Chapter 7, Problem 1CO

List some factors influencing the biocompatibility of materials and explain how those factors are related to chemical bonding.

Interpretation Introduction

Interpretation:

To list some factors influencing biocompatibility of materials and explain the interrelation of those factors to chemical bonding.

Concept introduction:

A bio-material can be defined as the non-living materials that are designed in specific way so that it can interact with the biological systems without causing much adverse effects. In such cases, it is of utmost importance that the used bio-material can elucidate appropriate responses in connection with the bio-material’s application in the body of an active organism. This is otherwise known as bio-compatibility. This bio-compatibility not only depends on the condition of host, but also on the properties of the material. In turn, the properties of the bio-material depend on the type of bonding that has taken place.

The factors that influence biocompatibility are size, shape, material composition, surface wettability, roughness and charge. For biocompatibility, the adverse reaction should be minimal at the interface between blood and the material or tissue. This means that the material interaction should be a nearly natural like the natural material in the surroundings of blood and tissue.

Answer to Problem 1CO

The following table illustrates the use of biomaterials.

Bio-Materials

Metals	Polymers	Ceramics	Semiconductors
Used for making orthopedic screws and fixation; dental implants	Drug delivering devices, skin/cartilage Ocular implants	Bone replacements, heart valves and dental implants	Implantable microelectrodes and biosensors

In general, a bio-material is chosen for an application depending upon the needed strength of the material, where it is used and the function it is expected to perform in the body. For example, the dental and bone replacements need structural strength, should not be brittle and should be able to withstand a lot of pressure. Accordingly, metals or ceramics are the biomaterial chosen for these tasks. Metals have metallic bond and ceramics are based on ionic bond, both of which are strong primary bonds. On the other hand, polymers are chosen for ocular implants and these polymers have covalent and secondary bonds which are weak bonds. If we observe to perform tougher jobs withstanding high stress like during grinding the food or incessant movement, metals or ceramics are chosen. For soft surfaces like skin and eye materials made of polymers are preferred.

Explanation of Solution

Th chosen material should have the properties that are needed for its proper performance in the body and at the same time biocompatible also. Considering biocompatibility, it is mainly surface phenomenon which interacts with the cells in the biological fluid.

In general, a biomaterial used can trigger foreign body reactions as these hydrophobic materials have high affinity to proteins. These protein biomaterial interactions help in the exposure of hidden protein structures. When exposed they become the receptor sites for inflammatory cells. This initiates the foreign body reactions. The surface properties of polymers influence the type and amount of bound proteins.

Thus, the primary bonding i.e. the strong bonds are covalent- formed due to sharing of electrons, ionic- formed due to the complete transfer of electrons and metallic bond where the metal cations are embedded in a ‘sea of electrons makes the material strong enough to withstand or resist induced stress without cracking or the shape being changed in anyway. In general, of the bonds in terms of strength is as follows:

Covalent bonds > ionic bonds > metallic bonds

In case of secondary bonding, there is no electron sharing − only electrical imbalance called dipole. They are the short-range attraction force that is the Van der Waals forces. This force is essential for the adhesion between molecules of liquids or molecular crystals. The other dipole − dipole interactive type hydrogen bonding forms hydrogen bridges between neighbouring molecules.

For example, the intraocular lens is basically made up of PMMA, silicones and acrylic. Poly methyl methacrylate (PMMA) and acrylic are polymers whereas silicones are soft lining materials

Conclusion

Thus, the factors influencing biocompatibility and chemical bonding are closely related in terms of how the biomaterial is chosen and modified according to size, shape, material composition, surface wettability, roughness and charge.