294 THOUGHTS A N D PROGRESS

Detachable Shape-Memory Sewing Ring for Heart Valves

Josef Jansen, Sebastian Willeke, Helmicl Reul, and Giinter RUM from Helmholtz Institute for Biomedical Engineering,

Technical University of Auchen, Auchen, Germany

Abstract: The novel sewing ring concept consists of a detachable ring snap connection between valve housing or stent and suture cuff. The suture cuff itself is a memory metal ring of nickel-titanium (NiTi) that is embedded within a textile fabric torus. After a special training pro- gram the ring can assume two geometric shapes when alternately cooled and heated. Due to this physical prop- erty, the sewing ring can be safely fixed within a groove of the housing or stent by changing between room and physiological temperature. This allows the surgeon first to implant the sewing ring followed by arbitrary positioning and final fixation of the valve by the shape-memory effect. Additional advantages are an increased stiffening of the valve base for minimizing potential leaflet dysfunction, and reduced time and risk of implantation, particularly when a reoperation is necessary. Key Words: Sewing ring-Heart valve prosthesis-Shape-memory effect- Detachable ring snap connector-Nickel-titanium mem- ory alloy.

Why look for a new suture ring concept? The objective of this study was to simplify the

sewing ring technique and to adapt this concept es- pecially to the requirements of flexible trileaflet poly- meric heart valves. Current suture ring configura- tions have the potential risk of damaging the leaflets while flexible valves are implanted. Furthermore, standard techniques of attaching the cuff to the valve are relatively complicated since the cuff is sutured by hand to the valve housing in such a way that the textile cuff material is tied and sutured into the groove of the valve ring in various shapes. There- fore, a novel sewing ring concept was developed, consisting of a detachable ring snap connector be- tween valve stent and sewing ring. For mechanical valves, the new concept facilitates free orientation of the occluder after insertion of the valve.

Requirements for a suture ring Besides the hemodynamic features of a heart

valve, the design and handling of its sewing ring are very important to the surgeon. Material and profile

Received August 1991; revised January 1992. Address correspondence and reprint requests to Dr. J . Jansen

at Helmholtz Institute for Biomedical Engineering, Pauwelsstr. 30, 5100 Aachen, Germany.

This work was presented in part at the VlIlth World Congress of the International Society for Artificial Organs, held August 19-23, 1991, Montreal, Canada.

of the suture ring are significant for the implantation itself as well as for the adaptation to the anatomical and physiological characteristics of the implantation site. The suture ring should be soft and pliant to facilitate coaptation to the host annulus.

A characteristic radiographic image of the individ- ual valve type is desired for postoperative investiga- tions, especially for nonvisible polymeric valves. The relatively soft plastic stent of such valves can potentially be deformed by implantation into a de- formed noncircular annulus, eventually leading to dysfunction of the flexible closing leaflets. While implanting the valve there are two more risks of damaging the leaflets with subsequent dysfunction: damaging the soft leaflets by stitching or by suture trapping at the tip of the stent posts, especially dur- ing mitral implantation. For mechanical valves it is highly desirable to have the possibility of free valve orientation after insertion.

Concept for a new suture ring The requirements of radiopacity and secured stiff-

ening of the valve stent can be fulfilled by placing a metal ring around the stent. The risk of damaging the soft leaflets can be avoided by applying a concept in which only the sewing ring is implanted, followed by insertion of the valve itself. This technique also facilitates free valve orientation. Ideally, the pros- thesis should be even detachable again from the sew- ing ring, in order to ease reoperation. A reoperation, which is sometimes necessary due to the limited lifetime of a prosthesis, represents a risk for the patient. Normally, the valve rings are removed from the annulus, including the ingrown tissue. This prep- aration damages the annulus tissue and therefore limits the number of reoperations.

The new concept avoids these complications since it consists of a detachable ring snap connection be- tween valve housing or stent and sewing ring.

Material, method and result A functional solution for a detachable ring snap

connector is a metal ring of nickel-titanium (NiTi) shape-memory alloy.

Description of the memory effect The memory effect is the property of certain metal

alloys of reassuming a preformed shape upon chang- ing temperature. If memory alloys are deformed in a cold state and are subsequently heated then they assume their original shape again. The metal thus is able to remember two totally different, previously formed shapes and to assume those by either heating or cooling. Referring to the intended application,

Ar/ i /" )rgun\ , V d . 16, N o . 3, 1YY2

THOUGHTS AND PROGRESS 295

FIG. 1. The one way principle: reversible martensite transformation.

this effect means that a ring can move between two diameters for connecting two circular parts. The so- called two way shape-memory effect is of interest for this application.

The physical basis for this extraordinary behavior is the reversible martensite transformation, whereby a shape-memory alloy assumes two different crystal- line structures as a function of temperature and/or external mechanical stresses. At lower temperatures the martensite phase is stable, and at higher tempera- tures the austensite phase is stable. This reversible martensite transformation is shown schematically in Fig. 1. By mechanical deformation, the material in the martensite phase changes during subsequent heating into the austensite phase and assumes again its original shape. This behavior is called the one way effect. By a special training treatment the mate- rial can obtain a two way shape effect. In that case, no external mechanical deformation is required. Re- peatable strains of up to 3% between the two fixed geometric states can be achieved (I) . The alloy can be composed with tight temperature limits (i.e., be- tween room and physiological temperature) for shaping the geometry of the part. Due to its corro- sion-resistant properties, the alloy is also suitable as implant material ( I ) . The main disadvantage of NiTi alloys is the high price for the material itself and for the machining of parts due to its ductile properties.

2 MARTENSITE deformed * ? * f a + **+ f

1 MARTENSITE CRYSTAL mechanical deformation e+-t.

-4 t * + a + * .

undeformed shape

deformed shape

Design o f t h e suture ring The sewing ring is a memory metal ring of NiTi

embedded in a textile fabric torus (Fig. 2 ) . The sew- ing ring can thereby be safely fixed, form or force locking within a groove of the housing or stent by changing approximately between room and body

. . . * & r ' '* 3 AUSTENSITECRYSTAL

original shape

temperature. To achieve this effect the memory part has first to undergo a special training program.

The training is shown schematically as straidtern- perature functions in Fig. 3 . Mechanical strain is applied by expanding the ring over a cone up to defined strain limits. This extension is done as the first step of each one way effect cycle. Then the part is heated until its permanent deformation reverses almost completely. After cooling down the part again, this deformation/tempering cycle is repeated several times. After approximately seven to ten such cycles the ring is trained to a stable process such that

FIG. 2. Valve fixation by temperature-induced cuff memory effect.

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296 THOUGHTS AND PROGRESS

FIG. 3. Training program for two-way shape-memory ring.

it assumes two geometric shapes when alternately cooled and heated.

The surgical technique may now be implemented as follows (Fig. 4). First, the ring is sutured into the annulus position. After cooling down the annulus area to room temperature (slightly below the blood temperature of the patient), the ring expands. The valve housing can then be inserted into the expanded cuff. The valve may now be rotated relative to its suture ring for precise in situ positioning. Subse- quently, the valve ring, including the surrounding tissue area, is heated to physiological temperature, which leads to shrinking of the ring and, thus, to safe fixation of the valve.

/ 1st. SUTURING OF CUFF 2nd: ARBITRARY POSITIONING

3rd. FINAL FIXATION BY SHAPE OF VALVE

MEMORY SEWING RING

\

FIG. 4. New valve insertion technique with shape-memory sewing ring.

Discussion and conclusion The surgical techniques for implantation of all

heart valve types, synthetic, mechanical, or biologi- cal, can be simplified, and new possibilities are: of- fered. With the new sewing ring concept, first the cuff is sutured into position, followed by arbitrary orientation and final fixation of the valve due to the shape-memory snap connection. The prospective advantages of such a sewing ring can be summarized as follows. The risk of damaging the soft leaflets of flexible valves by stitching or by suture trapping of stent posts is eliminated. The valve can be rotated relative to its suture ring for precise in situ position- ing. Secured stiffening of the valve housing (stent) minimizes ring deformation, thereby eliminating the potential for leaflet dysfunction. There is reduced time and risk for implantation, particularly when a reoperation is necessary. There is increased radio- pacity for improved valve monitoring. Finally, man- ufacturing of valve and sewing ring can be performed separately, followed by a very easy intraoperative coupling.

In order to demonstrate the feasibility of this new sewing ring concept, animal experiments are planned as the next step. To further improve: the properties of sewing cuffs, it might be necessary to additionally coat parts of the cuff by, e.g., pyrolitic carbon to retard pannus overgrowth from the sewing cuff to the valve housing. This precaution should secure the detachability of the valve from the sewing ring for explantation.

This concept of a detachable ring snap connector can also be applied to a variety of other connectors, e.g., as a quick connector between cannulae and blood pumps.

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THOUGHTS A N D PROGRESS 297

Acknowledgment: We thank Dr. Bensmann, Krupp Forschungsinstitut GmbH Essen, for providing free sam- ples of the NiTi alloy.

References I . Bensmann G, Baurngart F, Hartwig J.,, Untersuchungen der

Memory-Legierung Nickel-Titan und Uberlegungen zu ihrer Anwendung im Bereich der Medizin. Tech M i f f Krupp Forsc,h B c ~ 1979;37 : 2 1-34.

The Valvo-Pump, An Axial Blood Pump Implanted at the Heart Valve Position:

Concept and Initial Results

Krnichi Yamcizaki, Eiji Okamoto, Katsuyilki Ya mamo to , * Yos hin ori Mit a m uru, -t Ta kas h i Tanuku, and jRyohei Yozir f rom Division of

Biomedical Engineering, Facirlty of Engineering, H o k kaido University, Sup poro , * Depart men t of Elrctronic and Information Engineering, School

o f Engineering, Hokkaido Tokui University, Sup poro , t Yusir h isa Man i&ct wing Com pu n y ,

Tokyo, and $Department of Cardiovascular Sirrgery, School of Medicine, Keio University,

Tokyo , Japan

Abstract: The valvo-pump, an axial nonpulsatile blood pump implanted at the heart valve position, has been de- veloped. The valvo-pump consists of an impeller and a motor, which are encased in a housing. An impeller with 5 vanes (22.0 mm in diameter) is used. The impeller is connected to a samarium-cobalt-rare earth magnet direct current (DC) brushless motor measuring 21.3 mm in diam- eter and 18.5 mm in length. Sealing is achieved by means of a ferrofluidic seal. A pump flow of 10.5 Limin was obtained at a pump differential pressure of 3.3 kPa (25 mm Hg), and a flow of 4.9 Limin was obtained at 7.0 kPa (53 mm Hg). Sealing was kept perfect against a pressure of 29.3 kPa (220 mm Hg) at 9,000 rpm. Key Words: Artificial heart-Nonpulsatile blood pump-Axial flow pump.

In the severely diseased heart, the myocardium does not work as a pump. However, the myocardium can be used as a reservoir or housing for a blood

Received August 1991; revised January 1992. Address correspondence and reprint requests to Dr. Y . Mita-

mura at Department of Electronic and Information Engineering, School of Engineering, Hokkaido Tokai University, 5-1-1-1 Mi- narninosawa, Minami-ku, Sapporo 005, Japan.

This work was presented in part at the VIIIth World Congress of the International Society for Artificial Organs, held August 19-23. 1991, Montreal, Canada.

pump. If a miniature actuator can be implanted at the heart valve position, preserving the diseased heart muscle, the system will work as an artificial heart does. Because the actuator is implanted in the heart, it will be a simple nonpulsatile pump. We have named such an artificial heart the “valvo-pump.”

The valvo-pump has several advantages over a total artificial heart: it fits well anatomically with the circulatory system because it is implanted at the heart valve position, the blood-contacting surface of the valvo-pump is relatively small because the myocardium is used as a pump housing, and the valvo-pump is implanted as easily as an artificial heart valve replacement is. The valvo-pump was invented by Yozu ( I ) , and preliminary studies have been conducted ( 2 , 3 ) . In this study, the previous model of the valvo-pump was improved by designing a new impeller.

It is the purpose of this study to demonstrate the feasibility of the concept of the valvo-pump based on the improved valvo-pump.

Materials and methods The valvo-pump, an axial flow pump, consists of

a motor and an impeller. The impeller was designed after the conventional design procedure of an axial flow pump (4,5), and the motor was selected among commercially available motors. The characteristics required for the valvo-pump are a pump flow of 5 L/min and a pump head of 13.3 kPa (100 mm Hg).

The design points of the designed impeller were a pump flow of 25 L/min and a pump head of 13.3 kPa (100 mm Hg). The specific speed of the impeller was 1,130 (m, m3/min, rpm), and the rotational speed was 9,000 rpm.

The outer diameter of the impellers is 22 mm. This impeller has a boss ratio of 0.6 and a vane discharge angle of 11.1”. The number of vanes is five.

We selected a motor for the valvo-pump among commercially available motors, based on power and size. The efficiency of an impeller depends on the specific speed at an operating point. In an axial flow pump, the maximum efficiency of the impeller is 76% when the specific speed is 1,640 (5 ) . The impellers designed for the valvo-pump have a specific speed of 1,130, and the efficiency of this impeller is esti- mated at 74%. With this efficiency the power re- quired for the motor at the design point is 7.5 W. Since the rotational speed is 9,000 rpm, the torque required for the motor at the design point is 0.008 Nm. Considering these values and the size, we se- lected a DC brushless motor RBE-400 (Inland, Rad- ford, VA, U.S.A.) . This motor has a torque of 0.007

ArtifOrgnns, V o / , 16, No. 3 , 1992