Polyester is a type of polymer where the repetitive units are connected by ester linkage. Polyesters are naturally occurring chemicals; at the same time, they can also be synthesised in the laboratory. The most widely used biodegradable polymers are polyglycolic acid (PGA) and polylactic acid (PLA).
POLYGLYCOLIC ACID
Polyglycolic acid is the simplest aliphatic polymer. It is highly crystalline and has a high melting point and low solubility. The sutures made using PGA under the trade name dexon usually lose strength before 2 to 4 weeks. Degradation of PGA happens due to the hydrolysis of the unstable ester linkage.
Properties
- Excellent biodegradability
- Has thermoplastic properties
- Is semi crystalline
- Has excellent gas barrier properties
- Relative high strength
Medical applications
- Drug delivery systems
- Dental applications
- Orthopaedic applications
- Surgical applications like bone fractions
- In the field of tissue engineering
POLYLACTIC ACID
Polylactic acid is a polymer obtained by the condensation of lactic acid with the loss of water. As PLA is a D, L amorphous polymer, it is used for drug delivery often. PLA can be used in sutures and orthopaedic devices as it is semi-crystalline in nature. PLA is more hydrophobic than PGA, thus reducing the rate of hydrolysis.
Degradation of PLA inside the body occurs by the hydrolysis of the ester bond backbone. This degradation rate is dependent on the temperature and pH in the tissue.
Properties of PLA
- Semi-crystalline and 3D printable
- Biocompatible
- Is biodegradable
- Has higher processability
- Is hydrophobic
Medical applications of PLA
- Cardiovascular implants
- Orthopaedic applications
- Drug delivery systems
- Skin and tendon healing
- Used in medical tools and equipments
POLYLACTIC CO-GLYCOLIC ACID
Polylactic co-glycolic acid is a copolymer of PLA and PGA. It is very much biodegradable and biocompatible. It is an aliphatic polyester. Degradation of PLGA involves the breakage of the ester bonds in the polymer backbone due to hydrolysis
Properties of PLGA
- Properties are mostly dependent on the ratio of lactic acid and glycolic acid
- Has fast degradation rates
- Biocompatible and biodegradable
- Has adjustable mechanical properties
- Crystallinity depends on the block structures and molar ratio
Medical applications of PLGA
- Production of grafts
- Production of sutures
- Prosthetic devices
- Drug delivery device
- Production of implants
POLYCAPROLACTONE
Polycaprolactone (PCL) is a semi-crystalline polymer. As it shows a slower degradation rate than PLA, it can remain active as a drug delivery system for longer than a year. An implantable biodegradable contraceptive with the trade name Capronor comprises Polycaprolactone. which dissolves in the body after being implanted under the skin. The process of degradation occurs by the hydrolysis of ester linkages. This reaction is autocatalyzed by the carboxylic acid end groups of the polymer, forming carbon dioxide and water.
Properties of PCL
- Partially crystalline
- It is partially hydrophobic
- Solvent resistant
- Slow degradation
- Biocompatible
Medical applications of PCL
- Used in the replacement of the hard tissues where healing takes time
- Implantable contraceptives
- Drug delivery system
- Tissue engineering and regenerative medicine
- Surgical implants
POLYDIOXANONE
The ring-opening polymerization reaction of P-Dioxanone results in the formation of polydioxanone. It can also be referred to as poly (ether ester) or poly (oxyethylene glycolate). It is used as sutures as the filament loses only 50% of its initial breaking strength after 3 weeks. This serves as an advantage over other sutures for slow-healing wounds. Polydioxanone is also degraded by the mechanism of hydrolysis which leads to breakage in the ester links.
BACTERIAL POLYESTERS
These polyesters are biodegradable. This category of polyesters mostly comprises of Polyhydroxybutyrate (PHB), Polyhydroxyvalerate (PHV) and copolymers. The above-mentioned polyesters are synthesised by microorganisms for intracellular energy storage. The rate of degradation of this polymer can be controlled by varying the copolymer composition. PHB is highly crystalline and brittle as a homopolymer while its copolymer with hydroxy valeric acid is more flexible, less crystalline and can be processed. These bacterial polyesters are used in,
- Drug release
- Artificial skin
- Suturing
- Paramedical disposables
POLYANHYDRIDES
Polyanhydrides are highly reactive and they are hydrolytically unstable. They get easily degraded as they don’t need any kind of catalysts for the same. There are two types of polyanhydrides. Aliphatic polyanhydrides (CH2 in the backbone and the side chains) degrades within days while Aromatic polyanhydrides (benzene ring in the side chain) degrade over several years. As polyanhydride exhibit excellent biocompatibility, it is used in the delivery of insulin, angiogenesis inhibitors and enzymes.
POLYAMINO ACIDS
Poly amino acids are synthetic biopolymers made from repeating the amino acid units. They are mostly used for drug delivery as the amino acid side chains offer sites for drug attachment. Due to limited solubility, this cannot be widely used as a biomaterial. The polymers containing more than three amino acids may trigger an antigenic response.
POLY (γ GLUTAMIC ACID)
It is relatively more abundant than the other poly amino acids. Glutamic acid is a major product of the fermentation industry in the form of monosodium salt. Properties of poly(γ-glutamic acid) is given by,
- High molecular weight
- Water soluble
- Biodegradable
- Is of low cost
POLY (ε LYSINE)
Poly(ε-lysine) is produced by Streptomyces albulus, thereby giving it a selective antimicrobial property. It is most commonly used for the preparation of hydrogels.
POLY (α- ASPARTIC ACID)
The natural analogues of polyaspartate polymers play a key role in the regulation of mineralisation. Polyaspartate can be produced through the hydrolysis of poly(succinimide).
PSEUDO POLYAMINOACIDS
The polymers in this category would usually have ester or urea bonds. These polymers are developed in order to achieve better material properties and to overcome the material properties like high crystallinity and melting points.
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