Microfluidics Flashcards
Microchannels
A microchannel is a passive component used in almost any microfluidic system. It connects the various parts of the fluidic system together and can be fabricated using various methods in glass, polymers or silicon.
- WAFER BONDING SEALING OF BULK ETCHED CHANNELS: grooves and cavities in Si wafers can be formed using dry or wet anisotropic or isotropic Si and glass etching. Anisotropic etching in (110) Si can be used to fabricate densely packed flow channels. To bond the wafers together: Silicon Direct bonding or Silicon fusion bonding and Anodic bonding of silicon to glass.
- UNDERCUT ETCHING AND CHANNEL SEALING BY DEPOSITION: we can obtain surface micromachined channels, tipically a few microns high and 10-100 micrometers wide, by etching a sacrificial layer (SiO2, Poly-si or PSG) under a top layer (Poly or Silicon nitride). The etched channels are sealed in a deposition process.
- PLANTING SEAL: trade off between the first two. Channels are made by electroplating with thick photoresists as the sacrificial molds.
Micromixers
Mixing several fluids in chambers of micron scale is not easy because the Reynolds number for these dimensions is usually rather small so no turbulence occurs. Mixing can be accomplished through diffusion. Most micromixers are based on diffusion and some improve the mixing by multiplying the interface: subdividing, twisting, distorting and expanding the interfaces.
A chaotic micromixer is composed of a structure of 2-layer crossing channels. It uses two streams: when one stream is stopped, the other will bulge into both halves of the main channel.
Microfilters
Mechanical microfilters are typically membranes consisting of an array of precisely fabricated pores. The pore array is placed across the entire width of a microchannel. Microfilters are used for preventing a microchannel from blocking, for mechanical separation or isolation of large particles, such as blood cells, from the fluid solution, or for washing out waste products from the fluid.
Microneedles
Minimally invasive medical field: limited tissue damage and reduced pain.
To obtain the necessary fluid fllow a large number of microneedles on the same device area must be used.
-In-plane: most convenient because it can be fabricated with state-of-the-art planar technology, including surface micromachining and etching, and creates a good degree of flexibility with respect to different needle designs. BUT only one row of needles can be made per chip so the density of needles is limited.
-Out-of-plane: large number of parallel microneedles that may give the necessary redundancy in case some needles get blocked during operation.
Microfluidic actuators
The performance of active microvalves and micropumps depends strongly on the performance of the actuator so miniaturizing the actuator is important.
EXTERNAL:
-Fabricated separately from the devices;
-more expensive for commercial applications;
- Electromagnetic actuation: with a solenoid plunger or an integrated magnetic material in conjunction with an externally generated magnetic field.
-Piezoelectric actuation: disks or bimorphs, usually by adhesively attaching the piezoelectric devices to a micromachined device.
-Pneumatic actuation: tipically with small diameter tubes.
-Shape Memory Alloy (SMA) actuators coupled with micromachined devices.
INTEGRATED
-Fabricated as part of the devices
-Actuation force provided through electrostatic, electromagnetic, thermopneumatic, bimetallic or shape memory effect.
Microvalves
They are used for shutting off the flow or to control the amount of fluid travelling trough the valve.
- Active (with actuator): external or integrated actuator;
- Passive (without actuator): they get the working energy from the pressure difference across the valve;
- Fixed: based on asymmetric fluid behaviour.
Active microvalves (external actuator)
-ELECTROMAGNETIC ACTUATION: Electromagnetic actuation can be realized by using a solenoid plunger. The force developed by these actuators depends on the applied current and the number of turns of wire in the solenoid. Integration of such a valve is difficult because of the difficulty in obtaining sufficient turns in a solenoid and in providing a low-loss magnetic return path. Alignment and attachment of the actuator to the substrate generally requires more area than the active valve area.
-PIEZOELECTRIC ACTUATION: Piezoelectric actuators are commercially available as both disks and cantilever beams.
These actuators are usually bimorphs, where two layers of piezoelectric material are stacked so that the applied voltage causes one layer to contract and another to expand, thus providing for more efficient response. The deflection from bimorph cantilever beams can be relatively large, but the corresponding force is low (and viceversa).
-SHAPE MEMORY ALLOY ACTUATION: Shape memory alloys have the ability to return to a previously defined shape when subjected to an appropiate thermal procedure: when the temperature is raised or lowered, the crystal structure of the shape memory alloy transforms from a low temperature state to a high temperature state and when the wire is heated to the high temp state it will generate force/stress and recover its original shorter shape, thus it can be used as an actuator.
Small actuators can be made by using a shape memory alloy (SMA) spring working against a bias spring. The SMA coil returns to its original shape when it is heated above its critical temperature. It is difficult to control the displacement precisely in these devices, so they can only be used as on-off valves.
Active microvalves: integrated actuator
-ELECTROSTATIC ACTUATION: An electrostatic actuator in its simplest form has a movable planar electrode and a fixed electrode.
The generated force is proportional to the square of applied voltage and inversely proportional to the square of gap distance between the electrodes. For example, with an applied voltage of 10V, a gap of 10 microm (medium between the gap is air) generates electrostatic pressure of 4.4Pa. If the voltage is increased to 100V and the gap reduced to 5 microm, the pressure is increased by a factor of 400. The force generated by a simple cantilever type of valve is small when the electrode gap is large and the controllable pressure range is also limited.
-BUBBLE ACTUATED GATE VALVE: This valve consists of a single crystal silicon gate that is thrust across a channel by expanding bubbles.
It is electrically actuated; the electrically actuated bubbles expand to push the gate on demand.
The device can be easily integrated by fabricating multiple devices on the same die. It minimizes leakage by forcing a solid piece of silicon across the channel. Actuation occurs with only 116 microJ. Actuation frequencies as high as 3Hz have been achieved.
-BIMETALLIC ACTUATION: Use of an appropriate combination of the two metallic materials.
The generated pressure is proportional to the difference between the thermal expansion coefficients of the two materials and the temperature difference. Although a number of combinations of materials can be used for the bimetallic actuator, the use of a silicon diaphragm and an aluminum (or alternatively nickel) layer is one of the most attractive. A typical structure consists of a diaphragm with a central boss and a bimetallic actuator.
The actuator is a circular silicon diaphragm having diffused resistors and an anular aluminium region which are the elements of the bimetallic structure.
The temperature of the bimetallic structure on the diaphragm is used to control the force applied to the central diaphragm boss so as to control the gas flow. When the valve is cold, the aluminum and silicon are relaxed and the valve is closed. As temperature increases, the aluminum expands more than the silicon and goes into compression, creating an upward bowing force which lifts the boss off a central orifice and allows flow. This valve can control a gas flow ranging from 0 to 90 ml/min. The leakage flow is about 45 microl/min, which corresponds an on-to-off ratio of about 1600.
The relaxation time is the dominant factor in the response time.
Passive (check) microvalves
Microvalves without actuators are mainly used for check valves of micropumps.
In the use of micropumps, very small leakage under reverse applied pressure and large reverse-to-forward flow resistance ratio are required.
The response time which means transition time during open-to-close or close-to-open is also an important parameter of valves.
Check valves are operated by the pressure difference across the valves instead of an independently controllable actuator. Check valves tend to be small compared to valves with integrated or external actuators.
They behave like fluidic diodes.
-spring valve: the valve consists of a flat flapper attached to the channel wall by a spiral spring. The flapper is constrained to only move in one direction. When there is a pressure differential in the preferred direction, the flappers are pushed aside allowing the fluid to flow freely. When the pressure is opposite to the preferred direction, the flappers are pushed up against their stops and flow is obstructed.
Fixed microvalves
Such valves do not open and close, but due to their shape alone provide less flow resistance in one direction compared to that in the opposite direction.
An example is nozzle-diffuser valve in a piezoelectric actuation pump.
During the supply mode more fluid will flow through the inlet element than through the outlet element, while the opposite happens during the pump mode. The result is a net flow from the inlet to the outlet side of the pump. Two parallel pump chambers working in anti-phase are used to reduce the inlet and outlet pressure pulses and to increase pump flow performance.
Different designs of the same system have been realized with different materials (silicon, plastics, …) and technologies (DRIE, molding, …).
-Silicon-etched nozzle diffuser valve oriented-in-plane: consists of a special loop that connects two parts of a channel. As fluid flows in one direction, very little goes through the loop. In the opposite direction, much of the fluid flows into the loop instead of through the main channel. This, then empties into the main channel perpendicular to the main direction of flow. As a result, once again there is a lower fluidic resistance in the first direction.
Advantages of fixed microvalves:
-relatively simple and cheap to fabricate, and more robust;
-more freedom for design of pump resonance because the operating frequency is not limited by the time needed for mechanical valves to open and close.
Cons:
-They are good at creating a net flow when pulsed by alternating pressures, however, they have no substantial resistance to constant counter-pressure;
-they are dependent on inertial effects and become much less effective at low Reynolds numbers.
Mechanical pumps with moving parts
There are two kinds:
- reciprocating: consist of a pressure chamber having a flexible diaphragm driven by an actuator and passive microvalves (check valves);
- peristaltic.
Reciprocating pumps
-piezoelectric: A micropump using a piezo disk glued onto a glass diaphragm was the first developed. Piezoelectric micropumps can be made without check valves, but with nozzles or restrictors that act in a similar way. In fact one disadvantage of a micropump using check valves is the restriction on the driving frequency due to the slow actuation of the check valves. Without check valves the micropump can be driven at frequencies of hundreds of Hertz.
During the supply mode a larger amount of fluid flows into the pump chamber through the inlet element, which acts as a diffuser, than through the outlet element, which acts as a nozzle. The opposite is true during the pump mode.
Thus, during a complete pump cycle a net fluid volume is transported from the inlet to the outlet side.
-Electrostatic: An electrostatically driven micropump has been made which consists of four stacked silicon wafers. The liquid to be pumped is not subject to any electrical field, so that saline solutions or drugs can also be pumped.
Upon application of an electrical voltage, the membrane will bend outward, leading to a fluid flow through the inlet valve into the pump chamber. When the supply voltage is shut off, the membrane will bend back and the fluid is forced through the outlet valve.
Peristaltic piezoelectric pumps
The micro peristaltic pump is actuated by 3 piezo-discs located in a recess etched in the Pyrex. The fluid inlet and outlet holes are etched into the Pyrex wafer together with the pump membrane. Six wire bonding holes are also etched into the Pyrex wafer to provide electrical access. The PCR is to be achieved by introducing the reactant droplet of 1 µl into the inlet hole then, by driving the micro bi-directional pump, the reaction droplet is to be moved back and forth between these three reaction chambers at 90°C, 72°C and 55°C.
The position of the droplet is detected and controlled as a result of an optical signal detected by a pn-diode integrated in the chip as the droplet meniscus scatters the illumination light.
Pressure drive pumping
One of the most common methods of generating pressure for fluid flow in microfluidics is positive displacement pumping. Ultra-precise syringe pumps often are used for this purpose.
In pressure-driven flow, the flow rate Q (m3/s) is given by Q = DeltaP/R , where DeltaP is the pressure drop across the channel (Pa), and R is the channel resistance (Pas/m3 ).
The pressure drop can be created either by opening the inlet to atmospheric pressure and applying a vacuum at the outlet, or by applying positive pressure at the inlet (e.g.,via a syringe pump) and opening the outlet to atmospheric pressure.
Air bursting detonator pumping
Pressurized gas in a microreservoir can be utilized as an energy source to control fluidic sequencing on a disposable biochip. The gas can be compressed and stored for subsequent usage and this pressurized gas can be released upon “triggering” by a short electrical pulse.
Once an electric power pulse is sent to the micro-heater, thermal stress of the membrane will increase until the membrane is broken and the pressurized gas pushes the fluid sample into the microchannel through a broken hole on the membrane. Low power consumption is guaranteed since only one pulsed power is used to burst the pressurized gas. In case of a conventional micropump a continuous supply of electrical energy must be provided in order to
actuate the micropump. For the air-bursting detonator, the work done in displacing the fluid is accomplished by the potential energy stored in the compressed gas.
