Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 25

The design of a biomaterial for a specific application starts with the selection of a material with certain inherent properties. Then materials science, biological and engineering principles and procedures are utilized to design and create an implant with the requisite properties for the intended application. Important factors that should be considered in biomaterials design and selection are summarized in the following sections.

 Chemical composition: It is well known in materials science that the chemical composition (or atomic structure) of a material controls its intrinsic properties, such as mechanical strength and stiffness, whether the material is brittle or not, its ability to degrade in an aqueous environment, whether the material is electrically conducting or not, and so on. Consequently, the chemical composition is perhaps the first factor to be considered in selecting a material for use as a biomaterial.

 Biocompatibility: The ability of a material to perform with an appropriate host response in the intended application and its continued function is a critical factor in the selection of a biomaterial. Biocompatibility, as described earlier, is a property of both the biomaterial and the tissue system in which it is implanted, or with which it interfaces. The biomaterial should not, for example, be toxic or release ions or molecules that are toxic locally or systemically.

 Mechanical properties: All tissues (and organs) in the body have some characteristic mechanical properties, such as strength and stiffness. Consequently, the mechanical properties of a biomaterial are important in determining its performance in vivo. A useful guideline is that a biomaterial used to repair a diseased or damaged tissue should have mechanical properties that are comparable to the host tissue. However, this is often difficult to achieve. A large mismatch in mechanical properties between an implant and the host tissue can lead to adverse biological effects. The use of a strong metal implant to repair a bone defect can, for example, lead to resorption and weakening of the surrounding bone and, eventually, to bone fracture. The mechanical properties of a material can also influence the response of cells and, thus, they can determine the ability to regenerate a specific tissue. For example, a material that is optimal for regenerating bone typically would not be suitable for regenerating a soft tissue such as cartilage.

 Stability in the biological environment: A biomaterial should be nondegradable or should degrade at a desirable rate for the intended application. For a degradable biomaterial, a guideline often found in the literature is that the implant should degrade at a rate comparable to the rate at which new tissue is being formed. However, this can often be difficult to achieve in practice.

 Ease of fabrication: Typically, a biomaterial will have an external shape (or geometry), complex or simple, that is similar to the tissue or organ to be replaced or regenerated. Biomaterials can be 3D objects, fibers, coatings, films, or particles, depending on the application. The biomaterial might be required to have internal structural features that are also important. These internal structural features relate to the way in which the components or phases, such as the solid phase and porosity, are arranged within the biomaterial. The structure at a microscale and nanoscale, referred to as the “microstructure” and “nanostructure,” respectively, are often the major structural features of interest. The microstructure (or nanostructure) can be simple or complex, depending on the application. Materials used to create biomaterials should be capable of being formed economically into the desired external shape and internal microstructure (and/or nanostructure). In common with other technologies, the use of additive manufacturing to create biomaterials, particularly with complex shape and microstructure, has been increasing rapidly.

 Ability to be sterilized: A biomaterial should be capable of being sterilized by one of the sterilization methods that employ heat (steam autoclaving or dry heat), gas, radiation, or electron beam treatment prior to their use in studies in vitro or implantation in vivo. Lack of sterility will invariably lead to infection and destruction of cells, followed by potentially catastrophic failure in vivo.

 Other physicochemical properties: The applications of biomaterials are many and, consequently, one or more additional properties can be particularly important in certain applications. The capacity to absorb water can be crucial to the use of biomaterials in applications such as drug delivery and tissue engineering. It depends on the composition and the structure of the biomaterial. As water is composed of polar molecules, the capacity to interact with, and absorb, water molecules is favored by the presence of ionic charges, polar groups, or a combination of the two, in the molecular structure of the biomaterial. The biomaterial should also have an interconnected but rather expandable molecular structure to allow the migration of water molecules into the structure.

Whether a material is electrically conducting or insulting is important to the function of some medical devices. Electrically conducting metals are, for example, important for use as electrodes in pacemakers and neural stimulators. In comparison, electrically insulating materials are typically used as coatings to isolate or insulate sensitive electronic devices. The ability to conduct electrical signals is quantified by the electrical conductivity of the material or, less commonly, by the electrical resistivity, which is the inverse of the electrical conductivity. The capacity of a material to respond to a magnetic field is important for its ability to function in some treatments such as hyperthermia treatment of tumors and in diagnostic imaging.

The use of materials in devices such as contact lenses and intraoptical lenses is crucially dependent on their ability to transmit light (i.e. their transparency). Whether a material can conduct heat or not is quantified by its thermal conductivity. The thermal expansion coefficient quantifies the expansion or contraction of a material upon heating or cooling. These thermal properties are normally important for biomaterials that are subjected to sizable temperature changes during manufacture or use. As some biomaterials, particularly natural materials, can deteriorate when heated, their maximum processing or use temperature can also be important.