Surface properties and treatments

Surfaces are the interfaces through which devices interact with the external environment. They are separation elements where sets of atoms or molecules are organized in an interface, a discontinuity with a high level of energy, which can decrease through an optimal arrangement of the superficial molecules in function of the surrounding environment. Surface characteristics, topography, mechanical properties, and surface energy density together with adhesive and antifouling properties of a surface are essential in understanding how a substrate can direct cell adhesion, migration, or clustering. Suitable selection of bulk materials, combined with a specific superficial modification strategy, allows to obtain optimal conditions in terms of biocompatibility and functioning of the device. Physical, chemical, and topographical surface modification methods can be differentially utilized on the basis of the specific material surface (metals, polymers, ceramics, and glasses). Vapor deposition, solvent casting, chemical reactions, plasma techniques, polymer grafting, and photolithography are only some of the basilar techniques that can be used alone or in combination with other methods. They allow to obtain covering layers deposited on the chip surfaces or obtained by the alteration of the initial chip surface with physical/chemical, erosion, or patterning processes. In this chapter, our purpose is to point the attention on the surface properties influencing cell behavior and on those techniques that constitute innovative protocols which speed up and simplify the surface modification specific for microfluidic devices. Great attention is dedicated to the superficial treatments of the synthetic polymers that, because of their versatility in composition and optimal chemical–physical–mechanical properties and their easy processability, are largely used both as biomaterials and in microfluidics. Among the modification surface methods, recent advances in the application of polymers, glass, and silicon materials for biomedical applications has promoted the development of a new category of treatments based on biological modification methods. These methods include the incorporation of natural polymers (e.g., collagen, gelatin, laminin hyaluronic acid, chitosan, alginate) or other bioactive molecules on polymer surfaces or the incorporation of biomolecules on metals, glass, and ceramics to obtain a proteinic e/o polysaccharidic layer able to mediate cell adhesion onto a biocompatible surface. Surface topographical modifications, additionally, represent an ulterior recent strategy: it has in fact been shown that cells are able to perceive topological stimuli and regulate their functions on the basis of these. The surface architecture, in particular of polymers or of silicon-based materials, can be modified to create a substrate that has suitable surface topographical cues at the microscale and nanoscale 3D conformation level. Surfaces are the interfaces through which a device interacts with the external environment. The world around us is full of surfaces: each thing, each object, and all living organisms are defined by a surface. A surface is a separation element between a body and another, an object and another, a phase and another, a solid and a liquid, a gas and another solid, and so on. Each surface is constituted by a set of atoms or molecules organized in an interface, a discontinuity with high energy. However, chemistry teaches that the superficial molecules tend to arrange themselves in order to reach the minimum energy and so to make the system more stable. Obviously the interface energy also depends on the surrounding environment or on the phases which are in contact: a minimum change or perturbation of this level implies the research of a new equilibrium condition.