CAPABILITIES OF IMPLANTABLE TRANSDUCERS 
Eph Konigsberg* 
ABSTRACT 
The capabilities of implantable pressure transducers 
are set forth, both for standard specifications and for 
less commonly used criteria. Distinctions are made be- 
tween expected performance in bench tests and in vivo, 
and between short term and long term expectations. 
The effect of the transducer on the animal and the im- 
plantation environment on the transducer are also set 
forth. Some estimates are made on probable expectations 
for long term implants. 
INTRODUCTION 
In this paper we primarily discuss implant- 
able pressure transducers of the type which 
were first introduced some six years ago. These 
devices consist of a titanium disc 7.0 mm. in 
diameter, and 1.2 mm. thick, with a 2 mm. di- 
ameter stem, from which exits the transducer 
cable. Within this disc is an inner pressure 
reference cavity, behind the integral pressure 
sensing diaphragm. Aflfixed to the inner surface 
of the diaphragm are (usually) four semicon- 
ductor strain gages, arranged in a Wheatstone 
bridge. With proper excitation and compensa- 
tion for temperature effects, an output signal 
is produced proportional to the pressure applied 
to the face of the disc. The discussion below will 
apply, with some modifications, to comparable 
instruments of smaller diameter or slight dif- 
ferences in thickness. 
It is important to point out that we describe 
herein what can be expected on a 1 sigma prob- 
ability. Transducers with improved or inferior 
capabilities may be obtained, either by selection, 
more careful process or manufacturing control, 
improved specifications, etc. Further, we de- 
scribe herein, especially in our description of 
long term results, transducers which were de- 
signed and manufactured several years ago. 
Long term data regarding items now being pro- 
* Konigsberg Instruments, Inc., Pasadena, California. 
duced must await the results of further tests 
now being conducted. 
METHODOLOGY 
First, let us discuss the more standard specifi- 
cations, and the results to be expected from a 
transducer which is now, and which is bench 
tested in the laboratory. The ranges given in 
the table below cover transducers which are 
from 3 to 7 mm. in diameter. Generally (though 
not always) the larger instruments will have 
the more preferable accuracy specifications : 
Parameter 
Units 
Range 
Non-Linearity - 
-1- % F.S., B.S.L. 
0.2 to 0.5 
Hysteresis - 
. .. % F.S. 
0.1 to 0.3 
Non Repeatability 
% F.S. 
0.2 to 0.3 
Static Error Band . 
■+- % F.S., B.S.L. 
0.3 to 0.6 
( Combined Non-Linearity, 
Hysteresis, and Non-Repeat- 
ability) 
Zero Temperature Shift 
-+- mm. Hg./°C 
0.1 to 0.3 
Sensitivity Temperature Shift 
■+- % reading/°C 
0.2 to 0.3 
Pressure Range, Std. 
mm. Hg. 
300 
Output - - 
mV/300 mm. Hg. 
5 to 60 
Some comments on the above: First, non- 
linearity, hysteresis, and non-repeatability are 
generally such small numbers that they are al- 
most always quoted in combined form. Second, 
those of you familiar with typical implantable 
transducer specifications will note that the er- 
rors or inaccuracies quoted above are small, in 
relation to the specifications usually quoted. 
This is for two reasons : we are discussing one 
sigma probabilities, and specifications have to 
take into account the occasional instruments 
which are slightly less accurate. Further, there 
is some degradation of certain aspects of per- 
formance in vivo and specifications attempt to 
take this into account. 
There are additional accuracy and capabilities 
criteria which should be considered, and which 
1145 
