as used herein , the expression “ acoustic energy ” refers to focused continuous and pulse acoustic energy with frequency typically higher that 20 khz and less than 100 mhz . the wavelength corresponding to 20 khz is about 7 cm and 100 mhz is about 14 . 5 micrometers in aqueous medium . as used herein , the expressions “ standing wave ”, “ stationary wave ”, and the like mean a non - propagating wave that is generated by the interference of two waves moving in opposite directions . the frequencies of two waves must be the same to produce standing waves . standing waves have nodes and anti - nodes . at nodes , the displacement is zero , and at antinodes the displacement is maximum . see , for example , the discussion of standing wave at the web site http :// en . wikipedia . org / wiki / standing_wave . as used herein , the term “ node ” means the places where the medium does not move . as used herein , the term “ anti - node ” means the places where the medium experiences maximal vibration . fig1 shows an acoustic standing wave 10 having nodes 12 a , 12 b , 12 c , and 12 d , and anti - nodes 14 a , 14 b , and 14 c . fig2 a and 2b show separation of particles by means of acoustic radiation forces . in fig2 a , the particles 20 have not been separated . in fig2 b , the particles 20 have been concentrated at the node 22 and antinodes 24 a , 24 b . acoustic radiation forces urge the particles toward the node 22 , in accordance with the acoustic radiation force equation set forth previously . as used herein , the expressions “ ultrasonic transducer ”, “ acoustic energy transducer ” mean a device that converts electrical energy to mechanical energy , in the form of sound , and vice versa . ultrasonic transducers are further described in panametrics - ndt ™ ultrasonic transducers brochure 920 - 041c - en , 2008 , olympus ndt , waltham , mass ., pages 1 - 49 , incorporated herein by reference . as used herein , the expression “ delay line transducer ” means a single element longitudinal wave transducer used in conjunction with a replaceable delay line . the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves , thereby allowing the transducer to complete its “ sending ” function before it starts its “ listening ” function so that near surface resolution is improved . the delay allows the element of the transducer to stop vibrating before a return signal from a reflector can be received . the acoustic field generated by a transducer can be divided by two zones . the zones are characterized on the basis of their distance from the face of the transducer . the near field is the zone closer to the transducer , and the near field exhibits significant variations . the far field is defined as a zone starting with the end of the near field distal from the transducer , and the far field is stable and predictable . the delay line allows only the stable far field to be used . the key feature of the delay line is that the material selected needs to be transparent to the sound field , or to be minimally reflective of sound . as used herein , the expression “ acoustic impedance ” means product of the density of a material and acoustic velocity . at the interface of two different materials , the difference in acoustic impedances determines acoustic transmission and reflection at the interface . as used herein , the expression “ su - 8 photoresist ” means a negative photoresist . it is a very viscous polymer that can be spun or spread over a thickness ranging from one micrometer up to two millimeters and still be processed with standard mask aligner . su - 8 is mainly used for fabrication of devices for microfluidics . as used herein , the expressions “ micro mold ”, “ micro molding ”, and the like , refer to tooling and the use thereof for producing millimeter - sized and micrometer - sized parts for various fields , such as , for example , biomedical , pharmaceutical , electronics , microfluidics . see , for example , micro molds make micro parts , plastics technology , december 2002 , http :// www . pyonline . com / articles / 200212fa1 . html , incorporated herein by reference . as used herein , the expression “ soft lithography ” refers to a family of techniques for fabricating or replicating structures using “ elastomeric stamps , molds , and conformable photomasks ”. soft lithography uses elastomeric materials , such as , for example polydimethylsiloxane ( pdms ). soft lithography is generally used to construct features measured on the micrometer to nanometer scale . as used herein , the term “ photoresist ” means a light - sensitive material used in several industrial processes , such as photolithography and photoengraving to form a patterned coating on a surface . a positive photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer and the portion of the photoresist that is unexposed remains insoluble to the photoresist developer . a negative photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes relatively insoluble to the photoresist developer and the portion of the photoresist that is unexposed is dissolved by the photoresist developer . as used herein , the term “ micromachining ” means a process used to produce micromachinery or microelectromechanical systems . bulk micromachining produces structures inside a substrate . as used herein , the expression “ function generator ” means a piece of electronic test equipment or software used to generate electrical waveforms . these waveforms can be either repetitive , or single - shot , in which case some kind of triggering source is required . as used herein , the expression “ re power amplifier ” means a type of electronic amplifier used to convert a low - power radio - frequency signal into a larger signal of significant power , typically for driving the antenna of a transmitter . it is usually optimized to have high efficiency , high p1 db compression , good return loss on the input and output , good gain , and good heat dissipation . as used herein , the term “ particle ”, includes , but is not limited to , particulate material in whole blood , such as , for example , red blood cells , white blood cells , and platelets . insofar as possible , like reference numerals will be used to designate like components . a schematic diagram of the components that are used to generate acoustic standing waves is shown in fig3 . in fig3 , a function / waveform generator 100 provides a waveform or a standard function , e . g ., a sine wave , a square wave , to a rf ( radio frequency ) power amplifier 102 , which in turn provides amplification of the signal from the function / waveform generator 100 to an ultrasonic transducer 104 . a representative example of a function / waveform generator 100 suitable for use herein is agilent 33210a function / waveform generator , commercially available from agilent technologies . a representative example of a rf power amplifier 102 is intraaction model pa - 4 rf power amplifier , commercially available from intraaction corp ., bellwood , ill . a representative example of an ultrasonic transducer 104 is olympus accupath wedge 45 ° transducer , commercially available from olympus ndt , waltham , mass . an ultrasonic transducer 104 can be custom - made by assembling components that are commercially available from fuji ceramics corporation , yamamia 2320 - 11 , fujinomiya - shi , shizuoka - pref ., japan . in the embodiment shown in fig4 , 5 , 6 , and 7 , an apparatus 200 for carrying out a batch process for effecting separation of blood cells from blood plasma or blood serum in a test tube or plurality of test tubes is shown . a layer 202 having at least one receptacle 204 for at least one test tube 206 functions as a delay line . the layer 202 is in face - to - face contact with a layer 208 . the layer 208 functions as an acoustic energy transducer . the acoustic energy transducer 208 receives the appropriate function or waveform from the function / waveform generator 100 , amplified by the rf ( radio frequency ) power amplifier 102 , as shown in fig3 . the at least one receptacle 204 in the layer 202 has a shape that is congruent with the lower portion 206 a of the at least one test tube 206 . requirements and preferred features of a delay line are set forth in u . s . pat . no . 3 , 832 , 655 , incorporated herein by reference . requirements and preferred features of acoustic energy transducers are set forth in panametrics - ndt ™ ultrasonic transducers brochure 920 - 041c - en , olympus ndt , 2008 , waltham , mass ., pages 1 - 49 , previously incorporated herein by reference . the frequency can range from 20 khz to 100 khz . the frequency selected is based on the heights of the test tubes . a typical “ vacutainer ” test tube has the dimensions 13 mm ( diameter )× 75 mm ( length ). the frequency of 20 khz generates an acoustic wave having a wavelength of 7 cm . the speed of sound in water is approximately 1450 m / sec . the acoustic node of the standing wave is placed at a location near the bottom of the test tube . the forces of the moving blood cells are acoustic , the primary force , and gravitational , the secondary force . the air serves as a reflective medium at the interface of blood and air , because of the large difference in acoustic impedance between air ( close to zero ) and blood ( 1 . 483 × 10 5 g / cm 2 sec ). prior to the acoustic separation process , blood cells “ bc ” are substantially uniformly dispersed throughout the liquid in the test tube 206 , as shown in fig6 . after the acoustic separation process , blood cells “ bc ” are concentrated in the lower layer 206 a of the test tube 206 and plasma or serum “ p / s ” is presented in the upper layer 206 b of the test tube 206 , as shown in fig7 . the upper layer can then be removed from the test tube by manual aspiration , as by a pipette , or by robotic aspiration , as by a robotic pipette , for subsequent processes . fig8 , 9 , 10 a , 10 b , 11 a , 11 b , 11 c , 11 d , 12 , 13 , 14 , 15 , 16 , and 17 illustrate embodiments of articles that can be used to carry out a continuous process for effecting the separation of blood cells from blood plasma or blood serum in a flow channel . the flow channel is constructed with dimensions that allow the flow of the sample of blood to be laminar . fig8 and 9 illustrate how a flow channel can be constructed by combining three layers of material appropriate for preparing the apparatus described herein , wherein the two exterior layers form boundaries of the flow channel and the flow channel itself is formed in the interior layer . fig1 a illustrates a flow channel that employs a set of custom - designed ultrasonic transducers , which are indicated by triangular cross - sections . fig1 b illustrates a flow channel that employs a set of commercially available ultrasonic transducers , which are indicated by rectangular cross - sections . fig1 a illustrates acoustic standing waves generated by the ultrasonic transducers shown in fig1 a . fig1 b illustrates acoustic standing waves generated by the ultrasonic transducers shown in fig1 b , fig1 c illustrates particles , e . g ., blood cells and platelets , flowing parallel to the walls of the flow channel of fig1 a . fig1 d illustrates particles , e . g ., blood cells and platelets , flowing parallel to the walls of the flow channel of fig1 b . fig1 , 13 , 14 , and 15 illustrate various embodiments of inlet channels leading into and outlet channels leading out of the flow channel described herein . fig1 and 17 illustrate alternative means for removing blood cells from flow channels . fig8 shows an article 300 having a flow channel suitable for use for the method described herein . fig9 shows an exploded view of the article 300 shown in fig8 . referring now to fig8 and 9 , the article 300 comprises a first exterior layer 302 , a second exterior layer 304 , and an interior layer 306 . one major surface 306 a of the interior layer 306 is in face - to - face contact with one major surface 302 a of the first exterior layer 302 and the other major surface 306 b of the interior layer 306 is in face - to - face contact with one major surface 304 a of the second exterior layer 304 . a flow channel 308 is an aperture cut through the interior layer 306 . as shown in fig8 and 9 , the major surface 302 a of the first exterior layer 302 and the major surface 304 a of the second exterior layer 304 are spaced apart by means of the interior layer 306 to form a flow channel 308 . if the flow channel is merely cut into a major surface of the interior layer 306 , the first exterior layers 302 or the second exterior layer 304 can be dispensed with . the layers 302 and 304 also function as delay lines . the delay lines provide a means for removing unstable near field acoustic effects . the sound field of an ultrasonic transducer is divided into two zones , the near field and the far field . the near field is the region directly in front of the ultrasonic transducer where the echo amplitude goes through a series of maxima and minima and ends at the last maximum , at a distance n from the ultrasonic transducer . the location of the last maximum is known as the near field distance and is the natural focus of the ultrasonic transducer . the far field is the area beyond n where the sound field pressure gradually drops to zero . the near field effect is a phenomenon resulting from the geometry of the ultrasonic transducer . the sound that emanates from an ultrasonic transducer does not originate from a point , but rather from most of the surface of the ultrasonic transducer . sound waves from the surface of the ultrasonic transducer create diffraction effects on account of interference of the sound waves . extensive fluctuation in the intensity of the sound near the source of the sound waves is referred to as near field effects . near field effects are discussed in greater detail in panametrics - ndt ™ ultrasonic transducers brochure 920 - 041c - en , 2008 , olympus ndt , waltham , mass ., pages 1 - 49 , previously incorporated herein by reference . the flow channel 308 is preferably elongated , in order to provide a satisfactory rate of flow with a small volume of the sample . representative examples of the dimensions of the flow channel 308 can range from about 30 mm to about 60 mm in length ( i . e ., the direction of flow of the fluid ), from about 70 μm to about 700 μm , preferably from about 100 μm to about 200 μm , in width ( i . e ., the direction perpendicular to the walls of the flow channel upon which the ultrasonic transducers are mounted ), and from about 1000 μm to about 5000 μm in depth ( i . e ., the direction parallel to the walls of the flow channel upon which the ultrasonic transducers are mounted ). the depth is selected to be of such a magnitude as to minimize the effect of gravitational force on the blood cells &# 39 ; and the platelets . the selection of the width of the flow channel 308 is based upon the frequency selected . the frequency and width of the flow channel 308 are selected in such a manner as to drive the blood cells to the center of the flow channel 308 . for example , a frequency of approximately 7 . 5 mhz calls for a width of the flow channel 308 of approximately 100 μm , and a frequency of approximately 3 . 5 mhz calls for a width of the flow channel 308 of approximately 200 μm . the major surface 302 a of the first exterior layer 302 can be joined to the major surface 306 a of the interior layer 306 by means of adhesives or anodic bonding . the major surface 304 a of the second exterior layer 304 can be joined to the major surface 306 b of the interior layer 306 by means of adhesives or anodic bonding . the distance between the major surface 302 a of the first exterior layer 302 and the major surface 304 a of the second exterior layer 304 , which constitutes the width of the flow channel 308 , can range from about 70 μm to about 700 μm , preferably from about 100 μm to about 200 μm . the perpendicular distance between the edges of the flow channel 308 that are parallel to the direction of the flow of the fluid in the flow channel 308 is preferably greater than about 1000 μm , in order to provide an adequate level of throughput . the length of the flow channel is preferably greater than about 30 mm , in order to provide an adequate level of throughput . a plurality of ultrasonic transducers 310 a , 310 b , and 310 c are coupled with the layer 302 and a plurality of ultrasonic transducers 312 a , 312 b , and 312 c are coupled with the layer 304 . the ultrasonic transducers 310 a , 310 b , 310 c , 312 a , 312 b , and 312 c are arranged so that the incident angle of the wave relative to the first exterior layer 302 and the second exterior layer 304 is approximately 45 °. additional ultrasonic transducers ( not shown ) can be used . it is also possible to use fewer ultrasonic transducers . as shown in fig8 , 10 a , 11 a , and 11 c , the ultrasonic transducers 310 a , 310 b , 310 c , 312 a , 312 b , and 312 c , which have triangular cross - sections , are custom - designed ultrasonic transducers . as shown in fig1 b , 11 b , and 11 d , the ultrasonic transducers 310 a ′, 310 b ′, 310 c ′, 310 d ′, 310 e ′, 310 f ′, 312 a ′, 312 b ′, 312 c ′, 312 d ′, 312 e ′, and 312 f ′, which have rectangular cross - sections , are commercially available ultrasonic transducers . the arrangement of the ultrasonic transducers 310 a , 310 b , 310 c , 312 a , 312 b , and 312 c enables refraction and reflection of sound waves and the formation of standing waves 330 a , 330 b , 330 c , 330 d with nodes 332 a and 332 b at the center of the flow channel 308 . the arrangement of the ultrasonic transducers 310 a ′, 310 b ′, 310 c ′, 310 d ′, 310 e ′, 310 f ′, 312 a ′, 312 b ′, 312 c ′, 312 d ′, 312 e ′, and 312 f ′ enables refraction and reflection of sound waves and the formation of standing waves 330 a ′, 330 b ′, 330 c ′, 330 d ′, 330 e ′, and 330 f ′ with nodes 332 a ′, 332 b ′, 332 c ′ at the center of the flow channel 308 . when a sample of whole blood flows through the flow channel 308 in the direction indicated by the arrow “ a ”, blood cells , indicated by “ bc ” are concentrated in a central zone 334 of the flow channel 308 by acoustic energy . fig1 , 13 , 14 , and 15 illustrate various inlet ports and inlet channels by means of which a sample of whole blood can be introduced into the flow channel 308 along with various outlet ports and outlet channels by means of which blood cells and blood plasma or blood serum can be removed from the flow channel 308 . it should be noted that for the sake of simplification , the ultrasonic transducers are not shown in fig1 , 13 , 14 , and 15 . in fig1 , 13 , 14 , and 15 , the parallel , hatched lines represent the fluid in the flow channel 308 . in fig1 , a first inlet port 414 and a second inlet port 416 communicate with the flow channel 308 by means of inlet channels 418 a , 418 b , and 418 c . the inlet channel 418 c is formed in the second exterior layer 304 . a sample of whole blood can be introduced into the flow channel 308 by introducing the sample into the first inlet port 414 , the second inlet port 416 , or both inlet ports 414 and 416 . buffers can also be introduced into the flow channel 308 by introducing the buffers into the first inlet port 414 , the second inlet port 416 , or both inlet ports 414 and 416 . a first outlet channel 420 is formed in the second exterior layer 304 . the first outlet channel 420 enables removal of blood cells from the flow channel 308 by means of a first outlet port 422 . a second outlet channel 424 is formed in the first exterior layer 302 . the second outlet channel 424 enables removal of blood plasma or blood serum from the flow channel 308 by means of a second outlet port 426 . in fig1 , it can be seen that the inlet channels 418 a , 418 b , and 418 c and the outlet channels 420 and 424 extend beyond the outermost faces of the first exterior layer 302 and the second exterior layer 304 . in fig1 , 14 , and 15 , the inlet channels and the outlet channels terminate at the outermost faces of the first exterior layer 302 and the second exterior layer 304 . in fig1 , an inlet port 514 communicates with the flow channel 308 by means of an inlet channel 516 . the inlet channel 516 is formed in the second exterior layer 304 . a sample of whole blood can be introduced into the flow channel 308 by introducing the sample into the inlet port 514 . buffers can also be introduced into the flow channel 308 by introducing the buffers into the inlet port 514 . a first outlet channel 520 is formed in the second exterior layer 304 . the first outlet channel 520 enables removal of blood plasma or blood serum from the flow channel 308 by means of a first outlet port 522 . a second outlet channel 524 is formed in the first exterior layer 302 . the second outlet channel 524 enables removal of blood plasma or blood serum from the flow channel 308 by means of a second outlet port 526 . a third outlet channel 528 is formed in the first exterior layer 302 . the third outlet channel 528 enables removal of blood cells and platelets from the flow channel 308 by means of a third outlet port 530 . in fig1 , a first inlet port 614 communicates with the flow channel 308 by means of a first inlet channel 616 . the first inlet channel 616 is formed in the second exterior layer 304 . a second inlet port 618 communicates with the flow channel 308 by means of a second inlet channel 620 . the second inlet channel 620 is formed in the first exterior layer 302 . a sample of whole blood can be introduced into the flow channel 308 by introducing the sample into the first inlet port 614 , the second inlet port 618 , or both inlet ports 614 and 618 . buffers can also be introduced into the flow channel 308 by introducing the buffers into the first inlet port 614 , the second inlet port 618 , or both inlet ports 614 and 618 . a first outlet channel 622 is formed in the first exterior layer 302 . the first outlet channel 622 enables removal of blood plasma or blood serum from the flow channel 308 by means of a first outlet port 624 . a second outlet channel 626 is formed in the first exterior layer 302 . the second outlet channel 626 enables removal of blood plasma or blood serum from the flow channel 308 by means of a second outlet port 628 . in fig1 , an inlet port 714 communicates with the flow channel 308 by means of an inlet channel 716 . the inlet channel 716 is formed in the second exterior layer 304 . a sample of whole blood can be introduced into the flow channel 308 by introducing the sample into the inlet port 714 . buffers can also be introduced into the flow channel 308 by introducing the buffers into the inlet port 714 . a first outlet channel 720 is formed in the second exterior layer 304 . the first outlet channel 720 enables removal of blood plasma or blood serum from the flow channel 308 by means of and the first outlet port 722 . a second outlet channel 724 is formed in the first exterior layer 302 . the second outlet channel 724 enables removal of blood plasma or blood serum from the flow channel 308 by means of a second outlet port 726 . a third outlet channel 728 is formed in the first exterior layer 302 . the third outlet channel 728 enables removal of blood cells and platelets from the flow channel 308 by means of a third outlet port 730 . the width of the flow channel 308 in fig1 increases between the first inlet channel 716 and both the first outlet channel 720 and the second outlet channel 724 . the width of the flow channel 308 in fig1 decreases between the third outlet channel 728 and both the first outlet channel 720 and the second outlet channel 724 . requirements and preferred features of a delay line are set forth in u . s . pat . no . 3 , 832 , 655 , incorporated herein by reference . requirements and preferred features of acoustic energy transducers are set forth in panametrics - ndt ™ ultrasonic transducers brochure 920 - 041c - en , 2008 , olympus ndt , waltham , mass ., pages 1 - 49 , previously incorporated herein by reference . the frequency can range from about 1 mhz to about 10 mhz , preferably from about 3 mhz to about 6 mhz . at a collection zone 336 of the flow channel 308 , the width of the flow channel 308 is expanded gradually to maintain laminar flow of the sample and avoid turbulence . the expanded width of the collection zone 336 facilitates the collection of blood cells and the collection of serum and plasma . as shown in fig1 , blood cells can be removed by inserting a conduit 338 at the central zone 340 of the flow channel 308 in the center of the collection zone 336 . alternatively , as shown in fig1 , blood cells can be removed by creating a central outlet 342 at the distal end 344 of the flow channel 308 . the blood plasma or the blood serum can be removed through the outlets 346 and 348 , which flank the central outlet 342 . the plurality of ultrasonic transducers 310 a , 310 b , 310 c , 312 a , 312 b , 312 c along the flow channel 308 should be oriented so that the gravitational forces do not adversely affect the separation of blood cells from blood plasma or blood serum . the apparatus described herein and the components of the apparatus described herein can be formed from a variety of sub - components . samples and buffer solutions can be moved though the inlet flow channels and the outlet flow channels illustrated in fig1 , 13 , 14 , and 15 and the flow channel 308 by means of positive displacement pumps , such as , for example , syringe pumps , peristaltic pumps . the flow channel 308 can be formed in the interior layer 306 by any of several processes . in one process , a silicon wafer can be used for preparing the interior layer 306 . the flow channel 308 can be formed in the silicon wafer by micromachining , anisotropic wet etching , or by deep reactive ion etching . inlet channels , inlet ports , outlet channels , and outlet ports or recesses for receiving inlet channels , inlet ports , outlet channels , and outlet ports can also be formed in silicon wafers by micromachining , anisotropic wet etching , or by deep reactive ion etching . micromachining can be carried out by any of the methods described in the article located at the web site http :// en . wikipedia . orq / wiki / bulk_micromachining , incorporated herein by reference . anisotropic wet etching can be performed with potassium hydroxide or edp solution , which is a mixture of ethylene diamine and pyrocatechol . the portion of the silicon wafer that is not to be etched is masked by a photoresist , which has been applied in a desired pattern by a photolithographic process . a negative photoresist suitable for use is epoxy - based su - 8 , commercially available from microchem , newton , mass . deep reactive ion etching can be carried out by plasma etching . in another process , the flow channel 308 can be micro - molded by means of a high melt flow thermoplastic resin , wherein the melt flow index is equal to or greater than 20 . resins suitable for this technique include , but are not limited to , acrylic resins , polystyrene , polycarbonate , cyclo - olefin copolymers , and polyvinyl chloride . in still another process , the flow channel 308 can be prepared by soft lithography , wherein a negative photoresist is employed and the pattern stamp is formed with polydimethylsiloxane . a commercially available polydimethylsiloxane suitable for use in this process is “ sylgard ” 184 silicone elastomer , dow corning corporation . the first exterior layer 302 and the second exterior layer 304 are preferably made of optically and acoustically transparent material . such materials include , but are not limited to , ( a ) glass , such as , for example , polysilicate glass , e . g ., “ pyrex ” glass , “ pyrex ” 7740 glass , ( b ) silicon wafer , and ( c ) polymeric materials , such as , for example , acrylic resins , polystyrene , polycarbonate , cyclo - olefin copolymers , and polyvinyl chloride . the major surface 302 a of the first exterior layer 302 can be bonded to the major surface 306 a of the interior layer 306 and the major surface 304 a of the second exterior layer 304 can be bonded to the major surface 306 b of the interior layer 306 by means of adhesives , such as , for example , two - part epoxy adhesives , two - part urethane adhesives . the major surface 302 a of the first exterior layer 302 can be bonded to the major surface 306 a of the interior layer 306 and the major surface 304 a of the second exterior layer 304 can be bonded to the major surface 306 b of the interior layer 306 by means of anodic bonding , which requires high voltage , e . g ., greater than 1000 volts , and high temperature , greater than 250 ° c . a commercially available power supply is dc high voltage power supply , model ps310 , commercially available from stanford research systems . the inlet ports and the outlet ports can be machined by means of screw threads , after which machining process port adapters can be inserted in the ports . tubing connections can be effected by means of glue , interference fit , or solvent bonding . solvent bonding is preferred for polyvinyl chloride tubing . materials that are suitable for tubing include , but are not limited to , polyvinyl chloride , silicone , polytetrafluoroethylene , ultra low density polyethylene . separation efficiency depends on the particle size , the acoustic frequency , and the energy density of the acoustic standing waves . commercially available piezoelectric elements , such as pzt ( lead zirconate titanate ), can be used to generate acoustic waves . the dimensions of the flow channel 308 need to be adjusted to facilitate laminar flow . optimal separation can be achieved by adjusting one half of the wavelength to be the same width of the flow channel 308 , thereby allowing for the node to be aligned in the center of the flow channel 308 and antinodes to be adjacent to the walls of the flow channel , whereby cells accumulate at the center of the flow channel 308 . alternatively , optimal separation can be achieved by adjusting quarter wavelength with a node adjacent to one wall of the flow channel and an antinode adjacent to the opposing wall of the flow channel , whereby cells accumulate at the walls of the channel . for a continuous separation process , the positions of the nodes and antinodes of acoustic standing waves determine the location ( s ) where blood cells , platelets , and blood plasma or blood serum can be collected . the frequency required is in the ultrasound domain (& gt ; 20 , 000 hz ), so the wavelength is matched with the dimensions of the flow channel 304 . the plurality of ultrasonic transducers is preferably oriented with respect to the flow channel so that gravitational forces do not adversely affect the separation . see fig8 . removal of blood cells can be facilitated by the addition of a buffer , e . g ., phosphate buffered saline , to the sample of whole blood . laminar flow prevents intermixing of fluid streams , whereby blood plasma or blood serum can be collected at a location different from that where blood cells are collected . ultrasonic transducers are commonly manufactured with piezoelectric materials , such as lead zirconate titanate ( pzt ) or polyvinylidene fluoride , or magnetorestrictive materials , such as rare earth alloy , terfenol - d . the apparatus and methods described herein have numerous benefits . the apparatus described herein requires a smaller amount of space than do other types of separating equipment . the apparatus described herein costs less than do other types of separating equipment , such as , for example , centrifuges . moreover , the cost of maintenance of the apparatus described herein is much lower than that of other types of separating equipment , such as , for example , centrifuges . in order to operate the apparatus shown in fig4 , 5 , 6 , and 7 , a sample of whole blood is introduced into a test tube 206 . prior to the acoustic separation process , blood cells “ bc ” are substantially uniformly dispersed throughout the liquid in the test tube 206 , as shown in fig6 . a standing acoustic wave is then generated in such a manner by the function / waveform generator 100 , the rf ( radio frequency ) power amplifier 102 , and the ultrasonic transducer assembly , which comprises the delay line 202 and the acoustic energy transducer 208 , that the acoustic node of the standing wave is placed at a location near the bottom of the test tube . the forces of the moving blood cells are acoustic , the primary force , and gravitational , the secondary force . air serves as a reflective medium at the interface of blood and air , because large differences of acoustic impedances between air and the sample of blood , because large differences of acoustic impedances between air ( close to zero ) and blood ( 1 . 483 × 10 5 g / cm 2 sec ). after the acoustic separation process , blood cells “ bc ” are concentrated in the lower layer 206 a of the test tube 206 and plasma or serum “ p / s ” is presented in the upper layer 206 b of the test tube 206 , as shown in fig7 . the upper layer can then be removed from the test tube by manual aspiration , as by a pipette , or by robotic aspiration , as by a robotic pipette , for subsequent processes . in order to operate the apparatus shown in fig8 , 9 , 10 a , 10 b , 11 a , 11 b , 11 c , 11 d , 12 , 13 , 14 , 15 , 16 , and 17 , a sample of whole blood is introduced into the flow channel 308 , typically by means of an inlet port and an inlet channel . standing acoustic waves are then generated in such a manner by the function / waveform generator 100 , the rf ( radio frequency ) power amplifier 102 , the ultrasonic transducers 310 a , 310 b , 310 c , 310 a ′, 310 b ′, 310 c ′, 310 d ′, 310 e ′, 310 f ′, 312 a 312 b , 312 c , 312 a ′, 312 b ′, 312 c ′, 312 d ′, 312 e ′, and 312 f ′ that the nodes are formed in the center of the flow channel 308 , i . e ., equidistant from the walls , i . e ., the first exterior layer 302 and the second exterior layer 304 of the flow channel 308 . the red blood cells , the white blood cells , and the platelets , i . e ., “ bc ”, are concentrated at the nodes that are formed in the center of the flow channel 308 as the sample flows through the flow channel 308 . the red blood cells , the white blood cells , and the platelets , i . e ., “ bc ”, can be removed by one outlet channel or by one conduit , and the blood plasma or the blood serum can be removed by at least one other outlet channel or by at least one other conduit . various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention , and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein .
a method for separating , or removing , particulate material , e . g ., blood cells , from a sample of fluid , e . g ., whole blood of a patient , in which the particulate material is suspended . in the case of separating blood cells from blood plasma or blood serum , the resulting samples of blood plasma or blood serum can be used for in vitro diagnostic applications . in normal practice , a whole blood sample of a patient are provided and then introduced into an apparatus that contains a flow channel . an acoustic field , which contains acoustic standing waves from external ultrasonic transducers , is located within the flow channel . laminar flow is maintained in the flow channel . blood cells and platelets are separated from blood plasma or blood serum at the end of the flow channel and collected . the method described herein allows fluid components to differentially migrate to areas of preferred acoustic interaction . the parameters that affect separation of particles are size , density , compressibility of the particles , and the fluid surrounding the particles .