Abstract:
The invention is based on the realization that feedback control for the vertical drive of a pin head becomes unnecessary if one instead prealigns the relative heights of the spotting surface and the well plates such that the pins in the pin head are lowered to the same absolute height both for picking up liquid from the well plates and depositing it onto the spotting surface. By aligning the heights of the well plates and spotting surfaces, the vertical drive of the pin head needs only to be able to reproducibly and accurately arrive at a single lowered position. A pin head can provide this functionality by having a mechanically defined bottom point, obviating the need for a complex and costly feedback control system for height determination of the pins. A simple, low-cost micro-arrayer can thus be provided.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to microarraying and spotting, more especially but not exclusively to microarraying apparatus and to pin heads for microarraying apparatus. 
     Microarraying is a technique in widespread use in fields such as chemistry and biotechnology. Liquid samples are stored in the wells of a well plate. The liquid may be assays or any other biological or chemical sample of interest. To spot the liquid from a well, a pin is dipped in the well to retrieve an amount of the liquid. The pin carrying an amount of the liquid is then moved across to a spotting surface of a microscope slide or other suitable surface. A spot of liquid is deposited on the slide by bringing the pin into close proximity with the slide surface, or by physically contacting the tip of the pin with the slide surface. 
     In general, a plurality of slides and well plates are arranged on a surface of a microarraying apparatus. An array of pins is held in a pin holder which is mounted on a frame above the arraying surface. A motorized positioning system is provided to move the pin holder laterally about the frame so that the pins can be moved between the slides and well plates, and other areas such as a wash station. A vertical positioner or drive is also provided for causing vertical motion of the pins. This allows the pins to be dipped into the well plates and also allows the pins to be brought into contact with the slides for spotting. 
     Typically, microarraying apparatus of this type are large and costly pieces of equipment intended for large scale use, and are capable of handling a large volume of well plates and slides. Given the repetitive nature of the spotting procedure and the large lateral distances between the well plates and spotting surfaces of the slides, the vertical pin drive is required to have the capability of returning exactly to any desired absolute height position many thousands or tens of thousands of times without any long term drift. 
     For example, the pin drive must be able to dip the pins into the wells to a suitable depth to pick up liquid, and also to deposit the liquid as desired on a spotting surface. Spotting requires that the pin or pins approach the spotting surface with great precision, in order to be close enough for effective deposit of the liquid without the pin crashing into the slide. In other words, attaining an appropriate clearance height during spotting is critical, and must take account of factors such as use of slides of different thicknesses. Similarly, well plates may be of different thicknesses or have reservoirs filled to different levels. 
     To attain the necessary specification, the pin drive is typically provided with some kind of height sensing with feedback, such as an interferometric encoder system. Although precision feedback systems for accurate height sensing are effective and available with established technology, they are costly, and contribute substantially to the overall cost of a microarraying apparatus. 
     While cost is not a major factor for high volume applications, it is a considerable deterrent to the more widespread use of microarraying for smaller scale research projects. 
     SUMMARY OF THE INVENTION 
     The invention is based on the realization that feedback control for the pin drive becomes unnecessary if one instead prealigns the relative heights of the spotting surface and the well plates such that the pins in the pin head can be lowered to the same absolute height both for picking up liquid from the well plates and depositing it onto the spotting surface. By aligning the heights of the well plates relative to the spotting surfaces, the pin drive needs only to be able to reproducibly and accurately arrive at a single lowered position. Building from this realization, it has been further appreciated that a pin head can be designed to achieve this functionality purely with a mechanically defined bottom point, obviating the need for an encoder or other feedback control system for height determination of the pins. Moreover, it is a simple matter to provide a height adjustable platform for supporting the area carrying the well plates, or alternatively a height adjustable platform for supporting the area carrying the microscope slides for spotting onto. Through these measures it becomes possible to produce a microarraying apparatus of significantly reduced cost that can be used for small-scale experimentation. 
     Accordingly, a first aspect of the invention is directed to a pin head for a microarraying apparatus comprising: a pin holder for carrying an array of pins; a vertical drive operable to move the pin holder in a vertical axis; and a positioning mechanism that cooperates with the vertical drive to mechanically define a lowest point of travel for the pins in the vertical axis. 
     Moreover, a second aspect of the invention is directed to a microarraying apparatus comprising: an apparatus bed defining an arraying surface for carrying one or more slides; a well plate platform for carrying at least one well plate; and a height adjustment mechanism operable to alter the height of the well plate platform relative to that of the apparatus bed. 
     In use, the microarraying apparatus is aligned before use using the height adjustment mechanism, which is preferably manually actuatable for simplicity and cost reasons. Alignment is achieved by ensuring that the horizontal plane defined by the lowest point of travel of the pins lies an appropriate distance below the surface of the liquid held in the well plates and an appropriate distance above the spotting surface of the slides (which may be zero in the case of near-contact of the pin tips with the spotting surface). 
     The pin holder has simple vertical motion with a defined lowest position in which the pin holder can be readily located without the need for complex encoder systems. This arrangement permits a microarraying apparatus to be set up with its component parts positioned correctly with respect to the position of the pins when the pin holder is in the lowest position, so that spotting can be carried out without the need for control of the vertical position of the pins during the spotting process. 
     For some applications where pins are individually fired to the bottom position, the vertical drive may be connected directly to the pins rather than via the pin holder in which case there may be provided a pin head for a microarraying apparatus comprising: a pin holder carrying an array of pins; a vertical drive operable to actuate the pins in a vertical axis individually or collectively; and a positioning mechanism that cooperates with the vertical drive to mechanically define a lowest point of travel of the actuated pins in the vertical axis. 
     Advantageously, the positioning mechanism comprises a crank system having a bottom dead center and coupled to the pin holder so that the pin holder achieves its lowest position when the crank system is at bottom dead center. A crank system is a simple way of limiting the travel of the pin holder to define the lowest position. Also, the operation of a crank system means that the pin holder and pins therein move at very low velocity and acceleration as the crank system approaches bottom dead center, reaching a momentary static point at the lowest position even if the crank is continuously rotated. This smooth pin approach is highly desirable for ensuring good quality spots are deposited. 
     The vertical drive may comprise a rotary solenoid or a rotary motor which is coupled to the pin holder by the crank system, the crank system operable to transfer motion produced by the rotary solenoid to the pin holder. A rotary solenoid has the advantage of only requiring a very simple on/off actuation signal. For example, the bottom position may be associated with a deenergized state of the solenoid and a withdrawn position associated with an energized state of the solenoid (or vice versa). 
     Another aspect of the present invention is directed to a head apparatus combining the above described pin head with an additional height adjustment mechanism. More especially, the head apparatus comprises: a mount adapted to attach a pin head to a microarraying apparatus; a pin head as described above movably mounted on the mount; and a pin head height adjustment arrangement operable to allow the lowest position of the pins to be varied relative to the mount. 
     A still further aspect of the present invention is directed to a microarraying apparatus comprising: an arraying surface; a slide holder for holding one or more slides mounted on the arraying surface; a well plate platform for holding one or more well plates mounted on the arraying surface; and a platform height adjustment mechanism operable to alter the relative heights of the slide holder and the well plate platform. 
     The platform height adjuster may be provided with a distance scale which is indicative of the difference in the relative heights of the slide holder and the well plate platform. 
     The platform height adjuster facilitates setting up the apparatus so that the slides and well plates are at heights at which dipping and spotting can be performed with the same lowest vertical pin position. 
     It will be appreciated that further height adjustable platforms may be provided, for example for a wash station, so that pins are dipped into cleansing liquid in the wash station to a sufficient depth for cleaning. 
     The microarraying apparatus may further comprise a mounting frame mounted over the arraying surface; and a pin head mounted on the mounting frame. The pin head is provided with a drive system operable to move the pin head in a plane parallel to the arraying surface. The pin head comprises a pin holder for holding an array of pins, a vertical drive operable to move the pin holder along a vertical axis, and a positioning mechanism which defines a furthest point of travel of the pin holder along the vertical axis, the furthest point being the lowest position achievable by the pin holder. 
     In this way, a simple pin head which has a repeatable lowest position can be used for dipping and spotting. Specifically, precision, accuracy and reproducibility are achievable without the need for a feed-back-controlled pin head, since the pin head is only required to be able to find its bottom position precisely, accurately and reproducibly, which can be performed with a variety of simple mechanical or electromechanical solutions. 
     A still further aspect of the invention is directed to a spotting method using a microarrayer having an apparatus bed and a well plate platform, the method comprising: 
     arranging at least one slide on the apparatus bed to provide a spotting surface; 
     arranging a well plate filled to a level with spotting liquid on the well plate platform; 
     vertically aligning the well plate platform relative to the apparatus bed so that the spotting surface lies at a desired height at or below the level of the spotting liquid; 
     dipping a pin from a pin head into the spotting liquid by moving the pin to a lowered position; 
     moving the pin head across the microarrayer to a spotting position; and 
     depositing the spotting liquid onto the spotting surface by moving the pin once again to the lowered position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: 
     FIG. 1 shows a partially exploded perspective view of a first embodiment of a pin head employing a crank system and a rotary solenoid; 
     FIG. 2 shows a perspective rear view of the rotary solenoid of FIG. 1; 
     FIG. 3 shows a plan rear view of the rotary solenoid of FIG. 1; 
     FIG. 4 shows a schematic diagram of the crank system of FIG. 1; 
     FIG. 5 shows a graph of the variation of the vertical position of a pin holder comprising part of the pin head of FIG. 1; 
     FIG. 6 shows a partially exploded perspective view of a pin head according to a second embodiment also having a crank and based on an electric motor; 
     FIG. 7 shows a schematic diagram of the crank system of FIG. 6; 
     FIG. 8 shows a graph of the variation of the vertical position of a pin holder comprising part of the pin head of FIG. 6; 
     FIG. 9 shows a cross-sectional side view of part of a pin head with integral height adjustment; 
     FIG. 10 shows a cross-section top view of the pin head of FIG. 9; 
     FIG. 11 shows a perspective view of a microarraying apparatus embodying the invention; and 
     FIG. 12 shows a perspective view of the microarraying apparatus of FIG. 11 when housed. 
    
    
     DETAILED DESCRIPTION 
     First Embodiment of Pin Head 
     FIG. 1 shows a partially exploded view of an embodiment of a pin head. 
     The pin head  10  has a pin holder  12 , which comprises an upper plate  14  and a lower plate  16 . The plates  14 ,  16  each have a matrix of holes  18 , the holes  18  in the upper plate  14  being positioned vertically above the holes  18  in the lower plates  16 . The holes  18  receive pins  20 . Each pin  20  has a head  21  of a larger diameter than the holes  18 . A pin  20  passes through a hole in the upper plate  14  and through the correspondingly positioned hole  18  in the lower plate  16 . The head  21  of the pin  20  prevents the pin from passing completely through the hole  18  in the upper plate  14  so that the pin  20  is hangs freely in position in the pin holder. For the sake of clarity, FIG. 1 shows one pin  20   a  in position in the pin holder, and one pin  20   b  about to be inserted into the pin holder  12 . In use, each pair of holes  18  in the upper and lower plates  14 ,  16  will hold a pin  20 . The pin holder  12  in FIG. 1 holds twenty-four pins  20  in a six by four array, but other quantities of pins  20  may be accommodated, for example forty-eight. 
     This pin head arrangement is but one of a variety of possible arrangements. It will be understood that the manner in which the pins are accommodated in the pin head is not relevant for implementing the invention, so that a variety of pin housing schemes may be used, as will be familiar to a person skilled in the art. 
     The pin holder  12  is affixed to one side of an elongate vertical member  22 , towards the lower end of the vertical member. The same side of the upper end of the vertical member  22  has an elongate protrusion  24  positioned longitudinally along it. The protrusion  24  is substantially T-shaped in cross-section. An attachment member  26  has an elongate recess  28  also of substantially T-shape cross-section. The recess  28  receives the protrusion  24  which can slide freely within it. Thus the recess  28  and the protrusion  24  together form a linear bearing which allows the vertical member  22 , and the pin holder  12  mounted thereon, to slide vertically with respect to the attachment member  26 . 
     At the lower end of the vertical member  22 , on the reverse side to the pin holder  12 , there is a first peg  30 , for the purpose of coupling the pin holder to a vertical drive. The coupling is achieved by means of a crank system, comprising a first crank shaft  32  which is elongate and has a hole  33  at each end, and a second crank shaft  34  which is also elongate, and has a second peg  36  at one end. The first crank shaft  32  joins the vertical member  22  to the second crank shaft  34  by means of its two holes  33 , one of which receives the first peg  30  and the other of which receives the second peg  36 . The pegs  30 ,  36  can rotate freely in the holes  33 . The end of the second crank shaft  34  distal from the second peg  36  is coupled to a rotary solenoid  40  by means of an axle  38 . The rotary solenoid is connected to a source of reversible current (not shown), and comprises the vertical drive of the pin head. 
     In use, the pin head  10  is fixed to a microarraying apparatus by means of the attachment member  26 , so that the attachment member is fixed in relation to the microarraying apparatus. Therefore, the vertical member  22  and pin holder  12  are free to slide vertically in relation to the microarraying apparatus by means of the linear bearing formed by the protrusion  24  and the recess  26 . 
     The vertical motion of the pin holder  12  is controlled by the rotary solenoid  40 . Application of current to the rotary solenoid  40  causes the second crank shaft  34  to rotate first in one direction, to the position shown by the solid lines in FIG. 1, owing to magnetization of the solenoid in the conventional fashion. Reversal of the current causes rotation in the opposite direction, to the position indicated by the broken lines in FIG.  1 . The second crank shaft  34  is preferably constrained so that rotation from one extreme to the other covers 110°, although other angles of rotation can be used as appropriate. 
     The movement of the second crank shaft  34  is transferred via the first crank shaft  32  to the vertical member  22 , which is moved up and down within the linear bearing as the second crank shaft  34  rotates from side to side. The vertical member  22 , and hence the pin holder  12 , reach an uppermost position when the second crank shaft  34  is at either of the two extremes of rotation. The vertical member  22  and pin holder  12  reach a lowest position when the second crank shaft  34  is positioned vertically, midway between the two extremes of rotation. This is at the bottom dead center of the crank system, at which point the two crank shafts are extended to their fullest reach so that this is the furthest travel which the pin holder can achieve. The pins therefore reach the lowest position once per reversal of the alternating current, so that there is a defined pin position which is repeatedly achievable without the need for computer control or the like. In this way, the crank system acts as a positioning mechanism which defines the lowest vertical position which the pin holder, and hence the pins, can reach. This contrasts with pin holders controlled by encoded motorized drive systems in which encoded feedback control of the motor is required to locate the pins at any predetermined position. 
     When the current is removed from the rotary solenoid  40 , the solenoid  40  is demagnetized and no longer acts on the second crank shaft  34 . Therefore, the crank system falls freely under its own weight (and the weight of the pin head to which it is coupled) to the bottom dead center position. Hence, the pin holder falls to its lowest position. 
     This behavior means that the pin holder  12  always returns to the lowest position when no current is applied to the rotary solenoid  40 . Therefore, the pin holder  12 , and the pins  20  held therein revert to the defined lowest position, which can be repeatedly and consistently achieved merely by removing the current. This permits microarraying apparatus to be set up easily with respect to this defined lower pin position. 
     However, in some instances it may be desirable for the pin holder  12  and pins  20  to be retained in an upper position when not in use, while still being consistently locatable in the lower position. This affords protection to the pins, as they can be “stored” in an upper position, possibly within a housing or casing, which protects them from accidental damage. 
     FIG. 2 shows a perspective rear view of a rotary solenoid suitable for this purpose. The solenoid  40  has a lever  110  connected at one end to the axle  38  of the solenoid  40 . The lever  110  has a first post  112  protruding from its distal end. The distal end terminates in a handle portion  111  which extends beyond the edge of the solenoid  40 . A second post  114  protrudes from the rear of the solenoid  40 , positioned above the axle  38  and in line with the bottom dead center of the crank system. A coil spring  116  extends between the two posts  112 ,  114 . 
     The lever  110  has a sensor shutter  118  extending from its side, and having the form of a flat protrusion. A sensor gate  120  is located on the rear surface of the solenoid  40 , having an opening  122  which can receive the shutter  118 . An optical source (not shown) such as a light emitting diode (LED), and an optical detector (not shown), are located within the gate  120 , one on each side of the opening. Together these components form an optical position sensor. Cabling  124  carries power to the optical source and response signals from the optical detector. The power can be provided from the power source which drives the solenoid  40 . The response signals are passed to the solenoid power source, which is adapted to respond to the signals. 
     In use, a pulse of current is applied to energize the solenoid  40 , which moves the crank system in the manner previously described. The lever  110  is rotated with the second crank shaft  34  via its attachment to the axle  38 . The pulse is applied for a time sufficient to carry the crank system, and hence the lever  110 , from one extreme of rotation over roughly a third of second crank shaft&#39;s permitted travel. Once the pulse ceases, the solenoid  40  is de-energized, but the crank system and the lever  110  continue to travel, carried by inertia, over a further third of the permitted travel. The crank system therefore coasts through its bottom dead center, and the pins pass through their lowest position. 
     Following this, the spring  114 , which has been stretched by the lever passing the bottom dead center position, is able to contract and hence pulls the lever further round, carrying the crank system with it round to the other extreme of rotation. Braking can be achieved by shorting the solenoid coils which provides a back e.m.f., thus ensuring that the crank system is brought to a smooth halt. 
     In this manner, the spring carries the pins and pin holder to an upper position, corresponding to an extreme of rotation of the crank system, when the solenoid is demagnetized. Thus the pins can be protected when not in use. The device is therefore mechanically bi-stable, as it returns to one or other of the extreme positions when not in use. When the solenoid is demagnetized, with the pins in an upper position, the pins can be moved in their lowest position by means of the lever  110 . The lowest position is, however, unstable. The lever can be manually toggled back and forth against the force of the spring  116 . When the crank system is located at bottom dead center, the spring  116  is balanced and does not pull the lever  110  to either side. 
     FIG. 3 shows a schematic plan view of the rear of the solenoid  40 . The arrow indicates the movement of the lever  110  from one extreme of rotation to the other. 
     The purpose of the optical sensor is to determine the position of the crank system. At one extreme of rotation, the shutter  118  is received within the opening  112 , and hence blocks the light from the optical source so that nothing is detected by the detector. At the opposite extreme of rotation, the shutter  118  is removed from the opening  112  so that light from the optical source is detected by the detector. Hence it is possible to determine from the state of the detector signal in which of the two extremes of rotation the crank system is located. In turn, this determines which polarity of pulse needs to next applied to the solenoid  40 , to move the crank system to the other extreme of rotation. Therefore, the detector signal is fed to the solenoid power source to control the pulse polarity. 
     A crank system having a bottom dead center is a particularly advantageous way of configuring the positioning mechanism. 
     FIG. 4 shows a schematic diagram of the crank system in a number of positions, including bottom dead center. The second crank shaft  34  is shown in its two extreme positions (solid outline and broken outline), and also at a number of intermediate positions in which it is represented by solid lines for the sake of clarity. The first crank shaft  32  is represented by solid lines in all cases. 
     At the extreme left position of the second crank shaft (solid outline), the angle between the two crank shafts is at a minimum. The position of the lower end of the first crank shaft  32  is constrained by the linear bearing of the pin head  10  to lie on the vertical axis of movement of the pin holder  12 . At this minimum angle, the pin holder is at its uppermost position. As the second crank shaft  34  moves away from the extreme left position, the angle between the crank shafts increases, and the lower end of the first crank shaft moves down the vertical axis, carrying the pin holder  12  with it. This continues until the angle between the crank shafts reaches a maximum of 180°, at which point the crank system is at bottom dead center, and extended to its maximum reach, so that the pin holder has been moved to the lowest position achievable. As the second crank shaft moves past the bottom dead center position towards its extreme right position (broken outline), the reverse process occurs until the pin holder is once again in the uppermost position. 
     An equivalent effect can be achieved by locating the crank system in some orientation other than that shown in FIGS. 1 and 4, so that, for example, the lowest position of the pin holder  12  corresponds to one or both of the extreme positions of the second crank shaft  34 . More complex crank systems having more than two crank shafts may also be used. However, the use of the bottom dead center position to define the lowest position is to be preferred. It is to be understood throughout this application that the term bottom dead center refers also to the positions of other crank system orientations in which the lowest position is achieved. 
     FIG. 5 shows a graph of the variation of the vertical position x of the pin holder with time t, as the solenoid current is reversed. If the second crank shaft  34  was driven continuously around and around, the pin holder position would vary in a sinusoidal fashion as shown by line  100 . However, as the second crank shaft  34  is constrained to travel over only 110°, and also because the solenoid is powered by the reversing of the current supply, the movement of the pin holder  12  is limited to that portion of the sine wave under the line  102 . The pin holder moves between the uppermost position  104  and the lowest position  106 . 
     It is noted that, as the pin holder  12  approaches the lowest position, the slope of the graph becomes less and less, reaching zero at the lowest position. This indicates that the velocity and acceleration of the pin holder also reduce and reach zero as it approaches the lowest position (assuming continuous rotation of the crank through the turning point at bottom dead center). Therefore, the chances of damage to the pins  20  or to slides or other parts of a microarraying apparatus owing to unexpected impact are reduced, and the quality of the spotting is enhanced since ample time will be allowed for a controlled release of the spotting liquid carried by the pin onto the spotting surface. Also, the pins  20  are retained in the pin holder  12  more securely. As mentioned, the pins  20  hang freely under their own weight in the pin holder  12 , so that if the pin holder  12  moves downwards too quickly, the pins  20  can be left behind, and possibly not reach their lowest position before the pin holder  12  begins to move upwards again, collecting the pins  20  on the way. The low velocity and acceleration about the lowest position prevent this from occurring, so that the tips of the pins  20  reach their lowest position at the same time as the pin holder  12 . This low velocity behavior is an advantageous feature of driving the pin head  12  with a rotational drive coupled to a crank system, which imparts the sinusoidal motion to the pin head  12 . 
     Second Embodiment of Pin Head 
     As described above, the first embodiment uses a rotary solenoid as the vertical drive for the pin head. An alternative embodiment uses a rotary electric motor in place of the rotary solenoid. The structure and function of the pin head is otherwise identical to that of the first embodiment shown in FIG. 1, and the same reference numerals will be used to refer to corresponding parts. The rotary motor performs the same function as a rotary solenoid, in that the second crank shaft  34  is rotated so that the pin holder  12  reaches its lowest position when the crank system is at bottom dead center. 
     FIG. 6 shows a partly exploded perspective view of a pin head driven by a rotary motor. As before, the pin head  10  comprises a pin holder  12  for holding pins  20  which is mounted on one side of a vertical member  22  having a linear bearing at its upper end. A peg  30  on the reverse side of the vertical member  22  couples the pin head  10  to a crank system comprising a first crank shaft  32  and a second crank shaft  34 . The second crank shaft is coupled to an axle  38  driven by a rotary stepper motor  126 . The motor  126  is powered in the conventional manner. 
     In operation, the rotary motor rotates fully, so that the second crank shaft  34  performs complete revolutions. Once per revolution, the crank system therefore passes through its bottom dead center and its top dead center. The bottom dead center takes the pins in the pin holder to their lowest position, and the highest position of the pins occurs at top dead center. This is in contrast to the rotary solenoid embodiment described above, in which the second crank shaft moves back and forth over a fraction of a full revolution, so that the crank system never reaches bottom dead top. The crank system can be set to bottom dead center, and hence the pins set to their lowest position, by manual rotation of the motor until the position is reached. 
     FIG. 7 shows a schematic depiction of the rotation of the crank system. The first crank shaft  32  and the second crank shaft  34  are shown in an intermediate position which locates the pins between the highest and lowest positions. As the second crank shaft  34  is rotated by the motor  126 , as shown by the arrow in FIG. 7, it carries the first crank shaft with it so that the crank system passes first through top dead center  128  and then through bottom dead center  130 . 
     FIG. 8 shows a graph of the variation of the vertical position x of the pin holder with time t, as the motor rotates the crank system. The motion is sinusoidal, with a period T corresponding to one revolution. As before, for the rotary solenoid, the velocity of the pins is zero at the lowest position. The pins have maximum velocity at the position  134  intermediate between the lowest position  132  and the highest position  136 . 
     Pin Head Height Adjustment Mechanism 
     FIG. 9 shows a cross-sectional side view of a pin head provided with a vertical height adjustment arrangement. The pin head may be according to any of the embodiments described above. 
     The vertical member  22  is attached to the attachment member  26  by means of the linear bearing in the manner previously described. The attachment member  26  is fixed on the inside of a C-shaped (in vertical cross-section) mount  42 . The mount  42  is used to fasten the pin head  10  to a microarraying apparatus. 
     The pin head  10  is held within the mount  42  by an adjustment member  44 . The adjustment member is fixed to the solenoid  40  (or motor  126 ), and is held vertically within the mount by a biasing spring  46  and an adjusting screw  48 . The adjusting screw  48  screws upwards through a threaded hole in the mount  42 , and passes through the mount  42  so as to abut the underside of the adjustment member  44 . The adjustment member  44  is supported on the adjusting screw  48 . The biasing spring  46  extends between the upper side of the adjustment member  44  and the underside of the upper arm of the mount  42  so as to bias the adjustment member  44  and the mount  42  apart. 
     To adjust the height of the pin head  10 , the adjusting screw  48  is turned so as to move further into or out of the threaded hole. Because the adjustment member  44  rests on the screw  48 , the adjustment member moves up or down accordingly, and is held in contact with the adjusting screw  48  by the biasing spring  46 . The spring  46  thus ensures that the adjustment member follows the movement of the screw. 
     The adjustment member  44  is attached to the solenoid  40 , which is in turn coupled to the vertical member  22  by the crank shafts, so that as the adjustment member moves, the vertical member  22  slides up or down along the linear bearing. The vertical member  22  hence moves relative to the mount  42 . Therefore the height of the pin head relative to the mount  42  can be altered by turning the adjusting screw  48 . The mount  42  attaches the pin head  10  to a microarraying apparatus, so this has the effect of altering the lowest position which the pins can reach relative to the surface of the apparatus. A scale can be provided so that heights can be recorded and replicated. 
     FIG. 10 shows a further cross-sectional view of the pin head, along the direction ‘A’ in FIG.  9 . The mount  42  is C-shaped in horizontal cross-section. A locking screw  50  passes through a hole in one side  45  of the mount  42  and is screwed into a threaded hole on the opposite side  43  of the mount  42 . Tightening of the locking screw  50  pulls the two sides  43 ,  45  of the mount  42  towards one another. 
     Each side of the mount  42  is provided with a vertical channel  52 , which receive the adjustment member  44  with the solenoid  40  attached. As the locking screw  50  is tightened, the two sides  43 ,  45  of the mount  42  clamp the adjustment member  44  between them. 
     This feature allows the pin head  10  to be more firmly secured to the mount  42 . When the height of the pin head  42  is to be adjusted, the locking screw  50  is loosened so that the adjustment member  44  can slide within the channels  52 , and hence move up or down as the adjusting screw  48  is turned. After the height is set, the locking screw  50  is tightened to hold the pin head  10  firmly in position. 
     The purpose of this feature of vertical adjustment is to permit a user to manually and readily alter the lowest pin position achieved by the pin holder with respect to the microarraying apparatus, so that the lowest pin position can be set to coincide with a pin position which needs to be repeatedly achieved, such as contacting the surface of slides with the pins for spotting in a microarraying process. 
     Although similar adjustment can be achieved by moving both the apparatus bed and well plate platform together in the vertical direction, the provision of a mechanism on the pin head for adjusting the bottom pin position is more convenient, since it avoids having to move two or more components together (the apparatus bed, well plate platform and any other platform). In addition, with this approach, there is no need to provide a mechanism for adjusting the height of the surface supporting the microscope slides. In other words, the apparatus bed can be a simple non-adjustable block. 
     Microarrayer 
     FIG. 11 shows a perspective view of a microarraying apparatus  60  according to an embodiment of the invention, which may include a pin head according to any of the above described designs. The apparatus  60  comprises a microarraying surface  62 , constituted by the main bed of the apparatus. The surface  62  may optionally have in it a plurality of shallow depressions  64 , each of which has dimensions slightly larger than a standard microscope slide. The depressions  64  together form a slide holder or locator  65 , and each depression  64  receives a slide  66 . 
     The surface  62  also has a well plate platform  68 , which has a number of shallow depressions each large enough to receive and locate a well plate  70 . The well plate platform  68  is not integral with the surface  62 . Instead, it is a separate planar member which fits within an aperture  69  in the surface  62 . Underneath the well plate platform  68  is an elevating mechanism, for example a jack, operable to raise and lower the well plate platform  68  with respect to the surface  62 . The elevating mechanism is operated by means of a dial  72  protruding from the surface  62 . Turning the dial  72  in one direction raises the well plate platform  68 , and turning it in the opposite direction lowers the well plate platform  68 . The dial  72  has a scale marked on it indicating the amount by which the well plate platform  68  is raised or lowered. The scale also indicates when the well plate platform  68  is in a position in which the top of well plates  70  in the well plate platform  68  are level with the top of slides  66  in the slide holder  65 . 
     In this embodiment the elevating mechanism is purely mechanical for simplicity of construction, low cost and ease of use. However, in other embodiments any other known elevating mechanism could be used, for example using an electric motor. 
     A frame  74  is mounted over the surface  62 . The frame  74  comprises two elongate uprights  76  affixed at their lower ends to the surface  62 , and a cross-bar  78  joining the upper ends of the uprights  76 . A horizontal arm  80  is mounted on a track on the underside of the cross-bar  78 . 
     The arm  80  has on its underside a further track, mounted on which is a pin head  10 . The pin head  10  can be according to any of the embodiments described above; however, for the purposes of example only, the following description assumes a pin head  10  having a rotary solenoid and a crank system according to the first embodiment. A mount  42  having a height adjustment mechanism with a locking screw  50  couples the pin head  10  to the track. A housing  82  houses the vertical drive of the pin head  10 . 
     The frame  74  is motorized to provide movement of the pin head  10  in a plane parallel to the surface  62 . Control wires  84  to control and power the motorization pass along the underside of the arm  80  and the cross-bar  78  to a power supply and control box  86 . Further wires  84  connecting the vertical drive to the power supply are also provided. The pin head  10  can travel along the length of the arm  80 , riding on the track, and the arm  80  in turn can travel along the length of cross-bar  78 , also by riding on the track. Thus the pin head can move to a position above any slide  66  in the slide holder  65 , and any well plate  70  in the well plate platform  68 . Vertical movement of the pin head, towards and away from the surface  62 , is provided by the vertical drive. 
     To operate the microarraying apparatus, a slide  66  of the same thickness as slides to be spotted by the microarraying apparatus is placed into the slide holder. The pin head  10 , having pins in its pin holder, is moved to a position above the slide. No current is supplied to the solenoid, so the pins in the pin holder are in their lowest position. Using the head  46  of the adjustment screw on the mount  42 , the pin head  10  is lowered manually until the pins are in the position needed to spot the slide (for example, the tips of the pins just contact the slide surface, or are just above it, depending on the spotting technique to be applied). Application of current to the solenoid then moves the pin holder to its upper position, well clear of the slide surface. 
     Next, the height of the well plate platform  68  is adjusted, using the dial  72 . The height is first set to that at which the tops of well plates are level with the tops of slides, as this means that the lowest pin position will be such that the tips of the pins are approximately level with the top of the well plate. Given the depth of the wells in the well plates, the depth of fluid in the wells and the depth to which the pins need to dip into the fluid, the height of the well plate platform  68  is adjusted until the lowest pin position coincides with the pin position required to dip the pins into the wells. 
     Once the positions of the pin head  10  and the well plate platform have been set, well plates  70  are placed into the well plate platform  68  and slides  66  are placed into the slide holder  65 , and the microarraying procedure can be carried out. Movement of the pin head  10  over the surface  62  by control of the motorized frame  74  is carried out in the conventional way. Current is applied to the solenoid while the pin head  10  is travelling over the surface, to keep the pin holder in its upper position so that the pins are clear of the slides and well plates. However, each time the pins are required to dip into the wells or spot the slides, the current applied to the solenoid is reversed so that the pin holder moves from its upper position to its lowest position (where spotting or dipping occurs) and back to its upper position for further travel over the surface. 
     The microarraying apparatus depicted in FIG. 11 is of a small size, for the handling of modest quantities of well plates and slides. This small size makes the apparatus portable, and well-suited for use in situations where micro-arraying is carried out on a small or infrequent scale. 
     FIG. 12 shows a perspective view of a housing  88  for encasing the microarraying apparatus  60 . The housing  88  comprises an upper part  90  and a lower part  92  which in use, when encasing an apparatus, are sealed together. However, the two parts  90 ,  92  can be taken apart to allow the apparatus to be removed from the housing  88 , for maintenance, repair and the like. The upper part  90  has an openable transparent window  94 . Operation of the apparatus can be observed through the window, and it can be opened to give access to the apparatus. 
     Alternative Embodiments 
     Constructions alternative to that of FIGS. 9 and 10 may be used to provide the manual adjustment offered by the pin head mount. For example, a gearing system may be incorporated into the screw thread adjustment to allow finer control and more accurate control of the position of the pin holder. 
     Although the main embodiment describes a spotting surface without height adjustment in combination with a height adjustable well plate platform, it will be appreciated that the spotting surface may be height adjustable and the well plate non-height adjustable. Alternatively, both the well plate platform and spotting surface may be provided with height adjustment facilities. All that is required is that they be relatively adjustable by known amounts so that they can be set into the correct positions for spotting and dipping once the lowest pin holder position has been set to coincide with one of these positions. Indeed, if adjustment to both the slide holder and the well plate platform is provided, it is possible to dispense with the manual adjustment of the pin head. The slide holder and well plate platform can be locked together at the same height, and moved together so that the slide is in the spotting position under the pins in their lowest position. The holders can then be unlocked, and the well plate platform adjusted to provide the necessary height difference from the slide holder. 
     A variety of interchangeable well plate platforms can be provided, each accommodating different sizes of well plate, as these are commonly available in a number of sizes having varying numbers of wells. 
     It will also be understood that references to slides or microscope slides for providing the spotting surface are made for clarity. Of course, a wide variety of items may be used to provide a spotting surface, some of which may not normally be termed as slides in the art. The term slide as used in this document should be construed as meaning any item that provides a spotting surface. 
     The bottom position of the pins may preferably be defined by a de-energized or off state of an electrical actuation signal to the vertical drive for the pins, as described above for the rotary solenoid embodiment. Alternatively, the bottom position may be defined by an energized or on-state of the electrical actuation signal. Moreover, although the above-described simple two-state electrical actuation signal is preferred, a more complex multi-state or analog electrical actuation signal could be used. 
     Instead of a crank-based rotary drive, in another embodiment, a linear motor may be used to drive the pin head vertically, in combination with a positioning mechanism comprising an abutment against which the pin holder or vertical member abuts. The abutted position defines the lowest position of the pins. 
     It will be appreciated that although particular embodiments of the invention have been described, many modifications/additions and/or substitutions may be made within the spirit and scope of the present invention.