As+ implantation

Polyurethanes as Biomaterials. Much of the progress in cardiovascular devices can be attributed to advances in preparing biostable polyurethanes. Biostable polycarbonate-based polyurethane materials such as Corethane (9) and ChronoFlex (10) offer far-reaching capabiUties to cardiovascular products. These and other polyurethane materials offer significant advantages for important long-term products, such as implantable ports, hemodialysis, and peripheral catheters pacemaker interfaces and leads and vascular grafts. 

The materials which are currently specified in the National and International Standards for use as implants together with their eissociated mechanical properties are shown in Table 2.23. 

Duration ofcontact devices are considered transient if intended for continuous use for less than 60 minutes, short term if intended for continuous use for not more than 30 days, and long term if intended for continuous use for more than 30 days. Devices that are intended to be introduced into the body or to replace an epithelial surface or the surface of the eye by a surgical procedure for more than 30 days are considered as implants. 

Based on these studies, it is obvious that poly(N-acylhydroxy-proline esters) are very slowly degrading polymers. These materials may therefore be useful for long-term applications, such as implantable, multiyear contraceptive formulations. For such applications the degradation rates of poly (lactic acid)/poly (glycolic acid) devices would probably be too rapid. 

Figure 11. Grazing incidence X-ray diffraction patterns (a) and OD spectra (b) of the as-implanted Au-Fe sample and of the sample after annealing at 600 °C for 1 h in reducing atmosphere, (c) Hysteresis loops of the as implanted Au-Fe and of the only-Fe samples measured at 3 K.

Ethylene vinyl acetate has also found major applications in drug delivery. These copolymers used in drug release normally contain 30-50 wt% of vinyl acetate. They have been commercialized by the Alza Corporation for the delivery of pilocarpine over a one-week period (Ocusert) and the delivery of progesterone for over one year in the form of an intrauterine device (Progestasert). Ethylene vinyl acetate has also been evaluated for the release of macromolecules such as proteins. The release of proteins form these polymers is by a porous diffusion and the pore structure can be used to control the rate of release (3). Similar nonbiodegradable polymers such as the polyurethanes, polyethylenes, polytetrafluoroethylene and poly(methyl methacrylate) have also been used to deliver a variety of different pharmaceutical agents usually as implants or removal devices. 

The mechanical pump approach employs miniature mechanical devices, such as implantable and portable infusion pumps and percutaneous infusion catheters, to deliver drugs into appro priate blood vessels or to a discrete site in the body. When compared with the... 

Most of the bio-nanocomposites tested as implants for bone regeneration are based on the assembly of HAP nanoparticles with collagen, trying to reproduce the composition, biocompatibility and suitable mechanical properties of natural bone. 

As seen above, implanting cells without scaffold support or scaffolds without cells may not prove efficacious. NP allograft studies in a lapine disc degeneration model indicate that injection of NP cells alone does not mitigate degeneration as effectively as implanting NP allografts, and can result in the formation of fibrotic... 

Fig. 10. Hydrogen distribution in c-Si after annealing at the indicated temperature. The solid line indicates the initial (as implanted) distribution. [Courtesy of C.W. Magee and C.P. Wu]...

Fig. 14. The 1-H line at 6 K in n+ GaAsiSi implanted with a 500 nA current of 190 keV protons. The spectral resolution is 0.4 cm 1, (a) as implanted, (b) after 20 min. annealing at 200°C. (c) after additional 20 min. annealing at 400°C. The apparent increase of the absorption coefficient in (c) is due to the diffusion of hydrogen throughout the Si-doped layer. B. Pajot et al., Mat. Res. Soc. Symp. Proc, 104, 345 (1988). Materials Research Society.

Our researchers have worked very hard to accomplish our goals by doing things we felt would enhance our synthetic bone materials and their performance to enable them to equal and often exceed the performance of autograft as implants as well as in other types of bone augmentation and replacement. The nonporous tooth enamel solid calcium phosphate materials have flexural strengths of over 20,000 lb in.2 However, without pores it would take an extremely long time to resorb this nonporous bioceramic. 

The surface tension of polymers (synthetic polymers such as plastics, biopolymers such as proteins and gelatin) is indeed of much interest in many areas. In industry where plastics are used, the adhesion of these materials to other materials (such as steel, glass) is of much interest. The adhesion process is very complex since the demand on quality and control is very high. This is also because adhesion systems are part of many life-sustaining processes (such as implants, etc.). The forces involved in adhesion need to be examined, and we will consider some typical examples in the following text. 

In this chapter, we review the current status of doping of SiC by ion implantation. Section 4.2 examines as-implanted depth profiles with respect to the influence of channeling, ion mass, ion energy, implantation temperature, fluence, flux, and SiC-polytype. Experiments and simulations are compared and the validity of different simulation codes is discussed. Section 4.3 deals with postimplant annealing and reviews different annealing concepts. The influence of diffusion (equilibrium and nonequilibrium) on dopant profiles is discussed, as well as a comprehensive review of defect evolution and electrical activation. Section 4.4 offers conclusions and discusses technology barriers and suggestions for future work. 

In this section we will review the current knowledge of ion damage accumulation and chemical profiles in as-implanted single-crystal SiC. We will investigate the effects of the postimplant annealing process in Section 4.3. Results for the 4H- and 6H-SiC polytypes will also be presented. 

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