Digital microfluidics
Encyclopedia
Digital microfluidics is an alternative technology for lab-on-a-chip systems based upon micromanipulation of discrete droplets. Microfluidic processing is performed on unit-sized packets of fluid which are transported, stored, mixed, reacted, or analyzed in a discrete manner using a standard set of basic instructions.
, digital microfluidics works much the same way as traditional bench-top protocols, only with much smaller volumes and much higher automation. Thus a wide range of established chemistries and protocols can be seamlessly transferred to a nanoliter droplet format. Electrowetting
, dielectrophoresis
, and immiscible-fluid flows are the three most commonly used principles, which have been used to generate and manipulate microdroplets in a digital microfluidic device.
properties of liquid. For example, water placed on a hydrophobic surface will lower its contact with the surface by creating drops whose contact angle
with the substrate will increase as the hydrophobicity increases. However, in some cases it is possible to control the hydrophobicity of the substrate by using electrical fields. This is referred to as Electrowetting
on dielectric or EWOD. In thin layers of Teflon AF, FluoroPel V-polymer or CYTOP, for example, while no field is applied the surface will be extremely hydrophobic and a droplet of water will try to 'stay away' from the surface, resulting in a droplet with steep walls. When a field is applied, a polarized hydrophilic surface is created, and the water droplet tries to 'get closer' to the surface, resulting in much more spread out droplet. By controlling the localization of this polarisation it is possible to control the displacement of the droplet.
in 1987 http://www.nsf.gov/awardsearch/piSearch.do;jsessionid=D05E82394F781CBA17DB0C5AC8E3C0B8?SearchType=piSearch&page=1&QueryText=&PIFirstName=james&PILastName=brown&PIInstitution=cytonix&PIState=MD&PIZip=&PICountry=US&RestrictExpired=on&Search=Search#results and subsequently commercialized by Advanced Liquid Logic, there are two parallel glass plates, and the bottom plate contains a patterned array of individually controllable electrodes, and the top plate is coated with a continuous grounding electrode
. A dielectric
insulator
coated with a hydrophobic is added to the plates to decrease the wettability of the surface and to add capacitance
between the droplet and the control electrode. The droplet containing biochemical samples and the filler medium, such as the silicone oil
, a fluorinated oil or air are sandwiched between the plates; the droplets travel inside the filler medium. In order to move a droplet, a control voltage
is applied to an electrode
adjacent to the droplet, and at the same time, the electrode just under the droplet is deactivated. By varying the electric potential
along a linear array of electrodes, electrowetting
can be used to move droplets along this line of electrodes.
Basics
In analogy to digital microelectronics, these basic instructions can be combined and reused within hierarchical design structures so that complex procedures (e.g. chemical synthesis or biological assays) can be built up step-by-step. And in contrast to continuous-flow microfluidicsMicrofluidics
Microfluidics deals with the behavior, precise control and manipulation of fluids that are geometrically constrained to a small, typically sub-millimeter, scale.Typically, micro means one of the following features:* small volumes...
, digital microfluidics works much the same way as traditional bench-top protocols, only with much smaller volumes and much higher automation. Thus a wide range of established chemistries and protocols can be seamlessly transferred to a nanoliter droplet format. Electrowetting
Electrowetting
Electrowetting is the modification of the wetting properties of a surface with an applied electric field.-History:...
, dielectrophoresis
Dielectrophoresis
Dielectrophoresis is a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field. This force does not require the particle to be charged. All particles exhibit dielectrophoretic activity in the presence of electric fields...
, and immiscible-fluid flows are the three most commonly used principles, which have been used to generate and manipulate microdroplets in a digital microfluidic device.
Working Principle
Droplets are formed using the surface tensionSurface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
properties of liquid. For example, water placed on a hydrophobic surface will lower its contact with the surface by creating drops whose contact angle
Contact angle
The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...
with the substrate will increase as the hydrophobicity increases. However, in some cases it is possible to control the hydrophobicity of the substrate by using electrical fields. This is referred to as Electrowetting
Electrowetting
Electrowetting is the modification of the wetting properties of a surface with an applied electric field.-History:...
on dielectric or EWOD. In thin layers of Teflon AF, FluoroPel V-polymer or CYTOP, for example, while no field is applied the surface will be extremely hydrophobic and a droplet of water will try to 'stay away' from the surface, resulting in a droplet with steep walls. When a field is applied, a polarized hydrophilic surface is created, and the water droplet tries to 'get closer' to the surface, resulting in much more spread out droplet. By controlling the localization of this polarisation it is possible to control the displacement of the droplet.
Implementation
In one of various embodiments of EWOD-based microfluidic biochips, investigated first by CytonixCytonix
Cytonix is an American manufacturing company.The company was founded in 1984 as "Cyto Fluidics" at the University of Maryland's technology incubator to develop materials and applications for a "fluid transistor" concept based on electrowetting and digital microfluidics...
in 1987 http://www.nsf.gov/awardsearch/piSearch.do;jsessionid=D05E82394F781CBA17DB0C5AC8E3C0B8?SearchType=piSearch&page=1&QueryText=&PIFirstName=james&PILastName=brown&PIInstitution=cytonix&PIState=MD&PIZip=&PICountry=US&RestrictExpired=on&Search=Search#results and subsequently commercialized by Advanced Liquid Logic, there are two parallel glass plates, and the bottom plate contains a patterned array of individually controllable electrodes, and the top plate is coated with a continuous grounding electrode
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
. A dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
insulator
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
coated with a hydrophobic is added to the plates to decrease the wettability of the surface and to add capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
between the droplet and the control electrode. The droplet containing biochemical samples and the filler medium, such as the silicone oil
Silicone oil
A silicone oil is any polymerized siloxanes with organic side chains. They are formed of alternating silicon-oxygen atoms or siloxane, rather than carbon atoms . Other species attach to the tetravalent silicon atoms, not to the divalent oxygen atoms which are fully committed to forming the...
, a fluorinated oil or air are sandwiched between the plates; the droplets travel inside the filler medium. In order to move a droplet, a control voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
is applied to an electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
adjacent to the droplet, and at the same time, the electrode just under the droplet is deactivated. By varying the electric potential
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
along a linear array of electrodes, electrowetting
Electrowetting
Electrowetting is the modification of the wetting properties of a surface with an applied electric field.-History:...
can be used to move droplets along this line of electrodes.
External links
- UBC Okanagan Digital Microfluidics
- Advanced Liquid Logic - Powered by Digital Microfluidics
- Keck Graduate Institute Microfluidics Laboratory
- Max Planck Institute for Dynamics and Self-Organization Prof. Stephan Herminghaus, Dr. Ralf Seemann Group
- Wheeler Digital Microfluidics Group at the University of Toronto
- Digital Microfluidics at Duke University
- The University of Chicago
- Harvard - Weitz Group
- Digital Microfluidics at University of California, Irvine
- Applications of Digital Microfluidics - Garrell Group at UCLA
- UCLA Micromanufacturing Laboratory - Kim Group at UCLA
- IMNS - The University of Edinburgh
- Cytonix Fluid Transistor Group