Multiple phase flow system for detecting and isolating substances

A multiple flow system and method for detecting substances in a fluid is provided. More specifically, a first fluid tube containing a first fluid and a second fluid tube containing a second fluid are coupled to a common fluid tube via a connector, such that alternating discrete compartments of the first fluid and the second fluid flow through the common fluid tube. The first and second fluids are immiscible. A substance detector, having a flow chamber with an internal wall, is coupled to the common fluid tube. The alternating discrete compartments of the first and second fluids flow through the flow chamber and are analyzed by the substance detector.

BACKGROUND

Melanoma is the most deadly form of skin cancer. Although melanoma has a relatively low death rate compared with other cancers as it is generally easy to see on the skin and can be simply removed, it quickly becomes deadly if the superficial tumor metastasizes to other parts of the body. In metastasis, circulating melanoma cells (CMCs), cells that have broken off from the original tumor site and move through the blood or lymphatic system, can plant themselves elsewhere and create secondary tumors that are subsequently the cause for terminal melanoma. Accordingly, early diagnosis will likely be the cure to cancer, as the treatments have higher chances of success because the cancer has not fully manifested itself yet.

Furthermore, it is predicted that in 2010 over 68,000 Americans will be diagnosed with the disease and over 8,000 melanoma patients will die from the illness. Nevertheless, melanoma can be easily cured if it is detected early and quickly removed from the skin; however, if it metastasizes, the cancer can become lethal. Considering that a 1 mm diameter tumor typically consists of one million cells, larger more visible tumors can generally consist of billions of cells. That said, a patient who is diagnosed from a large tumor will have reduced options and will necessitate more physically demanding treatment than a patient who is diagnosed before secondary tumors can form. Unfortunately, patients must currently wait months to know if the superficial melanoma tumor has spread, because the current methods of diagnosing metastatic cancer are through imaging techniques, which require the presence of a visible metastatic tumor. Consequently, patients are oftentimes diagnosed too late.

Current methods for diagnosing metastatic cancer include lymph node biopsies and imaging techniques. However, lymph node biopsies can produce false negatives if the cancer did not interact with the lymphatic system and cannot be performed numerous times for disease monitoring. That said, circulating tumor cells can be an excellent source of information for diagnosis and monitoring of metastatic melanoma and other pathological diseases if they can be detected in the lymphatic system or blood stream. For that reason, many techniques are being investigated to detect and isolate these cells for both diagnostic and disease monitoring purposes. Some research has already been conducted to find CMCs, including RT-PCR, immunohistochemistry, magnetic cell sorting, fiber optic array scanning technology, and microfilters; however, high false negative rates, labeling, and long procedures limit chances of clinical implementation.

Unfortunately, the current methods used to diagnose metastatic disease are not sensitive to single metastatic cells. Patients must wait until secondary tumors are formed before they can be diagnosed and begin life saving treatment.

DETAILED DESCRIPTION

Embodiments of the present invention are directed to a label-free method to detect melanoma in vitro in combination with an isolation method that is simple, has a low margin for error, and effective. Metastatic melanoma cells are detected and then isolated from a patient sample, such as a centrifuged blood sample. Isolation of melanoma cells will aid not only in early diagnosis of the patient but there is a positive correlation between the number of circulating tumor cells present in the blood and the prognosis of the patient. Detection of circulating melanoma cells, cells that have broken off from the original tumor site and move through the blood or lymph system, can be diagnostic and monitoring tools for cancer. Being able to isolate and count melanoma cells could be used for monitoring therapies of cancer patients; high cell counts would indicate a severe prognosis, whereas low cell counts would indicate that the treatment is working well.

Detection of metastatic melanoma in the blood is imperative because it eliminates the waiting game for cancer patients. Using embodiments of the present invention, patients can have a simple blood sample scanned for cancer as often as desired instead of waiting months to see if a tumor is visible on a scan. These months could have been spent providing the patients with life saving therapies, as opposed to waiting to see if the cancer has spread, or monitoring whether current therapies are working.

Embodiments of the present invention decrease potential error involved with a normal flow system and allow for preliminary isolation of metastatic melanoma from the patient sample. In one embodiment, the detector includes a laser, transducer, and oscilloscope.

Having briefly described an overview of embodiments of the present invention, exemplary figures depicting embodiments of the present invention are described in order to provide a general context for various aspects of the present invention. With reference toFIGS. 1-4in particular, where like reference numerals identify like elements in the various views, an exemplary multiple phase flow system10is illustrated. Multiple phase flow system10generally includes a first pump12, a second pump14, a first fluid tube24, a second fluid tube26, a connector28, a multiple fluid tube30and a substance detector32. Although depicted as having two pumps, it will be appreciated that multiple phase flow system10may include any number of pumps. First pump12and second pump14are depicted inFIG. 1as being syringe pumps. However, it will be appreciated that any type of device that is used for the movement of fluids may be utilized as a pump. The flow rate for the multiple phase flow system used ranges from 0-1000 μL/sec. In further embodiments, the movement of fluids through the multiple phase flow system is based on pressure driving the flow of the respective fluid through the system. For example, the one or more fluids may be flowing through the multiple phase flow system based on gravity, resulting in an elevation head for the fluid.

With reference toFIG. 2, tubes24,26and30are a long hollow and typically cylindrical object, used for the passage of fluids. In one embodiment, tubes24,26and30are silicone tubing.

Referring briefly toFIG. 4, the flow chamber33within the detector chamber38is made from acrylamide gel shaped around a wire or other tube. The acrylamide gel is allowed to harden or solidify and the wire is removed to create a channel, such as flow chamber33defined by an internal wall, for fluid to flow through when it is within the detection chamber38. It will be appreciated that any variety of other methods and materials may be used to create flow chamber33, for fluid to flow through. In embodiments, flow chamber33is made from a material having an acoustic impedance close to the acoustic impedance of water. The multiple phase flow system10has an inner flow diameter of the flow chamber33from about 0-10 mm in diameter. The acrylamide is used as an acoustic path from the excited cell to the acoustic sensor.

With reference toFIGS. 1 and 2, a first fluid20and a second fluid22are fed from their respective tubes24and26into common fluid tube30using a connector28. A fluid is any state of matter which can flow with relative ease and tends to assume the shape of its container and may include a liquid, gas or plasma. In this embodiment, first fluid20is different from the second fluid22and is immiscible and/or creates bubbles with second fluid22. For example, first fluid20may be air while second fluid22is water or other fluid creating gas bubbles of the first fluid20within the second fluid22. In another example, first fluid20is oil while second fluid22is water or hydrophilic liquid. WhileFIGS. 1 and 2are depicted as having a first and second fluid, it will be appreciated that any number of multiple different fluids may be utilized in embodiments of the present invention. For example, a third fluid may flow into common fluid tube30via connector28. As such, as shown in the multiple phase flow system800ofFIG. 8, a third pump802may include a tube808coupled to the connector28.

First fluid20and second fluid22are fed into a common fluid tube30using a connector28. Common fluid tube30is a common channel for the fluids to flow through. The connector28depicted inFIGS. 1 and 2is a Y connector. It will be appreciated that the connector may be any shape, including a Y connector or T connector that allows for the first fluid20and second fluid22to be forced together into a single flow path without mixing together. In one embodiment, the multiple phase flow system may use three fluids, two of which mix together in the common fluid tube30while remaining separate from the third fluid. For example, if the three fluids are water, alginate and air, the water and alginate may be mixed together to form a gel discrete compartment and that remains separate from the air discrete compartment. In further embodiments, instead of two tubes meeting at a common tube, a multiple phase flow may be created based on multiple tubes meeting at the common fluid tube30.

First fluid20and second fluid22essentially take turns entering the common fluid tube30due to the immiscibility (e.g., oil and water) and/or gas bubble creation (e.g., air and water) of the two fluids. In one embodiment, the immiscibility is due to the hydrophobicity of the first fluid (such as oil) and the hydrophilicity of the second fluid (such as water soluble fluid). As the first fluid20enters common fluid tube30, second fluid22builds up pressure until the first fluid20stops flowing and second fluid22has its turn to flow. This creates alternating discrete compartments of first fluid20and second fluid22respectively. By introducing two fluids through a connector, such as a T-junction into a single flow path, a substance is compartmentalized. This is ideal because the exact discrete compartment of a first fluid or second fluid in which the substance to be detected is located can be identified and easily extracted. It will be appreciated that the discrete compartments of the first fluid20and second fluid22as shown inFIGS. 3A and 3Bmay vary in size depending on the diameter of common fluid tube30and flow rate of the fluids may be from 0-1 ml/s in size. Furthermore, more than two fluids may be utilized and biological fluids from multiple patients may also be used and separated in alternating discrete compartments.

In embodiments, first fluid20is a separator fluid such as air, gas and/or oil for creating discrete compartments of second fluid22. Second fluid22may include any type of fluid that includes a substance to be detected. In one embodiment, second fluid22may be any type of fluid containing a biological material or substance, such as cancer cells (breast cancer, melanoma, prostate cancer and the like), tissue, cells, pathogens, microorganisms (such as malaria), infectious diseases, pigment, non-optically absorbing substances with color inducement (e.g., dyes added to non-colored cancer cells or gold nanoparticles added to cells), compounds, non-biological substances and/or foreign bodies from an individual's biological sample including, but not limited to, blood, saliva, urine, fecal material, serum, plasma, tissue and spinal fluid. In another embodiment, second fluid22is a non-biological fluid containing biological or non-biological substances such as detecting lead (substance) in water (second fluid). As will be described in more detail below, in one embodiment second fluid22is centrifuged white blood cell solution of an individual which may or may not contain melanoma cells. In one embodiment, whole blood samples are taken from melanoma patients and centrifuged, leaving any melanoma in a white blood cell suspension.

The alternating discrete compartments of first fluid20and second fluid22flow through common tube30into the flow chamber33within the detection chamber38of detector32. With reference toFIGS. 1 and 4, generator34is a photoacoustic laser source. Detection chamber38includes an internal wall36defining an open interior of the detection chamber38. Laser source34includes a laser40and pulsator42. Although laser source34is depicted in one embodiment to be a photoacoustic detector, it will be appreciated that generator34may include any variety of excitation devices and/or mechanisms for converting energy into something that can be further processed to obtain information including radiation, laser, chemical, electrical semiconductor, fluorescence, radio waves, magnetic, tomography, thermography and ultrasound detection.

In one embodiment, a photoacoustic detector can detect or determine whether or not a fluid contains substances having pigment such as metastatic melanoma which inherently contains melanin, a small pigmented granule. As an individual discrete component of fluid passes through flow chamber33, it is irradiated by photoacoustic laser source34with a rapid pulse of light. It will be appreciated that the laser light may be directed transversely to the flow or along the axis of the flow using an optical waveguide as described with respect toFIG. 2B. When melanoma is irradiated with a rapid pulse of intense laser light the melanin undergoes thermo-elastic expansion. Thus, if the discrete component passing through the chamber33contains melanoma, the melanin in the melanoma undergoes thermo-elastic expansion and ultimately creates a photoacoustic wave. Irradiated melanoma cells produce photoacoustic waves which are detected with a piezoelectric transducer, while the white blood cells create no signals because they are optically transparent. It will be appreciated that a detector may be used to determine whether or not a fluid passing through the detector in a multiple phase flow system contains a particular substance.

In this embodiment, as shown in and discussed in more detail below with respect toFIGS. 5A-5F, the irradiation volume for each discrete compartment that has been irradiated by intense laser light may be communicated from an acoustic sensor (or other type of sensor) within flow chamber33via connection50to computer display device52or other output device. The irradiation volume for the discrete compartment is displayed on display device52and shown and described in more detail below with respect toFIGS. 5A-5F. The irradiation volume graphical displays include a spectrometer before irradiation point to ensure the expected medium is producing signals. Use of the irradiation volumes can allow for single melanoma cell isolation, because when a melanoma cell is detected it can visually be seen which discrete compartment of fluid melanoma cell(s) reside in.

Specific discrete compartment(s) detected to contain the substance being tested for, such as melanoma cell(s), can be extracted. With reference toFIG. 1, a collection container48may be used to collect the discrete compartment from the tube exit46. It will be appreciated that while the collection of the discrete compartment of fluid identified as containing the substance being tested for is depicted as being done manually in collection container48inFIG. 1, any variety of collection mechanisms, automatic or manual, may be used to collect the discrete compartment. Furthermore, alternative separate containers (not shown) may be used to collect other discrete compartments that have not tested positive for the substance being tested for. Fluid volumes containing substances, such as melanoma cells, may be sequestered by bubbles, allow for isolation of small volumes of fluid. Whereas, utilizing a continuous flow without bubbles it is difficult to determine what volume of liquid to collect. Collected discrete compartments of fluids can be sent for a variety of testing or further diluted.

The extracted discrete compartment may then be diluted and re-passed through the system10and repeated until each separate extraction volume of the second fluid22contains a single melanoma cell and no white blood cells. As such, the analysis of samples may be conducted iteratively until a desired concentration of a desired substance is detected and/or isolated. In one embodiment, each iteration of the repeated separation reduces the concentration of undesired particles (such as white blood cells) by a particular factor of dilution. For example, a discrete compartment demonstrating a photoacoustic effect (thus indicating that the discrete compartment contains a melanoma cell) can be diluted and re-passed through the system10iteratively until the desired melanoma cell is isolated from other undesired particles. The isolated melanoma cell (or other desired particle) can then be analyzed for particular testing, such as gene expression testing, PCR, and the like.

In embodiments, pigmentation is detected due to photoacoustic thermo-elastic expansion when an absorber is struck with a rapid pulse of light; colorless particles remain acoustically transparent allowing numerous cells to cross the detection beam at a time. Accordingly, a multi-phase flowmetry technique adjunct with photoacoustic detection to detect and capture CMCs in vitro is provided. Capture of the detected melanoma cells not only verifies that melanoma is truly being detected; it will also provide cancer biologists with early stage CMCs to study.

With reference toFIG. 2B, an optical waveguide54is produced by the fluid having a higher optical index of refraction than the surrounding material. Fluid that contains the cells could have an index of refraction of 1.39 by adding sugar or polyethylene glycol and the surrounding acrylamide flow chamber may have an index refraction of 1.35. This mismatch allows total internal reflection, making an optical waveguide much like an optical fiber. This allows the alternating components to be an optical wave guide so laser light is filling entire tube and every discrete compartment is filled with laser light. The laser light can irradiate the entire sample due to total internal reflection. Total internal reflection occurs when the light hits a material that has a lower index of refraction than the material it is current moving through. This allows for the laser light to stay inside the medium of choice as long as the index of refraction parameters are met. Therefore, the flow channel is an optical waveguide to ensure the entire discrete compartments are irradiated without wasting any laser energy. This is done in one embodiment by adding sugar, polyethylene or other substance to increase the index of refraction to the second fluid to increase to higher than the index of refraction of acrylamide.

The multi-phase flow system10allows for a sample of interest to be run across a detector to identify if a certain analytes and/or substances are present in the sample. Millions of cells can flow past the detector at a time due to the photoacoustic transparency of white blood cells. This allows for large volumes to be scanned very quickly for circulating melanoma cells.

Multiple Phase Flow Design

The multiple phase flow system utilized in Example 1 comprises cylindrical silicone tubing. However, it will be appreciated that other shapes such as square or rectangular could be used. The multiple phase flow system has an inner flow diameter of the channel of about 1.6 mm in diameter. The multiple phase flow system used ranged from 200-300 μL/min. However, the slugs that formed were only 2-3 microliters and the Capillary and Reynolds number stayed within microfluidic conditions. In addition, high flow rates were used so a large blood volume could be examined quickly.

The system utilized a polyvinylidene fluoride (PVDF) “clip” transducer. The sensor may be any piezoelectric material or other non-piezoelectric sound sensor. It will be appreciated that any material may be used so long as it conducts electrical current. Two brass sheets were cut with a hole in the middle and soldered to a BNC coaxial cable connector. Tape separated the brass sheets and PVDF was placed between the brass covering the holes. The transducer was placed into a Polydimethylsiloxane (PDMS) housing that kept both the transducer and flow chamber stationary. The transducer was fitted into the slit and a PDMS ring was placed above the transducer, exposing the PVDF but covering the brass. The PDMS was used to prevent acoustic reflections from the brass that could interfere with melanoma detection.

Next the flow chamber was placed on top of the PDMS ring such that the chamber was directly aligned with the PVDF element. A frequency-tripled, Q-switched Nd:YAG laser (Continuum) fired at the side of the flow chamber at a right angle from the transducer. The laser pulsed at 532 nm, 20 Hz, and between 5-8 mJ for 5 ns pulse duration.

The flow chamber was connected to two syringe pumps; one syringe contained the cell samples and the other contained air. The air is pumped at 0.2 mL/min and the cell samples are pumped at 0.1 mL/min. The syringe pump that housed the cells was set vertically to ensure the cells did not settle to the bottom, therefore the air needed a higher flow rate to compensate for the increased pressure.

Sample Preparation

Two-phase flow was created using a T-junction, which combines the two separate phases into one flow path while keeping the phases distinct. The phases chosen for use in Example 2 were both water and oil, and air and water. Air and water produced a water slug could easily be extracted without contaminating the sample with the neighboring phase. However, when using air and water the liquids build up pressure and purge the system. Tween 20 was used in the air/water two-phase flow system to reduce the interfacial tension between the phases. A 2% Tween 20 in PBS buffer was used as the water phase. It will be appreciated that other surfactants, such as 2% Tween 80 may also be used to reduce the tension between the phases.

Sample Preparation

Biological samples were used to show that the multi-phase system is applicable to a clinical environment. Melanoma cells were cultured at the University of Missouri, and all white blood cell samples were obtained from whole blood donated from lab members. The cell culture and enrichment techniques are described below.

Melanoma Cells Suspended in PBS.

An HS 936 melanoma cell line was cultured for use in photoacoustic experiments. The cells were fixed in ethanol and re-suspended in PBS. Approximately 15 minutes before experiments, the cells were diluted with a phosphate buffered saline (PBS)+2% Tween 20 (Fisher Scientific) solution to the desired cell concentration. The cells were then counted manually using a hemoctyometer.

White Blood Cells Suspended in PBS and Melanoma.

White blood cells suspended in PBS were prepared by first obtaining cancer-free whole blood donated from lab members. The blood was poured into a centrifuge tube that contained Histopaque 1077, a material whose density value is between that of white blood cells and red blood cells, and then centrifuged. The white blood cell layer and the contents adjacent to it were removed and then added to smaller diameter centrifuge tube. This tube was centrifuged again and the white blood cell layer was removed and diluted with PBS.

White blood cell samples spiked with melanoma suspended in PBS were prepared by first obtaining whole blood donated from lab members. The blood was again poured into a centrifuge tube that contained Histopaque 1077. Cultured melanoma cells from the HS 936 line were added into the same centrifuge tube. The centrifugation process and the separation procedure remained the same as the White Blood Cell preparation above. The melanoma cells and white blood cells settle to the same layer after centrifugation due to their similar densities.

This layer was isolated and diluted with PBS.

Flow Chamber Design

The photoacoustic multi-phase flow system utilized a PVDF piezoelectric transducer while the laser struck at 90° from the transducer element. The alternative is to introduce laser light along the axis of flow utilizing the optical waveguide nature of the system. A single cell is considered an optical point source and will therefore emit photoacoustic waves radially in all directions equally. The flow chamber was held together with an acrylic ring that had three holes drilled at 90° from each other. Masterflex tubing was fed through the holes and a wire with the same outer diameter as the tubing's inner diameter was suspended through the two opposing holes while a second wire was fed through the third hole to prevent acrylamide from entering the tubing hole.

Once the acrylic ring was prepared, Parafilm was stretched across the bottom of the ring and clear acrylamide was poured into the ring, gelling around the tubing and wire. The acrylamide was made from 10 mL of 20% acrylamide solution (Sigma Aldrish), 0.04 g of ammonium persulfate (Sigma Aldrich) and 20 μL of TEMED (Fisher Scientific). After adding the TEMED, the mixture was poured immediately to avoid premature gelling.

After gelling, the wires were removed and the tubing used to hold the optical fiber was pulled out approximately 4 mm. The flow chamber was then ready to be used in the system.

Blind Study

In order to prove that any clinician can successfully operate this system, a blind study was performed to determine if samples that do not contain melanoma can be easily distinguished from samples that do contain melanoma.

Twenty random Boolean values were obtained using a short program made in MATLAB: “0” represented PBS+2% w/v Tween 20 solution, and “1” represented 10 melanoma cells/μL suspended in PBS+2% w/v Tween 20.

Two scientists initially prepared the samples without revealing which samples contained melanoma and which samples did not. Then two different scientists ran the samples through the flow system and were asked to determine which samples had melanoma cells and which did not based on the photoacoustic waves that were seen on the oscilloscope.

The scientists correctly identified all 20 samples, as seen in Table 1. Photoacoustic signals of a baseline and a detected cell sample are shown inFIGS. 6 and 7. In addition to larger amplitudes, melanoma cells produce transient signals while baseline peaks remain constant throughout flow.

Photoacoustic Detection and Capture of Melanoma Amongst WBCs

The flow system was prepared using three separate inlets: one syringe pump contained air, one syringe pump contained WBCs in PBS+Tween 20, and one syringe contained cultured melanoma cells in PBS. The WBC+PBS and air formed slugs throughout the system and were used to show that no photoacoustic signals were produced from either air or WBC bubbles. The syringe pump containing white blood cells was stopped and the syringe that contained melanoma was used to manually introduce a melanoma bubble into the flow system in hopes to create photoacoustic waves and then be isolated from the system.

Results

It was observed that two phase flow using oil and air produced uniform, consistent bubbles without any backlogging of the flow system. Water and air.

It was observed that two phase flow using water and air produced inconsistent flow and pressure buildup occasionally purged the flow system.

After 2% Tween 20 was added to the water, consistent and uniform bubbles were produced without undergoing any backlogging.

Photoacoustic Melanoma Signals by Concentration

The system produced a photoacoustic signal from the melanoma bubbles and remained baseline for the WBC and air bubbles as seen inFIGS. 5A-5F. The detected melanoma bubble was then isolated as it dripped out of the system and the bubble was imaged along with a control WBC bubble.

FIGS. 5A-5Eare graphical representations of photoacoustic signals from different melanoma cell concentrations of discrete compartment detected utilizing the multi-phase flow system described above.FIGS. 5A-5Eare plotted as photacoustic amplitude (mV) v. time (μs).FIG. 5Ais a graphical representation of photoacoustic signals from a melanoma cell-free sample.FIG. 5Bis a graphical representation of a photoacoustic signal from a sample containing 10 cells/μL of melanoma.FIG. 5Cis a graphical representation of a photoacoustic signal from a sample containing 25 cells/μL of melanoma.FIG. 5Dis a graphical representation of a photoacoustic signal from a sample containing 100 cells/μL of melanoma.FIG. 5Eis a graphical representation of a photoacoustic signal from a sample containing 800 cells/μL of melanoma.

FIG. 5Fis a graphical representation of plot of voltage response vs. cell concentration. The amplitude of photoacoustic signals followed linearly with concentration until concentrations of 10 cells/μL as shown inFIG. 5F.

Although successful bubble formation was achieved using both oil/water and air/water+surfactant, due to the ease of which melanoma cells can be extracted from the flow system, water and air with the addition of a surfactant will be the choice fluids for further studies. The addition of Tween 20 was effective due to its ability to decrease interfacial tension between the two phases allowing water to slide past the air when pressure built up.

The concentration study showed a linear correlation between voltage amplitude and concentration until very low concentrations were investigated. This was expected because the cells do not disseminate homogenously, which is made apparent at low concentrations. Also, as expected the PBS+Tween 20 control did not produce any photoacoustic signals.

The blind study confirmed that this method can be used in a clinical setting to detect melanoma. The melanoma cell capture further proved that the system can effectively isolate detected melanoma cells from the system. In addition, this is a significant step towards both an efficient disease monitoring technique and will enable scientists to study these metastatic cells in hopes of discovering the mechanisms by which they metastasize, survive in the blood and lymphatic systems, and settle in other tissues, so that better therapies for cancer patients and other types of disease can be developed.

Capture and Isolation of Melanoma

The system produced no signals for the PBS baseline, and produced photoacoustic signals from each different concentration of melanoma slugs.FIGS. 6 and 7show the photoacoustic response of 1 melanoma cell/μL compared to the PBS baseline. Slugs that produced photoacoustic signals were isolated and then stained using the Fontana Masson stain, but the best results were from the 100 cells/μL samples, most likely due to the harsh staining procedure that require the slides to be washed numerous times, the nucleus stained red and melanin stained black. As shown inFIG. 6, a graphical view600of a photoacoustic amplitude of a sample of irradiated white blood cells yields no photoacoustic effect, with an isolated droplet of the cell suspension showing a white blood cell610. InFIG. 7, a graphical view700of a photoacoustic amplitude of a sample of irradiated melanoma cells among white blood cells yields photoacoustic waves. An isolated droplet of the cell suspension showing photoacoustic waves indicates the presence of pigmented melanoma cells710.