Role Of Water In Biomaterials · Water Is The "Universal Ethe

Role of water in Biomaterials · Water is the “universal ether†as it has been termed (Baier and Meyer, 1996),

Water plays a fundamental role in biomaterials primarily as a solvent system, owing to its unique chemical and physical properties. Often referred to as the “universal ether” (Baier and Meyer, 1996), water's ability to dissolve inorganic salts and large organic molecules such as proteins and carbohydrates makes it indispensable in biological systems (Pain, 1982). Its active participation in biological processes stems from its mediating properties, enabling a myriad of cellular functions, structural formations, and biochemical reactions (Andrade et al., 1981). In biomaterials science, water is typically the first molecule to interact with a material in vivo, influencing biocompatibility, material degradation, and cellular responses (Baier, 1978).

Water's Structural and Dynamic Properties in Biomaterials

Within biological contexts, water exhibits complex self-association behavior characterized by hydrogen bonding, forming a dynamic three-dimensional network that influences the properties of biological molecules and tissues (Franks, 1972). The tetrahedral hydrogen-bonding structure of water, with electrons participating as Lewis acid centers and lone pairs acting as Lewis bases, facilitates its solvent functions (Fig. 1A). These hydrogen bonds, being relatively weak (3–5 kcal/mole), are transient, often persisting only for tens of picoseconds (Berendsen, 1967; Luzar and Chandler, 1996). Nevertheless, at any instant, more than 75% of water molecules are interconnected in this network, underpinning the physical and chemical behavior of water in biological environments (Robinson et al., 1996).

Water's amphoteric nature allows it to share and donate electron density, forming a flexible network of hydrogen bonds essential for the stability of biomolecular structures (Fig. 1D). This self-association influences key properties such as density, partial molar volume, and solvation capacity, which are critical in understanding biomaterial interactions in vivo. For instance, the phenomenon of ice floating more than liquid water exemplifies how hydrogen bonding and molecular density are interconnected, impacting environmental and biological systems (Franks, 1972).

Water as a Solvent in Biomaterials: Physical and Chemical Perspectives

As a solvent, water's capacity to dissolve a wide range of solutes is crucial for maintaining cellular viability and facilitating biochemical reactions (Baier and Meyer, 1996). Its small size and highly transient hydrogen-bonded network enable rapid diffusion of solutes, which is essential for nutrient transport, waste removal, and enzyme activity within biological tissues (Pain, 1982). The molecular dimensions of water (~0.25 nm) and its network dynamics underpin its role in these processes, affecting the behavior and stability of biomaterials designed for implantation or tissue engineering (Andrade et al., 1981).

Hydrophobicity and Hydrophobic Interactions in Biomaterial Design

Hydrophobicity, derived from the Greek words for water (“hydro”) and fear (“phobicity”), refers to the property of a material or molecule to repel water, which is significant in biomaterials for controlling surface interactions and biological responses (Baier and Meyer, 1996). Hydrophobic materials like waxes, oils, and fats tend to exhibit high water contact angles (>90°), resisting wetting due to their nonpolar nature (Fig. 1C). The evaluation of hydrophobicity through contact angle measurements assists in predicting material behavior in biological environments (Young's equation). Materials with high hydrophobicity facilitate water repellency, reduce protein adsorption, and influence cell attachment, critical factors in designing implants and surface coatings (Kauzmann, 1959).

The hydrophobic effect also drives the aggregation of nonpolar molecules, such as fats, within aqueous environments, a principle exploited in the formation of lipid bilayers and micelles (Walter Kauzmann, 1959). This effect stems from the disruption of water's hydrogen bonding network when nonpolar substances aggregate, leading to a net increase in entropy and spontaneous association of hydrophobes (ChemWiki, 2023). In biomaterials, harnessing hydrophobic interactions enables the development of water-repellent coatings, anti-icing surfaces, and self-cleaning materials (Baier and Meyer, 1996).

Hydrophilic Coatings and Surface Wetting in Biomaterials

Hydrophilic coatings promote wetting and maintain a water film on surfaces, which can enhance biocompatibility by reducing protein adsorption and cellular adhesion (Pain, 1982). These coatings are employed to improve the surface properties of implants, medical devices, and tissue scaffolds, facilitating integration with biological tissues (Andrade et al., 1981). The extent of surface wetting is often quantified through contact angle measurements, where lower contact angles (

Superhydrophobic surfaces, exhibiting contact angles above 150°, mimic the lotus leaf's remarkable water-repellent properties, causing water droplets to bead and roll off, carrying dirt and contaminants away (Fig. 2C). These surfaces are created through micro- and nano-structuring, which traps air and minimizes the solid-liquid contact area (ChemWiki, 2023). In biomedical contexts, superhydrophobic coatings are applied for anti-icing, self-cleaning, and anti-fouling purposes, enhancing device longevity and functionality (Walter Kauzmann, 1959).

Water's Role in Protein and Cell Interactions with Biomaterials

At the interface between biomaterials and biological systems, water is integral in mediating protein adsorption and cell adhesion, which are critical for biocompatibility (Pain, 1982). It is hypothesized that biological responses are initiated within a thin surface layer—often less than 1 nm—dominated by water interactions and surface chemistry (Baier and Meyer, 1996). The surface energy and water-wettability properties influence the formation of a hydration layer, which can either promote or inhibit protein conformational changes and cellular attachment (Andrade et al., 1981).

Understanding how water interacts with biomaterial surfaces enhances the design of materials that are resistant to bacterial colonization or promote tissue integration. Hydrophilic surfaces tend to attract and stabilize proteins in their native conformations, reducing inadvertent immune responses, whereas hydrophobic surfaces may lead to denaturation and rejection (Baier, 1978). Consequently, surface modification techniques aim to manipulate water interactions to attain desired biological outcomes.

Conclusion

Water's versatile and active role in biomaterials is a cornerstone of biomaterials science. Its solvent capabilities, structuring behavior, and interactions with hydrophobic and hydrophilic surfaces influence the biocompatibility, functionality, and longevity of biomaterials. Advances in understanding water's behavior at interfaces and within biological systems contribute significantly to the development of innovative biomaterials for medical applications, ranging from tissue engineering to implantable devices. Future research continues to uncover the intricate mechanisms of water's interactions, promising enhanced biomaterial design tailored to specific biological environments.

References

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