Prototype miniature force transducer that utilizes the shape setting effect of nitinol and closes when heated to 39 degrees Celsius. No need for sutures. Unfortunately, it suffered from durability issues. Poster presented at the 19th Annual Meeting of the Scandinavian Society for Research in Cardiothoracic Surgery, Geilo, Norway, February 2009

1. NEW MINIATURE TRANSDUCERS FOR IN VIVO
CHORDAE TENDINEAE FORCE MEASUREMENTS
J.B. Askov1,2, M.O. Jensen1,2, S.L. Nielsen1, H. Nygaard1,2, J.M. Hasenkam1;
1Cardiothoracic Research Department T, Skejby, Aarhus University Hospital, Aarhus, Denmark
2Department of Biomedical Engineering, Engineering College of Aarhus, Aarhus, Denmark
BACKGROUND
In reconstructive surgery on the mitral valve leaflets a remodeling Figure 1: Chordae
annuloplasty ring is routinely applied as an adjunct procedure to tendineae connects the
support the repaired mitral valve. In order to evaluate the impact papillary muscles with
of flexible properties of different mitral annuloplasty ring designs the mitral valve leaflets.
on chordal tension it is necessary to design an easy implantable Chordal tension is
small force transducer capable of measuring chordal tension (fig. dictated by the flexible
properties of an
1). implanted mitral
annuloplasty ring (triple
HYPOTHESIS dashed line). This
tension can be
We hypothesize that chordal tension is dependent upon flexible measured by a small
characteristics of an implanted mitral annuloplasty ring and it is force transducer (red).
possible to develop a small force transducer capable of measuring
the chordal tension.
AIM Figure 2: Outputs for
The aim of this study is to investigate the possibility of designing different semiconductor
an easily implantable force transducer which is capable of based strain gauges and
foil based strain gauges
measuring the tension in chordae tendineae (CT).
depicted as a function of
chordal tension.
METHODS
Based on chordal tension the outputs from various strain gauge
models were simulated, which provided a thorough overview and
easy comparison between the different performance
characteristics (fig. 1).
A frame was needed to transfer the tension from the chordae to Figure 3: Strain gauges
strain gauge. Using 3D finite element modeling a clip was are mounted upon a
optimized to utilize the shape memory effect of nitinol and create clip. In order to utilize
sufficient amount of strain for the strain gauge (fig. 3). The clip the shape memory
was laser cut, bend into shape in a custom made tool and heat effect of nitinol the clip
must be optimized to
treated. create an even
distribution of high
RESULTS stress.
The shape memory effect of nitinol keeps the clip open at
temperatures below 5°C and closed at temperatures above 30°C.
This enables easy implantation as the clip closes around CT when
heated, see figure 4.
Based on strain gauge comparison the frame was equipped with
two strain gauge model E (fig. 4). This enabled the force
transducer to theoretically measure less than 0.01N of chordal
tension (fig. 5).
CONCLUSION
Simulation and a prototype of a chordal force transducer capable
Figure 4: The shape memory effect enables the clip to be bend open at
of measuring small changes in chordal tension have been
cold temperatures (<5°C) and to close when heated to warm temperatures
successfully accomplished. The principle of attaching the force (>30°C). The final frame measured 5mm by 2mm by 3mm. Left: Cooled clip
transducer to a single chordae without the use of suture has been in open state. Right: Clip attached to CT in warm state.
demonstrated and makes implantation easy. Thus, the basis for in
vivo and in vitro testing of the force transducer is established.
ACKNOWLEDGEMENTS
This research was made possible by support from The Danish Heart
Foundation grant 08-10-R68-A2164-B905-22500, Snedkermester Sophus
Jacobsen og Hustru Astrid Jacobsens Fond and The Augustinus
Foundation.
Figure 5: Close up of the final clip mounted
with semiconductor based strain gauges
and wires.