Patent Publication Number: US-2023150197-A1

Title: Three-dimensional (3d) printing systems, methods, and devices

Description:
TECHNICAL FIELD 
     The present disclosure is related generally to particle organization, and in particular to tunable patterning of particles via acoustophoresis. 
     STATEMENT OF GOVERNMENT RIGHTS 
     This invention was made with Government support under Grant (or Contract) No. W911NF-09-D-0001, awarded by the Army Research Office. The Government has certain rights in this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Pat. Application No. 16/853,518, filed on Apr. 20, 2020, which is a continuation of U.S. Pat. Application No. 15/554,079, filed on Aug. 28, 2017, now abandoned, which claims priority to PCT Application PCT/US2016/025259, filed on Mar. 31, 2016, which claims priority to U.S. Provisional Application No. 62/141,053, filed on Mar. 31, 2015, the disclosures of which are incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The manipulation of particles suspended in a solution is utilized in a number of applications, including filtration, biochemical diagnostics, and materials processing. Manipulation of these particles is typically accomplished via chemical, magnetic, electric, or rheological techniques that require either functionalized particles or a narrow range of particle/fluid properties. 
     It would be beneficial to develop a system and method of organizing and patterning particles suspended in a solution that allows particle orientation and patterning particles suspended in a solution that allows particle orientation and location to be controlled. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment provides a method of ordering particles suspended in a solution in a channel, wherein the particles are unordered when entering the channel. The method further includes applying sound waves to the channel, wherein the frequency of the sound wave is tuned to create one or more columns of particles oriented in the same direction. 
     Another embodiment provides a method for delivering ordered particles to a printhead of a three-dimensional (3D) printer. The 3D printer includes an acoustophoresis device, a piezoelectric device, and a printhead. The acoustophoresis device includes at least a first inlet, at least a first outlet, and a channel defined between the first inlet and the first outlet. The piezoelectric device is adhered to a surface of the acoustophoresis device to generate sound waves within at least a portion of the channel. The method includes delivering a solution to the first inlet of the acoustophoresis device. The solution includes anisotropic particles. The acoustic waves generated by the piezoelectric device organizes the particles into one or more columns along an axis parallel to a direction of flow of the solution. The first outlet delivers the one or more columns of particles to the printhead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an acoustophoresis device that utilizes acoustic waves to organize particles suspended in a fluid into one or more columns according to an embodiment of the present invention. 
         FIG.  2    is a magnified view that illustrates the organization of anisotropic particles within a channel of the acoustophoresis device according to an embodiment of the present invention. 
         FIG.  3   a    is a microscopic view of spherical shaped particles both in the absence of and presence of acoustic waves according to an embodiment of the present invention. 
         FIG.  3   b    is a microscopic view of rod-shaped particles both in the absence of and presence of acoustic waves according to an embodiment of the present invention. 
         FIG.  3   c    is a microscopic view of bow-tie shaped particles both in the absence of and presence of acoustic waves according to an embodiment of the present invention. 
         FIG.  3   d    is a microscopic view of rectangular shaped particles both in the absence of and presence of acoustic waves according to an embodiment of the present invention. 
         FIG.  4   a    is a cross-sectional view of an acoustophoresis device utilized to deliver anisotropic particles organized into a column according to an exemplary application of the present invention. 
         FIG.  4   b    is a cross-sectional view of an acoustophoresis device utilized to deliver anisotropic particles organized into a column according to an exemplary application of the present invention. 
         FIG.  4   c    is a cross-sectional view of an acoustophoresis device utilized to deliver anisotropic particles organized into a column according to an exemplary application of the present invention. 
         FIG.  5   a    is a microscopic view illustrating the relationship between columnar spacing and number of columns according to embodiments of the present invention. 
         FIG.  5   b    is a microscopic view illustrating the relationship between columnar spacing and number of columns according to embodiments of the present invention. 
         FIG.  5   c    is a graph illustrating the relationship between columnar spacing and number of columns according to embodiments of the present invention. 
         FIG.  6   a    illustrates stacking of anisotropic particles in the x-z plane in the presence of acoustic waves and maintenance of anisotropic particle organization following removal of the acoustic waves according to an embodiment of the present invention. 
         FIG.  6   b    illustrates stacking of anisotropic particles in the x-z plane without acoustic waves according to an embodiment of the present invention. 
         FIG.  6   c    illustrates stacking of anisotropic particles in the x-z plane in the presence of acoustic waves and maintenance of anisotropic particle organization following removal of the acoustic waves according to an embodiment of the present invention. 
         FIG.  6   d    illustrates stacking of anisotropic particles in the x-z plane in the presence of acoustic waves and maintenance of anisotropic particle organization following removal of the acoustic waves according to an embodiment of the present invention. 
         FIG.  6   e    illustrates stacking of anisotropic particles in the x-z plane in the presence of acoustic waves and maintenance of anisotropic particle organization following removal of the acoustic waves according to an embodiment of the present invention. 
         FIG.  7   a    illustrates the relationship between particle shape/size, flow rate, and separation efficiency according to embodiments of the present invention. 
         FIG.  7   b    illustrates the relationship between particle shape/size, flow rate, and separation efficiency according to embodiments of the present invention. 
         FIG.  7   c    illustrates the relationship between particle shape/size, flow rate, and separation efficiency according to embodiments of the present invention. 
         FIG.  7   d    illustrates the relationship between particle shape/size, flow rate, and separation efficiency according to embodiments of the present invention. 
         FIG.  8   a    illustrates the relationship between voltage applied to the piezoelectric device and formation of one or more columns according to an embodiment of the present invention. 
         FIG.  8   b    illustrates the relationship between voltage applied to the piezoelectric device and formation of one or more columns according to an embodiment of the present invention. 
         FIG.  8   c    illustrates the relationship between voltage applied to the piezoelectric device and formation of one or more columns according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a system and method of organizing particles using acoustic waves. In particular, the acoustic waves provide a primary focusing force that drives particles to the pressure node of the standing wave, and secondary scattering forces that interact with the particles to align the particles end-to-end with one another within a given column, and to repel particles aligned with separate columns. The combination of these competing forces allows particles to be organized into a plurality of parallel columns. By increasing the primary focusing force, the plurality of columns can be collapsed into a highly ordered column or array of particles. In this way, the present invention provides a system and method of selectively organizing particles in a way that is desirable for a particular application via selective modification/tuning of the acoustic waves applied. Benefits of utilizing acoustic waves to organize the orientation and location of particles include the ability to organize particles of various sizes and of different materials (i.e., material agnostic). 
       FIG.  1    is a perspective view of acoustophoresis system  10  that utilizes acoustic waves to organize particles suspended in a fluid into one or more columns according to an embodiment of the present invention. System  10  includes reservoir  12 , acoustophoresis device/chip  14 , and piezoelectric device  16 . Reservoir  12  holds a colloidal solution that includes microscopically dispersed insoluble particles. A variety of particle sizes may be utilized for particles with at least one dimension on the order of 100 nm or larger. Depending on the size, density, and/or buoyancy of the particles in the solution, sonicator  17  may be utilized to maintain a desired distribution of particles within the solution. For example, in one embodiment sonicator  17  is operated at a frequency of approximately 20 kHz to maintain the desired distribution of particles. In other embodiments, sonicator  17  may not be required to maintain the desired distribution of particles within the colloidal solution, and a passive reservoir may be utilized in isolation. 
     In the embodiment shown in  FIG.  1   , acoustophoresis device  14  is manufactured as a semiconductor device that includes glass layer  18  and semiconductor substrate  20 . Acoustophoresis device  14  includes inlet  22 , channel  24 , and outlets  26  and  28 . A variety of fabrication techniques may be utilized to fabricate acoustophoresis device  14 , although in other embodiments the device may be fabricated using traditional, rather than semiconductor fabrication techniques. In the embodiment shown in  FIG.  1   , semiconductor substrate  20  is a silicon substrate having a thickness of approximately 525 µm. The silicon substrate is coated with an aluminum oxide film via an atomic layer deposition process to serve as a hard etch mask, which is patterned using photolithographic techniques and etched using a buffered oxide etch. Channel  24  is etched into semiconductor substrate  20  using a deep-reactive ion etching technique to provide a channel depth of approximately 150 µm. The aluminum oxide film (e.g., hard etch mask) is subsequently removed via a buffered oxide etch (e.g., buffered HF or BOE process) and the processed substrate is anodically bonded to glass layer  18  having pre-drilled fluidic vias to serve as inlet  22  and outlets  26  and  28 . 
     Piezoelectric device  16  is bonded to the surface of acoustophoresis device  14  in the vicinity of channel  24 . An excitation signal provided to piezoelectric device  16  generates acoustic waves directed toward channel  24 . The magnitude and frequency of the acoustic waves are related to the magnitude and frequency of the excitation signal provided to piezoelectric device  16 , which may be selectively modified to achieve the desired result. As described in more detail below, modifying the amplitude and frequency of the acoustic waves allows the particles to be organized in a controlled and/or tunable manner. 
     In the embodiment shown in  FIG.  1   , the solution retained within reservoir  12  is supplied to acoustophoresis device  14  via inlet  22 . As described in more detail below, the rate at which solution is provided to inlet  22  determines the flow rate through acoustophoresis device  14 . Typically, organizing particles via traditional methods requires relatively low flow rates. A benefit of the present invention, as described below, is the ability to organize particles in the presence of flow. Upon entering channel  24  from inlet  22 , particles are evenly suspended and randomly organized within the solution. Application of acoustic waves from piezoelectric device  16  causes the particles to organize within channel  24 . In the embodiment shown in  FIG.  1   , the portion of channel  24  extending on the outlet side of piezoelectric device  16  is bifurcated such that particles located in the center of channel  24  are directed to outlet  26  and panicles located on the edges of channel  24  are directed to second or waste outlet  28 . In other embodiments, channel  24  may be designed with a different number of outlets and/or geometries based on the particulars of the application. 
       FIG.  2    is a magnified view that illustrates the organization of anisotropic particles within channel  24  of the acoustophoresis device  14  according to an embodiment of the present invention. In the embodiment shown in  FIG.  2   , the particles are rod-like in shape, and are therefore anisotropic particles that exhibit attributes based on the orientation of the particles (e.g., greater strength in one direction than another). Upon entering channel  24 , the anisotropic particles are distributed randomly throughout the channel and are oriented in random directions. The anisotropic particles flow in the z direction toward outlets  26  and  28 . As the particles move past piezoelectric device  16 , the acoustic waves generated by piezoelectric device  16  cause the particles to organize. As discussed in more detail below, two competing forces act on the particles to organize and pattern the particles. The first is a primary focusing force that drives particles that are denser (e.g., anisotropic particles in this example) to the pressure node of the standing acoustic wave. The second is a scattering force, which is attractive when the interacting particles are aligned parallel to the acoustic wave and repulsive when the particles are aligned perpendicular to the acoustic wave. As a result of the attractive scattering forces, anisotropic particles are snapped end-to-end in a given column (i.e., along the z axis). In addition, as illustrated in other embodiments, repulsive forces act to keep multiple assembled columns from collapsing into a single column. 
     In the embodiment shown in  FIG.  2   , however, anisotropic particles are aligned end-to-end in a single column near the middle of channel  24  (i.e., along the x axis). That is, the acoustic standing wave generated by piezoelectric device  16  has a wavelength approximately twice that of the channel width, and therefore has a pressure node near the center of channel  24 . Once organized, the particles tend to retain this organization even after leaving the region of the channel located under piezoelectric device  16 . As a result, particles organized into a column as shown in  FIG.  2    would be provided to outlet  26  (shown in  FIG.  1   ). A benefit of this arrangement is that outlet  26  is provided with a much higher volume fraction of particles than that provided at inlet  22 . In addition, the anisotropic particles shown in  FIG.  2    are oriented such that anisotropic attributes of the particles can be utilized. This is an important consideration in fields such as 3D printing, in which the strength of the printed device (i.e., the finished product) depends in part on the orientation of the particles deposited. One of the benefits of the embodiment illustrated in  FIGS.  1  and  2    is the ability to order the particles in the presence of flow (i.e., while the particles are flowing through the channel from inlet to outlet). As described in more detail below, the dimensions of channels  24  may also be modified to accommodate different applications. In some embodiments, the height (i.e., y-axis) may be increased to allow for the stacking of columns in the y direction. In other embodiments, the width (i.e. x-axis) of channel  24  may be modified to accommodate more or fewer columns stacked adjacent to one another. 
       FIGS.  3   a - 3   d    are microscopic views of various shaped particles both in the absence of and presence of acoustic waves according to an embodiment of the present invention. In each figure, the left-hand side illustrates the particles in the absence of acoustic waves, while the right-side illustrates the particles in the presence of acoustic waves.  FIG.  3   a    illustrates spherical particles (i.e., non-anisotropic) having a diameter of approximately 20 µm;  FIG.  3   b    illustrates rod-like particles (i.e., anisotropic) having a length of approximately 50 µm and a diameter of approximately 10 µm;  FIG.  3   c    illustrates bow-tie shaped particles (i.e., anisotropic) having a height of approximately 80 µm, a width of approximately 20 µm, and a length of approximately 40 µm; and  FIG.  3   d    illustrates brick shaped particles (also anisotropic) having a height of approximately 50 µm, a length of approximately 20 µm, and a width of approximately 10 µm. 
     The embodiment shown in  FIG.  3   a    illustrates how application of acoustic waves cases the spherical particles to group together within the approximate center of the channel at the pressure node of the standing acoustic wave. Similarly, in the embodiment shown in  FIG.  3   b   , application of acoustic waves causes the cylindrical particles to align within the approximate center of the channel. However, because the cylindrical shaped particles are anisotropic, scattering forces cause the particles to snap end-to-end, so that they are aligned along the long axis of each cylindrical particle. Similarly, the bow-tie shaped particles illustrated in  FIG.  3   c    and the brick-shaped particles shown in  FIG.  3   d    - due to anisotropic attributes - are snapped end-to-end. The embodiments shown in  FIGS.  3   b - 3   d    therefore illustrate the ability not only to control the location of the particles within the channel, but also to control the orientation of those particles. This is particularly beneficial in applications in which directional attributes of the particles provide advantageous benefits, such as in 3D printing applications, discussed below. It should be understood that the dimensions described with respect to  FIGS.  3   a - 3   d    are exemplary, and other embodiments may utilize particles of different sizes and/or geometries. 
       FIGS.  4   a - 4   c    are cross-sectional views of acoustophoresis devices utilized to deliver anisotropic particles organized into a column according to exemplary embodiments of the present invention. Each of the embodiments shown in  FIGS.  4   a - 4   c    illustrate use of acoustophoresis devices to deliver anisotropic particles to a 3D printhead, although it will be recognized that in other embodiments the acoustophoresis devices shown in  FIGS.  4   a - 4   c    may be utilized in other applications. 
       FIG.  4   a    illustrates a single-stage acoustophoresis device,  FIG.  4   b    illustrates a two-stage acoustophoresis device, and  FIG.  4   c    illustrates a two-stage acoustophoresis device that further includes a UV curing window 
     In the embodiment shown in  FIG.  4   a   , single stage acoustophoresis device  30  includes three-dimensional printhead  31 , which is used to deposit material onto substrate  32  layer by layer, allowing a three-dimensional object to be constructed. An important attribute to 3D printing is the volume fraction of particles (i.e., concentration) that is delivered to the printhead. A common problem with particle delivery to printhead  31  is clogging of the printhead. As described below, the ability to locate particles within the center of the channel and orient the particles along their long axis allows a high volume fraction of particles to be delivered to the printhead. A benefit of focusing particles in an ordered manner is that it allows for particle concentrations that otherwise would result in clogging of the printhead. 
     In the embodiment shown in  FIG.  4   a   , acoustophoresis device  30  includes channel  34 , piezoelectric device  36 , outlet  38 , and waste outlet  40 . A colloidal solution that includes suspended, anisotropic particles is flowed into channel  34 . Acoustic waves generated by piezoelectric device  36  force the denser particles to the center of channel  34 , to the pressure node of the standing acoustic wave. Scattering forces act on the anisotropic particles to snap them end-to-end, resulting in the cylinder shaped particles aligning into a single column in the middle of channel  34 . As a result, anisotropic particles forced to the center of channel  34  are flowed into outlet  38 , which provides the aligned particles to printhead  31 . The received particles are deposited by printhead  31  onto substrate  32 . 
     The embodiment shown in  FIG.  4   a    takes advantage of the ability to control the location and orientation of the particles for purpose of increasing the density of particles directed to printhead  31 . As a result, rather than providing randomly distributed and oriented particles to printhead  31 , a single column of particles - each snapped end-to-end - is presented to printhead  31  via outlet  38 . When the orientation of the particles is not controlled, the randomly oriented particles have the effect of clogging printhead  31 . Controlling the orientation of the particles allows a higher concentration of particles to be provided to printhead  31 , while preventing clogging of the printhead. Excess solution is routed via waste outlets  40  to a waste collection/recycling system. 
     Although not shown in  FIG.  4   a   , in some embodiments additional steps are taken to ensure the particles - once ordered - remain that way when provided to printhead  31 . This may be particularly important if the distance between piezoelectric device  36  and printhead  31  is substantial. This may include photopolymerization or thermal curing of the solution/particles upon exiting the portion of the channel in which the acoustic waves are generated. Photopolymerization utilizes light (e.g., ultra-violet (UV) light) to cure the colloidal solution (e.g., hydrogel or photocurable epoxy type solution) in which the particles are suspended, thereby preserving the ordering of the particles. In one embodiment, a light guide coupled to a LED is utilized to illuminate the colloidal solution for a time period sufficient to provide the desired photocuring. In other embodiments, particle ordering may be promoted by adding macromolecules that promote particle cohesion. For example, in one embodiment described in more detail with respect to  FIG.  6   d    utilizes non-specific binding with DNA. This provides the desired cohesion, but still provides sufficient flexibility to allow subsequent manipulation of the particle pattern in the solution. In still other embodiments, ordering is preserved by utilizing a fluid with sufficient viscosity and elasticity to retain the shape of the solution during deposition. For example, in one embodiment, an epoxy-type fluid may be utilized that includes fillers to obtain the desired viscosity and elasticity during the deposition process. In addition, embodiments utilizes an epoxy-type fluid may benefit from thermal curing, wherein the ordered anisotropic particles suspended in the epoxy-type fluid are thermally cured prior to deposition to help maintain the ordering of the particles. 
     As described above, a benefit of providing ordered particles to printhead  31  is that it provides a greater concentration of particles to the printhead without causing clogging of the printhead. In addition, the ability to orient anisotropic particles being deposited on substrate  32  allows the product being built to take advantage of the anisotropic benefits of the particles. For example, the cylinder shaped particles shown in  FIG.  4    may exhibit greater strength in one direction than another, and by depositing them in a desired orientation the product being constructed may take advantage of the directional attributes of the particles. An object constructed with all particles oriented in a particular direction may therefore exhibit additional strength over that provided by the same particles randomly oriented. 
     In the embodiment shown in  FIG.  4   b   , acoustophoresis device  42  is a two-stage device - including first stage  44  and second stage  46 . In the embodiment shown in acoustophoresis device  42  includes channel  48 , first piezoelectric device  50 , waste outlets  52 , first outlet  54 , second piezoelectric device  56 , and second outlet  58 . As described with respect to  FIG.  4   a   , a colloidal solution that includes suspended, anisotropic particles is flowed into channel  48 . In first stage  44 , acoustic waves generated by piezoelectric device  50  force the denser particles to the center of channel  48 , to the pressure node of the standing acoustic wave. Scattering forces act on the anisotropic particles to snap them end-to-end, resulting in the cylinder shaped particles aligning into one or more columns (e.g., two columns in the example illustrated in  FIG.  4 B ). As a result, anisotropic particles forced to the center of channel  48  are flowed into first outlet  54 . Rather than be provided directly to a printhead, particles flowed to first outlet  54  are provided to second stage  46  for additional organizing, as discussed below. Excess solution is routed via waste outlets  52  to a waste collection/recycling system (not shown). 
     In addition, the embodiment shown in  FIG.  4   b    includes a second stage  46 , which includes second piezoelectric device  56 . Anisotropic particles provided by first outlet  54  are further organized by second piezoelectric device  56 . In one embodiment the organization of particles provided by first and second stages  44  and  46 , respectively, may be fine-tuned for different purposes. For example, in one embodiment first stage  44  is configured to provide filtering of particles for provision to first outlet  54  (i.e., to provide a high concentration of particles), while second stage  46  is configured to provide organization into a desired column formation (e.g., one-column, two columns, etc.) for provision to printhead  58 . As shown in  FIG.  4   b   , anisotropic particles organized in second stage  46  are provided to printhead  58  for deposition onto substrate  60 . 
     In the embodiment shown in  FIG.  4   c   , acoustophoresis device  62  may be described again as a two-stage device - including first stage  64  and second stage  66 . However, the embodiment shown in  FIG.  4   c    further includes a UV curing of the particles prior to deposition. 
     In the embodiment shown in  FIG.  4   c   , acoustophoresis device  62  includes channel  68 , first piezoelectric device  70 , waste outlets  72 , first outlet  74 , second piezoelectric device  76 , and second outlet  78 . As described with respect to  FIG.  4   a   , a colloidal solution that includes suspended, anisotropic particles is flowed into channel  68 . In first stage  64 , acoustic waves generated by piezoelectric device  70  force the denser particles to the center of channel  68 , to the pressure node of the standing acoustic wave. Scattering forces act on the anisotropic panicles to snap them end-to-end, resulting in the cylinder shaped particles aligning into one or more columns (e.g., two columns in the example illustrated in  FIG.  4   c   ). As a result, anisotropic particles forced to the center of channel  68  are flowed into first outlet  74 . Rather than be provided directly to a printhead, particles flowed to first outlet  74  are provided to second stage  66  for additional organizing, as discussed below. Excess solution is routed via waste outlets  72  to a waste collection/recycling system (not shown). 
     Anisotropic particles provided by first outlet  74  are further organized by second piezoelectric device  76 . Once again, the organization of particles provided by first and second stages  64  and  66 , respectively, may be fine-tuned for different purposes. Anisotropic particles organized by second piezoelectric device  76  are provided to second outlet  78 . In the embodiment shown in  FIG.  4   c   , anisotropic particles provided to outlet  78  are combined with lubricating fluid provided by input channels  79 . The combination of anisotropic particles and lubricating fluid are provided to ultra-violet (UV) curing window  82 , wherein UV light is provided to cure the anisotropic particles in their patterned or organized state. Once cured, the anisotropic particles remain aligned within the patterned configuration, and can be provided to printed head  86  for deposition onto substrate  88 . In other embodiments, rather than photocuring via curing window  82 , the ordered anisotropic particles may be thermally cured after being deposited onto substrate  88 . As discussed above, epoxy-type fluids may be utilized which can be thermally cured to retain the shape/ordering of the anisotropic particles. 
     A benefit of the 3D or additive techniques described with respect to  FIGS.  4   a - 4   c    is the ability to implement net-shape manufacturing techniques in which the shape of the printed product is very close to the final shape of the product. That is, the embodiments shown in  FIGS.  4   a - 4   c    does not require subsequent cutting or molding of the ordered particles. In addition, ordering of the particles allows the particles to be deposited with a desired orientation to provide additional strength/resiliency attributes to the finished product.  FIG.  5   a    is a microscopic view of anistropic particles organized into two or more columns according to exemplary embodiments of the present invention. Microscopic view  100   a  illustrates organization of the particles into two columns; microscopic view  100   b  illustrates organization of the particles into three columns, microscopic view  100   c  illustrates organization of the particles into four columns, microscopic view  100   d  illustrates organization of the particles into five columns, microscopic view  100   e  illustrates organization of the particles into six columns, and microscopic view  100   f  illustrates organization of the particles into seven columns. 
     The embodiments shown in  FIG.  5   a    illustrate the ability to effectively tune the organization of particles into multiple columns. The organization of particles in this manner is a result of competition between the primary focusing force and the secondary scattering forces. Scattering forces are attractive when the interacting particles are aligned parallel to the acoustic wave and repulsive when the particles are aligned perpendicular to the acoustic wave. As a result of the attractive scattering forces, anisotropic particles are snapped end-to-end in a given column (i.e., along the z axis). In contrast, the repulsive forces act to keep multiple assembled columns from collapsing into a single column. For example, microscopic view  100   a  illustrates two columns of aligned anisotropic particles. Primary forces act on the particles to push them toward the pressure node of the standing acoustic wave (in this embodiment, to a point located between the two columns). However, repulsive scattering forces, balancing the primary force, act to maintain the columns a distance d from one another. As discussed in more detail below, increasing the magnitude (i.e., power) of the acoustic wave increases the magnitude of the primary force, which at some point overcomes the magnitude of the secondary scattering force and causes the two columns to collapse into a single column. 
     The ability to tune the organization of particles into two or more columns is illustrated in microscopic views  100   a – 100   f . Through controlling the particle concentration, the anisotropic particles can be selectively ordered into a plurality of columns (seven in the embodiment shown in microscopic view  100   f ). Increasing the magnitude of the acoustic wave has the effect of collapsing the columns into adjacent columns. In addition, as illustrated in  FIGS.  5   a - 5   c   , the distance between adjacent columns is related to the total number of columns. As the number of columns increases, the distance between each of the columns decreases. 
     This concept is illustrated in schematic fashion by  FIG.  5   b   , which illustrates how the changing distance between columns is a function of number of columns according to an embodiment of the present invention. The bottom axis of the graph shown in  FIG.  5   b    is defined as the position along the channel divided by the width of the channel. Although this axis would extend from -1 to 1, only -0.1 to 0.1 is shown, as columns are created within this region. As discussed above with respect to  FIG.  5   a   , as the number of columns increases (toward the top of the graph), the distance between the columns decreases.  FIG.  5   b    also illustrates how the columns are centered around the center of the channel, presumably the location of the pressure node of the standing wave. 
       FIG.  5   c    is a chart that illustrates spacing between columns as a function of both particle type and number of columns. The y-axis describes the distance d between columns, while the x-axis describes the total number of columns N c . The chart in  FIG.  5   c    illustrates the difference in spacing between anisotropic particles (cylindrical shaped) and non-anisotropic particles (spherical). In this case, particles were chosen with similar particle volumes so that the magnitude of the primary acoustic radiation force is similar between different particle shapes.  FIG.  5   c    once again illustrates that as the number of columns increases, the spacing between each of the columns decreases. In addition, the example shown in  FIG.  5   c    illustrates that the spacing between columns of anisotropic particles are greater than the spacing between columns of non-anisotropic particles. For example, when anisotropic particles are divided into two columns, the distance between the columns is approximately 38 µm. When the non-anisotropic particles are divided into two columns, the distance between the columns is approximately 32 µm. As the number of columns increases, the distance between the columns decreases, but the spacing between anisotropic particles remains greater than the spacing between non-anisotropic particles. Thus, the embodiment shown in  FIG.  5   c    illustrates the effect particle size and shape has on spacing between columns. 
       FIGS.  6   a - 6   e    illustrate an additional aspect of organizing particles, in which rather than particles being configured in a single column, the columns are essentially stacked on top of one another in the y-direction (as shown in  FIG.  2   ). In addition,  FIGS.  6   a - 6   e    illustrate various methods of maintaining this stacked array configuration of particles via microscopic masonry according to various embodiments of the present invention. 
     The view shown in  FIGS.  6   a  and  6   b    represents a top view of the channel taken in the x-z plane (i.e., looking down into the channel). In particular, this embodiment shows that in the presence of acoustic waves, assuming a channel with sufficient height, a stacked array of particles can be formed with the stack extending out of the page in the y direction. In the embodiment shown in  FIG.  6   a   , a voltage of approximately 40 V pp  is applied to piezoelectric device, although in other embodiments the voltage may be increased/decreased as appropriate based on the pattern desired and the particles being manipulated. When the piezoelectric device is turned On, the particles are stacked into an array near the center of the channel. When the piezoelectric device is turned Off, gravity begins to pull on the stacked array of particles as shown in  FIG.  6   b   , as the stacked array turns sideways to expose how the particles are stacked on top of one another in an array format. The stacked array may also be described as a ribbon-like structure. Benefits of the configuration of particles shown in  FIG.  6   b    is the shared orientation of the particles as well as the close-packing of the particles in a micro-masonry configuration. Benefits of this configuration include added strength associated with the shared orientation and dense packing of the particles. In the embodiment shown in  FIGS.  6   a - 6   e   , the particles are anisotropic particles that are well-suited for micro-masonry configurations. 
     In this way,  FIGS.  6   a  and  6   b    illustrate another way in which anisotropic particles may be organized. Rather than being organized into one or more plurality of columns, the particles are organized into a stacked array of particles, forming a ribbon-like structure. Although the particles - once patterned - tend to retain their shape for at least a nominal amount of time, in some embodiments it may be required or beneficial to retain the desired pattern. In the embodiment shown in  FIG.  6   c   , a photopolymerization of the hydrogel solution in which the particles are suspended is utilized to preserve the desired pattern. As discussed previously, photopolymerization utilizes a hydrogel solution that can be photocured using application of light to the solution surrounding the ordered particles. In 3D printing applications, the rates of polymerization must be balanced with the desired transport properties of the suspension being provided to the print head. In other embodiments, rather than photopolymerization, ordering is preserved by utilizing a fluid with sufficient viscosity and elasticity to retain the shape of the solution during deposition. For example, in one embodiment, an epoxy-type fluid (rather than a hydrogel) may be utilized that includes fillers to obtain the desired viscosity and elasticity during the deposition process. In addition, the epoxy-type fluid may be thermally cured in order to further retain the ordering of anisotropic particles. 
     In the embodiment shown in  FIG.  6   d   , pattern stability is promoted by adding macromolecules that promote particle cohesion according to another embodiment of the present invention. In one embodiment, the array or pattern of particles is held together via non-specific binding with DNA. For example, in the embodiment shown in  FIG.  6   d    (including the magnified portion), the stacked array of rod-shaped particles (in this case, glass rods) are focused in a chaotropic solution with dispersed DNA. The chaotrope drives DNA onto the surfaces of the rod-shaped particles, creating cohesive interactions between a single DNA strand and adjacent rods in packed arrays. In one embodiment the DNA concentration utilized is sufficient to coat the surfaces of the particles as well as the surface of the channel walls. 
     In the embodiment shown in  FIG.  6   e   , a sequence of still shots is taken illustrating how by sweeping the frequency to set up acoustic standing waves alternating from the x direction to the z direction can be used to fold the ribbon-like structure (shown in frames  102   a - 102   i ) multiple times onto itself, resulting in an ordered three-dimensional array. The embodiment illustrated in  FIG.  6   e    utilizes DNA dispersed within the solution to promote particle adhesion as described with respect to  FIG.  6   d   . 
     In first frame  102   a  the ribbon-like array of stacked particles are aligned in a single direction (i.e., parallel to the z-axis). In frames  102   b – 102   d , the direction of the acoustic wave generated by the piezoelectric device is modified such that particles - as they exist in the ribbon like structure - are driven to a position parallel with the x-axis as shown in frame  102   d . In this way, the ribbon-like structure is folded over on itself. In frames  102   e – 102   i  the direction of the acoustic waves generated by the piezoelectric device is returned to the original orientation such that the ribbon-like structure is again folded onto itself and the particles are once again aligned parallel to the z-direction. As a result of multiple folding operations, the initially two-dimensional ribbon (such as that shown in  FIG.  6   a   ) is folded onto itself a number of times, thereby creating a three-dimensional shape. 
       FIGS.  7   a - 7   d    illustrate separation/filtering results for anisotropic and non-anisotropic particles of various sizes by an acoustophoresis device according to embodiments of the present invention.  FIGS.  7   a  and  7   b    illustrate separation efficiencies for sphere-shaped particles (i.e., non-anisotropic) of different concentrations in suspension - 0.1 and 0.5 nominal wt%, respectively - at excitation voltages of 5 V pp , 10 V pp , and 15 V pp , and at flow rates of 10, 50, and 150 µL/min. Likewise,  FIGS.  7   c  and  7   d    illustrate separation efficiencies for micro-rod shaped particles (i.e., anisotropic particles) of different sizes - 0.1 and 0.5 nominal wt%, respectively - at excitation voltages of 5 V pp , 10 V pp , and 15 V pp , and at flow rates of 10, 50, 150, 300 and 600 µL/min. In each figure, separation of the particles at an excitation voltage of 5 V pp  is illustrated by a circle, 10 V pp  is illustrated by a square, and 15 V pp  is illustrated by a diamond. Flow rates are denoted along the x-axis, while separation efficiencies are denoted along the y-axis. Particle sizes were selected by volume to be approximately equal in order to ascertain the effect of particle shape on the separation efficiency achieved at each voltage. 
       FIGS.  7   a  and  7   b    illustrate separation efficiencies for micro-spheres. In particular, at low flow rates (i.e., 10 µL/min), the separation efficiency is approximately the same regardless of the excitation voltage applied. In general, as the flow rate increases the separation efficiency of the micro-spheres decreases. In addition, at higher flow rates the excitation voltage applied has a greater effect on the separation efficiency achieved. 
       FIGS.  7   c  and  7   d    illustrate separation efficiencies for micro-rods. In contrast with the microspheres, the micro-rods illustrate increased separation efficiency at increased flow rates (e.g., as the flow rate increases from 10 µL/min to 300 µL/min). Only at the very highest flow rates, when the flow rate approaches 600 µL/min does the separation efficiency of the micro-rods begin to decrease. This indicates that the anisotropic effect of the micro-rods improves separation efficiency in the presence of flow. 
       FIGS.  8   a - 8   c    illustrate the relationship between voltage applied to the piezoelectric device and formation of one or more columns according to an embodiment of the present invention. In  FIG.  8   a   , an excitation voltage of 20 V pp  is applied to piezoelectric device. The resulting balance of primary forces and secondary scattering forces result in the particles being organized into three parallel columns. That is, at this excitation, the scattering force between the various columns balances the primary force attempting to collapse the columns to the center (i.e., to the pressure node of the acoustic wave). In  FIG.  8   b   , the excitation voltage is increased to 24 V pp . The increase in excitation voltage results in an increase in the magnitude of the acoustic wave provided to the channel, and as a result an increase in the primary forces applied to the particles. In response, two of the columns are collapsed into a single column, resulting in a total of two columns. That is, at an excitation voltage of 24 V pp , primary forces and secondary scattering forces are balanced with two columns. Similarly, when the excitation voltage is increased from 24 V pp  to 41 V pp , the primary force once again outweighs the secondary scattering forces and causes the two columns shown in  FIG.  8   b    to collapse into a single column. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.