**5. Water sorption and diffusion**

Water sorption and diffusion are also very important in biomedicine because these properties play a very important role in cell survival, especially in tissue engineering applications [3]. Thus, acrylic hydrogels such as poly(2-hydroxyethyl methacrylate) or poly(2-hydroxyethyl acrylate), are very important hydrophilic materials as these polymers were able to absorb and swell retaining large amounts of water within their structure[77–80]. The excellent water sorption property has made these types of materials very promising in a wide range of biomedical applications such as controlled drug delivery, tissue engineering, wound healing, etc. [4, 81]. The ability of hydrogels to absorb water arises from hydrophilic functional groups attached to the polymeric backbone, while their resistance to dissolution arises from crosslinks between network chains [82]. However, these single-network hydrogels have weak mechanical properties and slow response at swelling. Therefore, they are in need of reinforcement, as already mentioned, which can also modify their water sorption properties. For example, the combination of hydrophilic and hydrophobic functional groups of acrylic polymers as multicomponent polymeric systems is shown in **Figure 1**.

**4. Thermal properties**

82 Acrylic Polymers in Healthcare

results of PHEMA/SiO<sup>2</sup>

(TiO<sup>2</sup>

and it was shown that the SiO<sup>2</sup>

) and ferric oxide (Fe2

of PMMA with TiO<sup>2</sup>

Poly(ethyl acrylate) [76], etc.

**5. Water sorption and diffusion**

tion temperature with loadings as low as 0.05 wt.% [49].

O3

O3

and Fe2

Even though biomaterials do not need to endure temperatures higher than that of the human body, the improvement of thermal properties can increase its long-term operation. Thus, for example, the incorporation of polyurethane into polyacrylamide network in the form of an interpenetrating polymer networks enhanced the thermal properties of these semi-IPNs due to higher crosslink density imparted by the hard segment content [18]. Though silica can improve the mechanical properties of acrylic polymers, the differential scanning calorimetry

Tg transition [69]. However, polymer nanocomposites with functionalized graphene sheets (FGNS) showed an unprecedented shift in glass transition temperature of up to 40 and 30°C in poly(acrylonitrile) with 1 wt.% of FGNS and in poly(methyl methacrylate) with only 0.05 wt.%, respectively [70]. Besides, the thermal stability of magnetite-graphene/poly(aryleneether-nitrile) nanocomposites were significantly enhanced by the incorporation of magnetitegraphene hybrids [44]. The nanocomposites of PMMA with chemically modified graphene (CMG) and GO fillers made by *in situ* polymerization showed large shifts in the glass transi-

Another strategy to improve the thermal properties of acrylic polymers is by nanoparticle filling. Thus, the thermal performance of well-known acrylic polymers such as PMMA can be significantly improved by filling of nanometric particles (5, 10 15 and 20%) of titanium oxide

Thermal degradation can also be improved in acrylic-based materials by nanoparticle filling. For example, the experimental results obtained by thermogravimetric analysis (TGA)

stability of PMMA by about 50°C by loading 5 wt.% of fillers [72]. The TGA also showed that the presence of small amounts of Pd nanoparticles (0.0005–0.005 vol%) in PMMA/Pd nanocomposites significantly improved the thermal stability of PMMA, as shown by a degradation initiation retarded by 75°C and a gain of 32°C at the maximum decomposition rate [73].

Acrylic hydrogels are hydrophilic polymers and are able to absorb large amounts of water in their biomedical applications due to contact with cells or tissue in the human body. Therefore, the thermal analysis of water and its influence on the swollen hydrogel properties becomes essential. Thus, many studies have been done in this way with acrylic hydrogels such as PHEMA [74], bulk and plasma-polymerized poly(2-hydroxyethyl acrylate) (PHEA) [75],

Water sorption and diffusion are also very important in biomedicine because these properties play a very important role in cell survival, especially in tissue engineering applications [3].

) by the solvent casting method [71, 72].

hybrids are complicated, showing two glass transition temperatures,

content is an important factor in influencing the shift of the

showed that these nanoparticles can improve the thermal

The reinforcement of acrylics through GO loading can modify the water sorption behavior of the polymers. Thus, the swelling rates of graphene oxide/poly(acrylic acid-co-acrylamide) nanocomposite hydrogels increased with increase in the GO loadings to 0.30 wt.% and then decreased with further increasing GO loadings. It is worth noting that the hydrogel with only 0.10 wt.% GO exhibited significant improvement of swelling capacity in neutral medium and could also retain relatively higher swelling rates to a certain degree in acidic and basic solutions. Therefore, these GO-based superabsorbent acrylic hydrogels have very potential applications in many fields such as biomedical engineering and hygienic products [47].

The mechanism of water diffusion [83] can also be altered by the reinforcement of acrylics through any of the methods shown in Section 1. Thus, poly(acrylic acid)-GO nanocomposite hydrogels shows non-Fickian anomalous diffusion and the deswelling ratio decreases with increasing GO content [48].

A new method (ultrasound synthesis) has been developed to prepare superabsorbent polymers of sodium lignosulfonate-grafted poly(acrylic acid-co-acryl amide). This superabsorbent acrylic-based polymer exhibited also a non-Fickian water diffusion transport and a maximum water absorbency of 1350 g⋅g−1 [84].

There are many acrylic hydrogels, which exhibit a non-Fickian diffusion behavior such as poly(2-hydroxyethyl acrylate) [79, 80]. Even though water sorption is not classically Fickian, it has been observed in a variety of polymers such as PHEMA, that an important waterswellable biomedical polymer is controlled by Fickian diffusion [85]. Thus, copolymeric hydrogels based on 2-hydroxyethyl methacrylate (HEMA) and epoxy methacrylate (EMA) synthesized by bulk polymerizations showed that the swelling process of these polymers also follows Fickian behavior and the equilibrium water content (EWC) decreased with increase in EMA content due to its hydrophobicity [86].

It is remarkable that the pH has a big influence in the swelling properties and diffusion mechanism of acrylic-based materials. Thus, the swelling properties of semi-interpenetrating polymer networks of acrylamide-based polyurethanes decreased in acidic pH, while a reverse trend was seen in basic pH. However, these semi-IPNs were found to be hydrolytically stable in phosphate buffer solution, which makes them to be a potential material for biomedical applications [18].

Polyacrylic acid is a pH-sensitive and biocompatible polymer that is being used in many biomedical fields [26]. It has attracted considerable interest because of its therapeutic use, due to its ability to swell reversibly with changes in pH. Thus, GO functionalized with PAA (GO-PAA) by *in situ* atom transfer radical polymerization (ATRP) showed potential use as an intracellular protein carrier using bovine serum albumin (BSA) as a model protein [87]. This application is very important because proteins participate in all vital body processes and these perform an essential function inside cells as enzymes, transduction signals and gene regulation. Another pH-sensitive terpolymer hydrogel, poly(acryl amide-co-2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamido glycolic acid), with applications in drug release showed a quasi-Fickian diffusion mechanism with partly chain relaxation controlled diffusion. These hydrogels demonstrated a sharp change in its water absorbency and molecular weight between crosslinks of the network with a change in pH of the swelling media [88].

The effect of temperature on swelling properties of acrylic hydrogels is also very important [86], and they can be modified to exhibit fast temperature sensitivity, and improved oscillating swelling-deswelling properties as, for example, in thermosensitive poly(N-isopropyl acrylamide-co-acrylic acid) hydrogels [89].
