ULTRASONIC WELDING OF FELT FILTER IN HARDSHELL RESERVOIR

Abstract: A polyester felt depth filter is attached to a rigid thermoplastic support frame in a cardiotomy filter/defoamer without the use of tie straps or bondants by ultrasonically welding the felt to the frame.

Description: ULTRASONIC WELDING OF FELT FILTER IN HARDSHELL RESERVOIR Field of the invention This invention relates to a method of manufacturing a cardiotomy filter in a hardshell venous reservoir, and more particularly to a method in which the filter material is ultrasonically welded to the filter frame. Background of the invention Cardiotomy blood filter/defoamer structures used in hardshell venous reservoirs conventionally include a rigid apertured cylindrical frame which supports defoaming layers and a filter layer. These layers defoam and filter the cardiotomy blood as it passes through them. The filter layer is typically formed from a polyester felt or screen. It is typically placed around the outer circumference of the frame and is secured to the frame in sealing relationship by using nylon tie straps and silicon glue or urethane potting. The use of silicon glue or other bondants, alone or in conjunction with tie straps, presents manufacturing problems that can be quite severe. For example, tie straps are typically washed in water after manufacture, and thereby acquire a substantial amount of pyrogens. Sterilization removes most of these pyrogens but low levels survive sterilization. If the pyrogen level in the straps was high initially, this may result in an unacceptable level of pyrogens in the filter/defoamer after sterilization. Extensive and costly rework and lot evaluation is then needed to determine if the manufacturing lot is usable or not. Also, current manufacturing methods are prone to cause worker ergonomic issues resulting from the repetitive movements involved in tie strap assembly. It is therefore desirable to secure the filter to the frame without the use of bondants or tie straps. Until the present invention, however, it was considered unfeasible to secure the polyester medium to the frame by a welding process because of the different melting points of the two materials. Summary of the invention The present invention achieves the objective of sealingly bonding the filter to the frame without bondants or tie straps, by ultrasonically welding the polyester filter material to a thermoplastic (e.g. polystyrene) frame while applying mechanical pressure thereto. Brief description of the drawings Fig. 1 is a vertical section of a hardshell venous resrvoir in which the invention is used; Fig. 2 is a vertical section of the cardiotomy frame and depth filter; Fig. 3 is a plan view of the frame and filter of Fig. 2 showing the ultrasonic horn, and Fig. 4 is a detail side elevation as indicated by 4 in Fig. 3; and Fig. 5 is a schematic diagram illustrating the ultrasonic welding system. Description of the preferred embodiment Fig. 1 shows the environment of the invention. A hardshell venous reservoir 10 contains a blood processing unit 12 which includes a cardiotomy filter/defoamer section 14 and a venous microair defoamer section 16. In the cardiotomy section 14, cardiotomy blood is introduced into the cardiotomy chamber 18 and flows from there into the reservoir 20 through the filter/defoamer assembly 22. The assembly 22 typically consists of an inner defoaming layer 24 of porous defoaming-chemical impregnated polyurethane foam; a rigid thermoplastic supporting frame 26; a polyester felt or screen filter 28 for filtering out bone chips, blood clots, flesh particulates or sutures suctioned out of the surgery field; a secondary defoaming layer 30 of porous defoaming-chemical impregnated polyurethane foam; and a secondary filtering and flow-retarding tricot sock 32. The thickness of the filter 28 may typically range from about 0.65 mm to 6.5 mm. Because it is essential that no solid matter in the cardiotomy blood be able to circumvent the depth filter 28, the filter 28 must be sealingly attached to the frame 26 along its top and bottom circumference 34, 36 (Fig. 2). To achieve this without the use of bondants or tie straps, the invention in its preferred embodiment uses a titanium ultrasonic horn 38 (Fig. 3) which is vibrated by a hydraulic ultrasonic vibrator 39 (Fig. 5) at a frequency of about 15-25 kHz. The face of the horn 38 takes the form of several parallel ridges 40 which, when the horn 38 is pressed against the felt layer 28 and frame 26, anchor the felt material into the frame material (Fig. 4). The vibrating action of the horn 38 under pressure against the workpiece heats and partially melts the polyester felt or screen 28 and the thermoplastic frame 26. As schematically indicated in Fig. 5, the ultrasonic vibrator 39 cyclically presses the horn 38 against the filter material 28 and frame 26 at the vibration frequency with a weld pressure. At the same time, the horn 38 is also pressed against the filter material 28 and frame 26 with a higher steady-state hold pressure by the holding device 41. In a representative practical embodiment of the invention, a 3.5 mm thick polyester felt 28 was welded to a polystyrene frame 26 with a 20 kHz horn 38 encompassing slightly more than 120° of the circumference of the weld. A weld pressure of 40 ± 10 PSIG was applied to the horn 38 for 0.42 ± 0.1 seconds while the horn was pressed against the felt 28, backed by the frame 26 at a hold pressure of 45 ± 10 PSIG for 0.36 ± 0.1 seconds. The longer duration of the weld pressure assured the presence of weld pressure during the entire holding time. The workpiece was then rotated 120°, and the above sequence was repeated twice more to complete the weld along the entire 360° circumference of the workpiece. The above-described welding method not only eliminates a major source of pyrogens in the manufacture of hardshell venous reservoirs, but also reduces ergonomic problems in the assembly of the reservoirs. It also meaningfully reduces the cost of the product by eliminating the straps and glue, and by reducing the assembly time. It is understood that the exemplary ultrasonic welding of polyester filter in hardshell reservoir described herein and shown in the drawings represents only a presently preferred embodiment of the invention. Indeed, various modifications and additions may be made to such embodiment without departing from the spirit and scope of the invention. Thus, other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of different applications.

Claims: What is claimed is: 1. A method of sealingly mounting a felt depth filter on a thermoplastic support frame in a cardiotomy filter, comprising the steps of: a) providing a rigid substantially cylindrical thermoplastic frame; b) positioning a layer of biocompatible plastic filtering material around the circumference of said frame; and c) ultrasonically welding said plastic material and frame by heating them while pressing them together. 2. The method of Claim 1 , in which said filtering material is polyester and said frame is made of polystyrene. 3. The method of Claim 2, in which said filtering material is a screen. 4. The method of Claim 2, in which said filtering material is felt. 5. The method of Claim 1, in which said felt and frame are welded along the top and bottom circumference of said frame. 6. The method of Claim 1 , in which said welding step is performed by an ultrasonic horn subjected to ultrasonic vibrations while being pressed against said felt. 7. The method of Claim 6, in which said felt is pressed into said frame by ridges formed on said horn. 8. The method of Claim 1, in which said welding step includes the steps of applying said horn to said filtering material while ultrasonically vibrating it for substantially 0.42 ± 0.1 seconds at a weld pressure of substantially 40 ± 10 PSIG while holding it against said felt filtering material for substantially 0.36 ± 0.1 seconds at a higher pressure. 9. The method of Claim 8, in which said higher pressure is substantially 45 ± 10 PSIG. 10. The method of Claim 8, in which said ultrasonic vibration has a frequency of substantially 15-25 kHz.

Inventors: VAN DRIEL MICHAEL R, VAN DRIEL, MICHAEL, R.
Applicants: MEDTRONIC INC, MEDTRONIC, INC.
Application year: 1997
Main classification: A61M1
IPC codes: A61M1, B01D29, B01J19, B29C65