Microelectromechanical Systems Based on Magnetic Polymer Films: Comparison
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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.

  • magnetic polymer films
  • design and fabrication
  • medical applications
  • Microelectromechanical systems (MEMS)

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.
Polymer Specific Properties Ref. *
Polydimethylsiloxane (PDMS) Ease of fabrication by rapid prototyping and good sealing, transparency in the UV-visible regions, low polarity, low electrical conductivity, and elasticity/flexibility; density of 970 kg/m3; Young’s modulus = 0.36–0.87 GPa, tensile or fracture strength is 3.5–7.65 MPa while elongation to break is 76%;

Good chemical stability, being compatible with the following solvents: water, nitromethane, dimethyl sulfoxide, ethylene glycol, perfluorotributylamine, perfluorodecalin, acetonitrile, and propylene carbonate. Higher swelling ratios are reported for diisopropylamine, triethylamine, pentane, and xylenes (1.41–2.13 defined as follow: S = 		D/D0, where D is the length of PDMS in the solvent and D0 is the length of the dry PDMS).

A major drawback of the PDMS-based materials is related to the high deformability. For instance, in the case of the microfluidic systems with thin wall, the diameter can be enlarged several times before failure.		 [18][19][20][21]
Polymethyl methacrylate (PMMA) Good transparency in the visible regions, but filters the UV light bellow 300 nm; durable density of 1180 kg/m3; the glass transition occurs between 100 and 130 °C; water absorption is 0.3%; Young Modulus is 2855 MPa, tensile or fracture strength is 70 MPa while the elongation to break is of 4.5%; PMMA is biocompatible and also biodegradable.

Generally, PMMA is stable in most inorganic chemicals, aliphatic hydrocarbons, cycloaliphatic compounds, fats and oils at room temperature, and also to diluted acids and concentrated solutions of most alkalis at temperatures up to 60 °C but is attacked by chlorinated hydrocarbons, ketones, esters, ethers, alcohols and aromatic compounds.		 [22]
Parylene (PAR) Transparent material, the glass transition temperature <90 °C; depending on the composition, the melting point can vary between 290 and 420 °C; tensile strength is 45–69 MPa for Parylene N/C, the Young’s modulus is 2.4–3.0 GPa (N-C-F) while the elongation to break is 20–200% for Parylene C, can reach 250% for Parylene N while for Parylene F 10–50% at most. Good water absorption (<0.1%); good barrier properties in general; inert to most solvents up to 150 °C; parylene C became soluble in chloro-naphtalene at 175 °C while parylene N at 265 °C (solvent boiling point); diluted inorganic reagents (including acids, alkali, etc.) have no effect bellow 75 °C but, under severe conditions (concentrated acids, 75 °C for 30mins) swelling is observed (ranging from 0.7% for HCl to 8.2% for chromic acid); these polymers are biocompatible. [23][24]
Polyimides (PI) Strong dependence of the properties and performances of the polyimides can be correlated with the composition and synthesis/processing; aromatic polyimides are usually dielectric, tensile strength 72 MPa, their Young modulus can be 3.8–12.2 GPa while the elongation to break is only 8% depending on composition and processing, transparent material in the visible range (80–92% transmittance in the 420–900 nm); excellent thermal stability but also good chemical properties; The solubility is strongly dependent on the nature of the polyimides; there are some polyimides soluble in polar solvents (such as dimethyl acetamide, dimethyl formamide, N-methyl pyrolidone, m-cresol, as well as in conventional polar solvents such as tetrahydrofuran and chloroform) but other polyimides can be very stable.

Loading polyimides with 0.5–1.4% carbon-based materials can greatly approve electrical properties; good biocompatibility and can act as an electrically triggering drug delivery support (if loaded with C).		 [25][26][27][28]
Polyethylene terephthalate (PET) Strong dependence of the properties and performances with the synthesis/ processing; Young Modulus is 2.8–3.17 GPa, tensile or fracture strength is 60–85 MPa while the elongation to break is of 20% but is also strongly dependent on the composition; transparent material in the visible range; good thermal stability, the melting point is 255–265 while the Tg is 67–140 °C; PET is insoluble in water, ethyl ether and most organic solvents but soluble in trifluoro acetic acid, DMSO, nitrobenzene, phenol and o-chlorophenol. The chemical stability in concentrated acids or alkali is poor, thus, limiting the use of this polymer. [29]
Polyethersulphone (PES) Polyethersulfone is a transparent material resistant to acids, alkalis, oils, greases, and aliphatic hydrocarbons and alcohols. It is attacked by ketones, ester, some halogenated and aromatic hydrocarbons, pyridine and aniline; it is very stable, being obtained up to 400 °C, being not oxidized up to 150–190 °C; the Tg is 190–290 °C; Young Modulus is 2.6 GPa, tensile or fracture strength is 83–85 MPa while the elongation to break is of 25–80% being strongly dependent on the composition and processing. [30]
Polystyrene (PS) Transparent, brittle, flammable, thermoplastic, stiff and hard material, obtained by polimerization of styrene. Polystyrene can be copolymerized or blended with other polymers, lending hardness and rigidity to many plastic materials. Flows when heated over 100 °C. Refractive index is 1.6. Soluble in benzene, toluene, ethylacetate, acetone, chloroform, trichloroethylene, cyclohexanone, MEK, THF, etc. Insoluble in water. [31][32]
Polyaniline

(PANI)		 It is one of the best known, studied and applicable conducting polymer. Its properties strongly depend on the type of dopant and its concentration. PANI has many amine functional groups which interact with negative charge anion owing to its inherent cationic nature, such as easy chemical/electrochemical synthesis in a large scale, nontoxicity and good environmental stability. [33][34]
* Some properties are extracted from: https://polymerdatabase.com (accessed on January 2022).

3. MEMS Type Devices Based on Magnetic Polymer Films

MEMS devices are, in fact, microsystems composed of micromechanical sensors, actuators, and microelectronic circuits. While technologies for manufacturing of microelectronic circuits are no longer a challenge, being a well-documented and experienced field over the years, current research in MEMS devices has been almost entirely engaged in the development of micromechanical sensors and actuators, known also as transducers [35]. During the last two decades, a wide range of MEMS were produced, most of them based on quite the same polymers and magnetic components, the major challenge being represented by their design [36][37][38][39][40][41][42][43]. Many approaches have been used in designing such devices, the biomimetic approach being also used in this field. For instance, Vignali et al. [38], designed and fabricated a 3D-printed pump mimicking the motion of the left ventricle using polyurethane (tensile strength: 0.8–1.5 MPa and elongation to break: 170–220%). Polymers are found in various medical instruments with several biological and physicochemical characteristics that make them appropriate for their use in the biomedical field, whether implantable devices or external devices are discussed. The flexibility of polymers is an essential feature that allows for the possibility to apply them in applications involving precise control of movable structures, such as MEMS actuators, with an emphasis on electromagnetic actuation [36].

3.1. Microvalves

A microvalve is a key element in microfluidics that regulates the flow between two ports (inlet and outlet). Over time, microvalves have shown limitations in pumping rates at macroscopic level. Thus, different types of microvalves have been designed and manufactured to overcome those limitations. Currently, there are two main types of microvalves: passive microvalves and active microvalves. The active microvalves can be further categorized into mechanical and non-mechanical microvalves. Based on the initial state, the microvalves are divided in normally open, normally closed, and bistable microvalves [44][45]. The microvalve is widely used in many applications, such as life sciences (especially PCR amplification of the nucleic acids [46]), chemical engineering, microfluidics, printing devices, drug delivery, etc.

3.2. Micromixers

Technological processes that take place at the macroscopic level often require mixing operations for various fields of interest from combustion engines or reactors to pharmaceutical and cosmetic formulations. This mixing operation had also become of interest in the context of the emergence of microfluidics at the microscopic scale [47]. Unfortunately, even if, from a mathematical point of view, diffusion is expected to be faster in the microscale, limitations occur due to low a Reynolds number, gives a laminar flow to the microscale system, slowing down the mixing capacity of two fluids in such a system [48]. Hence, microfluidics also focused on the idea of designing proper devices in order to enhance mixing performances. Micromixers are classified into two general types: passive and active. Passive micromixers are defined as devices that do not use any external stimuli to control the mixing process and that rely mainly on diffusion processes; mixing efficiencies may be enhanced by tuning microchannels in different types of geometries. 

3.3. Microsensors

Microsensors are another branch of microfluidics that have received special attention, primarily due to the progress in the use of elastic polymers that are easy to manufacture and that are sensitive to physiological variations. However, the design and integration of microsensors is currently a limitation, especially in terms of sensitivity, response time and working range. In an attempt to overcome these limitations, research focused on using different types of polymers, in particular to lower the detection limits of devices. An electrochemical impedance spectroscopy (EIS) microsensor, for example, was used to dose traces of glyphosate in water samples, after molecularly imprinted chitosan was covalently attached on the surface of a microelectrode previously treated with 4-aminophenylacetic acid (CMA). 

3.4. Magnetic Labeling and Separation

Several label-free biosensing systems based on liquid crystals (LCs) have been developed during the last decade. These include biodetection at the LC–glass interface, typically comprising a thin film of LC sandwiched between two glass substrates to produce an LC cell, and at the LC–aqueous interface in the form of LC films or LC-in-water droplets. Although the biosensing capability of both thermotropic and lyotropic LCs has been proven, most LC-based biosensors described to date have used the nematic 5CB as the sensing mesogen. LC–photopolymer composites have demonstrated important implications in biomedical applications, such as label-free and single-substrate biodetection.

4. Design and Fabrication of the Microfluidic Systems

The functionality of the microfluidic systems is strongly associated with the design and nature of the active and passive components; however, the influence of the microscale must also be considered. Mechanical and non-mechanical actuation can be used to generate movement. It is generally accepted that mechanical actuation is more advantageous in term of controllability, high vibration rate and large membrane deformation [17].  The use of the soft lithography can be used in developing the complex systems and can be conducted in several steps, starting with the development of the electromagnetic part, followed by the fabrication of the magnet-o mechanical part, and, finally, by attaching the two components using adequate epoxy resins. 3-D printing is an alternative way of generating even complex 3D-structures. Various printing approaches can be used, such as: stereolithography- SLA [49], inkjet (including multijet) [50][51] or fused deposition modeling (FDM) [52][53] techniques. 

5. Conclusions

Composite films based on several polymers and magnetic powder are increasingly developed for the development of the microfluidic systems; they include microvalves, micropumps, micromixers, microsensors, drug delivery micro-systems, magnetic labeling and separation microsystems, etc. Poly(dimethylsiloxane) and Parylene C/N seem to be suitable polymers having adequate stability in a wide range of solutions, including aqueous solution at moderate acidic or alkaline pH. Moreover, the mechanical properties including elongation to break and tensile strength are proper for the most applications. From the point of view of the preparation/processing and price, PDMS is most convenient and this is why PDMS-based films are more frequent comparing to the other polymers. To make the PDMS easily to be removed from the mould, a physical (to grease the mould with silicon oil) or a chemical approach (by functionalizing it with adequate silanisation agent) can be used. If superior properties are requested, PDMS-based materials coated with parylene, or even pure parylene films, can be used, thus, taking – about the compatibilization of the layers by using adequate techniques.

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