MICRO TOOLING FEATURE IN THE GRIP OF THINGS structure of <1000 C, obtained in <1s. The


use of SU-8 and the dimensions chosen ensure that any cell placed within the tips will remain unheated. Thermal losses from the structure are largely by conduction down the arms to the chip and across the gap between the outer and inner arms [3].


David Wood and Linzi Dodd from The School of Engineering and Computing Sciences at Durham University explore the dynamics of a multipurpose microgripper device for individual cell capture and analysis


T


here is a rapidly growing need for individual cell manipulation in a wide


range of applications including stem cell sorting, gene and molecular delivery, cellular diagnostics, and single cell-based assays. When compared with data-averaging over a cell population, direct physical manipulation offers more precise selection and understanding of cell properties. An electrothermally actuated


microgripper device that can hold and manipulate individual cells is presented here. The tool has been integrated into a commercial cell handling station, and represents a highly versatile multi- purpose instrument. As a proof of concept demonstration, the microgripper can hold and mechanically stress mouse oocytes of ~60μm diameter, while simultaneously monitoring the level of stress. We have also used the tool to


manipulate other cell types, both plant and animal, and a range of other objects on a similar size scale, such as hairs, fibres, sand particles and polystyrene beads. This design of cell microgripper can do more than just capture individual cells; it can simultaneously perform scientific measurements.


THE MICROGRIPPER The microgripper design presented here uses an electrothermal bimorph actuation system [1]. In this design (Figure 1), a current path exists between two contact pads. From the first pad, the current travels onto a chip, along an outer arm of the microgripper itself, back on to the chip, and along the other outer arm before exiting on the second contact pad. Via resistive heating, the current causes both outer arms to expand. The two inner arms are unaffected.


Because of this differential expansion, the microgripper will close, causing the gap at the end to narrow - in essence, it acts as an electrically operated pair of tweezers. Once the current stops, the arms cool quickly and the microgripper reverts to its original shape and dimensions. By alternatively locating the gold tracks on the inner arms, the device can be made to open on actuation instead. The microgripper is made with SU-8 as


the structural material, and chromium/ gold as the metallisation layer [2]. Design dimensions are variable, but typically the chip is 2mm square, the microgripper arms 1mm long and with a 60μm thickness. A drive current of ~10-15mA gives a maximum temperature on the


/ MICROMATTERS


TIP DESIGN AND CELL MANIPULATION The microgripper tips are designed for any particular application at the lithography stage [2]. Shapes including flat or curved ends, scoops and even ‘pestle and mortar’ arrangements have all been demonstrated. The microgripper chip is held in a rapid prototyping holder via a plastic clip, and the whole assembly placed on a cell handling station. Slight bending of the microgripper allows the structure to enter a Petri dish at a non-zero angle, and slide along the bottom of the dish to collect a cell.


Figure 1:


The cell microgripper. The tips (where a cell is held) are closest to the front of the image


References


[1] B. Solano and D. Wood, Microelectron. Eng., Vol. 84, pp 1219– 1222 (2007).


[2] R. Daunton, A. J. Gallant and D. Wood, Proc Materials Research Society Meeting, San Francisco, USA, April 9- 13 , 2012, vol 1463, paper mrss12-1463- qq02-10.


[3] B. Solano, S. Rolt and D. Wood, Proc. Inst. Mech. Engrs., part C, Vol. 222, pp 73-86 (2008). [4] R. Daunton, D. Wood, A. J. Gallant and R. Kataky, RSC Advances, Vol. 4, pp 50536-50541 (2014)


DEMONSTRATION Cells generally communicate via influxes and effluxes of electroactive species. The advantage of this microgripper sensor device is that the capture, stressing and ionic response can be investigated in real time from one cell, showing the evolution of key ions involved in intercellular communication due to mechanical external stresses. For this demonstration, an electrode was


fabricated in the microgripper structure to run all the way to the tip ends. The electrode was then converted electrochemically to enable sensitivity to potassium ions. Using mouse oocytes that have had their


zona pelucida stripped chemically, we have shown that capturing and stressing a cell will result in the excretion of potassium ions, which can simultaneously be detected by the ion selective electrode [4]. This is a first generation ion sensor with


competitive characteristics in selectivity, sensitivity, stability and temporal resolution to other non-miniaturised techniques. The microgripper technology is generic and the ability to deliver or measure voltage, current, light or mechanical stress makes the microgripper a flexible and highly adaptable tool for cell handling and analysis.


Durham University www.dur.ac.uk david.wood@durham.ac.uk


Enter 205 MICROMATTERS | SPRING 2015 15


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