Polymer gel is a special dispersion system with no fluidity, but it often contains a lot of liquid inside. The gel used in pharmaceutical preparations has the advantages of rich drug load, lasting effect and easy peeling, and it is a common external drug type with the feeling of ice and cool when applied to the skin. Generally, external dosage forms containing gel ingredients include gel paste (original cataplasm) and external gel (general daubing gel).

The Babu ointment type originated from Japan, and the word "Babu" is a transliteration of its Japanese name (パップ) to distinguish it from traditional plasters (ープ). In European and American countries, Babu ointment is usually translated as cataplam, Poultice or hydrogel patch. The traditional Japanese cataplasm refers to a paste like dressing, which originated in ancient times when Japanese women pounded vegetables and fruits to make a paste like film for their face. In the 1970s, with the industrial production of Babu ointment in Japan, this dosage form was widely used and introduced to the Chinese and European markets in the 1980s. The research and development of catapults in China began in the early 1980s. Cataplasm was officially included in the 2000 edition of the Chinese Pharmacopoeia for the first time. The 2010 edition of the Chinese Pharmacopoeia changed the name of cataplasm to gel paste.

Figure 1 Samples of Okayama Dapeng Pharmaceutical gel Patch

Gel paste usually consists of three layers: backing layer, paste layer and protective film. Among them, the paste layer, also known as the drug repository, is the main structural layer of gel paste, usually the thickest layer, composed of hydrogel matrix, in which Chinese (Western) drug powder or Chinese herbal extract is dispersed.

Fig. 2 Structure diagram of general gel paste

图源：互联网

Traditional rubber paste (commonly known as rubber paste), most specifications of home-made hot-melt adhesive paste are 4~10cm in length and width, while the size of gel paste is 13.6cm * 10cm commonly used at home and abroad; The area covered by gel plaster is 1.94~5.67 times that of the traditional rubber plaster (refer to the preparation specification, 4cm * 6cm, 7cm * 10cm). For the area of the back and waist that is relatively large and the range of daily activities is not intense, the traditional plaster needs 4~8 patches to "cover", while the gel plaster can be covered by 2~4 patches symmetrically.

Traditional rubber ointments have a thickness of only about 0.1mm, and due to poor drug compatibility of rubber, the drug content is usually only a few percent. The thickness of gel paste matrix can reach 1-3mm, and its unique three-dimensional network three-dimensional structure makes the drug capacity larger (can reach more than 30%). Therefore, the gel patch has a large drug loading capacity, which is very suitable for multi-component, large dosage drug use, especially to meet the demand for high drug loading of external patches of traditional Chinese medicine; In addition, gel paste preparations are generally more than 12g, and the paste matrix is a polymer material with good elasticity, which can greatly protect the affected parts from the inevitable external forces such as squeezing, pressing, and touching in daily life, play a buffer protection, reduce the stimulation and pain of external forces, and make the patients more comfortable to wear. gel paste can really be described as protecting the affected parts, and it is thick enough to carry drugs.

For patients with acute soft tissue injury in the early stage, if there is no skin damage to the affected area, cold compress can be applied locally within 24 hours of injury, which can cause skin and blood vessels to contract, tissue edema to subside, and play a role in hemostasis, swelling reduction, and pain relief. The gel paste takes away the excess heat in the body through the vaporization of water in the hydrophilic polymer gel to achieve the effect of cooling. The gel plaster can absorb the excess heat generated by the inflammatory reaction of the affected part for a long time while delivering the therapeutic drugs precisely locally, playing a certain physical cooling effect, reducing the pain of patients, and making the application more comfortable.

Although the non-woven fabric backing layer has been domestically produced, there is still a significant gap in its elastic performance and deformation recovery performance compared to imported non-woven fabrics. This is not only reflected in the stability and yield of industrial production preparations, but also in the performance of finished gel pastes. The gel pastes listed in Japan and the United States can have a large tensile range, and significantly reduce the adhesion of the surface of the pastes. For fingers, wrists, ankles, and other areas, cutting, stretching, wrapping, and applying can provide better wrapping and support for the affected area, making it more comfortable.

Even after punching, the breathability of rubber patches is still poor, and prolonged application can cause discomfort in whitening the local skin. The backing material of gel paste is usually non-woven fabric, which is permeable and breathable, does not hinder the normal breathing of the skin, and is comfortable to apply. The gel paste contains a lot of water (usually more than 50%), which helps soften the cuticle. In addition, its thermal insulation is also better, which is conducive to further opening the stratum corneum and enhancing transdermal absorption. Many gel plasters are also added with transdermal absorbent, which additionally improves the transdermal efficiency and is beneficial to the efficacy.

The gel plaster has excellent thickness, viscosity and elasticity, which makes it easier for patients to apply it. Even if they are accidentally curled and stuck, they can easily stretch and apply medicine again. The gel paste has a lasting viscosity, so it is also convenient and reliable to apply it again after taking part in bathing, swimming and other activities. The application site has strong comfort, no peeling pain, no odor, and no skin patches, significantly improving the patient's compliance.

Common sports injury sites