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Role of water in Biomaterials · The primary role water plays in biomaterials is as a solvent system. · Water is the “universal ether” as it has been termed (Baier and Meyer, 1996), dissolving inorganic salts and large organic macromolecules such as proteins or carbohydrates (solutes) with nearly equal efficiency (Pain, 1982). · Water is an active participant in biology, which simply could not and would not work the way it does without the special mediating properties of water. · Moreover, it is widely believed that water is the first molecule to contact biomaterials in any clinical application (Andrade et al., 1981; Baier, 1978). · This is because water is the majority molecule in any biological mixture, constituting 70 wt % or more of most living organisms, and because water is such a small and agile molecule, only about 0.25 nm in the longest dimension. · Consequently, behavior of water near surfaces and the role of water in biology are very important subjects in biomaterials science. · Self-association confers unique properties on water, many of which are still active areas of scientific investigation even after more than 200 years of chemical and physical research applied to water (Franks, 1972). WATER SOLVENT PROPERTIES A C B D FIG. Atomic structure of water illustrating (A) tetrahedral bonding arrangement wherein hydrogen atoms (H, light-colored spheres) are Lewis acid centers and the two lone-pair electrons on oxygen (O, dark-colored spheres) are Lewis base centers that permit water to hydrogen bond with four nearest-neighbor water molecules; (B) electron density map superimposed on an atomic-radius sphere model of water providing a more authentic representation of molecular water; (C) approximate molecular dimensions; and (D) five water molecules participating in a portion of a hydrogen-bond network. · Hydrogen bonds in water are relatively weak 3–5 kcal/mole associations with little covalent character (Iassacs et al., 1999; Marshall, 1999). · Hydrogen bonds are quite transient in nature, persisting only for a few tens of picoseconds (Berendsen, 1967; Luzar and Chandler, 1996). · Modern molecular simulations suggest, however, that more than 75% of liquid-water molecules are interconnected in a three-dimensional (3D) network of three or four nearest neighbors at any particular instant in time (Robinson et al., 1996). · In chemical terminology, less self-associated water has a greater chemical potential than more self associated water. · This is why water ice with a complete crystalline network is less dense than liquid water and floats upon unfrozen water, a phenomenon with profound environmental impact. · Water is amphoteric in this sense because, as illustrated in Figs. 1A and 1D, it can simultaneously share and donate electron density. Hydrogen atoms (the Lewis acids) on one or more adjacent water molecules can accept electron density from the unshared electron pairs on the oxygen atom (the Lewis bases) of another water molecule. · In this manner, water forms a 3D network through Lewis acid–base self-association reactions. · In chemical terminology, less self-associated water has a greater chemical potential than more self-associated water. · Interestingly, more self-associated water with a relatively more complete 3D network of hydrogen bonds must be less dense (greater partial molar volume) than less self-associated water because formation of linearly directed hydrogen bonds takes up space (Fig. 1C), increasing free volume in the liquid. 1 MSEG-402-Vijay 2 2 WHAT IS MEANT HYDROPHOBICITY ? Hydrophobicity comes also from the greek word Hydro(water) and Phobicity (fear) it refers to the physical property of a material that repels a mass of water. · Some of the common natural Hydrophobic materials are waxes, oil and fats. The evaluation of hydrophobicity is made through water contact angle measurements. A water droplet would be spherical so the water contact angle will be significantly high. • At the molecular level, the hydrophobic effect is important in driving protein folding formation of lipid bilayers and micelles, insertion of membrane proteins into the nonpolar lipid environment and protein-small molecule interactions. Substances for which this effect is observed are known as hydrophobes. HYDROPHOBIC EFFECT The hydrophobic effect represents the tendency of water to exclude non-polar molecules. The effect originates from the disruption of highly dynamic hydrogen bonds between molecules of liquid water. HYDROPHILIC COATING Hydrophilic coatings wet the surface very easily, and maintain the wetness for longer time. Thus, using hydrophilic coatings eliminates the need for additional lubricants.  MSEG-402-Vijay 10 SUPER HYDROPHOBIC COATING The process of coating the surface of a material with hydrophobic property material in order to avoid sticking of liquids on that surface. This is absolutely unique way of coating unlike conventional which shrink continuously during drying to produce low porosity films. Super hydrophobic technology makes water bounce, it stops it, rolls it off the surface. The surface of plant leafs, especially of the lotus flower, can show extreme hydrophobicity to water (large contact angels >150oC) Surface structure 20-100M MSEG-402-Vijay 12 Young’s equation is used to describe the interactions between the forces of cohesion and adhesion and measure what is referred to as surface energy. www.ramehart.com/contactangle.htm · A drop with a contact angle over 90° is hydrophobic. · This condition is exemplified by poor wetting, poor adhesiveness and the solid surface free energy is low. Hydrophobic interactions · Hydrophobic interactions describe the relations between water and hydrophobes (low water-soluble molecules). · Hydrophobes are nonpolar molecules and usually have a long chain of carbons that do not interact with water molecules. · The mixing of fat and water is a good example of this particular interaction. · The common misconception is that water and fat doesn’t mix because the Van der Waals · forces that are acting upon both water and fat molecules are too weak. Causes of Hydrophobic Interactions • American chemist Walter Kauzmann discovered that nonpolar substances like fat molecules tend to clump up together rather that distributing itself in a water medium, because this allow the fat molecules to have minimal contact with water. Hydrophobic interactions ChemWiki Formation of Hydrophobic Interactions · The mixing hydrophobes and water molecules is not spontaneous; however, hydrophobic interactions between hydrophobes are spontaneous. · When hydropobes come together and interact with each other, enthalpy increases ( is positive) because some of hydrogen bonds that form the clathrate cage will be broken. · Tearing down a portion of the clathrate cage will cause the entropy to increase ( is positive), since forming it decreases the entropy. According to the formula: ΔG= ΔH-TΔS ΔH=Small positive value ΔS=Large positive value Result :ΔG= Negative A negative ΔG indicates that hydrophobic interactions are spontaneous What does the strength of Hydrophobic Interactions depend on? In order of Effectiveness: · Temperature · Number of carbons on the hydrophobes · The shape of the hydrophobes APPLICATIONS · A primary purpose of hydrophobic coatings such as polytetrafluoroethylene(PTFE) or polyxylylene is to act as a barrier against water commonly seen in automobiles · Used in fabrication on metallic nano rod to prevent icing. · Its is widely used in aerospace industry for providing anti-icing coating on the surface of the aeroplane . · Hydrophobic self cleaning glasses are installed in trafficsensor control unit. · We induce hydrophobic recovery after plasma treatment, aphysical contact treatment (PCT) . Glucose molecules have polar hydroxyl(OH) groups in them and these attract the water to them. When sugar is in a crystal the molecules are attracted to the water and go into solution. Once in solution the molecules stay in solution at least in part because they become surrounded by water molecules. This layer of water molecules surrounding another molecule is called a hydration shell. Contact angle method Mostly water drop on the surface Cohesive force: molecules in a drop Adhesive force: molecules in a drop and those on the surface FIG. Four ways that the contact angle can be measured. (A) Sessile drop. (B) Captive air bubble method. (C) Capillary rise method. (D) Wilhelmy plate method. THE HYDROPHILIC EFFECT • The hydrophilic solutes with biomedical relevance would include cations such as Na+, K+, Ca2+, and Mg2+ or anions such as Cl−, HCO3-1 and HPO4-2 . THE SURFACE WETTING EFFECT • It is a very common observation that water wets certain kinds of surfaces whereas water beads up on others, forming droplets with a finite “contact angle.” WATER AND THE BIOLOGICAL RESPONSE TO MATERIALS It has long been assumed that the observed biological response to materials is initiated or catalyzed by interactions with material residing in the same thin surface region that affects water wettability, arguably no thicker than about 1 nm. In particular, it is frequently assumed that biological responses begin with protein adsorption hat biology does not “sense” or “see” bulk properties of a contacting material, only the outermost molecular groups protruding from a surface. But exactly how surfaces influence “biocompatibility” of a material is still not well understood. Both surface energy and water theories acknowledge that the principle interfacial events surfaces can promote or catalyze are adsorption and adhesion. Adsorption of proteins and/or adhesion of cells/tissues are known (or at least strongly suspected) to be involved in the primary interactions of biology with materials. Water is a very small, but very special, molecule. Properties of this universal biological solvent, this essential medium of life as we understand it, remain more mysterious in this century of science than those of the very atoms that comprise it. Self association of water through hydrogen bonding is the essential mechanism behind water solvent properties and understanding self association effects near surfaces is a key to understanding water properties in contact with biomaterials. Relationships Among Water Structure and Solvent Properties Assignment-2 Explain any two of each surface characterization and mechanical testing techniques for biomaterials MSEG-402-Vijay 2 MSEG-402-Vijay 2 2