Valves and valve actuation – Tube compressors
Reexamination Certificate
2000-08-25
2001-07-17
Fox, John (Department: 3753)
Valves and valve actuation
Tube compressors
C251S011000
Reexamination Certificate
active
06260818
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to thin film fluid control systems and, more particularly, to the fabrication of microelectromechanical shape-memory thin film fluid control systems such as peristaltic pumps, for example, and methods of fabricating such devices.
The shape-memory effect employed under the present invention arises in certain metal alloys that undergo reversible displacive transformations (‘martensite’ transformations) in which, on cooling, the crystal lattice shears to a new crystal symmetry, with no diffusion occurring, and very little volume change. The strain produced during a shape-memory transformation can produce very large forces that are useful to carry out work.
To produce a shape-memory displacement, the appropriate alloy is first cooled to a temperature at which the structure is fully martensitic, that is, to a temperature below the ‘martensite-finish’ temperature, hereinafter M
f
. If the cooling is done in the absence of stress, the martensite is self-accommodated, and contains a nearly random distribution of 24 variants of the sheared structure, such that the macroscopic deformation is nominally zero. If the martensite is now subjected to an applied stress it can be permanently deformed, but the deformation which takes place is not mediated by dislocation slip, but rather by a cooperative rearrangement of the individual martensite variants via twin-boundary motion. Shape-change occurs by motion of the intervariant boundaries in a way which increases the size of the shear-variants aligned with the deforming stress, while variants which have shears in the antisense of the applied stress shrink or disappear.
The shape-memory effect is manifest when this material is heated to a temperature at which the martensite reverts to the high-temperature phase. This process begins at A
s
, the austenite-start temperature, and is complete at A
f
, the austenite finish temperature. During the transformation, each variant transforms from its monoclinic (low-symmetry) structure back to the high temperature (cubic) structure; since the crystal symmetry is increased, each martensite variant has only one way it can shear to form the parent symmetry. This shear is always the exact opposite of the shear which initially formed the variant, and the material therefore recovers not only the transformational shears (which were self-accommodated) but also the shears imparted by the deforming stress, since the latter were ‘descended’ from the original variants. The result is a recovery (‘memory’) of the shape held prior the deformation of the martensite.
If the alloy is again cooled below M
f
, the transformation once again produces a ‘random’ martensite that will not exhibit shape-memory unless it is again deformed by application of external stress. Thus the exploitation of the effect to make a reversible, cyclic actuator requires the use of a ‘biasing’ element to provide the force needed to deform the martensite phase on cooling.
The so-called shape memory effect is described in relative detail in U.S. Pat. No. 5,325,880 by Johnson et al. which describes a microvalve in the form of a poppet formed of a shape memory alloy. The poppet is suspended within a pressure cavity defined by a base and a valve cap formed of silicon. By applying heat to the shape memory alloy, the alloy blocks the port to preclude fluid flow.
U.S. Pat. No. 5,619,177 which is also by Johnson et al. relates to a more elaborate microactuator wherein the actuator member is comprised of a shape memory alloy layer, an elastic substrate, a base including a first layer of silicon and a second layer of a charge carrying material. In order to activate the apparatus, a heating circuit and a clamping circuit are provided whereby one is held open and the other is held closed to open or close an orifice provided within the base.
While the devices described in the foregoing patents may be useful for the stated purposes, the apparatuses disclosed do not appear to be useful in association with tubular conduits generally and, more particularly, delicate conduits such as biological conduits, i.e., blood vessels and urethras by way of non-limiting example. Neither of the Johnson et al. references appear to be capable of a retrofit application, i.e., installed and used in association with an existing conduit. As such, there is a clear need for the thin film fluid control systems of the present invention.
SUMMARY OF THE INVENTION
In view of the foregoing, a high-efficiency fluid control system in accordance with the present invention generally comprises two principal elements: an actuating element in the form of a shape-memory alloy which produces a large strain when heated from the martensitic form to the austenitic form, and a spring element which stores some of the strain energy released by the shape memory alloy and makes such energy available to reset the martensitic form during the cooling phase of the actuation cycle.
By way of non-limiting example, a high-capacity thin film peristaltic pump in accordance with the teachings of the present invention uses a plurality of segregated titanium-nickel thin-film bands which are tensilized in the martensitic condition and wrapped around a rigid core to capture a flexible tubular member which optionally may be pre-primed with the fluid to be pumped. A properly phased driving signal sequentially excites the TiNi bands to effect peristaltic action. A simplified uniband version may function as a positive shutoff valve.
The bias forces of the present designs are derived directly from the resilience of the tubing as it is pinched, and augmented as necessary by additional forces generated from encapsulants and other packaging materials. No discrete assembly of special springs is required.
The fluid control system of the present invention, i.e., valves and pumps employing tubular conduits, offer numerous advantages over previously known fluid control systems. For example, the fluid control systems of the present invention can be produced at arbitrarily small scales. Tubular members with diameters less than 200 micrometers may be used in extremely compact systems, thus the size will be limited in principal only by the dimensions of the tubular member.
An additional advantage relates to applying the system to a pre-existing tubular member. As such, the fluid control system can be applied in biological systems to control fluid flow, i.e., wrapped around veins and arteries, gas delivery systems, intravenous drug, blood and nutrient transfer tubular members, by way of non-limiting example. It should therefore be understood that the term “fluid” as used herein includes gases and/or liquids generally.
Since the actuating element is mechanically stable in the sense that buckling instabilities are generally avoided, the system will also be adaptable to larger tubing, i.e. flexible tubular members limited primarily by the maximum thickness available based on known flexible tube processing techniques. In this regard it is estimated that a 25 micrometer thick (0.001″) titanium-nickel thin film will have the ability to generate closure forces on tubes exceeding 10 Newtons per millimeter, or approximately 50 pounds per lineal inch of tubing. These forces are more than sufficient to effect closure and peristalsis for a variety of tubing materials.
The systems of the present invention are simple and inexpensive to manufacture and are subject to low capital cost.
The systems of the present invention, through the use of high-transformation shape-memory alloy thin films such as TiNi systems, for example, are useful at high ambient temperature and under severe environments.
The systems of the present invention may also be retrofit; that is, the thin film fluid control systems of the present invention may be deployed in the field on a user-supplied, pre-primed tube such as intravenous fluid, drug delivery and metering, for example.
The systems of the present invention are also adaptable to applications requiring collocation of drive and/or sensor electronics on electronic gr
Board of Trustees operating Michigan State University
Fox John
Harness & Dickey & Pierce P.L.C.
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