**2.2 Overview of the most commonly used biodegradable polymers**

Both natural and synthetic polymers have been extensively studied as biodegradable biomaterials (Nair & Laurencin, 2007). At first, natural polymers were considered as promising candidates. Nevertheless, several studies quickly demonstrated that some of these materials involve different drawbacks: possibility of disease transmission, strong immunogenic reactions and major difficulties with the purification process (Nair & Laurencin, 2007). In a later stage, synthetic biodegradable polymers were developed by the synthesis of polymers with hydrostatically unstable linkages in their backbones (Middleton & Tipton, 2000). These hydrostatically unstable groups are esters, ortho-esters, anhydrides and amines (Middleton & Tipton, 2000). The major advantages of synthetic over natural polymers are (Nair & Laurencin, 2007):


Various synthetic biodegradable polymers were invented of which the aliphatic polyesters appear to be the most attractive for biomedical applications. For this reason, this chapter will only discuss this type of polymers. An aliphatic polyester is a thermoplastic polymer which contains hydrolysable aliphatic ester linkage in its backbone (Donglu, 2010). In theory all polyesters are degradable, however only aliphatic polyesters with reasonably short aliphatic chains between the ester bonds will degrade within a time interval suitable for biomedical applications (Nair & Laurencin, 2007). Such aliphatic polyesters can be derived from a wide range of monomers through ring-opening and condensation polymerisation routes, but in some cases bacterial processes can also be used. In what follows we will briefly discuss the most common biodegradable aliphatic polyesters. The structural chemical formula of each discussed polymer is given in Table 1.

rigid steel implant. This is in contrast to a biodegradable implant which degrades little by little and transfers by degrees the load from the implant to the fractured bone. This gradually movement of the load results in less bone re-fracture (Middleton & Tipton,

Secondly, another interesting application field for biodegradable polymers is tissue engineering. This research branch aims to produce completely biocompatible tissues which could be employed to replace damaged or diseased tissues in reconstructive surgery (Djordjevic et al., 2008). Today's focus in this field is the use of so called scaffolds. These scaffolds are 3D artificial matrices that guarantee optimal support and conditions for growth of tissue (Djordjevic et al., 2008). Optimally, these scaffolds should fulfil the following two

• having the ability to degrade over time while leaving behind a reproduced functional

Finally, biodegradable polymers can also contribute in controlled drug delivery (Amass et al., 1998). A gradual delivery of antibiotics can be beneficial for the treatment of deep skeletal infections after a surgery, while the healing process of a fractured bone can be enhanced by a delivery in stages of bone morphogenetic proteins (Agrawal et al., 1995,

Both natural and synthetic polymers have been extensively studied as biodegradable biomaterials (Nair & Laurencin, 2007). At first, natural polymers were considered as promising candidates. Nevertheless, several studies quickly demonstrated that some of these materials involve different drawbacks: possibility of disease transmission, strong immunogenic reactions and major difficulties with the purification process (Nair & Laurencin, 2007). In a later stage, synthetic biodegradable polymers were developed by the synthesis of polymers with hydrostatically unstable linkages in their backbones (Middleton & Tipton, 2000). These hydrostatically unstable groups are esters, ortho-esters, anhydrides and amines (Middleton & Tipton, 2000). The major advantages of synthetic over natural

Various synthetic biodegradable polymers were invented of which the aliphatic polyesters appear to be the most attractive for biomedical applications. For this reason, this chapter will only discuss this type of polymers. An aliphatic polyester is a thermoplastic polymer which contains hydrolysable aliphatic ester linkage in its backbone (Donglu, 2010). In theory all polyesters are degradable, however only aliphatic polyesters with reasonably short aliphatic chains between the ester bonds will degrade within a time interval suitable for biomedical applications (Nair & Laurencin, 2007). Such aliphatic polyesters can be derived from a wide range of monomers through ring-opening and condensation polymerisation routes, but in some cases bacterial processes can also be used. In what follows we will briefly discuss the most common biodegradable aliphatic polyesters. The structural chemical

• being capable of supporting initial cell growth and further proliferation

**2.2 Overview of the most commonly used biodegradable polymers** 

2000, Athanasiou et al., 1998).

requirements (Ryu et al., 2005):

Wang et al., 1990, Ramchandani & Robinson, 1998).

polymers are (Nair & Laurencin, 2007):

• they have more predictable properties

• synthetic biodegradable polymers are biologically inert

formula of each discussed polymer is given in Table 1.

• their characteristics can be tailored with a specific application in mind

tissue.

Table 1. Structural chemical formula of the most common biodegradable polymers.
