3.2.1 Necessary conditions and special performance...
When biomedical polymers are used for the skin beauty and the diagnosis and treatment of skin diseases，it is necessary to contact with human tissues，blood and body fluid.Due to the natural protective reaction of human body，when the material acts on human body or skin surface，rejection phenomenon is bound to occur，which will often produces very serious consequences，and even endanger life.Therefore，compared with general polymer materials，the requirements of biomedical polymer materials are very strict，and also have special specific requirements for properties.Before medical polymer materials are used in clinic，the physical and chemical properties，mechanical properties，and the mutual adaptability of the materials with organisms and human bodies must be evaluated in detail and comprehensively，and then they can be used in clinic only after being approved by the national administrative departments.Therefore，biomedical polymer materials must have the following properties.
1.Biosafety
Biomedical polymer materials must be non-toxic or have minimal side effects.Biomedical polymer materials，after contact with the body's tissues，will produce a series of complex biological，physical，and chemical reaction，and a variety of temporary or profound influences.On the one hand，the material requires high molecular compound purity，clean production and processing environment，less residual impurities，and no disease，sterile or non-toxic transmission.On the other hand，after contact with body fluid，the polymer materials must demonstrate no sensitization，irritation，genetic toxicity，or three causes（carcinogenic，teratogenic，and mutation），and reflects biological inertness.In other words，no changes in structure and composition occur in the body，or it can be degraded in the body，while no adverse reactions and no harmful by-products are produced.The first application of biodegradable biomedical polymer materials is absorbable suture，and the representative of which are poly（glycolic acid）（PGA），poly（lactic acid）（PLA），and poly（lactic-co-glycolic acid）（PLGA），as described in Table 3-1.
The influences of human environment on the polymer materials basically have the followings：body fluid can cause the degradation，cross-linking and phase change of polymers；free radicals in the body can cause materials oxidation and degradation；enzyme can cause polymers decomposition reaction.Under the action of body fluid，the additives in polymer materials may be dissolved out.In the meantime，the substance such as lipid，steroid and adipose in blood and body fluid also can seep into polymer materials，leading to falling mechanical strength.But for biomedical polymers，in some cases，the reactions of human environment to aging，decomposition and degradation of polymer materials are not always negative，and even have positive significance.The surgeon，for example，does not want the sutures to be chemically stable or biologically inert，but rather to be broken down，absorbed，or expelled by the tissue as soon as possible.In this case，the additional requirement for biomedical polymer materials is that no harmful by-products are produced during the decomposition process.
2.Physical，chemical and mechanical properties(https://www.daowen.com)
It should meet the design and function requirements of skin beauty and the diagnosis and treatment of skin diseases.Differing from high mechanical strength for dentistry and orthopedics，the strength requirements of polymer materials used in skin beauty and fractured wound repair are not that high；however they must demonstrate a certain degree of flexibility，especially dermal fillers，which should reflect similar or even consistent mechanical properties such as elasticity and elasticity to normal skin，and should have a certain degree of adhesion and adaptability with the skin，in order to match and integrate with the skin.The repair materials at the damaged part of the skin are physically combined with human skin tissue.Such soft tissue repair materials should have certain activity，which can not only promote the repair of the injured part，but also degrade after repair，and finally be absorbed and metabolized by the body.
3.Low clinical application cost
The materials with non-expensive can be suitable for civilian consumption.And the materials should be relatively easy to prepare and obtain，so they can finally achieve the wide application.