Synthetic Biodegradable Polymers:
Biodegradable polymers offer the advantage of being able to be eliminated from the body after fulfilling their intended use. Therefore, avoid the usual costly and complicated procedures to remove the implants or scaffolds. It is not surprising that biodegradable biomaterials are becoming more and more important in biomaterials and tissue engineering field. In this chapter, some of the important synthetic polymers will be reviewed.
Aliphatic Polyester:
Aliphatic polyesters presently are the most attractive synthetic biodegradable polymers for biomedical use. These biodegradable polymers all have hydrolysable ester linkages in their structure.
1- Poly(glycolide):
Poly( glycolide) (PGA) is synthesized through the ring opening polymerization of glycolide to yield high-molecular-weight materials. The monomer glycolide is synthesized from the dimerization of glycolic acid.
As shown in (12.4), PGA is the simplest linear aliphatic polyester. It is highly crystalline (45% -55%) with a high melting point (220°C -225°C) and a glass transition temperature of 35°C -40°C[3]. Because of its high degree of crystallization, it is not soluble in most organic solvents except for highly fluorinated organic solvents such as hexafluoroisopropanol.
PGA Fibers exhibit high strength and modulus because of its high degree of crystallinity. But it is too stiff to be used as sutures except as braided material. PGA was used to develop the first totally synthetic absorbable suture that has been marketed as Dexon since the 1960s by Davis and Geek. Sutures of PGA lose about 50% of their strength after two weeks and 100% after four weeks and are completely absorbed in 4 - 6 months .
Glycolide can be copolymerized with other monomers, such as lactide and trimethylene carbonate, to yield copolymer with various mechanical and biodegradation properties for different applications.
2- Poly( lactide):
Lactic acid is a chiral molecule, and exists in two stereo-isometric forms: DLA and L-LA. The monomer used to synthesis poly (lactide) (PLA) is lactide, which is the cyclic dimer of lactic acid. L-lactide, is the naturally occurring isomer, and DL-lactide is the synthetic blend of D-lactide and L-lactide. The polymerization of lactide is similar to that of glycolide (12. 5). Both poly(D-LA) (PDLA) and poly(L-LA) (PLLA) are crystalline polymers.
PLLA is the most commonly used form than PDLA because the degradation product L-lactic acid is the natural occurring stereoisomer of lactic acid . Poly( L-lactide) is about 37% crystalline with a melting point of 175°C -178°C and a glass transition temperature of 60°C -65°C .
Because PLLA and PDLA exhibit high tensile strengths and high modulus that makes them more applicable than the amorphous poly( DL-LA) (PDLLA) polymers for load-bearing applications such as in orthopedic fixation and sutures. However, the degradation time of PLLA is much longer than that of PDLLA. It takes more than 3 years for PLLA to be completely absorbed .
As a biodegradable polymer, PLLA has satisfactory biocompatibility in vivo .It is essentially non-toxic, and elicits only a mild inflammatory response. The hydrolysis product L-lactic acid is the normal intermediate of carbohydrate metabolism and will not accumulate in vital organs. It has been proposed and successfully applied in the reconstruction of bone, articular defects, suture materials, drug carriers, and fixation devices .
However, the slow in vivo degradation rate of PLLA opposes a problem for its application. It has been found that bone plates made of crystalline (PLLA) were used for the fixation of zygoma fractures. After 3 years, all patients showed a severe, well-defined swelling, which was strictly limited to the implantation site. In the case of seven patients, surgeries were performed to surgically remove the swelling. Light microscopic examination showed that the removed tissue was characterized by a foreign body reaction without signs of inflammation. Examination of the tissue samples using transmission electron microscopic revealed the presence of large amounts of highly crystalline PLLA particles. Therefore, it is believed that the swelling of the implantation site was caused by the aseptic inflammatory reaction of the host tissue to the small crystalline PLLA particles. It is also confirmed that the degradation of the PLLA crystals was very slow and that the total degradation time was much longer than 3 years .
To overcome this problem, D-lactic acid was added as co-monomer to obtain a copolymer with a lower crystallinity than PLLA and consequently a higher degradation rate.
3- Poly( lactide-co-glycolide):
Lactide and glycolide can be copolymerized to obtain a range of copolymers with various mechanical and biodegradation properties (12.6).
Copolymers of glycolide with both L-lactide and DL-lactide have been developed for both device and drug delivery applications. It has been found that there is not a linear relationship between the copolymer composition and the mechanical and degradation properties of the materials. For example, a copolymer of 50% glycolide and 50% DL-lactide degrades faster than both homopolymers . The degradation rate strongly depends on the crystallinity of the copolymers. Copolymers of 25% - 7 0% L-lactide with glycolide are amorphous due to the disruption of the regularity of the polymer chain by the other monomer . Therefore, these copolymer showed a much higher degradation rates than each individual homopolymers. Relationship between the copolymer composition and the mechanical and degradation properties of the materials is non-linear. For example, the degradation time of these copolymers were found to increase in the following order :
PLA100(6. 7months) < PGA100 (5months) < PLA75GA25(2. 5 weeks)
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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