Abstract:
An apparatus ( 10 ) for creating a three dimensional device ( 100 ). The apparatus comprises a dispensing head ( 14 ) for dispensing material, and a base member ( 16 ) for receiving the material dispensed from the dispensing head ( 14 ). A controller ( 20 ) is provided for controlling the operation of the apparatus ( 10 ). The apparatus is operable to create the three dimensional device ( 100 ) by depositing a series of line deposits of material from the dispensing head ( 14 ) based on predetermined commands sent by the controller ( 20 ). The controller ( 20 ) is operable to control how the line deposits are dispensed to improve the sealing properties of the device ( 100 ).

Description:
RELATED APPLICATION 
       [0001]    This application claims priority from Great Britain Patent Application No. 1506943.8 filed on Apr. 23, 2015, the contents of which are incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an apparatus for fused deposition modeling (FDM). 
         [0003]    FDM is an additive manufacturing technology commonly used for modeling, prototyping, and production applications. Additive manufacturing is also referred to as 3D-printing. 
         [0004]    FDM begins with a software process which processes a 3D CAD file, mathematically orienting and slicing the model for the build process. The next step is to take the sliced model and create tool paths and build process that builds a part with the desired properties. The model or part is produced by extruding a small bead of material along the tool path to form layers as the material hardens immediately after extrusion from the nozzle. Typically FDM machines include a plastic filament or metal wire which is unwound from a coil and is fed to an extrusion nozzle via drive rollers (or similar) at a controlled rate. Filament is not always used and in some instances beads or pellets are fed into the nozzle. The material is heated inside the nozzle to a semi-liquid state and is then extruded through the exit of the nozzle and deposited onto the part. 
         [0005]    The nozzle can be moved in both horizontal and vertical directions by a numerically controlled mechanism. The nozzle follows a tool-path controlled by a computer-aided manufacturing (CAM) software package, and the part is built from the bottom up, one layer at a time. 
         [0006]    One of the limitations of building up the material one layer at a time is that it is not possible to produce features which are unsupported by material on the previous layer. This means that is it not possible to have large overhangs of material, although small overhangs with limited slope are possible. 
         [0007]    U.S. Pat. No. 5,121,329 describes in more detail the principles behind FDM. 
         [0008]    Previous attempts to fabricate fluidic devices using FDM have had limited success. This is because the beads of material extruded and deposited by a FDM nozzle typically have a circular or near circular cross section and this results in gaps between neighbouring bead deposits in the device. These gaps form leak paths and when a fluid is pumped into the device a significant amount of the fluid will fill these gaps and leak out of the device. This invention resolves the issue of leak paths and enables the manufacture of sealed fluidic devices. 
         [0009]    FDM has the potential to provide significant benefits for the manufacture of fluidic devices, including the potential to fabricate devices in a wide range of materials, primarily polymers. This is potentially extremely useful in the research, development and manufacture of fluidic devices. One example is the development of devices for point of care diagnostic testing. In this area of R&amp;D it would be very useful to manufacture a broad range of fluid features including fluidic channels, channel networks, fluid reservoirs, fluid splitting junctions, fluid merging junctions, passive mixer structures, fluid connection ports, valve geometries, and flow cells. It would also be useful to change the material that the device is fabricated from. 
         [0010]    Many commercially available FDM machines use ABS polymer, however it has been found that it is possible to use a wide range of polymers including polypropylene, cyclic olefin copolymer (COC), polycarbonate, polystyrene, as examples. Being able to quickly manufacture in different materials is particularly useful in diagnostic or biological applications where there can be complex interactions between the fluids and the wetted surfaces. For example protein binding to polymer surfaces can be undesirable for the analysis biological samples. By trying different build polymers it would be possible to find a material that has low binding properties for a particular protein. In addition different polymers have different chemical resistance, optical, thermal and mechanical properties which again can be optimised by changing the build material. 
         [0011]    A specific example of such a device is a sensor for measuring glucose levels in a patient&#39;s blood stream. The device would typically include a port for injection of a blood sample, a fluidic channel where dried electrolytes are dissolved into the blood sample and an interface to the electrochemical sensor, which allows the sample to be brought into contact with the sensor. The final device may include some extra components such as gaskets and adhesive layers but the basic fluid structure would be fabricated using FDM. Once a suitable material and geometry has been found for the fluidic structure it would be possible to manufacture devices in medium volume by FDM. 
         [0012]    This invention is primarily focused on fluidic devices with features as described with a scale range from microfluidic channels with features sizes of 10 μm-1 mm (more typically 50 μm-1 mm) up to millimetre (milli-fluidic) scale devices with features sizes in the 1 mm-100 mm range. It is possible to also conceive of larger fluidic devices in the &gt;100 mm range, for example pipework and vessels for chemical and biological reactors. 
         [0013]    Traditional prototyping methods for fluidic devices have various drawbacks. For example stereolithography is limited to a narrow range of photo curable materials and there are also limitations around the length and cross sections of fluid channels that can be fabricated. In addition stereolithography suffers from slow build times which result in high manufacturing costs. Fluidic devices are often fabricated by CNC milling a channel network and then capping the channels with a sealing layer. The main disadvantage of this approach is that attaching the sealing layer is not a straightforward process and sealing processes such as laser welding often place limitations on the materials that can be used and the geometries that can be achieved. 
       SUMMARY OF THE INVENTION 
       [0014]    According to a first aspect of the present invention there is provided an apparatus for creating a three dimensional fluidic device containing at least one fluid channel, the apparatus comprising: 
         [0015]    a dispensing head comprising a passage for receiving a supply of material, and comprising a dispensing orifice at one end of the passage; 
         [0016]    a base member for receiving the material dispensed from the orifice of the dispensing head; 
         [0017]    an actuator means for moving the dispensing head relative to the base member; and 
         [0018]    a controller for sending a set of predetermined commands to each of the actuator means and the dispensing head; 
         [0019]    wherein the apparatus is operable to create the fluidic device by depositing a series of sequential layers of material from the dispensing head onto the base member, each layer formed of a series of adjacent line deposits of material, based on the predetermined commands sent by the controller to the actuator means and the dispensing head, 
         [0020]    wherein the apparatus is operable to overlap the line deposits to reduce leakage paths between the line deposits to improve the sealing properties of the at least one fluid channel in the fluidic device. 
         [0021]    The predetermined commands from the controller may include instructing the apparatus to: 
         [0022]    dispense a closed loop of material forming a portion of the perimeter wall of a fluid channel in the fluidic device in at least one of the layers of material, wherein the two ends of the closed loop of material overlap. 
         [0023]    The predetermined commands from the controller may include instructing the apparatus to dispense: 
         [0024]    a plurality of deposits of material forming a bottom portion of a perimeter wall of a fluid channel in the fluidic device in at least one of the layers of material; 
         [0025]    wherein the plurality of deposits of material are substantially parallel to the direction of fluid flow along the fluid channel when the fluid channel is in use. 
         [0026]    The predetermined commands from the controller may include instructing the apparatus to dispense: 
         [0027]    a first line deposit of material having a first pitch forming a first portion of a perimeter wall of a fluid channel in the fluidic device in at least one of the layers of material; and 
         [0028]    a second line deposit of material, which is located in the layer sequential to the layer containing the first line deposit, onto the line first deposit forming a second portion of the perimeter wall of the fluid channel in the fluidic device; 
         [0029]    wherein the second line deposit of material is laterally offset from the first line deposit of material, and overhangs into the fluid channel. 
         [0030]    The predetermined commands from the controller may include instructing the apparatus to dispense: 
         [0031]    a first line deposit of material forming a portion of a perimeter wall of a first transverse fluid channel in the fluidic device in at least one of the multiple layers of material; and 
         [0032]    a second line deposit of material which neighbours the first deposit of material in the at least one of the multiple layers of material, and which forms a portion of a perimeter wall of a second fluid channel in fluid communication with, and substantially perpendicular to, the first transverse fluid channel; and 
         [0033]    wherein the first line deposit and the second line deposit overlap at the interface of the first and second line deposits of material. 
         [0034]    The predetermined commands from the controller may include instructing the apparatus to dispense: 
         [0035]    a first line deposit of material forming a side wall of a fluid channel in the fluidic device in at least one of the multiple layers of material; 
         [0036]    a second line deposit of material, which is located in the layer sequential to the layer containing the first deposit, onto the first deposit forming a top wall of the fluid channel in the fluidic device, wherein the second line deposit of material extends transversely across the width of the fluid channel; 
         [0037]    a third line deposit of material, which is located in the same layer as the layer containing the second line deposit of material, wherein the third line deposit of material is adjacent to the second line deposit of material; and 
         [0038]    a fourth line deposit of material, which is located in the layer sequential to the layer containing the second and third deposits, onto the top wall; 
         [0039]    wherein the first, third and fourth deposits extend in a direction parallel to the length of the fluid channel. 
         [0040]    According to a second aspect of the present invention there is provided an apparatus for creating a three dimensional device, the apparatus comprising: 
         [0041]    a dispensing head comprising a passage for receiving a supply of material, and comprising a dispensing orifice having a central axis at one end of the passage; 
         [0042]    a base member for receiving the material dispensed from the orifice of the dispensing head; 
         [0043]    an actuator means for moving the dispensing head relative to the base member; and 
         [0044]    a controller for sending a set of predetermined commands based on pattern data derived from the required structure of the three dimensional device to each of the actuator means and the dispensing head; 
         [0045]    wherein the apparatus is operable to create the device by depositing a series of sequential layers of material from the dispensing head onto the base member, each layer formed of a series of adjacent line deposits of material, based on the predetermined commands sent by the controller to the actuator means and the dispensing head, 
         [0046]    wherein, for a line deposit of material in a region where the pattern data requires an abrupt change of direction of the line deposit, the controller is operable to instruct the apparatus to move the dispensing head in this region along an arcuate path. 
         [0047]    The controller may be operable to move the central axis of the dispensing orifice along the arcuate path. 
         [0048]    The arcuate path may have a radius of curvature of between 10%-200% of the maximum width of the line deposit. In some embodiments, the lower end of the above percentage range may represent a larger percentage, and may be 20%, 25%, 30%, 40% or 50%. The upper end of this percentage range may represent a smaller percentage, and may be 180%, 150%, 120% or 100%. 
         [0049]    The arcuate path may have a radius of curvature of between 20%-400% of the maximum depth of the line deposit. In some embodiments, the lower end of the above percentage range may represent a larger percentage, and may be 25%, 30%, 40% or 50%. The upper end of this percentage range may represent a smaller percentage and may be 350%, 300%, 250%, 200%, 150% or 100%. 
         [0050]    The arcuate path may have a radius of curvature of between 0.1 mm-0.4 mm. 
         [0051]    The first aspect of the present invention also provides a method for creating a three dimensional fluidic device containing at least one fluid channel using an apparatus comprising: 
         [0052]    a dispensing head comprising a passage for receiving a supply of material, and comprising a dispensing orifice at one end of the passage; 
         [0053]    a base member for receiving the material dispensed from the orifice of the dispensing head; 
         [0054]    an actuator means for moving the dispensing head relative to the base member; and 
         [0055]    a controller for sending a set of predetermined commands to each of the actuator means and the dispensing head; 
         [0056]    wherein the method comprises depositing a series of sequential layers of material from the dispensing head onto the base member, each layer formed of a series of adjacent line deposits of material, based on the predetermined commands sent by the controller to the actuator means and the dispensing head, 
         [0057]    wherein the method also comprises overlapping the line deposits to reduce leakage paths between the line deposits to improve the sealing properties of the at least one fluid channel in the fluidic device. 
         [0058]    The second aspect of the present invention also provides a method for creating a three dimensional device using an apparatus comprising: 
         [0059]    a dispensing head comprising a passage for receiving a supply of material, and comprising a dispensing orifice having a central axis at one end of the passage; 
         [0060]    a base member for receiving the material dispensed from the orifice of the dispensing head; 
         [0061]    an actuator means for moving the dispensing head relative to the base member; and 
         [0062]    a controller for sending a set of predetermined commands to each of the actuator means and the dispensing head based on pattern data derived from the required structure of the three dimensional device; 
         [0063]    wherein the method comprises depositing a series of sequential layers of material from the dispensing head onto the base member, each layer formed of a series of adjacent line deposits of material, based on the predetermined commands sent by the controller to the actuator means and the dispensing head, 
         [0064]    wherein the method also comprises, for a line deposit of material in a region where the pattern data requires an abrupt change of direction of the line deposit, moving the dispensing head in this region along an arcuate path. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0065]    The invention will now be described with reference to the accompanying Figures in which: 
           [0066]      FIG. 1  shows a perspective view of a machine for fused deposition modeling. 
           [0067]      FIG. 2A  shows a plan view of a microfluidic device created using the machine of  FIG. 1 ; 
           [0068]      FIG. 2B  shows a right end view of the microfluidic device shown in  FIG. 2A ; and 
           [0069]      FIG. 2C  shows a bottom end view of the microfluidic device shown in  FIG. 2A . 
           [0070]      FIG. 3  shows an exploded view of various slices of the device shown in  FIGS. 2A-2C . For each slice, a plan view is also shown in the Figure. 
           [0071]      FIGS. 4A and 4B  each shows a dispensing path of a prior art tool path for creating a closed loop of material for a fluid channel; 
           [0072]      FIGS. 4C-4E  each shows a dispensing path of a different tool path for creating a closed loop of material for a fluid channel; 
           [0073]      FIG. 4F  shows a section view of the closed loop of material of  FIG. 4A ; and 
           [0074]      FIG. 4G  shows a section view of the closed loop of material of  FIGS. 4C-4E . 
           [0075]      FIG. 5A  shows a cross-section view of a portion of the device shown in  FIGS. 2A-2C .  FIG. 5A  also shows plan views of the layers shown in the cross-section view; and 
           [0076]      FIG. 5B  shows a cross-section view of an alternative design to the design shown in  FIG. 5A . 
           [0077]      FIG. 6A  shows a plan view of a slice of the device shown in  FIGS. 2A-2C ; 
           [0078]      FIG. 6B  shows a plan view of a neighbouring slice to the slice shown in  FIG. 6A ; 
           [0079]      FIG. 6C  shows a plan view of the dotted region of the slice shown in  FIG. 6B ; 
           [0080]      FIG. 6D  shows a first section view of the dotted region of the device shown in  FIG. 6B ; and 
           [0081]      FIG. 6E  shows a second section view of the dotted region of the device shown in  FIG. 6B . 
           [0082]      FIG. 7A  shows a photograph of a deposit created by a tool following a first vector path; and 
           [0083]      FIG. 7B  shows a photograph of a deposit created by a tool following a second vector path. 
       
    
    
     DETAILED DESCRIPTION 
       [0084]    With reference to  FIG. 1 , there is shown a FDM machine  10 . The machine comprises a reel of material  12  which is fed into a robotic head  14  via a flexible tube  15 . Material from the head  14  is dispensed onto a base member  16  of the machine  10 . 
         [0085]    A heater is located in the robotic head  14  for heating the material passing through the head beyond the material&#39;s glass transition temperature prior to it being dispensed. 
         [0086]    The robotic head  14  can move relative to the base member  16  along a three dimensional Cartesian coordinate system. Movement of the robotic head  14  is controlled by an actuator means  18  located on the machine  10 . 
         [0087]    The base member  16  is preferably in the form of a flat plate and acts as the base plate onto which heated material dispensed from the robotic head  14  is deposited. 
         [0088]    A controller  20  is located on the machine which controls the operation of the head  14  and the actuator means  18 . A user interface  22  is connected to the controller to allow user control of the machine  10 . 
         [0089]    To make the machine  10  suitable for creating fluidic devices, the robotic head  14  comprises a dispensing orifice  24  through which material is dispensed which has a variable diameter of between 0.1 mm-1.0 mm. 
         [0090]    Smaller diameters than this may be used depending on the size of the smallest features from the fluidic device being manufactured. 
         [0091]    An example of a microfluidic device created using the apparatus shown in  FIG. 1  is shown in  FIGS. 2A-2C  and  FIG. 3 . The device shown in these Figures comprises a block  100  of material in which are located three fluid inlet ports  102 . Each fluid inlet port  102  defines a vertical channel  104  which extends from the bottom surface  100   a  of the block up into the thickness of the block  100 . Each vertical channel  104  is fluidly connected to a respective horizontal fluid channel  106 . The three horizontal channels  106  extend through a portion of the thickness of the block  100  and meet at a mixing point  108 . An outlet fluid channel  110  radiates from the mixing point  108  and extends through the block where it terminates at an exit channel  112 . The exit channel  112  extends vertically through the thickness of the block to a fluid outlet port  114  located on the bottom surface  100   a  of the block  100 . 
         [0092]    Block  100  is created by sequentially depositing multiple layers of material from the dispensing head  14  onto the base member  16  based on predetermined commands sent by the controller to the actuator means and the dispensing head. In each layer, as shown in  FIG. 3 , the head  14  is moved across the base member  16  and material is deposited from the head  14  to create a series of linear deposits  200  of material which together define the features of the microfluidic device in that layer. 
         [0093]    The device shown in  FIG. 3  is approximately 16 mm long; 15 mm wide; and 2 mm thick. Each inlet port  102  has a diameter of approximately 1000 μm, and each of the three horizontal channels  106  has an inner diameter of approximately 400 μm wide×200 μm deep. 
         [0094]    In light of the micro-size of these channels, there is the possibility of leakage between each of the linear deposits deposited by the head  14 . 
         [0095]    To minimise the extent of such leakage, the predetermined commands issued by the controller are carefully controlled. 
         [0096]    In one embodiment, the apparatus is configured to minimise leakage between each of the linear deposits  200  deposited by the head  14  as shown in  FIGS. 4C-4E . In  FIGS. 4A-4E , there are shown closed loops of material  202   a - 202   e  which each form a portion of the perimeter wall of a fluid channel in the microfluidic device. 
         [0097]    In the prior art operation of  FIG. 4A , the material  202   a  is deposited from a beginning position  206  with a curved end  206 ′ and around in a loop to an end position  208  with a curved end  208 ′ which abuts the curved end  206 ′ of the beginning position  206 . In this way, no overlap of material is created in the deposited layer in the space between the curved ends  206 ′; 208 ′ of the beginning and end positions  206 ; 208 . Due to the slight separation between the curved ends  206 ′; 208 ′ of the beginning and end positions  206 ; 208 , a leakage path  209  is created which allows fluid to escape between these two portions of the deposited loop of material  202   a.  A section view of the closed loop of material  202   a,  and leakage path  209 , is shown in  FIG. 4F . 
         [0098]    In  FIG. 4B , which also shows a prior art operation, once the head  14  reaches the end position  208 , the head  14  continues to deposit the layer around the outside of the closed loop of material  202   b.  In this operation, the curved ends  206 ′; 208 ′ also do not overlap and a leakage path  209  is still present as shown in  FIG. 4B  between the sides of the beginning and end positions  206 ; 208  which allows fluid to escape from the closed loop of material  202   b.    
         [0099]    In  FIG. 4C , the material  202   c  is also deposited from a beginning position  206  and around in a loop to an end position  208 . However, in this operation, the curved end  206 ′ of the beginning position  206  is located beyond the curved end  208 ′ of the end position  208  such that there is an overlap of material deposited in the vicinity of these two positions  206 ; 208  which prevents the leakage path as present in  FIGS. 4A and 4B  from forming. Due to the overlap of the two curved ends  206 ′; 208 ′, to prevent an excess of material building up, the deposited material dispensed from the dispensing head in the beginning and end portions of the closed loop of material  202   b  is carefully controlled so that the total thickness of the layer is uniform. The control of the dispensed deposited material can be achieved by varying the velocity of the dispensing head, or by changing the feed rate of material to the dispensing head  14 , in this area. 
         [0100]      FIG. 4D  shows a similar tool path to  FIG. 4C  except that the curved end  208 ′ of the end position  208  does not extend as far beyond the curved end  206 ′ of the beginning position  206 . 
         [0101]    In the operation as shown in  FIG. 4E , material is also deposited from a beginning position  206  and around in a loop to an end position  208 . In this operation however, the beginning position  206  is located just short of the end position  208 . To prevent leakage in this operation, additional material  210  is deposited at the beginning and/or end position  208  which flows to bridge the gap between the beginning and end positions  206 ; 208  to thereby create an overlap of the curved ends  206 ′; 208 ′ of the two positions  206 ; 208  to prevent the leakage path  209 . In this operation, the overlap can be created by decreasing the velocity of the dispensing head, or by increasing the feed rate of material to the dispensing head  14 , in the beginning and/or end positions  206 ; 208  to ensure sufficient material flows to bridge the gap. 
         [0102]    A section view of the closed loops of material  202   c - 202   e  from  FIGS. 4C-4E , which do not contain the leakage path  209 , is shown in  FIG. 4G . 
         [0103]    Another improvement for reducing leakage in microfluidic devices created using FDM is shown in  FIG. 5A . The cross-section view of  FIG. 5A  is taken through a section of the outlet fluid channel  110 . The outlet fluid channel  110  is formed of a series of linear deposits  200 . The bottom of the channel  110  is formed of a series of parallel deposits  250  which are largely parallel to the direction of fluid flow along the fluid channel  110  when the channel is in use. The side wall of the channel  110  is formed of another series of deposits  260  which are largely parallel to the deposits  250  making up the bottom of the channel  110 . The top surface of the channel is created by a deposit  270  which snakes in an alternating fashion as a series of abutting line portions which extend across the topmost deposit from the parallel deposits  260 . Deposits  274  which are largely parallel to the deposits  250 ; 260  are located on both sides of the snaking deposit  270 . 
         [0104]    To support the snaking deposit  270  as much as possible, the snaking deposit  270  extends along the width, and at an angle to rather than along the length, of the channel  110 . In this way, the snaking deposit  270  is located at a different orientation to each of the deposits  250 ; 260  making up the bottom and sides of the channel  110 . 
         [0105]    At the interface  272  of the topmost side deposit  260  and the snaking deposit  270 , which are located at different orientations, there is a potential leak path due to the mismatch in layer orientations. 
         [0106]    Conventionally, layers above the snaking deposit  270  would be deposited in a similar orientation/pattern to the snaking layer  270 . However,  FIG. 5A  shows an improved configuration with reduced leakage whereby the layers  280  above the snaking deposit  270  are deposited in a similar orientation to the deposits  250 ; 260  making up the bottom and sides of the channel  110 , and also deposits  274 . 
         [0107]    An alternative to using a snaking deposit  270  as the top surface of the channel is shown in  FIG. 5B . As shown in this Figure, some of the deposits  260  making up the sidewall of the channel  110  each partially overhang the deposit  260  on which it is deposited. The degree by which each of these deposits  260  overhangs depend on numerous factors, such as the rate of deposition of material, and also the properties of the material being deposited. In  FIG. 5B , each of the overhanging deposits  260  overhangs by approximately 30% of their width. The overhang percentage may be more than this however, for instance 50%. By overhanging the deposits in this way, the top of the channel  110  can be created without the need for a snaking deposit  270 . 
         [0108]    Another improvement for reducing leakage between a horizontal channel and a vertical channel in microfluidic devices created using FDM is shown in  FIGS. 6A-6E . These Figures focus on the particular interface between one of the horizontal fluid channels  106  and its respective vertical channel  104  in the device  100 . 
         [0109]      FIGS. 6D and 6E  each shows a section view of a portion of the dotted region of the device shown in  FIG. 6B . In each of  FIGS. 6D and 6E , there is shown a series of linear deposits  280  defining the base of the horizontal channel  106  and a series of different deposits  290  making up the wall of the vertical channel  104 . 
         [0110]    The plan view of  FIG. 6A  represents a first deposit layer  292  shown in  FIGS. 6D and 6E , whilst the plan view of  FIG. 6B  represents a second deposit layer  294  (also shown in  FIGS. 6D and 6E ) that is adjacent to the first deposit layer  292 . 
         [0111]    In the second deposit layer  294 , in conventional FDM deposition techniques, as shown in  FIG. 6D , a leak path  296  is created at the interface of the deposits  280 ; 290  making up the respective walls of the horizontal and vertical channels  106 ; 104 . 
         [0112]      FIG. 6E  shows an improvement to the prior art deposition technique shown in  FIG. 6D . In this embodiment, at the interface of the deposits  280 ; 290  making up the respective walls of the horizontal and vertical channels  106 ; 104 , the pitch between the deposits  280 ; 290  is reduced such that the two deposits overlap  280 ; 290  along their length to plug the leakage path  296 . The overlapping region where the reduction in pitch is present between the two deposits  280 ; 290  is shown in the dotted region X of  FIG. 6C . The overlap is created using any of the techniques used to create the overlaps described in relation to  FIG. 4C-4E  (the beginning and end positions  206 ; 208  as described in  FIGS. 4C-4E  are the interfacing portions of the deposits  280 ; 290  which overlap as shown from  FIGS. 6C and 6E ). 
         [0113]    An improvement to creating sharp corners in FDM is shown in  FIGS. 7A and 7B . Conventionally, as shown in  FIG. 7A , to create a sharp corner using FDM the dispensing head  14  is configured to dispense material along a first vector  500  representative of a first edge of the item being created. Once the head  14  reaches the sharp corner of the item being created, the dispensing head pauses and then moves along a second vector  502  representative of another edge of the item being created. 
         [0114]    Due to the sudden change in direction, and pause, of the dispensing head  14  between the two vectors  500 ; 502 , an excess of material is dispensed by the head  14  at the corner between these two vectors  502 ; 504 , thus resulting in a bulged corner  506  as shown in the photograph of  FIG. 7A . 
         [0115]    To obviate the formation of the bulge  506 , the dispensing head  14  is configured to follow an arcuate path  508  between the two vectors  502 ; 504 . By dispensing material along this arcuate path, no sudden changes in direction and/or pauses occur between these two vectors. This results in a sharper corner  510  with no bulge as shown in  FIG. 7B . 
         [0116]    The radius of curvature of the arcuate path may be dependent on the height or the width of the deposited material along vectors  502 ; 504 . Preferably, the radius of curvature is between 10%-200% of the width or 20%-400% of the depth of the line deposit, though narrower percentage ranges are also possible. The radius of curvature of the arcuate path may alternatively be a fixed amount, for instance between 0.1 mm-0.4 mm. 
         [0117]    In an alternative embodiment, the formation of the bulge  506  is reduced by decreasing the flow rate of material dispensed from the dispensing head in the region of the corner. 
         [0118]    Although the above improvements have been described in relation to the particular geometry of microfluidic device shown in the Figures, it will be appreciated that the deposition techniques herein described could be applied to any other device with different geometry.