Single cell viability and impact of heating by laser absorption

Abstract

Optical traps such as tweezers and stretchers are widely used to probe the mechanical properties of cells. Beyond their large range of applications, the use of infrared laser light in optical traps causes significant heating effects in the cell. This study investigated the effect of laser-induced heating on cell viability. Common viability assays are not very sensitive to damages caused in short periods of time or are not practicable for single cell analysis. We used cell spreading, a vital ability of cells, as a new sensitive viability marker. The optical stretcher, a two beam laser trap, was used to simulate heat shocks that cells typically experience during measurements in optical traps. The results show that about 60% of the cells survived heat shocks without vital damage at temperatures of up to 58 ± 2°C for 0.5 s. By varying the duration of the heat shocks, it was shown that 60% of the cells stayed viable when exposed to 48 ± 2°C for 5 s.

Optical stretching in continuous flows

“…We designed the setup in a minimalistic approach to change as few parameters as possible to keep the systems comparable. In this regard, we used the same lasers, optical fibers, and reservoirs as in previous studies concerning cancer research [4,8,39,59,60], various research questions of single cell rheology [38,40,41,61,62], and the principle of the Optical Stretcher as an optical rheometer for living cells [36,37,50]. The microfluidic surroundings have been adopted as far as possible, keeping the channel dimensions and path lengths, as well as flow velocities and pressure in the same order of magnitude.…”

“…The IFOS works with forces in the pN range inducing small strains over 0.5-2 s resembling the regime of physiological deformations. It directly probes the whole cytoskeleton and cells will most likely survive the measurement enabling subsequent experiments with the same cell population [50]. The chip-based design allows to easily add additional channels, e.g.…”

“…A specially designed Nd:YAG 1064 nm laser (Fibolux, FIBOTEC, Germany) with two separately controllable outputs and a maximum power of × 2 2 W has been attached. 1064 nm is outside of the absorption range of proteins and in a local minimum of the water absorption spectrum (0.13 cm −1 [53]), which significantly reduced heat damage on the cells [50].…”

Investigation of temperature effect on cell mechanics by optofluidic microchips

“…The obtained results on optical deformability showed a statistically significant difference (p < 10 −3 ) between all the five temperatures. As known from literature [10,21,22], laser power can induce a sudden increase of the cell temperature, making it difficult to compare the deformation values obtained at different optical power levels. As in our case the same optical power is applied to all OS measurements, we expect an increase of about 30°C in the irradiated region for all the samples, provided that absorption coefficient changes due to the different temperatures are negligible.…”

“…In addition the low-temperature CC measurements allow investigating cells' properties in a significantly different range of cell-temperatures. As a final remark regarding the cell viability, it is worth noting that in the case of stretching experiments performed on cells initially at 35°C, the impact of temperature on their viability has to be carefully considered [21], since during the measurement the cell is expected to exceed the damagethreshold temperature. Conversely, if low-temperature investigations are conducted, the effect of 5-s heating on viability is generally negligible, as discussed in [7].…”