Microelectromechanical systems (MEMS) have been increasingly used worldwide in a wide range of applications, including high tech, energy, medicine or environmental applications. Magnetic polymer composite films have been used extensively in the development of the micropumps and valves, which are critical components of the microelectromechanical systems.
1. Introduction
Films based on polymer and magnetic materials are extensively used in industrial, and environmental, but also medical, applications
[1]. The development of the magnetic films is especially beneficial because it can combine the advantages of the two components. Magnetic component is usually embedded into or coated onto the polymer film in order to confer to these films magnetic properties, hyperthermia capacity, monitoring capacity, etc., while the polymer assures some properties, including binder capacity for the magnetic powder, biocompatibility, release regulator capacity, stabilizing effect, especially in harsh conditions (acidic or chelating agents), etc.
[2].
Metal oxides (such as Fe3O4, Fe2O3) are especially preferred to be used because they are more stable in high magnetization compared with the pure metals (such as Fe or Co) while the magnetic properties are suitable. Moreover, both magnetite and hematite can also be used in medical applications, being stable and biocompatible; the accidental release of the ions do not induce significant pH change and the level of the Fe2+/Fe3+ is in a safe range and does not induce inflammation or irritation. Moreover, it can be easily functionalized with adequate molecules and with passivation or activation purposes, depending on the desired applications. The main disadvantage of the magnetite versus the permanent magnetic powders is related to the lack of the control on the recovery time.
The disposal of a small amount of liquids is of increasing interest in the field of developing MEMS; certainly, micropumps and valves are critical components of these devices. Polymer magnetic films have been developed for over 20 years; based on the literature survey, these films are used in many applications, such as membrane actuators, magnetic micro-pumps, micro-mixer, micro-robots, micro-sensors, micro-concentrators, etc.
[3][4][5][6][7][8][9][10][11][12][13][14]. These systems are based on several actuation principles such as: piezoelectric, electrostatic, thermopneumatic, electrochemical, bimetallic, shape memory alloy, and electromagnetic and their performances are gradually improved
[4][15]. Starting from the initial films which assured limited displacements of few microns (usually below 10–20 µm) the actual films can assure tens of µm even at low currents
[4][16][17].
2. Common Polymers Used in Developing Magnetic Composite Films
The most polymers used in the development of the polymeric films are: polydimethylsiloxane—PDMS; polymethylmethacrylate—PMMA, parylene or polyimides and the range of the polymers can be extended depending on the needs. Table 1 presents the main polymers used and their most important characteristics which recommend or limit their use in the development of the specific micro-devices.
The selection of the polymers is based on their overall performances but also on the final application and operational conditions. Depending on the final applications, more flexible or elastic materials are desired and from this point of view, the polymers mentioned in Table 1 cover the required range especially if consider also blends and layered structures.
Table 1. Properties of different polymers used in the development of the microfluidic devices.