Medical applications molecular weight

Purification of poloxamers has been extensively investigated due to their use in medical applications, the intention often being to remove potentially toxic components. Supercritical fluid fractionation and liquid fractionation have been used successfully to remove low-molecular weight impurities and antioxidants from poloxamers. Gel filtration, high-performance liquid chromatography (HPLC), and ultrafiltration through membranes are among the other techniques examined [5]. 

Polyelectrolytes such as the ion exchange plastics form an interesting group of materials because of their ability to interact with water solutions. They have been used in medical applications involving the removal of heavy metal ions from the human body. They can be used to interact with external electric fields and change their physical properties drastically as is illustrated by the fact that some electrically active liquid crystals are polyelectrolytes of low molecular weight. 

Polylactic acid (PLA) has been produced for many years as a high-value material for use in medical applications such as dissolvable stitches and controlled release devices, because of the high production costs. The very low toxicity and biodegradability within the body made PLA the polymer of choice for such applications. In theory PLA should be relatively simple to produce by simple condensation polymerization of lactic acid. Unfortunately, in practice, a competing depolymerization process takes place to produce the cyclic lactide (Scheme 6.10). As the degree of polymerization increases the rate slows down until the rates of depolymerization and polymerization are the same. This equilibrium is achieved before commercially useful molecular weights of PLA have been formed. 

In polyester synthesis via ring-opening polymerizations, metal catalysts are often used. For medical applications of polyesters, however, there has been concern about harmful effects of the metallic residues. Enzymatic synthesis of a metal-free polyester was demonstrated by the polymerization of l,4-dioxan-2-one using Candida antarctica lipase (lipase CA). Under appropriate reaction conditions, the high molecular weight polymer (molecular weight = 4.1 x 10" ) was obtained. 

IMS is a new, developing technique to visualize biomolecule maps in tissue. IMS has opened a new frontier in medicine as well as in clinical applications. Lipids and low-molecular-weight compounds in tissue sections cannot be observed with conventional microscopic or electron microscopic techniques therefore, no distribution map of these molecules in a tissue structure has been described in the scientific literature or in medical textbooks. However, IMS is bringing to light the characteristic distribution map of lipids (Fig. 21.11) this map made a major impact to lipid research. 

Processing requirements for thermoset composites, with specific examples of silicones, were recently reported.514 Composites based on the low molecular weight polysiloxanes for medical applications have been reviewed (in Russian).515 Silicone rubber/hydrogel composites have been evaluated for medical applications.516... 

Important for medical applications, COC is considered as a high purity product with low amounts of low molecular weight extractable compounds. 

The rationale for the development of such libers is demonstrated by their application in the medical field, notably hernoperfusion. where cartridges loaded with activated charcoal-filled hollow fiber contact blood. Low molecular weight body wastes dilTuse through the liber walls and arc absorbed in the fiber core. In such processes, the blood does not contact the active sorbent directly, but faces the nonloxic. blood compatible membrane. Other uses include waste industrial applications as general as chromates and phosphates and as specific as radioaclive/nudcar materials. 

The second method is azeotropic condensation polymerization of lactic acid, which produces high-molecular weight PLA without using chain-extenders or esterification-promoting adjuvants. This type of polymerization needs high reaction rates and thus uses catalysts however, due to the use of catalysts, the PLA produced by this method is not suitable for some applications, such as medical, since any residual catalyst offers toxicity within the polymer, which is harmful for medical applications. In addition to toxicity, residual catalyst degrades PLA in further processing (39). On the other hand, the level of residual catalyst can be reduced with the use of sulphuric acid (55,56). 

Polylactic acid was first discovered in the 1930s when a DuPont scientist, Wallace Caruthers, produced a low molecular weight PLA product. In 1954, DuPont patented Carothers process. Initially the focus was on the manufacture of medical grade applications due to the high cost of the polymer, but advances in fermentation of glucose, which forms lactic acid, has dramatically lowered the cost of producing lactic acid and significantly increased interest in the polymer. 

---