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Introduction
In recent years, polymers have been widely used for human body, especially in medical application. Not only the nature polymers but also synthetic ones play important role in medical area, and it has been found that mam-made polymers like polyesters and polyamides developed rapidly from time to time. Biomedical polymers have functions not only for making bulky materials used in tissue engineering, but also for the coating and carriers in the nano-particle size for drug release and delivery. This essay introduces some applications of polymers in medical area in terms of implant, plastic surgery and drug delivery system for human body.
Polymers for surgery
Synthetic polymers are the basic types of biomaterials which have wide use in implant surgery for both tissue engineering and plastic surgery. Cell transplantation is the main process in implant surgery which had been attempted 20 years ago to replace organ function. Nowadays scientists have designed new scaffolds which have three-dimensional structure made of natural and synthetic polymers in tissue engineering which can not only repair but also regenerate the tissue we want.
Polymers used for synthetic scaffolds are mostly biodegradable polymers which can be divided into four categories. The first type is common established bioresorbable polymer like polycaprolactone (PCL), poly-L-lactic acid (PLLA) and polyglycolide (PGA). The second one is di-block and tri-block polymer like polylactic acid (PLA). The third type is polymer in clinical trials like polyhydroxyalkanoate (PHA) and poly(etherester amide) (PEEA). The fourth one is polymer that can form by themselves like biomimetic materials and supramolecular polymers.
Apart from cell transplantation, polymers also play important role in the surgery process. Polyfumarate is crosslinkable and injectable material so it has the mainly use in bone tissue engineering. Polypyrrole (PPy) is widely used in nerve tissue engineering because it is electrodeposited, and in neuronal tissue regeneration electronic interaction is an important factor. Semicrystalline polymer like UHMWPE has extremely strength and high water-resistance which is mostly used as joint prostheses in orthopedic implant surgery.
Polymers made suture materials also played an important role in both implant and plastic surgery and the stability and degradability are the main factors to make the selection of these materials. They can be biological or synthetic and both of them can be resorbable or not depends on the function or body part. Resorbable suture materials like polyglactic acid (Vicryl) and collagen- based materials are usually used for tissue which can heal fast like inner organs while non-resorbable suture materials like silk, nylon and polyester are used for tissue which heal slowly like tendons and skin. In modern surgical techniques, suture material is replaced by tissue adhesive and sealants which have lower adhesion strength but can form preferred tight occlusion than suture. The main components used for biological sealants and adhesive are thrombin and fibrin. Gelatin, polysaccharide and collagen can also be used as adhesives but they need to combine with suture materials due to their limited strength.
In recent years, cosmetic surgery has become more and more popular and the domain of plastic surgery is correction of contour defect. Cross-linked silicone elastomer is the main material used in this kind of surgery because it can adhere to soft tissue and bone and help them augment. This kind of polymer can also be used in breast implant surgery as the outer shell or filled implant which can reduce the inflammation caused by contracture and encapsulation.
In facial plastic surgery, nose constructive is the popular but challenging one and the structural cartilage drives the most important role in this surgery. Polymers which are biodegradable like poly-L-lactic acid (PLLA), polyglycolic acid (PGA) and their co-polymers are used as template for cell transplantation to regenerate cartilage. The polymer solution is used as the coating of three-dimensional PGA fibre network and PLLA with high molecular weight is used to provide more strength.
Polymers for drug delivery system
It has been found that pumps and polymers are used for drug release in controlled delivery system which can predict the release time for at least 12 hours. Polymers are preferred because they are cost-effective and have particular size (can be very small) so they have been most commonly used in the system.
Polymers can release drugs in three classic mechanisms – diffusion, chemical reaction (biodegradable system) and solvent-activated system (..).
In diffusion system, drug can be covered by the polymer membrane like a capsule called reservoir system or drug can freely disperse in the whole solid polymer which called a matrix system. In both of these two systems, drug can be released through the pore or polymer backbone in the membrane and rate of this process is limited. Polymer made homogeneous films with no pore are mostly used as the membrane in this system and membranes with micropore can also be used but in this case, drug is released by the diffusion through pores filled with water rather than polymer phase (..). In the matrix system, drug can be dispersed or molecularly dissolved in the polymer matrix and in both of these two situations, diffusion can occur through water-filled pores or via solution-diffusion.
In biodegradable system, polymers used to cover the drug are degraded by enzyme or water which called erodible system, or the bond between polymer and drug is cleaved to release drug which called pendant chain system. Polymers used in biodegradable system should be degradable which means they need labile bonds in crosslinker of backbone to be hydrolysis and proteolysis easily to release drug. Dissolution mechanisms in erodible system are different so polymers used in biodegradable system differ. Polymers which are water-soluble like poly(vinylpyrrolidine) can be solubilized by degradable cross-links. Protonation and hydrolysis of side groups can solubilize polymers like maleic anhydride which are water-insoluble, and some other water-insoluble polymers like poly(ortho ester) and poly(anhydrides) can be solubilized to some small molecules that water-soluble by the cleavage of backbone-chain. However, whether the degradation products are safe is the main issue in biodegradable system. Different kinds of fragment sizes can distribute after degradation which make it challenging to determine their toxicity which may bring risk to patient.
In solvent-activated system, drug is released by the osmotic pressure caused by a tablet which has the osmotic agent inside and covered by a membrane. The inside agent can either be a salt or drug itself, and the outer membrane is semipermeable which means it can only allow water to transfer rather than the agent. There is a hole on the membrane which is single laser-drilled so that the drug can be driven out through this hole when water come in during the release process.
Apart from the release system, polymers can also be used in the delivery process. Common delivery systems like oral administration and injection of drug have difficulty in concentration because drug is delivered through the blood vessel which may make drug absorbed by other healthy tissue before it reaches to the target area. Scientists have invented a catheter device within the polymer matrix which can deliver drug locally to the target tissue by phonophoresis and iontophoresis. A return electrode is located in the body and a supply electrode is located in the catheter, once the current provided between two electrodes, drug in the polymer matrix can be directly delivered to target tissue which can get rid of the undesirable absorption by other tissue.
Conclusion
Biomedical polymers have widely application for human body. In terms of tissue engineering, they can be used as scaffolds for cell transplantation to grow and regenerate tissue because of their carbon-based chemistry which make them closer to the body tissue. At the same time, polymers can be used for some medical materials like suture and adhesives which make the surgery process advanced due to their biodegradability especially in the plastic surgery. In terms of drug delivery system, polymers not only contribute to the process of drug release, but also can be used as a matrix carrier to drive the drug to the target body part which increase the efficiency and decrease the possibility of useless absorption which make the targeted therapy possible. Overall, polymers play the vital role for human body and the development of biomedical polymers is very impressive.
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