Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 56

The complex interplay between the processes that underlie biomaterial
charge, ion interactions with ionized groups (including the formation of salt
bridges), and interactions of the resulting material structure with (bio)mo-
lecules from the aqueous phase can hardly be derived from the primary
material structure or from known ionization patterns of isolated functional
groups. Therefore, revealing the fundamental principles of interfacial and
bulk charge formation and related phenomena is of utmost importance for
optimizing the performance of biomaterials in demanding products and
technologies such as bioactive cell scaffolds and immunosorbent assays.
Electrokinetics is one of the oldest and most frequently applied methods
to investigate charging processes at material interfaces of various shape and
chemical nature. 18-20 The approach dictates that the interfacial charge of the
investigated system be unambiguously derivable from a given measured
electrokinetic quantity (e.g., streaming current/potential for planar macro-
scopic surfaces or electrophoretic mobility for particulate systems). The
fundamental theory for the interpretation of electrokinetic measurements at
hard surfaces, i.e., surfaces that are not permeable to water and electrolyte
ions, was developed by Smoluchowski about a century ago and proven to
provide valuable information on the impact of the electrical double layer on
numerous interfacial and colloidal phenomena. 18,19
The growing interest of the scientific community in complex systems and
the emerging fields of micro- and nanofluidics has focused attention on
basic interfacial phenomena associated with charge formation. 21 Advanced
experimental techniques and theories have been developed in recent years
that facilitate interpretation at a much higher level of sophistication. 21-25
The progress made in the quantitative interpretation of peculiar electro-
kinetic features of soft planar surfaces has its origin in recent theoretical
developments, and the option to simultaneously access film surface con-
ductivity and swelling. Measurements of the latter provide additional in-
formation on interfacial charging mechanisms and structure, which
significantly constrains modeling film electrokinetic properties.
In this chapter, we provide an overview of state of the art experimental
techniques and theory to analyze the charge and structure of soft planar
(bio)polymeric systems. To demonstrate the options arising from these de-
velopments, we discuss strategies and recent results reported for stimuli-
responsive polymer coatings, biohybrid hydrogels, and supported bilayer
lipid membranes. Finally, we provide an outlook on potential future devel-
opments in this field.

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3.2 Fundamental Principles and Theory

3.2.1 Streaming Current and Streaming Potential
Measurements in Rectangular Microchannels
The electrokinetic phenomena streaming current and streaming potential
are based on the transport of charge-compensating ions in the electrical