“Looking for circulating tumor cells in a blood sample is like looking for a needle in a haystack,” said Tony Jun Huang, professor of engineering science and mechanics. “Typically, the CTCs are about one in every one billion blood cells in the sample.”
Existing methods of separation use tumor-specific antibodies to bind with the cancer cells and isolate them, but require that the appropriate antibodies be known in advance. Other methods rely on size, deformability or electrical properties. Unlike conventional separation methods that centrifuge for 10 minutes at 3000 revolutions per minute, surface acoustic waves can separate cells in a much gentler way with a simple, low-cost device.
Acoustic-based separations are potentially important because they are non-invasive and do not alter or damage cells. However, in order to be effective for clinical use, they also need to be rapidly and easily applicable.
“In order to significantly increase the throughput for capturing those rare CTCs, device design has to be optimized for much higher flow rates and longer acoustic working length,” said Ming Dao, principal research scientist, materials science and engineering, Massachusetts Institute of Technology. “With an integrated experimental/modeling approach, the new generation of the device has improved cell sorting throughput more than 20 times higher than previously achieved and made it possible for us to work with patient samples.”
The researchers worked both experimentally and with models to optimize the separation of CTCs from blood. They used an acoustic-based microfluidic device so that the stream of blood could continuously pass through the device for separation. Using the differential size and weight of the different cells they chose appropriate acoustic pressures that would push the CTCs out of the fluid stream and into a separate channel for collection. They report their results in the Proceedings of the National Academy of Sciences.
Tilted-angle standing surface acoustic waves can separate cells using very small amounts of energy. The power intensity and frequency used in this study are similar to those used in ultrasonic imaging, which has proven to be extremely safe, even for fetuses. Also, each cell experiences the acoustic wave for only a fraction of a second. In addition, cells do not require labeling or surface modification. All these features make the acoustic separation method, termed acoustic tweezers, extremely biocompatible and maximize the potential of CTCs to maintain their functions and native states.
If two sound sources are placed opposite each other and each emits the same wavelength of sound, there will be a location where the opposing sounds cancel each other. Because sound waves have pressure, they can push very small objects, so a cell or nanoparticle will move with the sound wave until it reaches the location where there is no longer lateral movement, in this case, into the fluid stream that moves the separated cells along.
The researchers used two types of human cancer cells to optimize the acoustic separation — HELA cells and MCF7 cells. These cells are similar in size. They then ran an experiment separating these cells and had a separation rate of more than 83 percent. They then did the separation on other cancer cells, ones for which the device had not been optimized, and again had a separation rate of more than 83 percent.
“Because these devices are intended for use with human blood, they need to be disposable,” said Huang. “We are currently figuring out manufacturing and mass production possibilities.”
Physicians could use the devices to monitor how patients reacted to chemotherapy, for initial diagnosis and for determining treatment and prognosis.
“This work, involving a highly cross-disciplinary group of medical doctors, engineers, computational biologists, and device experts, has led to the design and development of a label-free platform for identifying and separating CTCs while preserving the integrity of the cell,” said Subra Suresh, president, Carnegie Mellon University and part of the research team. “It promises to offer new avenues for basic research into the pathology and metastasis, and for clinical diagnosis of rare tumor cells.”