Comprehensive analytical model for CW laser induced heat in turbid media

In this work, we present a new analytical approach to model continuous wave laser induced temperature in highly homogeneous turbid media. First, the diffusion equation is used to model light transport and a comprehensive solution is derived analytically by obtaining a special Greens' function. Next, the time-dependent bio-heat equation is used to describe the induced heat increase and propagation within the medium. The bio-heat equation is solved analytically utilizing the separation of variables technique. Our theoretical model is successfully validated using numerical simulations and experimental studies with agarose phantoms and ex-vivo chicken breast samples. The encouraging results show that our method can be implemented as a simulation tool to determine important laser parameters that govern the magnitude of temperature rise within homogenous biological tissue or organs.

Photothermal nanoblade for patterned cell membrane cutting

We report a photothermal nanoblade that utilizes a metallic nanostructure to harvest short laser pulse energy and convert it into a highly localized and specifically shaped explosive vapor bubble. Rapid bubble expansion and collapse punctures a lightly-contacting cell membrane via high-speed fluidic flows and induced transient shear stress. The membrane cutting pattern is controlled by the metallic nanostructure configuration, laser pulse polarization, and energy. Highly controllable, sub-micron sized circular hole pairs to half moon-like, or cat-door shaped, membrane cuts were realized in glutaraldehyde treated HeLa cells.

Ultra-fast photoacoustic flow cytometry with a 0.5 MHz pulse repetition rate nanosecond laser

In vivo photoacoustic (PA) flow cytometry (PAFC) has great potential for detecting disease-associated biomarkers in blood and lymph flow, as well as real-time control of the efficacy of photothermal (PT) and other therapies through the counting of circulating abnormal objects. We report on a high speed PAFC with a Yb-doped fiber laser having a 0.5-MHz pulse repetition rate at a wavelength of 1064 nm, pulse width of 10 ns, and energy up to 100 microJ. This is the first biomedical application of PA and PT techniques operating at the highest pulse repetition rate of nanosecond lasers that provide 100-fold enhancement in detection speed of carbon nanotube clusters, as well as real-time monitoring of the flow velocity of individual targets through the width of PA signals. The laser pulse rate limits for PT and PA techniques depending on the sizes of laser beam and targets and flow velocity are discussed. We propose time-overlapping mode and generation of periodic nano- and microbubbles as PA-signal and PT-therapy amplifiers, including discrimination of small absorbing targets among large ones. Taking into account the relatively low level of background signals from most biotissues at 1064 nm, our data suggest that a nanosecond Yb-doped fiber laser operating at high pulse repetition rate could be a promising optical source for time-resolved PA and PT cytometry, imaging, microscopy, and therapy, including detection of nanoparticles and cells flowing at velocities up to 2.5 m/s.

Photothermal and photoacoustic Raman cytometry in vitro and in vivo

An integrated Raman-based cytometry was developed with photothermal (PT) and photoacoustic (PA) detection of Raman-induced thermal and acoustic signals in biological samples with Raman-active vibrational modes. The two-frequency, spatially and temporally overlapping pump-Stokes excitation in counterpropagating geometry was provided by a nanosecond tunable (420-2300 nm) optical parametric oscillator and a Raman shifter (639 nm) pumped by a double-pulsed Q-switched Nd:YAG laser using microscopic and fiberoptic delivery of laser radiation. The PA and PT Raman detection and imaging technique was tested in vitro with benzene, acetone, olive oil, carbon nanotubes, chylomicron phantom, and cancer cells, and in vivo in single adipocytes in mouse mesentery model. The integration of linear and nonlinear PA and PT Raman scanning and flow cytometry has the potential to enhance its chemical specificity and sensitivity including nanobubble-based amplification (up to 10- fold) for detection of absorbing and nonabsorbing targets that are important for both basic and clinically relevant studies of lymph and blood biochemistry, cancer, and fat distribution at the single-cell level.