Understanding the deformability and associated biomechanical properties of red blood cells (RBCs) is crucial for many pathological analysis and diagnosis of human diseases. In such endeavors, optical tweezers have played an active role over the past decades. Here, we study the RBC deformability by employing a novel “tug of war” (TOW) optical tweezers consist of a pair of elongated diverging accelerating beams that can stably trap and stretch a single RBC under different osmotic conditions without any tethering or mechanical movement. With a viscous drag method, we compare directly the trapping force at different states of RBCs, and find that even one arm of the TOW tweezers can apply a force of over 18pN with only 100mW laser power, more than 2 times stronger than that from the Gaussian trap at the same condition. Without the need of two independent controls as in a conventional dual trap, the spacing between the two TOW traps can be increased conveniently from 0 to over 9m, resulting in nearly 15% of cell deformation. We obtain the shear modulus of the RBCs in different osmotic conditions, with the largest value of 3.36±0.95pN/μm in the hypertonic case, and compare with those previously reported results. Our work may bring about a new photonic tool for the study of biomechanical properties of living cells, promising for applications such as distinguishing healthy and diseased cells.
Biological samples often have various absorption bands that need to be either targeted or avoided in opto-fluidic micromanipulation or biomedical imaging. With nonlinear optics, it is possible for light to self-induce a waveguide. However, the desired wavelengths may not be suitable to exhibit nonlinear self-guiding due to the absorption bands or the light-bioparticle interaction is not strong enough. Here we study formation of waveguides in red blood cell suspensions for a range of different wavelengths. We utilize nonlinear optical response for self-trapping of a laser beam, forming light guides in RBCs suspended in a phosphate buffer solution. To improve the number of usable light wavelengths over purely self-guided propagation, we use the master-slave relation, in a manner similar to the pump-probe experiment: a master beam creates a waveguide first in a scattering bio-soft-matter suspension over a few centimeters, and then a “slave” beam uses this waveguide to propagate through the medium. The slave beam, injected simultaneously, has no appreciable nonlinear self-action itself but experiences the master waveguide akin to an optical fiber. This new approach can provide a path to guide a wide range of wavelengths, including those in the absorption bands at lower power so as not to damage the sample. The fact that we can guide a wide range of wavelengths may bring about new applications in medicine and biology, for instance, in developing alternative solutions to transmit energy and information through scattering media, as needed in deep-tissue imaging, treatment and diagnostics.
We design and demonstrate multi-trap tug-of-war (TOW) optical tweezers with object-adapted optical potentials for trapping and manipulating asymmetric particles and biological samples such as mutant bacterial cells. While dual TOW tweezers can effectively trap rod-shaped objects and even stretch them laterally, triangular TOW tweezers enable in-plane trapping of larger asymmetric objects which do not necessarily have mirror symmetry. When trapping with the dual TOW tweezers, we previously demonstrated that they are more stable than Gaussian beam-based dual traps, and the strong lateral pulling forces from the TOW optical tweezers can stretch and even break apart cellular clusters. Here we show multi-trap TOW (with 3 and 4 arms) optical tweezers can be employed to control and manipulate mutant Sinorhizobium meliloti bacterial cells, which are typically multi-pronged. We discuss the advantage of such TOW beam-based optical tweezers over traditional Gaussian beam-based holographic tweezers, and the potential applications of these TOW tweezers in studying cellular viscoelasticity, biomechanics, motility, and intercellular interactions.
In typical colloidal suspensions, the corresponding optical polarizability is positive, and thus enhanced scattering takes place as optical beams tend to catastrophically collapse during propagation. Recently, light penetration deep inside scattering suspensions has been realized by engineering dielectric or plasmonic nanoparticle polarizibilities. In particular, we have previously demonstrated two types of soft-matter systems with tunable optical nonlinearities - the dielectric and metallic colloidal suspensions, in which the effects of diffraction and scattering were overcome, hence achieving deep penetration of a light needle through the suspension.
In this work, we show that waveguides can be established in soft matter systems such as metallic nanosuspensions through the formation of plasmonic resonant solitons. First, we show that, due to plasmonic resonance, a 1064nm laser beam (probe) would not experience appreciable nonlinear self-action while propagating through 4cm cuvette containing the metallic nanosuspension of gold spheres (40nm), whereas a 532nm laser beam (pump) can readily form a spatial soliton due to nonlinear self-trapping. Second, we demonstrate effective guidance of the probe beam, which would otherwise diffract significantly through the nanosuspensions, due to the soliton-induced waveguide from the pump beam. Such guidance was observed when the power of the probe beam was varied from 20mW to 500mW at constant pump beam power, with more pronounced guidance realized from lower to higher probe beam power. Interestingly, due to the presence of the probe beam, the pump beam undergoes self-trapping at an even lower power. These results may bring about the possibility of engineering plasmonic soliton-based waveguides for many applications.