. 7 (Shah et al. 2010).

Fig. 7
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Effect of MBAN/mAA on the swelling ratio of superabsorbent polymer

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Some researchers reported that the number of functional groups of cross-linker has affected AC of polymer gels. Different AC of PGs of polyacrylamide (PAAm) and polyacrylamide sodium acrylate (PAAm/SA) was obtained using different type of cross-linker as shown in Fig. 8. AC via using 1,4-butanediol dimetacrylate (BDMA), ethylene glycol dimethacrylate (EGDMA), N,N′-methylenebisacrylamide (MPA), and trimethylolpropane triacrylate (TMPTA) were in the following order:

Fig. 8
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Linking sites of a 1,4-butanediol dimetacrylate (BDMA), b ethylene glycol dimethaacrylate (EGDMA), c N,N′-methylenebisacrylamide (MPA), d trimethylolpropane triacrylate (TMPTA)

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MPA > BDMA > EGDMA > TMPTA

Different AC is observed as a result of functional groups of each type of cross-linker. MPA, BDMA, and EGDMA are tetrafunctional cross-linkers and TMPTA is hexafunctional cross-linker. NH group of MPA increases SR and AC via causing new hydrophilic interaction. Using TMPTA cross-linker led to many cross-linking sites; therefore, the cross-linking density is higher than using other cross-linker at same concentration which leads to decreased AC (Ren and Sun 2010). The chemical structure of cross-linker can affect absorption properties. Cross-linking agent with hydrophilic property such as MPA enhances the AC as a result of presences of amide groups (Vital et al. 2008). In addition, water solubility, short chains, and high activity of MPA give this kind of cross-linker wide use in free radical polymerization (Turkington and Paradise 2005).

4.1.2 Initiator Content

Absorption capacity of SPGs is affected by the initiator content as a result of change in polymer molecular weight. At low molecular weight, the relative amount of polymer chain ends increases (polymer chain ends do not contribute to AC); therefore, AC will decrease at high content of initiator (D’Ulivo 2004). Moreover, when the content of initiator is low, the polymerization reaction takes place slowly and leads high molecular weight and network structure with big pore size. Network with large voids has less ability to hold water and less WAC. At high initiator content, the polymerization rate is high and the product has smaller space size and that prevents fluid molecules from entering the network (Lin et al. 2007). Figure 9 shows the effect of ammonium persulfate (Desrosiers et al. 2000) on water absorbency of copolymer of PAA-co-PAAm filled with inorganic clay (Li and Wang 2005). From above-mentioned, there is an optimum voids size to get maximum WAC and that can be controlled via initiator content.

Fig. 9
figure 9

Effect of mAPS/mAA on the swelling ratio of superabsorbent polymers

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4.1.3 Degree of Neutralization (DON)

DON is normally used between 0 and 80% for most polymerization process (Talpur et al. 2009; Pourjavadi et al. 2004). Neutralization of acrylic acid (AA) using sodium hydroxide (NaOH), the negatively charged carboxyl groups attached to polymer chains, produces an electrostatic repulsion (ESR) which leads to network expansion which means high AC is associated with high ESR. After certain degree of neutralization, AC decreases with a corresponding increase in chain stiffness and counterion condensation on the polyion “(screening effect)” (Dall and Sharples1991). At low DON, the ESR is low which ultimately decreasing the absorption capacity of the hydrogel networks. When DON start to increase the content of –CO2–Na groups increases leading to enhancement in osmotic pressure between the inside and outside network and enhancing AC of SPGs. However, AC stop’s at certain value of DON and starts to decline. The main cause of decline in AC is the presence of Na+ ions which leads to shielding of carboxylate anions on PAA chains and inhibits anion–anion repulsion forces (El Bakouri et al. 2008). Table 1 shows DON effect on water absorbency of PAA hydrogel at various cross-linking agent concentrations (Yadvinder et al. 2005).

Table 1 The effect of neutralization degree of acrylic acid at various cross-linking agent concentrations on water absorbency of the synthesized beads
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4.1.4 Solvent Volume and Concentration

Network structure of SPGs is affected by solvent concentration as a result of enhanced primary cyclization rate of multi-vinyl monomer during the polymerization process. Increasing the reaction solvent volume leads to decrease in contents reaction of cross-linking agent, initiator, and monomer and finally decreases in polymerization rate and cross-linking density. Low solvent volume means reaction contents’ viscosity is high and difficult movement for free radicals and monomer in the reaction (El Bakouri et al. 2008; Munshi et al. 1978). Solvent concentration affects network properties by affecting radical propagation dynamics. At low solvent concentration, the double bond concentration surrounding by free radical is relatively high. This leads to fast propagation step and less opportunity for free radicals to recycle by reacting with its own pendant double bond. Solvent type and quality have been reported to affect SPGs properties. Poor solvent use results in loose network structure, and firm networks are a result of using good solvent (Elliott and Bowman 2002). Previous study indicated that high solubility monomer and homopolymer in solvent lead to difficulty in grafting polymerization. In the same study, effect of solvent contents was determined by fixing other reaction contents. The result showed that the maximum percentage of grafting was obtained at 50-ml-total reaction mixture. As a conclusion of this section, solvent concentration can be used to control free radical polymerization in both microscopic and molecular levels.

4.1.5 Fiber Type and Content

AC of SPGs is highly affected by chemical composition and concentration of used fibers. Hydrophilic groups on fiber surface contribute to enhance network absorption capacity. If the fibers act as additional cross-linking agent inside SPGs network, high concentration of fibers leads to rigid network structure and less. Previous work on sodium alginate (Na–Ag) grafted with carboxymethylcellulose (CMC) showed that SR of polymer gels increase gradually with increase in Na–Ag content, and the maximum AC is achieved at 0.5 weight ratio of Na–Ag/CMC. After this ratio, AC decreases as a result of increase in viscosity of reaction mixture which hindered reactants movement (Yang et al. 2003; Shah et al. 2010). Kappa-carrageenan (kC) used as a fiber showed that WAC of the network was increased with increase in kC content. Same result has been obtained using CMC grafted with polyacrylonitrile (PAN), and starch-poly(sodium acrylate-co-acrylamide) (Pourjavadi et al. 2007a; Bonakdar et al. 2010; Seoudi 2008). Another study indicated that absorption properties of SPG were enhanced by filling the structure with inorganic clay (attapulgite) (Li and Wang 2005).

4.1.6 Chemical Structure and Monomer Molar Ratio

Blending of two monomers to produce desirable properties of SPGs has been studied intensively in the area of gels (Adem et al. 2009; Abd El-Rehim et al. 2006; Kadlubowski et al. 2007; Zhang et al. 2009; Mahdavinia et al. 2008). The effect of molar ratio of poly(acrylamide-acrylic acid) is shown in Fig. 10.

Fig. 10
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Effect of molar ratio of AM to AA content on water absorbency of superabsorbent polymer

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Increasing the polyacrylamide to more than 0.51 molar ratios leads to decrease AC, and counter result was observed at ratio less than this value. This study reported that the incorporation in suitable ratio of hydrophilic groups (–CONH, COONa, and COOH) inside the network reduces the repelling action and increases water absorbency (Li and Wang 2005). The chemical structure of polymer repeating unit affects SR and AC by changing the ratio of hydrophilic to hydrophobic groups (Chun et al. 2009). Network of high contents of hydrophilic groups swells to higher degree than those of high hydrophobic groups. Hydrophobic groups may collapse in the presence of water. The collapse chains minimize their exposure to water molecule as such lower absorption capacity and swelling rate.

4.1.7 pH of Absorbed Fluid

Swelling behavior and absorption capacity of SPGs are affected by surrounding medium (Peppas et al. 2000; Hongyan He 2006). Affect of swollen pH on absorption and swelling capacity of polyelectrolytes has been considered in many studies (Varshosaz and Falamarzian 2001; Liu et al. 2006; Sahoo et al. 2007; Yoo et al. 2000; Kim and Park 2010; Karppi et al. 2007, 2008; Zhang et al. 2004; Wang et al. 2008; Singh et al. 2007; Sheikh et al. 2010; Yang et al. 2009, Abd El-Rehim et al. 2006). Effect of pH medium on swelling ratio of PAA hydrogel in Fig. 11 indicates that the high swelling ratio was at pH 4.9 solution. The cause of this value is associated with pKa of polyacrylic acid (PAA) and swollen medium. PAA has pKa of 4.9; therefore, when pH of swollen sample is less than pKa of PAA that leads to high strength of H+ which affect carboxylic group ionization. At pH higher than pKa of PAA, more carboxylic ions are formed which enhances electrostatic repulsion and create more network free space and lead to increase in swelling ratio (Sheikh et al. 2010).

Fig. 11
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Swelling ratio of PAA in media of different pH values as function of time

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4.2 Releasing Behavior

Definition of SPG indicated that the network has the ability to release absorbed fluid as a response to environmental change (temperature, pH, ions, electric signal, sound, and magnetic field). Releasing behavior of SPGs draws great attention of many researches (Lee and Chen 2006; Kim and Peppas 2003; Tomic et al. 2007; Liu et al. 2006; Chen et al. 2009; Lee et al. 2008; Fundueanu et al. 2001; Siemoneit et al. 2006; Uchida et al. 2003; Issa et al. 1990; Huynh et al. 2009; Alemzadeh and Vossoughi 2002; Chun et al. 2005; Tada et al. 2005). Releasing character of SPGs via its sensitivity to environmental change grants this class of materials’ advantages to be used in drug control release in medical and water treatment applications. Theophylline release from P(AA-co-AAm) hydrogel in distilled water indicated that low release rate related with samples of high degree of cross-linking as illustrated in Fig. 12 (Katime et al. 1999).

Fig. 12
figure 12

Fractional release profiles of theophylline at 310 ± 0.2 K from glassy poly(acrylic acid-co-acrylamide) (50/50%)/MBAAm gels in water: (◯) (0.67%) MBAAm, (□) (1.00%) MBAAm, (△) (1.25%) MBAAm, (◊) (1.50%) MBAAm

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4.3 Mechanical Properties SPGs

Applications of SPGs are limited in some areas because of their poor mechanical properties (Tanaka et al. 2005). Mechanical properties of SPG are very important in applications such as drug delivery system and agriculture when the hydrogel has to withstand certain pressure in deep placement process (Omidian et al. 2005). There are some methods such as increasing the cross-linking density, copolymerizing of two kinds of polymer, and grafting with rigid substrates used to enhance mechanical properties of polymer gels in general (Cauich-Rodriguez et al. 1996; Katime et al. 2001). Mechanical properties of SPG can be controlled by changing system cross-linking degree. High cross-linking degree leads to more brittle and stronger network structure. Therefore, to achieve a relatively strong and elastic property network, suitable cross-linking degree has to be optimized. Also, mechanical properties are affected by the main monomers used to synthesize SPGs which means copolymerization of two types of repeating unit (hydrophilic–hydrophobic) (Katime et al. 2001; Patachia et al. 2007). The structural strength of polymer gels can be enhance by introducing a comonomer to the main networks which contribute to new hydrogen bonding and stable network structure (Solpan et al. 2003; Peppas et al. 2000). Mechanical properties are improved using nanomaterials, such as inorganic nanoclay and carbon nanotubes to produce nanocomposites network structure (Xiang et al. 2006; Pourjavadi et al. 2007a).

5 Application of Smart Polymer Gels

Unique properties of PGs such as absorption, swelling and de-swelling behavior, hydrophobicity, biocompatibility, and sensitivity to environmental change are the main reason for its wide applications. Homopolymer and copolymer of PGs have been used in various applications in our daily life such as agricultural, biomedical, water treatment, and other industrial applications (Katime et al. 1999, 2004; Liang et al. 2009; Solpan et al. 2008; Shukla et al. 2009; Zhao et al. 2004; M’Bareck et al. 2006; Devine et al. 2006; Nho et al. 2004; Hoffman 2002).

5.1 Biomedical Application

SPGs materials have wide application in biomedical and pharmaceutical because of their desirable properties such as minimal interfacial tension with surrounding biological fluids, gas permeation, diffusion of low molecular weight, less mechanical, and friction irritation to tissue (Cauich-Rodriguez et al. 1996). One of the difficulties biomedical experts are facing nowadays is how to deliver such a drug completely to particular place in the human body. Experts used a system called drug delivery system to manage release and deliver drug to specific positions (Wang et al. 2010; Blanco et al. 1996; Lukowski et al. 1992; Miao et al. 2010; Kakoulides et al. 1998; Kamath and Park 2010; Gong et al. 2009; Kaetsu 1996; Miyata et al. 2002; Geever et al. 2008; Nam et al. 2004; El-Hag et al. 2009; Bhattarai et al. 2010; Adnadjevic and Jovanovic 2009; Li et al. 2009; Cheddadi et al. 2011; Tu et al. 2010). SPGs of thermosensitive and pH sensitive are wildly used in medical application as drug delivery system because of their sensitivity to pH and temperature of human body. Many monomer and polymer were used in this kind of application but the most used are PAA and PAAm and their copolymers. PAA grafted with n-alkyl methacrylate was prepared to study swelling and release kinetics of theophylline and aminophylline. This study indicated that the release of theophylline is fairly independent of alkyl chain length while the release of aminophylline is highly dependent on alkyl chain length (Katime et al. 2001). Copolymer of P(AA-co-AAm) hydrogel was used for theophylline release in distilled water (Katime et al. 1999). SPG based of PAA filled with cotton fiber was synthesized for gentamicin release. The outcome of this study showed the release of drug from the grafted network is a chain relaxation controlled manner (Bajpai and Das 2011). SPG of PAA grafted with gelatin (Ge) and cotton fiber was prepared for gentamicin sulfate release (Changez et al. 2003; Bajpai and Das 2011; Changez et al. 2004). The result of these studies regarded to use hydrogel based on PAA to deliver gentamicin sulfate in controlled manner. Loaded gentamicin sulfate showed fair antibacterial action against Escherichia coli. Grafted PAA with polyethylene oxide using radiation polymerization was prepared as insulin carrier. This study indicated that insulin loaded on hydrogel has higher effect on lowering of blood glucose over injection insulin solution (Nho et al. 2004).

SPGs of pH-sensitive material are able to convert chemical energy to mechanical energy as such can serve as actuators or artificial muscles in medical applications. SPGs have also been used in many other biomedical application including diagnostic, therapeutic, and implantable devices such as catheters, biosensors (Pourjavadi et al. 2007b; Adhikari and Majumdar 2004), artificial skin (Devine et al. 2006), and tissue engineering (Kim et al. 2008; Landers et al. 2002).

5.2 Water Treatment Application

Water pollution is considered as one of the most important issues facing humanity these days. Usually, water is polluted by different undesirable ions and dyes. The sources of these contaminants are either industrial or agricultural. Most factories effluents are discarded into water (rivers, lakes, and seas). These ions and dyes change the biological system of rivers and lakes and affect life of several kinds of species. Wastewater has been treated using several methods such as biological treatment, chemical precipitation, supercritical water oxidation, steam-stripping, microwave radiation, ion exchange, and absorption methods. Absorption method gets high attention because of its low energy consumption and easy operation (Zheng and Wang 2009). Therefore, using the ability of such material to absorb and separate particular ions or dyes from solution can be helpful (Guilherme et al. 2007; Paulino et al. 2007). SPGs have been used in this felid for past forty years. Some researchers studied AC of copolymer of PAA-PAAm-polysaccharide to methylene blue (MB) from water. The result showed good AC of this copolymer and excellent methylene blue removal (48 mg of dye per 1 g of SAP) (Paulino et al. 2006). In another study, SPGs of PAA grafted with chitosan was prepared for ammonium removal applications. This study indicated that PGs have high absorption capacity of 30 mg/g ammonium nitrogen in a wide pH range of 4–9 (Zheng et al. 2009). In another work, grafted PAA/chitosan/vermiculite gels were synthesized for adsorption of methylene blue from water. The results showed that grafted hydrogel has high adsorption for MB dye and suggested that it has the potential as adsorbent for cationic dye removal from wastewater (Liu et al.). Many other SPGs based on PAA copolymer were synthesized and absorption of cationic dyes such as methylene blue, safranine-O (SO), magenta (M), Janus Green B (JGB), and ions such as lead (Pb+2), cadmium (Cd+2), copper (Cu+2), (Ni+2), iron (Fe+3), (Mn+2) were studied (Solpan et al. 2008; Xiong et al. 2009; Zheng et al. 2010; Duran et al. 1999; Li et al. 2011; Kangwansupamonkon et al. 2010; Dai et al. 2011; Zheng et al. 2011; El-Hag et al. 2003; Wei et al. 2009; Shirsath et al. 2011).

6 Conclusion

SPGs are polymer gels that response to the surrounding environmental change in their swollen and dry state. Conventional polymerization techniques were used to syntheses and prepare SPGs such as solution, suspension, emulsion, and radiation polymerization. Solution polymerization is the most widely used technique because of its advantage of pure product (free of surfactants and other contaminants). SPGs are classified depended on their sensitivity to thermo, pH, electric, magnetic, and sound-sensitive gels. Swelling, release, and mechanical properties are the most important criteria to select the correct SPGs for particular application. SPGs properties are affected by initiator content, cross-linker kind and content, chemical composition of polymer network, fiber content pH, and ion concentration of surrounding medium. SPGs properties granted these materials very wide applications in biomedical and water treatment.