ULTRASONIC BONDING OF A FELT FILTER TO A FRAME IN A 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 softening the surface of the frame and embedding the fibers of the felt therein under pressure.

Description: ULTRASONIC BONDING OF A FELT FILTER TO A FRAME IN A 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 a fibrous filter material is ultrasonically embedded in the filter frame. Related Cases This case is a continuation-in-part of copending application serial Number 08/728,875 filed 10. October.1996. 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 manufactured 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 adhesive bondants or tie straps. Until the present invention, however, it was considered unfeasible to secure the polyester felt medium to the frame by a heat- bonding process because of the different properties of the two materials. More specifically, the felt material, due to the presence of the air spaces between the fiber of the felt, tended to burn and tear before it had softened sufficiently to melt together with the more solid frame material. This was exacerbated by the fact that medical-grade polyester, and that any heat application sufficient to soften the felt would severely weaken or even destroy the frame. Summary of the Invention The present invention achieves the objective of sealingly bonding the filter to the frame without bondants or tie straps, by embedding the polyester filter material in a softened thermoplastic energy and mechanical pressure to the felt and frame. Brief Description of the Drawings Fig. 1 is a vertical section of a hardshell venous reservoir in which the invention is used; Fig. 2 is a vertical section of the cardiotomy frame end 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 in Fig. 3; and Fig. 5,, is a schematic diagram illustrating the ultrasonic embedding 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 polystyrene supporting frame 26 with a melting point of 80° C; a woven 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, and its polyester material may preferably have a melting point of about 200° C. Because it is essential that no solid matter in the cardiotomy blood be able to circumvent the 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 it 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 j^. 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 while being ultrasonically vibrated, push the fibers of the felt material into the softened surface of the frame 26 (Fig. 4). The vibrating action of the horn 38 under pressure against the workpiece is preferably so adjusted as to heat and partially melt the surface thermoplastic frame 26 but to leave at least a portion of its thickness intact. 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 vibration 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 holding device 41. In a representative practical embodiment of the invention, a 3.5 rnm thick polyester felt 28 was embedded into a polystyrene frame 26 with a 20 kHz horn 38 encompassing slightly more than 120° of the circumference of the frame. A vibration 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 frame 26, at a steady-state pressure of 45 _+ 10 PSIG for 0.36 _+ 0.1 seconds. The longer duration of the vibration pressure assured the presence of vibration 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 combination of vibration pressure and steady- state pressure firmly pounded the fibers of the felt 28 into the surface of the frame 26 which had been softened by ultrasonically generated heat energy. Upon withdrawal of the horn 38, the fibers remained trapped in the surface of the frame 26 until they firmly locked thereto by cooling and solidifying of the softened surface. The above-described ultrasonic heat-bonding 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 heat-bonding of an unsoftened polyester filter material to a polystyrene frame in a 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: Claims: 1. A method of sealing 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 having a relatively low melting temperature; b) positioning a layer of biocompatible plastic fibrous filtering material having a substantially higher melting temperature around the circumference of said frame; and c) ultrasonically heat-softening the surface of said frame while embedding said filtering material in an unsoftened state into said frame by pressing said material and frame together while said surface is softened. 2. The method of Claim 1, in which said filtering material is polyester and said frame is made polystyrene. 3. The method of Claim 2, in which said filtering material is a screen. 4. The method of Claim 2, in which filtering material is felt. 5. The method of Claim 1, in which said felt and frame are sealingly bonded along the top and bottom circumference of said frame. 6. The method of Claim 1, in which said neat-softening and pressing 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 heat-softening and pressing step includes the step of applying said horn to said filtering material while ultrasonically vibrating it for substantially 0.42 _+ 0.1 seconds at a vibration pressure of substantially 40 _+ 10 PSIG and simultaneously holding it against said felt filtering material for substantially 0.36 ± 0.1 seconds at a high steady-state 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: 1998
Main classification: A61M1
IPC codes: A61M1