Implantable medical devices biocompatibility

The lag between the time that nitinol, was first produced and the time it was used commercially in medical devices was due in part to the fear that nickel would leach from the metal and not be tolerable as a human implant. As it turns out, with a correct understanding of the surface electrochemistry and subsequent processing, a passivating surface layer can be induced by an anodizing process to form on the nitinol surface. It is comprised of titanium oxide approximately 20 mn thick. This layer actually acts as a barrier to prevent the electrochemical corrosion of the nitinol itself. Without an appreciation for the electrochemistry at its surface, nitinol would not be an FDA-approved biocompatible metal and an entire generation of medical devices would not have evolved. This is really a tribute to the understanding of surface electrochemistry within the context of implanted medical devices. 

Medical devices are used for a variety of functions in humans. This review elaborates on the types of tests used to evaluate biocompatibility of the interactions between man-made medical devices and host tissues and organs. The outcome of the response depends on the site of implantation, the species of the host, the genetic makeup of the host, the sterility of the implant, and the effect the device has on biological processes. Biological processes involved in host tissue responses to implantable medical devices reflect activation of a series of cascades that require blood proteins or other components found in the blood. 

The identification of the fundamental biological requirements of a biomaterial first requires an identification of how the biomaterial is to be used and in what type of implant medical device or prosthesis the biomaterial is to be used. Having identified the intended application of the biomaterial, the identification of the tissues that will contact the biomaterial and the implant duration of the biomaterial, medical device or prosthesis is necessary. Once these parameters have been defined, appropriate in vitro and in vivo assays can be sdected to identify the success or failure of the biomaterial and its intended application. A broad perspective in the selection of in vitro and in vivo biocompatibility assays is necessary to not only identify adverse reactions but also identify reactions that indicate the successful function of the biomaterial in its intended application. Adverse tissue responses do not necessarily reject a biomaterial from use in a medical device or prosthesis. Adverse tissue responses do require a risk... 

The above definition also stipulates that the biocompatibility of a material has to be qualified by reference to the specific application. The response to some very common and popular biomaterials may vary quite considerably and some of the major problems of implantable medical devices have been caused by a misunderstanding about transferability of biocompatibility data. To recognize the very effective performance of a material under one set of conditions but then to assume that the same material can perform equally well under entirely different circumstances is inherently dangerous since it takes into account neither the variations one might expect to see in the host response from site to site nor the fact that what is appropriate for one situation may not be appropriate for another. 

Several elastomer matrices have been used to improve the mechanical properties of PLA, but special attention has been paid to polyurethane elastomer (PU) ° and natural rubber (NR). Lt is well known that thermoplastic polyurethane elastomers are widely used in a diverse range of implantable medical devices due to a unique combination of toughness, durability, flexibility, biocompatibility and biostability. Ln addition, it... 

Figure 13.1 Textile science contribution to smart fibrous implantable medical devices designing. Biocompatibility and well-functioning implants in the postoperative stage will be focused on in this review.

This example of vascular grafts devices points out the evolution of fibrous implantable medical devices and highlights the great potential offered by each scale level of fibrous structures for biocompatibility improvements. Fibers as well as whole fibrous stmctures should be considered as implantable devices that have inherent abilities to interact with the biological environment at each of the three predetermined scale levels. Study of characteristics and specificities of fibers, fibrous siuface, and fibrous volume should then provide a more forward-looking approach in the textile substitute s area for design and achievement of smart medical implantable textile devices. 

Discontinuity is the basis of fibrous stmctures that are obtained by fibers interlacing. This implies a number of characteristics that can be exploited to promote biocompatibility of implantable medical devices. 

Fiber interlacing process allows to obtain cohesive fibrous stmctures that demonstrated clear benefits to be used as implantable medical devices with improved biocompatibility features. However, these fibrous cohesive structure features may need to be scaled and organized to better match the applications. These adjustments are mostiy related to textile processes. Besides simple fibrous cohesive structures, textile processes could help to give controlled fibrous patterns with desired scale, orientation, and physical properties. 

Material biocompatibility of fibrous implantable medical devices... 

Although the range of textile materials is very large, focusing on implantable devices narrows the scope. Chemical biocompatibility is an essential requirement. A close look reveals that only about 50 materials are commonly used compared to more than 1.5 Mio materials that exist. This shows how necessary it is to take into account chemical constraints of implantable medical devices to meet the requirements of standards and regulatory processes (Table 13.3). 

In the design process of an implantable medical device, the place and the role of material could be of several natures, which orients the material choice. Depending on the intended medical application, the material of the implantable device could be preferentially selected for its chemical, physical, mechanical, or other specific features. However, a suitable combination of these features is commonly recommended for improved biocompatibility. 

For instance, equiatomic nickel-titanium alloy (nitinol) is a very attractive material for biomedical applications. However, the high nickel content of the alloy and its potential influence on biocompatibility is an issue for nitinol-composed devices. Corrosion resistance of nitinol components from implantable medical devices should be assessed according to regulatory processes and standard recommendations. It is now well known that nitinol requires controlled processes to achieve optimal good life and ensure a passive surface, predominantly composed of titanium oxide, that protects the base material from general corrosion. Passivity may be enhanced by modifying the thickness, topography, and chemical composition of the surface by selective treatments [46]. 

In this chapter we have chosen not to focus on specific examples of smart textiles application in order to avoid narrowing the field of smart implantable fibrous medical devices to a few innovative textile properties. Contrariwise, fiber characteristics are pointed out to show that all of them, in a prospective designing approach, could achieve smart features in the implantable device area. However, we are limited in exploratory areas using new materials because a decline is needed to be certain that a material is accepted by the body. Given the diversity of appreciation of smart appearance, as well as the implantable medical device aspect, we have focused on the biocompatibility and biointegration of substitutes in their environment. This theme therefore needs to be complemented by other approaches such as the concepts of smart attitude and implantable device as related in Fig. 13.1. 

Biocompatibility is the first thing that the packaging engineer should consider when designing a hermetic package for an implantable medical device, as it is the package that makes direct contact with body tissue. It is critical that implantable medical devices do not elicit any undesirable local or systemic effects in the human body. In addition, the package materials should be stable and must be able to withstand attack from a harsh ionic body environment. 

Biocompatible materials that have been successfully used for implantable medical device packaging include titanium and its alloys, noble metals and their alloys, biograde stainless steels, some cobalt-based alloys, tantalum, niobium, titanium-niobium alloys, Nitinol, MP35N (a nickel-cobalt-chromium-molybdenum alloy). 

Diffusion bonding eliminates any foreign material as needed in brazing so it would be preferred for implantable medical device applications. Alumina can be diffusion bonded to a few biocompatible metals including tungsten, platinum, molybdenum, stainless steel, and niobium [58,61]. Zirconia has been successfully diffusion... 

The international standard organization ISO 10993 standard plays an important role in the assessment of the biocompatibility of a medical device. In principle, a great number of tests have to be undertaken depending on the intended use of the medical device. The standard describes tests on toxicity, carcinogenicity, and hemocompatibility, among others. Some of these tests are simple in vitro tests, while others require extensive animal experiments. For implanted medical devices,... 

Biocompatibility of conducting polymers has been studied for biomedical applications including implantable medical devices [53, 61, 76]. Biocompatible polymer... 

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