| Miniaturized instruments for the injection and positioning of single cells and embryos are becoming increasingly important in biological and genetic studies. Localized and accurate microinjection of genetic material into biological model systems, such as Drosophila, will enable a variety of studies in developmental biology and genetics. For such studies to be carried out in-vivo, the damage caused by the injection must be minimized. We study the force required for the penetration and injection into Drosophila embryos using a surface micromachined silicon-nitride probe with integrated, micrograting-based force sensors. The probe is supported by springs with a known spring constant, and the penetration force is determined from displacement measurements using a high-resolution, miniaturized optical encoder that is designed to only be sensitive to axial deflections of the probe. We also demonstrate penetration force minimization through ultrasonic actuation of silicon-nitride microinjectors. To facilitate parallel, high-throughput microinjection of Drosophila embryos, we demonstrate microfluidic self-assembled immobilization of Drosophila embryos and measure the positioning force acting on the embryos in the array. The positioning force is the most critical parameter in determining alignment errors, which in turn determines whether acceptable injection yields can be achieved. We operate the optical-encoder force sensor in reflection to characterize the positioning forces, and to study shape-matching, alignment tolerance and hysteresis of the self-assembly process as a function of pad geometry. An extended surface energy model is developed for simulations of the positioning forces and the affiliated potential energy wells created by the oil-based fluidic system between the ellipsoidal embryo and the flat pad. Both experimental and simulation results show a linear-spring like relationship between the force and displacement of the embryos, in contrast to the constant positioning force profile observed for self-assembly of flat silicon pieces. Our measurements also show significant hysteresis in the force vs. displacement, indicating that friction plays an important role in the self-assembly process. |
