Stefano Oberti

Monolithically Fabricated Microgripper With Integrated Force Sensor for Manipulating Microobjects and Biological Cells Aligned in an Ultrasonic Field

This paper reports an electrostatic microelectromechanical systems (MEMS) gripper with an integrated capacitive force sensor. The sensitivity is more than three orders of magnitude higher than other monolithically fabricated MEMS grippers with force feedback. This force sensing resolution provides feedback in the range of the forces that dominate the micromanipulation process. A MEMS ultrasonic device is described for aligning microobjects suspended in water using ultrasonic fields. The alignment of the particles is of a sufficient accuracy that the microgripper must only return to a fixed position in order to pick up particles less than 100 mum in diameter. The concept is also demonstrated with HeLa cells, thus providing a useful tool in biological research and cell assays

Manipulation of micrometer sized particles within a micromachined fluidic device to form two-dimensional patterns using ultrasound

Ultrasonic manipulation, which uses acoustic radiation forces, is a contactless manipulation technique. It allows the simultaneous handling of single or numerous particles (e.g., copolymer beads, biological cells) suspended in a fluid, without the need for prior localization. Here it is reported on a method for two-dimensional arraying based on the superposition of two in-plane orthogonally oriented standing pressure waves. A device has been built and the experimental results have been compared with a qualitative analytical model. A single piezoelectric transducer is used to excite the structure to vibration, which consists of a square chamber etched in silicon sealed with a glass plate. A set of orthogonally aligned electrodes have been defined on one surface of the piezoelectric. This allows either a quasi-one-dimensional standing pressure field to be excited in one of two directions or if both electrodes are activated simultaneously a two-dimensional pressure field to be generated. Two different operational modes are presented: two signals identical in amplitude and frequency were used to trap particles in oval shaped clumps; two signals with slightly different frequencies to trap particles in circular clumps. The transition between the two operational modes is also investigated.

A micro-particle positioning technique combining an ultrasonic manipulator and a microgripper

The acoustic radiation force acts on particles suspended in a fluid in which acoustic waves are present. It can be used to establish a force field throughout the fluid volume capable of positioning the particles in predictable locations. Here, a device is developed which positions the particles in a single line by the sequential use of two excitation frequencies which have been identified by a finite element model of the system. The device is designed such that at one end there is an opening which allows the fingers of a microgripper to enter the fluid chamber. Hence the gripper can be used to remove the last particle in the line. The high accuracy of the positioning of the particles prior to gripping means that the microgripper needs just to return to a fixed position in order to remove subsequent particles. Furthermore, the effects of the microgripper fingers entering the fluid volume whilst the ultrasound field is excited are examined. One result being the release of a particle stuck to a gripper finger. It is believed that this combination of techniques allows for considerable scope in the automation of microgripping procedures.

Towards the automation of micron-sized particle handling by use of acoustic manipulation assisted by microfluidics.

The use of acoustic radiation forces for the manipulation and positioning of micrometer sized particles has shown to be a promising approach. Resonant excitation of a system containing a particle laden fluid filled cavity, can (depending on the mode excited) result in positioning of the particles in parallel lines (1-D) or distinct clumps in a grid formation (2-D) due to the high amplitude standing pressure fields that arise in the fluid. In a broader context, the alignment of particles using acoustic forces can be used to assist manipulation processes which utilise an external mechanical tool, for instance a microgripper. In such a system, particles can be removed sequentially from a line formed by acoustic forces within a microfluidic channel, hence allowing a degree of automation. In order to fully automate the gripping process, the particles must be confined to a repeatable and accurate location in two dimensions (assuming that in the third dimension they sit on the lower surface of the channel). Only in this way it is possible to remove subsequent particles by simply bringing the gripper to a known location and activating its fingers. This combined use of acoustic forces and mechanical gripping requires that one extremity of the channel is open. However, the presence of the liquid-air interface which occurs at this opening, causes the standing pressure field to decay to zero towards the opening. In a volume of liquid in proximity to the interface positioning of particles by acoustic forces is therefore no longer possible. In addition, the longitudinal gradient of the field can cause a drift of particles towards the longitudinal center of the channel at some frequencies, undesirably moving them further away from the interface, and so further from the gripper. As a solution the use of microfluidic flow induced drag forces in addition to the acoustic force potential has been investigated.

Novel sample preparation technique for protein crystal X-ray crystallographic analysis combining microfluidics and acoustic manipulation

In order to perform X-ray crystallographic analysis, protein crystals are removed from their growing solution by means of a nylon loop, which is then mounted on a goniometer. As this process is repeated for a large number of crystals, there is a need for automation, especially with regard to the placement on the nylon loop. A novel technique involving the use of acoustic radiation forces and a micro-machined fluidic device is introduced here. After insertion into the micro-machined channel, the crystals are positioned in a row along its centre-line by excitation of a high-frequency standing pressure field, and then moved towards an orifice by applying a flow along the channel, which also ensures spatial separation. Once located in a defined orifice, the single crystals can be removed using a nylon loop. X-ray crystallographic analysis showed that application of ultrasound does not influence the diffraction properties of the crystals.