**10. Polyglycolide (PGA), Polylactide (PLA) and poly(Lactic-co-Glycolic Acid) (PLGA)**

Polyglycolide also named polyglycolic acid (PGA) is a biodegradable, thermoplastic poly‐ mer and the simplest linear, aliphatic polyester which contains the ester functional group in it's main chain [148]. It can be prepared starting from glycolic acid by means of polyconden‐ sation or ring-opening polymerization. PGA has been known since 1954 as a tough fiberforming polymer. Owing to its hydrolytic instability, its use has initially been limited [149]. *In vivo*, PGA initiates a marked host reaction around the implantations. This leads to the de‐ velopment of a foreign body response that comprises an initial acute inflammatory phase and a subsequent chronic inflammatory phase. For example, when a synthetic PGA scaffold seeding with adult-derived or somatic lung progenitor cells from mammalian lung tissue was implanted in an immunocompetent host, a serious foreign body response totally altered the integrity of the developing lung tissue [150].

Polylactic acid or polylactide (PLA) is another thermoplastic aliphatic polyester derived from renewable resources, such as corn starch, tapioca products, and sugarcanes [30]. A poly(L-lactide) (PLLA) coil stent has ever been implanted in pigs with no stent thrombosis and late restenosis [151]. However, PLA, as well as PLLA, and poly(D,L-lactide) (PDLA), in‐ duces a strong inflammatory response when they are implanted in the body due to their acidic products [152]. Aframian and coworkers implanted tubular PLLA, PGA coated with PLLA (PGA/PLLA), or nothing (sham-operated controls) in Balb/c mice either beneath the skin on the back, and found that inflammatory reactions were shorter and without epithe‐ lioid and giant cells in the sham-operated controls. Tissue responses to PLLA and PGA/ PLLA scaffolds are generally similar in areas subjacent to skin in the back and oral cavity. Biodegradation proceeded more slowly with the PLLA tubules than with the PGA/PLLA tu‐ bules. No significant changes in clinical chemistry and hematology were seen due to the im‐ plantation of tubular scaffolds. [153]. It was reported that, after the PLLA segments were swallowed *in vivo* by phagocytes, cell damage and cell death were obvious. The highest numbers of necrotic cells were observed on day 2 [154]. These reactions can result in an un‐ expected risk for patients and have strongly limited in clinical applications of this kind of biomaterials.

To date, numerous strategies have been investigated to overcome body reactions induced by this kind of biomedical devices [155]. As a result, most of the PLA, PLLA, and PDLA have been used as a composite or compound with some other biomaterials. For example, a PLLA and poly(ethylene oxide) (PEO) blend has been prepared by mechanical mixture and fusion of homopolymers [156]. A biodegradable star-shaped 8 arms PEG-b-PLLA block copolymer was synthesized by Nagahama and coworkers to create a novel implantable soft material with drastically lowered crystallinity, increased swelling ability, and desirable mechanical properties [157,158].

Currently PGA, PLA and their copolymers, such as poly(lactide-co-caprolactone) (PLCA), poly(glycolide-co-caprolactone) (PGC), and poly (glycolide-co-trimethylene carbonate) are widely used as biomaterials for the synthesis of absorbable sutures and tissue engineering scaffolds in the biomedical field [159,160]. For example, a resorbable PLGA bone fixation im‐ planted in craniofacial patients in 1996 resulted in 0.2 percent significant infectious compli‐ cations, 0.3 percent device instability, and 0.7 percent self-limiting local foreign-body reactions [161]. As long-term implants, the toxicity of the accumulated acidate products made the situations even worse [162]. Until the present, most of the implanted PGA, PLA and PLGA related biomaterials still encounter an immune tissue response due to tissue trau‐ ma during implantation and the presence of foreign body reactions [163]. Surface coating has become one of the research hot points for the implantable devices with poor biocompati‐ bilities. For instance, the biocompatibilities of some artificial polymer devices, such as heart valves, stents and vascular prosthesis that come into contact with bodily tissues or fluids particularly blood, have been improved by Venkatraman and coworkers with endotheliali‐ zation surface layers [164,165].

Similarly, when a polyvinyl acetate (PVA)/PLGA microsphere was implanted into the sub‐ cutaneous tissue of rats, acute inflammation with neutrophils was found at day 3. Chronic inflammation with multinucleate giant cells, fibrosis, and mixed inflammatory cells was found at day 30. Mineralization around the implant was found at day 60 [166]. On the con‐ trary, a dexamethasone/PLGA microsphere system can suppress the inflammation reaction by a fast releasing of dexamethasone [167]. A highly monodisperse and smooth PLGA-pacli‐ taxel microspheres against malignant brain tumors were fabricated using an electrohydro‐ dynamic atomization (EHDA) process [168]. In addition, PLA, PGA and PLGA can be tailored to meet mechanical performance and resorption rates required for applications ranging from non-structural drug delivery applications, nanoparticles (nanofibers), to re‐ sorbable screws and anchors [169,170].
