‘HIV is diagnosed based on counting CD4 cells,’ said Tony Jun Huang, associate professor of engineering science and mechanics at Penn State University. ‘Ninety per cent of the diagnoses are done using flow cytometry.’
According to a statement, Huang and his colleagues designed a mass-producible device that can focus particles or cells in a single stream and performs three different optical assessments for each cell. They believe that the device represents a major step toward low-cost flow cytometry chips for clinical diagnosis in hospitals, clinics and in the field.
‘The full potential of flow cytometry as a clinical diagnostic tool has yet to be realised and is still in a process of continuous and rapid development,’ the team said in a recent issue of Biomicrofluidics. ‘Its current high cost, bulky size, mechanical complexity and need for highly trained personnel have limited the utility of this technique.’
Flow cytometry is said to typically look at cells in three ways using optical sensors. Flow cytometers use a tightly focused laser light to illuminate focused cells and to produce three optical signals from each cell.
These signals are fluorescence from antibodies bound to cells, which reveals the biochemical characteristics of cells; forward scattering, which provides the cell size and its refractive index; and side scattering, which provides cellular granularity.
Processing these signals is said to allow diagnosticians to identify individual cells in a mixed cell population, identify fluorescent markers and count cells and other analyses to diagnose and track the progression of HIV, cancer and other diseases.
‘Current machines are very expensive costing $100,000 (£64,500),’ said Huang. ‘Using our innovations, we can develop a small one that could cost about $1,000.’
The university says that one reason current machines are so large and expensive is the method used to channel cells into single file and the necessary alignment of lasers and multiple sensors with the single-file cell stream. Currently, cells are guided into single file using a three-dimensional flow cell that is difficult to manufacture. More problematic is that these current machines need multiple lenses and mirrors for optical alignment.
‘Our approach needs only a simple one-layer, two-dimensional flow cell and no optical alignment is required,’ said Huang.
Huang and his team used microfluidic drifting to create a focused stream of particles.
The microfluidic chip’s channel begins as a main channel that contains the flow of carrier liquid and a second channel that comes in perpendicularly that carries the particles or cells. Immediately after these two channels join, the channel curves 90°, which moves all the cells into a horizontal line. After the curve, liquid comes into the channel on both sides, forcing the horizontal line of cells into single file. The cells then pass through a microlaser beam.
The microfluidic flow cytometry chip can be mass-produced by moulding and standard lithographic processes. The fibres for the optical-fibre delivered laser beams and optical signals already exist.
‘The optical fibres are automatically aligned once inserted into the chip, therefore requiring no bulky lenses and mirrors for optical alignment,’ said Huang. ‘Our machine is small enough it can be operated by battery, which makes it usable in Africa and other remote locations.’
The researchers tested the device using commercially available, cell-sized fluorescent beads. They are now testing the device with actual cells.