**3. Biopolymers**

A variety of natural, synthetic, and biosynthetic polymers such as poly(Llactide) (PLLA), polyhydroxyalkanoate (PHA), poly(ε-caprolactone) (PCL), poly glycolic acid (PGA), poly ethylene glycol (PEG), polyesteramide (PEA), and aliphatic copolyesters (PBSA). The biodegradability is capable of undergoing decomposition into carbon dioxide, methane, water, inorganic compounds, or biomass in which the predominant mechanism is the enzymatic action of microorganisms. The biodegradability of polymer depends on the chemical structure of the materials and on the constitution of the final product, and also depends on the raw materials used for its production. The polymer based on the C-C backbone tends to be nonbiodegradable, whereas heteroatom containing polymer backbones confer biodegradability. It is possible to engineer biodegradability of polymers using the judicious addition of chemical linkages such as anhydride, ester, or amide bonds, among others (**Figure 2**). The properties of some commercial biodegradable polymers are summarized in **Table 1**.

The most biomedical application of polymers are surgical dressings, sutures, adhesives, polymeric screws and nails, fiber/polymer composite bone plates, tendons/ligaments, reinforcing meshes, heart valves, joint reconstruction and bone cement, tubular devices, soft-tissue replacement materials for cosmetic reconstruction, drug delivery implants, artificial kidney/blood dialysis, artificial lung/blood oxygenator, and artificial heart.

#### **3.1 Poly(L-lactide)**

Among the aliphatic polyesters, poly(L-lactide) (PLLA) is considered to be the most promising biodegradable material, not only because it has excellent biodegradability, compatibility, and high strength but also due to the fact that it can be obtained totally from renewable resources. PLLA is a bio-based,

**85**

**Table 1.**

(g/m2 /day)

**Figure 2.**

Density (g/cm3

Water permeability WVTR at 25°C

*Properties of some commercial biopolyesters.*

*(PEA), aliphatic copolyesters (PBSA), aromatic copolyesters (PBAT).*

*Poly(L-Lactide) Bionanocomposites*

fields.

*DOI: http://dx.doi.org/10.5772/intechopen.85035*

biodegradable polymer which can be produced from renewable sources such as corn and has found numerous applications in the medical and pharmaceutical

*Bioabsorbable implants that have potential applications throughout the spine (a), an example of a bioabsorbable plate and pedicle screw, washer (b), and the in-vitro degraded bioabsorbable screw.*

**PLLA\* PHA\* PCL\* PEA\* PBSA\* PBAT\***

172 21 177 680 330 550

) 1.25 1.25 1.11 1.07 1.23 1.21

Melting point (°C) (DSC) 152 153 65 112 114 110–115 Glass transition (°C) (DSC) 58 5 −61 −29 −45 −30 Crystallinity (%) 0–1 51 67 33 41 20–35 Elastic modulus (MPa) 2050 900 190 262 249 52 Elongation at break (%) 9 15 >500 420 >500 >500 Tensile stress at break or max (MPa) — — 14 17 19 9 Biodegradation mineralization (%) 100 100 100 100 90 100

*\*Abbreviations: Poly(L-lactide) (PLLA), polyhydroxyalkanoate (PHA), poly(ε-caprolactone) (PCL), polyesteramide* 

*Peptide Synthesis*

conductivity.

**3. Biopolymers**

polymers are summarized in **Table 1**.

oxygenator, and artificial heart.

**3.1 Poly(L-lactide)**

**2.3 In situ polymerization**

high shear and elongation flow processing and about optimization of processing

The nanoparticles are dispersed in monomer and then the polymerization process is starts. As solution blends, functionalized nanoparticles can improve the initial dispersion of the nanoparticles in the liquid (monomer and solvent) and consequently in the nanocomposites. Furthermore, in situ polymerization methods enable covalent bonding between nanoparticles and the polymer matrix using various condensation reactions. Noteworthy extensions of in situ polymerization include infiltration methods in which the reactive agents are introduced into a

A variety of natural, synthetic, and biosynthetic polymers such as poly(Llactide) (PLLA), polyhydroxyalkanoate (PHA), poly(ε-caprolactone) (PCL), poly glycolic acid (PGA), poly ethylene glycol (PEG), polyesteramide (PEA), and aliphatic copolyesters (PBSA). The biodegradability is capable of undergoing decomposition into carbon dioxide, methane, water, inorganic compounds, or biomass in which the predominant mechanism is the enzymatic action of microorganisms. The biodegradability of polymer depends on the chemical structure of the materials and on the constitution of the final product, and also depends on the raw materials used for its production. The polymer based on the C-C backbone tends to be nonbiodegradable, whereas heteroatom containing polymer backbones confer biodegradability. It is possible to engineer biodegradability of polymers using the judicious addition of chemical linkages such as anhydride, ester, or amide bonds, among others (**Figure 2**). The properties of some commercial biodegradable

The most biomedical application of polymers are surgical dressings, sutures, adhesives, polymeric screws and nails, fiber/polymer composite bone plates, tendons/ligaments, reinforcing meshes, heart valves, joint reconstruction and bone cement, tubular devices, soft-tissue replacement materials for cosmetic reconstruction, drug delivery implants, artificial kidney/blood dialysis, artificial lung/blood

Among the aliphatic polyesters, poly(L-lactide) (PLLA) is considered to be the most promising biodegradable material, not only because it has excellent biodegradability, compatibility, and high strength but also due to the fact that it can be obtained totally from renewable resources. PLLA is a bio-based,

Controlling the alignment of nanofibers in polymer matrix is possible using melt mixing methods. For example, spinning of extruded melt samples is used for alignment of fillers in nanofibers/polypropylene nanocomposites with high dispersion of nanofibers. Up to now, various methods of nanofiber alignment techniques have been developed such as using further increment in residence time in the die channel or die design to control the orientation of nanofibers. Injection molding was also found to induce significant alignment in nanofibers/ polypropylene composites, as demonstrated by measurement of thermal expansion and electrical

parameters to provide good nanofibers dispersion.

nanoparticle structure and subsequently polymerized.

**84**

biodegradable polymer which can be produced from renewable sources such as corn and has found numerous applications in the medical and pharmaceutical fields.

#### **Figure 2.**

*Bioabsorbable implants that have potential applications throughout the spine (a), an example of a bioabsorbable plate and pedicle screw, washer (b), and the in-vitro degraded bioabsorbable screw.*


*\*Abbreviations: Poly(L-lactide) (PLLA), polyhydroxyalkanoate (PHA), poly(ε-caprolactone) (PCL), polyesteramide (PEA), aliphatic copolyesters (PBSA), aromatic copolyesters (PBAT).*

#### **Table 1.**

*Properties of some commercial biopolyesters.*

The PLLA has important characteristics over other biopolymers such as:


The commercialization of PLLA has been affected from three factors:


Copolymerization of LA with other monomers like glycolide or CL can significantly enhance the properties and broaden the use of PLLA. The PLLA is produced form polylactic acid. The asymmetric polylactic acid has two stereo isomeric forms, L- and D-isomers. The L-isomer exists in normal human carbohydrate metabolism, and the D-isomer is detectable in urine and in acidic milk. If a polymer formed by one type of monomer, it is called homopolymer PLLA. A copolymer consists of two types of monomers named g. poly(D, L)-lactic acid (PDLLA) (**Figure 3**).

Some large scale manufacturers are beginning to favor PLLA because it is renewable, conserves energy, and degrades easily. The ring-opening polymerization of L-lactide oligomers (LAs) yields the PLLA semi-crystalline polymer with a melting point of 180–190°C and a glass transition temperature of 55–60°C (**Figure 4**).

Up to now, the PLLA has limited biomedical applications as implanting devices because of its biodegradation effect. If incorporating different nanoparticles into the PLLA matrix could enhance the properties of this material significantly, this process would increase its applicability further. In addition, the PLLA showed shape memory effect and the original shape could be recovered up to glass transition temperature. However, the recovery strain of PLLA was relatively low and the

#### **Figure 3.**

*Stereoisomeric forms of lactic acid: lactic acid occurs in two, L(+) and D(−). Note the difference in location of the hydroxyl group in the chiral carbon.*

**87**

(**Figure 5**).

bioabsorption.

*Poly(L-Lactide) Bionanocomposites*

been considered.

**Figure 4.**

nucleation point [23, 25, 26].

neous nucleation points [19, 28].

**3.2 Poly(L-lactide-co-ε-caprolactone)**

*DOI: http://dx.doi.org/10.5772/intechopen.85035*

recovery temperature was high for using in the human body. In order to improve the shape memory and decrease the recovery temperature, copolymers with PCL has

*Molecular scheme of L-lactide as oligomer and poly(L-lactide) as homopolymer.*

The PLLA with high performance biodegradable and biocompatible homopolymer is under various studies due to significant properties. PLLA crystallization happens very slowly, even if nanoparticles are incorporated and treat heterogeneous

In some researches, the thermo mechanical properties of PLLA nanocomposites

reinforced with functionalized multi-walled carbon nanotubes (MWCNT-g-PLLAs) were determined. For functionalization, PLLA chains were grafted form the surface of MWCNTs. Then, the func.MWCNTs/PLLA composite is prepared by solution casting. The results show that the MWCNT-g-PLLAs were dispersed in PLLA matrix adequately. With increasing the weight percentage of MWCNT-g-PLLAs, up to 2 wt% led to gradual enhancement of the mechanical properties of nanocomposite. The thermal analysis also revealed the func.MWCNTs increase the melting point and the glass transition temperature of nanocomposite. Also, the DMA analysis results show that incrementing the concentrations of func.MWCNTs is also accompany with increasing Young modulus and the transition temperature of PLLA. The chain stiffness in amorphous phase of PLLA can also increase due to the van der Walls force and the homogenous dispersion of func.MWCNTs. In addition, the crystallinity of PLLA could be increased due to func.MWCNTs as heteroge-

Poly(ε-caprolactone) (PCL) is another important aliphatic polyester that is considered as a potential material in both biomedical and environmental fields. PCL is a biodegradable and nontoxic polyester. The ring-opening polymerization of ε-caprolactone oligomers (CLs) yields the PCL semi-crystalline polymer with a melting point of 59–64°C and a glass transition temperature of −60°C. The glass transition temperature can be increased by copolymerization with L-lactide, which also enhances the biodegradation of the polymer. PCL has good permeability to many therapeutic drugs and has been studied for long-term contraceptive delivery

The polymer has been regarded as tissue compatible and used as a biodegradable

suture. PCL exhibits a low glass transition temperature and melting point, high crystallinity and permeability, and good flexibility with a high elongation at break and low modulus. However, modification is highly necessary when it is applied to different requirements. Because the homopolymer has a degradation time on the order of 2 years, copolymers have been synthesized to accelerate the rate of

*Poly(L-Lactide) Bionanocomposites DOI: http://dx.doi.org/10.5772/intechopen.85035*

**Figure 4.**

*Peptide Synthesis*

The PLLA has important characteristics over other biopolymers such as:

• Possible modification of physical and mechanical properties using copolymer-

The commercialization of PLLA has been affected from three factors:

types of monomers named g. poly(D, L)-lactic acid (PDLLA) (**Figure 3**).

• High cost in comparison to other polymers due to its immature technology,

Copolymerization of LA with other monomers like glycolide or CL can significantly enhance the properties and broaden the use of PLLA. The PLLA is produced form polylactic acid. The asymmetric polylactic acid has two stereo isomeric forms, L- and D-isomers. The L-isomer exists in normal human carbohydrate metabolism, and the D-isomer is detectable in urine and in acidic milk. If a polymer formed by one type of monomer, it is called homopolymer PLLA. A copolymer consists of two

Some large scale manufacturers are beginning to favor PLLA because it is renewable, conserves energy, and degrades easily. The ring-opening polymerization of L-lactide oligomers (LAs) yields the PLLA semi-crystalline polymer with a melting point of 180–190°C and a glass transition temperature of 55–60°C (**Figure 4**).

Up to now, the PLLA has limited biomedical applications as implanting devices because of its biodegradation effect. If incorporating different nanoparticles into the PLLA matrix could enhance the properties of this material significantly, this process would increase its applicability further. In addition, the PLLA showed shape memory effect and the original shape could be recovered up to glass transition temperature. However, the recovery strain of PLLA was relatively low and the

*Stereoisomeric forms of lactic acid: lactic acid occurs in two, L(+) and D(−). Note the difference in location of* 

• Using renewable resources for production,

• Using carbon dioxide for manufacturing,

• Improving the farm economics by composting,

• Moisture absorption of in environment, and

• Modified processing conditions are needed.

• Considered as energy saver,

• Decline of landfill volumes, and

• Recyclable to lactic acid,

ization and blending.

**86**

**Figure 3.**

*the hydroxyl group in the chiral carbon.*

*Molecular scheme of L-lactide as oligomer and poly(L-lactide) as homopolymer.*

recovery temperature was high for using in the human body. In order to improve the shape memory and decrease the recovery temperature, copolymers with PCL has been considered.

The PLLA with high performance biodegradable and biocompatible homopolymer is under various studies due to significant properties. PLLA crystallization happens very slowly, even if nanoparticles are incorporated and treat heterogeneous nucleation point [23, 25, 26].

In some researches, the thermo mechanical properties of PLLA nanocomposites reinforced with functionalized multi-walled carbon nanotubes (MWCNT-g-PLLAs) were determined. For functionalization, PLLA chains were grafted form the surface of MWCNTs. Then, the func.MWCNTs/PLLA composite is prepared by solution casting. The results show that the MWCNT-g-PLLAs were dispersed in PLLA matrix adequately. With increasing the weight percentage of MWCNT-g-PLLAs, up to 2 wt% led to gradual enhancement of the mechanical properties of nanocomposite. The thermal analysis also revealed the func.MWCNTs increase the melting point and the glass transition temperature of nanocomposite. Also, the DMA analysis results show that incrementing the concentrations of func.MWCNTs is also accompany with increasing Young modulus and the transition temperature of PLLA. The chain stiffness in amorphous phase of PLLA can also increase due to the van der Walls force and the homogenous dispersion of func.MWCNTs. In addition, the crystallinity of PLLA could be increased due to func.MWCNTs as heterogeneous nucleation points [19, 28].

### **3.2 Poly(L-lactide-co-ε-caprolactone)**

Poly(ε-caprolactone) (PCL) is another important aliphatic polyester that is considered as a potential material in both biomedical and environmental fields. PCL is a biodegradable and nontoxic polyester. The ring-opening polymerization of ε-caprolactone oligomers (CLs) yields the PCL semi-crystalline polymer with a melting point of 59–64°C and a glass transition temperature of −60°C. The glass transition temperature can be increased by copolymerization with L-lactide, which also enhances the biodegradation of the polymer. PCL has good permeability to many therapeutic drugs and has been studied for long-term contraceptive delivery (**Figure 5**).

The polymer has been regarded as tissue compatible and used as a biodegradable suture. PCL exhibits a low glass transition temperature and melting point, high crystallinity and permeability, and good flexibility with a high elongation at break and low modulus. However, modification is highly necessary when it is applied to different requirements. Because the homopolymer has a degradation time on the order of 2 years, copolymers have been synthesized to accelerate the rate of bioabsorption.

For example, copolymers of CL with LA have yielded materials with more rapid degradation rates. Also, combining nanoparticles with PCL is an effective and operable approach to improving the properties of PLLA significantly. The copolymers of PLLA with other biopolymers such as PCL may increase its applications because with this procedure, it becomes possible to fabricate a various kinds of bioabsorbable polymers and composites with soft and elastic properties. Because the PCL has a low melting point, if PCL is introduced into segmented polyurethane as a soft segment, the shape memory effect would be expected. Hydrolysis of PCL yields 6-hydroxycaproic acid which enters the citric acid cycle and is metabolized.

PLLA is a biocompatible and biodegradable homopolymer with good mechanical properties and its copolymers with PCL may expand its applications. The CL appears to be a suitable comonomer for the preparation of copolymers with PLLA and PGA with mechanical properties ranging from rigid to elastomeric. The copolymer of PLLA and PCL possessed properties partly like that of PLLA and partly like that of PCL (**Figure 6**).

The poly(L-lactide-co-ε-caprolactone) PLACL has a lower tensile strength than higher elongation and substantially more rapid degradation time than PLLA. But PLACL has not enough sufficient characters for hard tissue engineering. The synthesis of LA/CL copolymers and other lactone polymers have been widely studied in recent years. Most studies have focused on random, diblock, and triblock copolymers. Both PLLA and PCL have shape memory properties. Hence the PLACL must have shape memory effect. It is found that the mechanical properties of PLACL are significantly affected by the polymer compositions. With the increment of CL content, the maximum stress decreases linearly and the strain at break increases gradually as can be seen in **Figure 6**. By adjusting the compositions of monomers, the copolymers exhibit excellent shape memory effects.

There are many research on reinforcing the PLACL using nanomaterials. As an example, PLACL reinforced with well-dispersed multi-walled carbon nanotubes (MWCNTs) were prepared using functionalized MWCNT by in situ polymerization. The surface functionalization of MWCNTs can effectively improve the

**Figure 5.**

*Molecular scheme of ε-caprolactone as oligomer and poly(ε-caprolactone) as homopolymer.*

**89**

*Poly(L-Lactide) Bionanocomposites*

PLACL nanocomposites [29].

MWCNT nanocomposite [33].

**4.1 Metal-based nanoparticles**

nonmetallic, ceramics, polymeric and so on..

**4. Nanoparticles**

[30] (**Figure 7**).

**Figure 7.**

**3.3 Poly(D,L-lactide-co-glycolide) (PLGA)**

*Molecular scheme poly(D,L-lactide-co-glycolide) as copolymer.*

*DOI: http://dx.doi.org/10.5772/intechopen.85035*

dispersion and adhesion of MWCNTs in PLACL and hence, it will have a significant effect on the physical, thermomechanical, and degradation properties of MWCNT/

The next nanocarrier that has been considered for sustained and targeted delivery of different agents is poly[L-lactide-co-glycolide] (PLGA)-based nanoparticles. Although PLGA have been applied many years ago, but the task of nanoparticles in mechanism of intercellular uptake, their trafficking, and sorting into different intercellular compartments, as well as their procedure of action for therapeutic efficacy of nanoparticles encapsulated agent at cellular level is recently considered

In addition, we know that the PLGA nanoparticles have deeper in vitro and in vivo effects in comparison with industrial nanoparticles in similar range such as ferrous oxide and zinc oxide. The effect of PLGA nanoparticles on cell viability was characterized by in vitro cytotoxicity analysis via a WST assay. The PLGA, silica, and ferrous oxide have a cell viability up to 75%, but for zinc oxide, particles cell viability significantly reduced [31]. The researchers found that nanoparticle mean size correlates linearly with polymer concentration is between 70 and 250 nm [32]. The PLGA/MWCNT composite was considered as a scaffold material to treat artificial bloods. PLGA/MWCNT nanocomposite is prepared using electrostatic technique, in which layers of MWCNTs are deposited on the PLGA. For in vivo and in vitro analysis, the fibrinogen is immobilized on PLGA/MWCNT composite and incubated in non-stimulated platelet-rich plasma (PRP) for platelet studies. The interaction of fibrinogen and PRP, are characterized on the prepared PLGA/

Nanomaterials consists of materials that the size of particle is less than 100 nm.

The widely used metallic nanoparticles in the field of medicine and biotechnology are gold (Au), platinum (Pt), silver (Ag), selenium (Se), copper (Cu), palladium (Pd), and gadolinium (Gd), also, the widely used metal oxide nanoparticles

All kinds of materials could be treating to be nanomaterials such as metallic,

**Figure 6.** *Molecular scheme poly(L-lactide-co-ε-caprolactone) as copolymer.*

*Poly(L-Lactide) Bionanocomposites DOI: http://dx.doi.org/10.5772/intechopen.85035*

*Peptide Synthesis*

that of PCL (**Figure 6**).

For example, copolymers of CL with LA have yielded materials with more rapid degradation rates. Also, combining nanoparticles with PCL is an effective and operable approach to improving the properties of PLLA significantly. The copolymers of PLLA with other biopolymers such as PCL may increase its applications because with this procedure, it becomes possible to fabricate a various kinds of bioabsorbable polymers and composites with soft and elastic properties. Because the PCL has a low melting point, if PCL is introduced into segmented polyurethane as a soft segment, the shape memory effect would be expected. Hydrolysis of PCL yields 6-hydroxycaproic acid which enters the citric acid cycle and is metabolized.

PLLA is a biocompatible and biodegradable homopolymer with good mechanical properties and its copolymers with PCL may expand its applications. The CL appears to be a suitable comonomer for the preparation of copolymers with PLLA and PGA with mechanical properties ranging from rigid to elastomeric. The copolymer of PLLA and PCL possessed properties partly like that of PLLA and partly like

The poly(L-lactide-co-ε-caprolactone) PLACL has a lower tensile strength than higher elongation and substantially more rapid degradation time than PLLA. But PLACL has not enough sufficient characters for hard tissue engineering. The synthesis of LA/CL copolymers and other lactone polymers have been widely studied in recent years. Most studies have focused on random, diblock, and triblock copolymers. Both PLLA and PCL have shape memory properties. Hence the PLACL must have shape memory effect. It is found that the mechanical properties of PLACL are significantly affected by the polymer compositions. With the increment of CL content, the maximum stress decreases linearly and the strain at break increases gradually as can be seen in **Figure 6**. By adjusting the compositions of monomers,

There are many research on reinforcing the PLACL using nanomaterials. As an example, PLACL reinforced with well-dispersed multi-walled carbon nanotubes (MWCNTs) were prepared using functionalized MWCNT by in situ polymerization. The surface functionalization of MWCNTs can effectively improve the

the copolymers exhibit excellent shape memory effects.

*Molecular scheme of ε-caprolactone as oligomer and poly(ε-caprolactone) as homopolymer.*

*Molecular scheme poly(L-lactide-co-ε-caprolactone) as copolymer.*

**88**

**Figure 6.**

**Figure 5.**

**Figure 7.** *Molecular scheme poly(D,L-lactide-co-glycolide) as copolymer.*

dispersion and adhesion of MWCNTs in PLACL and hence, it will have a significant effect on the physical, thermomechanical, and degradation properties of MWCNT/ PLACL nanocomposites [29].
