Femtosecond near-infrared lasers can preserve the integrity of cells when used as a microinjection method, a new study has found. The technique could one day allow doctors to perform nanosurgery on individual cells.The findings are being presented today by lead researcher Ingrid Wilke, assistant professor of physics at Rensselaer Polytechnic Institute, at the World of Photonics Congress, held in conjunction with LASER 2007.World of Photonics in Munich, Germany. Human illness begins and advances at the cellular level. Understanding how materials like proteins or drug ingredients affect an individual cell can give researchers important insight into how that material might impact the entire human body, according to Wilke. This makes discoveries at the cellular level extremely important. The new findings could serve as a set of guidelines for future research that requires precise microinjection of live single cells. Such research ranges from testing drugs for toxicity to targeting tumor cells with chemotherapy. “The technique will allow researchers to use unprecedented precision to microinject cells or even perform nanosurgery on cells,” Wilke said. “The problem with previous methods of single-cell injection was low cell viability and low efficacy,” Wilke said. Other physical microinjection methods are greatly hindered in living cells by the natural protective shield encasing mammalian cells. Breaking through this strong, microscopic fortress while still keeping the cell alive and undamaged has proven extremely difficult. The researchers used tightly focused femtosecond laser beam pulses that created a pore or opening in the cellular wall of living cells and encouraged the cell to take in different molecules. The laser beam serves as a “needle” that punctures the protective skin around the cell, encouraging the cell to take up the material surrounding it. In this case, the researchers used a yellow iodine dye as their nanoscale “vaccine” so the injection results could be easily viewed in microscopic images. A femtosecond is one billionth of one millionth of a second. The pulse from a femtosecond laser is so fast that it appears as a constant beam of light to the naked eye. The lasers emit radiation in the near-infrared (NIR) portion of the spectrum, meaning that the wavelength is too long to be seen by human eyes. Upon analysis, the femtosecond NIR lasers were found to preserve the integrity of the cells, Wilke said. But only up to a certain intensity. “The connections between laser intensity and the rate of injection had not been previously explored in-depth,” Wilke said. “We found that the size of the pores was highly dependent on the intensity of the laser. By modifying the strength of the laser, we could encourage the cell to uptake as little or as much of the materials as we desired. We also determined the intensity at which the cell could first be permeated and the level at which to would be disintegrated.” The researchers first microinjected living bovine aortic cells. They were able to create different-sized pores within the cells that would remain open while the laser continued to pulse and close after the laser beam was stopped. They later expanded the experiment to include clam eggs (Spisula solidissima oocytes). This form of microinjection is particularly important for cells that are resistant to any other forms of physical microinjection due to an extremely tough cellular membrane, Wilke said. The team also was able to microinject the clam eggs using the femtosecond NIR pulses. The research discovered that cells were permeated at laser intensities of 4 terawatts per square centimeter. The pore size grew larger as the intensity increased. When the intensity reached more than 35 terawatts per sq cm, the cellular structure disintegrated and the cell was no longer viable. “For the first time, we have shown a relationship between pore characteristics and laser beam intensity,” Wilke said. This level of control has not been previously quantified and Wilke says it will allow better regulation of the concentrations of molecules injected into cells. Wilke was assisted in her research, which originally appeared in the April 2007 edition of Physical Review E, by Acting Provost and biology professor Robert E. Palazzo and former Rensselaer graduate student Cheng Peng. For more information, visit: www.rpi.edu