Abstract
Interactions at the solid–body fluid interfaces play a vital role in bone tissue formation at the implant surface. In this study, fully atomistic molecular dynamics (MD) simulations were performed to investigate interactions between the physiological components of body fluids (Ca2+, HPO42–, H2PO4–, Na+, Cl–, and H2O) and functionalized parylene C surface. In comparison to the native parylene C (−Cl surface groups), the introduction of −OH, −CHO, and −COOH surface groups significantly enhances the interactions between body fluid ions and the polymeric surface. The experimentally observed formation of calcium phosphate nanocrystals is discussed in terms of MD simulations of the calcium phosphate clustering. Surface functional groups promote the clustering of calcium and phosphate ions in the following order: −OH > −CHO > −Cl (parent parylene C) ≈ −COO–. This promoting role of surface functional groups is explained as stimulating the number of Ca2+ and HPO42– surface contacts as well as ion chemisorption. The molecular mechanism of calcium phosphate cluster formation at the functionalized parylene C surface is proposed.
Highlights
Polymers have been widely used for the last 3 decades in medical applications, such as coated transducers, neural prosthesis, catheters, and parts of orthopedic implants.[1−5] A polymer vividly explored in this context and meeting sophisticated criteria for biomaterials is parylene C (poly(chloro-para-xylene))
It was shown that parylene C can be applied in biomedical devices as a versatile coating serving as a multifunctional anticorrosive,[8] biocompatible,[9] and anti-infection/therapeutic layer.[10−12] Notably, these functions are achievable on all classes of biomaterials, that is, ceramics,[13] metallic,[6] and polymeric.[14]
The parylene C surface becomes rough in the nanoscale, with nanocorrugations in the range of 60−200 nm (Figure 1A), and hydrophilic (Figure 1B), with a water contact angle θW of 0.1° and the corresponding surface free energy (SFE) of 72.9 mJ/ m2 with 48.6 and 24.3 mJ/m2 polar and dispersive components, respectively.[47]
Summary
Polymers have been widely used for the last 3 decades in medical applications, such as coated transducers, neural prosthesis, catheters, and parts of orthopedic implants.[1−5] A polymer vividly explored in this context and meeting sophisticated criteria for biomaterials is parylene C (poly(chloro-para-xylene)). When a biomaterial is placed in the human body, it first interacts with a biological fluid consisting of water enriched with ions, sugars, and proteins. Upon such exposure, the oriented adsorption of molecules creates a conditioned surface that is responsible for the subsequent cell−surface interactions.[15] One of the strategies to improve surface biocompatibility, in this context, is an incorporation of the functional groups of various chemical nature at the biomaterial surface, such as −OH, −COOH, −NH2, and −F.10,16,17. Because of a plethora of existing possibilities, there are three questions that need to be addressed in order to obtain the biomaterial tailored for the desired site in the body: (a) what is the required function? (b) Should functional groups be introduced? If yes, which one? (c) What will be the most effective surface coverage?
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