Elastomers biocompatibility

The thermo-chemical properties of PHA have been shown to be strongly related to their structure. Thus, PHAscl are mainly brittle thermoplastics with high melting and glass transition temperatures, whereas PHAscl are mainly elastomers. Biocompatibility is another interesting property of PHAs. In this case, PHAmcl have been shown to be more biocompatible than PHAscl, however, they are also more hydrophobic and the hydrophobicity may be an important drawback for such types of applications. Thus, postfermentation modifications of PHAs were also applied to modify their structure. Those modifications consist either of monomeric modifications,... 

Applications. Polymers with small alkyl substituents, particularly (13), are ideal candidates for elastomer formulation because of quite low temperature flexibiUty, hydrolytic and chemical stabiUty, and high temperature stabiUty. The abiUty to readily incorporate other substituents (ia addition to methyl), particularly vinyl groups, should provide for conventional cure sites. In light of the biocompatibiUty of polysdoxanes and P—O- and P—N-substituted polyphosphazenes, poly(alkyl/arylphosphazenes) are also likely to be biocompatible polymers. Therefore, biomedical appHcations can also be envisaged for (3). A third potential appHcation is ia the area of soHd-state batteries. The first steps toward ionic conductivity have been observed with polymers (13) and (15) using lithium and silver salts (78). 

Bakker D, Bakker D, van Blitterswijk CT, Daems WTH, and Grote JJ. Biocompatibility of six elastomers in vitro. J Biomed Mater Res, 1988, 22, 423-439. 

Hybrid organosilicon-organophosphazene polymers have also been synthesized (15-18) (structure ) (the organosilicon groups were introduced via the chemistry shown in Scheme 11). These are elastomers with surface contact angles in the region of 106°. Although no biocompatibility tests have been conducted on these polymers, the molecular structure and material properties would be expected to be similar to or an improvement over those of polysiloxane (silicone) polymers. 

Both types are hydrophobic materials that, depending on the side group arrangements, can exist as elastomers or as microcrystalline fiber- or film-forming materials. Preliminary studies have suggested that these two classes of polyphosphazenes are inert and biocompatible in subcutaneous tissue implantation experiments. 

Elastomers based on PTMEG have excellent microbial and fungus resistance. Their hydrolytic stability make these elastomers prime candidates for use in ground-contact applications, for example, as jacketing material for buried cables. Because of their good biocompatibility, they have also found uses in medical applications, such as catheter tubing. 

Elastomers of this type are usually cross-linked during fabrication, and often contain fillers such as carbon black or iron oxide to reduce the compliance of the elastomer (i.e. to provide a greater resistance to deformation). Such materials are depicted in Figure 3.1. They are used in technology because of their flexibility and elasticity at low temperatures (-60 °C), their resistance to hydrocarbon solvents, oils, and hydraulic fluids, and their fire resistance.145 For these reasons, they are utilized in aerospace and advanced automotive applications. Some interest exists in their development as inert biomaterials, mainly because of their surface hydrophobicity and consequent biocompatibility. 

Block copolymers, polymer blends, polymers at interfaces, liquid crystalline polymers, polymers with novel optical and electronic properties, cross-linked polymers (including elastomers and thermosets), and biocompatible polymers are all areas of active research that are beyond the scope of this chapter. 

Materials are indexed quite adequately. The broad MeSH heading Biomedical and Dental Materials includes such narrower terms as Alloys, Biocompatible Materials, Polymers, and Tissue Adhesives. More precise narrower terms au-e also provided, for example, the many different types of polymers (e.g., cyanoacrylates, elastomers, plastics, and silicones). 

Elastomers of silicone are widely used as biomaterials. In general, silicone elastomers have excellent biocompatibility, inducing only a limited inflammatory response following implantation. In fact, until very recently, it was assumed that silicones were almost completely inert in biological systems. It is now known, however, that certain silicone polymers can provoke inflammatory and immune responses. The biological response to implanted silicone, and the variability of that response among individuals, is the subject of considerable debate and interest. 

Starting with the silicone elastomer hydrocephalus shunt in 1955, silicone elastomer has become widely used as a soft, flexible, elastomeric material of construction for artificial organs and implants for the human body. When prepared with controls to assure its duplication and freedom from contamination, specific formulations have excellent biocompatibility, biodurability, and a long history of clinical safety. Properties can be varied to meet the needs in many different implant applications. Silicone elastomer can be fabricated in a wide variety of forms and shapes by most all of the techniques used to fabricate thermosetting elastomers. Radiopacity can be increased by fillers such as barium sulfate or powdered metals. It can be sterilized by ethylene oxide, steam autoclave, dry heat, or radiation. Shelf-life at ambient conditions is indefinite. When implanted the host reaction is typically limited to encapsulation of... 

Holter s successful development of a silicone elastomer hydrocephalus shunt (4) (Figures 1-2) in 1955 heralded the era of implants. No effective treatment for hydrocephalus was known at the time. Thus, by 1957, only two years after the shunt was first used, and continuing today, essentially every hydrocephalic child born in the developed countries of the world has received a silicone elastomer hydrocephalus shunt implant. Hydrocephalus occurs in approximately one out of every 400 to 600 children born alive. The hydrocephalus shunt is one of the oldest, and also one of the most widely used of all silicone elastomer implants. Some individuals have now had shunt implants for more than 25 years. The excellent biocompatibility of implant grades of silicone elastomer is evidenced by the essential absence of adverse biological response in this long term, large volume use. 

Medical grade silicone elastomers became available in the early 1960 s. "Medical grade" refers to silicone elastomers specifically formulated, manufactured and qualified for implant uses. The formulations contain no materials with potential for biodegradation or adverse biocompatibility. Manufacturing and processing are done under carefully controlled, clean conditions to assure batch-to-batch duplication, and freedom from adulteration, contamination, and cross contamination. Batch-to-batch tests include assessment of chemical, physical, and biological properties. The materials must elicit no cytotoxic reaction by direct contact tissue-cell culture testing (5,6). Qualification of a controlled formulation for implant use typically requires 2-year minimum biocompatibility (host and tissue reaction) ( 7) and... 

After qualification by acute and chronic biocompatibility and biodurability evaluations, clinical studies were conducted to confirm that the implants were highly durable. Medical grade high performance silicone elastomer has now become used in various biomedical applications including construction of flexible bone and joint implants as designed by Swanson ( 9) (Figures... 

Other cardiovascular uses have included coatings on pacemakers and pacemaker lead-wires for purposes of insulation and for achieving biocompatibility. Medical grade silicone elastomer has been widely used as a material of construction in experimental artificial hearts and heart assist devices. Silicone tubing is often preferred for use in roller-type blood pumps during cardiopulmonary bypass. Medical grade silicone elastomer contains no leachable or organic plasticizers and thus contributes minimal contamination in blood contact applications. 

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