Biomedical Engineering Reference
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pulses, compared with the use of conventional direct current
anodization. Three-dimensional complex nanostructures, lines,
curved features, and arrays can be machined in substrates in single-
step processing.
The method is based on the application of ultrashort voltage
pulses of nanosecond duration, which leads to the spatial
confinement of electrochemical reactions, e.g., dissolution of
material. The electrochemical dissolution rate of the material has
to be intentionally varied over the workpiece surface by applying
inhomogeneous current density distribution in the electrolyte and
at the workpiece surface. This can be achieved by the geometric
shape of the tool, locally very small tool-workpiece distances,
partial insulation of the tool or workpiece, and high overall current
densities, etc. This situation is illustrated in Fig. 1.
8
. The workpiece
is preferentially etched within the gap region between the front
face of the tool and the workpiece surface. This approach for local
confinement of electrochemical reactions is based on the local
charging of the electrochemical double layer (DL) and the resulting
direct influence on the electrochemical reaction rates.
Figure 1.8
Sketch of the experimental setup and principle of electrochemical
micromachining with ultrashort pulses. RE and CE are abbreviations for
reference and counter electrode, respectively.
The potentials of the workpiece and tool are controlled by the
low-frequency bipotentiostat. The voltage pulses are supplied by
the high-frequency pulse generator. An ultrashort voltage pulse
limits the charging of double layer capacitance to the vicinity of the
tool. The current distribution between the DL is also illustrated in
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