This invention pertains generally to the field of microfluidic systems and particularly to flow sensors for such systems.
There are many applications where it is necessary or desirable to manipulate fluids in small volumes, including rapid bioassays, microchemical reactions, and chemical and biological sensing. See, e.g., M. Freemantle, xe2x80x9cDown Sizing Chemistry,xe2x80x9d Chem. and Eng. News, Vol. 77, No. 8, 1999, pp. 27-36. Microfluidic devices which can manipulate fluids at small scales have been formed using lithographic techniques similar to those used in microelectronic processing, often with crystalline silicon as the substrate on which the devices are formed. The microfluid flow channels that are formed in or on such substrates may have relatively small dimensions, e.g., channel widths of 1,000 xcexcm or smaller.
For many applications, it is necessary to measure the rate of flow in the microchannels to properly operate the microfluidic device. Not surprisingly, the design challenge of producing flow sensors which are both accurate and economical at such dimensions is formidable. One approach to the sensing of flow rates in a closed microfluidic channel is to utilize pressure sensors embedded in the walls of the channel at either end of the channel; the differential in the pressure at the two sensors is used to determine the flow rate. Such an approach has several disadvantages. It requires an especially microfabricated channel or channel cap, which can be difficult and expensive to make. In addition, it is generally difficult to accurately calibrate the pressure sensors to obtain a good reading. Further, such an approach is limited to fully enclosed channels through which the fluid flows under pressure. The typical membrane type pressure sensors formed on the walls of the microfluidic channels generally require significant additional processing steps beyond that required to produce the flow channels themselves, and the cover that extends over the substrate to close the channel typically must be micromachined to allow integration of the sensor. Another approach to flow sensing in microchannels has utilized lift forces for sensing gas flow at moderately high Reynolds numbers. See Nikalas Svedin, et al., xe2x80x9cA New Silicon Gas Flow Sensor Based on Lift-Force,xe2x80x9d JMEMS, Vol. 7, No. 3, 1988, pp. 303-308. However, such devices are not well suited for sensing fluid flow in smaller microfluidic channels and at lower Reynolds number flow.
In accordance with the present invention, sensing of microfluidic flow is carried out by confining and directing a fluid along a surface in a primary direction of flow past a cantilever beam which is mounted at one end of the beam to the surface. The cantilever beam has opposite beam surfaces that are oriented at an angle off parallel to the primary direction of flow of the fluid. As the fluid is directed past the beam at a rate such that the drag forces imposed by the fluid on the opposite surfaces of the beam are greater than the inertial forces of the fluid on the beam, a differential force is applied to the beam that tends to pivot the beam about its mount to the surface or bend the beam, or both. The differential drag forces apply a net force to the beam to deflect it in a direction further away from a parallel to the primary direction of flow, i.e., deflection toward an upstream direction of the flowing fluid, rather than in a direction toward the parallel to the primary direction of flow (deflection in a downstream direction), as would be the case with a beam oriented off axis in a flowing stream under flow conditions that are typically encountered in the normal macro-world where inertial forces typically dominate drag forces. However, in accordance with the present invention, the flow rates and viscosity of the fluid flowing through relatively small channels, e.g., with lateral dimensions of 500 xcexcm or less, result in drag forces dominating, making such devices well suited for microfluidic flow sensing applications. The deflection of the beam in response to the differential drag forces imposed by the flowing fluid may then be detected to determine the rate of flow of the fluid.
Microfluidic flow sensing apparatus in accordance with the invention may include a substrate having a channel formed therein defined by side walls and a bottom wall, and a cover on the substrate enclosing the channel and defining a top wall for the channel. The channel is formed to guide flow of a fluid therethrough in a primary direction of flow. An elongated beam having preferably parallel opposite surfaces is cantilever mounted at one end of the beam to a wall of the channel, and contained within the channel, with the surfaces of the beam oriented at an angle off of parallel to the primary direction of flow of fluid in the channel. Various means may be provided for detecting deflection of the beam in the channel in response to differential drag forces on the beam imposed by fluid flowing through the channel in the primary direction of flow. These include strain sensors formed in the wall of the channel to which the beam is mounted to sense strain in the wall caused by pivoting of the beam, photodiodes formed in the wall of the channel to which the beam is mounted at, a position beneath a portion of the beam such that the beam partially blocks light incident on the photodiodes and blocks more or less light as the beam is deflected, and an optical observation system projecting light through the cover and optically detecting deflections of the beam. Such detection means are by way of example only and any other type of detector which can detect deflection of a beam may also be utilized. The cover that is mounted to the substrate to close the channel may be formed of any of a variety of materials, including polymers, since the cover does not need to be micromachined in order to accommodate the microbeam mounted within the channel. If desired, the sensing structure can be formed on a single chip by an IC manufacturer specializing in microelectronic integration, and the cover can be formed by a microfluidic foundry specializing in fabricating plastic or elastomeric parts. Typical dimensions for the channel include a height and width of less than 1,000 xcexcm, preferably from 1 xcexcm to 500 xcexcm, a thickness of the beam less than 200 xcexcm, and a length of the beam generally at least 10 times the thickness of the beam. The substrate in which or on which the channel is formed may be various materials, including crystalline silicon, which may be machined or patterned with flow paths using well developed micromachining and patterning techniques.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.