Imagine a tool the size of a credit card that can analyze single cells with a throughput of more than 2 million cells per second. Moreover, the tool retains each cell of interest for downstream molecular analysis. The first steps towards this vision are being made in the Belgian research institute Imec. Experts in holographic imaging, high-speed image processing, microfluidics, and cell biology are working together to realize this lab-on-chip tool for applications related to circulating tumor cells, white blood cells, stem cells, and bacteria.
Shrink the Tool and Magnify Its Usage
Cytometry and cell sorting is a workhorse in medical research and practice. The goal is to discriminate cells based on size, morphology, cell pigments, protein expression level, etc. The most common method used today for this purpose are cell imaging microscopes and fluorescence-activated cell sorting (FACS). These instruments are readily available at university hospitals and research institutes. They offer a high quality but are costly and large. What if those two instruments could be combined into one, with the same quality at a much lower price and size? Then, these cell sorting tools could become mainstream in hospital and doctors’ offices.
This trend towards miniaturization into so-called lab-on-chip (LOC) systems is clearly ongoing in the life science arena. Microfluidic FACS systems are under development today. Although slower than their “big brother,” they are smaller, cheaper, and disposable. (See Figure 1)
Research institutes around the world are developing LOCs for cell analysis and sorting. Imec began some 30 years ago as an expertise center in microelectronics and is currently collaborating with top semiconductor companies worldwide to improve the performance of silicon chips. The center brought biologists and bioengineers on board about ten years ago, believing that bringing together expertise in electronic devices and biology would make brilliant things happen.
One such thing is its recent development: a lab-on-chip for cell sorting that incorporates unique imaging technology and microfluidics. It promises higher throughput, faster results, and higher cell sorting flexibility than the LOC systems on the market today, mainly due to the use of lens-free on-chip imaging.
A cell sorter device consists of different building blocks. Let’s illustrate this by following the path of the cells through the LOC device. (See Figure 2)
(1) A blood sample is injected into the inlet of the microfluidic channel. The volume largely depends on the application. For example, when hunting for rare circulating cells, a volume of 1 ml should be analyzed, thus requiring a very high throughput.
(2) With a speed of >1m/sec, the cells flow over the imaging area. Above the microfluidic channel, this area consists of a small laser that is used as light source. The light, which is scattered by the cell, is recorded by the image sensor at the bottom of the microfluidic channel. This principle of using scattered light to reconstruct an object is called holographic lens-free imaging. It uses no lenses, and off the shelf optical components, making it compact. The currently achieved optical resolution is 0.7 um, which is more than enough to image subcellular features.
(3) The images are processed and reconstructed using dedicated algorithms. Within 100μsec the cell type is identified and classified based on the images.
(4) Depending on the cell type, the cells are pointed towards a specific fluidic channel by microfluidic switches based on microbubbles. Thermal bubble generators, similar to the generators used in thermal bubble inkjet printers, are integrated at the top surface of the microfluidic channels. The tiny and short-lived steam bubbles that they create are used to gently deflect the cells towards the desired outlet.
(5) In this way, cells are sorted based on cell type. The cells are still viable after passing through this sequence of steps. This makes it possible to analyze them further. For example, the DNA of circulating tumor cells can be analyzed to collect more information on the type of cells causing the metastasis, and on cancer mutations or drug resistances.
(6) A blood biopsy based on a cell sorter is more likely to be included as a routine clinical test in follow-up cancer patients than the more invasive and labor intensive biopsies that are currently used in cancer diagnostics.
At Imec, a lab-on-chip device is under development with three key advantages over other similar systems. It’s label-free. The cell sorter employs lens-free holographic imaging techniques to analyze single-cell morphology in microfluidic channels. No targets or labels are required to identify or sort cells. It’s scalable. The target is to sort 2,000 cells per second, per channel. By using semiconductor manufacturing processes, thousands of channels can be processed next to each other, reaching composite sorting speeds of up to 2 million cells per second. And, it’s compact and inexpensive. The system consists of a few cheap optical components, as most of the optical system is replaced by digital lens-free imaging algorithms. The disposable chip is no larger than a microscope slide, making it accessible at the point of need. And, doctors and nurses can use it at the patient’s bedside or in the doctor’s office.
The concept of each of the subcomponents has been proven and the main expertise to further develop the device are present. Future work will focus on developing successive prototypes with dedicated and optimized image sensors, processing platforms, microheaters, and microfluidics with thousands of channels. Together with medical device manufacturers and clini cal end users the concept will then be further translated towards specific applications. If everything goes according to plan, we expect it will take three to four years to fully develop this LOC device.
Imec (Interuniversity Microelectronics Center), Leuven, Belgium http://www2.imec.be/be_en/home.html