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Ultrasonic hot embossing of polymers is an alternative to reduce fabrication costs of microreactors. An ultrasonic welding machine is used to melt a stack of thermoplastic foils and adapting them to a short-time milled aluminum mold showing the inversed design of the desired microfluidic cavities. Two micromixers were fabricated this way providing a low degree of axial dispersion and pressure loss. Stability analysis is successfully performed for a wide temperature range and high pressure. Mixing of colored aqueous solutions and neutralization reactions are implemented to both systems for defined volume flow rates and optically investigated via microscope. Reaction progress is automatically determined with a MATLAB script by reference to the consequential color change of the neutralization reaction with a color indicator. Typical mixing characteristics are identified for both mixers.

Through-mask electrochemical micromachining (TMEMM) is a versatile microfabrication technique, which combines anodic metalshaping and -finishing steps in a single operation for the purpose of microstructuring a wide range of electrically conductive materials. It has been extensively utilized for the manufacture of precision-engineered components in microsystems, such as microlenses, microprobes, microactuators, microchannels and microvalves. This is because TMEMM is able to mass-produce parts; it is applicable to difficult-to-etch materials and operable under polishing conditions. This review provides an in-depth summary of the current state of TMEMM and presents key concepts relevant to its application. To this end, the interplay between the current distribution, mass-transfer effects and surface-film formation is discussed, and its effect on the machining rate, shape profile and surface finish is highlighted. Also, past incarnations of TMEMM are reviewed in terms of their masking method, substrate material, electrolyte composition and technical application. Techno-economic aspects of TMEMM are debated in relation to potential rival techniques, taking microreactor fabrication as an example for a novel area of application. This is followed by an overview of process variations and recent developments.

Electrochemical impedance spectra recorded during the mass-transfer limited dissolution of commercially pure aluminum rotating disk electrodes in concentrated phosphoric acid are presented. The influence of rotation rate and potential on the observed capacitive and inductive semi-circles is discussed and the presence of a salt film on top of the oxide film is debated. The impedance spectra are interpreted hypothesizing the presence of a compact, barrier-type Al2O3 film only. The capacitive and inductive features are explained in terms of the capacitive charging of the barrier film, the relaxation of a surface charge at the film/solution interface and the perturbation of the film thickness. The properties of the film such as electric field strength, half-jump distance and polarizability of the film/solution interface are calculated in agreement with literature values.

Aluminum micro-channels have been machined in phosphoric acid via mass-transfer limited electrochemical dissolution through photoresist masks. The results of shape evolution experiments using a rotating disk electrode are presented in terms of the dimensions, shape profile and uniformity of the machined micro-channels. The influences of applied potential, cumulative passed charge and hydrodynamic conditions on the shape evolution process are discussed. Experimental results are compared with a shape evolution model assuming the rate of aluminum removal is solely controlled by diffusive mass transfer. The degree of agreement between experimental and simulated results depends mainly on the hydrodynamic conditions in the electrochemical cell and indicates a shift from purely diffusive to mixed convective-diffusive mass transfer. Through-Mask Electrochemical Micromachining (TMEMM) is an unconventional machining process, in which a metal substrate is made the anode in an electrochemical cell and thereby dissolved.1,2 Localized material removal is achieved by covering the substrate with an insulating mask. The use of common photolithography processes for the creation of the mask enables the fabrication of micrometer-sized structures. Thereby, TMEMM can be used in two ways to fabricate microfluidic devices: firstly, cavities and channels can be machined into the substrate surface and secondly, through-holes and slits can be machined into metal foils or shims.1 Compared to classical wet chemical and dry etching processes, TMEMM offers higher rates of material removal, better control of surface finish, more selective machining of substrates and enhanced process safety. [3][4][5] In addition, combining photolithography with electrochemical dissolution processes is expected to enable the fabrication of a large number of microfluidic devices in parallel and at low cost. 6Aluminum is widely used as a construction material for the manufacture of microfluidic devices such as micro-heat exchangers, 7 microreactors 8 and micro-fuel cells. 9 Other areas of interest are the fabrication of intricate parts in optical, electronic, automotive and aerospace systems. 10,11 Its chief advantages compared to other materials used in these fields are its high thermal conductivity, low electrical resistivity, easy machinability and low density. In addition, it is possible to modify the surface structure of aluminum substrates via the formation of highly ordered, porous oxide films along the surface. 12,13 It has been demonstrated, that these films can be subsequently used to incorporate noble metal catalysts along the surface of micro-channels within a micro-reactor. [14][15][16] Electrochemical dissolution of aluminum in phosphoric acid is known to produce reflective aluminum surfaces with sub-micrometer surface roughness at sufficiently high electrolyte concentration and temperature.17 This property makes it attractive for TMEMM of micrometer-sized structures made from aluminum. The observed leveling and brightening effect is caused by the m...