Characteristics of implantable

Management approaches have therefore emphasized the need for prevention through the addition of good sterile technique at the time of insertion. Manufacturers have also responded by using materials and creating surface characteristics of implanted materials inclement to microbial attachment. Likewise the use of prophylactic antibiotics at the time of insertion of deep-seated devices such as joint and heart valve prostheses has further reduced the risk of infection. Once a medical device becomes infected, management is difficult. Treatment with agents such as flucloxacillin, vancomycin and most recently linezolid is often unsuccessful and the only course of action is to remove the device. 

First, we note the improvement of the mechanical characteristics of implantable devices that incorporate carbon nanotubes. Even if it is quite difficult to identify which one of stmctural, material chemistry, mechanical strength or other features prevails in a fibrous implantable device s success, it is assumed that carbon nanotubes strength may have a positive impact on soft tissues matrix. 

Microelectronics applications require the ability to pattern features with dimensions as small as t pn. To enable processing by techniques that are standard in the industry, there are numerous materials requirements on solubility, thermal stability, and morphology. In this section, we discuss the materials requirements for proces-sible films, as well as the characteristics of implanted polymers that enable their processing. 

Biomaterials. Just as stem designs have evolved in an effort to develop an optimal combination of specifications, so have the types of metals and alloys employed in the constmction of total joint implants. Pure metals are usually too soft to be used in prosthesis. Therefore, alloys which exhibit improved characteristics of fatigue strength, tensile strength, ductihty, modulus of elasticity, hardness, resistance to corrosion, and biocompatibiUty are used. 

Properties of the deposit The changes in surface properties of implanted materials which can be achieved are very considerable. Physical and chemical properties can be widely varied to produce special characteristics, some of which cannot be obtained in any other way. 

While it would be difficult to enumerate all of the efforts in the area of implants where plastics are involved, some of the significant ones are (1) the implanted pacemaker, (2) the surgical prosthesis devices to replace lost limbs, (3) the use of plastic tubing to support damaged blood vessels, and (4) the work with the portable artificial kidney. The kidney application illustrates an area where more than the mechanical characteristics of the plastics are used. The kidney machine consists of large areas of a semi-permeable membrane, a cellulosic material in some machines, where the kidney toxins are removed from the body fluids by dialysis based on the semi-permeable characteristics of the plastic membrane. A number of other plastics are continually under study for use in this area, but the basic unit is a device to circulate the body fluid through the dialysis device to separate toxic substances from the blood. The mechanical aspects of the problem are minor but do involve supports for the large amount of membrane required. 

A method of assessing the toxicity of implants has been proposed based on the effects on cell ultrastructure in organ cultures, on cell surface characteristics, and cell population doubling times. The effects have been correlated with hemorrhage, fibrosis, and necrosis, respectively (103). Poly-e-caprolactone was stated to give minimal tissue reaction and could not be scored in these tests. 

Figure 45. Current—overpotential characteristics of porous gold electrodes on YSZ at 770 °C and = 1 atm, comparing YSZ that has, and has not, been implanted with 8 X 10 atoms/cm of Fe at 15 kV. (Reprinted with permission from ref 99. Copyright 1992 Elsevier.)... 

Figure 6.6 (a) Structure of an implanted emitter BJT. (b) I-V characteristics of an implanted emitter 4H-SIC. The device shows greater common emitter current gain in the reverse-active region. 

Figure 6.28 shows the I-V characteristics of a single epitaxial emitter cell. A maximum current gain of about 15 is obtained. This current gain is extremely sensitive to the base contact implant spacing from the edge of the emitter mesa (in the... 

The effects of various formulation factors on the in vitro release characteristics of spherical polymethylmethacrylate implants were studied. Physical and mathematical models were proposed to describe the in vitro release profiles. The in vitro release data could be described by a biexponential equation of the following type fraction of tobramycin remaining in the implant at time t=Aerai+BQ, where a, and P represent the rate constants for the initial rapid and subsequent slow phases of release. The influence of drug loading, volume of dissolution medium, implant size and type of cement and the incorporation of water-soluble additives on the release profiles and a and P rate constants is described. 

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