7 commonly used medical plastics

1. The cost is low, and it can be reused without disinfection, and is suitable for the production of disposable medical devices;

2. The processing is simple, and its plasticity can be processed into various useful structures, while metal and glass are difficult to manufacture into products with complex structures;

3. Tough, flexible, not as easy to break as glass;

4. It has good chemical inertness and biological safety.

These performance advantages make plastics have a wide range of applications in medical equipment, including polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), polystyrene (PS), polycarbonate (PC), ABS, Polyurethane, polyamide, thermoplastic elastomer, polysulfone and polyether ether ketone, etc. Blending can improve the properties of plastics, and reflect the best performance of different resins, such as polycarbonate/ABS, polypropylene/elastomer and other blending modification.

Due to the contact with the liquid medicine or the human body, the basic requirements of medical plastics are chemical stability and biological safety. To put it simply, the components in the plastic material cannot be precipitated into the liquid medicine or the human body, will not cause toxicity and damage to the tissues and organs, and are non-toxic and harmless to the human body. In order to ensure the bio-safety of medical plastics, medical plastics that are usually sold on the market are certified and tested by medical authorities, and users are clearly informed which grades are medical grade. Medical plastics in the United States usually pass FDA certification and USPVI biological testing, and medical-grade plastics in China are usually tested by Shandong Medical Device Testing Center. At present, there are still a considerable number of medical plastic materials in China that have not undergone strict biosafety certification, but with the gradual improvement of laws and regulations, these conditions will be more and more improved.

According to the structure and strength requirements of the equipment products, we choose the appropriate plastic type and the appropriate brand, and determine the processing technology of the material. These properties include processing performance, mechanical strength, use cost, assembly method, sterilization and so on. The processing properties and physical and chemical properties of several commonly used medical plastics are now introduced.

1. Polyvinyl chloride (PVC,  Polyvinyl chloride) 

PVC is one of the largest plastic varieties in the world. PVC resin is white or light yellow powder. Pure PVC is atactic, hard and brittle, and is rarely used. According to different purposes, different additives can be added to make PVC plastic parts present different physical and mechanical properties. Adding an appropriate amount of plasticizer to the PVC resin can make a variety of hard, soft and transparent products. Hard PVC does not contain or contains a small amount of plasticizer, has good tensile, bending, compression and impact resistance, and can be used as a structural material alone. Soft PVC contains more plasticizers, its flexibility, elongation at break, and cold resistance increase, but its brittleness, hardness, and tensile strength decrease. The density of pure PVC is 1.4g/cm3, and the density of PVC plastic parts added with plasticizers and fillers is generally in the range of 1.15~2.00g/cm3. According to market estimates, approximately 25% of medical plastic products are PVC. Mainly because of the low cost of the resin, wide application range, and easy processing. Medical PVC products include: hemodialysis tubing, breathing mask, oxygen tube, etc.

2.  Polyethylene (PE,  Polyethylene) 

Polyethylene plastic is the most produced variety in the plastic industry. It is milky white, odorless, odorless and non-toxic glossy waxy particles. It is characterized by low price and good performance. It can be widely used in industry, agriculture, packaging and daily industries, and it occupies a pivotal position in the plastics industry.

PE mainly includes low-density polyethylene (LDPE), high-density polyethylene (HDPE), and ultra-high molecular weight polyethylene (UHDPE). HDPE has fewer branches on the polymer chain, higher relative molecular weight, crystallinity and density, higher hardness and strength, poor opacity, and higher melting point. It is often used in injection molded parts. LDPE has many branches, so the relative molecular weight is small, the crystallinity and density are low, it has good flexibility, impact resistance and transparency. It is often used in blown film and is an alternative to the widely used PVC. . It is also possible to mix HDPE and LDPE according to performance requirements. UHDPE has high impact strength, low friction, stress crack resistance and good energy absorption characteristics, making it an ideal material for artificial hip, knee and shoulder connectors.

3. PP (polypropylene) 

Polypropylene is colorless, odorless and non-toxic. It looks like polyethylene, but it is more transparent and lighter than polyethylene. PP is a kind of thermoplastic with excellent performance. It has the advantages of small specific gravity (0.9g/cm3), non-toxicity, easy processing, impact resistance and flexural resistance. It has a wide range of applications in daily life, including woven bags, films, turnover boxes, wire shielding materials, toys, car bumpers, fibers, washing machines, etc. Medical PP has higher transparency, better barrier properties and radiation resistance, making it widely used in medical equipment and packaging industries. Non-PVC material with PP as the main body is a substitute for the widely used PVC material. 

4. Polystyrene (PS) and K resin

PS is the third largest plastic species after polyvinyl chloride and polyethylene. It is usually processed and applied as single-component plastics. Its main characteristics are light weight, transparency, easy dyeing, and good molding and processing properties, so it is widely used in Japan. Use plastics, electrical parts, optical instruments and cultural and educational supplies. The texture is hard and brittle, and has a high thermal expansion coefficient, therefore, it limits its application in engineering. In recent decades, the development of modified polystyrene and styrene-based copolymers has overcome the shortcomings of polystyrene to a certain extent. K resin is one of them. 

K resin is a copolymer of styrene and butadiene. It is an amorphous polymer, transparent, odorless, non-toxic, with a density of about 1.01g/cm3 (lower than PS and AS), and higher impact resistance than PS , Good transparency (80～90%), heat distortion temperature of 77℃, the hardness of the butadiene component contained in the  K material is also different, due to the good fluidity of the K material, the processing temperature range is wide, so its Good processing performance.  Main uses in daily life include cups, lids, bottles, cosmetic packaging, hangers, toys, PVC substitute products, food packaging and medical packaging supplies, etc.  

5. ABS, acrylonitrile-butadiene-styrene copolymer (Acrylonitrile Butadiene Styrene copolymers) 

ABS has certain rigidity, hardness, impact resistance and chemical resistance, radiation resistance and resistance to ethylene oxide disinfection. The medical application of ABS is mainly used as surgical tools, roller clamps, plastic needles, tool boxes, diagnostic devices and hearing aid housings, especially the housings of some large-scale medical equipment.

6. Polycarbonate (PC, Polycarbonate) 

The typical characteristics of PC are toughness, strength, rigidity and heat-resistant steam sterilization. These characteristics make PC a priority choice for blood dialysis filters, surgical tool handles and oxygen tanks (when in surgical heart surgery, this instrument can remove blood from the Carbon dioxide, increase oxygen);  PC's medical applications also include needle-free injection systems, perfusion instruments, blood centrifuge bowls and pistons. Taking advantage of its high transparency, the usual myopia glasses are made of PC.

7. PTFE (Polytetrafluoroethylene) 

PTFE resin is white powder, waxy in appearance, smooth and non-sticky, and is the most important plastic. PTFE has excellent performance, which is not comparable to ordinary thermoplastics, so it is known as the "Plastic King". Its friction coefficient is the lowest among plastics, and it has good biocompatibility. It can be made into artificial blood vessels and other devices directly implanted in the human body.