**2. Mechanical properties**

The improvement of the mechanical properties of acrylics is one of the *hot topics* in the field of bioengineering, and many research groups have been working on this topic for many years. Acrylics can be reinforced through many kinds of methods and techniques: polymers with microphase-separated morphologies such as block copolymers, in which hydrophobic and hydrophilic domains alternate [5], increasing crosslinking density [6], by means of binary systems composed of two or more mixed polymers as interpenetrating polymer networks (IPNs) [7], self-reinforced composite materials composed of fibers embedded in a matrix of the same acrylic polymer [8], by plasma grafting of a hydrophilic acrylic polymer onto a hydrophobic acrylic substrate [9, 10] and with the sol-gel reaction to produce nanosilica reinforcement [11]. However, more recent studies have shown new procedures to improve the mechanical properties of acrylics with the incorporation of graphene (GN) (2010 Nobel Prize in Physics) and other carbon nanomaterials such as carbon nanotubes (CNT) [12]. Chemically modified graphenes (CMGs) such as graphene oxide (GO) [13, 14] or reduced graphene oxide (rGO) [15], have also been shown to be very good nanofillers to reinforce acrylics and improve many other properties, especially acrylic hydrogels which, in the swollen state, show very low mechanical properties.

#### **2.1. IPNs**

Acrylic-based interpenetrating polymer networks (IPN) have gained greater attention during last decades, mainly due to their biomedical applications as reinforced polymer networks. The use of an IPN, which consists of two separate but interwoven polymer networks, is a chemical procedure that is often used in polymer science to control, enhance and/or combine functional properties. These are advanced multicomponent polymeric systems of crosslinked polymer networks without any covalent bonds between them, where at least one of them is synthesized and/or crosslinked within the immediate presence of the other. It is important to differentiate between the six basic multicomponent polymeric structures (**Figure 1**).

If a crosslinker is present in the polymeric system, fully IPN [17] result, while in the absence of crosslinking, a network having linear polymers embedded within the first crosslinked network is formed (semi- or pseudo-IPN) [18, 19]. Acrylic-based IPNs are prepared usually in the form Latest Improvements of Acrylic-Based Polymer Properties for Biomedical Applications http://dx.doi.org/10.5772/intechopen.68996 77

products for various applications have been approved by the US Food and Drug Administration (FDA) and are expected to produce massively. However, many of their potential uses required for many biomedical applications are sometimes hindered by their low mechanical strength, biological interactions, electrical and/or thermal properties, water sorption and diffusion, antibacterial activity, porosity, etc. when they are synthesized as scaffolds for tissue engineering applications. Thus, new advanced acrylic-based materials have been developed and are currently under intensive research to solve all these problems by means of multicomponent polymeric systems or by combination with other materials and/or nanomaterials to form com-

The improvement of the mechanical properties of acrylics is one of the *hot topics* in the field of bioengineering, and many research groups have been working on this topic for many years. Acrylics can be reinforced through many kinds of methods and techniques: polymers with microphase-separated morphologies such as block copolymers, in which hydrophobic and hydrophilic domains alternate [5], increasing crosslinking density [6], by means of binary systems composed of two or more mixed polymers as interpenetrating polymer networks (IPNs) [7], self-reinforced composite materials composed of fibers embedded in a matrix of the same acrylic polymer [8], by plasma grafting of a hydrophilic acrylic polymer onto a hydrophobic acrylic substrate [9, 10] and with the sol-gel reaction to produce nanosilica reinforcement [11]. However, more recent studies have shown new procedures to improve the mechanical properties of acrylics with the incorporation of graphene (GN) (2010 Nobel Prize in Physics) and other carbon nanomaterials such as carbon nanotubes (CNT) [12]. Chemically modified graphenes (CMGs) such as graphene oxide (GO) [13, 14] or reduced graphene oxide (rGO) [15], have also been shown to be very good nanofillers to reinforce acrylics and improve many other properties, especially acrylic hydrogels which, in the swollen state, show very

Acrylic-based interpenetrating polymer networks (IPN) have gained greater attention during last decades, mainly due to their biomedical applications as reinforced polymer networks. The use of an IPN, which consists of two separate but interwoven polymer networks, is a chemical procedure that is often used in polymer science to control, enhance and/or combine functional properties. These are advanced multicomponent polymeric systems of crosslinked polymer networks without any covalent bonds between them, where at least one of them is synthesized and/or crosslinked within the immediate presence of the other. It is important to

If a crosslinker is present in the polymeric system, fully IPN [17] result, while in the absence of crosslinking, a network having linear polymers embedded within the first crosslinked network is formed (semi- or pseudo-IPN) [18, 19]. Acrylic-based IPNs are prepared usually in the form

differentiate between the six basic multicomponent polymeric structures (**Figure 1**).

posites or nanocomposites with or without interconnected porous morphology.

**2. Mechanical properties**

76 Acrylic Polymers in Healthcare

low mechanical properties.

**2.1. IPNs**

**Figure 1.** Schematic representation of (a) mechanical blends, (b) graft copolymers, (c) block copolymers, (d) AB-crosslinked copolymer, (e) semi-IPNs and (f) full-IPNs. Modified from Sperling and Mishra [16].

of simultaneous interpenetrating polymer networks (SINs), in which the precursors of both networks are mixed and the two networks are synthesized at the same time, or in the form of sequential IPNs, by swelling of a single-polymer network into a solution containing the mixture of monomer, initiator and activator, usually with a crosslinker. Thus, urethane acrylate resin networks were greatly reinforced by epoxy networks in SINs composed of both resins [20]. Full IPNs and semi-IPNs of the epoxy resin and poly(ethyl methacrylate) (PEMA) were also prepared by the sequential mode of synthesis, and these showed a gradual decrease of modulus and tensile strength properties with consequent increase in elongation at break and toughness for both types of IPNs with increases in PEMA content [21]. Combinations of different kinds of IPNs have been synthesized using simultaneous photopolymerization, which gave rise to simultaneous semi-interpenetrating polymer networks (semi-SINs) of epoxy resinacrylate polyurethane semi-interpenetrating networks having very high compatibility [22].

Pseudo-SIPNs were prepared by melt blending of poly(methyl methacrylate) (PMMA), and double-C60-end-capped poly(ethylene oxide) (FPEOF) exhibited a storage modulus of 16 times larger than that of PMMA, which are as good as those of PMMA/carbon nanotube nanocomposites [19].

Acrylic-based IPN hydrogels are also developed with the aim of enhancing the mechanical strength and swelling/deswelling response of these acrylic hydrogels [23]. For example, interpenetrating polymer network (IPN) hydrogels composed of chitosan and poly(acrylic acid) (PAA) are synthesized by employing the UV irradiation method and showed that even in the swollen state, the present chitosan/PAA IPNs possessed good mechanical properties [24].

'Smart' hydrogels are able to significantly change their volume/shape in response to small alterations of certain parameters of the environment. These responsive hydrogels have numerous applications, most of them focussing on biological and therapeutic demands [25, 26] and sensing applications [27].

Although IPNs based on hydrogels have been extensively reported, the combination of liquid crystalline (LC) property-based hydrogels has been rarely explored. In this case, the anisotropic and molecular order of liquid crystals can be combined with the responsive isotropic properties of hydrogels. Thus, advanced stimuli-responsive materials based on interpenetrating liquid crystal-hydrogel polymer networks have been recently fabricated consisting of a cholesteric liquid crystalline network that reflects color and an interwoven poly(acrylic acid) network that provides a humidity and pH response [28].

#### **2.2. Composites**

Several kinds of chemical modifications of acrylic hydrogels do not have a significant change of the overall mechanical strength because the main structural skeletons of these polymers or copolymers are still weak. On the contrary, the method of fiber reinforcement to produce composites is different because the added fabrics impart high strength to the networks which not only just embed inside the membranes but also form the main skeleton of the composites. A fiber-reinforced polymer is a composite material consisting of a polymer matrix imbedded with high-strength fibers such as glass, aramid and carbon [29]. In such kind of materials, the mechanical properties are presumed to be improved and the biocompatible characteristics of the acrylic polymer should remain the same. Thus, acrylic resin polymers have been reinforced with glass fibers for dental applications [30], and acrylic hydrogels such as poly(2-hydroxyethyl methacrylate), which is one of the most popular biomaterials, have been manufactured by adding various kinds of weaved and knitted fabrics and fibers, in order to improve overall qualities of the poly(2-hydroxethyl methacrylate) (PHEMA)-based artificial skin for advanced wound dressing usage [31]. However, in the recent decades, natural fibers as an alternative reinforcement in polymer composites have attracted the attention of many research groups due to their advantages over conventional glass and carbon fibers [32]. These natural fibers include flax, hemp, jute, sisal, kenaf, coir, kapok, banana, henequen and many others, which offer various advantages over man-made glass and carbon fibers such as low-cost, low-density, comparable specific tensile properties, non-abrasive to the equipment, non-irritation to the skin, reduced energy consumption, less health risk, renewability, recyclability and biodegradability [33]. Thus, ultra-long chitin natural fibers were incorporated into PMMA resin to prepare PMMA/chitin nanocomposites with improved properties [34]. This achievement is a significantly environmental move toward the sustainable utilization of marine-river crab shell wastes for biomedical applications.

#### **2.3. Nanocomposites**

Another alternative and very promising way of reinforcing acrylic polymers consists of the incorporation of nanomaterials such as silica, graphene and its derivatives, nanofibers or many other nanoparticles. Silica is a biocompatible material and has been reported to possess bioactive properties [35]. Silica can improve the mechanical properties of acrylics through nanosilica filling or the well-known sol-gel process, which offers a new approach to the synthesis of nanocomposite materials with domain sizes approaching the molecular level [36]. Thus, a biphasic matrix of a hybrid (inorganic-organic) nanocomposite materials of poly(2 hydroxyethyl acrylate) with a silica network obtained by an acid-catalyzed sol-gel process of tetraethoxysilane (TEOS) showed a very significant improvement of the mechanical properties of the pure hydrogel [37].

Although IPNs based on hydrogels have been extensively reported, the combination of liquid crystalline (LC) property-based hydrogels has been rarely explored. In this case, the anisotropic and molecular order of liquid crystals can be combined with the responsive isotropic properties of hydrogels. Thus, advanced stimuli-responsive materials based on interpenetrating liquid crystal-hydrogel polymer networks have been recently fabricated consisting of a cholesteric liquid crystalline network that reflects color and an interwoven poly(acrylic acid)

Several kinds of chemical modifications of acrylic hydrogels do not have a significant change of the overall mechanical strength because the main structural skeletons of these polymers or copolymers are still weak. On the contrary, the method of fiber reinforcement to produce composites is different because the added fabrics impart high strength to the networks which not only just embed inside the membranes but also form the main skeleton of the composites. A fiber-reinforced polymer is a composite material consisting of a polymer matrix imbedded with high-strength fibers such as glass, aramid and carbon [29]. In such kind of materials, the mechanical properties are presumed to be improved and the biocompatible characteristics of the acrylic polymer should remain the same. Thus, acrylic resin polymers have been reinforced with glass fibers for dental applications [30], and acrylic hydrogels such as poly(2-hydroxyethyl methacrylate), which is one of the most popular biomaterials, have been manufactured by adding various kinds of weaved and knitted fabrics and fibers, in order to improve overall qualities of the poly(2-hydroxethyl methacrylate) (PHEMA)-based artificial skin for advanced wound dressing usage [31]. However, in the recent decades, natural fibers as an alternative reinforcement in polymer composites have attracted the attention of many research groups due to their advantages over conventional glass and carbon fibers [32]. These natural fibers include flax, hemp, jute, sisal, kenaf, coir, kapok, banana, henequen and many others, which offer various advantages over man-made glass and carbon fibers such as low-cost, low-density, comparable specific tensile properties, non-abrasive to the equipment, non-irritation to the skin, reduced energy consumption, less health risk, renewability, recyclability and biodegradability [33]. Thus, ultra-long chitin natural fibers were incorporated into PMMA resin to prepare PMMA/chitin nanocomposites with improved properties [34]. This achievement is a significantly environmental move toward the sustainable utilization of marine-river crab

Another alternative and very promising way of reinforcing acrylic polymers consists of the incorporation of nanomaterials such as silica, graphene and its derivatives, nanofibers or many other nanoparticles. Silica is a biocompatible material and has been reported to possess bioactive properties [35]. Silica can improve the mechanical properties of acrylics through nanosilica filling or the well-known sol-gel process, which offers a new approach to the synthesis of nanocomposite materials with domain sizes approaching the molecular level [36].

network that provides a humidity and pH response [28].

shell wastes for biomedical applications.

**2.3. Nanocomposites**

**2.2. Composites**

78 Acrylic Polymers in Healthcare

The combination of the reinforcement through interpenetrated polymer networks and nanosilica filling is another strategy that has also been used in the past. For example, simultaneous polyurethane/poly(ethyl methacrylate) interpenetrating polymer network with silica filler consisting of very fine powders with an approximate diameter of 5 nm also showed an important improvement of material strength [38].

Graphene (GN) is a two-dimensional (2D) monolayer of sp2 -bonded carbon atoms, which has attracted increasing attention [39] owing to its excellent electrical and thermal conductivities [40, 41] and great mechanical strength [42]. Besides, graphene promotes adherence of human osteoblasts and mesenchymal stromal cells [43], which render this nanomaterial also very promising material in the biomedical field. Thus, it has shown potential applications in nanocomposites such as magnetite-GNs/poly(arylene-ether-nitrile) nanocomposites because their mechanical properties were significantly enhanced by the incorporation of magnetite-GNs hybrids [44]. Besides, GN enhances the shape memory of poly(acrylamide-co-acrylic acid) and self-healing ability when the content of graphene is in the range of 10–30%, even though this copolymer itself has poor shape memory ability [45]. There are some reports that graphene oxide (GO) nanosheets can also enhance the mechanical strength of polymer substrates such as poly(acrylamide) (PAM) hydrogels [46]. GO is also a 2D nanomaterial prepared from natural graphite that can be easily exfoliated into monolayer sheets. GO has many hydrophilic oxygenated functional groups, including hydroxyl (-OH), epoxy (-C-O-C-), carbonyl (-C=O) and carboxyl (-COOH), which enable its dispersion in water solution [47]. The diversity of unique properties of GO, including great tensile modulus (1.0 TPa), ultimate strength (130 GPa) and electrical and thermal properties [48], renders graphene oxide an ideal carbon nanomaterial for variety of applications toward the development of new advanced materials. Thus, GO added into PAM hydrogels improved very much the mechanical performance of the original PAM hydrogels, which generally exhibit pronounced weakness and brittleness [46]. In the same way, the addition of GO nanosheets increased the Young's modulus and maximum stress of poly (acrylic acid)/gelatin composite hydrogels significantly as compared with control (0.0 wt.% GO). The highest Young's modulus was observed for hydrogel with GO (0.2 wt.%)/PAA (20 wt.%), whereas the highest maximum stress was detected for GO (0.2 wt.%)/ PAA (40 wt.%) specimen. These results suggested that the application of GO nanosheets could be used to improve mechanical properties of hydrogel materials, which is very beneficial for tissue engineering applications [14]. The other derivatives of graphene such as chemically modified graphene (CMG) fillers have been used in nanocomposites of PMMA and were compared with the GO filling. These results showed an elastic modulus of GO/PMMA and RG-O/ PMMA composites improved by 28% by just loading 1 wt.%. Fracture strength increased for GO/PMMA composites but decreased for RG-O/PMMA composites [49].

Single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes (MWCNTs) as well as carbon nanofibers (CNFs) are being used for reinforcing polymer matrices such as poly(methyl methacrylate) by melt blending. Thus, for example, using an amount of carbon nanofibers of 5 wt.%, the nanocomposites improved over 50% of their axial tensile modulus as compared to the control PMMA. The PMMA/CNFs nanocomposite fibers also show enhanced thermal stability, significantly reduced shrinkage and enhanced modulus retention with temperature, as well as improved compressive strength [50]. The electrical properties and electromechanical responses of acrylic materials such as acrylic elastomers and styrene copolymers can be improved toward electroactive applications such as artificial muscle and/or micro-electromechanical systems (MEMS) devices [51].

The other novel nanocomposite hydrogels such as those prepared with polyacrylamide (PAM) as a matrix material reinforced with natural chitosan nanofibers via *in situ* free-radical polymerization showed that these nanofibers acted as a multifunctional crosslinker and a reinforcing agent in the hydrogel system producing a compression strength and a storage modulus significantly higher than those of pure PAM [52].

Reinforcement can be also performed with plant fiber-based nanofibers by a successful fibrillation of wood pulp fibers into nanofiber bundles, which are thin enough to work, as well as bacterial cellulose in maintaining the transparency of resin [53].

The other nanoparticles such as clay have been employed to reinforce acrylic polymers. These polymer-clay nanocomposites such as PMMA/clay constitute a class of materials in which the polymer matrix is reinforced by uniformly dispersed inorganic particles (usually 10 wt.% or less) having at least one dimension in the nanometre scale and exhibiting enhanced mechanical and thermal properties when compared to pure polymer or conventional composites [54].
