The use of lasers to optically trap, sort, guide and transfect individual mammalian cells can reduce the cost of diagnostic processes and research. These chemical-free optical processes can increase the ease and speed with which research is conducted, says the Council for Scientific and Industrial Research’s (CSIR’s) National Laser Centre (NLC) biophotonics head researcher Dr Patience Mthunzi.
“Micromanipulation (manipulating tiny objects) using lasers has significant biomedi- cal applications,” she explains, adding that, for example, cancerous cells can be sorted from noncancerous cells in minute sample volumes by using a tightly focused laser beam spot.
One of Mthunzi’s projects focuses on the human immunodeficiency virus (HIV-1) and part of her work is to use photonics-based approaches to devise cost-effective diagnostic tools for HIV-1. Since sample volumes range in the microlitre-size regime, anything between 20 µℓ and 80 µℓ, to be specific, the reduced amount of reagents used leads to cost savings.
Using lasers to tweeze or trap cells, enables researchers to specifically grab, move and analyse single mammalian cells or micro-particles within a mixed sample population at will and without affecting surrounding cells or particles.
The optical tweezing technique has advanced many innovative methodologies in the biomedical and biophotonics research arenas. For example, various microscopic devices, such as microfluidic chips, can be coupled to optical tweezing systems, which then paves the way for future prospects of designing multifunctional microscopic devices, she says.
Tiny laser beam spots (1 µm to 5 µm beam diameter) are used in the tweezing process and micrometre-sized particles (mammalian cells vary between 5 µm and 12 µm in size) can be manipulated using this technique. However, there have been reports of the technique being used on nanometre- scale-sized objects, such as gold and silver nanoparticles. This, then, means that subcellular observation and control of biological processes are possible.
Further, the beam can be shaped to produce different geometries or even a matrix of six to eight spots that can be used as multiple traps for particles, says Mthunzi.
To avoid optocution (damage by heating effects), optical manipulation of biological materials is normally performed using near-infrared (NIR) wavelengths. It is important to note that this range of the light spectrum is invisible to the naked human eye.
Further, most mammalian cells are transparent and they, therefore, do not absorb NIR light, leading to reduced risks of heating effects that could potentially cause cytotoxicity (cell death) during experiments.
“This makes the technique noninvasive. This is significant because, when one is working with biological materials like cells, one wants to ensure that there is as little interference as possible with the phy- siology and genetic composition of the cell,” she explains.
Meanwhile, phototransfection, which is the introduction of foreign genetic material into a cell through the use of pulsed laser light rather than chemicals, enables researchers to differentiate pluripotent stem cells into different cell types using a femtosecond laser light source.
Femtosecond lasers are systems that deliver extremely short pulses of light (10-15 seconds) with high peak intensities, and can be used to observe and possibly control biological processes.
A specific intensity laser is used to ablate the cell membrane, essentially causing a transient microexplosion, which creates a small self-healing opening in the cell’s plasma membrane. However, if the cell is submerged in a medium containing a genetic material of interest, then this genetic material is transported into the cell (through osmosis) before the opening closes. This procedure can be employed in stem cell differentiation, whereby specialised cells can be produced, enabling researchers to study these new cells further for cell-based therapy and tissue regeneration, she explains.
Mthunzi published a paper on this research in the Journal of Biomedical Optics, and now focuses on improving the technology.
“The beauty of this technique is that it eliminates the use of chemicals and only uses cell culture medium (food for cells) and buffers (that prevent pH variations),” she says.
“This is why we are promoting the optical treatment of stem cells because we want to see if we can get to a point where patients can be treated using stem cell therapy,” she emphasises.
Phototransfection also boasts transfection efficiencies equal to or greater than 80%, which rivals many chemically induced cell transfection methodologies, she notes.
“I can say with confidence that all the projects mentioned are feasible. However, the availability of funding to do such research is a challenge and can hamper our efforts,” she says.
“We have international collaborators, we know how to do the work and are establishing a biophotonics – optical tweezing/ guiding/sorting and transfection – laboratory here at the NLC, which should be operational by the end of May this year,” concludes Mthunzi.