Patent Publication Number: US-9895699-B2

Title: Circuit-based optoelectronic tweezers

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application is continuation of U.S. patent application Ser. No. 14/051,004, filed Oct. 10, 2013, which is a non-provisional (and thus claims the benefit of the filing date of) U.S. provisional patent application No. 61/724,168 filed Nov. 8, 2012, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Optoelectronic microfluidic devices (e.g., optoelectronic tweezers (OET) devices) utilize optically induced dielectrophoresis (DEP) to manipulate objects (e.g., cells, particles, or the like) in a liquid medium.  FIGS. 1A and 1B  illustrate an example of a simple OET device  100  for manipulating objects  108  in a liquid medium  106  in a chamber  104 , which can be between an upper electrode  112 , sidewalls  114 , photoconductive material  116 , and a lower electrode  124 . As shown, a power source  126  can be applied to the upper electrode  112  and the lower electrode  124 .  FIG. 1C  shows a simplified equivalent circuit in which the impedance of the medium  106  in the chamber  104  is represented by resistor  142  and the impedance of the photoconductive material  116  is represented by the resistor  144 . 
     Photoconductive material  116  is substantially resistive unless illuminated by light. While not illuminated, the impedance of the photoconductive material  116  (and thus the resistor  144  in the equivalent circuit of  FIG. 1C ) is greater than the impedance of the medium  106  (and thus the resistor  142  in  FIG. 1C ). Most of the voltage drop from the power applied to the electrodes  112 ,  124  is thus across the photoconductive material  116  (and thus resistor  144  in the equivalent circuit of  FIG. 1C ) rather than across the medium  106  (and thus resistor  142  in the equivalent circuit of  FIG. 1C ). 
     A virtual electrode  132  can be created at a region  134  of the photoconductive material  116  by illuminating the region  134  with light  136 . When illuminated with light  136 , the photoconductive material  116  becomes electrically conductive, and the impedance of the photoconductive material  116  at the illuminated region  134  drops significantly. The illuminated impedance of the photoconductive material  116  (and thus the resistor  144  in the equivalent circuit of  FIG. 1C ) at the illuminated region  134  can thus be significantly reduced, for example, to less than the impedance of the medium  106 . At the illuminated region  134 , most of the voltage drop is now across the medium  106  (resistor  142  in  FIG. 1C ) rather than the photoconductive material  116  (resistor  144  in  FIG. 1C ). The result is a non-uniform electrical field in the medium  106  generally from the illuminated region  134  to a corresponding region on the upper electrode  112 . The non-uniform electrical field can result in a DEP force on a nearby object  108  in the medium  106 . 
     Virtual electrodes like virtual electrode  132  can be selectively created and moved in any desired pattern or patterns by illuminating the photoconductive material  116  with different and moving patterns of light. Objects  108  in the medium  106  can thus be selectively manipulated (e.g., moved) in the medium  106 . 
     Generally speaking, the unilluminated impedance of the photoconductive material  116  must be greater than the impedance of the medium  106 , and the illuminated impedance of the photoconductive material  116  must be less than the impedance of the medium  106 . As can be seen, the lower the impedance of the medium  106 , the lower the required illuminated impedance of the photoconductive material  116 . Due to such factors as the natural characteristics of typical photoconductive materials and a limit to the intensity of the light  136  that can, as a practical matter, be directed onto a region  134  of the photoconductive material  116 , there is a lower limit to the illuminated impedance that can, as a practical matter, be achieved. It can thus be difficult to use a relatively low impedance medium  106  in an OET device like the OET device  100  of  FIGS. 1A and 1B . 
     U.S. Pat. No. 7,956,339 addresses the foregoing by using phototransistors in a layer like the photoconductive material  116  of  FIGS. 1A and 1B  selectively to establish, in response to light like light  136 , low impedance localized electrical connections from the chamber  104  to the lower electrode  124 . The impedance of an illuminated phototransistor can be less than the illuminated impedance of the photoconductive material  116 , and an OET device configured with phototransistors can thus be utilized with a lower impedance medium  106  than the OET device of  FIGS. 1A and 1B . Phototransistors, however, do not provide an efficient solution to the above-discussed short comings of prior art OET devices. For example, in phototransistors, the light absorption and electrical amplification for impedance modulation are typically coupled and thus constrained in independent optimization of both. 
     Embodiments of the present invention address the foregoing problems and/or other problems in prior art OET devices as well as provide other advantages. 
     SUMMARY 
     In some embodiments, a microfluidic apparatus can include a circuit substrate, a chamber, a first electrode, a second electrode, a switch mechanism, and photosensitive elements. Dielectrophoresis (DEP) electrodes can be located at different locations on a surface of the circuit substrate. The chamber can be configured to contain a liquid medium on the surface of the circuit substrate. The first electrode can be in electrical contact with the medium, and the second electrode can be electrically insulated from the medium. The switch mechanisms can each be located between a different corresponding one of the DEP electrodes and the second electrode, and each switch mechanism can be switchable between an off state in which the corresponding DEP electrode is deactivated and an on state in which the corresponding DEP electrode is activated. The photosensitive elements can each be configured to provide an output signal for controlling a different corresponding one of the switch mechanisms in accordance with a beam of light directed onto the photosensitive element. 
     In some embodiments, a process of controlling a microfluidic device can include applying alternating current (AC) power to a first electrode and a second electrode of the microfluidic device, where the first electrode is in electrical contact with a medium in a chamber on an inner surface of a circuit substrate of the microfluidic device, and the second electrode is electrically insulated from the medium. The process can also include activating a dielectrophoresis (DEP) electrode on the inner surface of the circuit substrate, where the DEP electrode is one of a plurality of DEP electrodes on the inner surface that are in electrical contact with the medium. The DEP electrode can be activated by directing a light beam onto a photosensitive element in the circuit substrate, providing, in response to the light beam, an output signal from the photosensitive element, and switching, in response to the output signal, a switch mechanism in the circuit substrate from an off state in which the DEP electrode is deactivated to an on state in which the DEP electrode is activated. 
     In some embodiments, a microfluidic apparatus can include a circuit substrate and a chamber configured to contain a liquid medium disposed on an inner surface of the circuit substrate. The microfluidic apparatus can also include means for activating a dielectrophoresis (DEP) electrode at a first region of the inner surface of the circuit substrate in response to a beam of light directed onto a second region of the inner surface, where the second region is spaced apart from the first region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a perspective view of a simplified prior art OET device. 
         FIG. 1B  shows a side, cross-sectional view of the OET device of  FIG. 1A . 
         FIG. 1C  is an equivalent circuit diagram of the OET device of  FIG. 1A . 
         FIG. 2A  is a perspective view of a simplified OET device according to some embodiments of the invention. 
         FIG. 2B  shows a side, cross-sectional view of the OET device of  FIG. 2A . 
         FIG. 2C  is a top view of an inner surface of a circuit substrate of the OET device of  FIG. 2A . 
         FIG. 3  is an equivalent circuit diagram of the OET device of  FIG. 2A . 
         FIG. 4  shows a partial, side cross-sectional view of an OET device in which the photosensitive element of  FIGS. 2A-2C  comprises a photodiode and the switch mechanism comprises a transistor according to some embodiments of the invention. 
         FIG. 5  shows a partial, side cross-sectional view of an OET device in which the photosensitive element of  FIGS. 2A-2C  comprises a photodiode and the switch mechanism comprises an amplifier according to some embodiments of the invention. 
         FIG. 6  shows a partial, side cross-sectional view of an OET device in which the photosensitive element of  FIGS. 2A-2C  comprises a photodiode and the switch mechanism comprises an amplifier and a switch according to some embodiments of the invention. 
         FIG. 7  is a partial, side cross-sectional view of an OET device having a color detector element according to some embodiments of the invention. 
         FIG. 8  illustrates a partial, side cross-sectional view of an OET device with an indicator element for indicating whether a DEP electrode is activated according to some embodiments of the invention. 
         FIG. 9  illustrates a partial, side cross-sectional view of an OET device with multiple power supplies connected to multiple additional electrodes according to some embodiments of the invention. 
         FIG. 10  illustrates an example of a process of operating an OET device like the devices of  FIGS. 2A-2C and 4-9  according to some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     This specification describes exemplary embodiments and applications of the invention. The invention, however, is not limited to these exemplary embodiments and applications or to the manner in which the exemplary embodiments and applications operate or are described herein. Moreover, the Figures may show simplified or partial views, and the dimensions of elements in the Figures may be exaggerated or otherwise not in proportion for clarity. In addition, as the terms “on,” “attached to,” or “coupled to” are used herein, one element (e.g., a material, a layer, a substrate, etc.) can be “on,” “attached to,” or “coupled to” another element regardless of whether the one element is directly on, attached, or coupled to the other element or there are one or more intervening elements between the one element and the other element. Also, directions (e.g., above, below, top, bottom, side, up, down, under, over, upper, lower, horizontal, vertical, “x,” “y,” “z,” etc.), if provided, are relative and provided solely by way of example and for ease of illustration and discussion and not by way of limitation. In addition, where reference is made to a list of elements (e.g., elements a, b, c), such reference is intended to include any one of the listed elements by itself, any combination of less than all of the listed elements, and/or a combination of all of the listed elements. 
     As used herein, “substantially” means sufficient to work for the intended purpose. The term “ones” means more than one. 
     In some embodiments of the invention, dielectrophoresis (DEP) electrodes can be defined in an optoelectronic tweezers (OET) device by switch mechanisms that connect electrically conductive terminals on an inner surface of a circuit substrate to a power electrode. The switch mechanisms can be switched between an “off” state in which the corresponding DEP electrode is not active and an “on” state in which the corresponding DEP electrode is active. The state of each switch mechanism can be controlled by a photosensitive element connected to but spaced apart from the switch mechanism.  FIGS. 2A-2C  illustrate an example of such a microfludic OET device  200  according to some embodiments of the invention. 
     As shown in  FIGS. 2A-2C , the OET device  200  can comprise a chamber  204  for containing a liquid medium  206 . The OET device  200  can also comprise a circuit substrate  216 , a first electrode  212 , a second electrode  224 , and an alternating current (AC) power source  226 , which can be connected to the first electrode  212  and the second electrode  224 . 
     The first electrode  212  can be positioned in the device  200  to be in electrical contact with (and thus electrically connected to) the medium  206  in the chamber  204 . In some embodiments, all or part of the first electrode  212  can be transparent to light so that light beams  250  can pass through the first electrode  212 . In contrast to the first electrode  212 , the second electrode  224  can be positioned in the device  200  to be electrically insulated from the medium  206  in the chamber  204 . For example, as shown, the circuit substrate  216  can comprise the second electrode  224 . For example, the second electrode  224  can comprise one or more metal layers on or in the circuit substrate  216 . Although illustrated in  FIG. 2B  as a layer inside the circuit substrate  216 , the second electrode  224  can alternatively be part of a metal layer on the surface  218  of the circuit substrate  216 . Regardless, such a metal layer can comprise a plate, a pattern of metal traces, or the like. 
     The circuit substrate  216  can comprise a material that has a relatively high electrical impedance. For example, the impedance of the circuit substrate  216  generally can be greater than the electrical impedance of the medium  206  in the chamber  204 . For example, the impedance of the circuit substrate  216  can be two, three, four, five, or more times the impedance of the medium  206  in the chamber  204 . In some embodiments, the circuit substrate  216  can comprise a semiconductor material, which undoped, has a relatively high electrical impedance. 
     As shown in  FIG. 2B , the circuit substrate  216  can comprise circuit elements interconnected to form electric circuits (e.g., control modules  240 , which are discussed below). For example, such circuits can be integrated circuits formed in the semiconductor material of the circuit substrate  216 . The circuit substrate  216  can thus comprise multiple layers of different materials such as undoped semiconductor material, doped regions of the semiconductor material, metal layers, electrically insulating layers, and the like such as is generally known in the field of forming microelectronic circuits integrated into semiconductor material. For example, as shown in  FIG. 2B , the circuit substrate  216  can comprise the second electrode  224 , which can be part of one or more metal layers of the circuit substrate  216 . In some embodiments, the circuit substrate  216  can comprise an integrated circuit corresponding to any of many known semiconductor technologies such as complementary metal-oxide semiconductor (CMOS) integrated circuit technology, bi-polar integrated circuit technology, or bi-MOS integrated circuit technology. 
     As shown in  FIGS. 2B and 2C , the circuit substrate  216  can comprise an inner surface  218 , which can be part of the chamber  204 . As also shown, DEP electrodes  232  can be located on the surface  218 . As best seen in  FIG. 2C , the DEP electrodes  232  can be distinct one from another. For example, the DEP electrodes  232  are not directly connected to each other electrically. 
     As illustrated in  FIGS. 2B and 2C , each DEP electrode  232  can comprise an electrically conductive terminal, which can be in any of many different sizes, shapes, and locations on the surface  218 . For example, as illustrated by the DEP electrodes  232  in the middle column of DEP electrodes  232  of  FIG. 2C , the conductive terminal of each DEP electrode  232  can be spaced apart from a corresponding photosensitive element  242 . As another example, and as illustrated by the left and right columns of DEP electrodes  232  in  FIG. 2C , the conductive terminal of each DEP electrode  232  can be disposed around (entirely as shown or partially (not shown)) and extend away from a corresponding photosensitive element  242 , and those terminals can comprise an opening  234  (e.g., a window) through which a light beam  250  can pass to strike the photosensitive element  242 . Alternatively, the terminals of such DEP electrodes  232  can be transparent to light and thus can cover a corresponding photosensitive element  242  without having an opening  234 . Although the DEP electrodes  232  are illustrated in  FIGS. 2B and 2C  (and in other figures) as comprising an electrically conductive terminal, one or more of the DEP electrodes  232  can alternatively comprise merely a region of the surface  218  of the circuit substrate  216  where one of the switch mechanisms  246  is in electrical contact with the medium  206  in the chamber  204 . Regardless, as can be seen in  FIG. 2B , the inner surface  218  can be part of the chamber  204 , and the medium  206  can be disposed on the inner surface  218  and the DEP electrodes  232 . 
     As noted above, the circuit substrate  216  can comprise electric circuit elements interconnected to form electrical circuits. As illustrated in  FIG. 2B , such circuits can comprise control modules  240 , which can comprise a photosensitive element  242 , control circuitry  244 , and a switch mechanism  246 . 
     As shown in  FIG. 2B , each switch mechanism  246  can connect one of the DEP electrodes  232  to the second electrode  224 . In addition, each switch mechanism  246  can be switchable between at least two different states. For example, the switch mechanism  246  can be switched between an “off” state and an “on” state. In the “off” state, the switch mechanism  246  does not connect the corresponding DEP electrode  232  to the second electrode  224 . Put another way, the switch mechanism  246  provides only a high impedance electrical path from the corresponding DEP electrode  232  to the second electrode  224 . Moreover, the circuit substrate  216  does not otherwise provide an electrical connection from the corresponding DEP electrode  232  to the second electrode  224 , and thus there is nothing but a high impedance connection from the corresponding DEP electrode  232  to the second electrode  224  while the switch mechanism  246  is in the off state. In the on state, the switch mechanism  246  electrically connects the corresponding DEP electrode  232  to the second electrode  224  and thus provides a low impedance path from the corresponding DEP electrode  232  to the second electrode  224 . The high impedance between the corresponding DEP electrode  232  while the switch mechanism  246  is in the off state can be a greater impedance than the medium  206  in the chamber  204 , and the low impedance connection from the corresponding DEP electrode  232  to the second electrode  224  provided by the switch mechanism  246  in the on state can have a lesser impedance than the medium  206 . The foregoing is illustrated in  FIG. 3 . 
       FIG. 3  illustrates an equivalent circuit in which the resistor  342  represents the impedance of the medium  206  in the chamber  204  and the resistor  344  represents the impedance of a switch mechanism  246 —and thus the impedance between one of the DEP electrodes  232  on the inner surface  218  of the circuit substrate  216  and the second electrode  224 . As noted, the impedance (represented by resistor  344 ) between a corresponding DEP electrode  232  and the second electrode  224  is greater than the impedance (represented by resistor  342 ) of the medium  206  while the switch mechanism  246  is in the off state, but the impedance (represented by resistor  344 ) between a corresponding DEP electrode  232  and the second electrode  224  becomes less than the impedance (represented by resistor  342 ) of the medium  206  while the switch mechanism  246  is in the on state. Turning a switch mechanism  246  on thus creates a non-uniform electrical field in the medium  206  generally from the DEP electrode  232  to a corresponding region on the electrode  212 . The non-uniform electrical field can result in a DEP force on a nearby micro-object  208  (e.g., a micro-particle or biological object such as a cell or the like) in the medium  206 . Because neither the switch mechanism  246  nor the portion of the circuit substrate  216  between the DEP electrode  232  and the second electrode  224  need be a photosensitive circuit element or even comprise photoconductive material, the switch mechanism  246  can provide a significantly lower impedance connection from a DEP electrode  232  to the second electrode  224  than in prior art OET devices, and the switch mechanism  246  can be much smaller than phototransistors used in prior art OET devices. 
     In some embodiments, the impedance of the off state of the switch mechanism  246  can be two, three, four, five, ten, twenty, or more times the impedance of the on state. Also, in some embodiments, the impedance of the off state of the switch  246  can be two, three, four, five, ten, or more times the impedance of the medium  206 , which can be two, three, four, five, ten, or more times the impedance of the on state of the switch mechanism  246 . 
     Even though the switch mechanism  246  need not be photoconductive, the control module  240  can be configured such that the switch mechanism  246  is controlled by a beam of light  250 . The photosensitive element  242  of each control module  240  can be a photosenstive circuit element that is activated (e.g., turned on) and deactivated (e.g., turned off) in response to a beam of light  250 . Thus, for example, as shown in  FIG. 2B , the photosensitive element  242  can be disposed at a region on the inner surface  218  of the circuit substrate  216 . A beam of light  250  (e.g., from a light source (not shown) such as a laser or other light source) can be selectively directed onto the photosensitive element  242  to activate the element  242 , and the beam of light  250  thereafter can be removed from the photosensitive element  242  to deactivate the element  242 . An output of the photosensitive element  242  can be connected to a control input of the switch mechanism  246  to switch the switch mechanism  246  between the off and on states. 
     In some embodiments, as shown in  FIG. 2B , control circuitry  244  can connect the photosensitive element  242  to the switch mechanism  246 . The control circuitry  244  can be said to “connect” the output of the photosensitive element  242  to the switch mechanism  246 , and the photosensitive element  242  can be said to be connected to and/or controlling the switch mechanism  246 , as long as the control circuitry  244  utilizes the output of the photosensitive element  242  to control the impedance state of the switch mechanism  246 . In some embodiments, however, the control circuitry  244  need not be present, and the photosensitive element  242  can be connected directly to the switch mechanism  246 . Regardless, the state of the switch mechanism  246  can be controlled by the beam of light  250  on the photosensitive element  242 . For example, the state of the switch mechanism  246  can be controlled by the presence or absence of the beam of light  250  on the photosensitive element  242 . 
     The control circuitry  244  can comprise analog circuitry, digital circuitry, a digital memory and digital processor operating in accordance with machine readable instructions (e.g., software, firmware, microcode, or the like) stored in the memory, or a combination of one or more of the forgoing. In some embodiments, the control circuitry  244  can comprise one or more digital latches (not shown), which can latch a pulsed output of the photosensitive element  242  caused by a pulse of a light beam  250  directed onto the photosensitive element  242 . The control circuitry  244  can thus be configured (e.g., with one or more latches) to toggle the state of the switch mechanism  246  between the off state and the on state each time a pulse of the light beam  250  is directed onto the photosensitive element  242 . 
     For example, a first pulse of the light beam  250  on the photosensitive element  242 —and thus a first pulse of a positive signal output by the photosensitive element  242 —can cause the control circuitry  244  to put the switch mechanism  246  into the on state. Moreover, the control circuitry  244  can maintain the switch mechanism  246  in the on state even after the pulse of the light beam  250  is removed from the photosensitive element  242 . Thereafter, the next pulse of the light beam  250  on the photosensitive element  242 —and thus the next pulse of the positive signal output by the photosensitive element  242 —can cause the control circuitry  244  to toggle the switch mechanism  246  to the off state. Subsequent pulses of the light beam  250  on the photosensitive element  242 —and thus subsequent pulses of the positive signal output by the photosensitive element  242 —can toggle the switch mechanism  246  between the off and the on states. 
     As another example, the control circuitry  244  can control the switch mechanism  246  in response to different patterns of pulses of the light beam  250  on the photosensitive element  242 . For example, the control circuitry  244  can be configured to set the switch mechanism  246  to the off state in response to a sequence of n pulses of the light beam  250  on the photosensitive element  242  (and thus n corresponding pulses of a positive signal from the photosensitive element  242  to the control circuitry  244 ) having a first characteristic and set the switch mechanism  246  to the on state in response to a sequence of k pulses (and thus k corresponding pulses of a positive signal from the photosensitive element  242  to the control circuitry  244 ) having a second characteristic, wherein n and k can be equal or unequal integers. Examples of the first characteristic and the second characteristic can include the following: the first characteristic can be that the n pulses occur at a first frequency, and the second characteristic can be that the k pulses occur at a second frequency that is different than the first frequency. As another example, the pulses can have different widths (e.g., a short width and a long width) like, for example, Morrse Code. The first characteristic can be a particular pattern of n short and/or long width pulses of the light beam  250  that constitutes a predetermined off-state code, and the second characteristic can be a different pattern of k short and/or long width pulses of the light beam  250  that constitutes a predetermined on-state code. Indeed, the foregoing examples can be configured to switch the switch mechanism  246  between more than two states. Thus, the switch mechanism  246  can have more and/or different states than merely an on state and an off state. 
     As yet another example, the control circuitry  244  can be configured to control the state of the switch mechanism  246  in accordance with a characteristic of the light beam  250  (and thus the corresponding pulse of a positive signal from the photosensitive element  242  to the control circuitry  244 ) other than merely the presence or absence of the beam  250 . For example, the control circuitry  244  can control the switch mechanism  246  in accordance with the brightness of the beam  250  (and thus the level of a corresponding pulse of a positive signal from the photosensitive element  242  to the control circuitry  244 ). Thus, for example, a detected brightness level of the beam  250  (and thus a level of a corresponding pulse of a positive signal from the photosensitive element  242  to the control circuitry  244 ) that is greater than a first threshold but less than a second threshold can cause the control circuitry  244  to set the switch mechanism  246  to the off state, and a detected brightness level of the beam  250  (and thus a level of a corresponding pulse of a positive signal from the photosensitive element  242  to the control circuitry  244 ) that is greater than the second threshold can cause the control circuitry  244  to set the switch mechanism  246  to the on state. In some embodiments, there can be a two, five, ten, or more times difference between the first brightness level and the second brightness level.  FIG. 7 , which is discussed below, illustrates an example in which the control circuitry  244  can control the state of the switching mechanism  246  in accordance with the color of the light beam  250 . Again, the foregoing examples can be configured to switch the switch mechanism  246  between more than two states. 
     As still another example, the control circuitry  244  can be configured to control the state of the switch mechanism  246  in accordance with any combination of the foregoing characteristics of the light beam  250  or multiple characteristics of the light beam  250 . For example, the control circuitry  244  can be configured to set the switching mechanism  246  to the off state in response to a sequence of n pulses within a particular frequency band of the light beam  250  and to the on state in response to the brightness of the light beam  250  exceeding a predetermined threshold. 
     The control module  240  is thus capable of controlling a DEP electrode  232  on the inner surface  218  of the circuit substrate  216  in accordance with the presence or absence of a beam of light  250 , a characteristic of the light beam  250 , or a characteristic of a sequence of pulses of the light beam  250  at a different region (e.g., corresponding to the location of the photosensitive element  242 ) of the inner surface  218 , where the different region is spaced apart from the first DEP electrode  232 . The photosensitive element  242 , the control circuitry  244 , and/or the switch element  246  are thus examples of means for activating a DEP electrode  232  at a first region (e.g., any portion of a DEP electrode  232  not disposed over a corresponding photosensitive element  242 ) on an inner surface (e.g.,  218 ) of a circuit substrate (e.g.,  216 ) in response to a beam of light (e.g.,  250 ) directed onto a second region (e.g., corresponding to the photosensitive element  242 ) of the inner surface  218 , where the second region is spaced apart on the inner surface  218  from the first region. 
     As illustrated in  FIGS. 2B and 2C , there can be multiple (e.g., many) control modules  240  each configured to control a different DEP electrode  232  on the inner surface  218  of the circuit substrate  216 . The OET device  200  of  FIGS. 2A-2C  can thus comprise many DEP electrodes in the form of DEP electrodes  232  each controllable by directing or removing a beam of light  250  on a photosensitive element  242 . Moreover, at least a portion of each DEP electrode  232  can be spaced apart on the inner surface  218  from the corresponding photosensitive element  242 —and thus the region on the inner surface where light  250  is directed—that controls the state of the DEP electrode  232 . 
     The illustrations in  FIGS. 2A-2C  are examples only, and variations are contemplated. For example, as noted, there need not be control circuitry  244 , and the photosensitive elements  242  can be connected directly to the switch mechanisms  246 . As another example, each control module  240  need not include control circuitry  244 . Instead, one or more instances of the control circuitry  244  can be shared among multiple photosensitive elements  242  and switch mechanisms  246 . As yet another example, DEP electrodes  232  need not include distinct terminals on the surface  218  of the circuit substrate  216  but can instead be regions of the surface  218  where the switch mechanisms  246  are in electrical contact with the medium  206  in the chamber  204 . 
       FIGS. 4-6  illustrate various embodiments and exemplary configurations of the photosensitive element  242  and the switch mechanism  246  of  FIGS. 2A-2C . 
       FIG. 4  illustrates an OET device  400  that can be similar to the OET device  200  of  FIGS. 2A-2C  except that the photosensitive element  242  can comprise a photodiode  442  and the switch mechanism  246  can comprise a transistor  446 . Otherwise, the OET device  400  can be the same as the OET device  200 , and indeed, like numbered elements in  FIGS. 2A-2C and 4  can be the same. As noted above, the circuit substrate  216  can comprise a semiconductor material, and the photodiode  442  and transistor  446  can be formed in layers of the circuit substrate  216  as is known in the field of semiconductor manufacturing. 
     An input  444  of the photodiode  442  can be biased with a direct current (DC) power source (not shown). The photodiode  442  can be configured and positioned so that a light beam  250  directed at a location on the inner surface  218  that corresponds to the photodiode  442  can activate the photodiode  442 , causing the photodiode  442  to conduct and thus output a positive signal to the control circuitry  244 . Removing the light beam  250  can deactivate the photodiode  442 , causing the photodiode  442  to stop conducting and thus output a negative signal to the control circuitry  244 . 
     The transistor  446  can be any type of transistor, but need not be a phototransistor. For example, the transistor  446  can be a field effect transistor (FET) (e.g., a complementary metal oxide semiconductor (CMOS) transistor), a bipolar transistor, or a bi-MOS transistor. 
     If the transistor  446  is a FET transistor as shown in  FIG. 4 , the drain or source can be connected to the DEP electrode  232  on the inner surface  218  of the circuit substrate  216  and the other of the drain or source can be connected to the second electrode  224 . The output of the photodiode  442  can be connected (e.g., by the control circuitry  244 ) to the gate of the transistor  446 . Alternatively, the output of the photodiode  442  can be connected directly to the gate of the transistor  446 . Regardless, the transistor  446  can be biased so that the signal provided to the gate turns the transistor  446  off or on. 
     If the transistor  446  is a bipolar transistor, the collector or emitter can be connected to the DEP electrode  232  on the inner surface  218  of the circuit substrate  216  and the other of the collector or emitter can be connected to the second electrode  224 . The output of the photodiode  442  can be connected (e.g., by the control circuitry  244 ) to the base of the transistor  446 . Alternatively, the output of the photodiode  442  can be connected directly to the base of the transistor  446 . Regardless, the transistor  446  can be biased so that the signal provided to the base turns the transistor  446  off or on. 
     Regardless of whether the transistor  446  is a FET transistor or a bipolar transistor, the transistor  446  can function as discussed above with respect to the switch mechanism  226  of  FIGS. 2A-2C . That is, turned on, the transistor  446  can provide a low impedance electrical path from the DEP electrode  232  to the second electrode  224  as discussed above with respect to the switch mechanism  226  in  FIGS. 2A-2C . Conversely, turned off, the transistor  446  can provide a high impedance electrical path from the DEP electrode  232  to the second electrode  224  as described above with respect to the switch mechanism  226 . 
       FIG. 5  illustrates an OET device  500  that can be similar to the OET device  200  of  FIGS. 2A-2C  except that the photosensitive element  242  comprises the photodiode  442  (which can be the same as described above with respect to  FIG. 4 ) and the switch mechanism  246  comprises an amplifier  546 , which need not be photoconductive. Otherwise, the OET device  500  can be the same as the OET device  200 , and indeed, like numbered elements in  FIGS. 2A-2C and 5  can be the same. As noted above, the circuit substrate  216  can comprise a semiconductor material, and the amplifier  546  can be formed in layers of the circuit substrate  216  as is known in the field of semiconductor processing. 
     The amplifier  546  can be any type of amplifier. For example, the amplifier  546  can be an operational amplifier, one or more transistors configured to function as an amplifier, or the like. As shown, the control circuitry  244  can utilize the output of the photodiode  442  to control the amplification level of the amplifier  546 . For example, control circuitry  244  can control the amplifier  546  to function as discussed above with respect to the switch mechanism  226  of  FIGS. 2A-2C . That is, in the absence of the light beam  250  on the photodiode  442  (and thus the absence of an output from the photodiode  442 ), the control circuitry  244  can turn the amplifier  546  off or set the gain of the amplifier  546  to zero, effectively causing the amplifier  546  to provide a high impedance electrical connection from the DEP electrode  232  to the second electrode  224  as discussed above with respect to the switch mechanism  246 . Conversely, the presence of the light beam  250  on the photodiode  442  (and thus an output from the photodiode  442 ) can cause the control circuitry  244  to turn the amplifier  546  on or set the gain of the amplifier  546  to a non-zero value, effectively causing the amplifier  546  to provide a low impedance electrical connection from the DEP electrode  232  to the second electrode  224  as discussed above with respect to the switch mechanism  246 . 
     The OET device  600  of  FIG. 6  can be similar to the OET device  500  of  FIG. 5  except that the switch mechanism  246  (see  FIGS. 2A-2C ) can comprise a switch  604  in series with an amplifier  602 . The switch  604  can comprise any kind of electrical switch including a transistor such as transistor  442  of  FIG. 4 . The amplifier  602  can be like the amplifier  546  of  FIG. 5 . The switch  604  and amplifier  602  can be formed in the circuit substrate  216  generally as discussed above. 
     The control circuitry  244  can be configured to control whether the switch  604  is open or closed in accordance with the output of the photodiode  442 . Alternatively, the output of the photodiode  442  can be connected directly to the switch  604 . Regardless, when the switch  604  is open, the switch  604  and amplifier  602  can provide a high impedance electrical connection from the DEP electrode  232  to the second electrode  224  as discussed above. Conversely, while the switch  604  is closed, the switch  604  and amplifier  602  can provide a low impedance electrical connection from the DEP electrode  232  to the second electrode  224  as discussed above. 
       FIG. 7  illustrates a partial, side cross-sectional view of an OET device  700  that can be like the device  200  of  FIGS. 2A-2C  except that each of one or more (e.g., all) of the photosensitive elements  242  can be replaced with a color detector element  710 . One color detector element  710  is shown in  FIG. 7 , but each of the photosensitive elements  242  in  FIGS. 1A-1C  can be replaced with such an element  710 . The control module  740  in  FIG. 7  can otherwise be like the control module  240  in  FIGS. 1A-1C , and like numbered elements in  FIGS. 1A-1C and 7  are the same. 
     As shown, a color detector element  710  can comprise a plurality of color photo detectors  702 ,  704  (two are shown but there can be more). Each pass color detector  702 ,  704  can be configured to provide a positive signal to the control circuitry  244  in response to a different color of the light beam  250 . For example, the photo detector  702  can be configured to provide a positive signal to the control circuitry  244  when a light beam  250  of a first color is directed onto the photo detectors  702 ,  704 , and the photo detector  704  can be configured to provide a positive signal to the control circuitry  244  when the light beam  250  is a second color, which can be different than the first color. 
     As shown, each photo detector  702 ,  704  can comprise a color filter  706  and a photo sensitive element  708 . Each filter  706  can be configured to pass only a particular color. For example, the filter  706  of the first photo detector  702  can pass substantially only a first color, and the filter  706  of the second photo detector  704  can pass substantially only a second color. The photo sensitive elements  708  can both be similar to or the same as the photo sensitive element  242  in  FIGS. 2A-2C  as discussed above. 
     The configurations of the color photo detectors  702 ,  704  shown in  FIG. 7  are an example only, and variations are contemplated. For example, rather than comprising a filter  706  and a photo sensitive element  708 , one or both of the color photo detectors  702 ,  704  can comprise a photo-diode configured to turn on only in response to light of a particular color. 
     Regardless, the control circuitry  244  can be configured to set the switch mechanism  246  to one state (e.g., the on state) in response to a beam  250  pulse of the first color and to set the switch mechanism  246  to another state (e.g., the off state) in response to a beam  250  pulse of the second color. As mentioned, the color detector element  710  can comprise more than two color photo detectors  702 ,  704 , and the control circuitry  244  can thus be configured to switch the switch mechanism  246  among more than two different states. 
       FIG. 8  is a partial, side cross-sectional view of an OET device  800  that can be like the device  200  of  FIGS. 2A-2C  except that each control module  840  can further include an indicator element  802 . That is, the device  800  can be like the device  200  of  FIGS. 2A-2C  except a control module  840  can replace each control module  240 , and there can thus be an indicator element  802  associated with each DEP electrode  232 . Otherwise, the device  800  can be like device  200  in  FIGS. 2A-2C , and like numbered elements in  FIGS. 2A-2C and 8  are the same. 
     As shown, the indicator element  802  can be connected to the output of the control circuitry  244 , which can be configured to set the indicator element  802  to different states each of which corresponds to one of the possible states of the switch mechanism  246 . Thus, for example, the control circuitry  244  can turn the indicator element  802  on while the switch mechanism  246  is in the on state and turn the indicator element  802  off while the switch mechanism  246  is in the off state. In the foregoing example, the indicator element  802  can thus be on while its associated DEP electrode  232  is activated and off while the DEP electrode  232  is not activated. 
     The indicator element  802  can provide a visional indication (e.g., emit light  804 ) only when turned on. Non-limiting examples of the indicator element  802  include a light source such as a light emitting diode (which can be formed in the circuit substrate  216 ), a light bulb, or the like. As shown, the DEP electrode  232  can include a second opening  834  (e.g., window) for the indicator element  802 . Alternatively, the indicator element  802  can be spaced away from the DEP electrode  232  and thus not covered by the DEP electrode  232 , in which case, there need not be a second window  834  in the DEP electrode  232 . As yet another alternative, the DEP electrode  232  can be transparent to light, which case, there need not be a second window  834  even if the DEP electrode  232  covers the indicator element  802 . 
       FIG. 9  is a partial, side cross-sectional view of an OET device  900  that can be like the device  200  of  FIGS. 2A-2C  except that the device  900  can comprise not only the second electrode  224  but one or more additional electrodes  924 ,  944  (two are shown but there can be one or more than two) and a corresponding plurality of additional power sources  926 ,  946 . Otherwise, the device  900  can be like device  200  in  FIGS. 2A-2C , and like numbered elements in  FIGS. 2A-2C and 9  are the same. 
     As shown, each switch mechanism  246  can be configured to connect electrically a corresponding DEP electrode  232  to one of the electrodes  224 ,  924 ,  944 . A switch mechanism  246  can thus be configured to selectively connect a corresponding DEP electrode  232  to the second electrode  224 , a third electrode  924 , or a fourth electrode  944 . Each switch mechanism  246  can also be configured to disconnect the first electrode  212  from all of the electrodes  224 ,  924 ,  944 . 
     As also shown, the power source  226  can be connected to (and thus provide power between) the first electrode  212  and the second electrode  224  as discussed above. The power source  926  can be connected to (and thus provide power between) the first electrode  212  and the third electrode  924 , and the power source  946  can be connected to (and thus provide power between) the first electrode  212  and the fourth electrode  944 . 
     Each electrode  924 ,  944  can be generally like the second electrode  224  as discussed above. For example, each electrode  924 ,  944  can be electrically insulated from the medium  206  in the channel  204 . As another example, each electrode  924 ,  944  can be part of a metal layer on the surface  218  of or inside the circuit substrate  216 . Each power source  926 ,  946  can be an alternating current (AC) power source like the power source  226  as discussed above. 
     The power sources  926 ,  946 , however, can be configured differently than the power source  226 . For example, each power source  226 ,  926 ,  946  can be configured to provide a different level of voltage and/or current. In such an example, each switch mechanism  246  can thus switch the electrical connection from a corresponding DEP electrode  232  between an “off” state in which the DEP electrode  232  is not connected to any of the electrodes  224 ,  924 ,  944  and any of multiple “on” states in which the DEP electrode  232  is connected to any one of the electrodes  224 ,  924 ,  944 . 
     As another example of how the power sources  226 ,  926 ,  946  can be configured differently, each power source  226 ,  926 ,  946  can be configured to provide power with a different phase shift. For example, in an embodiment comprising the electrodes  224 ,  924  and the power sources  226 ,  926  (but not the electrode  944  and power source  946 ), the power source  926  can provide power that is approximately (e.g., plus or minus ten percent) one hundred eighty ( 180 ) degrees out of phase with the power provided by the power source  226 . In such an embodiment, each switch mechanism  246  can be configured to switch between connecting a corresponding DEP electrode  232  to the second electrode  224  and the third electrode  924 . The device  900  can be configured so that the corresponding DEP electrode  232  is activated (and thus turned on) while the DEP electrode  232  is connected to one of the electrodes  224 ,  924  (e.g.,  224 ) and deactivated (and thus turned off) while connected to the other of the electrodes  224 ,  924  (e.g.,  924 ). Such an embodiment can reduce leakage current from a DEP electrode  232  that is turned off as compared to the device  200  of  FIGS. 2A-2C . 
     It is noted that one or more of the following can comprise examples of means for activating a DEP electrode at a first region of the inner surface of the circuit substrate in response to a beam of light directed onto a second region of the inner surface, where the second region is spaced apart from the first region; activating means further for selectively activating a plurality of DEP electrodes at first regions of the inner surface of the circuit substrate in response to beams of light directed onto second regions of the inner surface, where the each second region is spaced apart from each the first region; activating means further for activating the DEP electrode in response to the beam of light having a first characteristic, and deactivating the DEP electrode in response to the beam of light having a second characteristic; activating means further for activating the DEP electrode in response to a sequence of n pulses of the beam of light having a first characteristic; and activating means further for deactivating the DEP electrode in response to a sequence of k pulses of the beam of light having a second characteristic: the photosensitive element  242 , including the photodiode  442  and/or the color detector element  710 ; the control circuitry  244  configured in any manner described or illustrated herein; and/or the switch mechanism  246  include the transistor  446 , the amplifier  546 , and/or the amplifier  602  and switch  604 . 
       FIG. 10  illustrates a process  1000  for controlling DEP electrodes in a microfluidic OET device according to some embodiments of the invention. As shown, at step  1002 , a micro-fluidic OET device can be obtained. For example, any of the microfluidic OET devices  200 ,  400 ,  500 ,  600 ,  700 ,  800 ,  900  of  FIGS. 2A-2C and 4-9 , or similar devices, can be obtained at step  1002 . At step  1004 , AC power can be applied to electrodes of the device obtained at step  1002 . For example, as discussed above, the AC power source  226  can be connected to a first electrode  212  that is in electrical contact with the medium  206  in the chamber  204  and a second electrode  224  that is insulated from the medium  206 . At step  1006 , DEP electrodes of the device obtained at step  1002  can be selectively activated and deactivated. For example, as discussed above DEP electrodes  232  can be selectively activated and deactivated by selectively directing light beams  250  onto and removing light beams  250  from photosensitive elements  242  (e.g., the photodiode  442  of  FIGS. 4, 5, and 6 ) to switch the impedance state of the switching mechanism  246  (e.g., the transistor  446  of  FIG. 4 , the amplifier  556  of  FIG. 5 , and the switch  602  and amplifier  604  of  FIG. 5 ) as discussed above. 
     Although specific embodiments and applications of the invention have been described in this specification, these embodiments and applications are exemplary only, and many variations are possible.