**2.2.1 Polyglycolic acid (PGA)**

Polyglycolic acid (PGA) or polygycolide is the most simple linear biodegradable polymer and known as a rigid thermoplastic polymer (Vroman & Tighzert, 2009). Due to its high crystallinity (44-55 %) PGA provides excellent mechanical properties and it exhibits a low solubility in most organic solvents. Alow solubility (Nair & Laurencin, 2007). Nevertheless, it is soluble in highly-fluorinated solvents, such as hexafluoroisopropanol (Donglu, 2010). PGA has initially been studied for use as resorbable synthetic sutures and was for the first time commercialised in 1962 under the name DEXON® (Gilding & Reed, 1979). Today, due to their excellent biodegradability, good cell viability and good initial properties, PGA nonwoven fabrics are also extensively utilized as scaffolds for tissue engineering (Nair & Laurencin, 2007).

PGA loses its strength in 1 to 2 months when hydrolyzed and its mass within 6 to 12 months (Nair & Laurencin, 2007). When inserted in the body, PGA breaks down into glycolic acid. Glycolic acid is not toxic and can be excreted in the urine or converted into H2O and CO2 and subsequently removed from the body via the respiratory system (Maurus & Kaeding, 2004). Despite the above-mentioned non-toxicity of glycolic acid, it may result in an increased and localized acid concentration leading to tissue dammage (Gunatillake & Adhikari, 2003, Taylor et al., 1994). This presents in particular problems for orthopaedic applications where implants with substantial dimensions are needed (Gunatillake & Adhikari, 2003). Together with the high degradation rate and low solubility, these acidic degradation products have hampered the use of PGA for biomedical engineering applications.

### **2.2.2 Polylactic acid (PLA)**

The monomer building block of polylactic acid, lactic acid, is formed by converting sugar or starch from vegetable origin (e.g. wheat, corn, rice, etc.) via either bacterial fermentation or via a petrochemical process (Rasal et al., 2010). If PLA is implanted, it hydrolyses to its building block lactic acid which is a normal human metabolic by-product (Gunatillake & Adhikari, 2003). Lactic acid is degraded into H2O and CO2 which can be further removed by the respiratory system (Nair & Laurencin, 2007, Maurus & Kaeding, 2004). As can be seen in Table 1, lactic acid is a chiral molecule and therefore different forms of PLA occur. The two most important forms are poly(L-lactic acid) (PLLA) and poly(DL-lactic acid) (PDLLA).

Similar to PGA, PLLA has a high degree of crystallinity (± 37% depending on molecular weight and production processes) (Nair & Laurencin, 2007). Compared to PGA, PLLA slowly degrades: when PLLA is hydrolyzed, it loses its strength in circa 6 months. However, no mass loss is observed for a very long time and total degradation amount up to several years (Middleton & Tipton, 2000, Nair & Laurencin, 2007, Bergsma et al., 1995). Next to this slow degradation, PLLA offers good tensile strength, a high tensile modulus and low extension and can therefore be applied in load bearing applications like in orthopaedic fixation devices (Nair & Laurencin, 2007). PLLA fibres are also often used as surgical sutures, while PLLA composites, porous membranes or sponges can be employed as scaffolding matrices for tissue regeneration (Hu & Huang, 2010, Heino et al., 1996, Lam et al., 1995, Vaquette et al., 2008, Ma et al., 2006, Chen & Ma, 2004). For some other applications, the long degradation time of PLLA however presents a major concern.

PDLLA has an amorphous nature resulting in a substantial lower strength compared to PLLA (Nair & Laurencin, 2007). Moreover, PDLLA loses its strength in 1 to 2 months and its mass within 12 to 16 months (Maurus & Kaeding, 2004). Taking into account this low strength and its fast degradation rate, PDLLA can be employed as drug delivery system or as low strength scaffolding matrix for tissue engineering (Nair & Laurencin, 2007, Xie & Buschle-Diller, 2010).

### **2.2.3 Poly(lactic-co-glycolic acid) (PLGA)**

A lot of research has been carried out on the development of a full range of poly(lactic-coglycolic acid) (PLGA) polymers. This research has indicated that the degradation rate of PLGA strongly depends on the lactic acid/glycolic acid ratio (Gilding & Reed, 1979, Reed & Gilding, 1981, Miller et al., 1977). It is common knowledge that the intermediate co-polymers are much more unstable than the homo-polymers: a 50/50 PLGA and an 85/15 PLGA degrade in 1-2 months and 5-6 months respectively (Middleton & Tipton, 2000). This opportunity to tune the degradation rate of the polymer by varying the monomer ratio has made PLGA an ideal candidate for biomedical applications in the drug delivery and tissue engineering domain. However, the first commercial use of the co-polymer PLGA was as suture material under the name Vicryl ®(Nair & Laurencin, 2007, Gunatillake & Adhikari, 2003).
