3.1.2　Advances in biomedical polymers and skin bea...

3.1.2　Advances in biomedical polymers and skin beauty，diagnosis and treatment of skin diseases

With the development of biomedical polymers，it has become an indispensable material in the field of modern medicine，especially in the past 30 years，which has experienced a rapid development globally.Biomedical polymers，as the model of interdisciplinary，integrate polymer chemistry and physics，biochemistry，synthetic material technology，pathology，pharmacology，anatomy，and clinical medicine.It also involves many engineering issues，including the design and manufacturing of various medical devices.The collaboration and penetration of the above disciplines has promoted more varieties of biomedical polymer materials being applied to clinical medical treatment and daily protection，showing outstanding performance and advanced functions.Biomedical polymer materials are related to where and how they are used.At the same time，different application fields require materials to have certain special functions.In the fields of skin beauty，skin health，skin disease diagnosis and treatment，and daily protection，biomedical polymer materials have been widely accepted and applied.

The skin covers the surface of the whole body and is the largest organ of the human body.The total area of adult skin is 1.5-2 m2，accounting for 5％-15％of the total weight.The skin is in direct contact with the external environment.The first chapter of this textbook“Structure and Function of the Skin”demonstrated that the skin has important functions，including protecting the human body from heat，microorganisms or pathogens，protecting the deep tissues and organs of the human body，excreting，regulating body temperature，and feeling external stimuli.As people get older，their skin will gradually loosen，sag，reduce elasticity，and appear wrinkles.Now，injecting the polymeric supporting materials or fillers into the dermis and/or subcutaneous tissue to fill and increase the volume of the tissue will help correct or increase the function of the soft tissue clinically，and finally achieve the effect and propose of skin beauty.

The ideal facial fillers must demonstrate the good biocompatibility，safety and excellent cosmetic effects.The facial fillers can be divided into two types：non-absorbable and absorbable fillers.Non-absorbable fillers are prepared from non-degradable polymer materials，including poly（methyl methacrylate）（PMMA），polyacrylamide（PAM），expanded polytetrafluoroethylene（PTFE）and silica gel，etc.Absorbable fillers are prepared from degradable biomedical polymer materials，including collagen，hyaluronic acid（HA）and poly（L-lactic acid）（PLLA），etc.Expanded PTFE is prepared from a biaxial stretching process of PTFE to form“fibrous nodules”，showing the porous with the diameters less than 1μm.As a result，cells can be easily grown in and demonstrate excellent biocompatibility，therefore considered as an excellent facial filling material.Expanded PTFE has the advantages of natural lifelike，safe and permanent，and strong plasticity，so it is usually used for rhinoplasty，soft tissue repair，facial filling，mandibular prosthesis，nasolabial filling，etc.In addition，absorbable injectable fillers are the future developing direction of a novel generation of medical implants，which can be spontaneously degraded after being injected into the body for a certain period of time.Moreover，the degradation products have good biocompatibility and can be eliminated by the body through metabolism.At present，in addition to the usage for skin wrinkle removal propose，absorbable plastic implants are also widely used for depression filling，scar repair，and facial plasticity.Its main polymer materials include sodium hyaluronate gel，collagen implants，creotoxin and 3D PLA microsphere materials，etc.

The skin is the human body's first line of physical defense，and it also participates in various functional activities of the body.Therefore，abnormal conditions and even lesions of the body can also be reflected through the skin.If the physiological function of the skin is affected or damaged，it will cause the skin disease.The common pathogenic factors in skin diseases are infection and allergy，but with the degenerative changes of aging，senile skin diseases and skin cancer appear as the vital skin diseases.In addition，damage or loss of skin integrity may affect the function of the skin，leading to severe disability or even death.Therefore，wound healing is very important to the human body and is a major challenge faced by scientific research and clinical medicine.For a detailed introduction to skin diseases，please refer to the chapter 2“Skin Health and Representative Skin Diseases”.Biomedical polymers also play a significant role in the diagnosis and treatment of skin diseases and have been widely used，including skin wound dressings，pharmaceutical excipients，and composite artificial skins.

Healing of the skin wounds is one of the basic，yet key issues encountered by the entire human and medical circles.The process of wound healing is complicated，and an ideal surgical dressing can protect the wound，prevent infection，and promote healing.In recent years，the treatment of skin wounds and the variety of products are increasing day by day（see extended reading 3）.Traditional dressings such as cotton gauze only have simple functions of protection and reducing infection，and play a limited protective role in covering wounds and absorbing the exudate.New synthetic dressings constructed from biomedical polymer compounds，such as hydrogels，alginate dressings and foam dressings，are more effective for skin wound care，and can interact with the wound.In addition to absorbing the exudate，it also shows the effect of allowing gas exchange，preventing the invasion of external microorganisms and preventing wound infections，and creates an environment conducive to wound healing，which can alleviate the pain of patients and reduce scars and shrinkage.The dressing material itself can promote and accelerate wound healing.With the establishment and improvement of epidermal cell culture methods，active artificial skin，namely tissue engineered skin，is gradually born on the basis of the research of biological materials and synthetic materials.Biomedical polymer dressings and tissue engineered skins have significant advantages such as“wet”environment and good healing function that they provide for wounds，therefore leading to wide clinical applications.

Natural biomedical polymer materials，such as alginate and chitosan dressings，have been widely used in the repair of skin wounds.Dressing extracted from natural seaweed plants，contains calcium alginate ingredients.The calcium ions in the dressing exchange with the sodium salt secreted by the blood and wounds，forming gels with the effect of clotting and stopping bleeding and stabilizing biofilm，protect the wound surface and promote the growth of granulation tissue.Commercially available alginate dressing products include AlgiSite，Sorbsan and Kaltostat，etc.，which are suitable for wounds with moderate to large amounts of exudation，mild bleeding wounds，and yellow rotting and necrotic tissue wounds，but cannot be used for dry and eschar wounds.Most products are not self-adhesive and require secondary dressings to be fixed，with relatively high cost.Many synthetic biomedical polymer materials，such as polyvinylon，polypropylene，polyamide（Nylon），polyurethane，poly（vinyl alcohol），polyacrylonitrile，polycaprolactone，poly（lactic acid），poly（amino acid），polytetrafluoroethylene，and silicone elastomer，can also be used as a dressing.The more important and widely used ones are polyurethane and silicone.

Membrane-like dressings（e.g.，OpSite and Tegaderm）which are made of polyurethane，have a certain degree of elasticity，and at the same time reflect good adhesion performance，and the water vapor can smoothly permeate.Yet，water and bacteria cannot permeate the membranelike dressings made of polyurethane，and they do not interfere with the normal healing process of the wound.The prepared film has high transparency and is easy to observe.In addition，due to good biocompatibility，the film will not cause immune response，and can stay on the wound until the wound heals.Sometimes，it needs to be used in combination with other materials.For example，the melanocyte transplantation is used to treat vitiligo.After the cells are transplanted to the ground surface of the lesion，a layer of silica gel and a dressing impregnated with F12 medium are attached immediately，and then covered with Tegaderm.The FlexiGrid unidirectional breathable membrane produced by S & N in the UK，whose main components are polyurethane and polyacrylic，has the advantage of being a“molecular valve“function.The rancid gas decomposed by the necrotic tissue in the wound can outward penetrate the membrane smoothly，while the bacteria and air outside the membrane cannot penetrate the membrane to penetrate the skin wound，endowing the great permeability to the wound.In addition，polyether-based polyurethane has strong membrane permeability，so the moisture of the wound exudate can pass through and precipitate，and the protein and other components are transformed into gel under the dressing，which can relieve pain.

Generally，synthetic polymer dressings can be divided into film type，foam type，hydrogel，hydrocolloid and composite type according to the different status.The commercialized products are generally the composite materials.The advantages and disadvantages of the common dressings and the scope of applications are shown in Table 3-2.(https://www.daowen.com)

Scar is a general term for the appearance and histopathological changes of normal skin tissues caused by various kinds of wounds，including skin cutting injury，skin burn，skin infection，surgical operation，etc.Scar is an inevitable product in the process of human wound repair.When the growth of scar exceeds to a certain limit，all kinds of complications will occur，such as damage of appearance and dysfunction of function and activity，which will bring great physical and mental pain to the patients，especially scars left after burns，scalds and other severe trauma.The polymer material usually used to treat scar is cross-linked polydimethylsiloxane，commonly known as silicone，which forms a covering that is as soft and stretchable as human skin.Silicone used for scar treatment usually has three forms of ointment，film and gel，which can prevent the formation of new scars and relieve the symptoms and signs of existing hypertrophic scars，including erythema，hardness，size and pruritus.The use of polyurethane film alone or combined with pressure therapy can also reduce the color and hardness thickness of scar.The usage is similar to that of silicone products，12 to 24 hours per day and last for 2 to 4 months.Advantages lie in transparent appearance，suitable for exposed parts such as the head，face and hands，and not easy to macerate.

The cross-linked polydimethylsiloxane films also help moisturize the skin well and can be used in skin care products or cosmetics，as well as medical skin applications.Robert Langer from the Massachusetts Institute of Technology，collaborating with Massachusetts General Hospital，Living Proof and Olivo Laboratories，jointly developed the“second skin”elastic thin film materials.The material，a cross-linked polymer layer based on polydimethylsiloxane，is breathable，has“no feeling”against the skin and has not yet been found to have side effects on humans.Most importantly，it can effectively restore the mechanical properties of skin，eliminate the users'face bag and make the skin more moisture at the same time.If the product is successfully marketed，it is expected to be directly used to treat patients'skin or prevent UV exposure，for example，treating skin diseases such as eczema.When sunscreen is added，the“second skin”film also protects against damage from the sunlight.

Skin encapsulation therapy is a kind of therapy in which non-permeable films（e.g.，plastic wrap，plastic bags，bandages，gloves and medical dressings）are enclosed and wrapped on the surface of the affected part coated with drugs，so as to achieve the therapeutic purpose.Topical application of glucocorticoid encapsulation is the traditional method for the treatment of chronic inflammatory dermatosis，including chronic hypertrophic dermatitis，keratosis dermatosis，verrucous hyperplastic dermatosis，etc.The materials of packets developed from the initial polyethylene plastic materials to the present water gel sticker，and glucocorticoids can be made into a composite preparation with the encapsulated material or used separately.Topical use of glucocorticoids combined with hydrocolloid coated DuoDerm is effective for the treatment of psoriasis，which is also used for the treatment of refractory atopic dermatitis and other chronic inflammatory skin diseases.Another hydrocolloidal film Actiderm plus medi-effect hormone was used for external treatment of metatarsal pustular disease，and the effect was far better than that of strong hormone alone.Hydrocolloidal 3M AcneDressing can significantly improve the condition of acne patients by reducing hand exposure and ultraviolet radiation.These methods which use the hydrocolloidal external materials with strong adhesion and closure，can promote drug absorption，and can reduce scratching.Scratching has the effect of promoting chronic local skin inflammation，thus reduction of scratching has the therapeutic effect.