Materials for Biomedical Engineering. Mohamed N. Rahaman

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Название Materials for Biomedical Engineering
Автор произведения Mohamed N. Rahaman
Жанр Химия
Серия
Издательство Химия
Год выпуска 0
isbn 9781119551096



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field of sufficient strength and frequency to generate heat in the particles. This heat is transferred to the surrounding tumor and if the temperature is maintained above ~42 °C for a sufficient period (~30 minutes or more), the tumor can be destroyed. This process is attractive because of the possibility of eradicating the intended tumor without causing significantly adverse effects to healthy tissue. However, there are constraints on the magnitude of H and f that healthy tissues can tolerate and questions about whether a sufficient number of nanoparticles can be delivered to the tumor to ensure the generation of sufficient heat over the required time to eradicate the tumor.

      As the majority of biomedical applications occur at or near the body temperature, the thermal properties of biomaterials in these applications are of little consequence. On the other hand, thermal properties are important for biomaterials used in dental restorations such as crowns and bridges, for example, where they are required to match the thermal properties of the existing tooth structure. Thermal properties are also important for biomaterials whose processing or manufacturing process involves large changes in temperature. In the area of physical properties, the important thermal properties relevant to biomaterials are their thermal conductivity and coefficient of thermal expansion.

      4.7.1 Thermal Conductivity

      Whether a material can conduct heat or not is quantified by its thermal conductivity κ, defined by the equation

      Transport of heat in solids occurs by flow of electrons and by vibration of atoms (or ions) in the crystal lattice. Thus, we can write

      (4.46)equation

      where, κe and κl are the contributions to the thermal conductivity due to the electrons and lattice vibrations, respectively. Depending on the material, one or the other conduction mechanism often dominates. In pure metals with a high degree of metallic bonding, such as copper, thermal conduction occurs predominantly by the sea of nearly free electrons that surround the metal ions. Because of the large number of electrons and ease with which they flow, metals are good thermal conductors, that is, they have a high thermal conductivity. As the flow of electrons is responsible for both thermal and electrical conduction, metals are both good electrical conductors and good thermal conductors, a relationship quantified by the Weideman–Franz law given by

      (4.47)equation

      where, σ ′ is the electrical conductivity, T is the temperature and L is a constant equal to 2.44 × 10−8 WΩ/K2.

Schematic illustration of (a) the vibration of a sphere connected by springs. (b) The atomic vibrations in a crystal lattice are connected which leads to a wave behavior. Schematic illustration of chart showing the thermal conductivity values for a variety of materials.

      In polymers, heat conduction occurs by vibration and rotation of the long chain molecules. Polymers have a low thermal conductivity, although, for a semicrystalline polymer, the conductivity increases with increasing volume fraction of crystalline regions because phonon scattering in the crystalline region is lower than that in an amorphous region of the same composition.

      4.7.2 Thermal Expansion Coefficient

      The expansion or contraction of a material upon heating or cooling is commonly quantified by its linear coefficient of thermal expansion α defined by the equation

      (4.48)equation

      where, Δl is the change in length of a specimen of length lo due to a change in temperature ΔT. The thermal expansion coefficient has the unit °C−1 (or K−1) but is often expressed in units of 10−6 °C−1 because of its low value. The thermal expansion coefficient of one material relative to another is dependent on its interatomic bonding energy versus displacement curve (Section 2.2).

      Although α varies slightly, depending on the temperature range of measurement, ceramics and glasses typically have low α values relative to other classes of materials, in the range ~5 × 10−6 to ~15 × 10−6 °C−1 over a temperature range of a few hundred degrees Celsius above room temperature. On the other hand, some glasses, such as fused silica glass, and a few glass‐ceramics have values as low as ~0.5 × 10−6 °C−1. Metals have higher α values, in the range ~10 × 10−6 to ~25 × 10−6