Triple beam optical trap for microsyringe construction

A limited range of instruments are available which allow the controlled injection of sub-picolitre volumes; microfluidic devices and commercially produced mechanical microinjection systems accounting for the majority. We present an optically controlled microsyringe capable of dispensing femtolitres of liquid. Triple beam optical tweezers are used to manipulate hollow glass microneedles and also polymer microspheres which were used as 'handles' to assist the manipulation of microneedles and 'plungers' to dispense liquid from the microneedle.

Standard optical tweezers were used with the addition of a Ronchi ruling (250 lines per inch) mounted in the image relay telescope. The diffraction pattern generated by the Ronchi ruling produced three optical traps in the sample chamber. Trap spacing was controlled by translating the ruling along the axis of beam propagation within the image relay telescope.

Utilizing the three-beam trap, it was possible to manipulate pulled, borosilicate capillaries (5-150 mu m in length, 1-10 mu m in diameter) both perpendicular and parallel to the axis of the capillary. Rolled SiO/SiO2 microtubes (4 mu m diameter, 50 mu m long) were also manipulated, however in this case polymer microspheres were used as 'handles'. In both cases the microneedles did not align vertically along the propagation axis; an advantage over using a single beam optical trap. Tweezing a microsphere within a microneedle dispenses femtolitres of liquid from the needle. The force exerted on microneedles is calculated to be in the order of picoNewtons so may have applications where femtolitre volumes must be controllably delivered beyond a barrier, such as single cell microinjection.