MEMS-based Drosophila embryo injection system for high-throughput screens

The fruit fly Drosophila is one of the most important model organisms for human development and disease. The availability of the Drosophila genome sequences and techniques for gene silencing, such as RNA interference, offer unprecedented opportunity to study functions of genes and molecular mechanisms of development and disease. In order to automate genetic experiments, new instruments are needed. We are developing microinjector chips for use in automated Drosophila embryo injection systems for high-throughput genetic screens. Such injection technologies can be used for example for delivery of double-stranded RNA for specific, embryo-wide gene silencing through RNA interference.

The injector needle is defined in a surface micromachining process on a silicon wafer, using silicon nitride structural layers and a phosphosilicate glass sacrificial layer. The fabrication process allows for integration of mechanical on-chip filters and differential piezoresistive pressure sensors directly at the injection needle for monitoring of injection processes.

The size of a single injector chip and a fluidic interconnection hole are defined in a deep reactive ion etching process. A structured Pyrex wafer is bonded to the backside of the silicon wafer and contains a 500 nl sized reservoir for solutions to be injected. A plastic package enables handling of single injector chips and application of air pressure pulses in the range of tens of microseconds to both ends of the U-shaped reservoir for precise injection of typically 60 pl into a single embryo.

Packaged microinjectors are used in a fully automated, computer-controlled system based on motorized xyz-stages and two CCD cameras for machine vision. The system performs alignment between injector chip and alignment marks on the embryo glass slide, screens the glass slide for embryos, and precisely injects each single embryos with 60 pl of liquid. The video below demonstrates reliable, automated injection of Drosophila embryos with nominally 60 pl of green food coloring, as seen through the embryo glass slide by one of the above described CCD cameras.

Drosophila embryo injection system for generation of transgenic flies

A powerful method for genetic studies is the generation of transgenic animals. In Drosophila, generation of transgenic animals typically requires injection of DNA, carrying for example a gene of interest, into the posterior end of a developing embryo, right where the so-called pole cells are formed that later on give rise to germ cells of the adult fly. In order to enable injection specifically into the posterior end of an embryo, we are working on a microfluidic device with a microinjector integrated at the end of a microfluidic channel.

The microfluidic channel is shaped so that one end of the embryos is precisely aligned with the embedded microinjector. Fluid flow exerts a drag force on the embryo that presses the embryo against the microinjector, until the embryo membrane is pierced and DNA is injected. A redesign of the here shown device will exhibit control mechanisms for reliable transport of a single embryo from a reservoir into the chip and subsequent on-chip embryo tracking, transport, and injection.