Variable frequency ultrasonic matter enrichment system and method

Abstract

A material enrichment system has been disclosed, comprising: a sound field generating device for generating a variable-frequency ultrasonic sound field to control the enrichment of material within a fluid cavity; and an imaging device for acquiring the distribution state of the enriched material within the fluid cavity in real time and monitoring the migration and enrichment process of the material under the action of the variable-frequency ultrasonic sound field. The sound field generating device includes a signal generator, a power amplifier, and an ultrasonic transducer connected in sequence. The ultrasonic transducer is positioned relative to the fluid cavity, which contains a fluid containing the material to be enriched. The fluid cavity has a reflective interface, such that the variable-frequency ultrasound incident on the fluid cavity by the ultrasonic transducer interferes with the reflected ultrasound to form a standing wave sound field. The ultrasonic transducer, through frequency changes of the variable-frequency ultrasound, causes the sound pressure nodes of the standing wave sound field to move in the direction of ultrasound propagation. The material to be enriched is captured at the sound pressure nodes by the sound radiation force and migrates with the nodes, achieving directional enrichment.

Description

Technical Field

[0001] This disclosure belongs to the field of biomedical technology, and specifically relates to a variable frequency ultrasonic material enrichment system and method. Background Technology

[0002] In the field of material enrichment, although some methods (such as density gradient centrifugation) can achieve both separation and enrichment of materials to a certain extent, the enrichment of particles (especially micron / nano-scale particles) still faces several common technical challenges in broader applications. Separation processes often involve particle loss and / or the introduction of additional liquid volumes, easily leading to a decrease in the overall concentration of the target particles, thus requiring additional enrichment steps for compensation. Furthermore, in the subsequent enrichment process, the enrichment effect is usually constrained by recovery rate, particle structural integrity, and system stability, easily resulting in an inherent trade-off between the enrichment factor and particle loss, agglomeration, or interfacial retention. Meanwhile, while some existing enrichment or capture technologies are relatively effective in particle capture, they still have shortcomings in terms of gentle particle release, controllable recovery, and repeatability, thus limiting the implementation of continuous processing, high-throughput operations, and subsequent analysis or application workflows. In addition, under complex system conditions such as high salinity, high protein, or high viscosity, the enrichment process is more sensitive to operating conditions and system parameters, making it difficult to consistently guarantee enrichment efficiency and repeatability, thus restricting the standardization and large-scale application of related technologies. To address these challenges, researchers have conducted extensive studies aimed at better controlling the directional enrichment of matter to increase its concentration at desired locations. To this end, they have explored the possibility of using external fields (such as magnetism, light, and sound) to concentrate matter at specific sites, further enhancing the efficiency of directional enrichment.

[0003] External field manipulation of matter typically utilizes various external physical fields to act on specific substances, actively and relatively efficiently enriching them at desired locations. Compared to traditional enrichment methods, external field manipulation of matter typically utilizes various external physical fields to act on specific substances, actively and relatively efficiently enriching them at desired locations.

[0004] Compared to traditional enrichment methods, external field-controlled material enrichment strategies typically offer advantages such as contactless or low-contact operation, programmable control, spatial localization, and faster response. They can also reduce dependence on complex fluid conditions, device geometry, or chemical labeling steps, thereby improving the controllability and operational flexibility of the enrichment process. However, different external physical fields still have their limitations in practical applications. For example, in magnetic field schemes, the target material often needs to be integrated with superparamagnetic components or introduced as a magnetic carrier. Furthermore, the structure of the magnetic guidance system is usually quite complex, and the spatial resolution of the magnetic field is relatively limited, often on the order of centimeters, making high-precision control of the material enrichment process challenging. In addition, effective focusing of magnetic fields is difficult, constructing strong anisotropic magnetic fields is challenging, and under given magnetic field conditions, magnetic materials are usually driven by a uniformly oriented force, which limits their fine-grained manipulation and application range in complex scenarios. For example, optical field schemes are susceptible to scattering / absorption by the medium and potential photothermal effects. In turbid, strongly scattering, or multiphase systems, the beam propagation path and focal spot morphology are easily distorted, leading to a reduction in the effective volume and decreased positioning stability, which in turn affects the enrichment location and enrichment efficiency. For larger samples or targets at greater depths, the depth of the optical field is limited, often making it difficult to maintain a sufficient intensity gradient in regions far from the incident interface to generate usable maneuvering force. Meanwhile, to obtain considerable maneuvering force or enrichment rate, it is often necessary to increase the optical power or extend the irradiation time, which may introduce undesirable effects such as local temperature rise, thermal convection, thermophoresis, and photochemical reactions, causing disturbances in the system's flow field, changes in particle aggregation state, or affecting the activity of biological samples. Furthermore, optical enrichment is highly dependent on optical windows, alignment, and imaging systems, and its adaptability to device packaging and field applications is relatively limited. In high-throughput or large-area enrichment tasks, it often faces a trade-off between efficiency and energy consumption. For electric field-based methods, in addition to being affected by the conductivity of the medium, they are also typically constrained by the combined effects of electrode interfacial processes and thermal effects. In samples with high ionic strength or high conductivity, Joule heating is significantly enhanced, easily leading to temperature rise, thermal convection, and dielectric drift, thereby altering the force conditions on particles and reducing enrichment stability and repeatability. Simultaneously, electrochemical side reactions may occur near the electrode (such as electrolysis, pH drift, bubble generation, and electrode contamination / passivation), which not only interfere with the electric field distribution and fluid environment but may also introduce additional particle loss or cause the enrichment region to drift. For methods such as dielectrophoresis that rely on electric field gradients, the electrode microstructure and gap size significantly affect performance, posing challenges to manufacturing consistency and scale-up. In complex media, non-specific adsorption and fluctuations in particle surface charge / polarization characteristics with environmental changes also lead to a narrowing of the enrichment threshold and parameter window.Furthermore, if the target system is sensitive to electrical stimulation (e.g., samples containing living cells or biomacromolecules), electroporation caused by the electric field, changes in the local electrochemical environment, or shear flow coupling effects may limit the integrity and activity of the sample.

[0005] Currently, acoustic manipulation for material enrichment faces challenges such as low enrichment rates and low spatial resolution, making it difficult to accurately concentrate large quantities of material at desired locations. Ultrasound's label-free and biocompatible nature makes it one of the most powerful tools for material enrichment, capable of precisely manipulating materials of various sizes.

[0006] Ultrasound's label-free and sub-millimeter resolution makes it one of the most powerful tools for enriching matter. Sound waves can manipulate matter without disrupting the surrounding environment and enrich it separately based on size or acoustic contrast coefficient. Sound enriches matter primarily by generating acoustic radiation forces and acoustic flow. Acoustic radiation forces in standing waves stand out in matter enrichment due to their high controllability, with increasing matter size leading to stronger radiation forces. The acoustic radiation forces generated by the sound field on larger matter can overcome fluid resistance caused by drag. Therefore, larger particles can move faster and farther towards low sound pressure nodes, while the movement of smaller matter is dominated by the drag of background streamlines. This difference in particle size is widely used for particle separation and enrichment. Acoustic flow effects can also be used for matter enrichment. Among these, acoustic vortices have significant advantages, including high controllability, rapid formation, and ease of adjustment. When matter enters the sound field region, the microflow can change its trajectory according to particle size: larger matter moves towards the vortex center and is confined within closed streamlines; while smaller matter can pass through the vortex. However, acoustic manipulation enrichment techniques based on traditional standing wave sound fields and vortex sound fields can only enrich substances near their focal point (within about 2 wavelengths), resulting in low enrichment efficiency.

[0007] Therefore, the main challenge of ultrasound-controlled material enrichment is that ultrasound-mediated material enrichment has low expected enrichment efficiency, making it difficult to achieve large-scale, precise, and directional enrichment of materials. Summary of the Invention

[0008] This disclosure addresses the problem that existing acoustic manipulation technologies are difficult to use for large-scale, precise, and directional enrichment of materials, and proposes a system and method for directional enrichment of materials using a variable frequency ultrasonic sound field.

[0009] One aspect of this disclosure is a material enrichment system, comprising:

[0010] A sound field generator is used to produce a variable frequency ultrasonic sound field for manipulating the enrichment of substances within a fluid cavity.

[0011] The imaging device is used to acquire the distribution of enriched substances in a fluid cavity in real time and monitor the migration and enrichment process of substances under the action of a variable frequency ultrasonic field.

[0012] The sound field generating device includes a signal generator, a power amplifier, and an ultrasonic transducer connected in sequence, wherein...

[0013] The ultrasonic transducer is positioned relative to a fluid cavity, which is used to contain a fluid containing the substance to be enriched.

[0014] The fluid cavity has a reflective interface, which causes the frequency-converted ultrasound incident on the fluid cavity by the ultrasonic transducer to interfere with the reflected ultrasound to form a standing wave sound field.

[0015] The ultrasonic transducer causes the sound pressure nodes of the standing wave sound field to move in the direction of ultrasonic propagation by changing the frequency of the variable frequency ultrasound. The material to be enriched is captured at the sound pressure node by the sound radiation force and migrates with the node, thus achieving directional enrichment.

[0016] One aspect of this disclosure is a method for material enrichment based on frequency conversion ultrasound, comprising:

[0017] A variable-frequency ultrasonic wave with a frequency that varies with time is generated by an ultrasonic transducer, forming a standing wave sound field in a fluid cavity with a reflective interface. The standing wave sound field includes sound pressure nodes.

[0018] The material to be enriched, dispersed in the fluid within the fluid cavity, is captured at the sound pressure node by utilizing acoustic radiation force.

[0019] By changing the frequency of the variable-frequency ultrasound, the sound pressure node moves in the direction of ultrasound propagation, thereby driving the material to migrate synchronously with the sound pressure node and achieving directional enrichment of the material at the target location. Attached Figure Description

[0020] The above and other objects, features, and advantages of this disclosure will become readily apparent from the following detailed description of exemplary embodiments, taken in conjunction with the accompanying drawings. Several embodiments of this disclosure are illustrated in the drawings by way of example and not limitation, in which:

[0021] Figure 1 A schematic diagram of the sound field enrichment principle according to one embodiment of the present disclosure.

[0022] Figure 2 According to one embodiment of the present disclosure, the initial position of matter in an ultrasonic field is simulated using multiphysics simulation software.

[0023] Figure 3 A schematic diagram illustrating the changes in the position of matter within various ultrasonic fields using multiphysics simulation software, according to one embodiment of this disclosure.

[0024] Figure 4A schematic diagram illustrating the changes in the position of matter within various ultrasonic fields using multiphysics simulation software, according to one embodiment of this disclosure.

[0025] Figure 5 A schematic diagram illustrating the changes in the position of matter within various ultrasonic fields using multiphysics simulation software, according to one embodiment of this disclosure.

[0026] Figure 6 A schematic diagram of the experimental apparatus according to one embodiment of this disclosure.

[0027] Figure 7 Experimental diagrams of various sound field enriched materials according to one embodiment of this disclosure.

[0028] Figure 8 Experimental diagrams of various sound field enriched materials according to one embodiment of this disclosure.

[0029] Figure 9 Experimental diagrams of sound field enrichment materials according to one embodiment of this disclosure. Detailed Implementation

[0030] The main challenges of existing ultrasound-mediated material enrichment techniques are low enrichment efficiency and low spatial resolution. Current acoustic-based enrichment methods only enrich materials near their focal point (within approximately two wavelengths). They fail to enrich most other materials distributed in the fluid. To address these challenges, the purpose of this disclosure is to utilize variable-frequency ultrasound for large-scale, directional enrichment of materials within a spatial range, achieving broad-area enrichment of materials at the desired location and improving enrichment efficiency.

[0031] Therefore, this disclosure proposes a material enrichment technology based on frequency conversion ultrasound to achieve large-scale enrichment of material at the desired location and improve the enrichment efficiency. The main frequency conversion ultrasound sound field in this disclosure utilizes the change in the distance between standing wave nodes in the sound field to generate sound nodes that move along the propagation direction, ultimately enriching material over a large area at the target location.

[0032] in, Figure 1 This is a schematic diagram of the sound field enrichment principle. Figure 2 The initial particle positions in the simulated sound field are shown. Figure 3 This demonstrates the positions of particles in a silent field. Figure 4 It demonstrates the particle enrichment in a standing wave sound field. Figure 5 The particle enrichment in the frequency conversion sound field is shown. Figure 6 This is the experimental apparatus proposed in this publication. Figure 7 An experiment demonstrating silent field particles was presented. Figure 8 The experiment demonstrated the enrichment of particles in a standing wave acoustic field. Figure 9 The experiment on particle enrichment in a frequency-converting sound field was demonstrated.

[0033] According to one or more embodiments, this disclosure proposes an ultrasound-based material enrichment system, which mainly includes:

[0034] (a) An ultrasonic transducer module, wherein the transducer is a piezoelectric or equivalent type ultrasonic transducer, used to convert externally input electrical signals into mechanical vibrations, thereby exciting an ultrasonic field with spatial characteristics in the medium; the transducer is installed on one side or below the fluid cavity, with its acoustic axis aligned with the target enrichment area, so that the ultrasonic energy forms a sound pressure enhancement zone within the predetermined focal region. The center frequency, focal length, and aperture of the transducer can be selected or adjusted according to the enrichment type, target depth, and material size, but are not limited to a specific value.

[0035] (b) A drive and control module, comprising a function signal generator and a power amplifier, electrically connected to form a drive link. The function signal generator directly outputs a frequency-converted sinusoidal electrical signal, and the power amplifier amplifies the voltage amplitude of the frequency-converted sinusoidal electrical signal to a range suitable for driving the ultrasonic transducer, and inputs the amplified electrical signal to the ultrasonic transducer via a cable. The function signal generator has a built-in frequency conversion output mode for presetting the starting frequency. With termination frequency and preset frequency conversion time The function signal generator during the frequency conversion time The internal frequency conversion function continuously changes the output frequency from the starting frequency to the ending frequency according to a linear function relationship. This can be represented as the frequency change within one period,

[0036]

[0037]

[0038]

[0039] This generates a time-varying frequency sinusoidal drive signal, which is used to drive the ultrasonic transducer to produce a frequency-varying sound field. Here, S is the frequency change rate, and t is the time for the frequency sweep.

[0040] (c) Fluid cavity module, used to contain the fluid containing the substance to be processed, with an acoustic reflection interface formed at its top, so that the incident wave and the reflected wave interfere with each other to form a standing wave sound field; in external applications, this cavity is a closed fluid cavity.

[0041] (d) Material Module: The material is a substance that can be manipulated by acoustic radiation force. Its size is smaller than the wavelength of ultrasound in the medium to ensure stable capture and migration with the acoustic nodes in the standing wave sound field. The material can be one or more of polystyrene, biodegradable polymers, liposomes, or inorganic substances, and is dispersed in the medium to form a suspension system, serving as the object of acoustic field manipulation. Through the action of acoustic radiation force, the material can be directionally transported within the fluid cavity and enriched at the target treatment site.

[0042] (e) Imaging and Monitoring Module: This module includes an optical microscope and an ultrasonic imaging device, used to acquire optical and ultrasonic images of the material distribution within the fluid cavity in real time. This allows for the localization of the acoustic focal region and dynamic monitoring of the migration and accumulation of substances under the influence of the scanning acoustic field. In in vitro experiments, optical microscopy can be prioritized for high-resolution visualization and monitoring of material accumulation behavior; in in vivo applications, ultrasonic imaging can be prioritized for non-invasive localization and real-time imaging of organ cavity structures and material accumulation areas.

[0043] The connection and working relationships of the various components are as follows:

[0044] (1) The drive and control circuit module is electrically connected to the ultrasonic transducer via a cable and is used to input a frequency-converted sinusoidal electrical signal to the transducer;

[0045] (2) The ultrasonic transducer converts the input electrical signal into ultrasonic waves and couples the energy into the fluid cavity;

[0046] (3) The top of the fluid cavity or the surface of the target tissue serves as a reflective interface, forming an acoustic resonance space with the transducer to construct a standing wave sound field;

[0047] (4) The substance is suspended in the fluid cavity and is within the ultrasonic propagation path and the area covered by the standing wave sound field;

[0048] (5) The imaging and monitoring module is optically or acoustically aligned with the fluid cavity to synchronously record the migration and enrichment state of the material.

[0049] The enrichment principle of this system is explained as follows:

[0050] Standing wave sound fields can propel particles to sound pressure nodes or anti-nodes through acoustic radiation forces. When ultrasound waves enter a fluid cavity or biological cavity and encounter a reflecting interface, the incident and reflected waves interfere, forming a standing wave sound field in the direction of propagation. Its sound pressure distribution can be expressed as follows:

[0051]

[0052] Where x represents the distance between the material and the reflective interface, t is time, and p is the sound pressure amplitude. Let be the velocity of sound in the medium, and f(t) be the time-varying driving frequency. In this standing wave sound field, the distance between adjacent acoustic nodes is related to the wavelength of the sound wave, and the node spacing can be expressed as:

[0053]

[0054] Where x0 is the distance between the material and the sound source. In the frequency conversion sound field, the material is captured by the initial frequency at this position, and d represents the distance the material moves within one frequency offset period.

[0055] Therefore, under the condition that the sound velocity in the medium is approximately constant, when the driving frequency changes from the initial frequency... Gradually increase to the termination frequency As the frequency increases, the wavelength decreases, the distance between acoustic nodes decreases continuously, and the entire acoustic node array shifts towards the reflection interface along the direction of ultrasonic propagation.

[0056] Matter within this sound field is captured and stably remains at the acoustic node under the influence of acoustic radiation force. Its dynamic behavior can be described using the Gor'kov potential model. The time-averaged acoustic radiation force experienced by the matter in the sound field is...

[0057]

[0058] Among them, Gor'kov's potential ( ) represents

[0059]

[0060] Where r is the radius of the substance. and These are the time-averaged squares of sound pressure and particle velocity, respectively. and These are the densities of the particles and the medium, respectively. and Let be the sound velocity of the particles and the medium. For substances with a density greater than that of the medium (such as polystyrene particles used in this disclosure), the direction of the acoustic radiation force always points to the location of the minimum acoustic potential energy, causing the material to spontaneously migrate and stably accumulate at the pressure node, thereby forming an acoustic potential well structure in space.

[0061] Compared to a static standing wave sound field, an ultrasound-based frequency-modulated sound field, due to the time-varying driving frequency, makes the position of the acoustic nodes a function of time, thus forming an array of acoustic nodes that move along the propagation direction. During each incremental frequency shift iteration, the node spacing gradually decreases, and the acoustic nodes continuously drift towards the reflection interface. Material in the sound field is "locked" into the node potential wells under the dominance of acoustic radiation force and migrates synchronously with the nodes. At the end of one frequency conversion cycle, when the driving frequency jumps from its maximum value back to its minimum value, the material is recaptured at a new adjacent acoustic node and continues to move towards the reflection interface in the next frequency conversion process. Through multiple frequency conversion iterations, the material initially dispersed within the cavity is continuously transported towards the reflection interface or the acoustic focus region under the synergistic effect of acoustic radiation force and node drift, ultimately forming a high-density enrichment area at the target treatment location. Utilizing this node migration mechanism driven by a scanning sound field can significantly improve the directional enrichment efficiency of material at the acoustic focus.

[0062] According to one or more embodiments, an ultrasound-based material enrichment method can enrich material in a sound field by using a frequency-converted sound field, thereby improving the enrichment efficiency of material at a target location. This embodiment also provides a comparison of simulated material enrichment in multiphysics simulation software.

[0063] In this embodiment, the positional variation of the sound pressure nodes was numerically simulated using the commercial COMSOL Multiphysics finite element software. A two-dimensional model was established to simulate the motion of matter in different sound fields. A focused sound field was used, with water as the material of the physical field. Offset range: f start = 3.5 MHz, f end = 4.0 MHz, Δf = 0.5 MHz. Initial position as follows Figure 2 In this disclosure, the radius of the material is much smaller than the wavelength in the surrounding medium (approximately 370 μm, 4 MHz). The enrichment effect is as follows: Figure 3 , 4 5. In the variable frequency sound field, matter is concentrated at the ultrasonic focal point ( Figure 6 ).

[0064] According to one or more embodiments, to demonstrate the universality of the enrichment method, the frequency-converted sound field provided in this embodiment is used for the directional enrichment of matter within a flow field cavity. This embodiment provides a comparison of the material enrichment efficiency at the target location using a sound field without sound, a static standing wave sound field, and a frequency-converted sound field.

[0065] This involves setting the sound field, which is generated by a transducer. The focal point of the transducer is determined based on the target location where the material needs to be enriched. In this embodiment, the transducer is a focusing transducer (center frequency 4.0 MHz), with a focusing distance of 20 mm and an aperture of 18.4 mm. The target location is located on the plane of the transducer's focal point. A reflecting interface is formed at the top of the fluid cavity, causing the incident and reflected waves to interfere with each other, forming a standing wave field.

[0066] Furthermore, the sound field is modulated by installing a transducer at the bottom of the fluid cavity as the sound field emission source to generate an ultrasonic field. The driving frequency of the transducer can be continuously or incrementally adjusted within a certain range, thereby changing the node spacing in the sound field. In this embodiment, a frequency conversion sound field is used with a frequency conversion range of 3.5-4.0 MHz, a frequency scan time of 1.5 s, and an electrical signal amplitude of 2.5 Vpp driving the transducer. As a control, a non-ultrasonic sound field is used, where no ultrasonic sound field is set within the fluid cavity. As a control, a static standing wave sound field is set within the fluid cavity, with a frequency of 4.0 MHz and an electrical signal amplitude of 2.5 Vpp driving the transducer. A schematic diagram of the experimental setup is shown below. Figure 6 .

[0067] In this embodiment, the fluid cavity is used to contain the fluid containing the substance to be enriched. The enriched substance is selected as PS microparticles with a particle size of 20 μm, which are dispersed in pure water. The microparticles are captured at acoustic nodes under the action of acoustic radiation force and migrate with the movement of the nodes. This ultimately achieves a directional enrichment effect at the acoustic focal point, such as... Figure 9 The enrichment efficiency of microparticles was measured by statistically analyzing the enrichment area. Within 5 minutes, the enrichment area of ​​the microparticles was 200 times that of the group without ultrasound. Figure 7 It is a static sound field group ( Figure 8 10 times that of ).

[0068] In summary, the acoustically manipulated material enrichment system proposed in this disclosure includes a sound field generating device, an acoustically responsive material, an ultrasonic real-time imaging device, and a terminal device. The sound field generating device includes a signal generator, a power amplifier, and a transducer for emitting a sound field and adjusting its parameters. The acoustically responsive material includes acoustically responsive substances such as microspheres, bubbles, liposomes, and cells. The ultrasonic real-time imaging device is a microscope or medical imaging equipment used to observe the sound field enrichment effect. The terminal device is a computer device used to control the sound field generating device and the ultrasonic real-time imaging device. The applicable ultrasonic frequency for the variable-frequency sound field is between 0.01 MHz and 10 MHz. The sound field enrichment method uses planar transducers and focusing transducers. The spatial location of the enrichment position in the variable-frequency sound field is controllable, allowing for precise directional enrichment of most materials in the flow field.

[0069] It should be understood that in the embodiments of this disclosure, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0070] It is worth noting that although the foregoing has described the spirit and principles of this disclosure with reference to several specific embodiments, it should be understood that this disclosure is not limited to the disclosed specific embodiments, and the division of aspects does not imply that the features in these aspects cannot be combined; such division is merely for the convenience of expression. This disclosure is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A material enrichment system, characterized in that, include: A sound field generator is used to produce a variable frequency ultrasonic sound field for manipulating the enrichment of substances within a fluid cavity. The imaging device is used to acquire the distribution of enriched substances in a fluid cavity in real time and monitor the migration and enrichment process of substances under the action of a variable frequency ultrasonic field. The sound field generating device includes a signal generator, a power amplifier, and an ultrasonic transducer connected in sequence, wherein... The ultrasonic transducer is positioned relative to a fluid cavity, which is used to contain a fluid containing the substance to be enriched. The fluid cavity has a reflective interface, which causes the frequency-converted ultrasound incident on the fluid cavity by the ultrasonic transducer to interfere with the reflected ultrasound to form a standing wave sound field. The ultrasonic transducer causes the sound pressure nodes of the standing wave sound field to move in the direction of ultrasonic propagation by changing the frequency of the variable frequency ultrasound. The material to be enriched is captured at the sound pressure node by the sound radiation force and migrates with the node, thus achieving directional enrichment.

2. The system according to claim 1, characterized in that, The signal generator is used to output a frequency conversion drive signal, and the power amplifier is used to amplify the frequency conversion drive signal to a level that drives the ultrasonic transducer.

3. The system according to claim 1, characterized in that, The ultrasonic transducer is a piezoelectric focusing transducer or a piezoelectric planar transducer.

4. The system according to claim 1, characterized in that, The fluid cavity is a closed fluid cavity, and the reflective interface is located at the top of the fluid cavity.

5. The system according to claim 2, characterized in that, The frequency of the variable frequency drive signal varies linearly within the range of 0.01 MHz to 10 MHz.

6. The system according to claim 1, characterized in that, The system also includes a terminal device for controlling the sound field generating device and the imaging device.

7. A method for enriching a substance, characterized in that, include: A variable-frequency ultrasonic wave with a frequency that varies with time is generated by an ultrasonic transducer, forming a standing wave sound field in a fluid cavity with a reflective interface. The standing wave sound field includes sound pressure nodes. The material to be enriched, dispersed in the fluid within the fluid cavity, is captured at the sound pressure node by utilizing acoustic radiation force. By changing the frequency of the variable-frequency ultrasound, the sound pressure node moves in the direction of ultrasound propagation, thereby driving the material to migrate synchronously with the sound pressure node and achieving directional enrichment of the material at the target location.

8. The method according to claim 7, characterized in that, The reflective interface is located at the top of the fluid cavity or on the surface of the target tissue, opposite to the direction of ultrasound incident. The sound pressure node moves toward the reflective interface during the frequency conversion process.

9. The method according to claim 7, characterized in that, The substance is a microparticle with acoustic response characteristics, selected from polystyrene microspheres, biodegradable polymer microspheres, liposomes, or inorganic microparticles, and its size is smaller than the wavelength of ultrasound in the medium.

10. The method according to claim 7, characterized in that, After one cycle of the frequency-converting ultrasound, the frequency jumps back to the starting frequency, and the material is recaptured at the new sound pressure node. Through multiple frequency-converting cycles, the material is continuously transported to the target location and enriched.

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