Lightsheet optical tweezer (LOT) for optical manipulation of microscopic particles and live cells

Abstract

Optical trapping and patterning cells or microscopic particles is fascinating. We developed a light sheet-based optical tweezer to trap dielectric particles and live HeLa cells. The technique requires the generation of a tightly focussed diffraction-limited light-sheet realized by a combination of cylindrical lens and high NA objective lens. The resultant field is a focussed line (along x-axis) perpendicular to the beam propagation direction (z-axis). This is unlike traditional optical tweezers that are fundamentally point-traps and can trap one particle at a time. Several spherical beads undergoing Brownian motion in the solution are trapped by the lightsheet gradient potential, and the time (to reach trap-centre) is estimated from the video captured at 230 frames/s. High-speed imaging of beads with increasing laser power shows a steady increase in trap stiffness with a maximum of 0.00118 pN/nm at 52.5 mW. This is order less than the traditional point-traps, and hence may be suitable for applications requiring delicate optical forces. On the brighter side, light sheet tweezer (LOT) can simultaneously trap multiple objects with the distinct ability to manipulate them in the transverse (xy) plane via translation and rotation. However, the trapped beads displayed free movement along the light-sheet axis (x-axis), exhibiting a single degree of freedom. Furthermore, the tweezer is used to trap and pattern live HeLa cells in various shapes and structures. Subsequently, the cells were cultured for a prolonged period of time (> 18 h), and cell viability was ascertained. We anticipate that LOT can be used to study constrained dynamics of microscopic particles and help understand the patterned cell growth that has implications in optical imaging, microscopy, and cell biology.

The effects of mechanical force on fibroblast behavior in cutaneous injury

“…Use of “optical tweezers,” which consist of coherent light beams, have been used to trap, manipulate, and apply force to cells in a non-contact manner. These protocols have allowed for study of application of force to individual cells and study of mechanotransduction on a uni-cellular level ( 49 – 51 ). The development and implementation of atomic force microscopy has allowed for high-resolution and fluorescent live visualization of cells as they are stretched and undergo mechanical force ( 52 – 54 ).…”

Using flat-top light sheet generated by femtosecond-pulsed laser for optical manipulation of microscopic particles

“…The gradient force (𝐹 𝑔𝑟𝑎𝑑 ) within this trap is along the 𝑦 − 𝑧 plane with its upper limit at the focal point along the propagation direction. Hence, a stable line-trap is constructed when 𝐹 𝑔𝑟𝑎𝑑 overpowers 𝐹 𝑠𝑐𝑎𝑡 , Brownian motion and the effect of gravity 24 .…”

“…This leads to a trapped particle being confined in onedimension by a LS, and thus possessing a single degree of freedom. 24 In this case, a 100×/1.3 NA objective lens and a single 2 µm silica microspheres were used to enhance visualization and trap strength (as the restoring force improves as particle gets bigger due to increase in viscous drag). 12 As a result of single particle trapping, we quantified the trap parameters using the QPD.…”

“…The laser operating at a wavelength of 1064 nm and at a power of 52.5 mW successfully trapped multiple 1-m spherical particles in a line and oriented the particles at various angles along the transverse plane. 24 The bottleneck of applying this tool is that the line trap requires greater power than traditional OT and the trap strength is an order of magnitude weaker due to the spread of intensity from a single point to a line. However, the high power of this CW light sheet OT produces unwanted thermal effects which can eventually damage biological specimens.…”

Dielectrophoresis-Based Selective Droplet Extraction Microfluidic Device for Single-Cell Analysis

“…By trapping cells in the flow path, various aspects of cell behavior can be examined, including cell interactions [ 1 ], drug responses [ 2 , 3 ], and protein expression [ 4 , 5 ]. Several approaches, such as those based on gravity [ 2 , 3 , 4 , 6 ], hydrodynamics [ 1 , 7 , 8 ], optical tweezers [ 9 ], and dielectrophoresis (DEP) [ 10 , 11 ], have been applied to microfluidic devices to capture single cells. In addition, many studies require selective extraction of the cells from the device after screening for subsequent off-chip analysis of more specific responses.…”