HUN-REN Biological Research Centre, Szeged 
Jun 28, 2024 | News
Single-cell investigation methods such as single-cell genetics, proteomics or imaging-based morphological classification have risen to the forefront of biological research in the last decade. These methods require precisely controlled physical manipulation of the individual cells on the microscopic scale. This manipulation on the single cell level means their transportation and rotation in a controlled manner, for which several methods have been developed in the last decades. These cutting-edge methods use active movable microtools such as microgrippers with the size similar to that of the cells, complex electrophoretic systems that use high-frequency electric fields to move the cells or optothermal traps that create liquid flow through localized laser heating. The technique of optical tweezers fits into this line being one of the most efficient single cell manipulation methods and which was awarded a Nobel prize in 2018. Optical tweezers consist of a highly focused laser beam, in the focus of which micrometer-sized objects, cells among others, can be trapped. A more robust and effective way of taking hold of the cells is their indirect trapping using intermediate objects as handles, which provide considerably larger tapping force and less photodamage for the sensitive cells. These handles, which can take the form of a simple microsphere or a more complex structure, are strongly bound to the cells, requiring some sort of physico- or biochemical treatment of both parties. Besides the inevitable advantages of such handles, the treatment itself, and the fact that they cannot be removed from the cells, are often disadvantageous.
A solution for this problem was offered by the group of researchers led by Lóránd Kelemen at the Institute of Biophysics in the HUN-REN Biological Research Centre, Szeged, in cooperation with their Slovakian partners, who developed a novel family of deformable single cell manipulation microtools that can be transiently attached to the cells. The key feature of the structures is their elasticity, which enables them to hold the cells without the need for any kind of treatment on them or on the cell. These tools act as tiny robots that can grab, hold still, rotate, move and release the cells under study. The microtools are operated with multifocus optical tweezers and can undergo very high degree of deformation in order to accommodate the cells. The deformable parts of the structures are as thin as 300 nanometers which is necessary for the optical tweezers to bend them. The microstructures are also manufactured by laser, using an additive microfabrication technique, called two-photon polymerization. This methodology is essential for the production of the microrobots, since this is one of the few techniques, which are capable of preparing arbitrary shaped 3D polymer objects with feature size down to even 100 nanometers.
The researchers demonstrated the capability of the technique through three types of structures specifically designed for three distinct cell manipulation tasks. The first is a cell transporter designed to enclose and move a cell that is selected from the many surrounding cells in a microfluidic environment and transport it with practically no force applied on it. It consists of two half spheres that are about twice the size of the cell, and which can be opened up with the tweezers to conveniently accommodate the cell. Its cargo then can be transported to and released at any area defined by the user. The second type is somewhat the opposite: it is designed to hold the cell firmly, allowing it to fluctuate only 50 nanometers. The purpose of this tool is to keep the cell still for imaging with the huge additional benefit of being able to rotate it to any preferred orientation under the microscope to access imaging directions at will. The potential of this microtool was demonstrated by using it to improve the optical resolution of 3D fluorescence imaging of the cell. Finally, the third member of the microrobot group is actually a pair of structures that is used to initiate cell-cell interaction with precise spatial and temporal control over the process. One of the structures is holding a cell firmly, while the other one is maneuvering a second cell to the first one with the optical tweezers and pressing them to each other. The main advantage of this setup is that the onset of the interaction can be accurately defined in contrast to experiments where it is randomly initiated in a test tube by simply mixing the cell suspensions. The tools enable the precise observation of time evolution of how the two cells react to each other, which is essential to discover the details of their behavior even at the molecular level. The researchers believe that the presented optical tweezers-based methodology using task-specific microtools will allow the investigation of single, non-adherent cells in a much more efficient way and proves that soft micro robotics has huge potential in biological applications.
A video on how microrobots work is available on the X website: https://x.com/AdvSciNews/status/1800468454199181564
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