The introduction of membrane impermeable substances such as foreign DNA into a cell (transfection) is a ubiquitous problem in cell biology. This technique is particularly challenging when it is desirable to target specific cells for treatment, as most transfection technologies (such as electroporation and liposomal transfection) are based on treating a global population of cells simultaneously [1]. However, laser-assisted cell poration, or ‘photoporation’, offers the distinct advantage of such cell specificity while maintaining high transfection efficiency, good post-transfection cell viability and overall ease of operation. In the technique of photoporation, a laser beam is typically focused through a high numerical aperture microscope objective lens onto the outer membrane of the targeted cell. The brief presence of the highly localised laser beam serves at least to modify the permeability of the cell membrane, if not creating a transient pore, thereby allowing foreign DNA in the surrounding medium to enter before the cell heals itself. 

When photoporation was first reported more than 20 years ago, a single nanosecond laser pulse at a wavelength of 355 nm was used to punch a self-healing hole in a cell membrane [2]. Subsequently, the technique of photoporation has been successfully achieved with a variety of continuous-wave [3, 4] and pulsed [5-8] laser sources spanning the ultraviolet, visible and near-infrared spectral regions. Most recently, the femtosecond titanium sapphire laser [9-11] has proven to be highly successful as a choice of laser source for photoporation, due mainly to the superior peak powers of femtosecond pulses. The photoporation mechanism in the case of tightly-focused near-IR femtosecond pulses relies strongly on two-photon processes. However, the single-photon approach can also be effective, as we have demonstrated in our laboratory with a continuous-wave violet diode laser in a highly simplified opto-mechanical environment [12].
 

One of the problems in with using a tightly focussed Gaussian beam is that there is a very narrow range of focus across which transfection is successful [13]. This makes the dream of, for example, computer automating the process of transfecting an entire area of a cell culture monolayer quite difficult. To this end, we have investigated novel beam shaping methodologies, such as the Bessel Beam, in order to obviate the need for tight focussing [14]. 
    

DNA is not the only membrane impermeable substance that can be loaded into a cell by photoporation. RNA [15], quantum dots [15], dextran [15], protein [15], ethidium bromide [16], sucrose [16], Trypan Blue [13], propidium iodide [17], merocyanin 540, [17], Sytox Blue [15], Sytox Green [15] are only a small sample of the diverse array of compounds a researchers is able to load into an individual cell with this technique.

Further reading: Optical Trapping Group research page on photoporation

1. S. Mehier-Humbert, and R. H. Guy, "Physical methods for gene transfer: Improving the kinetics of gene delivery into cells," Advanced Drug Delivery Reviews 57, 733-753 (2005).
2. M. Tsukakoshi, S. Kurata, Y. Nomiya, Y. Ikawa, and T. Kasuya, "A Novel Method of DNA Transfection by Laser Microbeam Cell Surgery," Applied Physics B-Photophysics and Laser Chemistry 35, 135-140 (1984).
3. H. Schneckenburger, A. Hendinger, R. Sailer, W. S. L. Strauss, and M. Schmidtt, "Laser-assisted optoporation of single cells," Journal of Biomedical Optics 7, 410-416 (2002).
4. G. Palumbo, M. Caruso, E. Crescenzi, M. F. Tecce, G. Roberti, and A. Colasanti, "Targeted gene transfer in eucaryotic cells by dye-assisted laser optoporation," Journal of Photochemistry and Photobiology B-Biology 36, 41-46 (1996).
5. Y. Shirahata, N. Ohkohchi, H. Itagak, and S. Satomi, "New technique for gene transfection using laser irradiation," Journal of Investigative Medicine 49, 184-190 (2001).
6. J. S. Soughayer, T. Krasieva, S. C. Jacobson, J. M. Ramsey, B. J. Tromberg, and N. L. Allbritton, "Characterization of cellular optoporation with distance," Analytical Chemistry 72, 1342-1347 (2000).
7. S. Sagi, T. Knoll, L. Trojan, A. Schaaf, P. Alken, and M. S. Michel, "Gene delivery into prostate cancer cells by holmium laser application," Prostate Cancer and Prostatic Diseases 6, 127-130 (2003).
8. S. K. Mohanty, M. Sharma, and P. K. Gupta, "Laser-assisted microinjection into targeted animal cells," Biotechnology Letters 25, 895-899 (2003).
9. U. K. Tirlapur, and K. Konig, "Cell biology - Targeted transfection by femtosecond laser," Nature 418, 290-291 (2002).
10. U. K. Tirlapur, and K. Konig, "Femtosecond near-infrared laser pulses as a versatile non- invasive tool for intra-tissue nanoprocessing in plants without compromising viability," Plant Journal 31, 365-374 (2002).
11. E. Zeira, A. Manevitch, A. Khatchatouriants, O. Pappo, E. Hyam, M. Darash-Yahana, E. Tavor, A. Honigman, A. Lewis, and E. Galun, "Femtosecond Infrared Laser - An Efficient and Safe in Vivo Gene Delivery System for Prolonged Expression," Molecular Therapy 8, 342-350 (2003).
12. L. Paterson, B. Agate, M. Comrie, R. Ferguson, T. K. Lake, J. E. Morris, A. E. Carruthers, C. T. A. Brown, W. Sibbett, P. E. Bryant, F. Gunn-Moore, A. C. Riches, and K. Dholakia, "Photoporation and cell transfection using a violet diode laser," Optics Express 13, 595-600 (2005).
13. D. Stevenson, B. Agate, X. Tsampoula, P. Fischer, C. T. A. Brown, W. Sibbett, A. Riches, F. Gunn-Moore, and K. Dholakia, "Femtosecond optical transfection of cells: viability and efficiency," Optics Express 14, 7125-7133 (2006).
14. X. Tsampoula, V. Garces-Chavez, M. Comrie, D. J. Stevenson, B. Agate, C. T. A. Brown, F. Gunn-Moore, and K. Dholakia, "Femtosecond cellular transfection using a nondiffracting light beam," Appl. Phys. Lett. 91, 053902-053903 (2007).
15. I. B. Clark, E. G. Hanania, J. Stevens, M. Gallina, A. Fieck, R. Brandes, B. O. Palsson, and M. R. Koller, "Optoinjection for efficient targeted delivery of a broad range of compounds and macromolecules into diverse cell types," Journal of Biomedical Optics 11 (2006).
16. V. Kohli, J. P. Acker, and A. Y. Elezzabi, "Reversible permeabilization using high-intensity femtosecond laser pulses: Applications to biopreservation," Biotechnology and Bioengineering 92, 889-899 (2005).
17. S. K. Mohanty, M. Sharma, and P. K. Gupta, "Laser-assisted microinjection into targeted animal cells," Biotechnol.Lett. 25, 895-899 (2003).