Membrane Actuation for Micropumps
Membrane actuation for micropumps refers to the reciprocating periodic motion of a thin flexible layer – diaphragm or membrane made of silicon or other materials – bounding one side of a displacement micropump to create volume and pressure oscillations in a fluid (i.e., liquid or gas) stored in the chamber of the micropump that is rectified by other means to accomplish a net fluid flow through the micropump. The mechanical energy necessary for the membrane actuation in micropumps is generally derived from electrical, thermal, optical, or other forms of energy.
Reciprocating displacement micropumps have been designed with a wide range of geometry and performance characteristics for various applications. Most reported reciprocating displacement micropumps are roughly planar structures between 1 and 4 mm thick . The membranes in these micropumps are also mostly planar with thickness values ranging from 5 μm to several hundreds of micrometers. Nonplanar membrane geometries with bosses at their centers have also been applied to a limited extent in reciprocating displacement micropumps. Most membranes are circular in shape; however, square-shaped membranes have also been used in few micropumps reported in the literature. The overall size of the membrane, which must be large enough to produce the required volume displacements, depends on the micropump capacity and the actuation driver force capacity. Even if the driver is capable of supplying effectively large force, the membrane displacement is limited by the diaphragm’s failure criteria, which scale unfavorably with decreasing membrane diameter. Scaling down the membrane diameter presents a significant challenge for designers of reciprocating displacement micropumps.
The choice of the passive membrane material is critical to the performance of micropump and is often dictated by the type of actuator and the fabrication method selected. For micropumps driven by low-frequency and/or low-force actuators, a low-modulus material generally allows the volume displaced by the membrane to be maximized, favorably impacting performance. Commonly used low-modulus materials for membranes are elastomers, polyimide, perylene, and polydimethylsiloxane (PDMS) and silicone rubber. However, since the membranes act against with the working fluid in the micropump, soft polymer membranes suffer from stability issues. For actuators capable of operating at a high frequency and adequate force, the fast mechanical response of a stiff membrane generally yields the best performance. For this reason, silicon, glass, and plastic are the most commonly used materials for membranes driven by piezoelectric actuation, which is capable of operating at very high frequencies.
The most common method for fabricating passive silicon membranes for micropumps is micromachining combined with glass bonding layers. The micromachining techniques generally involve photolithography followed by either wet etching or deep reactive ion etching (DRIE) from one side of the wafer. Integration of actuators and electrodes with the passive membrane involves additional fabrication steps that are often complicated. For example, a composite of thin-film piezoelectric material layer sandwiched between two electrode layers is formed on the membrane by a variety of techniques including physical vapor deposition techniques (ion beam sputtering, RF planar magnetron sputtering, dc magnetron sputtering), chemical vapor deposition (CVD) techniques (metal-organic-chemical vapor deposition or MOCVD), chemical solution deposition (CSD) techniques (including sol-gel routes, metal-organic decomposition), and pulsed laser deposition (PLD) . A number of reciprocating displacement micropumps have been fabricated through means other than traditional silicon/glass micromachining such as precision machining of brass, stereolithography of an ultraviolet-photocurable polymer. Improvements in techniques for fabricating precision components from plastic have led to increasing use of plastics in reciprocating displacement micropumps.
Purpose of the Membrane Actuation in Micropumps
The main purpose of the membrane actuation in a displacement micropump is to create volume, and therefore, pressure oscillations in the chamber of the micropump to enable displacement of the fluid stored in the micropump chamber. During its operation, an actuator drives the membrane displacing the membrane from its initial flat configuration in a transverse (up and down) motion in a periodic manner. The membrane actuation frequencies vary widely from DC to several kilohertz in the micropumps reported depending on the application. The working principle of a reciprocating displacement micropump can be described by a cyclic process, which is divided into a supply mode (the pump chamber volume increases) and a pump mode (the pump chamber volume decreases). The alternating stroke volume ΔV of the membrane causes increase and decrease in the dead (initial) volume V 0 of the micropump chamber cyclically leading to pressure oscillations of amplitude | Δp | in the pump chamber. During the supply mode, low pressure is generated in the pump chamber, which causes fluid to be drawn into the pump chamber through the inlet valve, as soon as Δp becomes larger than the inlet valves threshold pressure Δp crit. During the pump mode, overpressure occurs in the pump chamber, which transfers liquid from the pump chamber into the outlet as soon as Δp becomes larger than the outlet valves threshold pressure Δp crit . In this stage, the inlet valve is designed to prevent a reverse flow, and likewise the outlet valve does during the supply phase. Thus, the membrane volume oscillations are rectified into a net flow through the micropump over a two-stroke pump cycle by means of valves at the inlet and outlet oriented to favor flow into and out of the micropump chamber during the chamber expansion stroke and contraction stroke, respectively.
This section details the actuation techniques used to drive the membrane, the modeling techniques for designing a membrane actuator, and the fabrication techniques employed to realize membrane actuators.
Membrane Actuation Techniques
The periodic motion of the passive membrane structure in a micropump is achieved by means of an actuator that is either external to the membrane or integrated with the membrane. A wide range of actuation principles involving both external and integrated actuators have been applied to drive membranes in micropumps. The membrane actuator stroke, stiffness, frequency of operation, response time, and hence, the performance of the micropump, are directly impacted by the choice and design of the actuator principle. External actuation principles used for membranes in micropumps include electromagnetic actuators with solenoid plunger and external magnetic field, disk-type or cantilever-type piezoelectric actuators, stack-type piezoelectric actuators, pneumatic actuators, and shape memory actuators. While external actuators have the advantage of imparting relatively large forces and displacement to the membranes, their bulkiness restricts the packaged size of the micropumps. Integrated actuators are micromachined with the pumps. Most common integrated actuators are thin-film-type piezoelectric actuators, electrostatic actuators, thermopneumatic actuators, electromagnetic actuators, and thermomechanical (bimetallic) actuators [3, 4]. Other less commonly used actuation principles used for membrane actuation in micropumps include shape memory alloys, magnetostrictive materials, electrowetting, bimetallic drivers, and magnetoelastic drivers. The three most common integrated actuator principles are discussed in more detail below.
The thermopneumatic actuation has the ability to produce relatively large stroke and pressures but needs a large amount of thermal energy, and consequently, such actuators consume considerable amount of electric power. The temporal response of thermopneumatic actuators is limited by the rate of heat transfer into and out of the secondary working fluid, and so thermopneumatically driven membranes typically operate at relatively low frequencies. Other drawbacks of thermopneumatic actuation include high temperatures and complicated thermal management. The thermopneumatic principle, illustrated in Fig. 2d, involves heating (usually with an integrated thin-film resistive heater) of a secondary working fluid causing it to expand, and thereby, actuate the membrane. In order to maximize the volume displaced by the membrane, low-modulus materials such as polyimide, pyrelene, elastomer, and polydimethylsiloxane (PDMS) are often used for membranes driven by thermopneumatic principle.
Key Research Findings
Membrane actuation used in reciprocating micropumps has enabled numerous applications involving precise fluid handling very small volumes of the working fluid over the past two decades. The piezoelectrically driven micropumps, which can be traced to a class of ink jet printheads developed in the 1970s, have received considerable attention and such micropumps are now available commercially. Micropumps using membrane actuation have been developed for dispensing therapeutic agents into the body . Among such micropumps, implantable insulin delivery systems for maintaining diabetics’ blood sugar levels without frequent needle injections are currently available. Micropumps might also be used to dispense engineered macromolecules into tumors or the bloodstream. Micropumps are also found in a few current-generation micro total analysis systems (μTAS) employed in chemical and biological analysis.
Future Directions for Research
The membrane actuation is crucial for micropumps as researchers are working on developing new applications for micropumps and further miniaturization of micropumps for precise handling of even smaller amounts fluids on the order of nano- and picoliters. On the new applications side, micropumps are being developed for use in cryogenic spot cooling of microelectronic devices and micropropulsion for space exploration . Control of smallest amounts of fluids is of increasing interest in modern bioanalytical and pharmaceutical research and industry. Micropumps including those utilizing membrane actuations will see a more widespread application especially in the life sciences area as researchers are seeking to address issues related to their reliability, complexity of microfabrication, power consumption, cost, and biocompatibility. Research is also ongoing to improve the performance of currently available membrane actuation technologies and active materials for membrane actuation.