View source for FlowFET

A '''flowFET''' is a [[Microfluidics|microfluidic]] component which allows the rate of flow of [[liquid]] in a microfluidic channel to be modulated by the [[Electric potential|electrical potential]] applied to it. In this way, it behaves as a microfluidic analogue to  the [[Field-effect transistor|field effect transistor]],<ref name="Schasfoort">{{cite journal
 | last1 = Schasfoort
 | first1 = Richard B. M.
 | last2 = Schlautmann
 | first2 = Stefan
 | last3 = Hendrikse
 | first3 = Jan
 | last4 = van den Berg
 | first4 = Albert
 | title = Field-Effect Flow Control for Microfabricated Fluidic Networks
 | journal = Science
 | volume = 286
 | issue =5441
 | pages = 942–945
|date=29 October 1999
 | language =
 | url =https://ris.utwente.nl/ws/files/6645318/Schasfoort99field.pdf
 | doi =10.1126/science.286.5441.942| pmid = 10542145
 }}</ref> except that in the flowFET the flow of liquid takes the place of the flow of [[electric current]]. Indeed, the name of the flowFET is derived from the naming convention of electronic FETs (e.g. [[MOSFET]], [[Multigate device#FINFET|FINFET]] etc.).

== Mechanism of action ==
{{Main|Electro-osmosis}}
[[File:FlowFET I-V curve.png|thumb|300px|The simulated flow velocity profile across a microfluidic channel as a function of depth into the channel from the wall (y-axis) for several different applied voltages.<ref name="Kerkhoff" />]]
A flowFET relies on the principle of [[electro-osmosis|electro-osmotic flow]] (EOF). In many liquid-solid [[Interface (matter)|interfaces]], there is an [[Double layer (surface science)|electrical double layer]] that develops due to interactions between the two [[phase (matter)|phases]]. In the case of a microfluidic channel, this results in a charged layer of liquid on the periphery of the fluid column which surrounds the bulk of the liquid. This electric double layer has an associated [[voltage|potential difference]] known as the [[zeta potential]]. When an appropriately-oriented electrical field is applied to this interfacial double layer (i.e. parallel to the channel and in the plane of the electric double layer), the charged liquid ions experience a motive [[Lorentz force]]. Since this layer sheaths the fluid column, and since this layer moves, the entire column of liquid will begin to move with a speed <math>\nu_{EOF}</math>. The velocity of the fluid layer "[[momentum diffusion|diffuses]]" into the bulk of the channel from the periphery towards the centre due to viscous coupling.<ref name="Schasfoort" /> The speed is related to the strength of the electric field <math>E</math>, the magnitude of the zeta potential <math>\zeta</math>, the [[electrical permittivity|permittivity]] <math>\epsilon</math> and the [[viscosity]] <math>\eta</math> of the fluid:<ref name="Schasfoort" />

<math>\nu_{EOF}={\epsilon \over \eta} \zeta E</math>

In a FlowFET, the zeta potential between the channel walls and the fluid can be altered by applying an electrical field ''perpendicular'' to the channel walls. This has the effect of altering the motive force experienced by the mobile liquid atoms in the double layer. This change in the zeta-potential can be used to control both the magnitude and direction of the electro-osmotic flow in the microchannel.<ref name="Schasfoort" />

The controlling voltage need only be in the range of 50 V for a typical microfluidic channel,<ref name="Kerkhoff">{{cite journal
 | last1 = Kerkhoff
 | first1 = H.G.
 | last2 = Barber
 | first2 = R.W.
 | last3 = Zhang
 | first3 = X.
 | last4 = Emerson
 | first4 = D.R.
 | title = Fault modelling and co-simulation in FlowFET-based biological array systems
 | journal = Proceedings – Third IEEE International Workshop on Electronic Design, Test and Applications, DELTA 2006
 | pages = 177–182
|date=2006
}}</ref> since this correlates to a gradient of 1.5 MV/cm due to the channel size.<ref name="Schasfoort" />

=== Operational limitations ===
Variation of the FlowFET dimensions (e.g. insulating layer thickness between the channel wall and gate electrode) due to the manufacturing process can lead to inexact control of the zeta potential. This can be exacerbated in the case of wall contamination, which can alter the channel wall surface's electrical properties adjacent to the gate electrode. This will affect the local flow characteristics, which may be especially important in chemical synthesis systems whose [[stoichiometry]] are directly related to the transport rate of reaction [[precursor (chemistry)|precursors]] and reaction products.<ref name="Kerkhoff" />

There are constraints placed on the fluid that can be manipulated in a FlowFET. Since it relies on EOF, only fluids producing an EOF in response to an applied electric field may be used.<ref name="Kerkhoff" />

While the controlling voltage need only be on the order of 50V,<ref name="Kerkhoff" /> the EOF-producing voltage along the channel axis is larger, on the order of 300V.<ref name="Design">{{cite journal
 | last1 = Kerkhoff
 | first1 = H
 | last2 = Barber
 | first2 = R
 | last3 = Emerson
 | first3 = D
 | last4 = Van Der Wouden
 | first4 = E
 | title = Design and Test of Micro-Electronic Fluidic Systems
 | journal = Proceedings Workshop on MEMS, DATE05 Workshops
 | pages = 47–52
|date=2005
}}</ref> It is noticed experimentally that [[Electrolysis of water|electrolysis]] may occur at the [[electrode]] contacts. This water electrolysis can alter the [[pH]] in the channel and adversely affect [[Cell (biology)|biological cells]] and [[biomolecule]]s, while [[Bubble (physics)|gas bubbles]] tend to "clog" microfluidic systems.<ref name="electrode">{{cite journal
 | last1 = Erlandsson
 | first1 = P. G.
 | last2 = Robinson
 | first2 = N. D.
 | title = Electrolysis-reducing electrodes for electrokinetic devices
 | journal = Electrophoresis
 | volume = 32
 | issue = 6–7
 | pages = 784–790
|date=2011
 | doi=10.1002/elps.201000617
 | pmid = 21425174
| s2cid = 1045087
 | url = http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-67312
 }}</ref>

In further analogy with [[microelectronic]] systems, the switching time for a flowFET is [[Proportionality (mathematics)#Inverse proportionality|inversely proportional]] to its size. Scaling down a flowFET results in a reduction in the amount of time for the flow to equilibrate to a new flow rate following a change in the applied electrical field. It should be noted, however, that the frequency of flowFET is many orders of magnitude slower than with an electronic FET.

== Applications ==
A FlowFET sees potential uses in massively parallel microfluidic manipulation,<ref name="Schasfoort" />  for example in [[DNA microarray]]s.<ref name="Kerkhoff" />

Without using a FlowFET, it is necessary to control the rate of EOF by changing the magnitude of the EOF-producing field (i.e. the field parallel to the channel's axis) while leaving the zeta potential unaltered. In this arrangement, however, simultaneous control of EOF in channels connected with each other cannot easily be accomplished.<ref name="Schasfoort" />

A FlowFET provides a way of controlling microfluidic flow in a way that uses no moving parts.<ref name="Schasfoort" /><ref name="Kerkhoff" /><ref name="Design" /> This is in stark contrast to other solutions including [[Peristaltic pump#Microfluidic pumps|pneumatically-actuated peristaltic pumps]] such as presented by Wu et al.<ref name="Wu">{{cite journal
 | last1 = Wu
 | first1 = Min Hsien
 | last2 = Huang
 | first2 = Song Bin
 | last3 = Cui
 | first3 = Zhanfeng
 | last4 = Cui
 | first4 = Zheng
 | last5 = Lee
 | first5 = Gwo Bin
 | title = Development of perfusion-based micro 3-D cell culture platform and its application for high throughput drug testing
 | journal = Sensors and Actuators, B: Chemical
 | volume = 129
 | issue = 1
 | pages = 231–240
|date=2008
 | doi=10.1016/j.snb.2007.07.145
| bibcode = 2008SeAcB.129..231W
 }}</ref> Fewer moving parts allows less opportunity for [[wear|mechanical breakdown]] of a microfluidic device. This may be increasingly relevant as large future iterations of large  microelectronic fluidic (MEF) arrays continue to increase in size and complexity.

The use of bi-directional electronically-controlled flow has interesting options for particle and bubble cleaning operations.<ref name="Kerkhoff" />

== See also ==
{{Portal|Biology|Technology}}
* [[Fluidics]]
* [[Microfluidics]]
* [[Electro-osmosis]]
* [[Lab-on-a-chip]]

== References ==
{{Reflist}}

[[Category:Fluid dynamics]]
[[Category:Nanotechnology]]
[[Category:Microfluidics| ]]
[[Category:Biotechnology]]