This review gives a brief overview of microvalves, and focuses on the actuation mechanisms and their applications. One of the stumbling blocks for successful miniaturization and commercialization of fully integrated microfluidic systems was the development of reliable microvalves. Applications of the microvalves include flow regulation, on/off switching and sealing of liquids, gases or vacuums. Microvalves have been developed in the form of active or passive microvalves employing mechanical, non-mechanical and external systems. Even though great progress has been made during the last 20 years, there is plenty of room for further improving the performance of existing microvalves.

http://iopscience.iop.org/article/10.1088/0960-1317/16/5/R01/pdf

A review of microvalves

TiNi thin films have attracted much attention in recent years as intelligent and functional materials because of their unique properties. TiNi thin film based micro-actuators will become the actuator of choice in many aspects in the rapidly growing field of micro-electro-mechanical systems (MEMSs). In this review paper, some critical issues and problems in the development of TiNi thin films are discussed, including preparation and characterization considerations, residual stress and adhesion, frequency improvement, fatigue and stability, modeling of behavior as well as functionally graded or composite thin films. Comparison is made of TiNi SMA micro-actuation with other micro-actuation methods. Different types of TiNi thin film based microdevices, such as microgrippers, microswitches, microvalves and pumps, microsensors, etc. are also described and discussed.

/pdf/tini-based-thin-films-in-mems-applications-a-review-23wqos5t8n.pdf

TiNi-based thin films in MEMS applications: a review

This paper briefly overviews progress on the development of MEMS-based micropumps and their applications in drug delivery and other biomedical applications such as micrototal analysis systems (μTAS) or lab-on-a-chip and point of care testing systems (POCT). The focus of the review is to present key features of micropumps such as actuation methods, working principles, construction, fabrication methods, performance parameters and their medical applications. Micropumps have been categorized as mechanical or non-mechanical based on the method by which actuation energy is obtained to drive fluid flow. The survey attempts to provide a comprehensive reference for researchers working on design and development of MEMS-based micropumps and a source for those outside the field who wish to select the best available micropump for a specific drug delivery or biomedical application. Micropumps for transdermal insulin delivery, artificial sphincter prosthesis, antithrombogenic micropumps for blood transportation, micropump for injection of glucose for diabetes patients and administration of neurotransmitters to neurons and micropumps for chemical and biological sensing have been reported. Various performance parameters such as flow rate, pressure generated and size of the micropump have been compared to facilitate selection of appropriate micropump for a particular application. Electrowetting, electrochemical and ion conductive polymer film (ICPF) actuator micropumps appear to be the most promising ones which provide adequate flow rates at very low applied voltage. Electroosmotic micropumps consume high voltages but exhibit high pressures and are intended for applications where compactness in terms of small size is required along with high-pressure generation. Bimetallic and electrostatic micropumps are smaller in size but exhibit high self-pumping frequency and further research on their design could improve their performance. Micropumps based on piezoelectric actuation require relatively high-applied voltage but exhibit high flow rates and have grown to be the dominant type of micropumps in drug delivery systems and other biomedical applications. Although a lot of progress has been made in micropump research and performance of micropumps has been continuously increasing, there is still a need to incorporate various categories of micropumps in practical drug delivery and biomedical devices and this will continue to provide a substantial stimulus for micropump research and development in future.

https://www.nectec.or.th/mems/download/MEMS-based%20Micropumps%20in%20Drug.pdf

MEMS-based micropumps in drug delivery and biomedical applications

This paper briefly reviews recent research and developments of micropump designs with a particular emphasis on mechanical micropumps and summarizes their applications in biomedical fields. A comprehensive description of the actuation schemes, flow directing concepts and liquid chamber configurations for micro pumping is provided with illustrative diagrams. Then, a comparative study of current mechanical micropump designs highlighting their advantages and limitations for various applications is presented, based on performance criteria such as actuation voltage and power consumption, ranges of operating frequency and maximum flow rate and backpressure. This study compiles and provides some basic guidelines for selection of the actuation schemes and flow rate requirements in biomedical applications. Different micropumps in biomedical applications, such as blood transport and drug delivery also have been reviewed.

Current micropump technologies and their biomedical applications

Micro-dosing/drug delivery control system is a bio-chip in practice. It is mostly developed by Micro-electro-mechanical Systems (MEMS) technology. In micro-dosing or drug delivery control systems, the driving power source with driving methodology and bio-compatibility are the two key issues that a great deal of researchers are truly interested in. Since the micro-dosing and drug delivery systems are applied on human bodies, there inevitably exist inherent limitations. Our study is aimed at driving technology review from all aspects. Comparisons are made to unveil the advantages and shortcomings of different driving designs. In addition, bio-compatibility is addressed and discussed, especially upon the currently-used and potential bio-materials in bio-MEMS.

Review of MEMS-based drug delivery and dosing systems

The internal stress in SMA/Si bimorph structure was investigate din this paper. The shape memory alloy (SMA) thin films were sputter-deposited onto single crystal silicon substrates at room temperature and thin annealed at high temperature for crystallization. The internal stresses in SMA films before and after crystallization were measured by substrate-curvature method based on S. Timoshenko's theory. The results show that the internal stress changes from compressive to tensile after film crystallization. The intrinsic stress in the sputter-deposited SMA films is almost relaxed completely during the crystallization annealing and the internal stress in crystallized SMA film is dominated by thermal stress. By varying the sputtering power during deposition, the interface status and intrinsic stress can be controlled and excellent SMA/Si bimorph actuation structure can be obtained.

Control of internal stress in SMA/Si bimorph microactuators

A novel micropump actuated by NiTi/Si diaphragm has been developed. In order to optimize the actuating performance of the micropump, the dynamic actuating properties were studied in different actuating conditions such as different actuating currents, frequencies and duty cycles. The experimental result show that there is a maximum displacement when increasing the actuating current and frequency. The influence of duty cycle on maximum displacement when increasing the actuating current and frequency. The influence of duty cycle on maximum displacement with water flow and without water flow is different. The higher the displacement of the diaphragm is, the larger the flow rate is for a given frequency. The displacement of the pump diaphragm depends not only on the flow rate, but also on the moving frequency. The change of the resistance of NiTi strip indicates that the A - M phase transformation is completed partly during dynamic actuating processes. The maximum flow rate of 360 (mu) l/min was obtained in about 50Hz with 1:1 duty cycle in our experiment.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Dynamic actuation behavior of NiTi/Si diaphragm micropump

CVD diamond film is an attractive potential material for microelectronics and MEMS application, but patterning ofdiamond is one of the main difficulties hindering diamond electronics. In this paper, we studied the RIE process for precisepatterning of diamond films. Ni/Cr, Cu/Cr or NiTi films were used as maskants, oxygen as reactive gas. The etching resultsshowed that the NiTi film has high etching selectivity when used as mask for diamond etching and it is very convenient tobe patterned by the special developed chemical etchant. Using 02 as main etching gas, the RIE can etch the diamond filmeffectively. The process parameters such as RE power, vacuum pressure have marked influence on the etching rate and theetched surface. Consider the etching rate and the smoothness of the etched surface, the optimized process parameters forpatterning of diamond film has been defined. Combined with sacrifice layer process and electroplating through masktechnique, diamond micro hinge has been fabricated on silicon wafer.Keywords: CVD diamond, RIE

Patterning of diamond films by RIE and its MEMS applications

A novel micropump actuated by NiTi/Si composite diaphragm was described in this paper. The driving principle, microfabrication processes and characteristics of the pump were reported. The pump is composed of a deformable chamber and two silicon flap check valves. The outer dimension of the pump is 6 mm ×6 mm ×1.5 mm , with the diaphragm size of 3 mm ×3 mm ×20  μm. The fabrication processes include silicon micromachining and AuSi eutectic bonding. By using the recoverable force of NiTi thin-film and biasing force of silicon membrane, the actuation diaphragm realized reciprocating motion effectively. Experimental results show that the pump has superior performance, such as high pumping yield (up to 340 μl/min), high working frequency (up to 100 Hz), and long fatigue life time (more than 4×107 working cycles).

Characteristics and fabrication of NiTi/Si diaphragm micropump

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Papers

Control of internal stress in SMA/Si bimorph microactuators

Dynamic actuation behavior of NiTi/Si diaphragm micropump

Patterning of diamond films by RIE and its MEMS applications

Characteristics and fabrication of NiTi/Si diaphragm micropump