Patent Grant
9416344
The field of biotechnology has grown tremendously in the last 20 years. This growth has been due to many factors, some of which include the improvements in the equipment available for bioreactors, the increased understanding of biological systems and increased knowledge as to the interactions of materials (such as monoclonal antibodies and recombinant proteins) with the various systems of the human body.
Improvements in equipment have allowed for larger volumes and lower cost for the production of biologically derived materials such as recombinant proteins. This is especially prevalent in the area of pharmaceuticals, where the successes of many types of new drug therapies have been directly due to the ability to mass produce these materials through protein-based manufacturing methods.
One of the key components that is utilized in the manufacturing processes of new biologically based pharmaceuticals is the bioreactor and the ancillary processes associated therewith. An area of growth in the bioreactor field has been with the perfusion process. The perfusion process is distinguished from the fed-batch process by its lower capital cost and higher throughput.
In the fed-batch process, a culture is seeded in a bioreactor. The gradual addition of a fresh volume of selected nutrients during the growth cycle is used to improve productivity and growth. The product is recovered after the culture is harvested. The fed batch bioreactor process has been attractive because of its simplicity and also due to carryover from well-known fermentation processes. However, a fed-batch bioreactor has high start-up costs, and generally has a large volume to obtain a cost-effective amount of product at the end of the growth cycle. After the batch is completed, the bioreactor must be cleaned and sterilized, resulting in nonproductive downtime.
A perfusion bioreactor processes a continuous supply of fresh media that is fed into the bioreactor while growth-inhibiting byproducts are constantly removed. The nonproductive downtime can be reduced or eliminated with a perfusion bioreactor process. The cell densities achieved in perfusion culture (30-100 million cells/mL) are typically higher than for fed-batch modes (5-25 million cells/mL). However, a perfusion bioreactor requires a cell retention device to prevent escape of the culture when byproducts are being removed. These cell retention systems add a level of complexity to the perfusion process, requiring management, control, and maintenance for successful operation. Operational issues such as malfunction or failure of the cell retention equipment has previously been a problem with perfusion bioreactors. This has limited their attractiveness in the past.
The present disclosure relates, in various embodiments, to a system for producing biomolecules such as recombinant proteins or monoclonal antibodies, and for separating these desirable products from a cell culture in a bioreactor. Generally, a fluid medium containing the cells and the desired products are passed or flowed through a filtering device
Disclosed in various embodiments is a system comprising a bioreactor and a filtering device. The bioreactor includes a reaction vessel, an agitator, a feed inlet, and an outlet. The filtering device comprises: an inlet fluidly connected to the bioreactor outlet for receiving fluid from the bioreactor; a flow chamber through which the fluid can flow; and a sleeve surrounding the flow chamber, the sleeve including at least one ultrasonic transducer and a reflector located opposite the at least one ultrasonic transducer, the at least one ultrasonic transducer being driven to produce a multi-dimensional standing wave in the flow chamber.
The filtering device may further comprise a product outlet through which desired product is recovered. The filtering device can also further comprise a recycle outlet for sending fluid back to the bioreactor.
The multi-dimensional standing wave may have an axial force component and a lateral force component which are of the same order of magnitude. The bioreactor can be operated as a perfusion bioreactor.
The sleeve may be separable from the flow chamber. Sometimes, the filtering device further comprises a jacket located between the sleeve and the flow chamber, the jacket being used to regulate the temperature of the fluid in the flow chamber. The jacket, the sleeve, and the flow chamber can be separable from each other and be disposable.
In particular embodiments, the ultrasonic transducer comprises a piezoelectric material that can vibrate in a higher order mode shape. The piezoelectric material may have a rectangular shape.
The ultrasonic transducer may comprise: a housing having a top end, a bottom end, and an interior volume; and a crystal at the bottom end of the housing having an exposed exterior surface and an interior surface, the crystal being able to vibrate when driven by a voltage signal. In some embodiments, a backing layer contacts the interior surface of the crystal, the backing layer being made of a substantially acoustically transparent material. The substantially acoustically transparent material can be balsa wood, cork, or foam. The substantially acoustically transparent material may have a thickness of up to 1 inch. The substantially acoustically transparent material can be in the form of a lattice. In other embodiments, an exterior surface of the crystal is covered by a wear surface material with a thickness of a half wavelength or less, the wear surface material being a urethane, epoxy, or silicone coating. In yet other embodiments, the crystal has no backing layer or wear layer.
The multi-dimensional standing wave can be a three-dimensional standing wave.
The reflector may have a non-planar surface.
These and other non-limiting characteristics are more particularly described below.
The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.
The present disclosure may be understood more readily by reference to the following detailed description of desired embodiments and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings.
Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.
The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
The term “comprising” is used herein as requiring the presence of the named component and allowing the presence of other components. The term “comprising” should be construed to include the term “consisting of”, which allows the presence of only the named component, along with any impurities that might result from the manufacture of the named component.
Numerical values should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.
All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 2 grams to 10 grams” is inclusive of the endpoints, 2 grams and 10 grams, and all the intermediate values). The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and/or values.
The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). When used in the context of a range, the modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the range of “from about 2 to about 10” also discloses the range “from 2 to 10.”
It should be noted that many of the terms used herein are relative terms. For example, the terms “upper” and “lower” are relative to each other in location, i.e. an upper component is located at a higher elevation than a lower component in a given orientation, but these terms can change if the device is flipped. The terms “inlet” and “outlet” are relative to a fluid flowing through them with respect to a given structure, e.g. a fluid flows through the inlet into the structure and flows through the outlet out of the structure. The terms “upstream” and “downstream” are relative to the direction in which a fluid flows through various components, i.e. the flow fluids through an upstream component prior to flowing through the downstream component. It should be noted that in a loop, a first component can be described as being both upstream of and downstream of a second component.
The present application refers to “the same order of magnitude.” Two numbers are of the same order of magnitude if the quotient of the larger number divided by the smaller number is a value less than 10.
Bioreactors are useful for making biomolecules such as recombinant proteins or monoclonal antibodies. Very generally, cells are cultured in a bioreactor vessel with media in order to produce the desired product, and the desired product is then harvested by separation from the cells and media. The use of mammalian cell cultures including Chinese hamster ovary (CHO), NS0 hybridoma cells, baby hamster kidney (BHK) cells, and human cells has proven to be a very efficacious way of producing/expressing the recombinant proteins and monoclonal antibodies required of today's pharmaceuticals. Two general types of bioreactor processes exist: fed-batch and perfusion.
While fed-batch reactors are the norm currently, due mainly to the familiarity of the process to many scientists and technicians, perfusion technology is growing at a very fast clip. Many factors favor the use of a perfusion bioreactor process. The capital and start-up costs for perfusion bioreactors are lower, smaller upstream and downstream capacity is required, and the process uses smaller volumes and fewer seed steps than fed-batch methods. A perfusion bioreactor process also lends itself better to development, scale-up, optimization, parameter sensitivity studies, and validation.
Recent developments in perfusion bioreactor technology also favor its use. Control technology and general support equipment is improving for perfusion bioreactors, increasing the robustness of perfusion processes. The perfusion process can now be scaled up to bioreactors having a volume up to 1000 liters (L). Better cell retention systems for perfusion bioreactors result in lower cell loss and greater cell densities than have been seen previously. Cell densities greater than 50 million cells/mL are now achievable, compared to fed-batch cell densities of around 20 million cells/mL. Lower contamination and infection rates have improved the output of perfusion bioreactors. Higher product concentrations in the harvest and better yields without significant increase in cost have thus resulted for perfusion processes.
A separate aspect of the use of high cell concentration bioreactors is the “dewatering” of the materials at the end of a bioreactor run. The “dewatering” or removal of interstitial fluid from a bioreactor sludge is important for improving the efficiency of recovery of the intended bioreactor product. Currently, high energy centrifuges with internal structures (known as disk stack centrifuges) are utilized to remove the interstitial fluid from the bioreactor sludge at the end of a run. The capital cost and operating costs for a disk stack centrifuge is high. A simpler method of removing the interstitial fluid from the remaining bioreactor sludge that can be performed without the high capital and operating costs associated with disk stack centrifuges is desirable. In addition, current methods of filtration or centrifugation can damage cells, releasing protein debris and enzymes into the purification process and increasing the load on downstream portions of the purification system.
Briefly, the present disclosure relates to the generation of three-dimensional (3-D) acoustic standing waves from one or more piezoelectric transducers, where the transducers are electrically or mechanically excited such that they move in a “drumhead” or multi-excitation mode rather than a “piston” or single excitation mode fashion. Through this manner of wave generation, a higher lateral trapping force is generated than if the piezoelectric transducer is excited in a “piston” mode where only one large standing wave is generated. Thus, with the same input power to a piezoelectric transducer, the 3-D acoustic standing waves can have a higher lateral trapping force compared to a single acoustic standing wave. This can be used to facilitate proteinaceous fluid purification of the contents of a bioreactor. Thus, the present disclosure relates to processing systems comprising a bioreactor and a filtering device, the filtering device using acoustophoresis for separation of various components.
Through utilization of an acoustophoretic filtering device that incorporates a 3-D standing wave, maintaining flux rates and minimizing cross-contamination risk in a multiproduct system can also be achieved. Other benefits, such as cleaning procedures and related demands often detailed and validated within standard operating procedures (SOP), can also be realized through the use of a 3-D acoustic standing wave capable apparatus. The cross-contamination risk can be eliminated between the bioreactor and outside processes.
Acoustophoresis is a low-power, no-pressure-drop, no-clog, solid-state approach to particle removal from fluid dispersions: i.e., it is used to achieve separations that are more typically performed with porous filters, but it has none of the disadvantages of filters. In particular, the present disclosure provides filtering devices that are suitable for use with bioreactors and operate at the macro-scale for separations in flowing systems with high flow rates. The acoustophoretic filtering device is designed to create a high intensity three dimensional ultrasonic standing wave that results in an acoustic radiation force that is larger than the combined effects of fluid drag and buoyancy or gravity, and is therefore able to trap (i.e., hold stationary) the suspended phase (i.e. cells) to allow more time for the acoustic wave to increase particle concentration, agglomeration and/or coalescence. The present systems have the ability to create ultrasonic standing wave fields that can trap particles in flow fields with a linear velocity ranging from 0.1 mm/sec to velocities exceeding 1 cm/s. This technology offers a green and sustainable alternative for separation of secondary phases with a significant reduction in cost of energy. Excellent particle separation efficiencies have been demonstrated for particle sizes as small as one micron.
The ultrasonic standing waves can be used to trap, i.e., hold stationary, secondary phase particles (e.g. cells) in a host fluid stream (e.g. cell culture media). This is an important distinction from previous approaches where particle trajectories were merely altered by the effect of the acoustic radiation force. The scattering of the acoustic field off the particles results in a three dimensional acoustic radiation force, which acts as a three-dimensional trapping field. The acoustic radiation force is proportional to the particle volume (e.g. the cube of the radius) when the particle is small relative to the wavelength. It is proportional to frequency and the acoustic contrast factor. It also scales with acoustic energy (e.g. the square of the acoustic pressure amplitude). For harmonic excitation, the sinusoidal spatial variation of the force is what drives the particles to the stable positions within the standing waves. When the acoustic radiation force exerted on the particles is stronger than the combined effect of fluid drag force and buoyancy/gravitational force, the particle is trapped within the acoustic standing wave field. The action of the acoustic forces on the trapped particles results in concentration, agglomeration and/or coalescence of particles. Additionally, secondary inter-particle forces, such as Bjerkness forces, aid in particle agglomeration.
Generally, the 3-D standing wave(s) filtering system is operated at a voltage such that the protein-producing materials, such as Chinese hamster ovary cells (CHO cells), the most common host for the industrial production of recombinant protein therapeutics, are trapped in the ultrasonic standing wave, i.e., remain in a stationary position. Within each nodal plane, the CHO cells are trapped in the minima of the acoustic radiation potential. Most cell types present a higher density and lower compressibility than the medium in which they are suspended, so that the acoustic contrast factor between the cells and the medium has a positive value. As a result, the axial acoustic radiation force (ARF) drives the CHO cells towards the standing wave pressure nodes. The axial component of the acoustic radiation force drives the cells, with a positive contrast factor, to the pressure nodal planes, whereas cells or other particles with a negative contrast factor are driven to the pressure anti-nodal planes. The radial or lateral component of the acoustic radiation force is the force that traps the cells. The radial or lateral component of the ARF is larger than the combined effect of fluid drag force and gravitational force. For small cells or emulsions the drag force FD can be expressed as:
where Uf and Up are the fluid and cell velocity, Rp is the particle radius, μf and μp are the dynamic viscosity of the fluid and the cells, and {circumflex over (μ)}=μp/μf is the ratio of dynamic viscosities. The buoyancy force FB is expressed as:
For a cell to be trapped in the ultrasonic standing wave, the force balance on the cell must be zero, and therefore an expression for lateral acoustic radiation force FLRF can be found, which is given by:
FLRF=FD+FB.
For a cell of known size and material property, and for a given flow rate, this equation can be used to estimate the magnitude of the lateral acoustic radiation force.
The theoretical model that is used to calculate the acoustic radiation force is based on the formulation developed by Gor'kov.17 The primary acoustic radiation force FA is defined as a function of a field potential U, FA=−∇(U),
where the field potential U is defined as
and f1 and f2 are the monopole and dipole contributions defined by
where p is the acoustic pressure, u is the fluid particle velocity, Λ is the ratio of cell density ρp to fluid density ρf, σ is the ratio of cell sound speed cp to fluid sound speed cf, Vo is the volume of the cell, and < > indicates time averaging over the period of the wave.
The lateral force of the total acoustic radiation force (ARF) generated by the ultrasonic transducers of the present disclosure is significant and is sufficient to overcome the fluid drag force at linear velocities of up to 1 cm/s. This lateral ARF can thus be used to retain cells in a bioreactor while the bioreactor process continues. This is especially true for a perfusion bioreactor.
The filtering devices of the present disclosure, which use ultrasonic transducers and acoustophoresis, can also improve the dewatering of the leftover material from a bioreactor batch (i.e bioreactor sludge), and thus reduce the use of or eliminate the use of disk stack centrifuges. This simplifies processing and reduces costs.
In a perfusion bioreactor system, it is desirable to be able to filter and separate the cells and cell debris from the expressed materials that are in the fluid stream (i.e. cell culture media). The expressed materials are composed of biomolecules such as recombinant proteins or monoclonal antibodies, and are the desired product to be recovered.
An acoustophoretic filtering device can be used in at least two different ways. First, the standing waves can be used to trap the expressed biomolecules and separate this desired product from the cells, cell debris, and media. The expressed biomolecules can then be diverted and collected for further processing. Alternatively, the standing waves can be to trap the cells and cell debris present in the cell culture media. The cells and cell debris, having a positive contrast factor, move to the nodes (as opposed to the anti-nodes) of the standing wave. As the cells and cell debris agglomerate at the nodes of the standing wave, there is also a physical scrubbing of the cell culture media that occurs whereby more cells are trapped as they come in contact with the cells that are already held within the standing wave. This generally separates the cells and cellular debris from the cell culture media. When the cells in the standing wave agglomerate to the extent where the mass is no longer able to be held by the acoustic wave, the aggregated cells and cellular debris that have been trapped can fall out of the fluid stream through gravity, and can be collected separately. To aid this gravitational settling of the cells and cell debris, the standing wave may be interrupted to allow all of the cells to fall out of the fluid stream that is being filtered. This process can be useful for dewatering. The expressed biomolecules may have been removed beforehand, or remain in the fluid stream (i.e. cell culture medium).
Desirably, the ultrasonic transducer(s) generate a three-dimensional standing wave in the fluid that exerts a lateral force on the suspended particles to accompany the axial force so as to increase the particle trapping capabilities of the acoustophoretic filtering device. Typical results published in literature state that the lateral force is two orders of magnitude smaller than the axial force. In contrast, the technology disclosed in this application provides for a lateral force to be of the same order of magnitude as the axial force.
The acoustic filtering devices of the present disclosure are designed to maintain a high intensity three-dimensional acoustic standing wave. The device is driven by a function generator and amplifier (not shown). The device performance is monitored and controlled by a computer. It may be necessary, at times, due to acoustic streaming, to modulate the frequency or voltage amplitude of the standing wave. This may be done by amplitude modulation and/or by frequency modulation. The duty cycle of the propagation of the standing wave may also be utilized to achieve certain results for trapping of materials. In other words, the acoustic beam may be turned on and shut off at different frequencies to achieve desired results.
Turning now to the perfusion bioreactor 202 on the right-hand side, again, the bioreactor includes a reaction vessel 220 with a feed inlet 222 for the cell culture media. An agitator 225 is used to circulate the media throughout the cell culture. An outlet 224 of the reaction vessel is fluidly connected to the inlet 232 of a filtering device 230, and continuously feeds the media (containing cells and desired product) to the filtering device. The filtering device is located downstream of the reaction vessel, and separates the desired product from the cells. The filtering device 230 has two separate outlets, a product outlet 234 and a recycle outlet 236. The product outlet 234 fluidly connects the filtering device 230 to a containment vessel 240 downstream of the filtering device, which receives a concentrated flow of the desired product (plus media) from the filtering device. From there, further processing/purification can occur to isolate/recover the desired product. The recycle outlet 236 fluidly connects the filtering device 230 back to a recycle inlet 226 of the reaction vessel 220, and is used to send the cells and cell culture media back into the reaction vessel for continued growth/production. Put another way, there is a fluid loop between the reaction vessel and the filtering device. The reaction vessel 220 in the perfusion bioreactor system 202 has a continuous throughput of product and thus can be made smaller. The filtering process is critical to the throughput of the perfusion bioreactor. A poor filtering process will allow for only low throughput and result in low yields of the desired product.
The perfusion systems of the present disclosure also use an acoustophoretic filtering device. The contents of the bioreactor are continuously flowed through the filtering device to capture the desired products.
It may be helpful now to describe the ultrasonic transducer(s) used in the acoustophoretic filtering device in more detail.
Screws (not shown) attach an aluminum top plate 82a of the housing to the body 82b of the housing via threads 88. The top plate includes a connector 84 to pass power to the PZT crystal 86. The bottom and top surfaces of the PZT crystal 86 are each connected to an electrode (positive and negative), such as silver or nickel. A wrap-around electrode tab 90 connects to the bottom electrode and is isolated from the top electrode. Electrical power is provided to the PZT crystal 86 through the electrodes on the crystal, with the wrap-around tab 90 being the ground connection point. Note that the crystal 86 has no backing layer or epoxy layer as is present in
The transducer design can affect performance of the system. A typical transducer is a layered structure with the ceramic crystal bonded to a backing layer and a wear plate. Because the transducer is loaded with the high mechanical impedance presented by the standing wave, the traditional design guidelines for wear plates, e.g., half wavelength thickness for standing wave applications or quarter wavelength thickness for radiation applications, and manufacturing methods may not be appropriate. Rather, in one embodiment of the present disclosure the transducers, there is no wear plate or backing, allowing the crystal to vibrate in one of its eigenmodes with a high Q-factor. The vibrating ceramic crystal/disk is directly exposed to the fluid flowing through the flow chamber.
Removing the backing (e.g. making the crystal air backed) also permits the ceramic crystal to vibrate at higher order modes of vibration with little damping (e.g. higher order modal displacement). In a transducer having a crystal with a backing, the crystal vibrates with a more uniform displacement, like a piston. Removing the backing allows the crystal to vibrate in a non-uniform displacement mode. The higher order the mode shape of the crystal, the more nodal lines the crystal has. The higher order modal displacement of the crystal creates more trapping lines, although the correlation of trapping line to node is not necessarily one to one, and driving the crystal at a higher frequency will not necessarily produce more trapping lines.
In some embodiments, the crystal may have a backing that minimally affects the Q-factor of the crystal (e.g. less than 5%). The backing may be made of a substantially acoustically transparent material such as balsa wood, foam, or cork which allows the crystal to vibrate in a higher order mode shape and maintains a high Q-factor while still providing some mechanical support for the crystal. The backing layer may be a solid, or may be a lattice having holes through the layer, such that the lattice follows the nodes of the vibrating crystal in a particular higher order vibration mode, providing support at node locations while allowing the rest of the crystal to vibrate freely. The goal of the lattice work or acoustically transparent material is to provide support without lowering the Q-factor of the crystal or interfering with the excitation of a particular mode shape.
Placing the crystal in direct contact with the fluid also contributes to the high Q-factor by avoiding the dampening and energy absorption effects of the epoxy layer and the wear plate. Other embodiments may have wear plates or a wear surface to prevent the PZT, which contains lead, contacting the host fluid. This may be desirable in, for example, biological applications such as separating blood. Such applications might use a wear layer such as chrome, electrolytic nickel, or electroless nickel. Chemical vapor deposition could also be used to apply a layer of poly(p-xylylene) (e.g. Parylene) or other polymer. Organic and biocompatible coatings such as silicone or polyurethane are also usable as a wear surface.
In some embodiments, the ultrasonic transducer has a 1 inch diameter and a nominal 2 MHz resonance frequency. Each transducer can consume about 28 W of power for droplet trapping at a flow rate of 3 GPM. This translates to an energy cost of 0.25 kW hr/m3. This is an indication of the very low cost of energy of this technology. Desirably, each transducer is powered and controlled by its own amplifier. In other embodiments, the ultrasonic transducer uses a square crystal, for example with 1″×1″ dimensions. Alternatively, the ultrasonic transducer can use a rectangular crystal, for example with 1″×2.5″ dimensions. Power dissipation per transducer was 10 W per 1″×1″ transducer cross-sectional area and per inch of acoustic standing wave span in order to get sufficient acoustic trapping forces. For a 4″ span of an intermediate scale system, each 1″×1″ square transducer consumes 40 W. The larger 1″×2.5″ rectangular transducer uses 100 W in an intermediate scale system. The array of three 1″×1″ square transducers would consume a total of 120 W and the array of two 1″×2.5″ transducers would consume about 200 W. Arrays of closely spaced transducers represent alternate potential embodiments of the technology. Transducer size, shape, number, and location can be varied as desired to generate desired three-dimensional acoustic standing wave patterns.
The size, shape, and thickness of the transducer determine the transducer displacement at different frequencies of excitation, which in turn affects separation efficiency. Typically, the transducer is operated at frequencies near the thickness resonance frequency (half wavelength). Gradients in transducer displacement typically result in more trapping locations for the cells/biomolecules. Higher order modal displacements generate three-dimensional acoustic standing waves with strong gradients in the acoustic field in all directions, thereby creating equally strong acoustic radiation forces in all directions, leading to multiple trapping lines, where the number of trapping lines correlate with the particular mode shape of the transducer.
To investigate the effect of the transducer displacement profile on acoustic trapping force and separation efficiencies, an experiment was repeated ten times using a 1″×1″ square transducer, with all conditions identical except for the excitation frequency. Ten consecutive acoustic resonance frequencies, indicated by circled numbers 1-9 and letter A on
As the oil-water emulsion passed by the transducer, the trapping lines of oil droplets were observed and characterized. The characterization involved the observation and pattern of the number of trapping lines across the fluid channel, as shown in
In the present systems, the system is operated at a voltage such that the particles (i.e. biomolecules or cells) are trapped in the ultrasonic standing wave, i.e., remain in a stationary position. The particles are collected in along well defined trapping lines, separated by half a wavelength. Within each nodal plane, the particles are trapped in the minima of the acoustic radiation potential. The axial component of the acoustic radiation force drives particles with a positive contrast factor to the pressure nodal planes, whereas particles with a negative contrast factor are driven to the pressure anti-nodal planes. The radial or lateral component of the acoustic radiation force is the force that traps the particle. It therefore must be larger than the combined effect of fluid drag force and gravitational force. In systems using typical transducers, the radial or lateral component of the acoustic radiation force is typically several orders of magnitude smaller than the axial component of the acoustic radiation force. However, the lateral force generated by the transducers of the present disclosure can be significant, on the same order of magnitude as the axial force component, and is sufficient to overcome the fluid drag force at linear velocities of up to 1 cm/s.
The lateral force can be increased by driving the transducer in higher order mode shapes, as opposed to a form of vibration where the crystal effectively moves as a piston having a uniform displacement. The acoustic pressure is proportional to the driving voltage of the transducer. The electrical power is proportional to the square of the voltage. The transducer is typically a thin piezoelectric plate, with electric field in the z-axis and primary displacement in the z-axis. The transducer is typically coupled on one side by air (i.e. the air gap within the transducer) and on the other side by the fluid of the cell culture media. The types of waves generated in the plate are known as composite waves. A subset of composite waves in the piezoelectric plate is similar to leaky symmetric (also referred to as compressional or extensional) Lamb waves. The piezoelectric nature of the plate typically results in the excitation of symmetric Lamb waves. The waves are leaky because they radiate into the water layer, which result in the generation of the acoustic standing waves in the water layer. Lamb waves exist in thin plates of infinite extent with stress free conditions on its surfaces. Because the transducers of this embodiment are finite in nature, the actual modal displacements are more complicated.
The transducers are driven so that the piezoelectric crystal vibrates in higher order modes of the general formula (m, n), where m and n are independently 1 or greater. Generally, the transducers will vibrate in higher order modes than (2,2). Higher order modes will produce more nodes and antinodes, result in three-dimensional standing waves in the water layer, characterized by strong gradients in the acoustic field in all directions, not only in the direction of the standing waves, but also in the lateral directions. As a consequence, the acoustic gradients result in stronger trapping forces in the lateral direction.
In embodiments, the pulsed voltage signal driving the transducer can have a sinusoidal, square, sawtooth, or triangle waveform; and have a frequency of 500 kHz to 10 MHz. The pulsed voltage signal can be driven with pulse width modulation, which produces any desired waveform. The pulsed voltage signal can also have amplitude or frequency modulation start/stop capability to eliminate streaming.
The transducer(s) is/are used to create a pressure field that generates forces of the same order of magnitude both orthogonal to the standing wave direction and in the standing wave direction. When the forces are roughly the same order of magnitude, particles of size 0.1 microns to 300 microns will be moved more effectively towards regions of agglomeration (“trapping lines”). Because of the equally large gradients in the orthogonal acoustophoretic force component, there are “hot spots” or particle collection regions that are not located in the regular locations in the standing wave direction between the transducer and the reflector. Hot spots are located in the maxima or minima of acoustic radiation potential. Such hot spots represent particle collection locations which allow for better wave transmission between the transducer and the reflector during collection and stronger inter-particle forces, leading to faster and better particle agglomeration.
Referring to
Acoustophoretic separation has been tested on different lines of Chinese hamster ovary (CHO) cells. In one experiment, a solution with a starting cell density of 8.09×106 cells/mL, a turbidity of 1,232 NTU, and cell viability of roughly 75% was separated using a system as depicted in
In another experiment, a solution with a starting cell density of 8.09×106 cells/mL, a turbidity of 1,232 NTU, and cell viability of roughly 75% was separated. This CHO cell line had a bi-modal particle size distribution (at size 12 μm and 20 μm). The result is shown in
In other tests at a flow rate of 10 L/hr, 99% of cells were captured with a confirmed cell viability of more than 99%. Other tests at a flow rate of 50 mL/min (i.e. 3 L/hr) obtained a final cell density of 3×106 cells/mL with a viability of nearly 100% and little to no temperature rise. In yet other tests, a 95% reduction in turbidity was obtained at a flow rate of 6 L/hr.
Further testing was performed using yeast as a stimulant for CHO for the biological applications. For these tests, at a flow rate of 15 L/hr, various frequencies were tested as well as power levels. Table 1 shows the results of the testing.
In biological applications, it is contemplated that all of the parts of the system (e.g. the flow chamber, tubing leading to and from the bioreactor or filtering device, the sleeve containing the ultrasonic transducer and the reflector, the temperature-regulating jacket, etc.) can be separated from each other and be disposable. Avoiding centrifuges and filters allows better separation of the CHO cells without lowering the viability of the cells. The transducers may also be driven to create rapid pressure changes to prevent or clear blockages due to agglomeration of CHO cells. The frequency of the transducers may also be varied to obtain optimal effectiveness for a given power.
The present disclosure has been described with reference to exemplary embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/761,717, filed on Feb. 7, 2013. This application is also a continuation-in-part of U.S. patent application Ser. No. 14/026,413, filed on Sep. 13, 2013, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/708,641, filed on Oct. 2, 2012, and is also a continuation-in-part of U.S. Ser. No. 13/844,754, filed Mar. 15, 2013, which claimed the benefit of U.S. Provisional Patent Application Ser. No. 61/611,159, filed Mar. 15, 2012, and of U.S. Provisional Patent Application Ser. No. 61/611,240, also filed Mar. 15, 2012, and of U.S. Provisional Patent Application Ser. No. 61/754,792, filed Jan. 21, 2013. These applications are incorporated herein by reference in their entireties.
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