Thermoplastic polyurethane(TPU) in medical applications

Thermoplastic polyurethane (TPU) is a linear block polymer consisting of soft chain segments (long-chain oligomeric diols) and hard chain segments (diisocyanates and chain expanders). Due to the strong polarity of the hard chain segments, the microphase regions of the hard chain segments are formed by hydrogen bonding and distributed in the soft chain segment matrix to form a kind of physical crosslinking point, thus giving the elastomer the elastic recovery properties of vulcanised rubber. This structure makes the polyurethane biocompatible due to the difference in polarity between the soft and hard chain segments in its molecular structure. Thanks to its easy handling and excellent properties, TPU has been used in a wide range of medical and healthcare products, for example as a medical material for long-term and short-term implantation in the human body.

The properties of TPUs depend on the type and ratio of the raw material diisocyanate, oligomeric diol and short chain diol extender. The two commonly used diisocyanates in the preparation of medical polyurethane elastomers are the aromatic 4,4'-diphenylmethane diisocyanate (MDI) and the aliphatic methylene dicyclohexyl diisocyanate (HMDI).Commonly used oligomeric polyols include polytetramethylene ether glycol (PTMEG), polyester diols and polycarbonate diols. Due to the excellent mechanical strength, water resistance and biocompatibility of polyurethane elastomers made from PTMEG and MDI or HMDI, PTMEG is an important oligomeric diol and polyester diol for medical grade polyurethane materials as well. Currently, attention is focused on polyester diols on polycarbonate, the most commonly used chain extender being 1,4-butanediol.

Examples of several medical grade TPU raw materials, properties and brands are as follows:

(1) TPUs based on HMDI or hexamethylene diisocyanate (HDI), PTMEG and BD as aliphatic polyether urethanes have good biocompatibility, strength and processability. For example, the medical polyurethane Tecoflex from Thermedics Company (formerly Thermo Electron) is composed of PT-MEG-HMDI-BD. In the 1980s, the medical polyurethane Adiprenelw 500 from DuPont in the USA was made from PTMEG-HDI-BD.

(2) TPUs such as aromatic polyether urethanes are based on MDI, PTMEG and BD. They have similar biocompatibility compared to aliphatic polyether urethanes and their physical strength, solvent resistance and high temperature processability are superior to those of aliphatic TPUs. they are an early type of TPU with a higher grade. The disadvantage is that it will turn yellow if exposed to light or gamma rays for long periods of time.Methylene-4,4'-diphenyldiamine (MDA) may be formed if the operating temperature is not suitable. Trade names are: BF Goodrich's Estane, DOW Chemical Company's (formerly Upjohn Company) Pellethane, Thermedics Company's Tecothane, Bayer USA's Texin, JPS USA Elastomers' Stevens range of TPUs MP-1880, MP-1882 and MP1890, Elastollan SP806 from Elastogran, Germany, etc.

(3) Aliphatic polycarbonate polyurethane, this TPU, is based on HMDI and polycarbonate diols. It has similar biocompatibility to polyether urethanes, better resistance to oxidative degradation and biological stability, and good strength and workability. Commodities include Carbothane from Thermedics and Chronoflex from PolyMedica Industries. this TPU is resistant to enzyme-induced degradation and can be used in artificial hearts and blood vessels.

(4) Aromatic polycarbonate urethanes are made from MDI, polycarbonate diol and BD. it has similar biostability and biocompatibility to aliphatic polycarbonate urethanes and the strength and solvent resistance of aromatic polyurethanes. The disadvantage is that it can produce MDA and yellowing. Commodities include Bionate from the US Polymer Technology Group, among others.

(5) Aromatic polyester urethanes are made from polycaprolactone or polyadipate MDI and BD. These TPUs are stronger than PTMEG polyurethanes but are susceptible to hydrolysis and microbial attack. Commodities include BF Goodrich's Estane and Dow Chemical's Pellethane.

(6) Polyurethane-urea elastomers such as Texin 5590 from Bayer (USA) is an aliphatic polyether urea TPU designed for contact with blood and for short-term implantable devices such as tubular materials and catheters. The medical polyurethane elastomer Biomer from Ethicon Corporation (USA) is formed by reacting PTMEG and MDI to form prepolymers with isocyanate capped ends.Linear polyurethane-urea block polymer cast in dimethylformamide (DMF) solvent with ethylenediamine as chain enhancer. corvita is a TPU elastomer material developed by Corvita in the 1990s for use in human organ implants, the composition of which is not disclosed.It is said to have properties similar to those of the widely used aromatic polyether polyurethanes, but is not affected by human enzymes. Degradation. The company is also developing polyurethane copolymers and composites for internal implants as well as other polymers.

In addition to TPUs, some polyurethane elastomer prepolymers and copolymers can also be used for medical applications. For example, Cardiothane from Controltron USA is a polyurethane-siloxane elastomer.It is a cross-linked polymer formed by adding a larger molecular weight siloxane cross-linker (terminal acetoxysiloxane) to a prepolymer made from MDI and polytetrahydrofuran ether diol and cross-linking it with a trace of water, using the solution cast method.Avcothane is a polyurethane-polydimethylsiloxane block copolymer developed by Avco-Ereto in the USA. The material is used in external circulation blood pumps. It can be impregnated and moulded and has good mechanical properties. The mechanical properties can meet the demanding requirements of an artificial heart.Polyurethane elastomers can be used in various forms for medical treatments. For example, they can be sold as PTMEG-MDI prepolymers, which can be cast in situ according to the desired shape. It is also possible to make microporous elastomers.

Applications of polyurethane elastomers. Polyurethane was first used as a material for fracture repair and then successfully as a secondary coating for sutures in vascular surgery. Since the 1970s, polyurethane has received attention as a medical material. In the early 1980s, polyurethane elastomers were successfully used in artificial heart transplants and further developed the use of polyurethane materials in biomedical applications.

The artificial heart assist device is used for the elastomer and encapsulation of the artificial heart septum and requires good biocompatibility and blood compatibility, as well as excellent mechanical strength, especially resistance to bending. If the heart beats at 70 beats per minute, the implanted human heart will have to withstand 40 million bends per year, or 400 million if implanted for 10 years.The artificial heart was developed in the 1960s and the artificial heart balloon was made of silicone rubber and natural rubber. Although it was biocompatible, it did not meet the requirements for flexural resistance. The polyurethane elastic balloon, which was successfully developed in the early 1980s, has been tested to withstand more than 500 million flexion cycles and has been successfully used in artificial hearts.Therefore, all countries in the world tend to use polyurethane materials for the development of artificial hearts and assistive devices. Polyurethane elastomeric diaphragm materials for use in artificial hearts are polyether thermoplastic polyurethanes, which are usually composed of polytetrafuran ether ethylene glycol (PTMEG)-MDI (or HDI)-BD or ethylenediamine, solution cast or injection moulded. To take advantage of the excellent biocompatibility of silicone rubber, polyurethane-silane block copolymers are also used.