Techniques to measure and exert forces in micro Newton range - Applied to genetic model organisms

Siddharth M. Khare
Department of Physics, Indian institute of Science
3 PM Monday, 9/26
351 Regents Hall 

Locomotive forces applied by genetic model organisms and their response to external mechanical forces can provide insight into the effect of mechanosensory and muscle mutations on the organism behavior as well as biomechanics of undulatory and crawling motion. This work presents novel techniques for measurement and application of such force in micro Newton range applied to Caenorhabditis elegans (C. elegans) and Drosophila melanogaster larvae.

Colored Polydimethyl Siloxane (PDMS) micropillar array developed to measure forces eliminates the common drawbacks of micro-machined force sensing tools such as highly complicated, fragile construction and low throughput. The color of the micropillars enhances contrast, facilitating the detection of pillar tip deflection as the organism crawls between or on top of the pillars. A custom built semi-automated graphical user interface to analyze images for pillar deflections reduces the data processing time per animal to minutes. The wild type C. elegans exertes an average force of ~1 μN on individual pillar and a total average force of ~7.68μN. The total average force is not affected by the defective sense of external mechanical touch. It is also observed that the ability to exert force does not necessarily reflect in the larger activity of the animals. The colored PDMS micropillar device also forms the first device to enable mapping of forces exerted by freely moving Drosophila larvae.

The conventional mechanosensory assay for C. elegans is accomplished using an eyelash hair to exert force on the animal body. The manual application of such forces using an eyelash can produce forces ranging from below 5 μN up to 200 μN. To mitigate this, a system that generates an air microjet with diameter restricted to ~800 μm was developed. The microjet can exert total force up to ~ 84 μN on a flat surface from distances as long as 6 mm. The long working distance simplified the operation and non-contact approach eliminates mechanical damage to the animal body and reduced contamination risks. The response of wild type C. elegans to the air jet was similar to that observed when the animal is touched manually by an eyelash. This demonstrates the applicability of the technique for C. elegans mechanosensory assay.