Bioinspired material designs with multifunctional characteristics aim to offer properties similar to their counterparts found in nature, which in some cases have evolved over millions of years. For example, natural tissue comprises layers of hard and soft tissue material called tropocollagen and hydroxyapatite are organized into staggered arrays to form collagen fibrils.
1 The ratio between the soft and hard material phases varies according to the tissue’s functionality, e.g., muscular or epithelial tissue.
2 The layers form a compact but strong and flexible lamella with load bearing, lubricious, and wear resistant characteristics. The soft material phase has elastomer characteristics and provides a dampening effect, whereas the hard material phase provides strength and load bearing characteristics. Another example involves micturition, a urethral function that relaxes the urethra voluntarily when it senses the bladder is full.
3 Here, the combination of the surface energy of the material, which determines the water contact angle (WCA), and the frictional properties of the tissue plays a significant role in defining the wetting and fluid flow characteristics down the urethral tissue.
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5 This principle could be applicable to cases where the ability to modify a surface between hydrophilic and hydrophobic can, for example, be used to increase engine component lifetimes. In a modern stop–start engine for example, increased wetting during the
stop-phase could be used to prevent premature engine wear by always maintaining a thin oil film layer in areas where there is potential of metal-to-metal contact. Since the WCA of a material affects its wetting properties, the ability to modify this characteristic by the way of incorporating a secondary low-surface energy material is investigated. In fact, modifying the surface energy of a metal layer with the addition of a hydrophobic layer has been reported before, where tungsten is coated with PTFE to a form a hydrophobic surface.
6 Incorporating a low-surface energy material such as PTFE with NiTi as the initial choice of materials in this study is driven by their biocompatibility and the need for intelligent in vivo drug delivery applications, where current challenges include the need to administer the correct dose and quantity of drugs when most needed by a patient.
7 Thin films comprising a soft PTFE phase, which has hydrophobic characteristics, have been combined with a harder NiTi metal phase, a shape memory alloy (SMA) material that offers shape memory functionality in response to heat or stress. It is well documented that SMAs transform from a low temperature, low allotrope martensite B19′ monoclinic phase, to a high temperature, high-symmetry allotrope austenite B2 cubic phase. NiTi undergoes such a B2–B19′ phase transformation but is limited to a maximum operating temperature of around 100°C.
8 Thus, the combination of the material biocompatibility and the shape memory functionality could, in theory, be applicable in drug delivery systems as part of a nano-fluid sensor which triggers a dose of drugs that is administered in response to a patient’s body temperature. Thin films are prepared by sputter physical vapor deposition (PVD) as this allows for the control of individual film thicknesses at a nano-level, permitting a greater control of film microstructure.
9 The objective is to modify the surface energy by varying the amount of PTFE and NiTi content in the coating and investigate the resultant wetting and microstructural properties. This study continues our initial research on such coatings where the tribological properties such as the coefficient of friction were measured and the concept of introducing a polymer-shape memory alloy nanocomposite film was initially proposed.
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