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Nanosecond pulsed electric fields (nsPEFs) induce changes in the plasma membrane (PM), including PM permeabilization (termed nanoporation), allowing free passage of ions into the cell and, in certain cases, cell death. Recent studies from our laboratory show that the composition of the PM is a critical determinant of PM nanoporation. Thus, we hypothesized that the biological response to nsPEF exposure could be influenced by lipid microdomains, including caveolae, which are specialized invaginations of the PM that are enriched in cholesterol and contain aggregates of important cell signaling proteins, such as caveolin-1 (Cav1). Caveolae play a significant role in cellular signal transduction, including control of calcium influx and cell death by interaction of Cav1 with regulatory signaling proteins. Present results show that depletion of Cav1 increased the influx of calcium, while Cav1 overexpression produced the opposite effect. Additionally, Cav1 is known to bind and sequester important cell signaling proteins within caveolae, rendering the binding partners inactive. Imaging of the PM after nsPEF exposure showed localized depletion of PM Cav1 and results of co-immunoprecipitation studies showed dissociation of two critical Cav1 binding partners (transient receptor potential cation channel subfamily C1 (TRPC1) and inositol trisphosphate receptor (IP3R)) after exposure to nsPEFs. Release of TRPC1 and IP3R from Cav1 would activate downstream signaling cascades, including store-operated calcium entry, which could explain the influx in calcium after nsPEF exposure. Results of the current study establish a significant relationship between Cav1 and the activation of cell signaling pathways in response to nsPEFs.

Direct observation of rapid membrane potential changes is critical to understand how complex neurological systems function. This knowledge is especially important when stimulation is achieved through an external stimulus meant to mimic a naturally occurring process. To enable exploration of this dynamic space, we developed an all-optical method for observing rapid changes in membrane potential at temporal resolutions of ∼25 ns. By applying a single 600-ns electric pulse, we observed sub-microsecond, continuous membrane charging and discharging dynamics. Close agreement between the acquired results and an analytical membrane-charging model validates the utility of this technique. This tool will deepen our understanding of the role of membrane potential dynamics in the regulation of many biological and chemical processes within living systems.

Nanosecond pulsed electric fields (nsPEFs) induce changes in the plasma membrane (PM), including PM permeabilization (termed nanoporation), allowing free passage of ions into the cell and, in certain cases, cell death. Recent studies from our laboratory show that the composition of the PM is a critical determinant of PM nanoporation. Thus, we hypothesized that the biological response to nsPEF exposure could be influenced by lipid microdomains, including caveolae, which are specialized invaginations of the PM that are enriched in cholesterol and contain aggregates of important cell signaling proteins, such as caveolin-1 (Cav1). Caveolae play a significant role in cellular signal transduction, including control of calcium influx and cell death by interaction of Cav1 with regulatory signaling proteins. Present results show that depletion of Cav1 increased the influx of calcium, while Cav1 overexpression produced the opposite effect. Additionally, Cav1 is known to bind and sequester important cell signaling proteins within caveolae, rendering the binding partners inactive. Imaging of the PM after nsPEF exposure showed localized depletion of PM Cav1 and results of co-immunoprecipitation studies showed dissociation of two critical Cav1 binding partners (transient receptor potential cation channel subfamily C1 (TRPC

Nanosecond pulsed electric field (nsPEF) exposure to cells causes a myriad of bioeffects with great potential to translate into beneficial technology. However, a general lack of fundamental knowledge of how the field is interacting with the cell limits the advancement of predictive models and maximal exploitation. Despite 30 years of research, this same dearth of mechanistic understanding remains for longer pulse exposures. Fundamental to determining what is occurring as these strong electric fields are applied to cells is measuring the induced change in membrane potential on the time scale of the exposure. Such measurements are critical to validating commonly used electric circuit-based continuum models for electroporation, but have remained elusive due to limits in signal-to-noise and fluorescent reporters. In a previous publication, we described a high-speed fluorescent imaging modality that combined a streak camera and a high power laser source termed a high speed streak camera microscope (SCM) to resolve membrane charging during a single nsPEF. In this paper, we use the SCM to quantify changes in membrane potential in CHO-K1 cells exposed to unipolar and bipolar 600ns PEF within the context of the recently discovered “bipolar cancellation” phenomenon. Immediately after a unipolar pulse exposure, we see a prolonged “depolarization” of the cell that is roughly 50-100mV in amplitude. Such a prolonged depolarization is not seen in bipolar exposures nor is it predicted by membrane charging models. We postulate that this lasting membrane depolarization, seen only in unipolar pulse exposure, is either the cause of later uptake of impermeable ions or signifies the acute (during the pulse) breakdown of the plasma membrane (nanoporation). The lack of lasting depolarization in bipolar pulse exposures may be fundamental to “bipolar cancellation” and explain why uptake of ions is substantially reduced as compared to unipolar pulse exposures.

Above a threshold electric field strength, 600 ns-duration pulsed electric field (nsPEF) exposure substantially porates and permeabilizes cellular plasma membranes in aqueous solution to many small ions. Repetitive exposures increase permeabilization to calcium ions (Ca2+) in a dosage-dependent manner. Such exposure conditions can create relatively long-lived pores that reseal after passive lateral diffusion of lipids should have closed the pores. One explanation for eventual pore resealing is active membrane repair, and an ubiquitous repair mechanism in mammalian cells is lysosome exocytosis. A previous study shows that intracellular lysosome movement halts upon a 16.2 kV/cm, 600-ns PEF exposure of a single train of 20 pulses at 5 Hz. In that study, lysosome stagnation qualitatively correlates with the presence of Ca2+ in the extracellular solution and with microtubule collapse. The present study tests the hypothesis that limitation of nsPEF-induced Ca2+ influx and colloid osmotic cell swelling permits unabated lysosome translocation in exposed cells. The results indicate that the efforts used herein to preclude Ca2+ influx and colloid osmotic swelling following nsPEF exposure did not prevent attenuation of lysosome translocation. Intracellular lysosome movement is inhibited by nsPEF exposure(s) in the presence of PEG 300-containing solution or by 20 pulses of nsPEF in the presence of extracellular calcium. The only cases with no significant decreases in lysosome movement are the sham and exposure to a single nsPEF in Ca2+-free solution.

Nanosecond electric pulses (nsEP's) are a well-studied phenomena in biophysics that cause substantial alterations to cellular membrane dynamics, internal biochemistry, and cytoskeletal structure, and induce apoptotic and necrotic cell death. While several studies have attempted to measure the effects of multiple nanosecond pulses, the effect of pulse repetition rate (PRR) has received little attention, especially at frequencies greater than 100 Hz. In this study, uptake of Propidium Iodide, FM 1–43, and YO-PRO-1 fluorescent dyes in CHO-K1 cells was monitored across a wide range of PRRs (5 Hz–500 KHz) using a laser-scanning confocal microscope in order to better understand how high frequency repetition rates impact induced biophysical changes. We show that frequency trends depend on the identity of the dye under study, which could implicate transmembrane protein channels in the uptake response due to their chemical selectivity. Finally, YO-PRO-1 fluorescence was monitored in the presence of Gadolinium (Gd3+), Ruthenium Red, and in calcium-free solution to elucidate a mechanism for its unique frequency trend.

Optoacoustic (OA) microscopy using an all-optical system based on the probe beam deflection technique (PBDT) for detection of laser-induced acoustic signals was investigated as an alternative to conventional piezoelectric transducers. PBDT provides a number of advantages for OA microscopy including (i) efficient coupling of laser excitation energy to the samples being imaged through the probing laser beam, (ii) undistorted coupling of acoustic waves to the detector without the need for separation of the optical and acoustic paths, (iii) high sensitivity and (iv) ultrawide bandwidth. Because of the unimpeded optical path in PBDT, diffraction-limited lateral resolution can be readily achieved. The sensitivity of the current PBDT sensor of 22μV/Pa and its noise equivalent pressure (NEP) of 11.4Pa are comparable with these parameters of the optical micro-ring resonator and commercial piezoelectric ultrasonic transducers. Benefits of the present prototype OA microscope were demonstrated by successfully resolving micron-size details in histological sections of cardiac muscle.

Electric-field induced physical phenomena, such as thermal, mechanical and electrochemical dynamics, may be the driving mechanism behind bioeffects observed in mammalian cells during exposure to nanosecond-duration electric pulses (nsEP) in-vitro. Correlating a driving mechanism to a biological response requires the experimental measurement and quantification of all physical dynamics resulting from the nsEP stimulus. A passive and electromagnetic interference (EMI) immune sensor is required to resolve these dynamics in high strength electric fields. The probe beam deflection technique (PBDT) is a passive and EMI immune optical method for quantifying and imaging refractive index gradients in liquids and gases, both dynamic and static, with nanosecond temporal resolution. In this work, a probe beam deflection imaging system was designed to acquire 2-D time-lapse images of thermal/mechanical dynamics resulting from monopolar and bipolar nsEP stimulus.

Pulsed infrared (IR) laser energy has been shown to modulate neurological activity through both stimulation and inhibition of action potentials. While the mechanism(s) behind this phenomenon is (are) not completely understood, certain hypotheses suggest that the rise in temperature from IR exposure could activate temperature- or pressure-sensitive ion channels or create pores in the cellular outer membrane, allowing an influx of typically plasma-membrane-impermeant ions. Studies using fluorescent intensity-based calcium ion ([Formula: see text]) sensitive dyes show changes in [Formula: see text] levels after various IR stimulation parameters, which suggests that [Formula: see text] may originate from the external solution. However, activation of intracellular signaling pathways has also been demonstrated, indicating a more complex mechanism of increasing intracellular [Formula: see text] concentration. We quantified the [Formula: see text] mobilization in terms of influx from the external solution and efflux from intracellular organelles using Fura-2 and a high-speed ratiometric imaging system that rapidly alternates the dye excitation wavelengths. Using nonexcitable Chinese hamster ovarian ([Formula: see text]) cells and neuroblastoma-glioma (NG108) cells, we demonstrate that intracellular [Formula: see text] receptors play an important role in the IR-induced [Formula: see text], with the [Formula: see text] response augmented by ryanodine receptors in excitable cells.

Fluorescent lifetime imaging is an optical technique that facilitates imaging molecular interactions and cellular functions. Because the excited lifetime of a fluorophore is sensitive to its local microenvironment,1, 2 measurement of fluorescent lifetimes can be used to accurately detect regional changes in temperature, pH, and ion concentration. However, typical state of the art fluorescent lifetime methods are severely limited when it comes to acquisition time (on the order of seconds to minutes) and video rate imaging. Here we show that compressed ultrafast photography (CUP) can be used in conjunction with fluorescent lifetime imaging to overcome these acquisition rate limitations. Frame rates up to one hundred billion frames per second have been demonstrated with compressed ultrafast photography using a streak camera.3 These rates are achieved by encoding time in the spatial direction with a pseudo-random binary pattern. The time domain information is then reconstructed using a compressed sensing algorithm, resulting in a cube of data (x,y,t) for each readout image. Thus, application of compressed ultrafast photography will allow us to acquire an entire fluorescent lifetime image with a single laser pulse. Using a streak camera with a high-speed CMOS camera, acquisition rates of 100 frames per second can be achieved, which will significantly enhance our ability to quantitatively measure complex biological events with high spatial and temporal resolution. In particular, we will demonstrate the ability of this technique to do single-shot fluorescent lifetime imaging of cells and microspheres.

Nanosecond pulsed electric fields (nsPEF) have proven useful for transporting cargo across cell membranes and selectively activating cellular pathways. The chemistry and biophysics governing this cellular response, however, are complex and not well understood. Recent studies have shown that the conductivity of the solution cells are exposed in could play a significant role in plasma membrane permeabilization and, thus, the overall cellular response. Unfortunately, the means of detecting this membrane perturbation has traditionally been limited to analyzing one possible consequence of the exposure – diffusion of molecules across the membrane. This method has led to contradictory results with respect to the relationship between permeabilization and conductivity. Diffusion experiments also suffer from “saturation conditions” making multi-pulse experiments difficult. As a result, this method has been identified as a key stumbling block to understanding the effects of nsPEF exposure. To overcome these limitations, we recently developed a nonlinear optical imaging technique based on second harmonic generation (SHG) that allows us to identify nanoporation in live cells during the pulse in a wide array of conditions. As a result, we are able to explore and fully test whether lower conductivity extracellular solutions could induce more efficient nanoporation. This hypothesis is based on membrane charging and the relative difference between the extracellular solution and the cytoplasm. The experiments also allow us to test the noise floor of our methodology against the effects of ion leakage. The results emphasize that the electric field, not ionic phenomenon, are the driving force behind nsPEF-induced membrane nanoporation.

Pulsed infrared light can excite action potentials in neurons; yet, the fundamental mechanism underlying this phenomenon is unknown. Previous work has observed a rise in intracellular calcium concentration following infrared exposure, but the source of the calcium and mechanism of release is unknown. Here, we used fluorescence lifetime imaging of Oregon Green BAPTA-1 to study intracellular calcium dynamics in primary rat hippocampal neurons in response to infrared light exposure. The fluorescence lifetime of Oregon Green BAPTA-1 is longer when bound to calcium, and allows robust measurement of intracellular free calcium concentrations. First, a fluorescence lifetime calcium calibration curve for Oregon Green BAPTA-1 was determined in solutions. The normalized amplitude of the short and long lifetimes was calibrated to calcium concentration. Then, neurons were incubated in Oregon Green BAPTA-1 and exposed to pulses of infrared light (0-1 J/cm2; 0-5 ms; 1869 nm). Fluorescence lifetime images were acquired prior to, during, and after the infrared exposure. Fluorescence lifetime images, 64x64 pixels, were acquired at 12 or 24 ms for frame rates of 83 and 42 Hz, respectively. Accurate α1 approximations were achieved in images with low photon counts by computing an α1 index value from the relative probability of the observed decay events. Results show infrared light exposure increases intracellular calcium in neurons. Altogether, this study demonstrates accurate fluorescence lifetime component analysis from low-photon count data for improved imaging speed.

Short infrared laser pulses induce a variety of effects in cells and tissues, including neural stimulation and inhibition. However, the mechanism behind these physiological effects is poorly understood. It is known that the fast thermal gradient induced by the infrared light is necessary for these biological effects. Therefore, this study tests the hypothesis that the fast thermal gradient induced in a cell by infrared light exposure causes a change in the membrane fluidity. To test this hypothesis, we used the membrane fluidity dye, di-4-ANEPPDHQ, to investigate membrane fluidity changes following infrared light exposure. Di-4-ANEPPDHQ fluorescence was imaged on a wide-field fluorescence imaging system with dual channel emission detection. The dual channel imaging allowed imaging of emitted fluorescence at wavelengths longer and shorter than 647 nm for ratiometric assessment and computation of a membrane generalized polarization (GP) value. Results in CHO cells show increased membrane fluidity with infrared light pulse exposure and this increased fluidity scales with infrared irradiance. Full recovery of pre-infrared exposure membrane fluidity was observed. Altogether, these results demonstrate that infrared light induces a thermal gradient in cells that changes membrane fluidity.

Short infrared laser pulses have many physiological effects on cells including the ability to stimulate action potentials in neurons. Here we show that short infrared laser pulses can also reversibly block action potentials. Primary rat hippocampal neurons were transfected with the Optopatch2 plasmid, which contains both a blue-light activated channel rhodopsin (CheRiff) and a red-light fluorescent membrane voltage reporter (QuasAr2). This optogenetic platform allows robust stimulation and recording of action potential activity in neurons in a non-contact, low noise manner. For all experiments, QuasAr2 was imaged continuously on a wide-field fluorescent microscope using a Krypton laser (647 nm) as the excitation source and an EMCCD camera operating at 1000 Hz to collect emitted fluorescence. A co-aligned Argon laser (488 nm, 5 ms at 10Hz) provided activation light for CheRiff. A 200 mm fiber delivered infrared light locally to the target neuron. Reversible action potential block in neurons was observed following a short infrared laser pulse (0.26-0.96 J/cm2; 1.37-5.01 ms; 1869 nm), with the block persisting for more than 1 s with exposures greater than 0.69 J/cm2. Action potential block was sustained for 30 s with the short infrared laser pulsed at 1-7 Hz. Full recovery of neuronal activity was observed 5-30s post-infrared exposure. These results indicate that optogenetics provides a robust platform for the study of action potential block and that short infrared laser pulses can be used for non-contact, reversible action potential block.

Exposure to nanosecond pulsed electrical fields (nsPEFs) results in a myriad of observable effects in mammalian cells. While these effects are often attributed to the direct permeabilization of both the plasma and organelle membranes, the underlying mechanism(s) are not well understood. We hypothesize that nsPEF-induced membrane disturbance will initiate complex intracellular lipid signaling pathways, which ultimately lead to the observed multifarious effects. In this article, we show activation of one of these pathways--phosphoinositide signaling cascade. Here we demonstrate that nsPEF initiates phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis or depletion from the plasma membrane, accumulation of inositol-1,4,5-trisphosphate (IP3) in the cytoplasm and increase of diacylglycerol (DAG) on the inner surface of the plasma membrane. All of these events are initiated by a single 16.2 kV/cm, 600 ns pulse exposure. To further this claim, we show that the nsPEF-induced activation mirrors the response of M1-acetylcholine Gq/11-coupled metabotropic receptor (hM1). This demonstration of PIP2 hydrolysis by nsPEF exposure is an important step toward understanding the mechanisms underlying this unique stimulus for activation of lipid signaling pathways and is critical for determining the potential for nsPEFs to modulate mammalian cell functions.

Remote chemically specific detection of trace impurities in the atmosphere from distances on the order of kilometers is an important problem from both an environmental and a national defense viewpoint. A new scheme is discussed consisting of the remote generation of a backward propagating stimulated Raman pulse. This pulse is then used to drive a coherent anti-Stokes Raman scattering scheme, resulting in a strong chemically specific signal propagating back to the detector.

We recently developed a nonlinear optical imaging technique based on second harmonic generation (SHG) to identify membrane disruption events in live cells. This technique was used to detect nanoporation in the plasma membrane following nanosecond pulsed electric field (nsPEF) exposure. It has been hypothesized that similar poration events could be induced by the thermal gradients generated by infrared (IR) laser energy. Optical pulses are a highly desirable stimulus for the nervous system, as they are capable of inhibiting and producing action potentials in a highly localized but non-contact fashion. However, the underlying mechanisms involved with infrared neural stimulation (INS) are not well understood. The ability of our method to non-invasively measure membrane structure and transmembrane potential via Two Photon Fluorescence (TPF) make it uniquely suited to neurological research. In this work, we leverage our technique to understand what role membrane structure plays during INS and contrast it with nsPEF stimulation. We begin by examining the effect of IR pulses on CHO-K1 cells before progressing to primary hippocampal neurons. The use of these two cell lines allows us to directly compare poration as a result of IR pulses to nsPEF exposure in both a neuron-derived cell line, and one likely lacking native channels sensitive to thermal stimuli.

Plasma membrane disruption can trigger a host of cellular activities. One commonly observed type of disruption is pore formation. Molecular dynamic (MD) simulations of simplified lipid membrane structures predict that controllably disrupting the membrane via nano-scale poration may be possible with nanosecond pulsed electric fields (nsPEF). Until recently, researchers hoping to verify this hypothesis experimentally have been limited to measuring the relatively slow process of fluorescent markers diffusing across the membrane, which is indirect evidence of nanoporation that could be channel-mediated. Leveraging recent advances in nonlinear optical microscopy, we elucidate the role of pulse parameters in nsPEF-induced membrane permeabilization in live cells. Unlike previous techniques, it is able to directly observe loss of membrane order at the onset of the pulse. We also develop a complementary theoretical model that relates increasing membrane permeabilization to membrane pore density. Due to the significantly improved spatial and temporal resolution possible with our imaging method, we are able to directly compare our experimental and theoretical results. Their agreement provides substantial evidence that nanoporation does occur and that its development is dictated by the electric field distribution.

Cells exposed to nanosecond-pulsed electric fields (nsPEF) exhibit a wide variety of nonspecific effects, including blebbing, swelling, intracellular calcium bursts, apoptotic and necrotic cell death, formation of nanopores, and depletion of phosphatidylinositol 4,5-biphosphate (PIP2) to induce activation of the inositol trisphosphate/diacylglycerol pathway. While several studies have taken place in which multiple pulses were delivered to cells, the effect of pulse repetition rate (PRR) is not well understood. To better understand the effects of PRR, a laser scanning confocal microscope was used to observe CHO-K1 cells exposed to ten 600ns, 200V pulses at varying repetition rates (5Hz up to 500KHz) in the presence of either FM 1-43, YO-PRO-1, or Propidium Iodide (PI) fluorescent dyes, probes frequently used to indicate nanoporation or permeabilization of the plasma membrane. Dye uptake was monitored for 30 seconds after pulse application at a rate of 1 image/second. In addition, a single long pulse of equivalent energy (200V, 6 μs duration) was applied to test the hypothesis that very fast PRR will approximate the biological effects of a single long pulse of equal energy. Upon examination of the data, we found strong variation in the relationship between PRR and uptake in each of the three dyes. In particular, PI uptake showed little frequency dependence, FM 1-43 showed a strong inverse relationship between frequency and internal cell fluorescence, and YO-PRO-1 exhibited a “threshold” point of around 50 KHz, after which the inverse trend observed in FM 1-43 was seen to reverse itself. Further, a very high PRR of 500 KHz only approximated the biological effects of a single 6 μs pulse in cells stained with YO-PRO-1, suggesting that uptake of different dyes may proceed by different physical mechanisms.

Time-correlated single photon counting (TCSPC) enables acquisition of fluorescence lifetime decays with high temporal resolution within the fluorescence decay. However, many thousands of photons per pixel are required for accurate lifetime decay curve representation, instrument response deconvolution, and lifetime estimation, particularly for two-component lifetimes. TCSPC imaging speed is inherently limited due to the single photon per laser pulse nature and low fluorescence event efficiencies (

Time-correlated single photon counting (TCSPC) is the most robust method for fluorescence lifetime imaging using laser scanning microscopes. However, TCSPC is inherently slow making it ineffective to capture rapid events due to the single photon product per laser pulse causing extensive acquisition time limitations and the requirement of low fluorescence emission efficiency to avoid bias of measurement towards short lifetimes. Furthermore, thousands of photons per pixel are required for traditional instrument response deconvolution and fluorescence lifetime exponential decay estimation. Instrument response deconvolution and fluorescence exponential decay estimation can be performed in several ways including iterative least squares minimization and Laguerre deconvolution. This paper compares the limitations and accuracy of these fluorescence decay analysis techniques to accurately estimate double exponential decays across many data characteristics including various lifetime values, lifetime component weights, signal-to-noise ratios, and number of photons detected. Furthermore, techniques to improve data fitting, including binning data temporally and spatially, are evaluated as methods to improve decay fits and reduce image acquisition time. Simulation results demonstrate that binning temporally to 36 or 42 time bins, improves accuracy of fits for low photon count data. Such a technique reduces the required number of photons for accurate component estimation if lifetime values are known, such as for commercial fluorescent dyes and FRET experiments, and improve imaging speed 10-fold.

Short infrared (IR) laser pulses on the order of hundreds of microseconds to single milliseconds with typical wavelengths of 1800-2100 nm, have shown the capability to reversibly stimulate action potentials (AP) in neuronal cells. While the IR stimulation technique has proven successful for several applications, the exact mechanism(s) underlying the AP generation has remained elusive. To better understand how IR pulses cause AP stimulation, we determined the threshold for the formation of nanopores in the plasma membrane. Using a surrogate calcium ion, thallium, which is roughly the same shape and charge, but lacks the biological functionality of calcium, we recorded the flow of thallium ions into an exposed cell in the presence of a battery of channel antagonists. The entry of thallium into the cell indicated that the ions entered via nanopores. The data presented here demonstrate a basic understanding of the fundamental effects of IR stimulation and speculates that nanopores, formed in response to the IR exposure, play an upstream role in the generation of AP.

Recently, infrared light has been shown to both stimulate and inhibit excitatory cells. However, studies of infrared light for excitatory cell inhibition have been constrained by the use of invasive and cumbersome electrodes for cell excitation and action potential recording. Here, we present an all optical experimental design for neuronal excitation, inhibition, and action potential detection. Primary rat neurons were transfected with plasmids containing the light sensitive ion channel CheRiff. CheRiff has a peak excitation around 450 nm, allowing excitation of transfected neurons with pulsed blue light. Additionally, primary neurons were transfected with QuasAr2, a fast and sensitive fluorescent voltage indicator. QuasAr2 is excited with yellow or red light and therefore does not spectrally overlap CheRiff, enabling imaging and action potential activation, simultaneously. Using an optic fiber, neurons were exposed to blue light sequentially to generate controlled action potentials. A second optic fiber delivered a single pulse of 1869nm light to the neuron causing inhibition of the evoked action potentials (by the blue light). When used in concert, these optical techniques enable electrode free neuron excitation, inhibition, and action potential recording, allowing research into neuronal behaviors with high spatial fidelity.

Nanoporation occurs in cells exposed to high amplitude short duration (< 1μs) electrical pulses. The biophysical mechanism(s) responsible for nanoporation is unknown although several theories exist. Current theories focus exclusively on the electrical field, citing electrostriction, water dipole alignment and/or electrodeformation as the primary mechanisms for pore formation. Our group has shown that mechanical forces of substantial magnitude are also generated during nsEP exposures. We hypothesize that these mechanical forces may contribute to pore formation. In this paper, we report that alteration of the conductivity of the exposure solution also altered the level of mechanical forces generated during a nsEP exposure. By reducing the conductivity of the exposure solutions, we found that we could completely eliminate any pressure transients normally created by nsEP exposure. The data collected for this proceeding does not definitively show that the pressure transients previously identified contribute to nanoporation; however; it indicates that conductivity influences both survival and pressure transient formation.

Previous work from our laboratory demonstrated nanopore formation in cell membranes following exposure to nanosecond pulsed electric fields (nsPEF). We observed differences in sensitivity to nsPEF in both acute membrane injury and 24 h lethality across multiple cells lines. Based on these data, we hypothesize that the biological response of cells to nsPEF is dependent on the physical properties of the plasma membrane (PM), including regional cholesterol content. Results presented in this paper show that depletion of membrane cholesterol disrupts the PM and increases the permeability of cells to small molecules, including propidium iodide and calcium occurring after fewer nsPEF. Additionally, cholesterol depletion concurrently decreases the “dose” of nsPEF required to induce lethality. In summary, the results of the current study suggest that the PM cholesterol composition is an important determinant in the cellular response to nsPEF.

Previous work from our laboratory demonstrated significant nanopore formation in cellular membranes following exposure of cells to nanosecond pulsed electric fields (nsPEF). We hypothesize that the sensitivity of cells to nsPEF is dependent on the properties of the plasma membrane, including lipid microdomains. Results show that depletion of membrane cholesterol increases the sensitivity of cells to nsPEF. Cholesterol depletion increases the permeability of cells to small molecules, including propidium iodide and calcium, at shorter nsPEF exposures. In contrast, depletion of caveolin, an important protein component of membrane lipid microdomains, renders the cells less sensitive to nsPEF. The results of the current study suggest that plasma membrane cholesterol and proteins are important determinants in the cellular response to nsPEF.

Exposure of cells to nanosecond pulsed electric fields (nsPEF) causes a rapid increase in intracellular calcium. The mechanism(s) responsible for this calcium burst remains unknown, but is hypothesized to be from direct influx through nanopores, the activation of specific ion channels, or direct disruption of organelles. It is likely, however, that several mechanisms are involved/activated, thereby resulting in a complex chain of events that are difficult to separate by slow imaging methods. In this letter, we describe a novel high-speed imaging system capable of determining the spatial location of calcium bursts within a single cell following nsPEF exposure. Preliminary data in rodent neuroblastoma cells are presented, demonstrating the ability of this system to track the location of calcium bursts in vitro within milliseconds of exposure. These data reveal that calcium ions enter the cell from the plasma membrane regions closest to the electrodes (poles), and that intracellular calcium release occurs in the absence of extracellular calcium. We believe that this novel technique will allow us to temporally and spatially separate various nsPEF-induced effects, leading to powerful insights into the mechanism(s) of interaction between electric fields and cellular membranes.

Permeabilization of cell membranes occurs upon exposure to a threshold absorbed dose (AD) of nanosecond pulsed electric fields (nsPEF). The ultimate, physiological bioeffect of this exposure depends on the type of cultured cell and environment, indicating that cell-specific pathways and structures are stimulated. Here we investigate 10 and 600 ns duration PEF effects on Chinese hamster ovary (CHO) cell nuclei, where our hypothesis is that pulse disruption of the nuclear envelope membrane leads to observed cell death and decreased viability 24 h post-exposure. To observe short-term responses to nsPEF exposure, CHO cells have been stably transfected with two fluorescently-labeled proteins known to be sequestered for cellular chromosomal function within the nucleus - histone-2b (H2B) and proliferating cell nuclear antigen (PCNA). H2B remains associated with chromatin after nsPEF exposure, whereas PCNA leaks out of nuclei permeabilized by a threshold AD of 10 and 600 ns PEF. A downturn in 24 h viability, measured by MTT assay, is observed at the number of pulses required to induce permeabilization of the nucleus.

The mechanism(s) responsible for the breakdown (nanoporation) of cell plasma membranes after nanosecond pulse (nsEP) exposure remains poorly understood. Current theories focus exclusively on the electrical field, citing electrostriction, water dipole alignment and/or electrodeformation as the primary mechanisms for pore formation. However, the delivery of a high-voltage nsEP to cells by tungsten electrodes creates a multitude of biophysical phenomena, including electrohydraulic cavitation, electrochemical interactions, thermoelastic expansion and others. To date, very limited research has investigated non-electric phenomena occurring during nsEP exposures and their potential effect on cell nanoporation. Of primary interest is the production of acoustic shock waves during nsEP exposure, as it is known that acoustic shock waves can cause membrane poration (sonoporation). Based on these observations, our group characterized the acoustic pressure transients generated by nsEP and determined if such transients played any role in nanoporation. In this paper, we show that nsEP exposures, equivalent to those used in cellular studies, are capable of generating high-frequency (2.5 MHz), high-intensity (>13 kPa) pressure transients. Using confocal microscopy to measure cell uptake of YO-PRO®-1 (indicator of nanoporation of the plasma membrane) and changing the electrode geometry, we determined that acoustic waves alone are not responsible for poration of the membrane.

Currently, no methods exist for the definitive diagnosis of AD premortem. β-amyloid, the primary component of the senile plaques found in patients with this disease, is believed to play a role in its neurotoxicity. We are developing a nanoshell substrate, functionalized with sialic acid residues to mimic neuron cell surfaces, for the surface-enhanced Raman detection of β-amyloid. It is our hope that this sensing mechanism will be able to detect the toxic form of β-amyloid, with structural and concentration information, to aid in the diagnosis of AD and provide insight into the relationship between β-amyloid and disease progression. We have been successfully able to functionalize the nanoshells with the sialic acid residues to allow for the specific binding of β-amyloid to the substrate. We have also shown that a surface-enhanced Raman spectroscopy response using nanoshells is stable and concentration-dependent with detection into the picomolar range.

Nanosecond electrical pulse (nsEP) exposure activates signaling pathways, produces oxidative stress, stimulates hormone secretion, causes cell swelling and induces apoptotic and necrotic death. The underlying biophysical connection(s) between these diverse cellular reactions and nsEP has yet to be elucidated. Using global genetic analysis, we evaluated how two commonly studied cell types, U937 and Jurkat, respond to nsEP exposure. We hypothesized that by studying the genetic response of the cells following exposure, we would gain direct insight into the stresses experienced by the cell and in turn better understand the biophysical interaction taking place during the exposure. Using Ingenuity Systems software, we found genes associated with cell growth, movement and development to be significantly up-regulated in both cell types 4 h post exposure to nsEP. In agreement with our hypothesis, we also found that both cell lines exhibit significant biological changes consistent with mechanical stress induction. These results advance nsEP research by providing strong evidence that the interaction of nsEPs with cells involves mechanical stress.

As an increasing number of studies use gold nanoparticles (AuNPs) for potential medicinal, biosensing and therapeutic applications, the synthesis and use of readily functional, bio-compatible nanoparticles is receiving much interest. For these efforts, the particles are often taken up by the cells to allow for optimum sensing or therapeutic measures. This process typically requires incubation of the particles with the cells for an extended period. In an attempt to shorten and control this incubation, we investigated whether nanosecond pulsed electric field (nsPEF) exposure of cells will cause a controlled uptake of the particles. NsPEF are known to induce the formation of nanopores in the plasma membrane, so we hypothesized that by controlling the number, amplitude or duration of the nsPEF exposure, we could control the size of the nanopores, and thus control the particle uptake. Chinese hamster ovary (CHO-K1) cells were incubated sub-10 nm AuNPs with and without exposure to 600-ns electrical pulses. Contrary to our hypothesis, the nsPEF exposure was found to actually decrease the particle uptake in the exposed cells. This result suggests that the nsPEF exposure may be affecting the endocytotic pathway and processes due to membrane disruption.

The underlying mechanism(s) responsible for nanoporation of phospholipid membranes by nanosecond pulsed electric fields (nsEP) remains unknown. The passage of a high electric field through a conductive medium creates two primary contributing factors that may induce poration: the electric field interaction at the membrane and the shockwave produced from electrostriction of a polar submersion medium exposed to an electric field. Previous work has focused on the electric field interaction at the cell membrane, through such models as the transport lattice method. Our objective is to model the shock wave cell membrane interaction induced from the density perturbation formed at the rising edge of a high voltage pulse in a polar liquid resulting in a shock wave propagating away from the electrode toward the cell membrane. Utilizing previous data from cell membrane mechanical parameters, and nsEP generated shockwave parameters, an acoustic shock wave model based on the Helmholtz equation for sound pressure was developed and coupled to a cell membrane model with finite-element modeling in COMSOL. The acoustic structure interaction model was developed to illustrate the harmonic membrane displacements and stresses resulting from shockwave and membrane interaction based on Hooke’s law. Poration is predicted by utilizing membrane mechanical breakdown parameters including cortical stress limits and hydrostatic pressure gradients.

Second Harmonic Generation (SHG) imaging is a useful tool for examining the structure of interfaces between bulk materials. Recently, this technique was applied to detecting subtle perturbations in the structure of cellular membranes following nanosecond pulsed electric field (nsPEF) exposure. Monitoring the cell’s outer membrane as it is exposed to nsPEF via SHG has demonstrated that nanoporation is likely the root cause for size-specific, increased cytoplasmic membrane permeabilization. It is theorized that the area of the membrane covered by these pores is tied to pulse intensity or duration. The extent of this effect along the cell’s surface, however, has never been measured due to its temporal brevity and minute pore size. By enhancing the SHG technique developed and elucidated previously, we are able to obtain this information. Further, we vary the pulse width and amplitude of the applied stimulus to explore the mechanical changes of the membrane at various sites around the cell. By using this unique SHG imaging technique to directly visualize the change in order of phospholipids within the membrane, we are able to better understand the complex response of living cells to electric pulses.

Nanosecond pulsed electric fields (nsPEF) cause the formation of small pores, termed nanopores, in the membrane of cells. Current nanoporation models treat nsPEF exposure as a purely electromagnetic phenomenon, but recent publications showing pressure transients, ROS production, temperature gradients, and pH waves suggest the stimulus may be physically and chemically multifactorial causing elicitation of diverse biological conditions and stressors. Our research group's goal is to quantify the breadth and participation of these stressors generated during nsPEF exposure and determine their relative importance to the observed cellular response. In this paper, we used advanced imaging techniques to identify a possible source of nsPEF-induced acoustic shock waves. nsPEFs were delivered in an aqueous media via a pair of 125 μm tungsten electrodes separated by 100 μm, mirroring our previously published cellular exposure experiments. To visualize any pressure transients emanating from the electrodes or surrounding medium, we used the Schlieren imaging technique. Resulting images and measurements confirmed that mechanical pressure waves and electrode-based stresses are formed during nsPEF, resulting in a clearer understanding of the whole exposure dosimetry. This information will be used to better quantify the impact of nsPEF-induced acoustic shock waves on cells, and has provided further evidence of non-electrical-field induced exposures for elicitation of bioieffects.

Cellular delivery of pulsed IR laser energy has been shown to stimulate action potentials in neurons. The mechanism for this stimulation is not completely understood. Certain hypotheses suggest the rise in temperature from IR exposure could activate temperature- or pressure-sensitive channels, or create pores in the cellular outer membrane. Studies using intensity-based Ca 2+- responsive dyes show changes in Ca 2+ levels after various IR stimulation parameters; however, determination of the origin of this signal proved difficult. An influx of larger, typically plasma-membrane-impermeant ions has been demonstrated, which suggests that Ca 2+ may originate from the external solution. However, activation of intracellular signaling pathways, possibly indicating a more complex role of increasing Ca 2+ concentration, has also been shown. By usingCa 2+ sensitive dye Fura-2 and a high-speed ratiometric imaging system that rapidly alternates the excitation wavelengths, we have quantified the Ca 2+ mobilization in terms of influx from the external solution and efflux from intracellular organelles. CHO-K1 cells, which lack voltage-gated Ca 2+ channels, and NG-108 neuroblastoma cells, which do not produce action potentials in an early undifferentiated state, are used to determine the origin of the Ca 2+ signals and investigate the role these mechanisms may play in IR neural stimulation.

Random lasers are a developing class of light sources that utilize a highly disordered gain medium as opposed to a conventional optical cavity. Although traditional random lasers often have a relatively broad emission spectrum, a random laser that utilizes vibration transitions via Raman scattering allows for an extremely narrow bandwidth, on the order of 10 cm−1. Here we demonstrate the first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium, with conversion efficiencies on the order of a few percent. Furthermore, Monte Carlo simulations are used to study the complex spatial and temporal dynamics of nonlinear processes in turbid media. In addition to providing a large signal, characteristic of the Raman medium, the random Raman laser offers us an entirely new tool for studying the dynamics of gain in a turbid medium., Unlike conventional lasers that require a uniform resonant cavity to operate, random lasers use a highly disordered gain medium in which scattering is dominant. Hokr et al. report Raman lasing from a bulk three-dimensional disordered medium whose intensity exceeds that of other random lasers by many orders of magnitude.

The requirement of center asymmetry for the creation of second harmonic generation (SHG) signals makes it an attractive technique for visualizing changes in interfacial layers such as the plasma membrane of biological cells. In this article, we explore the use of lipophilic SHG probes to detect minute perturbations in the plasma membrane. Three candidate probes, Di-4-ANEPPDHQ (Di-4), FM4-64, and all-trans-retinol, were evaluated for SHG effectiveness in Jurkat cells. Di-4 proved superior with both strong SHG signal and limited bleaching artifacts. To test whether rapid changes in membrane symmetry could be detected using SHG, we exposed cells to nanosecond-pulsed electric fields, which are believed to cause formation of nanopores in the plasma membrane. Upon nanosecond-pulsed electric fields exposure, we observed an instantaneous drop of ∼50% in SHG signal from the anodic pole of the cell. When compared to the simultaneously acquired fluorescence signals, it appears that the signal change was not due to the probe diffusing out of the membrane or changes in membrane potential or fluidity. We hypothesize that this loss in SHG signal is due to disruption in the interfacial nature of the membrane. The results show that SHG imaging has great potential as a tool for measuring rapid and subtle plasma membrane disturbance in living cells.

The time-temperature effects of laser radiation exposure are investigated as a function of wavelength. Here, we report the thermal response of bulk tissue as a function of wavelength from 700 to 1064 nm. Additionally, Monte Carlo simulations were used to verify the thermal response measured and predict damage thresholds based on the response.

Nanoelectroporation of biomembranes is an effect of high-voltage, nanosecond-duration electric pulses (nsEP). It occurs both in the plasma membrane and inside the cell, and nanoporated membranes are distinguished by ion-selective and potential-sensitive permeability. Here we report a novel phenomenon of bioeffects cancellation that puts nsEP cardinally apart from the conventional electroporation and electrostimulation by milli- and microsecond pulses. We compared the effects of 60- and 300-ns monopolar, nearly rectangular nsEP on intracellular Ca(2+) mobilization and cell survival with those of bipolar 60 + 60 and 300 + 300 ns pulses. For diverse endpoints, exposure conditions, pulse numbers (1-60), and amplitudes (15-60 kV/cm), the addition of the second phase cancelled the effects of the first phase. The overall effect of bipolar pulses was profoundly reduced, despite delivering twofold more energy. Cancellation also took place when two phases were separated into two independent nsEP of opposite polarities; it gradually tapered out as the interval between two nsEP increased, but was still present even at a 10-µs interval. The phenomenon of cancellation is unique for nsEP and has not been predicted by the equivalent circuit, transport lattice, and molecular dynamics models of electroporation. The existing paradigms of membrane permeabilization by nsEP will need to be modified. Here we discuss the possible involvement of the assisted membrane discharge, two-step oxidation of membrane phospholipids, and reverse transmembrane ion transport mechanisms. Cancellation impacts nsEP applications in cancer therapy, electrostimulation, and biotechnology, and provides new insights into effects of more complex waveforms, including pulsed electromagnetic emissions.

Nonlinear optical probes, especially those involving second harmonic generation (SHG), have proven useful as sensors for near-instantaneous detection of alterations to or ientation or energetics within a substance. This has been exploited to some success for observing conformational changes in proteins. SHG probes, therefore, hold promise for reporting rapid and minute changes in lipid membranes. In this report, one of these probes is employed in this regard, using nanosecond electric pulses (nsEPs) as a vehicle for instigating subtle membrane perturbations. The result provides a useful tool and methodology for the observation of minute membrane perturbation, while also providing meaningful information on the phenomenon of electropermeabilization due to nsEP. The SHG probe Di-4-ANEPPDHQ is used in conjunction with a tuned optical setup to demonstrate nanoporation preferential to one hemisphere, or pole, of the cell given a single square shaped pulse. The results also confirm a correlation of pulse width to the amount of poration. Furthermore, the polarity of this event and the membrane physics of both hemispheres, the poles facing either electrode, were tested using bipolar pulses consisting of two pulses of opposite polarity. The experiment corroborates findings by other researchers that these types of pulses are less effective in causing repairable damage to the lipid membrane of cells.

Despite 30 years of research, the mechanism behind the induced breakdown of plasma membranes by electrical pulses, termed electroporation, remains unknown. Current theories treat the interaction between the electrical field and the membrane as an entirely electrical event pointing to multiple plausible mechanisms. By investigating the biophysical interaction between plasma membranes and nanosecond electrical pulses (nsEP), we may have identified a non-electric field driven mechanism, previously unstudied in nsEP, which could be responsible for nanoporation of plasma membranes. In this investigation, we use a non-contact optical technique, termed probe beam deflection technique (PBDT), to characterize acoustic shockwaves generated by nsEP traveling through tungsten wire electrodes. We conclude these acoustic shockwaves are the result of the nsEP exposure imparting electrohydraulic forces on the buffer solution. When these acoustic shockwaves occur in close proximity to lipid bilayer membranes, it is possible that they impart a sufficient amount of mechanical stress to cause poration of that membrane. This research establishes for the first time that nsEP discharged in an aqueous medium generate measureable pressure waves of a magnitude capable of mechanical deformation and possibly damage to plasma membranes. These findings provide a new insight into the longunanswered question of how electric fields cause the breakdown of plasma membranes.