3.2.2　Composition and classification of biomedical...

3.2.2　Composition and classification of biomedical polymers

Biomedical polymer materials can be divided into natural and synthetic biomedical polymer materials according to their sources.These two kinds of materials have their own advantages and disadvantages.Usually，the natural polymers have good biocompatibility and the synthetic polymers have good mechanical properties.Among them，natural biomedical polymer materials are usually a kind of macromolecular materials extracted from living organisms or directly obtained in the natural environment，mainly including natural polysaccharides and natural protein materials，animals or human body's own acellular matrix materials，and natural polymer materials from microbial fermentation.Synthetic biomedical polymer materials can also be subdivided into non-degradable and degradable biomedical polymer materials.

The chemical structures of some common natural biomedical polymers are shown in Figure 3-1.Natural polysaccharides materials mainly include starch/cellulose/chitosan and their derivatives，alginate，hyaluronic acid，heparin and chondroitin sulfate，etc.Natural protein materials mainly include collagen，gelatin，silk fibroin，albumin and soybean protein.The acellular matrix materials of animal or human body can come from small intestine，bladder，skin，bone，heart，heart valve and others.The natural macromolecular materials derived from microbial fermentation include poly（3-hydroxybutyrate），poly（y-glutamic acid）and poly（ε-lysine），etc.Natural biomedical polymer materials are abundant and easy to obtain，generally display good biocompatibility and biodegradability.They have been widely used in pharmaceutical excipients，absorbable sutures，medical isolation membranes，wound repair membranes，wound dressings，medical adhesives，artificial skin，degradable tissue engineering scaffolds，and nano-drug carriers.However，natural biomedical polymer materials also have some defects，such as potential immunogenicity，complex structure，difficult to chemical modification and poor mechanical properties of proteins and acellular matrix materials.

Among the synthetic polymer materials，non-degradable materials are the first polymer materials used in the field of biomedicine.They are characterized by long-term stability under the physiological environment，not prone to degradation，cross-linking reaction or physical wear，and have excellent mechanical properties.Simultaneously，they generally have poor adhesion to cells and tissues，poor matching of mechanical properties with surrounding tissues，and poor adaptability.They are mainly used in various medical devices and consumables，including packaging materials for medical products，in vitro diagnostic equipment，disposable medical products，medical protective equipment，interventional embolization materials，blood bags，ophthalmic lenses，artificial tissues，artificial organs，etc.The chemical structures of some common synthetic non-degradable biomedical polymers are shown in Figure 3-2.

Compared with non-degradable polymer materials，synthetic degradable materials are the new generation of medical materials and the hot-spot in the field of scientific research.Meanwhile，in recent years，synthetic degradable materials have been widely applied in the field of medicine and have achieved success in clinical practice，making them an important direction of future development.Degradable biomedical polymer materials refer to a kind of polymer materials that can be gradually degraded into low molecular weight compounds or monomers through hydrolysis and enzymolysis in biological environment，and the degradation products can be excreted from the body or participate in the normal metabolism in the body and disappear.Commonly-used degradable polymer materials include poly（lactic acid）（PLA），poly（glycolic acid）（PGA），poly（lactic-co-glycolic acid）（PLGA），polycarbonate（PC），poly（p-dioxanone）（PPDO），poly（e-carprolactone）（PCL），poly（amino acid）（PA），polyphosphoester（PPE），poly（ortho ester）（POE），polyanhydride，polyphophazene，etc.The corresponding chemical structures are shown in Figure 3-3.(https://www.daowen.com)

In addition，biomedical polymers can also be divided into soft biomedical polymers，semi-crystalline biomedical polymers and other related biomedical polymers according to their structural characteristics，as shown in Table 3-4.