Background Art Techniques are rapidly developing for parallel performance of a large number of chemical and biological assays and synthesis operations. One approach uses a nanotiter plate having a high density platen of through-hole wells with hydrophilic interiors and openings surrounded by hydrophobic material. This is described, for example, in U. S.

Patent 6, 387, 331 and U. S. Patent Application 20020094533, the contents of which are incorporated herein by reference. One specific commercial example of a nanotiter plate system is the Living ChipTM made by Biotrove, Inc. of Cambridge, MA. Nanotiter plate technology relies on the ability to handle very small volumes of fluid samples, typically, 100 nanoliters or less. The various considerations taken into account in handling such small liquid samples are known as microfluidics.

Transferring of large collections of fluids such as libraries of small molecule drug candidates, cells, probe molecules (e. g. , oligomers), and/or tissue samples stored in older style 96-or 384-well plates into more efficient high density arrays of microfluidic receptacles such as a nanotiter plate can consume one or more hours, during which time samples may evaporate, degrade or become contaminated. It is therefore advantageous to submerse the array in a bath of immiscible fluid. The fluid is ideally electrically insulating, non-conductive and nonflammable, with a relative permittivity >1. One class of fluids that serves this purpose is perfluorinated hydrocarbons, such a perfluorodecalin, perfluorooctane, perfluoropentane, longer chained perfluorocarbons or mixed populations of perfluorocarbons. Hydrocarbons or silicone fluids would also work but are flammable and tend to extract compounds from the sample.

A microfluidic volume of a liquid sample may be loaded into a target receptacle by various means. One established method for transferring a liquid sample to a surface or to another liquid uses a transfer pin loaded with the sample liquid. For example, pins or

arrays of pins are typically used to spot DNA samples onto glass slides for hybridization analysis. Pins have also been used to transfer liquids such as drug candidates between microplates or onto gels (one such gel system is being developed by Discovery Partners, San Diego, CA). Many pin types are commercially available, of various geometries and delivery volumes. V&P Scientific of San Diego, CA makes slotted, grooved, cross- hatched, and other novel-geometry pins. The Stealth Pin by ArrayIt is capable of delivering hundreds of spots in succession from one sample uptake, with delivery volumes of 0.5nL to 2.5nL. Majer Precision Engineering sells pins having tapered tips and slots such as the MicroQuil 2000.

U. S. Patent 6,149, 815 describes an approach for dispensing liquid samples electrokinetically. A complex apparatus positions a capillary tube receiver reservoir and a non-conducting liquid dispenser between a ground plate and a high voltage plate, neither plate being electrically connected to a sample. An accurate volume of liquid sample is transferred from the dispenser to the receiver reservoir by precisely controlling the time that a high voltage is applied to the dispenser, the longer the voltage is applied, the greater the volume of sample transferred, and vice versa. As shown in Fig. 1 of the'815 patent, it is important to provide an insulating gap between the electrically charged dispenser and the electrically grounded receiver reservoir. Moreover, the'815 patent approach requires determining by visual observation the relationship between time, voltage, and volume of liquid transferred.

Summary of the Invention A representative embodiment of the present invention includes a liquid dispenser for a microfluidic assay systems including systems for arraying liquid samples for storage, screening and synthesis. The dispenser includes at least one transfer pin for transferring a microfluidic volume of sample liquid to a target receptacle. A pin tip at one end of the transfer pin is structured to cooperate with an opening in the target receptacle. The pin tip creates a liquid bridge to the target receptacle to dispense sample liquid from the pin tip to the target receptacle. Dispensed sample liquid is retained in the target receptacle by means of surface tension.

In a further embodiment, the target receptacle is one of an array of through-holes wells or closed-end wells in a platen. The target receptacle may have hydrophilic walls

that attract the sample liquid. The target receptacle may have an opening surrounded by hydrophobic material. The target receptacle may be filled with a porous hydrophilic material. A transfer pin array may include multiple transfer pins for transferring multiple samples to corresponding target receptacles. Individual transfer pins in the array may be individually actuable, as would be useful for producing sample patterns or layered sample patterns. Typically the spacing of pins in the array will match a subset of a source array such as a 384 well microtiter plate as well as the spacing of the receptacle array. At least one transfer pin in the array may be independently positionable to align the at least one independently positionable pin with respect to the opening of a target receptacle.

Positioning systems are typically capable of accurate movement in at least the x, y and z coordinates. Individual transfer pins in the array may be free floating or spring loaded.

In various embodiments, the dispensed sample may be from 0.2 to 100 nanoliters.

The transfer pin may have a diameter greater than the opening of the target receptacle. The sample liquid may be a polar liquid such as aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solutions. An optional high voltage differential may be applied across the pin-receptacle gap to aid in the transfer. The high voltage potential, for example between 100V and 5kV, may be applied to at least one of the pin tip and the target receptacle to aid the capillary action.

In a further embodiment, a voltage control module controls when the high voltage potential is applied to and removed. The voltage control module may operate to apply the high voltage potential before or after the transfer pin is positioned at the target receptacle, and to remove the high voltage potential before or after the transfer pin is moved away from the target receptacle. The voltage control module may include a resistor network and/or a controllable switch in series with the transfer pin.

The at least one transfer pin may be able to dispense multiple samples without replenishment. The capillary action may be aided by vibrating at least one of the pin tip and the target receptacle. The pin tip may have a slotted end, for example, an X-shaped slotted end. The pin tip may have one or more structural features that increase the surface area of the sample liquid at the pin tip. These structural features may include one or more features from the group of slots, grooves, and spirals. The target receptacle may be substantially empty prior to transferring the sample liquid.

Embodiments of the present invention also include a method for use in dispensing a

microfluidic sample of a liquid. The method includes providing at least one transfer pin for transferring a microfluidic sample of liquid to a target receptacle. One end of the transfer pin may have a pin tip structured to cooperate with an opening in the target receptacle.

Capillary action between the pin tip and the target receptacle is used to transfer the sample liquid from the at least one transfer pin to the target receptacle.

In such an embodiment, the target receptacle may be a through-hole well or a closed-end well in a platen array. The target receptacle also may include hydrophilic walls that attract the sample liquid and/or an opening surrounded by hydrophobic material. A high voltage potential, for example, between 100V and 5kV, may be applied to either the transfer pin or the target receptacle to aid the capillary action. The high voltage potential may be applied before or after the transfer pin is positioned at the target receptacle, and removed after the transfer pin is moved away from the target receptacle. The controlling step may use a resistor network and/or a controllable switch in series with the transfer pin.

The method may also include providing a transfer pin array including multiple transfer pins for transferring multiple samples to corresponding multiple target receptacles.

Individual transfer pins in the array may be individually-actuable, either sequentially or in parallel. At least one transfer pin in the array may be independently positionable for alignment with respect to the opening of a target receptacle. Individual transfer pins in the array may be free floating or spring loaded.

In such a method, the dispensed sample may be from 0.2 to 100 nanoliters. The transfer pin may have a diameter greater than the opening of the target receptacle. The sample liquid may be a polar liquid such as aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solutions. The at least one transfer pin may be able to dispense multiple samples without replenishment.

The method may further include applying evaporation control measures to the target receptacle. This may include immersing the target receptacle in an immiscible liquid such as a perfluorinated hydrocarbon. Alternatively, or in addition, the evaporation control measures may include at least one of humidity control, fluid pressure, and receptacle cooling.

The method may also further include positioning the transfer pin in direct contact with target receptacle, or positioning the transfer pin near the target receptacle without direct contact. The method may also include sequentially transferring multiple samples to

the target receptacle to produce a layered sample pattern.

An embodiment may aid the capillary action by vibrating the transfer pin or the target receptacle. The pin tip may have a slotted end, for example, an X-shaped slotted end. The pin tip may have one or more structural features that increase the surface area of the sample liquid at the pin tip. These structural features may include one or more features from the group of slots, grooves, and spirals. In the method, the target receptacle may be substantially empty prior to transferring the sample liquid.

Another embodiment of the present invention includes a microfluidic assay system.

The system includes at least one liquid sample storage device including multiple storage receptacles, a microfluidic dispenser, and a dispenser positioning module. The microfluidic dispenser has at least one transfer pin for using capillary action to transfer a microfluidic sample of liquid to a target storage receptacle, one end of the transfer pin having a pin tip structured to cooperate with an opening in the target storage receptacle.

The dispenser positioning module positions the liquid dispenser to enable the transfer pin to cooperate with the target receptacle for transferring the sample liquid.

In a further such embodiment, the storage device may be a platen array of through- holes or wells. The target storage receptacle may include hydrophilic walls that attract the sample liquid and/or an opening surrounded by hydrophobic material. The liquid dispenser may also include a transfer pin array including multiple transfer pins for transferring multiple samples to corresponding multiple target storage receptacles. Transfer pins in the array may be individually actuable, either sequentially or in parallel. At least one transfer pin in the array may be independently positionable for alignment with respect to the opening of a target storage receptacle. Individual transfer pins in the array also may be free floating or spring loaded.

In such a system, the dispensed sample may be from 0.2 to 100 nanoliters. The transfer pin may have a diameter greater than the opening of the target storage receptacle.

The sample liquid may be a polar liquid such as aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solutions.

An embodiment may further include a voltage controller that applies a high voltage potential, for example, 100V to 5kV, between the pin tip and the target receptacle to aid the capillary action. The voltage controller may apply the high voltage potential before or after the transfer pin is positioned at the target storage receptacle, and removes the high

voltage potential after the transfer pin is moved away from the target storage receptacle.

The voltage controller also may use a resistor network and/or a controllable switch in series with the transfer pin.

In a system, the storage device may use evaporation control measures to control evaporation of dispensed samples from the storage receptacles. This may include immersing the storage receptacles in an immiscible liquid such as a perfluorinated hydrocarbon and/or at least one of humidity control, fluid pressure, and receptacle cooling.

The positioning module may position the dispenser so that the at least one transfer pin makes direct contact with target storage receptacle for transferring the sample liquid, or so that the at least one transfer pin is near the target storage receptacle without direct contact for transferring the sample liquid.

The liquid dispenser may operate to sequentially transfer multiple samples to the target storage receptacle to produce a layered sample pattern. The at least one transfer pin may be able to dispense multiple samples without replenishment.

The capillary action may be aided by vibrating the transfer pin or the target receptacle. The pin tip may have a slotted end, for example, and X-shaped slotted end. The pin tip may have one or more structural features that increase the surface area of the sample liquid at the pin tip. These structural features may include one or more features from the group of slots, grooves, and spirals. The target receptacle may be substantially empty prior to transferring the sample liquid.

Brief Description of the Drawings The present invention will be more readily understood by reference to the following detailed description taken with the accompanying drawings, in which: Figure 1 shows a cut away view of a nanotiter plate having one of its through wells being loaded by a transfer pin bearing a liquid sample according to one embodiment of the present invention.

Figure 2 shows an elevated side view of an array of transfer pins according to one embodiment of the present invention.

Detailed Description of Specific Embodiments Various embodiments of the present invention are directed to using capillary action

to transfer a microfluidic volume of a liquid sample from a transfer pin to a suitable target receptacle. The target storage receptacle typically will have an affinity for the sample liquid, and could be a flat surface; a surface with indentations, close ended wells, or pores; a membrane or filter; a gel; or a platen with close-ended wells or through-hole wells. In one specific embodiment, the target receptacle is one or more wells in an array of through- hole wells as part of a parallel and/or series sample transfer process. In other embodiments, the target storage receptacle may be a hydrophilic spot or divot in a hydrophobic background. Such an environment may be established on a coated glass slide such as the ones available from Erie Scientific of Portsmouth, NH.

Figure 1 shows a cut away view of a nanotiter plate having one of its through-hole wells being loaded by a transfer pin bearing a liquid sample according to one embodiment of the present invention. Platen 10 contains a large number of through-hole wells 12 that traverse the platen 10 from one planar surface 14 to the other opposing planar surface (not shown). The platen 10 is may be from 0.1 mm to more than 10 mm thick; for example, around 0.3 to 1.52 mm thick, and commonly 0. 5 mm. The thickness of platen 10 is also the length of the through-hole wells 12 when they are oriented perpendicularly to planar surface 14. The length and volume of the wells 12 can be increased somewhat by orienting them at an angle to surface 14. The wells 12 are the target receptacles for the liquid samples from the transfer pin.

Typical microfluidic volumes of the through-hole wells 12 could be from 0.1 picoliter to 1 microliter, with common volumes in the range of 0.2-100 nanoliters.

Capillary action or surface tension of the sample liquid may be used to load the wells 12.

To enhance the drawing power of the wells 12, the target area of the receptacle, interior walls 42, may have a hydrophilic surface that attracts the sample liquid. Alternatively, the wells 12 may contain a porous hydrophilic material that attracts the sample liquid. To prevent cross-contamination (crosstalk), the exterior planar surfaces 14 of platen 10 and a layer of material 40 around the openings of wells 12 may be of a hydrophobic material.

Thus, each well 12 has an interior hydrophilic region bounded at either end by a hydrophobic region.

In some systems, the well 12 may be submersed in an immiscible, non-conducting liquid such as perfluorinated hydrocarbon, hydrocarbon, or silicone fluid. An immiscible liquid prevents evaporation of the sample liquid from the wells 12 and further protects the

dispensed samples from cross-communication. Of course, other evaporative control measures may also be useful, including without limitation, humidity control, fluid pressure, platen cooling, etc.

Transfer pin 20 is generally dowel-shaped, made of stainless steel, titanium, or other durable material, with a flat, rounded, tapered, or cupped tip. Typically, although not necessarily, the diameter of the transfer pin 20 is greater than the diameter of the wells 12 in order to have more rigidity in the pin and to allow the pin to reliably contact the side walls of the well to facilitate docking between the pin and the well that would otherwise require much more precise positioning measures.

Transfer pin 20 may also have slots, grooves or spirals cut into it to increase volumetric capacity and/or to better meter the dispensing action. The slots, grooves, or spirals also increase the surface area of the sample liquid available for contact with hydrophilic receptacle walls. Transfer pin 20 may be capable of holding and/or delivering anywhere from 0.1 picoliters to more than 10 microliters, but typically holds 0.1 nanoliters to 4 microliters.

Figure 1 shows an embodiment of the transfer pin 20 having a tapered tip with a tapered slot that holds the sample liquid. In such an embodiment, the tapered end is small enough to fit inside the well 12, but the overall pin diameter is still larger than the diameter of the well. In the embodiment shown, the tapered end of the transfer pin 20 forms a 40 degree angle, and the tapered slot within this end forms a 14 degree angle. This transfer pin 20 holds adequate amounts of sample liquid (-0. 5 Ill), facilitates wicking of the sample liquid to the tip of the pin, and can fill multiple wells 12 in succession without replenishment. The slotted end may use two approximately perpendicular slots forming an X-shape. In an alternative embodiment, the transfer pin 20 is a simple stainless steel dowel with one or more slots in the end. In another embodiment, the transfer pin 20 is a simple stainless steel dowel with a rounded tip and one or more slots in the end.

Transfer pin 20 may be free to move perpendicular to the surface 14 of the platen 10, but movement may be constrained in a plane parallel to the surface; this implementation is referred to as a floating pin. However, alternative embodiments of the invention may also be implemented with fixed transfer pins 20 as well. It is generally desirable to achieve good contact between the transfer pin 20 and the target area, but not to damage the target receptacle, well 12. This objective may be achieved by using a floating model transfer pin

20. Floating gravity-fed or spring-loaded transfer pins 20 help with reliable positioning of multiple pins to properly contact corresponding wells 12 to overcome minor errors in alignment. In some embodiments, spring-loaded transfer pins 20 may be used, preferably with"soft"springs having a spring constant that allows for relatively large displacement with a small applied force. In other embodiments, gravity-fed floating transfer pins 20 may be more advantageous in applying minimum force to a target well 12. However, gravity- fed transfer pins 20 may occasionally stick in one position following a sample dispensing cycle. One solution to this problem is to use a pressure or vacuum manifold to assist with pin positioning, such as a vacuum manifold that sucks the transfer pin 20 back into position between dispensing cycles. Floating transfer pins 20 may also use magnetism or electro-magnetism for pin positioning, such as use of a strong magnetic field for uniformly extending pins, use of magnetic pins, or by accelerating and rapidly decelerating individual pins or the entire array.

Typically, transfer pin 20 is loaded with sample liquid for transfer to platen 10. In typical embodiments, the sample liquid may be an aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solution. Then, transfer pin 20 is moved to a position over the well 12 to be loaded. The transfer pin 20 is lowered until contact is made with the opening of the well 12. When the tip of transfer pin 20 is tapered, as shown in Fig. 1, there is maximal contact between the outer surface of the pin and the surface of the interior walls 42 of well 12. Such maximal contact between pin tip and well wall is desirable because the sample liquid held in the transfer pin 20 needs to contact the interior wall 42 of the well 12 in order for transfer from the pin to be initiated. Furthermore, a tapered pin tip can correct for slight errors in pin placement with respect to the wells, as the taper of the transfer pin 20 guides it into the exact desired position.

Proper positioning of the transfer pin 20 (or arrays of transfer pins) relative to the well 12 (or array of wells) is important for making contact sufficient to effect transfer the sample liquid from the transfer pin 20 to the well 12. This may be accomplished, for example, by use of precisely machined guide plates that holds the transfer pin 20 (or an array of pins) in proper position via at least one hole in the guide plate. In one embodiment, the guide plate holes are slightly larger in diameter than the transfer pins to allow the pins to slide into the optimal position as they are brought into contact with the wells 12. In another embodiment, two guide plates may be used to position the transfer pin

20, a lower guide plate having a smaller diameter guide hole than the upper guide plate. A transfer pin 20 may have a shoulder on it that restricts its downward travel, but which allows the transfer pin 20 to move upward in response to a force such as produced when the pin docks with the well 12. Fine positioning of the transfer pin 20 and well 12 may be aided by vibrating either.

Once the transfer pin 20 is positioned in contact with the opening of well 12, a portion of the sample liquid in the pin will be wicked by capillary action into the well 12 (and displace any immiscible liquid which may previously have been stored therein). The volume of sample liquid that is transferred is self-metered by the volume of the well 12, and subject to other environmental variables, such as the action of the layers of hydrophilic and hydrophobic materials, whether the target area is under an immiscible fluid, and if so, the height of the immiscible fluid over the target area, the duration of contact with the area, the speed of withdrawal from the area, and various of the other variables listed above with respect to pin transfer. In the prior art, transfer of sample liquid has been into wells such as in a 384-well plate that are usually pre-filled with a liquid and the pins have been substantially smaller than the receptacle wells. In some embodiments of present invention, the well 12 may be substantially empty before transferring the sample liquid into it.

Initializing the wicking action and wetting the interior walls 42 of the well 12 is an important point in the transfer process. Some embodiments will have little difficulty establishing good contact between the sample liquid held by the transfer pin 20 and the interior walls 42 of the well 12 sufficient to cause self-metered transfer of the sample liquid to the well 12. Several factors affect the ability for this action to happen, among which are that the interior walls 42 must be sufficiently hydrophilic, and the use of one or more slots in the pin tip of the transfer pin 20 to maximize the exposed surface area of the sample liquid. Preferably the slots will have a reservoir cavity as well as geometry that minimizes the coefficients of variance of loading (related to the standard deviation of volume dispensed in multiple loading cycles) by causing the amount transferred to be a small fraction of the total volume held by the slot reservoir. A slot reservoir in the transfer pin 20 also provides the option of multiple sample transfers from the pin from a single load of sample from a source microplate.

Occasionally, for a variety of reasons, not all of which are well understood, there

will be difficulty establishing this wicking flow. Some embodiments overcome such difficulties in initiating the transfer of sample liquid to a storage receptacle by applying a high voltage electric potential. Although this approach may be useful for non-polar sample liquids, it is especially useful for transferring polar sample liquids such as aqueous, DMSO, dimethylformamide (DMF), or acetonitrile solutions can be transferred into a target well 12 by contacting a transfer pin 20 filled with sample liquid and applying a high voltage with low current (typically less than 5 microamps). Initiation of capillary action may also be aided by vibrating the transfer pin 20 or the target well 12.

Embodiments of the present invention use the existing transfer pin and platen well arrangement described with respect to Fig. 1 above, and add a high voltage potential to the transfer pin 20, or at least the tip of the pin. Such an arrangement differs from that described in the'815 patent in that it avoids the need for a complex plate insulation arrangement (as shown in its Fig. 1), and it does not use the electrokinetic relationship of voltage-time to volume transferred. In embodiments where transfer pin 20 is in direct contact with the receptacle target area, the electric charge applied to transfer pin 20 is not directly related to the duration of the sample liquid transfer or the amount dispensed. That purpose is accomplished by hydrophilic attraction of the interior walls 42 and the self- metering action of the platen wells 12. Rather the electrical charge on the transfer pin 20 serves as an activation energy that excites the sample liquid held by the pin to encourage the wetting of a liquid bridge flow channel between the transfer pin 20 and the interior walls 42 of well 12. The amount of sample liquid that is dispensed in a specific embodiment is dependent upon a multitude of variables such as pin geometry, pin coating, sample liquid surface tension, wetted depth, speed of transfer, sample liquid viscosity, sample liquid conductivity, the concentration of particles in the sample liquid, voltage level, voltage duration, voltage frequency, and loading environment (e. g. , air vs. under liquid). Careful control of these variables is required. In some embodiments, it may be useful to apply the voltage to the well 12 rather than to the transfer pin 20.

The voltage necessary to effect transfer of the sample liquid depends on the physical properties of the sample liquid and the receptacle, i. e., well 12, including their affinity for each other. In addition, the choice between AC and DC voltage supplies may affect the voltage necessary for transfer of the sample liquid, but both types of supplies are acceptable. Generally, the voltage will be between 10V and 50 kV, typically in the range

of 100V to 5 kV. The choice of voltage level is affected by effects of ohmic-related heating and material breakdown characteristics. With a high dielectric constant liquid, a high voltage of large voltage pulse may be applied without electrical breakdown.

It is desirable to limit the current flowing from the transfer pin 20 in order to prevent electrical heating, etching and ionization of the sample liquid, receptacle, pins, air, or immiscible fluid. Therefore, it is important to use a high-voltage, low current system.

Examples high-voltage, low current sources include a Van De Graaf generator, or a standard high voltage source in series with a high-voltage, high-resistance resistor.

In one specific embodiment, the voltage is applied to the transfer pin 20 after it is positioned at the opening of the desired well 12, and the voltage is removed after the sample liquid has been transferred to the well 12 and the transfer pin 20 has been withdrawn from the opening of the well 12. In other embodiments, the voltage may be applied to the transfer pin 20 before it is positioned at the opening of the desired well 12, and the voltage is removed after the sample has been transferred to the well 12 but before the transfer pin 20 has been withdrawn from the opening of the well 12.

In addition, voltage aided transfer of sample liquid in various embodiments may be based on either full, partial, or no physical contact between the transfer pin 20 and the target well 12. That is, in some embodiments, the end of the transfer pin 20 may be brought into substantial physical contact with a portion of the target well 12 in order to transfer the sample liquid from the pin to the well. In other embodiments, the transfer pin 20 approaches the opening of the target well 12 without actually establishing significant contact in order to transfer the sample liquid from the pin to the well. Some embodiments with or without contact may benefit from electrospray effect to transfer the sample liquid from the transfer pin 20 to the target well 12.

In various embodiments, either the target well 12 or the entire platen 10 may be electrically grounded. In other embodiments, the platen 10 and well 12 may be ungrounded. Either approach may be successful so long as there is an appropriate voltage difference between the transfer pin 20 and the target well 12. In addition, the platen 10, itself, may be made of conductive material, or non-conductive material. Moreover, specific embodiments may not necessarily require a combination of hydrophilic and hydrophobic materials as described with respect to Fig. 1, but may be able to exploit the invention using receptacle structures without any significant hydrophobic or hydrophilic

characteristics, or in ones with all hydrophobic or all hydrophilic materials.

The efficiency of voltage aided transfer of the sample liquid also may depend on the relative geometries of the transfer pin 20 and the target well 12. For example, a transfer pin 20 with a tapered point such as shown in Fig. 1, may be more effective than a different shaped end such as a flat one. In one specific embodiment in which the well 12 is 280 microns in diameter, a pointed pin tip of less than 200 microns, e. g., 140 microns, may be most effective. In some specific embodiments, a blunt pin tip also may work, but in other embodiments, such as under dense fluids, a blunt pin tip without a sufficient point on its end may not be operable in a voltage aided transfer arrangement since the sample liquid may climb the sides of the transfer pin 20.

In addition to use of an individual transfer pin 20 as shown in Fig. 1, an embodiment may be based on a multiple pin array 30, such as the one shown in Figure 2, which is designed so that each transfer pin 20 is spaced to address a unique well 12 in the platen 10.

In Fig. 2, multiple transfer pins 20 are held in an array by an electrical insulating plate 32.

The bottoms of the transfer pins 20 may be slotted as shown in Fig. 2, or have some other geometry for holding the sample liquid for dispensing. In addition, the bottoms of the transfer pins 20 may be squared off as shown in Fig. 2, or may be tapered as in Fig. 1, or have some other shape geometry.

The top side of each of the transfer pins 20 may be electrically connected either directly or via a resistor, switch, or transistor to a voltage source. The voltage may be specific for each transfer pin 20, or multiple transfer pins 20 may share a common voltage source.

The top sides of the transfer pins 20 are electrically connected to pin voltage sources 36 in a voltage control array 34, which may optionally include a voltage control port 38 addressable by an external processor. Each individual pin voltage source 36 may be, for example, a resistor element in a resistor network (i. e., the voltage control array 34) connected to a high voltage source so that each transfer pin 20 is connected via its own resistor to the high voltage source. To reduce the cost and size of the system, a single source resistor may be placed between the high voltage source and the resistor network, which allows the use of smaller, cheaper lower resistance resistors in the network together with a single bulky, more expensive, high-resistance resistor at the source. For example, the source resistor could be a 1 to 10 gigohm resistor, and the pin resistors could be 1 to 10

megohms each. However, it may be advantageous in terms of uniformity of transfer throughout the pin array 30 to have a higher resistance on the pin resistors, for example each pin having a gigohm resistor.

To individually actuate at least one transfer pin 20 using voltage application, a controllable switch may be placed in series with each actuable pin. These switches may be, for example, high voltage transistors or relays, and also may be controlled by a microprocessor. In one specific embodiment, each spring-loaded transfer pin 20 may be loaded on a spring, which also acts as an electrical contact to a printed circuit board voltage control array 34. The printed circuit board voltage control array 34 may contain the resistor network and connections to the high voltage source. In some embodiments, the printed circuit board voltage control array 34 also may contain the switch networks and connections to the computer or other device for selecting a sample dispensing pattern.

Thus, in one embodiment, each transfer pin 20 in a multiple pin array 30 is individually addressable for purposes of applying a high voltage potential to the pin. In such a pin array 30, one transfer pin 20 at a time may be actuable, multiple pins may be actuable at one time, or all of the pins in the array may be actuable at one time. The more transfer pins 20 that are actuated at any one time, the greater the parallel processing of the system. By actuating different patterns of multiple transfer pins 20 (in a manner analogous to an ink jet computer printer) patterns of dispensed samples may be developed. By repeating this process, layered sample patterns may be developed, including the synthesis of organic molecules such as peptides, small molecules or oligonucleotides.

In another embodiment, a pin array may be equipped with a controller for selectively extending or retracting a subset of transfer pins 20 to cause contact or removal from contact of those pins for the purpose of dispensing a pattern of sample. For example, an array of solenoids could be used to retract those transfer pins 20 that are not desired to contact the receptacle well 12. The solenoids may act directly on the transfer pin 20, or by a remote drive mechanism such as an array of pistons positioned slidably in an array of tubes. Alternatively, an array of controllable valves connected to a vacuum or pressure manifold may be used to selectively retract or extend a subset of transfer pins 20. Moving the pins in the array 30 so that only transfer pins 20 selected for sample transfer approach the opening of selected wells 12 avoids inadvertent transfer of the sample liquid from non- selected pins to non-selected wells, such as by wetting, which may occur even when no

voltage is applied to a non-selected pin. It may be desirable to both selectively actuate a pattern of transfer pins 20 using both movement controllers and application of high voltage to the selected pins in order to prevent inadvertent dispensing, such as by electrospray.

Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention.