A growing number of microsystem technology (MST) applications, particularly in the field of microfluidics with its applications in the life sciences, have a need for novel fabrication methods which account for substrates other than silicon or glass. We present in this paper an overview of existing polymer microfabrication technologies for microfluidic applications, namely replication methods such as hot embossing, injection molding and casting, and the technologies necessary to fabricate the molding masters. In addition, techniques such as laser ablation and layering techniques are examined. Methods for bonding and dicing of polymer materials, which are necessary for complete systems, are evaluated.

Polymer microfabrication methods for microfluidic analytical applications

Considerable scientific and technological efforts have been devoted to develop neuroprostheses and hybrid bionic systems that link the human nervous system with electronic or robotic prostheses, with the main aim of restoring motor and sensory functions in disabled patients. A number of neuroprostheses use interfaces with peripheral nerves or muscles for neuromuscular stimulation and signal recording. Herein, we provide a critical overview of the peripheral interfaces available and trace their use from research to clinical application in controlling artificial and robotic prostheses. The first section reviews the different types of non-invasive and invasive electrodes, which include surface and muscular electrodes that can record EMG signals from and stimulate the underlying or implanted muscles. Extraneural electrodes, such as cuff and epineurial electrodes, provide simultaneous interface with many axons in the nerve, whereas intrafascicular, penetrating, and regenerative electrodes may contact small groups of axons within a nerve fascicle. Biological, technological, and material science issues are also reviewed relative to the problems of electrode design and tissue injury. The last section reviews different strate- gies for the use of information recorded from peripheral interfaces and the current state of control neuroprostheses and hybrid bionic systems.

A critical review of interfaces with the peripheral nervous system for the control of neuroprostheses and hybrid bionic systems

Capillary electrophoresis and related techniques on microchips have made great strides in recent years. This review concentrates on progress in capillary zone electrophoresis, but also covers other capillary techniques such as isoelectric focusing, isotachophoresis, free flow electrophoresis, and micellar electrokinetic chromatography. The material and technologies used to prepare microchips, microchip designs, channel geometries, sample manipulation and derivatization, detection, and applications of capillary electrophoresis to microchips are discussed. The progress in separation of nucleic acids and proteins is particularly emphasized.

Capillary electrophoresis on microchip

To achieve selective electrical interfacing to the neural system it is necessary to approach neuronal elements on a scale of micrometers. This necessitates microtechnology fabrication and introduces the interdisciplinary field of neurotechnology, lying at the juncture of neuroscience with microtechnology. The neuroelectronic interface occurs where the membrane of a cell soma or axon meets a metal microelectrode surface. The seal between these may be narrow or may be leaky. In the latter case the surrounding volume conductor becomes part of the interface. Electrode design for successful interfacing, either for stimulation or recording, requires good understanding of membrane phenomena, natural and evoked action potential generation, volume conduction, and electrode behavior. Penetrating multimicroelectrodes have been produced as one-, two-, and three-dimensional arrays, mainly in silicon, glass, and metal microtechnology. Cuff electrodes circumvent a nerve; their selectivity aims at fascicles more than at nerve fibers. Other types of electrodes are regenerating sieves and cone-ingrowth electrodes. The latter may play a role in brain-computer interfaces. Planar substrate-embedded electrode arrays with cultured neural cells on top are used to study the activity and plasticity of developing neural networks. They also serve as substrates for future so-called cultured probes.

Selective electrical interfaces with the nervous system.

Complex metal structures are formed in microminiature dimensions utilizing multiple mask exposures which are capable of producing substantially arbitrary three-dimensional shapes. A first layer of a primary metal is electroplated onto an exposed plating base to fill a void in a photoresist, the photoresist is removed, and a secondary metal, which can constitute a sacrificial metal, is electroplated over the first layer and the plating base. The exposed surface of the deposited secondary metal is then machined down to a height which exposes the first metal to produce a substantially flat, uniform surface extending across both the primary and secondary metals. The secondary metal facilitates machining of the primary and secondary metals because of these mechanical stability provided by the secondary metal to the primary metal. After the first layers of the primary and secondary metals have been machined, another photoresist layer can be applied over the primary and secondary metals, and a second layer of primary metal is electroplated. A further layer of secondary metal can be also electroplated if desired. The process of machining and electroplating can be repeated until the desired structure is formed. Secondary metal is removed by an etchant which does not etch the primary metal.

Formation of microstructures by multiple level deep X-ray lithography with sacrificial metal layers

For fabricating microstructures with extreme structural heights a technology has been developed which is based on deep-etch lithography and subsequent replication processes. A particularly high precision is achieved when the lithographic process is carried out by means of synchrotron radiation. Electroforming and molding processes are used for the replication of microstructures from a large variety of materials. The field of application comprises sensors, electrical and optical microconnectors, components for fluid technology, micromechanical components, microfiltration systems and novel composite materials.

D. Münchmeyer

Papers

Three-dimensional microfabrication using synchrotron radiation