Gel-Filled Blood Pressure Transducer

The several features of the present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of the invention;

FIG. 2 is a disassembled view of the invention partly in section; and

FIG. 3 is a cross-sectional view taken along lines 3--3 of FIG. 1.

As shown in the drawings, the transducer 10 is, in operation, connected to a dome 11. The dome 11 has a cavity 12 and two passageways 13 and 14 that communicate with the cavity 12. Passageway 13 is connected to a catheter. Passageway 14 is connected to a saline solution supply, the saline solution filling all of the passageways and the cavity 12 up to the patient's blood vessel. Thus, there is a column of fluid directly from the patient's blood vessel to the cavity. That column of fluid will directly reflect the variations in the patient's blood pressure. The cavity 12 is closed by a rubber diaphragm 20 having a rim 21 imparting a cup-shaped configuration to the diaphragm. The rim 21 is seated and sealed in an annular channel in the dome so as to confine the saline solution in the cavity 12. The diaphragm 20 is in direct contact with the saline solution and, hence, will expand and contract with variations in the blood pressure of the patient.

The reusable transducer 10 has a body 25 having a cavity 26. The cavity 26 has a large opening 28 at one side of the body 25. An annular channel 29 surrounds the opening 28. A cup-shaped nitrile rubber diaphragm 30 having a rim 31 is disposed over the opening 28 with the rim 31 being sealed in the channel 29 to provide an air and liquid-tight seal of the diaphragm with respect to the body.

The body has a tapered bore 35 extending from the cavity 26 to the lower surface 36 of the body. An alumina substrate 40 is adhesively secured to the surface 36 of the body 25 and covers the bore 35. The substrate itself has a hole 41 which, when the substrate is adhesively-secured to the body, communicates with the bore 35. A silicon chip 42 has a pressure-sensitive diaphragm, such as that found on piezoresistive type sensors, that is connected by electrical circuits into a Wheatstone bridge. Other circuit components, including temperature compensation circuit, if necessary, are printed on the alumina substrate. Plastic tabs 44 surround the substrate projecting from the surface 36 to provide positioning elements for the proper location of the substrate on the surface of the body. The substrate is protected by a bottom cap which is secured to the lower surface.

As shown in FIG. 3, the body has still another bore 45 in the side of the body that communicates with the tapered bore 35. The bore 45 is provided for the purpose of filling the cavity 26 with a gel-forming fluid indicated at 50.

In order to fill the body with the fluid 50, the following procedure is carried out.

The fluid 50 initially has two parts, A and B. These parts are the system Visilox V-191 of Visilox Systems, Inc., Troy, N.Y. Part A is a polydimethylsiloxane and includes a platinum catalyst. Part B is also polydimethylsiloxane with a methyl hydrogen cross linker. When the gel is formed, it is a clear, very soft silicone material having the following properties:

Before cured, both parts A and B have a viscosity of 400 cps. In the process of filling the cavity 50, equal parts of A and B are poured together in a container. A magnetic stirring rod is placed in the container. The container is placed on a stirring plate and turned on. The stirring plate is placed in a vacuum chamber so that during the stirring, or mixing, all of the air is evacuated from the liquid.

The liquid is slow to cure. At room temperature, cure time is approximately 24 hours. While still liquid, the mixture is placed in a syringe and the syringe is used to introduce the mixture through the bore 45 into the cavity 26 so as to fill the cavity 26, the tapered bore 35, the hole 41 in the substrate 40 and a portion of the bore 45. Those steps are performed in a vacuum to assure the exhaustion of all of the air in the cavity and replacing it with the gel. A plug 46 is then introduced into the bore 45 to seal the bore 45 off. The amount of liquid in the bore 45, after filling, is sufficiently great that upon introduction of the plug, a portion of it must be pushed toward the cavity, thereby causing a slight distention or stressing of the diaphragm 30. This feature of the invention assures the complete filling of the cavity and elimination of any air. The slightly distended diaphragm will have a good fit with the diaphragm 20 of the dome 11.

The thus totally filled and closed body 25 is heated for four hours at 65.degree. C. (150.degree. F.) to cure the mixture and create the gel having the properties listed above.

The gel will not permeate through small cracks and holes in the body. Rather, the gel acts like a sealant, not allowing air in the cavity or gel out of the cavity.

The gel will not prematurely deteriorate the RTV adhesive used in the pressure sensor assembly. The gel-filled units have a lower volumetric displacement and higher resonant frequency than oil-filled units. Finally, the gel-filled units perform better over time. After 70,000,000 pressure cycles, representing approximately 400 days of use, the transducer's sensitivity change was seven times less on gel-filled units than oil-filled units.

From the above disclosure of the general principles of the present invention and the preceding detailed description of a preferred embodiment, those skilled in the art will readily comprehend the various modifications to which the present invention is susceptible. Therefore, we desire to be limited only by the scope of the following claims and equivalents thereof;