Abstract:
A high throughput microarraying or colony picking robot with an automatic mechanism for exchanging pin heads and an automated washing and drying apparatus. The robot allows a dirty pin head to be deposited on the automated washing and drying apparatus for cleaning without use of the xyz-positioner, while spotting or picking can continue by picking up a clean pin head without having to wait for the dirty pin head to be cleaned. In this way, the speed of operation can be increased without any increase in the acceleration or speed of the x-, y- and z-drives. The dead time normally associated with the washing and drying cycle, which is usually several minutes, is therefore eliminated, and replaced with the much shorter dead time, of only a few tens of seconds, needed for changing between heads.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a spotting and picking processes using pin heads, and to robotic apparatuses for carrying out such processes using pin heads. 
     Microarraying is a technique in widespread use. Conventional microarraying is based on standard multi-well plates having a 4.5 mm grid and 384 wells, although other sizes are available. 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. Sub-samples of the liquid within the well plates are carried to and deposited on a spotting surface, typically microscope slides, as required. Usually many such deposits are needed and microarraying is a process whereby multiple deposits can be made simultaneously and under machine automation. 
       FIG. 1  shows a typical microarraying robot highly schematically in plan view. The robot comprises a main bed  10  onto which well plates  4  are delivered by an automated well plate feeder  6  with a number of delivery lanes (two in the illustration) through a delidding/relidding mechanism (not shown). Each delivery lane has a feed port  7  and a restack port  8  where well plates are stacked before and after visiting the main bed  10  respectively. 
     The process of microarraying is one of spotting the liquid from the well plates  4  onto spotting surfaces provided by the microscope slides  2  which are also arranged on the main bed  10 . This process is carried out by a pin head  12  which is moved around the robot by x- y- and z-positioners  14 ,  16  and  18  respectively. The pin head  12  is carried by the z-positioner, which is in turn carried by the y-positioner, which is carried by the x-positioner. These items are shown schematically with dashed lines. The pins of the pin head can be pneumatically actuated between retracted and extended positions. The process of pneumatically actuating the pins into the extended position is referred to as “firing” and is the process used to deposit liquid from the pins onto the spotting surface. 
     In operation, the head  12  is initially positioned so as to align its pins with the required section of one of the well plates  4 . The head  12  is then actuated so as to partially immerse the pins in the liquid to be spotted. Surface tension ensures that samples of fluid remain on the pins as they are lifted away from the well plate. The pin holder is then carried by the head  12  to a chosen location above one of the microscope slides  2  for spotting where the pins are fired downwards to deposit the carried fluid at the chosen location. The liquid is deposited in a set pattern of many closely spaced spottings. One or more dense grids of spottings is usually generated by each pin. Dense spot grids, typically 11×11 squares, are generated with a 4.5 mm separation between adjacent grids. 
     After one round of spotting, the pin head needs to be cleaned before the pins can be used to pick up another group of samples from the well plates. For cleaning, the robot is provided with a washing station  20  and a drying station  24  which are usually arranged adjacent to each other for convenience. The washing station  20  is illustrated as including first, second and third baths  21 ,  22  and  23 . The drying station  24  also includes halogen lamps  26  to assist drying by heating the pins. 
     After completion of a spotting run, the x- y- and z-positioners  14 ,  16  and  18  move the head over to the washing station  20 . The pins are actuated into their extended positions and dipped into the first wash bath  21 , containing water for example. The first bath  21  may be fitted with upstanding brushes immersed in the water, in which case the x- and y-positioners are used to move the pins over the brushes in a rotary motion in the xy-plane. The head is then moved to the second bath  22 , which also contains water for further cleaning, and may also include bleach. The head is then moved to the third and last bath  23 , which contains ethanol for more cleaning. Ethanol is used in the last bath  23  in view of its volatility which assists the subsequent drying of the pins. It will be appreciated that the named cleansing agents are mentioned to give concrete examples. Other cleansing agents are sometimes used. 
     The pin head then leaves the washing station  20  and is moved over to above the drying station  24 . The halogen lamps  26  are switched on to heat the pins. An air blower is then switched on to cool the pins to ambient temperature. 
     The pin head  12  is now clean and ready to proceed with further spotting, so is moved over to the target well plate to pick up further liquid and the microarraying process repeats. 
     Colony picking uses similar apparatus to spotting, but may be considered to be the reverse process in which a pin head is used to move samples from a colony-bearing surface to a well plate. Namely, pins of a pin head are used to pick up samples from colonies and deposit them in a liquid held in wells of well plates. The colonies are provided in petri-dishes, Q-trays or omni-trays for example. Once all the pins have been used, they need to be cleaned before further inoculation to avoid cross-contamination using a similar washing and drying process to that described above for micro arraying. 
     The speed of picking and spotting processes is often limited significantly by the duration of the the washing and drying steps. The washing and drying steps are critical in biological applications as cross-contamination due to “carry-over” can often be a problem, so should not be compromised. The conventional way of increasing throughput of the robots is thus to increase the speed and/or acceleration of the drives used for the x- y- and z-positioners. However, this poses increased constraints upon the construction of the equipment and generally requires a disproportionate increase in cost. 
     SUMMARY OF THE INVENTION 
     The invention provides high throughput by providing a microarraying or colony picking robot with an automatic mechanism for clamping and releasing pin heads from the xyz-positioner, thereby allowing heads to be swapped, together with an automated washing and drying apparatus. This allows a dirty pin head to be deposited on the automated washing and drying apparatus for cleaning without use of the xyz-positioner, while spotting or picking can continue by picking up a clean pin head without having to wait for the dirty pin head to be cleaned. In this way, the speed of operation can be increased without any increase in the acceleration or speed of the x-, y- and z-drives. The dead time normally associated with the washing and drying cycle, which is usually several minutes, is therefore eliminated, and replaced with the much shorter dead time, of only a few tens of seconds, needed for changing between heads. Another way of considering the invention is to view it as replacing the conventional simple washing baths and drying chamber with an automated washing/drying apparatus which does not need involvement of the xyz-positioner, thereby freeing up the xyz-positioner for further spotting or picking activity. 
     The invention therefore provides an automated spotting or picking process, comprising: performing spotting or picking alternately between multiple pin heads, wherein spotting or picking is performed by one of the pin heads clamped to a positioning apparatus, and dirty pin heads are cleaned by depositing them into an automated washing and drying apparatus, while spotting or picking continues with another one of the pin heads clamped to the positioning apparatus. 
     The automated spotting or picking process uses a robot with a positioning apparatus for moving a head attached to it around the robot, wherein heads can be clamped to and released from the positioning apparatus in an automated manner by an attachment mechanism. 
     The process can be broken down into the following steps:
     (a) performing spotting or picking using a first pin head attached to the positioning apparatus;   (b) depositing the first pin head for it to be cleaned in an automated washing and drying apparatus;   (c) picking up a second pin head that is clean;   (d) performing spotting or picking with the second pin head attached to the positioning apparatus;   (e) depositing the second pin head for it to be cleaned in the automated washing and drying apparatus;   (f) picking up the first pin head that has been cleaned in the automated washing and drying apparatus;   (g) performing spotting or picking with the first pin head attached to the positioning apparatus; and   (h) repeatedly carrying out (b) to (g).   

     In one embodiment, the automated washing and drying apparatus comprises first and second automated washing and drying stations. Each washing and drying station is advantageously of the same or similar construction and independently operable. Each washing and drying station has a parking bay into which a pin head can be deposited. The first automated washing and drying station can be used for washing and drying the first pin head and the second automated washing and drying station can be used for washing and drying the second pin head. 
     In an alternative embodiment, the automated washing and drying apparatus comprises a single automated washing and drying station. This reduces cost and complexity in the robot. If there is only one washing and drying station, additional provision needs to be made for shuffling between the clean and dirty pin heads during change over. One possibility is to provide first and second parking bays in which pin heads can be deposited, and a drive for shuttling between them so that a pin head in either parking bay can be washed and dried. A clean pin head can thus sit in one parking bay while a dirty pin head is deposited in the other parking bay. The clean pin head can then be picked up and then the dirty pin head moved across to a washing and drying position using the shuttling mechanism. (The shuttling mechanism could be activated after cleaning and before change over, instead of after change over.) Another possibility is to provide the automated washing and drying station comprises a parking bay, and the robot with at least first and second further parking bays, so that the positioning apparatus can swap a dirty pin head for a clean pin head by: (i) depositing the dirty pin head in the first further parking bay; (ii) picking up the clean pin head from the washing and drying station and depositing it in the second further parking bay; (iii) picking up the dirty pin head from the first further parking bay and depositing it in the parking bay of the automated washing an drying station; and (iv) picking up the clean pin head from the second further parking bay. However, this latter possibility is less preferred, since there are a greater number of head release and pick-up actions which results in a slower process. 
     According to another aspect of the invention there is provided an automated washing and drying station for a microarraying or colony picking robot, comprising:
     (a) a parking bay into which a pin head can be deposited;   (b) a washing station comprising a washing area in which the pins can be washed;   (c) a drying station comprising a drying area in which the pins can be dried; and   (d) a drive for moving at least one of the parking bay, the washing station and the drying station so that a pin head parked in the parking bay can be brought together with the washing station for washing, and then together with the drying station for drying.   

     The automated washing and drying station preferably includes at least one inlet and outlet connected to pass cleansing liquid, typically water or a water based mixture, through the washing area. The washing area may advantageously comprise a washing chamber bounded by a top plate provided with an array of holes arranged and dimensioned to receive the pins of the pin head. The automated washing and drying may also include an air duct arranged in fluid communication with the drying area and/or at least one drying lamp, such as a halogen lamp, arranged in light communication with the drying area. 
     There are various possibilities for automating the washing and drying activities. The preferred solutions use a single drive only for simplicity, although there is no significant technical difficulty in using multiple drives. In the best mode, the parking bay and drying station are static and the washing station motorized. Namely, the drive is operable to move the washing station into and out of a washing position, with the parking bay and the drying station remaining static. An alternative is for the drive to move the parking bay between the washing station and the drying station. 
     The invention also provides a microarraying or colony picking robot comprising a positioning apparatus operable to move a pin head attached to it to perform spotting or picking and including an attachment mechanism to allow swapping between different pin heads by automated deposit and pick up, and at least one automated washing and drying station operable to clean pin heads deposited by the positioning apparatus. 
    
    
     
       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  is a schematic plan view of a microarraying robot according to the prior art; 
         FIG. 2  is a perspective view of a microarraying robot according to an embodiment of the invention; 
         FIG. 3  is a close up of  FIG. 2  showing the robot in more detail; 
         FIG. 4  is a perspective view of a combined washing/drying station; 
         FIG. 5  is a perspective section view of one end of a combined washing/drying station with a pin head above; 
         FIG. 6  is a perspective view of a wash carriage; 
         FIG. 7  shows the wash carriage of  FIG. 6  with the top plate removed; 
         FIGS. 8A–8D  show section side views of the washing/drying station for different positions of the wash carriage as it moves into its washing position, with  FIG. 8A  showing the wash carriage at an intermediate position of the drive midway between the washing and drying positions,  FIG. 8B  showing the wash carriage advanced to the furthest left abutting the end wall of the main frame,  FIG. 8C  showing the wash carriage moving upwards towards the pins, and  FIG. 8D  showing the wash carriage fully elevated in the washing position with the pins immersed in the washing chamber; 
         FIG. 9  is a side view of the head latching unit that is attached to the z-positioner motor drive; 
         FIG. 10  is an exploded perspective view of the head latching mechanism of the head latching unit; 
         FIG. 11  is an exploded perspective view of the head latching unit with the head latching mechanism of  FIG. 10  partly assembled; 
         FIG. 12  is a perspective view of a pin head showing parts used to secure the head to the head securing unit; and 
         FIG. 13  shows an alternative washing/drying station. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 2 and 3  are perspective views of a microarraying robot according to an embodiment of the invention, with  FIG. 3  being a close up of  FIG. 2 . The robot comprises a main bed  10  contained within a housing  11 . Access to the main bed  10  is provided by doors  32  and  34 . Well plates  4  are delivered to the main bed  10  of the robot by an automated well plate feeder  6  which, in the illustration, has four lanes. Each lane has a de-lidding/re-lidding mechanism  31  for removing and replacing the well plate lids as the well plates pass through. Each delivery lane is supplied from a feed port (hidden behind the end panel of the housing) to which is docked a cassette filled with well plates. Each feed port has a supply mechanism that deposits well plates, one at a time, onto the delivery lane. Behind each of the feed ports there is arranged a restack port (also hidden) which has a pneumatically driven lifting mechanism for inserting well plates returned from the main bed of the apparatus into a further cassette. On their return, the well plates pass through the feed ports before reaching the restack ports. For the sake of brevity, the mechanical design of the well plate feeder is not described further. For further details, reference is made to copending U.S. patent application Ser. No. 10/133,904, the contents of which are incorporated herein by reference. Other embodiments may not use an automated well plate feeder and instead rely on manual placement of well plates on the main bed  10 . 
     First and second automated washing and drying stations  30  are arranged to the left of the main bed  10  either side of the well plate delivery lanes. Each washing and drying station  30  comprises a parking bay  36  into which a pin head  12  can be deposited. In the illustration, the washing and drying station  30  situated furthest away from the access doors  32  and  34  is shown with a pin head docked in the parking bay, whereas the washing and drying station  30  nearest the access doors  32  and  34  is shown empty. A second pin head  12  can be seen attached to the positioning apparatus. The positioning apparatus comprises x-, y- and z-positioners  14 ,  16  and  18  respectively. The z-positioner  18  has an attachment mechanism in the form of a head latching unit  15  for releasably connecting a pin head  12  to it. The z-positioner  18  is carried by the y-positioner  16 , and the y-positioner  16  is carried by the z-positioner  14 , as is conventional. The z-positioner  14  is largely obscured in the illustration by the roof of the housing  11 . The main bed  10  has arranged thereon a number of microscope slides which provide spotting surfaces  2  onto which liquid contained in the well plates  4  can be spotted to form regular grids of spots during microarraying. It will be appreciated that any suitable spotting surface can be provided. 
     In operation, the head  12  held by the latching unit  15  in the Z-positioner  18  can be moved between the well plates  4  at the end of their delivery lanes and the spotting surfaces  2  in order to perform spotting. Once all the pins of the pin head are used (i.e. dirty), the dirty pin head can be deposited into the vacant parking bay  36 . The dirty pin head can then be washed and dried in the automated washing and drying station  30 . After depositing the dirty pin head into the vacant parking bay  36 , the positioner can then move to pick up the other pin head from the parking bay  36  of the other washing and drying station  30 , this pin head having been cleaned in the other washing and drying station  30 . In this way, spotting is only interrupted for the short time needed to exchange heads and does not need to wait for a washing and drying cycle to complete before continuing to spot. 
     Having described the basic layout of the machine, the automated washing and drying station  30  is now described in detail, followed by a detailed description of the automated head attachment mechanism  15  which allows pin heads to be deposited in the parking bays and then subsequently picked up. 
       FIG. 4  is a perspective view of one of the automated washing and drying stations  30 . The washing and drying station  30  is shown with a pin head  12  lying in the parking bay  36  of the park station  37 . The pin head  12  is connectable to the attachment mechanism  15  of the z-positioner using a latch shoe  74  which can be seen in the illustration. Directly below the park station  37  there is arranged a drying station  38 . The drying station  38  is connected to an air duct  48  through which air is blown into the drying station to dry the pins of the pin head  12  after washing. The washing and drying station  30  is designed with static park and drying stations  37  and  38  and a mobile washing station. The washing station comprises a carriage  40  supported by a cradle  46  which is drivable between a washing position, in which the pins of the pin head are immersed in cleansing liquid held in the washing carriage  40 , and a drying position, in which the washing carriage  40  and cradle  46  are withdrawn away from the parking station to allow drying of a pin head  12  held in the parking station  37  by the drying station  38 . 
     The cradle  46  for the washing carriage  40  can travel between the washing and drying positions using pairs of guide rollers  55  which tangentially engage either side of a guidance rail  50 . Motion of the cradle  46  along the guidance rail  50  is driven by an electric motor  52  which connects via a motor spindle  54 , drive belt  56  and pulley  58  to a lead screw  60  that is in threaded engagement with a lead screw nut  62 . The lead screw nut  62  is connected to a plate which is fixed to the cradle  46  to impart the motion of the lead screw nut  62  to the cradle  46 . The various parts of the washing and drying station  30  are supported in a main frame  44  to which are attached two pairs of mounting brackets  42  which allow the washing and drying stations  30  to be secured to the underside of the main bed  10 . 
       FIG. 5  is a perspective section view of one end of the combined washing and drying station  30  with a pin head  12  shown vertically above the parking bay  36 . The pin head  12  is also shown in the same section. The latch shoe  74  and pins  72  of the pin head  12  are evident. Various parts of the washing and drying station  30  that have already been described can be seen, namely the park station  37 , parking bay  36 , main frame  44 , mounting bracket  42 , cradle  46  and washing carriage  40 . It can be seen that the washing carriage  40  comprises a main body  68  and a top plate  70 . The lead screw  60  and the lead screw nut  62  are also evident. The lead screw  60  is journalled in a bearing  64  held in a bearing housing  66 . Further details of the drying station are also visible. Namely, it can be seen that the drying station comprises a chamber  39  into which the air duct vents from below. Moreover, extending across the drying chamber, a number of halogen bulbs  35  are provided. The embodiment uses three halogen bulbs  35 , one of which is visible in the illustration. The halogen bulbs  35  provide a light source to assist drying of the pins  72  by radiative heating. The pin temperatures can be elevated in this way up to 200° C. Air from the air duct is then used to cool the pins to bring them back to ambient temperature ready for further use. An example of a suitable halogen lamp is standard quartz 300 W linear lamps of the R7 type, e.g. Osram Haloline 64701 or Philips Plusline. In an example drying process, a pin head is illuminated for a period of 4–30 seconds after which an air blower is switched on to blow air through the air duct for a period of 5–50 seconds, for example, to allow cooling of the pins to ambient temperature. 
       FIG. 6  is a perspective view of the wash carriage  40  in which its main body  68  and top plate  70  are evident. It can be seen that the top plate  70  is perforated by an array of pin holes  76  matched to the pin array of the pin head  12 . It will be appreciated that the embodiment shown is for a 384-pin pin head with a 16×24 array of pins  72  conforming to the 384-well well plate standard. It will be appreciated that other standards could be catered for by simple modification. Each hole has a diameter 0.2 mm greater than the external pin diameter to allow passage of the pins  72  through the holes  76 . In addition to the square-pitch array of 16×24 pin holes  76 , it can be seen that a further two rows of dummy pins  80  are provided. These pins extend into the interior of the washing carriage  40 . Their purpose is described further below. The wash carriage  40  has four rollers  78  which locate the carriage  40  in the cradle  46 , as will be described with reference to the following figures. The main body  68  is provided at one end with a pair of liquid inlets  82  and a pair of liquid outlets  84  which provide for the circulation of water or other cleansing liquid through the wash carriage  40 . Standard flexible plastic tubing is used with standard fittings to connect the inlets and outlets to suitable liquid supplies and drains. 
       FIG. 7  shows the wash carriage again, but with the top plate  70  removed to reveal the internal construction of the main body  68  which provides for circulation of the cleansing liquid. Water injected into the washing carriage via the inlets  82  fills an entry chamber  83  bounded by a weir  86  that is perforated with holes  88 . Running parallel along the washing carriage are a series of channels  98 , one for each row of pins or pin holes. The channels  98  are defined by dividing walls  96  which run over the full length covered by the pin holes  76 . Two of the dummy pins  80  are also illustrated by way of example. The dummy pins  80  extend into the inlet ends of the channels  98  adjacent to the weir  86  so that, when water is injected into the channels through the holes  88 , the dummy pins  80  induce turbulent flow prior to the cleansing liquid reaching the first row of pins of the pin head which are held in the channels  98  during washing. This ensures that the first row of pins in the pin head is cleaned efficiently. Once water has passed along the channels  98  it reaches an end chamber  90 . At each side of the end chamber  90  there is a drain hole  92  (not visible) which forms one end of a bore. Two bores thus extend along each side of the main body  68  parallel to the channels and emerge at the outlets  84 , thus completing the circulation path for the cleansing liquid. As well as the carriage support rollers  78 , the figure also shows a pair of end rollers  94  arranged at the opposite end of the wash carriage  40  to the water feed connections  82  and  84 . The use of these end rollers  94 , and also the rollers  78 , is described with reference to the following figures. 
       FIGS. 8A–8D  show section side views of the washing and drying station  30  at different positions of the washing carriage  40  as it moves into its washing position. Each of the figures shows the pin head  12  located in the parking bay  36  with its latch shoe  74  upstanding. The pins  72  of the pin head  12  are also shown and point down towards the drying chamber  39  arranged in the base of the washing and drying station  30 . 
       FIG. 8A  shows the washing carriage is in an intermediate position of the drive, midway between the washing and drying positions. The lead screw nut  62  is thus positioned midway along the lead screw  60  and is moving the wash carriage  40  from right to left in the figure by rolling of the guide rollers  55  along the guidance rail  50 . As the cradle  46  moves from right to left, the washing carriage  40  remains stationary in the cradle with the rollers  78  resting at the bottom of a pair of ramps  102 . One of the end rollers  94  is also illustrated. The end rollers  94  are approaching an end wall  100  of the main frame  44  in the course of the motion. 
       FIG. 8B  shows the washing carriage  40  advanced to a position at which the end rollers  94  have just touched the end wall  100  of the main frame. From this point onwards, further actuation of the drive to move the lead screw nut  62  and thus the cradle  46  further towards the end wall  100  (i.e. to the left) results in the wash carriage rollers  78  being forced to move up the ramps  102  and also the end rollers  94  to roll vertically up the end wall  100 . 
       FIG. 8C  shows the wash carriage  40  after it has been raised by motion of the rollers  78  part way up the ramps  102  as the cradle  46  moves further towards the end wall  100 . At this point it can be seen that the ends of the pins  72  have inserted into the pin holes of the top plate  70 . 
       FIG. 8D  shows the final stage in the motion in which the wash carriage  40  is at its maximum vertical height with the pins  72  inserted near fully into the wash carriage  40 . The rollers  78  are near the top of the ramps  102  and the frame  46  is at its closest position to the end wall  100 . It is noted that even at this maximum height, there is a small clearance between the bottom face of the parking bay  36  and the top plate  70  of the wash station. The position illustrated in  FIG. 8D  is the washing position. 
     It will be appreciated that the wash station is separated from the pin head using the reverse procedure to that described. Namely the wash carriage  40  is lowered by motion of the rollers  78  down the ramps  102 , as induced by motor driven motion of the frame  46  away from the end wall  100 . Once the rollers  78  have reached the bottom of the ramps  102 , the wash carriage  40  remains static in the frame  46  and the frame  46  and carriage  40  move together away from the parking bay  36  and drying chamber  39 . The frame  46  and wash carriage  40  move further to the right than shown in  FIG. 8A  into a drying position in which the end rollers  94  are beyond (i.e. to the right of) the drying chamber  39 , thereby providing free space between the drying chamber  39  and parking bay  36  so that uninhibited drying of the pins  72  can take place. 
     The attachment mechanism used for automated exchange of pin heads is now described in detail. 
       FIG. 9  is a side view of the attachment mechanism which takes the form of a latching unit  220  that is attached to the z-positioner motor drive and serves to allow different pin heads to be loaded and unloaded. 
       FIG. 10  is an exploded perspective view of the head latching mechanism of the head latching unit  220 . 
       FIG. 11  is an exploded perspective view of the head latching unit  220  with the head latching mechanism of  FIG. 10  partly assembled. 
     The head latching unit  220  comprises a housing made of top and bottom plates  243  and  240  and left and right end plates  242  and  241 . Non-structural side plates  244  are also provided (omitted from  FIG. 9 , but evident in  FIG. 11 ). The bottom plate  240  is provided on its lower side with locating stubs  227  to assist location of the head latching unit in corresponding holes in the heads. 
     The latching mechanism is driven by a push-me-pull-you pneumatic piston assembly comprising a delatch driving piston  250  and a latch driving piston  260  with respective cylinders  226  and  266 . The cylinders  226  and  266  are rotatably mounted by spigots  252  (see  FIG. 10 ) located in blocks  245  (see  FIG. 11 ) secured to the base plate  240  in a manner similar to a howitzer. 
     The cylinders act on a knee joint  255  connecting to a floating upper joint  257  by an upper link  247 . The knee joint  255  is also connected to a lower fixed joint  256  by a lower link  248 . The fixed joint  256  is located in the side plates  246  (only one of which is shown in  FIG. 10 ) by a locating pin. The knee joint  255  is connected to both pistons  250 ,  260  by respective connectors  251  assisted by a locating pin  253 . The knee joint  255  has a pin  253  running through it which fits into a pair of arcuate slots  223  in the side plates  246  retained by a pair of outside circlips  254 . The floating upper joint  257  also has a pin  253  running through it which fits in a pair of vertical slots  225  in the side plates  246  retained by a pair of outside circlips  254 . The floating upper joint pin also pivotally mounts a latching arm  222 . The latching arm  222  extends obliquely down to a non-jointed bend  258  with a hole through which a further pin  253  passes which is retained by outside circlips  254  and fits into an L-shaped slot  224  in the side plates  246 . After the non-jointed bend  258 , the latching arm  22  extends vertically down and then at right angles thereto extends further to form a latch support which bears a latch  221 . 
     Before describing the latching action by which heads are attached to and detached from the latching unit  220 , the latching parts of the heads are first described. 
       FIG. 12  is a perspective view of a pin head  12  showing parts used to secure the head to the head securing unit. The head  12  is secured to the z-positioner via a securing plate  149  on its upper side. The securing plate  149  also provides a host for an electrical contact pad  150  which mates with a corresponding contact pad on the z-positioner. The contact pad  150  provides the head  12  with electrical power feeds for actuating valves for pneumatically firing the pins and also communication feed-throughs for providing communication lines from the main robot to logic within the head. For example each head has a unique identifier so that the robot can interrogate the head to receive identification data and thus determine the head&#39;s identity, which can be used to define the head&#39;s home parking bay, for example. It is also mentioned that each parking bay is fitted with a microswitch to communicate to the control system whether it is occupied or vacant. In this way, the system is robust against an operator removing a parked head manually for inspection and returning it to a different parking bay. 
     Locating holes  233  for accepting the locating stubs  227  on the head latching unit  220  can be seen. In addition there are four locating stubs  232  on the head  12  which locate in corresponding holes in the head latching unit  15  (not visible in  FIGS. 9–11 ). The stubs  232  are mounted on a plate  231  which mounts a lower latch  74  which has a fixed position and is shaped and dimensioned to engage with the latch  221  of the head latching unit. 
     The latching mechanism has two basic positions, a latched position, in which the head is gripped by the latch and securely engaged with the head latching unit  15 , and a delatched pre-engagement position.  FIGS. 9–11  illustrate the mechanism in the latched position. In view of this, movement of the mechanism from the latched position to the delatched position is described. This is the movement that would be performed when a head had been returned to its parking bay and was to be released by the head latching unit to allow the z-positioner with head latching unit to be moved away to pick up a different head. 
     Referring to  FIG. 9 , the delatching piston  250  is pneumatically actuated by compressed air through a feed line (not shown) which pushes the knee joint  255  to the right (as viewed in the figures) which pushes the pin around the arcuate slot  223 . This forces the floating upper joint  257  vertically down guided and constrained by the vertical slot  225 . Initially, the non-jointed bend  258  is also forced vertically down guided by the upright part of the L-shaped slot  224 . During this phase of the delatching, the latch  221  is being moved vertically down, allowing it to become free of the head&#39;s latch  74  (assuming the head  12  is supported in its parking bay). Once the downward motion proceeds to the point at which the base of the L-shaped slot has been reached by the non jointed bend  258  of the latching arm  222 , the latching arm  222  is forced to rotate by the pin through the non-jointed bend  258  being confined to move in the base of the L-shaped slot until it abuts the end of the base section of the L-shaped slot. The mechanism is now in the delatched position, with the knee joint  255  at its maximum bend and the latching arm  222  at is maximum swing angle away from vertical, swung into a space  261  provided within the plate  231 . At this point, the latching arm  222  has swung away sufficiently to allow the latching unit  15  to be lifted away from the head  12  by a simple vertical motion of the z-positioner. It will now be appreciated why the piston assemblies are pivotally mounted, namely to accommodate movement of the pins in the slots  223 ,  224  and  225 . 
     Motion from the delatched to the latched position proceeds approximately in the reverse sequence. It is initiated by actuation of the latching piston  260  which acts on the knee joint  255  to straighten it out. The reverse sequence of slot motion is performed in relation to the latch-to-delatch motion with the latching arm  222  being forced to move vertically upward in the L-shaped slot  224  when the pin through the non-jointed bend  258  reaches the corner of the L-shaped slot  224 . Worthy of mention is the fact that it in the final stages of straightening when moving into the latched position, the design allows a high amount of clamping force to be exerted, with the head&#39;s latch  74  being squeezed between the latching unit&#39;s latch  221  and the bottom of the lower link  248 . This feature provides a highly secure latching of the head. 
       FIG. 13  shows an alternative embodiment of washing and drying station  30 . The design is essentially the same as the main embodiment, but the park station is no longer static, but provided with a lateral drive for moving the park station relative to the drying station in a direction at right angles to the drive direction of the wash station, as indicated in the figure with the arrow. The park station  37  is provided with first and second parking bays  36  and is movable between two positions, so as to arrange a pin head parked in either parking bay in readiness for washing or drying. The park station  37  is mounted so as to be drivable relative to the main frame  44  used to support the drying and washing stations. The advantage of this alternative embodiment is that the robot only needs to be provided with a single washing and drying station instead of two. 
     Although the main embodiment describes a process involving two pin heads, it will be appreciated that three or more pin heads could be used. A suitable number of parking bays would of course need to be provided, where the number of parking bays is preferably at least equal to the number of heads. If, for example, three pin heads were provided, the robot could be fitted with three washing/drying stations, one for each pin head, two washing/drying stations, or a single shared washing/drying station. 
     It will also be appreciated that alternative designs of the wash station are possible. The design of the main embodiment is but one possibility. For example, the washing process need not be based on liquid circulation channels, but could involve directly spraying the pins with water jets, and/or could incorporate a brushing action using brushes. 
     For colony picking applications, it will be appreciated that as well as multiple pin heads for performing the picking, it will often be useful to provide a liquid handling head for filling of well plates with a buffer solution, a master mix etc. in preparation for picking. A gel coring head or any other head type could also be fitted if desired.