Patent Publication Number: US-9415610-B2

Title: System and method for forming hydrophobic structures in a porous substrate

Description:
TECHNICAL FIELD 
     This disclosure relates generally to systems and methods for controlling the deposition of a hydrophobic material in a porous substrate and, more particularly, to systems and methods for forming a hydrophobic material in paper as part of a chemical assay device to control diffusion of a fluid through the paper. 
     BACKGROUND 
     Paper-based chemical assay devices include a paper substrate, wax that forms fluid channels and other fluid structures in the paper, and one or more reagents. Common examples of paper-based chemical assay devices include biomedical testing devices that are made of paper and perform biochemical assays and diagnostics in test fluids such as blood, urine and saliva. The devices are small, lightweight and low cost and have potential applications as diagnostic devices in healthcare, military and homeland security to mention a few. The current state of the art paper diagnostic device is limited on fluidic feature resolution and manufacturing compatibility due to uncontrolled reflow of the wax channel after the wax is printed on the paper. 
       FIG. 10A  and  FIG. 10B  depict the prior art processes for melting wax that is formed on a paper substrate in a reflow oven. The melting process is required for the wax to penetrate into the paper instead of remaining in a layer on the surface of the paper. In  FIG. 10A , a reflow oven heats a paper substrate with solidified wax to a temperature of approximately 150° C. The entire paper and the wax are heated to the same temperature in an isotropic manner. As depicted in  FIG. 10B , the wax melts and spreads both into the porous paper and across the surface of the paper in a roughly uniform manner. The prior art reflow oven cannot control the direction of flow for the melted wax, and the melted wax tends to spread across the surface of paper to a greater degree than is desired. In a biomedical testing device, the wax is formed in lines and other structures that act as barriers and channels to fluids that diffuse through the paper substrate. The uncontrolled spread of the wax presents difficulties in forming the barriers and liquid channels with precise shapes. Consequently, improvements to the control the flow of a hydrophobic material that is deposited on a porous substrate would be beneficial. 
     SUMMARY 
     In one embodiment, an apparatus that distributes a hydrophobic material in a substrate has been developed. The apparatus includes a first roller, a second roller configured to engage the first roller to form a nip, a first heater operatively connected to the first roller and configured to heat the first roller to a first temperature that is greater than a second temperature of the second roller, and a substrate transport configured to move a substrate through the nip at a predetermined velocity to enable the first roller to engage a first side of the substrate and the second roller to engage a second side of the substrate, the second side of the substrate bearing the hydrophobic material that penetrates into the substrate in response to a temperature gradient in the nip between the first roller and the second roller. 
     In another embodiment, a method for distribution of a hydrophobic material in a substrate has been developed. The method includes engaging a first roller with a second roller to form a nip, heating the first roller with a first heater operatively connected to the first roller to heat the first roller to a first temperature that is greater than a second temperature of the second roller, and moving a substrate having a first side and a second side through the nip at a predetermined velocity with a substrate transport to enable the first roller to engage the first side of the substrate and the second roller to engage the second side of the substrate, the second side of the substrate bearing the hydrophobic material that penetrates into the substrate in response to a temperature gradient in the nip between the first roller and the second roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of an apparatus that controls the distribution of a hydrophobic material on a substrate are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1  is a diagram of an inkjet printer that includes an apparatus that applies heat and pressure to hydrophobic material on a surface of a substrate to enable the hydrophobic material to penetrate the substrate. 
         FIG. 2  is a diagram of another embodiment of the apparatus that applies heat and pressure to hydrophobic material on a surface of a substrate. 
         FIG. 3  is a diagram depicting a temperature gradient that is formed in the apparatus of  FIG. 1  or  FIG. 2  to urge the hydrophobic material to penetrate the substrate. 
         FIG. 4  is a diagram of another embodiment of an inkjet printer configuration that applies multiple layers of a hydrophobic material to a surface of a substrate before the apparatus of  FIG. 1  or  FIG. 2  applies heat and pressure to enable the hydrophobic material to penetrate the substrate. 
         FIG. 5  is a diagram of another embodiment of an inkjet printer configuration that applies multiple layers of a hydrophobic material to a surface of a substrate before the apparatus of  FIG. 1  or  FIG. 2  applies heat and pressure to enable the hydrophobic material to penetrate the substrate. 
         FIG. 6  is a diagram of an inkjet printer that applies multiple layers of a hydrophobic material to a first drum and applies heat and pressure to the hydrophobic material and a substrate to enable the hydrophobic material to penetrate the substrate. 
         FIG. 7  is a diagram of an inkjet printer that ejects liquid drops including reagents or other chemicals onto fluid channels in the substrate that are defined by the hydrophobic material in the substrate. 
         FIG. 8  is a cross-sectional view and a plan view of a biomedical test device formed in a substrate with fluid channels in the substrate that are formed by the hydrophobic material. 
         FIG. 9  is a block diagram of a process for applying heat and pressure to a hydrophobic material formed on a surface of a substrate to enable the hydrophobic material to penetrate the substrate. 
         FIG. 10A  is a diagram of a prior art reflow oven that melts a hydrophobic material formed on a surface of a substrate. 
         FIG. 10B  is a diagram depicting the spread of hydrophobic material on a substrate in the reflow oven of  FIG. 10A  in a prior art spreading process. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that produces images with resins or colorants on media, such as digital copiers, bookmaking machines, facsimile machines, multi-function machines, or the like. In the description below, a printer is further configured to deposit a melted wax, phase-change ink, or other hydrophobic material onto a porous substrate, such as paper. While the printers described below are inkjet printers and the hydrophobic phase change material can be a phase-change ink in some embodiments, in some configurations the hydrophobic material is an optically transparent wax or other material that does not have a particular color. The visual representations of the hydrophobic material that are presented below are for illustrative purposes only, and different embodiments described below use hydrophobic materials with no coloration or with any coloration that is suitable for use with a chemical assay device. 
     The printer is optionally configured to apply a temperature gradient and pressure to the substrate that spreads the hydrophobic material and enables the hydrophobic material to penetrate into the porous substrate to form hydrophobic structures including channels and barriers that control the capillary flow of liquids, including water, through the substrate. 
     As used herein, the terms “hydrophilic material” and “hydrophilic substrate” refer to materials that absorb water and enable diffusion of the water through the material via capillary action. One common example of a hydrophilic substrate is paper, such as cellulose filter paper, chromatography paper, or any other suitable type of paper. The hydrophilic substrates are formed from porous materials that enable water and other biological fluids that include water, such as blood, urine, saliva, and other biological fluids, to diffuse into the substrate. As described below, a hydrophobic material is embedded in the hydrophilic substrate to form fluid channel barriers and other hydrophobic structures that control the diffusion of the fluid through the hydrophilic substrate. 
     As used herein, the term “hydrophobic material” refers to any material that resists adhesion to water and is substantially impermeable to a flow of water through capillary motion. When embedded in a porous substrate, such as paper, the hydrophobic material acts as a barrier to prevent the diffusion of water through portions of the substrate that include the hydrophobic material. The hydrophobic material also acts as a barrier to many fluids that include water, such as blood, urine, saliva, and other biological fluids. As described below, the hydrophobic material is embedded in a porous substrate to form channel walls and other hydrophobic structures that control the capillary diffusion of the liquid through the substrate. In one embodiment, the substrate also includes biochemical reagents that are used to test various properties of a fluid sample. The hydrophobic material forms channels to direct the fluid to different locations in the substrate that have deposits of the chemical reagents. The hydrophobic material is also substantially chemically inert with respect to the fluids in the channel to reduce or eliminate chemical reactions between the hydrophobic material and the fluids. A single sample of the fluid diffuses through the channels in the substrate to react with different reagents in different locations of the substrate to provide a simple and low-cost device for performing multiple biochemical tests on a single fluid sample. 
     As used herein, the term “phase-change material” refers to a hydrophobic material with a solid phase at room temperature and standard atmospheric pressure (e.g. 20° C. and one atmosphere of pressure) and a liquid phase at an elevated temperature and/or pressure level. Examples of hydrophobic phase-change materials used herein include wax and phase-change ink. As used herein, the term “phase-change ink” refers to a type of ink that is substantially solid at room temperature but softens and liquefies at elevated temperatures. Some inkjet printers eject liquefied drops of phase-change ink onto indirect image receiving surfaces, such as a rotating drum or endless belt, to form a latent ink image. The latent ink image is transferred to a substrate, such as a paper sheet. Other inkjet printers eject the ink drops directly onto a print medium, such as a paper sheet or an elongated roll of paper. In a liquid state, the phase-change material can penetrate a porous substrate, such as paper. 
     In a traditional inkjet printer, the phase change ink is transferred to one side of a substrate, with an option to transfer different phase change ink images to two sides of a substrate in a duplex printing operation. The printer spreads the phase change ink drops on the surface of the substrate, and the phase change ink image cools and solidifies on the surface of the print medium to form a printed image. The embodiments described below, however, apply heat and pressure to phase-change ink or another hydrophobic material on the surface of the substrate to enable the hydrophobic material to penetrate through the porous material in the substrate to form a three-dimensional barrier through the thickness of the substrate that controls the diffusion of fluids through the substrate. 
       FIG. 1  depicts an inkjet printer  100  that includes an apparatus  180  for applying a heat gradient and pressure to a hydrophilic substrate, such as paper, to enable a flow of a hydrophobic material into pores of the substrate to form barriers and channels that control diffusion of a fluid through the hydrophilic substrate. As used herein, a reference to the term “apparatus,” unless expressly referred to otherwise, refers to a device that applies a heat gradient and pressure to a substrate to enable a hydrophobic material formed on a surface of the substrate to penetrate into the substrate with an anisotropic spread pattern. The apparatus is optionally incorporated into a printer, such as an inkjet printer. As described below, while the apparatus  180  is depicted in  FIG. 1  as part of an indirect inkjet printer  100 , the apparatus  180  can be incorporated into other printing devices or can be an independent device that is configured to process substrates that have a hydrophobic material formed on a surface using an inkjet printer or any other suitable application device. 
     The printer  100  includes an imaging drum  104 , transfix roller  108 , imaging drum heater  112 , rotating actuator  116 , and substrate heater  120 . The printer  100  includes one or more inkjet printheads  124  that eject liquefied drops of a phase-change ink or other hydrophobic material onto a surface of the imaging drum  104 . The imaging drum  104  and transfix roller  108  engage each other in a nip  106 . In the printer  100 , mechanical, pneumatic, or hydraulic actuators hold the imaging drum  104  and transfix roller  108  together to form the nip  106  and apply pressure to a substrate that passes through the nip  106 . In some embodiments, the actuators also move the imaging drum  104  and transfix roller  108  into engagement to form the nip  106  or out of engagement. The rotating actuator  116  is, for example, an electric motor that rotates the imaging drum  104  at a range of selected velocities. The transfix roller  108  rotates in response to the motion of the imaging drum  104  when engaged to the imaging drum  104 . 
     In the apparatus  180 , a substrate transport propels a substrate in a direction indicated by the arrow  130  to pass through the nip  106 . The substrate transport includes one or more actuators and belts, rollers, and other transport devices that move the substrate through the nip  106  in synchronization with the motion of the imaging drum  104  and transfix roller  108 . The imaging drum  104  and transfix roller  108  are part of the substrate transport system that propels the substrate through the nip  106 . In an embodiment where the apparatus  180  is incorporated in an inkjet printer, the media transport system in the printer transports the substrate to the apparatus  180  and the substrate moves through the nip  166  formed between the first roller  154  and second roller  158  in the apparatus  180 . 
     In the apparatus  180 , the cleaner roller  174  is formed with a silicone surface layer or another surface layer that removes the phase-change ink or other hydrophobic material from the surface of the second roller  158 . The second roller  158  is typically coated with a low surface energy material, such as polytetrafluoroethylene or another suitable coating, to reduce the adhesion between the second roller  158  and the hydrophobic material  144 . During operation, a small portion of the hydrophobic material  144  may adhere to the second roller  158 , and the cleaner roller  174  removes the residual hydrophobic material to prevent contamination of subsequent substrates that pass through the nip  166 . 
       FIG. 1  depicts a configuration of the apparatus  180  in an embodiment where the apparatus  180  is part of an inkjet printer. In  FIG. 1 , a digital electronic control unit (ECU), which is depicted as the controller  190 , receives digital image data corresponding to predetermined patterns and shapes for the hydrophobic material that are to be formed on the substrate. In the apparatus  180 , the printheads  124  eject drops of a phase-change ink onto the surface of the imaging drum  104  to form the latent ink image  140 . In one embodiment, the imaging drum  104  completes multiple rotations past the printheads  124  and the printheads  124  form an additional layer of phase-change ink during each rotation that is transferred to the substrate  152 . In one embodiment of the printer  100 , an actuator (not shown) removes the transfix roller  108  from engagement with the imaging drum  104 . The actuator  116  rotates the imaging drum  104  past the printhead  124  and the imaging drum  104  receives a latent ink image from the printhead  124  over the course of two or more rotations. In one embodiment, the printhead  124  forms four layers of a single latent ink image on the surface of the imaging drum  104 . Once the latent ink image is formed on the imaging drum  104 , the transfix roller  108  engages the imaging drum  104  and the substrate  152  passes through the nip  106  to receive the multi-layer latent ink image. 
     In another embodiment of  FIG. 1 , the printer  100  passes the substrate  152  through the nip  106  two or more times to form a printed image from multiple layers of ink. For example, in  FIG. 1  a first layer of the phase-change ink  142  is formed on the surface  156  of the substrate  152 . The media transport moves the substrate  152  as indicated by path  130  to pass through the nip  106  a second time as the imaging drum  104  carries an additional layer of phase-change ink  140  that is ejected by the printhead  124 . The media path  130  does not include a duplexing unit that is commonly used for two-sided printing in a printer to enable the side  156  of the substrate  152  to engage the imaging drum  104  during each pass through the nip  106 . In one configuration, the controller  190  operates the printhead  124  to form the same image during each pass of the substrate  152  so that a single printed pattern of the phase-change ink is formed on one side of the substrate. For example, the latent ink image  140  is aligned with the ink image  142  that is already formed on the substrate  152 , to form the combined image  144  that includes the combined volumes of phase-change ink in the images  140  and  142 . The multiple passes enable the printer  100  to deposit a greater amount of the phase-change ink on the substrate  152  than is commonly used for conventional printing operations. In some embodiments, the printer  100  passes the substrate  152  through the nip  106  four times to form four layers of the phase-change ink on one side of the substrate  152 . 
     In  FIG. 1 , the substrate transport moves the substrate  152 , such as a sheet or elongated roll of paper, through the nip  106 . In one embodiment, the imaging drum heater  112  heats the surface of the imaging drum  104  to 57° C. and the actuator  116  rotates the imaging drum  104  and transfix roller  108  at a linear surface velocity of five inches per second (IPS) to transfer the latent hydrophobic material image  144  to one side  156  of the substrate  152 . In alternative embodiments, the transfix velocity is faster or slower to adjust for the “dwell time” of the print medium  152  in a nip. As used herein, the term “dwell time” refers to an amount of time that a given portion of the print medium  152  spends in a nip to receive heat and pressure from the rollers that form the nip. The amount of dwell time is related to the surface areas of the rollers that form the nip and the linear velocity of the substrate through the nip. For example, in the nip  166  the dwell time is related to the surface areas of the rollers  154  and  158  and the linear velocity of the substrate  152  through the nip  166 . The dwell time is selected to enable the hydrophobic material to penetrate the substrate to form walls for fluid channels and other hydrophobic structures in the substrate. The selected dwell time can vary based on the thickness and porosity of the print medium  152 , the temperature gradient in the nip  166 , the pressure in the nip  166 , and the viscosity characteristics of the hydrophobic material. Larger rollers typically form a nip with a larger surface area. Thus, one embodiment of the apparatus  180  with larger roller diameters operates with a higher linear velocity to achieve the same dwell time as another embodiment of the apparatus  180  with smaller diameter rollers and a lower linear velocity. In different operating modes of the apparatus  180 , the selected dwell time is in a range of approximately 0.1 seconds to 10 seconds. 
     A blank side  160  of the print medium  152  engages the transfix roller  108  during an imaging operation. The heat and pressure in the nip  106  spreads the hydrophobic  140  material on the surface of the substrate  152  to form a printed image on the first side  156 , with the hydrophobic material  140  combining with the hydrophobic material  142  in the multi-pass embodiment of  FIG. 1 . After the imaging operation that is depicted in  FIG. 1 , a substantial portion of the hydrophobic material  144  remains on or near the surface  156  of the substrate  152 . 
     In the printer  100 , the media transport moves the substrate  152  to the apparatus  180  after one or more passes of the substrate  152  to receive the printed image  144 . The media transport moves the substrate as indicated by the path  134  to the apparatus  180 . The apparatus  180  includes a first roller  154 , a second roller  158 , an optional substrate heater  170 , and a cleaner roller  174 . The first roller  154  and second roller  158  engage each other to form a nip  166 . In the apparatus  180 , mechanical, pneumatic, or hydraulic actuators hold the rollers  154  and  158  together to form the nip  166  and apply pressure to a substrate that passes through the nip  166 . The first roller  154  and second roller  158  apply pressure to the substrate  152  and hydrophobic material  144  with a pressure of 1,000 pounds per square inch (PSI) in the embodiment of  FIG. 1 . In other configurations, the pressure in the nip  166  is between 800 PSI and 3,000 PSI and is selected based on the properties of the substrate and composition of the hydrophobic material. A heater  162  is operatively connected to the first roller  154  and is configured to heat the first roller  154  to a higher temperature than the second roller  158 . The media transport moves the substrate  152  through the nip  166  after the hydrophobic material  144  has been transferred to the side  156  of the substrate  152 . 
     In the example of  FIG. 1 , an actuator  168  rotates the first roller  154  to enable the substrate  152  to move through the nip  166  in the direction  134  while the second roller  158  rotates freely while engaging the printed side  156  of the substrate  152  that bears the hydrophobic material  144 . The elevated temperature of the first roller  154  forms a temperature gradient in the nip  166 , and the first roller  154  engages the second side  160  of the substrate  152 . As described below, the temperature gradient enables the printed pattern of the hydrophobic material  144  to penetrate the thickness of the substrate  152  while reducing the lateral spread of the hydrophobic material  144 . 
     In the apparatus  180  the optional substrate heater  170  elevates the temperature of the substrate to a predetermined temperature as the substrate passes through the nip  166 . In one embodiment, the substrate heater  170  heats the substrate to 60° C. as the substrate approaches the nip  166 . The roller heater  162  heats the surface of the first roller  154  to approximately 100° C. while the surface of the second roller  158  remains at a lower temperature of approximately 60-70° C. In one embodiment, the second roller  158  includes a larger diameter than the first roller  154  to enable the surface of the second roller  158  to cool after engaging the higher temperature first roll  154  in the nip  166 . In other embodiments, the rollers are substantially equal in size or the first roller  154  is larger in diameter than the second roller  158 . The roller heater  162  and substrate heater  170  are embodied as electric radiant heaters in the apparatus  180 . In the embodiment of  FIG. 1 , the actuator  168  rotates the first roller  154  and second roller  158  at a linear velocity of approximately one inch per second as the substrate  152  passes through the nip  166 . The linear velocity of the substrate  152  is inversely proportional to the dwell time in the nip  166 . The dwell time is affected by the surface areas of the rollers  154  and  158 , which affect the physical size of the nip  166 , and the linear velocity of the substrate  152 . In the apparatus  180 , the dwell time is between approximately 0.1 seconds to 10 seconds and the controller  190  adjusts the linear velocity of the substrate  152  to produce a selected dwell time in the nip  166 . 
     In alternative embodiments, the operating parameters of the apparatus  180  are adjusted to modify the temperature gradient in the nip  166  and the dwell time of the substrate  152  in the nip  166  to control the penetration of the hydrophobic material  144  through the substrate  152 . In different embodiments of the apparatus  180 , the temperature gradient and pressure in the nip  166 , and the dwell time of the substrate  152  in the nip  166  are adjusted to produce a selected dwell time for rollers with different diameters. 
       FIG. 1  depicts the apparatus  180  as the substrate  152  that already bears hydrophobic material  144  on one side  156  passes through the nip  166  where pressure and a temperature gradient are applied to the substrate  152  to enable the hydrophobic material  144  to penetrate into the porous material of the substrate  152 . In  FIG. 1 , the substrate transport moves the substrate through the nip  166  with the side  156  bearing the hydrophobic material engaging the second roller  158  while the blank side  160  engages the first roller  154 . The apparatus  180  is depicted in the printer  100  where the printer  100  forms printed patterns of the hydrophobic material. In another embodiment, however, the apparatus  180  receives a substrate and hydrophobic material that are formed in a separate inkjet printing device or through any suitable deposition process that forms the hydrophobic material on one surface of the substrate. 
       FIG. 2  depicts another embodiment of an apparatus  280 . The apparatus  280  includes some of the components from the apparatus  180 , including the first roller  154 , second roller  158 , first roller heater  162 , actuator  168 , and substrate heater  170 . The apparatus  280  also includes an intermediate web  272  that engages the surface of the second roller  158  and the surface of the substrate  152  in the nip  206  that is formed between the first roller  154  and the second roller  158 . In one embodiment, the intermediate web  272  is formed from an endless or cycling silicone rubber belt that engages the substrate  152  and hydrophobic material  144  in the nip  206 . The endless belt  272  prevents transfer of the hydrophobic material  144  to the second roller  158 , and can be cleaned of any hydrophobic material that transfers to the belt  272  in the nip  206 . In another configuration, the intermediate web  272  is a sacrificial material, such as a plastic film or coated paper web, which passes through the nip  206  once and is subsequently discarded or recycled. The apparatus  280  in  FIG. 2  applies pressure and a temperature gradient to the hydrophobic material  144  and substrate  152  in a similar manner to the apparatus  180  to enable the hydrophobic material to penetrate the substrate  152 . 
       FIG. 3  depicts the penetration of the hydrophobic material  144  into the substrate  152  in more detail. The elevated temperature and pressure in the nip  106  melt the solidified hydrophobic material  144  and the liquefied hydrophobic material spreads both horizontally and vertically into the porous material in the substrate  152 . The spreading distance L of the liquefied hydrophobic material is provided by Washburn&#39;s equation: 
             L   =         γ   ⁢           ⁢   Dt       4   ⁢   η               
where γ is the surface tension of the melted hydrophobic material  144 , D is the pore diameter of pores in the substrate  152 , t is the dwell time of the substrate in the nip during which the temperature gradient and pressure in the nip reduce the viscosity of the hydrophobic material  144 , and η is the viscosity of the melted hydrophobic liquid. The surface tension γ and viscosity η terms are empirically determined from the properties of the hydrophobic material  144 . The pore diameter D is empirically determined from the type of paper or other hydrophilic material that forms the substrate  152 . The apparatus  180  has direct or indirect control over viscosity η of the hydrophobic material as the hydrophobic material and substrate move through the temperature gradient that is produced in the nip  166  and time t for how long the hydrophobic material remains in a liquefied state in the nip  166 . Hydrophobic materials such as wax or phase-change inks transition into a liquid state with varying levels of viscosity based on the temperature of the material and pressure applied to the hydrophobic material. The viscosity of the liquefied hydrophobic material is inversely related to the temperature of the material. The temperature gradient in the nip reduces the viscosity of the hydrophobic material in the higher-temperature region near the side  160  and roller  154  to a greater degree than on the cooler side  156  and cooler roller  158 . Thus, the temperature gradient enables the ink in the higher temperature regions of the temperature gradient to penetrate a longer distance compared to the ink in the cooler regions due to the reduced viscosity at increased temperature.
 
     As is known in the art, the pressure applied in the nip  166  also reduces the effective melting temperature of the hydrophobic material  144  so that the temperature levels required to melt and reduce the viscosity level of the hydrophobic material  144  in the nip  166  are lower than the melting temperature at standard atmospheric pressure. Once a portion of the substrate  152  exits the nip  166 , the pressure and temperature levels drops rapidly, which enables the hydrophobic material  144  to return to a solidified state in a more rapid and controlled manner than in the prior art reflow oven depicted in  FIG. 10A . The dwell time of each portion of the substrate  152  in the nip  166  also affects the amount of time that the hydrophobic material  144  spends in the liquid state. 
     In the nip  166 , the temperature gradient produces anisotropic heating of the melted hydrophobic material  144 . The higher temperature of the first roller  154  on the side  160  reduces the viscosity η of the hydrophobic material  144  to a greater degree than on the cooler side  156 . Thus, the temperature gradient enables the hydrophobic material  144  to flow into the porous material of the substrate  152  toward the side  160  for a longer distance than the horizontal flow of the hydrophobic material  144  along the length of the substrate  152 . In  FIG. 3 , the longer arrow  220  depicts the longer distance of flow L for the hydrophobic material  144  through the porous material in the substrate toward the high temperature side  160  of the substrate  152 , while the shorter arrows  224  indicate a shorter flow distance along the length of the substrate  152 . For a phase-change ink hydrophobic material, the reduced viscosity η of the ink as the ink penetrates the substrate  152  towards the higher temperature roller  154  enables the phase-change ink to penetrate through the substrate from the printed surface  156  to the second side  160 , which forms a layer of the phase-change ink through the entire thickness of the substrate  152 . 
     The apparatus  180  generates the anisotropic temperature gradient and liquid flow patterns for the hydrophobic material  144  to form printed lines and other printed features with the hydrophobic material  144  that exhibit less spread along the length of the substrate  152  and improved penetration through the substrate  152  to from the printed side  156  to the blank side  160 . For example, in one embodiment the horizontal width of a printed channel barrier line that is formed with the apparatus  180  is approximately 650 μm while the prior-art reflow oven embodiment of  FIG. 10A  spreads the same printed line to a width of approximately 1000 μm. Furthermore, the anisotropic temperature gradient in the apparatus  180  enables the hydrophobic material  144  to penetrate into the substrate  152  to a greater degree than the prior art reflow oven with the isotropic temperature distribution depicted in  FIG. 10B . The narrower width of the barriers enables the production of smaller devices with finer feature details, and also improves the effectiveness of the fluid channels that control the capillary diffusion of fluids through the substrate. 
       FIG. 4  depicts another embodiment of an inkjet printer  400  that deposits a pattern of hydrophobic material  444  onto a substrate  452 . The inkjet printer  400  is a direct inkjet printer where multiple sets of printheads, such as printheads  424 A and  424 B, in a print zone eject the hydrophobic material directly onto the substrate  452 . The substrate  452  is illustrated as an elongated media web. Heaters  420  heat the substrate  452  to a predetermined temperature, such as 60° C., as the substrate  452  enters the print zone. In the example of  FIG. 4 , the printheads  424 A and  424 B form two layers of the hydrophobic material  444  in a predetermined pattern on the substrate  452 , although other printer embodiments include additional printheads to form the printed patterns with additional layers of the hydrophobic material. In the printer  400 , the rollers  430  are part of a media transport that moves the substrate through the print zone as indicated by the arrow  434 . The substrate  452  subsequently moves through the apparatus  180  or  280  that apply the temperature gradient and pressure to enable the hydrophobic material  444  to penetrate through the substrate  452 . The apparatus  180  or  280  is incorporated into the printer  400  in one embodiment. In another embodiment, the apparatus  180  or  280  receives the substrate  452  during a finishing or other processing that occurs after printing with the printer  400 . 
       FIG. 5  depicts another configuration of an inkjet printer  500  that deposits a pattern of hydrophobic material  544  onto a substrate  552 . In the printer  500 , the substrate  552  is an elongated web of paper or another substrate material that passes multiple printheads, such as printheads  524 A and  524 B, in a print zone to receive a printed pattern with multiple layers of a hydrophobic material  544 . While  FIG. 5  depicts two sets of printheads  524 A and  524 B that form two layers of the hydrophobic material in the substrate  552 , another configuration of the printer  500  forms three or more layers of the hydrophobic material using additional printheads. The printer  500  includes substrate heaters  520  that heat the substrate  552  as the substrate  552  approaches the print zone. In the printer  500 , the substrate  552  engages a rotating backer roller  528  that supports the substrate  552  as the substrate  552  moves past the printheads  524 A and  524 B in the print zone. The backer roller  528  includes a heater  562  to maintain the temperature of the substrate  552  at a predetermined temperature (e.g. 60° C.) during the printing process. The substrate  552  subsequently exits the print zone as indicated by arrow  534  and enters the apparatus  180  or  280 . The apparatus  180  or  280  is incorporated into the printer  500  in one embodiment. In another embodiment, the apparatus  180  or  280  receives the substrate  452  during a finishing or other processing that occurs after printing with the printer  500 . 
       FIG. 6  depicts another embodiment of an inkjet printer  600  that incorporates the functionality of the apparatuses  180  and  280 . The inkjet printer includes an imaging drum  604 , transfix roller  608 , substrate heaters  620 , printheads  624 A and  624 B, and a cleaning roller  674 . The imaging drum  604  engages the transfix roller  608  to form a nip  606 . The printheads  624 A and  624 B each eject a layer of a phase-change ink or other hydrophobic material to form a hydrophobic material image  644  on the surface of the imaging drum  604 . As with the embodiments of  FIG. 4  and  FIG. 6 , the printer  600  optionally includes additional printheads to form additional layers of the hydrophobic material on the imaging drum  604 , or the imaging drum  604  completes multiple rotations past one or more printheads to form a multi-layer printed image in a multi-pass printing configuration prior to moving the substrate  642  through the nip  606 . 
     In the printer  600 , imaging drum  604  optionally includes a heater  612  that heats the surface of the imaging drum  604  to a predetermined temperature (e.g. 60° C.) as the imaging drum  604  rotates past the printheads  624 A and  624 B. The printer  600  also includes one or more electrical, pneumatic, or hydraulic actuators that engage the imaging drum  604  and the transfix roller  608  in the nip  606  with a predetermined pressure level, such as a 1,000 PSI pressure level. The transfix roller  608  includes another heater  662  that heats the surface of the transfix roller  608  to a higher temperature than the surface of the imaging drum  604  in the nip  606 . For example, in one embodiment the surface temperature of the transfix roller  608  in the nip  606  is approximately 100° C. while the surface temperature of the imaging drum  604  is approximately 60°-70° C. 
     During operation, the printer  600  forms a temperature gradient in the nip  606  in a similar manner to the configurations of the apparatuses  180  and  280 . The hydrophobic material pattern  644  on the lower temperature imaging drum  604  transfers to one side  646  of the substrate  642  in the nip  606 , and the temperature gradient in the nip  606  enables the hydrophobic material  644  to penetrate through the substrate  642  toward the side  650  that engages the higher temperature transfix roller  608 . In the configuration of the printer  600 , the transfix roller  608  acts as the higher temperature first roller from the apparatuses  180  and  280  and the imaging drum  604  acts as the lower temperature second roller in the apparatuses  180  and  280 . The imaging drum  604  continues rotation through the nip  606  and passes the cleaning roller  674 , which removes and residual phase-change ink or other hydrophobic material from the surface of the imaging drum  604 . 
     The inkjet printers and apparatuses described above form predetermined patterns of hydrophobic material on a hydrophilic substrate, such as a paper, to form fluid channels and other features that control the diffusion of a liquid through the substrate. As described above, chemical assay devices are one example of a class of devices include a substrate with fluid channels that are formed with the hydrophobic material. Selected regions of the chemical assay include a variety of chemicals, including reagents, catalysts, indicators, buffers, and the like that are used with the biomedical testing device. In some embodiments, an inkjet printer applies the chemicals to different regions of the substrate after the hydrophobic material has been applied to the substrate to form the fluid channels. 
       FIG. 7  depicts an embodiment of an inkjet printer  700  with printheads  724 A- 724 C. In the configuration of  FIG. 7 , each of the printheads  724 A- 724 C ejects liquid drops that include a chemical for use with a chemical assay device that is configured as a biomedical sensor formed from the substrate  742 . In another configuration, the printer includes a different number of printheads for printing a different combination of chemicals or each of the printheads is configured with multiple liquid reservoirs to enable the ejection of different types of chemical from different groups of inkjets in a single printhead. In the substrate  742 , the regions  744  are formed from a hydrophobic material that substantially penetrates the entire thickness of the substrate  742  to form hydrophobic structures including barriers and fluid channels for liquids that are absorbed by exposed regions of the substrate  742 , such as the regions  704 A,  704 B, and  704 C. The printheads  724 A- 724 C eject drops of liquid that include one or more selected chemicals onto different exposed regions in the substrate  742 . For example, the printheads  724 A- 724 C eject liquid drops with different chemicals into the regions  704 A- 704 C, respectively, in the configuration of  FIG. 7 . The liquid drops include a carrier chemical such as water, alcohol, or another solvent that carries the chemical as a solution or suspension. After passing the printheads  724 A- 724 C, the liquid carrier dries from the substrate  742  and leaves the chemical deposited in the substrate  742  for later use in a chemical assay or biomedical testing device. 
       FIG. 8  depicts an example of a printed pattern in a biomedical test device  850  that includes a deposit location and fluid channels formed from the hydrophobic material in the substrate to direct the fluid to different locations where chemical reagents react with the fluid. The substrate  152  includes the barriers  824  and  828  that are formed from the hydrophobic material  144 . The apparatus  180  enables the hydrophobic material in the barrier hydrophobic structures  824  and  828  to penetrate through the thickness of the substrate  152  between the sides  156  and  160  to fully surround a fluid channel  808 . The hydrophilic substrate  152  absorbs a fluid sample and the fluid moves through the channel  808  through capillary diffusion, while the barriers  824  and  828  prevent the fluid from leaving the channel  808 . The biomedical detection device  850  includes the substrate  152 , the hydrophobic barriers that are formed in the substrate to control the diffusion of fluids, a deposit site  854 , and a set of reaction sites such as the reaction sites  858  and  862 . During operation, a fluid sample is deposited in the central deposit site  854 . While not depicted in  FIG. 8 , a mask layer is typically formed over the printed device  850  to ensure that fluid samples are only absorbed at the deposit site  854 . The fluid sample propagates through the hydrophilic substrate  152  through the channels that are formed by the hydrophobic material and to an array of reaction sites. Each of the reaction sites includes a chemical reagent that is embedded in the substrate  152 . The chemical reagents react with different chemical compounds in the fluid sample and change color or produce another indicator that can be used to analyze the fluid sample. For example, the reaction site  858  tests for anemia while the reaction site  862  tests for the glucose (blood sugar) level in a single blood sample that is placed in the deposit site  854 . 
       FIG. 4  depicts a block diagram of a process  900  for applying and spreading a hydrophobic material through a substrate. The process  900  is described in conjunction with the apparatus  180  of  FIG. 1 , the apparatus  280  of  FIG. 2 , the illustrative example of the nip and temperature gradient of  FIG. 3 , and the biomedical testing device  850  of  FIG. 8  for illustrative purposes. 
     Process  900  begins with an optional process of forming a hydrophobic material on a surface of a substrate (block  904 ). As described above, in one embodiment the apparatus  180  is incorporated in an inkjet printer, and the inkjet printheads  124  eject liquid drops of the hydrophobic material in predetermined patterns, such as the pattern depicted in the biomedical testing device  850 . The substrate transport moves a blank substrate through the nip and the hydrophobic material is transferred to a printed side of the substrate. 
     Process  900  continues as the substrate with the hydrophobic material passes through a nip formed from two rollers that are heated to different temperatures with a blank side of the substrate engaging the roller with the higher temperature and the side of the substrate that bears the hydrophobic material engaging the roller with the lower temperature (block  908 ). As depicted above in  FIG. 1 , in one configuration the heater  162  heats the first roller  154  to 100° C. while the second roller  158  remains at a lower temperature of approximately 60-70° C. In alternative configurations, the heater  162  heats the first roller  154  to an elevated temperature of 70° C. to 140° C. The second roller  158  remains at a lower temperature than the first roller  154  to produce the temperature gradient in the nip  166 . As the substrate moves through the nip  166 , the blank side  160  is heated to a higher temperature and the printed side  156  is heated to a lower temperature due to the temperature gradient between the rollers  154  and  158 . As depicted in  FIG. 3 , the hydrophobic material liquefies and flows through the thickness of the substrate  152 . The temperature gradient in the nip  166  enables the hydrophobic material to flow in an anisotropic manner with a greater portion of the liquid flow being directed into the substrate from the printed side  156  to the blank side  160  to form barriers and channels that control the diffusion of fluids through the hydrophilic material in the substrate  152 . 
     Process  900  continues with the optional application of reagents or other chemicals to the regions of the hydrophilic substrate that are defined by the hydrophobic fluid channel barriers (block  912 ). As depicted above with reference to  FIG. 7 , an inkjet printer can eject liquid drops that include various chemicals onto regions of the substrate that are defined by the hydrophobic material. A single chemical assay or biomedical testing device can include multiple chemicals that are deposited into different regions of the substrate and are isolated from each other by the fluid channels that are formed by the hydrophobic material. In the process  900 , the chemicals are formed on the substrate after the formation of the fluid channels with the hydrophobic material to prevent cross-contamination between different chemicals that are ejected onto a single substrate and because the application of heat and pressure to enable the hydrophobic material to penetrate the substrate may produce undesirable reactions with many chemicals that are used in chemical sensors or biomedical testing devices. 
     It will be appreciated that various of the above-disclosed and other features, and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.